摘要:

J. MATER. CHEM., 1994, 4(12), 1811-1814 Polyaniline Alloys with Poly(3-sulfonato-4-hydroxystyrene) Motomichi Inoue,*a Felipe Medrano,a Masanobu Nakamura,at Michiko 9. Inouea3band Quintus Fernando" a ClPM, Universidad de Sonora, Apartado Postal 730, Hermosillo, Sonora, Mexico Department of Chemistry, University of Arizona, Tucson, AZ 85727, USA Oxidation of aniline with ammonium peroxodisulfate in aqueous solutions containing poly(3-sutfonato-4-hydroxystyrene)gave poly- mer alloys (or polymer blends) with a general composition of {(-C,H,NH-)[-CH2-CH(C,H3.0H .SO,-)-~H,SO&*ZH~O}~. Their electrical conductivities depended on the composition and varied between and 0.5 S cm-' at 300 K. The polyaniline salts were soluble in water and exhibited an electronic absorption band characteristic of emeraldine salts at 800nm.The corresponding alkaline solutions showed a band due to emeraldine bases at 600nm. These bands showed a pH dependence that is a consequence of an interchain interaction between two kinds of polymer chains. A polymer with x =0.4, y =1.2 and z =3.6 was soluble enough to observe EPR spectra, whose pH dependence showed that, with increasing pH, deprotonation occurs at polarons prior to deprotonation at bipolarons. Preparation of polymer blends (or polymer alloys) of polyani- line is one of the more versatile approaches for the modifi- cation of the physical properties (including electrical, electrochemical and mechanical properties) of the electrically conducting polymer. It has been reported that the electro- chemical polymerization of aniline in polymer electrolytes such as poly(acry1ic acid), poly(vinylsu1fonic acid) and poly( 4- sulfonatostyrene) gives polymer alloys whose electrochemical properties are different from those of polyaniline Polyanilines in the form of water-insoluble particles of col- loidal dimensions also have been obtained by chemical oxi- dation of aniline in aqueous solutions containing polymers as stabilizers, and the morphology and electronic absorption spectra of these polyanilines have been When polyaniline is doped with organic or polymer acids such as alkylbenzenesulfonic acids and poly(alky1phosphonic acids), the resulting polyaniline salts are soluble in organic solvent^.^.^ Recently, it was reported that water-soluble poly- anilines were obtained by polymerizing aniline monomers on a polymer template, although the specific reaction conditions and the compositions of the polyanilines were not elucidated." Poly(4-~ulfonatostyrene)-dopedpolyaniline was also reported to be water-soluble when it was prepared by using a dopant which had been prepurified by dialysis;" earlier papers reported that this polymer alloy was water-in~oluble,~,~ and the origin of this discrepancy is not clear.The solubilities of these polyaniline salts are induced by the surfactant nature of the counter-anions; soluble polyaniline can be obtained when an appropriate dopant is selected. The introduction of an -OH group into poly(styrene) leads to significant changes in physical properties such as solubility and miscibility, owing to the formation of hydrogen bonding through the -OH group in the resulting poly(4- hydroxystyrene).12-14 This observation suggests that a sulfo- nated poly( 4-hydroxystyrene) may form a polyaniline alloy with higher solubility than the polyaniline-poly(su1fonatosty-rene) alloy.In our preliminary paper, we reported that water- insoluble polyaniline was soluble in an aqueous solution containing 50% sulfonated poly(4-hydroxystyrene), suggest- ing the formation of a polymer ~ornplex.'~ In the present study, we have used 100% sulfonated poly( 4-hydroxystyrene), i.e. poly (3-sulfonato-4-hydroxystyrene) (abbreviated as PSHS) as counter-anions, and obtained polymer alloys that are water-soluble and also electroconductive.This paper reports t On leave from Cosmo Oil Co. Ltd., Tokyo, Japan under a contract with the Japan International Cooperation Agency. poly (3-sulfonato-4-hydroxystyrene), PSHS the syntheses, the solution electronic absorption spectra and the EPR spectra of these new polyaniline alloys. Experimenta1 Materials Poly(3-sulfonato-4-hydroxystyrene sodium salt), NaPSHS, was supplied as a 32.7 wt.% aqueous solution from Cosmo Oil Co. Ltd., Tokyo, Japan. The mean molecular weights were: M, =4500 and M, =27 500. Aniline suppliad from Merck was distilled before use. Ammonium peroxodisulfate supplied from Aldrich was used without further purification. Polymerization Polymer I was synthesized in the molar ratio AN :SHS-unit = 1 :1.1, as follows.4.2 ml (6.2 mmol for the monomer unit) of the NaPSHS solution were diluted up to 20m1, and passed twice through a column of strongly acidic ion-exchauge resin (DOWEX 50x8-100). The eluent was concentrated to 20 ml. In the resulting solution was dissolved 0.52 ml (5.7 nzmol) of aniline (AN), and solid ammonium peroxodisulfate (1.3 g, 5.7 mmol) was added slowly with stirring. After the reaction mixture had been stirred for 2 h, the dark solid 1hat was formed was separated by filtration, washed with 50ml of 0.05 mol 1-' sulfuric acid and with a small amount of water, and dried in vacuum. Analytical data are shown in Table 1. Polymer I1 was obtained for the molar ratio AN :SHS =1:3; other reaction conditions were identical with those for polymer I.The product that was formed as a suspension was separated by centrifugation. Polymer I11 was synthesized by using NaPSHS in dilute sulfuric acid in the molar ratio AN:SHS= 1: 1.1. 4.2 ml (6.2 mmol) of the NaPSHS solution were mixed wiith 20 ml of 1 mol 1-' sulfuric acid containing 0.52 ml (5.7 mmol) of aniline. To the resulting mixture was added 1.3 g (5,7 mmol) of solid ammonium peroxodisulfate with stirring. 4fter the reaction had continued for 2 h, the dark solid that was formed J. MATER. CHEM., 1994, VOL. 4 Table 1 Powder electrical conductivity, CT,at 298 K and analytical data of {(-C,H4NH-)[-CH, -CH(C,H,.OHSO;)-],( H,SO,);zH,O}, ' composition found(%)(required) polymer X Y 2 a/S cm-' C H I 0.3 0.8 1.4 0.5' 40.40 4.58 (39.63) (4.55) I1 0.4 1.2 3.6 10-5 31.78 5.02 (3 1.27) (4.96) I11 0.6 0.2 1.5 0.2b 50.24 4.93 (50.42) (4.93) 'Sulfate ions are shown as sulfuric acid, but some of them may be involved as anilinium sulfate.temperature limit was 0.070 eV for polymer I and 0.08 eV for polymer 111. N S 5.41 13.50 (5.50) (13.85) 3.91 15.01 (3.96) (14.51) 5.52 10.24 (5.44) (9.97) The activation energy at the high-was collected by filtration, washed with 50 ml of 0.05 mol 1-' sulfuric acid followed by a small amount of water, and dried in vacuum. Physical Measurements The electrical conductivity of a compressed pellet was deter- mined by van der Pauw's four-probe method or the standard two-probe method.The IR spectra of the compounds in KBr pellets were obtained with a Perkin-Elmer 1600 FTIR spec- trometer. The electronic absorption spectra were recorded with a Perkin-Elmer Lambda 2 UV-VIS spectrophotometer. For the measurements of pH dependence, sample solutions were prepared as follows. 200 mg of polymer 11, for example, were dissolved in 50 ml of 0.1 rnol I-' sodium sulfate. The pH of a 5 ml aliquot of the solution was adjusted by adding 1 mol 1-' NaOH or 1 mol 1-l H2S04; each of the resulting solutions was diluted to 16 ml with 0.1 rnol 1-' sodium sulfate so that the ionic strengths of the sample solutions were almost identical with one another. For polymers I and 111, a small amount of insoluble components was eliminated by filtration prior to the sample preparation.The EPR spectra were obtained with the aid of a Bruker ESP-300E spectrometer operating at a microwave frequency of 9.65 GHz. Each sample solution was sealed in a quartz tube under vacuum. Results and Discussion Table 1 shows the compositions determined by elemental analyses, together with the electrical conductivity data. The elemental analyses showed that SO,2-ions are intrinsically involved in addition to PSHS- anions; the former anions were not removed by washing with water. In Table 1, sulfate ions are shown as sulfuric acid, but some of them may be involved as anilinium sulfate; the real charge density located on a polyaniline unit may differ from that calculated from the composition shown in Table 1.The temperature dependence of the electrical conductivities of polymers I and 111 fits the equation, 0=go exp(-E/kT) at high temperatures, but gradu- ally deviates from the semiconductive relation with decreasing temperature. The IR spectra of polymers I and I1 are shown in Fig. 1. The strong IR bands of the phenol group in PSHS are superimposed on the IR bands of polyaniline. The apparent difference between the spectra of the two polymers is due to the difference in the PA:PSHS ratios. In the spectrum of polymer 1, a strong band at ca. 1160 cm-', which is character- istic of conducting p~lyaniline,'~,'~ can be identified. This band is shifted in the spectrum of polymer 11.This suggests that the chain conformation of polyaniline is altered by interchain interactions with PSHS. Polyaniline I1 (ca. 8 mg) was completely dissolved in 1 ml of water to give a clear solution; this polymer was partly 4000 3000 2000 1000 450 wavenurnberkm-' Fig. 1 IR spectra of ((-C6H,NH-)[ -CH, -CH(C6H3 * OH-SO,-)-I,( H2S04)y-zH,O},: (a) polymer 1, x=0.3 and y=0.8; (b)polymer 11, x=0.4 and y= 1.2. The spectrum of poly(3-sulfonato-4-hydroxystyrene) (PSHS) is shown for comparison (c).The position corresponding to ca. 1160 cm-', where there is a band characteristic of polyaniline, is marked x. soluble in dimethyl sulfoxide (DMSO) and N-methylpyrrolidi- none (NMP). Polyanilines I and 111contained a small amount of water-insoluble component, and had a higher solubility in DMSO and NMP than in water, although a small amount of insoluble component also remained in the organic solvents.The solubilities of the polyaniline salts in water are the result of the interaction of polyaniline with PSHS, which is highly water-soluble owing to the hydrophilic groups. Self-doped polyaniline, which has an -SO3-substituent in the ring system, is soluble in a dilute NaOH solution.18 In contrast to this polymer, our polymer is soluble in acidic solution in the protonated state as well as in basic solution in the depro- tonated state. Fig. 2 shows the solution electronic spectra observed for polymer I1 at different pH values. The spectra did not show the increasing baseline with decreasing wavelength due to light scattering that might be observed if fine particles were dispersed in the solution.An aqueous solution of polymer I1 had a pH of 4.3 and its spectrum was identical to spectrum B observed for the Na2S04 solution of the polymer. PSHS showed a sharp peak at 285 nm, and no absorption band at wavelengths above 350 nm. The polymer alloy showed broad bands characteristic of emeraldine salts in the 400 and 800 nm regions. This is evidence for the formation of emeraldine with a significant number of benzenoid-quinoid linkages in the polymer alloy, although the molecular weight of the polyani- line cation is not expected to be large. The molar absorptivities of the 800nm bands observed for acid solutions were of the order of 500 1 mol-' cm-'; these molar absorptivities are given in the legend for Fig.2. Essentially the same spectra were observed in acid solutions, but the 800 nm band showed a small red shift with increasing pH. In alkaline solutions, a J. MATER. CHEM., 1994, VOL. 4 Ahm Fig. 2 Solution electronic absorption spectra of polymer I1 ((-C6H,NH -)[ -CH2 -cH(C6H3 .OH *so3-)-H2S04),,2* 3.6H20}, at different pH values (the polymer concentrations were identical for all the sample solutions): pH (a) 1.6 (~=5201 mol-'cm-'); (b)4.3 (530); (c) 6.3 (450); (d) 8.3 (300); (e) 9.1 (420); (f)11.1 (390). band characteristic of an emeraldine base was observed at 600 nm. and also a band attributable to deprotonated phenol groups was observed at 310nm; the 400nm and 800nm bands disappeared at high pH values.In the pH range of 7-9, the nature of the spectrum changed significantly with pH, and at pH z 8 a very broad band was observed with an absorption maximum at 700nm. Essentially the same pH dependence was observed for polymer I, but the 700nm band appeared at a lower pH of 7 (Fig. 3). Polymer I11 showed a large spectrum change in the region pH 7-8 (Fig. 4). Jiang and Dong" reported the pH dependence of solution spectra of a soluble polyaniline involving inorganic counter- anions; in the spectrum of polyaniline with inorganic counter- anions, the intense band that was observed at 830nm for strongly acidic solutions weakened with increasing pH without showing appreciable chromatic shift and disappeared at pH 35.0; at pH ~4.2, a 620 nm band appeared and strength- ened with increasing pH without a chromatic shift.The significant differences between the present polymer alloys and the inorganic anion salts are: (1)the 800 nm band disappeared at a higher pH in our polymer alloy; (2) the polymer alloy showed a 700 nm band at intermediate pH values; and (3) the 800 nm band showed a red shift with increasing pH. These differences in the pH dependence indicate the presence of an interaction between polyaniline and the PSHS chains. The sulfonic acid protons of PSHS are completely dissociated in I I I I I I I 1 1 200 400 600 800 1000 A/nm Fig. 3 Solution electronic absorption spectra of polymer 1 { (-c6H4NH-)[-CH, CH(C6H3 -OH * SO3-H2S04),,, * ~ 1.4H2O).at different pH values (a small quantity of insoluble components was removed by filtration; the polymer concentrations were identical for all the sample solutions): pH (a) 1.0; (b)3.0; (c) 5.0 (d) 7.0; (e) 8.5; (f)11.0. 800 E 700 t \ 4 600 Fig. 4 pH dependence of the electronic absorption band in the region 600-800 nm: 0,polymer I; 0,polymer 11; V , polymer I11 solution throughout the pH range studied. In contrast, the dissociation of the hydroxy protons is dependent on pH, and consequently the conformation of PSHS varies with pH. The conformational change of a PSHS chain influences the confor- mation of a neighbouring polyaniline chain. The 600 nm band characteristic of an emeraldine base has been attributed to a charge transfer between the quinoid and benzenoid units;I8 the 800 nm band arises from the protonated-nitrogen yuinoid and/or the benzenoid Both the 600nm ;md the 800 nm bands are sensitive to the conjugation, which is related to the conformation or the ring distortion of the polyaniline chains.18 Thus, the different conformations of PSHS at differ-ent pH values result in the chromatic shift of the (100 and 800 nm bands.When a polyaniline molecule is surrounded by PSHS chains, the local pH around the polyaniline may be different from that of the entire solution, owing to interactions between aniline cations and sulfonate anions. This effect results in the disappearance of the 800nm band at a higher pH in the polymer alloy than in the polyaniline containing inorganic counter-anions.The observation of the 700 nrn band at pHx7 suggests the presence of a partially protonated polyaniline chain, whose conformation is different from that in acidic or in alkaline solution. The solubility of polymer I1 in water was high enough for observation of EPR spectra, which were successfully obtained for the saturated aqueous solutions (8 mg in 1 ml H,O) at different pH values. An acidic solution with pH 1.5 exhibited a sharp signal at g=2.005 with a maximum-slope width, W, of 0.8 G, which was almost identical with W=0.7 G orserved for the solid sample. The EPR spectra were broadened with increasing pH: W=1.8 G at pH 5.8, 2.5 G at 8.4.The signal broadening is caused by the decreasing spin-diffusion ~~~elocity and/or spin concentration. An important observation is that the line broadening occurs even at pH 5.8, where the KO0 nm band is observed without appreciable change in in tensity. This behaviour can be explained by assuming that: (3 ) pola-rons and bipolarons coexist in the polymer;21 (2) the KO0 nm band is due mainly to bipolarons (protonated-nitrogen quin- oids) rather than polarons (anilinium radical cations:\ which are responsible for the EPR signal; (3) with increasing pH, deprotonation occurs at polarons prior to deprotona tion at bipolarons. In conclusion, the new polyaniline alloys containinq PSHS as counter-anions are water-soluble and exhibit novel sbectro- scopic properties.These properties have not been olxerved for polyaniline salts with inorganic or organic counter-anions, and can be explained as the result of an interchain interaction between the two component polymers. 1814 J. MATER. CHEM., 1994, VOL. 4 References 13 S. Arichi, N. Sakamoto, S. Himuro, M. Miki and M. Yoshida, Polymer, 1985,26, 1175. 1 G. Bidan and B. Ehui, J. Chem. SOC.,Chem. Commun., 1989,1568. 14 M. M. Coleman, C. J. Serman and P. C. Painter, Mucromolecules, 2 J. H. Hwang and S. C. Yang, Synth. Met., 1989,29, E271. 1987,20, 226. 3 S. Li, Y. Cao and Z. Xue, Synth. Met., 1987,20, 141. 15 M. Inoue, F. Medrano, M. M. Castillo-Ortega. K. Asano and 4 Y. Kang, M.-H. Lee and S. B. Rhee, Synth. Met., 1992,52, 319.M. Nakamura, Synth. Met., 1993,55-57, 1057. 5 S. P. Armes and M. Aldissi, J. Chem. SOC.,Chem. Commun., 1989, 16 W. R. Saleneck, B. Liedberg, 0. Inganas, R. Erlandsson, 88. I. Lundstrom, A. G. MacDiarmid, M. Halpern and N. L. D. 6 J-M. Liu and S. C. Yang, J. Chem. Soc., Chem. Commun., 1991, Somasiri, Mol. Cryst. Liq.Cryst., 1985, 121, 191. 1529. 17 J. Tang, X. Jing, B. Wang and F. Wang, Sjmth. Mot., 1988,24, 255. 7 N. Gospodinova, P. Mokreva and L. Terlemezyan, J. Chem. SOC., 18 J. Yue and A. J. Epstein, J. Am. Chem. Soc., 1990,112,2800;J. Yue, Chem. Commun., 1992,923. A. J. Epstein and A. G. MacDiarmid, Mol. Cryst. Liq. Cryst., 1990,8 Y. Cao, P. Smith and A. J. Heeger, Synth. Met., 1992,48,91. 189, 255. 9 I. Kulszewicz-Bajer, J. Pretula and A. Pron, J. Chem. Soc., Chem. 19 R. Jiang and S. Dong, Synth. Met., 1988,24, 255.Commun., 1994,641. 20 M. Tnoue, R. E. Navarro and M. B. Inoue, Swh. Met., 1989,10 M. Angelopoulos, N. Patel, J. M. Shaw, N. C. Labianca and 30, 199.S. A. Rishton, J. Vuc.Sci. Technol. B, 1993,11,2794. 21 M. B. Inoue, K. W. Nebesny, Q. Fernando and M. Inoue,11 K. Shannon and J. E. Fernandez, J. Chem. Soc., Chem. Commun., J. Muter. Chem., 1991,1, 213. 1994,643. 12 K. Nakamura, T. Hasegawa and H. Hatakeyama, Polymer, 1983, 24, 871. Paper 4/02880E; Received 16th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401811

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12). 1815-1819 Chemical Vapour Deposition of ZrO, Thin Films monitored by IR Spectroscopy Benjamin J. Gould,+ Ian M. Povey,* Martyn E. Pemble and Wendy R. Flavell* Department of Chemistry, UMlST, PO Box 88, Manchester, UK M60 7QD Thin films of ZrO, have been grown under kinetic control by decomposition of zirconium tetra-tert-butoxide onto quartz substrates. The resulting films have been characterised by optical and electron microscopy and X-ray diffraction. A phase transition from a poorly crystalline, metastable form of zirconia to the monoclinic phase showing a strong preferred orientation takes place as the substrate temperature is raised from 450 to 500°C. The decomposition of the precursor has been followed by ex situ infrared spectroscopy, allowing monitoring of the gas-phase products as a function of substrate temperature.The possible mechanism for the decomposition reaction is discussed. Metal-organic chemical vapour deposition (MOCVD) is a versatile technique which has been used in the electronics industry to prepare semiconductor devices with controlled structures, proving particularly useful in the preparation of compound 111-V and 11-VI materials. More recently, interest has been growing in the use of this technique in the production of technologically useful oxide materials, ranging from super- conductors to model catalysts.' One of the advantages of the technique is that it may be possible to produce structures which are not easily formed by other routes. Depending on the conditions used, the deposition may be either 'kinetically controlled' or 'mass-transfer controlled',2 which can lead to the formation of products which are not thermodynamically stable under the temperatures and pressures of the deposition process, as illustrated here.One of the primary disadvantages of MOCVD in the growth of ternary and more complex oxides is that unless the metal-organic precursor materials are particularly well chosen, it may prove difficult to achieve the required cation stoichiometry in the deposited film. Thus it may be necessary to conduct extensive investigations into the deposition of simple binary oxides containing the cations of choice, before deposition of more complex materials can be optimised. Here we investigate the MOCVD growth of ZrO, as a function of reactor conditions. Zr02 is a technologically useful material with applications stemming from its high relative permittivity and resistance to chemical attack, and, in its cubic modification, from its high ionic conductivity.The study of the deposition of this material (together with TiO,) is a first step along the route to the deposition of complex oxides such as PbTiO,, PbZr03 and Pb(Ti,Zr)O, which are employed in a wide range of electronic and electo-optic devices. While the deposition of TiO, by MOCVD has been quite well studied, there have been relatively few attempts to deposit ZrO, thin films via this Most studies have used /?-diketonates as prec~rsors,~-~ although the metal alkox- ides zirconium tetraisopr~poxide~ and zirconium tetra-tert- butoxide (ZTB)* have also been used in a limited number of investigations.ZTB decomposes more readily than the iso-propoxide, and is therefore potentially better suited as a precursor for MOCVD growth. Here we study the deposition of ZrO, from ZTB as a function of susceptor temperature, and use ex situ infrared (IR) spectroscopy to study the decomposition of the precursor material. t Current address: Department of Electrical and Electronic Engineering, The Queen's University of Belfast, Ashby Building, Stranmillis Rd, Belfast, UK BT9 5AH. 2 Current address: Department of Chemistry, University of Leicester, University Road, Leicester, UK LE17RH.Experimental Materials Thin films were deposited using ZTB precursor (Inorgtech, >99%). The carrier gas used was White Spot nitrogen. dried using a UCAR SG6170 gas purifier. Films were deposited onto BQ quartz plate supplied by Heraeus. Apparatus The films were deposited in an MOCVD system consisting of a gas handling system, a horizontal silica reactor, a r,idiant heater and an exhaust and pressure control system. The system offers accurate control over the carrier gas-flow rates, reactor pressure, precursor and substrate temperatures. The gas lines were heated to ca. 5 "C above the precursor tempera- ture to prevent condensation in the pipework. Before they were loaded into the reactor, the substrates were cleaned in boiling propan-2-01 for 20min and then rinsed in distilled water and dried.The conditions used in the deposition of the films are presented in Table 1. Film Characterisation The thickness of the deposited films was measured usjng an Olympus B0721 optical microscope. The crystalline structure of the deposited material was investigated by X-ray diffraction using a Scintag XDS 2000s diffractometer. The film morpho- logies were examined using a Phillips 505 scaning electron microscope (SEM). IR Studies The reactor exhaust gas was analysed at substrate tempera- tures in the range 100-700°C by ex situ IR spectroscopy using a Nicolet 20 DXB FTIR spectrometer with an external mercury cadmium telluride (MCT) detector. Table 1 Growth conditions used for the deposition of ZrO, films gas flow through precursor/ml min- total flow through reactor/ml min-' substrate temperature/"C precursor temperature/"C reactor pressure/Torr growth time/min 100 900 350-500 (increment 50) 70 10 10 J.MATER. CHEM., 1994, VOL. 4 Results and Discussion Film Thickness The measured film thicknesses are shown in Table 2. The measurements were used to determine the average growth rate in each case, and these are also shown in the table. The growth rate was found to be temperature dependent over the range studied. This observation contrasts with the work of Takahashi et a/., who found that the growth rate of ZrO, from ZTB reached a maximum at 450"C, after which the growth rate decreased with increasing temperature.* The temperature dependence observed in the work described here indicates that under the conditions of these experiments the MOCVD process is subject to kinetic control rather than mass-transfer control (which can give rise to a temperature- independent growth rate').Whilst a true temperature-independent growth regime is not attained in the work of Takahashi et al., the more rapid movement out of the kin- etically controlled regime in their experiment may be associ- ated with the fact that the decomposition in this case was carried out in the presence of 0,;oxygen partial pressure has been shown to affect the growth rate of Zr02 grown from /I-dike ton ate^.^ Our data were used to calculate an apparent activation energy of 57k9 kJ mol-' from the Arrhenius plot shown in Fig.1. This relatively low value suggests that the rate-determining step of the deposition process occurs at a surface, rather than in the gas phase. Possible mechanisms for the deposition are discussed further below. X-Ray Diffraction Fig. 2 shows the X-ray patterns collected from the films deposited at different temperatures. The peaks in the patterns collected from the films deposited at 350 and 400°C are broad, indicating poor crystallinity. The peak positions can be matched to strong reflections in the standard patterns of cubic, tetragonal and orthorhombic zirconia,"-' but the width of the peaks does not allow us to distinguish unambigu- Table 2 Film thickness (determined by optical microscopy) and average growth rate, as a function of substrate temperature growth temperature: film thickness"/ average growth rate/ C w pm min-' 350 2 0.017 400 4 0.033 450 12 0.100 500 15 0.125 pm.---1.5T 5 -2.5 \* \ \ \ \ \ .\9, -3.5 \ \ -1 12 1:3 1.4 1:5 1.6 1:7 lo3KIT Fig. 1 Arrhenius plot showing the dependence of film growth rate on the deposition temperature in the preparation of ZrO, thin films by MOCVD 1 1 0 20 40 60 80 2Bldeg rees Fig. 2 X-Ray diffraction patterns from ZrO, films dcposited on quartz, as a function of deposition temperature: (a)350 C, (h)400 'C, (c) 450 'C, and (d) 500 ^C ously between these structurally related forms. This is dis-cussed further below. The pattern from the film deposited at 500 "C shows two sharp peaks at 28=24.35 and 50.00" with low-intensity shoulders at 23.97 and 49.10".These are assigned to reflections from the (01 l), (022), ( 110) and (220) planes of monoclinic zirconia, re~pectively.'~ Both the phase and orien- tation of the deposited material are sensitively dependent on the growth temperature, changing from poorly crystalline metastable zirconia to a highly oriented monoclinic zirconia [showing preferred (01 1) and (110) orientation] at higher temperatures. The X-ray data were also used to estimate the crystallite sizes of the material grown, using the Scherrer ( 1) and Warren (2) formulae which are presented below:I4 where g, is the mean grain size, 2 is the wavelength of the X-ray source and OB is the Bragg angle of the peak used in the determination.B is the angular broadening of that peak relative to a reflection of similar QB and peak width B, in the pattern of a large grain size standard: B2=Bm2-B,2 where B, is the full width at half-maximum (FWHM) of the sample peak. The resulting average crystallite sizes are shown in Table 3. The crystallite size increases with deposition temperature, particularly in the temperature range 400-500 "C, where the transition to the monoclinic phase is observed. Takahashi et al. observed a similar temperature dependence, with the material deposited being completely monoclinic at 450 C.8 Undoped Zr02 deposited below 350°C was observed to be tetragonal.8 Mazdiyasni et al.observed a similar dependence of the phase on decomposition temperature in the formation of zirconia powders from zirconium alkoxides. l5 These authors observed a transition from metastable cubic to metastable tetragonal zirconia between 300 and 305 -C, and a gradual Table3 Mean grain size (calculated from peak widths in X-ray diffraction) as a function of growth temperature growth temperature/'C mean grain sizea/,& 350 57 400 60 450 110 500 180 J. MATER. CHEM., 1994, VOL. 4 transformation of the metastable tetragonal to the monoclinic form between 305 and 400°C.15 In the light of these obser- vations, it appears likely that the metastable phase deposited at low temperatures in our experiments is tetragonal zirconia, although this cannot be confirmed from our own XRD data.SEM The morphologies of the films at different deposition tempera- tures are shown in Fig. 3(u)-(dj. The micrographs show the dependence of film morphology on deposition temperature. The changes clearly reflect the phase transformation observed in the diffraction patterns. Fig. 3(u) shows a cracked film with few features; as the deposition temperature is raised, large, poorly crystalline agglomerates are seen to form [Fig. 3(bj and (c)]. The sample morphology changes dramatically between deposition temperatures of 450 and 500°C. At the higher temperature, the film is seen to be made up of numerous particles which appear highly crystalline and of even size [Fig.3(d)]. Comparison of the particle sizes measured from the micrographs with those measured from the diffraction data suggest that those seen in Fig. 3(dj are agglomerations of crystallites. This change in morphology between deposition temperatures of 450 and 500 “C corresponds to the transition from poorly crystalline metastable zirconia, to well crystallised monoclinic zirconia observed in the XRD patterns of Fig. 2. In the undoped state, the monoclinic form of zirconia is thermodynamically most stable under the conditions used for all the deposition experiments. IR Studies The reactor exhaust gases were analysed at different susceptor temperatures in order to identify the volatile reaction products with a view to elucidating the mechanism for the decompo- sition of the precursor.This was achieved by passing the exhaust through a cell placed between the spectrometer and the external detector. The experiments were carried oui using a reactor well coated in the deposits from previous growth experiments. Similar experiments have been reported investi- gating the growth of 111-V semiconductors.16 The experjmen- tal conditions used were similar to those used in the depsition of the films described above (given in Table l), the prrnciple difference being the reactor pressure. In the IR studies it was found to be necessary to keep the reactor at atmorlpheric pressure (of precursor and carrier gas) in order to prolrtuce a detectable signal from the precursor decomposition products at the IR detector.Two regions of the spectra at different decomposition temperatures are shown in Fig. 4(u) and (h) along with a spectrum of the undecomposed precursoi. The only IR-active product observed at decomposition temperatures of 300 and 400°C was isobutene. This 1s indi-cated by the presence of the bands at 890,2945 and 2980 cm -’. The increase in the intensities of these bands between the two decomposition temperatures indicates a greater forma tion of isobutene at the higher temperature. This is coincident with a decrease in the intensities of the bands due to the precursor, i.e. those at 749, 918, 1014, 1140, 1215, 1331, 2971 and 2984 cm- ’,the last forming an incompletely resolved doublet. When the decomposition temperature was increased to 500 -C, Fig. 3 SEM images of ZrO, films deposited on quartz as a function of deposition temperature: (a) 350 ‘C, (b)400 T,(c) 450 -C and (d 1 500 “C J 0 L v)21 B 3100 3000 2900 2800 wavenumbedcm-' Fig.4 IR spectra collected during the decomposition of ZTB, as a function of susceptor temperature: (a) 70 "C,(b)300 "C, (c) 400 "C and (d) 500°C.A, Region from 700 to 1500cm-l, mainly dominated at low temperature by peaks resulting from the undecomposed precursor. B, Region from 2750 to 3100cm-' (see text). In all spectra, one division on the ordinate corresponds to 5% transmittance. a characteristic band centred at 2143 cm-' with strong P and R branches appeared in the spectrum, indicating the presence of carbon monoxide.This was acconipanied by a further increase in intensity of the bands due to isobutene and a decrease intensity of those due to the precursor, which are barely distinguishable from the noise at this temperature. Further increases in susceptor temperature were accompanied by an increase in intensity of the carbon monoxide band and a decrease in the intensities of all other bands. The variation with temperature of the peak intensities of a strong feature associated with each decomposition product is shown in Fig. 5. In each case, a band was chosen which appeared to be free from overlap with bands due to other products of the decomposition reaction over the full temperature range of the experiment. These were for ZTB the band at 1371 cm-', for isobutene the band at 890 cm-' and for carbon monoxide the band at 2143 cm-l.The peak areas are measured by inte- gration (in absorbance) and then normalised so that the the intensity of each signal has an arbitrary value of unity when the corresponding signal is most intense. This gives an indication of the variation of the exhaust composition with susceptor temperature. Bradley has proposed a mechanism for the thermal decomposition of zirconium tertiary alk~xides.'~-~' The reac- tion is initiated by hydrolysis of the alkoxide, giving isobutyl alcohol in the case of zirconium tetra-tert-butoxide: (ButO),Zr + 2H20+Zr02 + 4(CH3),COH (3) J. MATER. CHEM., 1994, VOL. 4 0.8I-OIv).-c C3 0.2 0.0 0 T/"C Fig.5 Normalised variation in infrared peak intensities of ZTB and decomposition products with susceptor temperature: (u) ZTB, (b) isobutene, and (c) carbon monoxide The isobutyl alcohol is then dehydrated to give isobutene and water: (CH3),COH+(CH3),C=CH + H,O (4) Thus a chain reaction is set up as the water formed initiates the hydrolysis of more alkoxide molecules. 'The dehydration step (4)is rate-determining," but it is a rapid heterogeneous process,2o which would give rise to a relatively low apparent activation energy for deposition, as is observed in our experi- ments. Bradley has observed that the reaction can be catalysed by glass reactor walls," and that the initial source of water for the hydrolysis may be water or hydroxy groups adsorbed on the internal surfaces of such a reactor." In our FTIR studies, the reactor used was purposely coated with the deposits from previous growth experiments, and great care was taken to exclude water from the reactor during both the growth experiments and the IR studies. Bradley has proposed that in such circumstances thermolysis of the alkoxide gener- ates water via the tertiary alcohol formed according to the following reaction:18 (5) The oxide alkoxide formed is thought to undergo rapid disproportionation to the oxide and alkoxide.Strong OH stretching bands due to water were observed in the IR spectra at all substrate temperatures. Unfortunately, these are caused by small fluctuations in humidity in the external gas environment which the IR beam passes through, leading to miscancellations between background and sample spectra.It is therefore impossible to ascertain whether water is detected as a decomposition product. These bands are also sufficiently strong to obscure the region of the spectrum where the OH stretching vibration of isobutyl alcohol would be expected to occur, so that the first product of decomposition may not be observed directly. However, the observation of isobutene in concentrations initially inversely related to the concentration of unreacted precursor (Fig. 5) appears to be generally consist- ent with the proposed mechanism. At temperatures above 500"C, appreciable amounts of CO are produced, and the amount of isobutene detected decreases.This may be due to oxidation of the alkene or hydrocarbon fragments resulting from the cracking of the alkene over the ZrO, deposit coating the reactor walls, i.e. the deposited oxide may be acting as an oxidation catalyst. The oxygen incorpor- ated into the CO would then come from the oxide lattice, leaving the ZrO, oxygen deficient. In related experiments on the growth of TiO, from titanium tetraisopropoxide, we see evidence from variation in lattice parameters as a function of deposition temperature for increasing oxygen deficiency in J. MATER. CHEM., 1994, VOL. 4 Ti02-x as the deposition temperature is raised into the range where oxidation of the initial alkene decomposition product occurs.21 In the present case of ZrO, growth, the phase transformation which occurs within the growth range studied precludes us from making a similar analysis.However, com- parison between the two sets of data leads us to suppose that oxidation of the alkene occurs over ZrO, at elevated tempera- tures, leaving an oxygen-deficient film. We note that ZrO, is quite an efficient catalyst for production of alkenes, alkanes and alcohols from CO in the presence of H2,,, leading us to suppose that at elevated temperatures in the absence of H,, the reverse process may occur. Conclusion Low-temperature MOCVD growth of ZrO, thin films results in the formation of a form of zirconia which is not thermo- dynamically most stable under the growth conditions used, illustrating the kinetic control of this process.Under the conditions used, a transition to the monoclinic form (showing a strong preferred orientation) is observed between substrate temperatures of 400 and 500 "C. IR studies of the composition of the reactor exhaust as a function of substrate temperature show the appearance of increasing amounts of isobutene as the concentration of unreacted ZTB decreases, generally con- sistent with the decomposition mechanism proposed by Bradley.''-20 Further experiments, using a deuteriated precur- sor would confirm the mechanism, in particular allowing D20 from the proposed dehydration of the tertiary alcohol to be observed. At temperatures above 500 "C,the amount of isobut- ene produced decreases, to be replaced by CO, which may be formed by oxidation of isobutene at the oxide-coated surfaces of the reactor. Again, labelling experiments would be useful in determining the origins of the oxygen incorporated into CO.B.J.G. thanks SERC for the award of a studentship. References 1 e.g. Z-X. Chen and A. Derking, J.Muter. Chem., 1993,3,1137. 2 C. E. Morosanu, Thin Films Science and Technology 7, Thin Films by Chemical Vapour Deposition, Elsevier, Amsterdam, 1990, ch.5. 3 M. Balog and M. Schieber, Thin Solid Films, 1977,47, 109 4 M. Balog and M. Schieber, J. Electrochem. Soc., 1979,126. 1203. 5 M. Okada and K. Taominaga, J. Appl. Phys., 1992,71,1955. 6 L. Ben Dor, A. Elshtein and J. Shappir, Proc 4th European Conference on Chemical Vapour Deposition, Eindhoven, 1983, p.444. 7 M. Okada, K. Taominaga, T. Araki, S. Katayarna and Y. Sakashita, Jpn. J. Appl. Phys., 1990,29, 718. 8 Y. Takahashi, T. Kawae and M. Nasu, J. Crystal Growti?, 1986, 74,409. 9 M. J. Ludowise, J. Appl. Phys., 1985,58, R31. 10 JCPDS Powder Diffraction File, JCPDS, International Center for Diffraction Data, Swathmore, PA, Card 27-0977. 11 Ref. 10, Card 17-0923. 12 Ref. 10, Card 37-1413. 13 Ref. 10, Card 37-1484. 14 B. E. Warren, X-Ray Diffraction, Addison-Wesley, Readirg, MA, 1969. 15 K. S. Mazdiyasni, C. T. Lynch and J. S. Smith, J. Am. Ceram. Soc., 1966,49, 286. 16 G. H. Fan, R. Hoare, M. E. Pemble, I. M. Povey, A. G. Taylor and J. 0.Williams, J. Crystal Growth, 1992, 124,49. 17 D. C. Bradley and M. M. Faktor, Trans. Faraday Soc , 1959, 55,2117. 18 D. C. Bradley and M. M. Faktor, J. Appl. Chem., 1959,435. 19 D. C. Bradley, Chem. Rev., 1989,89, 1317. 20 D. C. Bradley, Phil. Trans. R. SOC.London, Ser. A, 1990,330,167. 21 B. J. Gould, I. M. Povey, M. E. Pemble and W. R. Flavell, unpub- lished; B. J. Gould, Ph.D. Thesis, UMIST, 1993. 22 e.g. S. C. Tang, N. Jackson and J. Ekerdt, J. Catal., 1988, 109,284. Paper 4/04628E; Received 28th Jul.s, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401815

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1821-1826 Studies of the Effects of NF, on the Growth of Polysilicon Films by Low-pressure CVD Part 1.-Effect on Growth Rate Michael L. Hitchman,* Junfu Zhaot and Sarkis H. Shamlian Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgo w, UK G7 1x1 The control of crystalline quality of polysilicon prepared by low-pressure chemical vapour deposition (LPCVD) is important for device applications. A number of reaction parameters affect this quality and, in general, it has been found previously that layers grown in the amorphous state show good structural perfection and low strain on annealing. We have investigated a new strategy for producing good crystalline polysilicon by attempting to etch layers during the deposition process in order to reduce the size of crystallites in the layers.In this paper we report on the effect of nitrogen trifluoride on the growth rate of polysilicon. We interpret the decreased growth rate in the presence of NF, in terms of an etching effect by molecular fluorine and as a result of growth inhibition by strong surface adsorption of the NF3. The mechanism of this inhibition is discussed. One of most widely used materials for the fabrication of modern VLSI circuits is polycrystalline silicon, commonly referred to as polysilicon. It is used for the gate electrode in MOS devices, for the fabrication of high-value resistors, for diffusion sources to form shallow junctions, for conduction lines, and for ensuring ohmic contact between crystalline silicon substrates and overlying metallisation structures.The standard method of preparation of polysilicon layers in modern integrated circuit technology is the technique of low-pressure chemical vapour deposition (LPCVD).' Typically, SiH, is pyrolysed with a temperature in the range 580-650 "C and at a pressure in the range 25-130 Pa. However, the crystalline quality of the polysilicon deposited can, depending on deposition parameters, be very variable. For most, if not all, of the applications mentioned above there is the need to control carefully the degree of crystallinity of the layers since poorly crystallised material can lead to high internal stress (e.g. dislocations and stacking faults) which, in turn, can produce unwanted defects in an active device area.In recent years considerable efforts have been made to establish the deposition conditions required for obtaining high-quality crystalline layers. This topic has recently been reviewed.' Factors affecting the crystalline structure of polysilicon have been shown to include deposition temperature,2 partial press- ure of silane,, length of time for the dep~sition,~ and type of dopant used.5 All of these factors have an influence on the relative rates of deposition or layer growth (j,) and crystalline growth (jCg).In general, whenjd>jcg then the deposited film will be largely amorphous or consist of very small crystallites, and when j, <jcgthen the film will show extensive crystallis- ation.Actually, the situation is a little more complicated than this simple picture would suggest because crystallisation depends upon a nucleation step and associated with this there is usually an induction time, which means in some instances j, <jcgbut, nevertheless, films can remain arnorph~us.',~ Not-withstanding this complication, the important feature from a practical device point of view is that it has been found2 for polysilicon layers grown in the amorphous state that on post- deposition annealing the highly desirable properties of good structural perfection and low strain are obtained. This is in contrast to material which is either partially crystalline or fully crystalline on deposition and which on annealing shows ~~ t Present address: Department of Applied Chemistry, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P.R.China. considerable lattice disturbances with some poor and some good crystallisation, leading to device defects. The need to control carefully the degree of crystallinity of polysilicon layers grown by LPCVD, which effectively means conti olling the relative rates of layer growth, is therefore apparent. The influence of deposition parameters such as deposition temperature, pressure and time as well as the presencc of in situ dopants on degree of crystallinity has been investigated previously, as mentioned above. We have considercd an alternative strategy for trying to produce good crystalline polysilicon by introducing into the CVD reactor an add] tional reactant which could partially etch the polysilicon as it is deposited.The rationale behind this strategy is that gas etching of the layer during the deposition process will ieduce the size of crystallites in the layer leading to an eflective decrease in crystalline growth rate. In particular, high-mergy surface sites such as would be associated with cryslalline defects and protuberances could be preferentially etched and this would lead to a better quality crystalline structure. To investigate this concept we have chosen NF, as a possible etchant gas for a number of reasons. It has been shown, for example, that NF, can be used to etch polys~licon~ and single-crystal silicon7 with etch rates as high as 1 pm min-'. This, though, has been in plasma reactors which will give rise to charged as well as neutral fluorine-conttiining molecules for reaction with silicon to form volatile SiF, species.However, it has also been shown that silicon can be etched by both molecular8~9 and atomic fluorine" and, fL rther-more, it has been reported'' that at temperatures in excess of 300°C that NF, dissociates homogeneously to forin F,; thermodynamic calculations (see discussion below) lent 1 sup-port to this. Experimental studies', have also shown that NF, undergoes spontaneous dissociative chemisorption on :,ingle- crystal silicon. Although at low temperatures up to 200°C it has been shown that13 NF3 does not spontaneously etch Si, we found in preliminary experiments that for typical polysil- icon deposition temperatures etching by NF, of a polyca'ystal- line silicon layer on a sapphire wafer did, in fact, OCCUI.Also a mass-spectroscopic investigation of the effect of temperature on NF, decomposition has shown14 that it does undergo pyrolysis at 600 "C. Therefore, NF, seemed a good candidate to investigate the idea of in situ etching during deposition in order to try and improve polysilicon layer quality. Other fluorine-containing compounds such as HF, F,, CIF, and XeF, might also be suitable, but NF, is less drfficult J. MATER. CHEM., 1994, VOL. 4 and dangerous to handle than most of these alternative materia1s.l5.l6 In this paper we examine the effect of NF, on polysilicon growth rate and discuss reasons for the observed effects.In a second paper we shall report on the effect of NF, on the degree of crystallinity of as-deposited polysilicon layers and on the crystalline quality of such layers after thermal ar~nealing.'~A third paper will discuss the effect of NF, on the chemical composition of polysilicon deposited by LPCVD.~~ Experimental The LPCVD reactor, the gas-handling system and typical deposition procedures and conditions have been described in detail previ~usly.'~~~' Briefly, the reactor consisted of a 95 mm (id) vitreous silica furnace tube with a three-zone heater. A flat temperature profile of various temperatures in the range 560-670 "C was maintained to within f1"C in the reactor zone.Pure SiH, and NF, (Air Products plc) were used as separate source gases with He being used as a diluent gas for some experiments. Depositions were made with 100% SiH, and with mixtures of SiH,-He, SiH,-NF, and SiH,-He-NF,. In all cases the total gas flow was kept constant at 100 sccmt Silane flows were in the range 5-100 sccm, NF, flows in the range 1-5 sccm and He flows adjusted accordingly to give a total flow of 100 sccm. The total pressure in the reactor was kept constant at 65 Pa by means of a control valve in the exhaust line to the rotary pump. Films were deposited on either 2 in sapphire ( 1102) or silicon (111) substrates. The wafers were placed parallel to each other and perpendicular to the flow with a spacing of 5mm in a fused silica boat capable of holding 50 wafers.Slots not used by test wafers were filled with dummy silicon wafers. Measurements of film thickness, crystallinity and composition were made, but in this paper we only report on thickness measurements. These were made by etching a concentric ring pattern for layers deposited on sapphire substrates2' and then measuring a thickness profile with a Sloan Dektak IIA profilometer. Results on layer crystallinity and composition are reported on in subsequent publication^.^^*^^ Results and Discussion Fig. 1 shows the variation of growth rate (j)with mole fraction of silane (x,)at constant temperature (580 "C), total pressure (65 Pa) and total gas flow (100 sccm). In the absence of NF, the dependence of j on x, is comparable to that reported earlier2' and is characteristic of a reaction system following Langmuir-Hinshelwood kinetics.'l The growth rate varies from 1.9 to 6.2 nm min-' for x, in the range 0.05-0.6.On adding a constant amount of NF, (1 sccm) to the SiH,-He mixture there is a dramatic fall-off in growth rate. For example, at a silane mole fraction of 0.15 the growth rate is 0.01 nm min-' with NF, compared with ca. 3 nm min-' at the same value of x, but in the absence of NF,. At x,=O.6 the values of growth rate with and without NF, are 3.8 and 6.2nm min -respectively. For x,<0.15 any polysilicon film deposited, even over relatively long run times (e.g. several hours), was too thin to measure reliably. All of these results indicate that, as expected, there could be some etching of polysilicon because of NF, addition.This effect is shown in a different way in Fig. 2 where growth rate is plotted as a function of mole ratio (y) of NF, : SiH, for a constant mole fraction of silane. However, it can be seen that if etching is occurring then the rate is not t Standard cm3 min-' "i 0 0 0 7 Ic 0.-E 0 0 0 0 0 0 Fig. 1 Dependence of polysilicon growth rate (j)on mole fraction of silane (x,) from a mixture of SiH, and He: 0,without NF,; 0,with NF, (1 sccm); & = 580 "C; P, =65 Pa; total gas flow, 100 sccm 30r I 0.00 0.04 0.08 0.12 Y Fig. 2 Dependence of polysilicon growth rate (j)on mole ratio (y) of NF, and SiH,: Td=580 "C; PT=65 Pa; total gas flow, 100 sccm; .x$=0.5 simply related to the gas-phase concentration of NF, since the fall in growth rate is not a linear function of 7.It can be also seen from Fig. 2 that the growth rate does not fall to zero for large y. In Fig. 1 the largest value of y, corresponding to x,=O.15, is 0.067 for which j~0.In Fig. 2 for the same value of y, j z7.0 nm min -'. The main experimental difference between the results in the two figures is the deposition temperature, 580°C for the results in Fig. 1 and 650°C for those in Fig. 2. The effect of temperature on growth rate is shown in more detail in Fig. 3. The total overall growth rate (j,) under any specific set of conditions can be taken as the difference between the deposition rate (j,) and the etch rate (j,) (i.e.jT=jd-je), each of which can be expressed in the form of general rate equations, and where kd and k, are the appropriate rate constants andf'(C,) and g(cN) are functions of the concentrations of SiH, (C,) and NF, (CN),respectively. The temperature dependence of J. MATER. CHEM.. 1994, VOL. 4 40 r 30 t7 t 0 10 -0 00 0 0 560 600 640 680 deposition temperature/"C Fig. 3 Dependence of polysilicon growth rate (j) on deposition tem- perature from a mixture SiH, and He: ?, without NF,; @, with NF, ( 1 sccm); P, = 65 Pa: Y, =0.5;total gas flow = 100 sccm the two rates can then be written as and (4) From the data in Fig. 3 for deposition without NF,, an Arrhenius-type plot can be made and the activation energy for deposition (Ea,d) can therefore be obtained.From the difference between growth without and with NF, the acti- vation energy for etching (Ea,e)can similarly be obtained. Fig. 4 shows the plots of eqn. (3) and (4) and from the slopes Ea,d is calculated to be 134.1 k0.3 kJ mol-I and Ea,e to be 130.4k0.8kJ mol-'. Clearly, the differences in growth rate between 580 and 650'C in the presence of NF, cannot be due to differences in the energies of activation of the deposition and etching processes. The second term on the right-hand side of eqn. (3) and (4) would therefore appear to be more deposition ternperature/OC 680 640 600 560 61 II I I 31.,., 1.04 1.08 1.12 1.16 1.20 10%~ Fig.4 Arrhenius-type plots for polysilicon total overall growth with NF, (jT),for growth without NF, (j,) and for 'etching'(j,): A, without NF, (j,): @, with NF, (1 sccm) (jT);W,je=jd-jT dominant in this context, and, since pre-exponential factors (A)are usually only weakly temperature dependent, the depen- dence of concentrations of active species on temperaturc may have a significant influence on growth and etching. We therefore now briefly examine the thermodynamics of formation of possible etchant species. Gibbs energies of forma- tion of such species can be calculated using data from JANAF thermochemical tables.22 For NF, gas-phase dissociation we assume that there are three possible reactions which can occur NF, $ NF,+F (1) NF, $ NF+F, (11) 2NF, S-N,+3F2 (1111 The Gibbs energy changes associated with each of these reactions as a function of temperature are shown in Fig.5 from which it can be seen that only reaction (111) is energeti- cally feasible. In fact, under typical deposition conditiclns of 560-67O'C and a total pressure of 65 Pa the extent cf this reaction is effectively loo%, therefore F, but not F' could be an etchant species. If F' were able to be formed then it would probably be more effective at etching polysilicon than F, since the activation energies for the production of fluorinated silicon species (e.g. SiF, and SiF,) from etching with F' lie in the range 8.7-14.5 kJ molp','' while for the formation of the predominant species SiF, from etching with F, the actiLation energy is in the range 38.2-66.5 kJ mol-';*,9 this is not unexpected since the reaction of F, requires breaking of an F-F bond.Of particular interest for the results presented here is that an activation energy in the range of 38.2-66.5 kJ mol-' for F, etching is significantly lower than the activation energy of 130 kJ mol-' determined from Fig. 4 for apparent etching. Therefore, if F, does etch the polysilicon as it is deposited then this process probably does not incorpor- ate the rate-determining step. Another possible etchant species to be considered is HF which could be generated by reaction between NF, and H,. In our system the H, would come from the decomposition of the SiH, when silicon is deposited.Possible reactions are 2NF,+H2 + 2HF+4F+N2 (IV) 2NF3+3H, $ 6HF +N2 (V) 2NF, +H, + 2HF +2NF2 (VI) 2NF, +H, +-2HF +2F2+N2 (VII) NF,+H2 $2HF+NF (VTTT) 0 -2001 ' I 800 850 900 950 1000 TIK Fig.5 Calculated dependence of Gibbs energies of reaction (hG) on thermodynamic temperature for reactions (I)-( 111): 2,reacti,m (1); @, reaction (11); M, reaction (111) The Gibbs energy changes for all of these reactions are large and negative under typical deposition conditions. For example, at ca. 600°Cand a total pressure of 65 Pa the AG values range from ca. -418 kJ mol-' for reaction (VI) to ca. -1750kJ mol-' for reaction (V). Therefore, energetically, HF could be readily formed in the gas phase and made available as an etchant for Si.Possible etching reactions by HF of silicon can be represented by the general equation Si + nHF +SiF, + n/2H2 (IX) where 1<n<4. If we assume that NF, in the reaction is completely converted to HF by reaction with H2 then the maximum partial pressure of HF, corresponding to 5 cm3 min-' of NF,, will be 10 Pa and the Gibbs energy changes for all cases of reaction (IX), except when n= 1, are negative and so etching by HF might occur. However, to our knowl- edge, no thermal etching of Si by HF at the temperature we have used has been reported. Indeed, anhydrous gaseous HF can be used to etch SiO, on Si without attack on the underlying material.23 There is therefore probably a kinetic hindrance to silicon etching.So if etching is occurring, leading to the effective fall in growth rate, then it would seem to have to involve molecular fluorine. The situation could be considerably complicated, though, by the fact that thermal dissociation of NF, may occur not only homogeneously but also heterogeneously with incorporation of nitrogen and fluorine at the surface.', This could then lead to growth rate inhibition analogous to that suggested in the case of polysilicon growth in the presence of PH, for in situ n-type d~ping,'.'~,~~ and for SIPOS growth in the presence of N20.25,26 In the case of PH, because of its lone pair of electrons one might expect an interaction with dangling bonds at a silicon surface, and evidence for the strong dissociative adsorption of PH, has been obtained by However,Meyerson and co-w~rkers.~~~~~ the strength of the Si-P bond (364kJ mol-')29 and its electronegativity difference (0.3)29are both significantly lower than the corre- sponding values for an Si-N bond (439kJ mol-' and 1.1, respectively) and, especially, for an Si-F bond (553kJ mol-' and 2.1).Also, all of the values are greater than those for an Si-Si bond (226kJ mol-') and an Si-H bond (299 kJ mol-' and 0.2).Therefore, NF, would be expected to be readily adsorbed on a silicon surface and to prevent the adsorption of the growth species SiH,. This adsorption could be dissoci- ative', and could involve, as indicated above, bonding with either N or F. The overall reaction scheme could then be represented by the following sequence of reactions: g SiH,(a) eSiH2(a) Si,H,(a) 3-x NF,(a)+T F,(g) (XV) SiH,(a)-+Si(s)+ 2H,(a) (XVII) Si2H,(a)+2Si(s) + 3H,(a) (XVII) SiH,(a)+Si(s) + H2(a) (XTX J.MATER. CHEM., 1994, VOL. 4 The equilibrium of reaction (X) lies well o~er to the left.30331 The reaction between silane and silylene h,ts been shown to be extremely and therefore the equilibrium lies well to the right. Tn the absence of any surface inhibition process, though, the primary route for polysilicon growth will be uia SiH, adsorption and diss~ciation,~~ although there is probably some growth uia SiH, and Si2H, as well. Silane itself is not expected to be strongly adsorbed and there is good evidence that this is the case.27,35,36 Silylene, on the other hand, as a biradical would be expected to interact re,tdily with silicon dangling bonds and to be strongly adsorbed.37 Disilane is certainly not as strongly adsorbed as silylene, but is consider-ably more strongly adsorbed than ~ilane.~"~, The argument for strong NF, adsorption has been given above.Hydrogen is known to inhibit polysilicon Since in our system no hydrogen gas is introduced into the reactor as a carrier gas and the only hydrogen present will be that from SiH, dissociation [reactions (X), (XVI1)-(XIX)], we assume this is readily desorbed and the equilibrium of reaction (XVI) will lie well to the left-hand side. The total growth rate (j,) of polysilicon can be represented as the sum of the growth rates from all silicon species adsorbed and undergoing heterogeneous decomposition where ki' is the heterogeneous rate constant for decomposition of adsorbed species SiHi, n is the total number of surface sites and OSiH, is the fractional coverage of any species SiHi.Following the same algebraic arguments given previously24 it can be shown that the inverse of the observed growth rate ( 1/',) is related to the mole ratio (1)) of NF, to SiH, by where A and B contain expressions involving kinetic and thermodynamic constants for the set of reactions (X)-(XIX) and n, is the gas-phase concentration of SiH4. Thus a plot of l/', us. y should be linear with an intercept corresponding to the deposition of polysilicon in the absence of NF,.Fig. 6 shows such a plot with data taken from Fig. 2 and the predicted relationship is found to hold. Note that in the case of PH, adsorption a similar plot was only linear up to y !z 3 x lop3but for greater values of y than this there was a deviation below linearity. This was attributed" to the frac- tional coverage of PH, reaching a limiting value of less than unity. Hence the inhibition effect would fall away and the 0.00 0.00 0.04 0.08 0.12 Y Fig.6 Test of eqn. (6), where 7 is the mole ratio of NF, to SiH,. Data points taken from Fig. 2 J. MATER CHEM., 1994. VOL. 4 growth rate would tend to level off. The results with NF, do not show any such deviation and this would be consistent with the greater adsorption tendencies of NF, mentioned above. Another interesting and significant difference between the results obtained with PH, and NF, arises from the fact that in the former case there is a marked variation of growth rate with radial distance on a wafer, but with NF, the layers are uniform across a wafer.For example, for polysilicon growth in the presence of PH, with y=4 x lo-, the growth rate at the edge of a wafer was found to be about 50% higher than in the centre,,, but with all of the mole fractions of NF, used in this study the deposited layers were uniform to within a few per cent across a wafer. In the case of PH, the radial variation of growth rate has been attributed to the blocking of kinetically controlled growth through SiH, adsorption and decomposition [reactions (XII) and (XVII)] and the trans- port-limited growth through SiH, and Si,H, adsorption and decomposition [reactions (XIIT), (XIV), (XVIII) and (XIX)].This mechanism seems not to be operating in the case of NF, and this could be because of the stronger affinity between Si and F. Any very reactive silylene or disilane formed in the gas phase by reactions (X) and (XI) could rapidly be removed by NF, to form Si-F containing species which would be very stable and would not be potential growth precursors. In this case growth of polysilicon would continue to be from 1825 are adsorbed more strongly on a silicon surface, may show a lower overall activation energy for silicon growth. In any case, the effect on E, is relatively small but with NF, there is a significant change in E, from 149 to 97 kJ mol-’.At a low mole fraction of silane a higher activation energy than in the absence of NF, is not unexpected if the NF, is inhibiting growth and adsorption of silane becomes the rate-determining step. As the mole fractron of silane increases then again the equilibrium of reaction (X) will be pushed over to the right, but now there is compctition between a homogeneous reaction with NF, and a hetero-geneous reaction with Si for the SiH, and Si,H, formal. The reactions with NF, can be written as SiH, $ SiH,+H, (XI SiH, +NF, -+A SiH, +SiH, +Si,H, (XI) Si,H, +NF, +B I XXT) where A and B represent NF,-Si complexes. The r..tte of formation of these NF,-Si complexes can be readily shown to be given by d([A] + [B])/dt zKO.’ [SiH,] (ko[NF,] +k, [SiH,] 1 (7) where K is the equilibrium constant for reaction (X), A.and k,SiH, alone and this process is always under kinetic ~ontrol~~.~’ and no radial variation of growth rate would be expected. If this is the case, the NF, is acting as a scavenger, rather like the wafer cages used in the deposition of P-doped polysilicon and semi-insulating polysilicon (SIPOS) to reduce the radial variation of growth rate. This could be an interesting and useful alternative approach for obtaining uniform layers in these systems. Some support for the concept of homogeneous reaction between NF, and Si,H, or SiH, is given by results for the apparent activation energies determined in the absence and presence of NF, as a function of silane mole fraction (Fig.7). Without NF, the apparent activation energy varies from a maximum value of 134 kJ mo1-l to a minimum value of 124 kJ mol-’. This is understandable if one remembers that there are alternative routes for polysilicon deposition and that as the mole fraction of silane increases so equilibria (X) and (XI) will be pushed towards the formation of SiH, and Si,H,, respectively, and the decomposition of these products, which \\\ 0.2 0.4 0.6 0.8 1.0 1.2 XS Fig. 7 Apparent activation energies as a function of silane mole fraction: m, without NF,; a, with NF, (1 sccm); P,=65 Pa; total gas flow, 100 sccm are the rate constants for reactions (XX) and (XXI), respectively, and it has been assumed that Si,H, is a reactive intermediate.Thus, for a fixed mole fraction of NF,, increasing the mole fraction of SiH, will increase its rate of disappearance through the reaction of SiH, and Si,H, with NF,, mak,ing it less available for deposition. As the temperature increascs this rate of consumption in the reactor before the test wafer will be enhanced so that there will be proportionally less SiH, for deposition at the test wafer at higher temperatures than at lower temperatures. Now it should be remembered that plots of the form shown in Fig. 4 are not true Arrhenius plots since the ordinate is not In k but rather In j,; i.e. the logarithm of the reaction rate rather than of the rate constant.The reaction rate in addition to reflecting the variation of the rate con- stant with temperature also contains the concentration of reac-tant, and if this concentration is less at higher tem-peratures than at lower temperatures then this will lead to an apparent lower activation energy than would have been obtained if the reactant concentration had remained constant over the whole temperature range. Conclusions The rationale behind this study of the effect of NF, on polysilicon growth by LPCVD was to try and introduce an etchant into the reactant gas mixture to reduce the size of deposited crystallites and to obtain films which show good crystalline quality on annealing. The results do indeed show an effective etching effect because of a decreased growth rate on addition of NF,, and thermodynamic consideration:, have suggested that etching could occur by molecular fluorine. However, it has also been suggested that the fall in growth rate with NF, addition could be enhanced because of arlsorp- tion of this species on the silicon surface leading to a blocking of adsorption by the growth precursor, SiH,.There is also evidence that NF, consumes the highly reactive molecule Si2H6 and the biradical SiH, in the gas phase, which means that, unlike other inhibited silicon growth processes, such as phosphorus-doped polysilicon and SIPOS growth, then is no radial non-uniformity of layer thickness on a wafer. The fact, though, that NF, reduces the polysilicon growth rate by adsorption and blocking of surface sites instead of orily by 1826 J.MATER. CHEM., 1994, VOL. 4 etching, as originally expected, does not necessarily mean that it will not assist in producing better quality layers of polysil-icon when they are subsequently annealed. It has been shown,2 for example, that phosphorus addition, which reduces the polysilicon growth rate in the same manner that NF, does, can lead to the production of higher-quality films. In the next paper in this series we shall report on the effect of NF, on the crystalline quality of polysilicon. 13 14 15 16 17 18 D. E. Ibbotson, J. A. Mucha, D. L. Flamm and J. M. Cook, J. Appl. Phys., 1984,56,2939. M. L. Hitchman and H. C. Shi, to be published. T. R. Torkelson, F. Oyen, S. E.Sadek and V K. Rowe, Toxicol. Appl. Pharmacol., 1962,4, 770. E. H. Vernot, C. C. Haun, J. D. McEwen and G. F. Egan, Toxicol. Appl. Pharmucol., 1973,26, 1. M. L. Hitchman, J. F. Zhao and S. H. Shamlinn, J. Muter. Chem., 1994,4, 1827. M. L. Hitchman, J. F. Zhao and S. H. Shamlian, J. Muter. Chern., 1994, 4, 1835. We gratefully acknowledge the award of an SERC grant for equipment, and of financial support from Taiyuan University of Technology, Air Products plc and Epichem Ltd for J.F.Z. and from the University of Strathclyde for J.F.Z. and S.H.S. 19 20 K. F. Jensen, M. L. Hitchman and W. Ahmed, in Proc. 5th Eur. Con$ CVD, ed. J. 0. Carlsson and J. Lindstrom, University of Uppsala, Uppsala, 1985, p. 144. M. L. Hitchman, J. Kane and A. E. Widmer, Thin Solid Films, 1979,59,231. Air Products plc and Epichem Ltd.are also thanked for the provision of gases for this research. 21 22 A. Clark, The Theory of Adsorption and ('atu Press, New York, 1970. JANAF Thermochemical Tables, 3rd edri. J. Phys. Chem. Rej. Data, 1985, 14, suppl. 1. References 23 24 R. Cleavelin and G. Duranko, Semiconduct. In[., 1987,10( 12), 94. M. L. Hitchman, W. Ahmed, S. Shamlian and M. Trainor. 1 2 3 4 5 6 7 8 9 10 11 M. L. Hitchman and K. F. Jensen, in Chenzicul Vupor Deposition-Principles and Applicutions, ed. M. L. Hitchman and K. F. Jensen, Academic Press, London, 1993, p. 159. G. Harbeke, L. Krausbauer, E. F. Steigmeier, A. E. Widmer, H. F. Kappert and G. Neugebauer, RCA Rev., 1983,44,187. P. Joubert, B. Loisel, Y. Chouan and L.Haji, J. Electrochem. Soc., 1987,134,2541. E. Kinsbron, M. Sternheim and R. Knoell, Appl. Phys. Lett., 1983, 42, 835. M. L. Hitchman, C. W. Jones, J. F. Zhao and S. H. Shamlian, Adv. Muter. Opt. Electron., 1993,2, 123. A. J. Woytek, J. T. Lileck and J. A. Barkanii, Solid State Technol., 1984,27( 3). 172. K. M. Eisele, in Proc. Syrnp. Plasma Etching, Electrochemical Society, Pennington, NJ, 1981, p. 174. J. A. Mucha, V. M. Donnelly, D. L. Flamm and L. M. Webb, J. Phys. Chem., 1981,85, 3529. M. Chen, V. J. Minkiewicz and K. Lee, J. Electrochem. Soc., 1979, 126,1946. M. J. Vasile and F. A. Stevie, J. Appl. Phys., 1982,53, 3799. J. A. Barkanic, D. A. Bohling and D. M. Reynolds, A Review and Safety Considerations of Dry Etching Using Nitrogen TriJEuoride, Air Products and Chemicals Inc., Allentown, PA, 1986. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Chemtronics, 1987,2, 147. M. L. Hitchman and J. Kane, J. Crystal Grourh, 1981, 55,485. M. L. Hitchman, Vacuum, 1984,34,377. B. S. Meyerson and W. Olbricht, J. Electroclrem. Soc., 1984, 131, 2361. B. S. Meyerson and M. L. Yu, J. Electrochm. Soc., 1984, 131, 2366. D. R. Lide, Handbook of Chemistry and Ph! sics, 71st ed. CRC Press, Boca Raton, FL, 1990-1991, p9-86: J. H. Purnell and R. Walsh, Proc. R. Soc. London, A, 1966, 293, 543. C. G. Newman, H. E. O'Neal, M. A. Ring, F. Leska and N. Shipley, Znt. J. Chem. Kinet., 1979,21, 1167. P. John and J. H. Purnell, J. Chem. Soc., 1973,69, 1455. G. Inoue and M. Suzuki, Chem. Phys. Lett., 1985, 122,361. J. M. Jasinski and J. 0.Chu, J. Chem. Phys., 1988,88, 1678. S. M. Gates, D. B. Beach, R. Inbihl, B. A. Scott and J. E. Denmuth, J. Vac. Sci. Technol., 1987, 5, 628. R. J. Buss, P. Ho, W. G. Breiland and M. E. Coltrin, J. Appl. Phys., 1988,63, 2808. J. G. E. Gardiniers, L. J. Giling, F. de Jong and J. P. van der Eerden, J. Crystal Growth, 1990, 104, 727. K. F. Roenigk and K. F. Jensen, J. Electrocht-Bm. So(,., 1985, 132, 448. 12 J. A. Shorter, J. G. Langan and J. I. Steinfeld, Surf Sci., 1989, 219, L560. Paper 4/02718C; Receired 9th Muy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401821

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1827-1834 Studies of the Effects of NF, on the Growth of Polysilicon Films by Low-pressure CVD Part 2.-Effect on Crystallinity Michael L. Hitchman,* Junfu Zhaot and Sarkis H. Shamlian Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK GI IXL In this paper the effects of silane mole fraction, deposition temperature and the addition of NF, on the crystallinity of as-deposited polysilicon films and on the crystalline quality of annealed films have been investigated with the aid of Raman spectroscopy and scanning electron microscopy (SEM). It has been found that without the addition of NF, it is necessary not only to have a low deposition temperature, as previously suggested in the literature, but also to have a high silane mole fraction in order to obtain amorphous films from which high-quality material can be obtained upon annealing.With the addition of NF, it is has been found that amorphous layers may be grown, and hence good quality annealed films obtained, at much higher deposition temperatures than is normally possible. As has been discussed in the Introduction to the previous paper,' polycrystalline silicon is widely used in VLSI fabri- cation. In all of the applications it is necessary to control the crystallinity of the layer carefully, since poorly crystallised material can give rise to high internal stress which can produce undesirable defects in active device areas. Factors affecting the crystalline structure of polysilicon include2 deposition temperature,, silane partial pressure,, deposition time' and dopant type.6 From the point of view of layer and device quality it has been found3 that partially or fully crystalline as-deposited polysilicon layers when annealed show significant lattice disturbances whereas post-deposition annealing of as- deposited amorphous layers gives material with the desirable features of good structural perfection and low strain.The most successful strategy3 for achieving these desirable proper- ties has been to grow at a low temperature where the deposition rate (j,) is greater than the crystalline growth rate ( and largely amorphous material is obtained. High .jcg)235 silane partial pressure4 and short deposition times5 can also favour the deposition of amorphous layers, but less attention has been paid to these factors.We have investigated an alternative approach of trying to etch polysilicon partially as it deposits with the aim of reducing crystallite size, leading to an effective decrease in crystalline growth rate, and of removing preferentially high- energy surfaces sites which might be associated with crystalline defects. As reported in the previous paper,' the gas we chose to explore this concept, NF,, did lead to a net decrease in growth rate and, hence, an apparent etching effect. However, while a consideration of the thermodynamics of the decompo- sition of nitrogen trifluoride did suggest that the observed fall in growth rate with the addition of this reactant could be due to in situ etching of the deposited layer by molecular fluorine, the results also suggested that adsorption of the NF, and the blocking of adsorption by the growth precursor SiH4 could occur. However, the fact that NF, reduces the polysilicon growth rate by inhibiting growth instead of just by etching, as originally expected, does not necessarily mean that it will not help in the production of high-quality crystalline layers on annealing.The n-type dopant PH, also reduces the polysil- icon growth rate through an adsorption rnechani~m,~ but at the same time it produces better quality crystalline material on annealing than undoped polysilicon grown under identical -tPresent address: Department of Applied Chemistry, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P.R.China. conditiom2 Therefore, we have investigated the effect of NF, on the crystalline quality of polysilicon and report on the results in this paper. Experimental The LPCVD reactor, the gas-handling system and typical deposition procedures and conditions have been described in detail previo~sly.~,~ The particular conditions for this study have been given in the previous paper.' Briefly, depositions were made with 100% SiH, and with mixtures of SiH4-He, SiH,-NF, and SiH,-He-NF,. The total gas flow was always 100sccmS with silane flows in the range 5-100sccm, NF, flows in the range 1-5 sccm and He flows adjusted to give the balance of 100 sccm.Films were deposited on silicorx(111) substrates with the wafers held in a fused silica boat p.iralle1 to each other and perpendicular to the gas flows. Annealing of samples was carried out in a double-v alled, fused silica tube. The annealing gas, nitrogen, was Ibassed between the two walls before being allowed to the inner tube in order to preheat it. Standard loading and unloading procedures were used with the time taken for these steps being ca. 30min. Annealing was carried out at 950'C and at atmospheric pressure for 2-3 h; there were no observable changes in the crystallinity of the layers after annealing ft )r 2 h. The degrees of crystallinity of as-deposited layers arid the crystalline quality of annealed layers were examined by R aman ~cattering.~ The experimental arrangement for R aman measurements of the films has been described in detail The technique employed an argon laser with a wavelength of 514.5 nm and 500 mW power.The slit width used was either 5 or lOcm-', depending on the sample, and the spectrum was scanned from 400 to 600cm-'. The sample was held in a holder at the optimum angle of 35" for maximum Raman intensity," and the laser light was focused onto a 1 mm diameter spot on the sample. The Raman-scattered light was collected at the standard angle of 90" to the incident light. The spectra were either recorded directly as a trace on a plotter or from a computer printout. The spectra of anilealed samples were compared with that obtained from single-crystal silicon.The surface morphology of as-deposited and annealed layers was also examined by SEM. The instrument used was a JEOL $: Standard cm3 min-'. JSM-840A with a magnification of x30000 and a beam energy of 15 or 25 keV. Results and Discussion Effect ,of Mane Mole Fraction on Crystallinity Fig. 1 shows the effect of silane mole fraction (x,) on the Raman spectra of as-deposited layers of silicon grown from silane-helium mixtures at a temperature of 580°C. The fre- quency and lineshape of the Raman spectrum of silicon films reflect the degree of crystallinity., In crystalline silicon the line due to lattice vibrations occurs at 522cm-', but for amorphous silicon the lack of lattice periodicity allows scat- tering from all vibrational modes and this produces a broad, less intense line centred at 483 cm-'.For films grown under conditions where there is a transition between a low degree of crsytallinity (i.e. mainly amorphous) and full crystallinity (ie. fully polycrystalline) a superposition of the two types of spectra will be obtained. In Fig. 1 this range of spectra can be seen. At a high mole fraction of silane (x,=O.5) a broad, very low intensity peak at ca. 481 cm-' is observed, corre- sponding to amorphous material. Harbeke et al., reported that for Td <580 "C amorphous silicon is grown from pure SiH,. However, they did not examine the effect of the partial pressure of SiH,. In their deposition system they had a total pressure (P,) of 65 Pa (0.5 Torr) and no other gas was added.So their silane partial pressure was also ca. 65 Pa (cf. our value of ca. 33 Pa). From the results of Joubert et a/., one would expect for both these partial pressures and for &= 580'C the as-deposited films to be largely amorphous, as indeed is found. At a partial pressure of ca. 3 Pa, corresponding approximately to x,=0.05, Joubert's results would indicate a fully crystalline layer. This is seen to be the case here since the spectrum in Fig. 1 for x,=O.O5 is a sharp peak at ca. 522 cm-', although there is a slight shoulder on the low- wavenumber side of the peak which probably indicates that the film is not fully crystalline. The SEM pictures also clearly showed the transition from crystalline to amorphous layers as the mole fraction of silane was increased. Fig.2 gives I.,.,.,.,. T13000 0.3 0.4 0.5 300 400 500 600 waven urnbedcm-' Fig. 1 Effect of silane mole fraction (x,) on the Raman spectra of as- deposited silicon films: gas mixture, SX-FIe; & =580 "C; P, =65 Pa J. MATER. CHEM., 1994, VOL. 4 -1pm Fig. 2 SEMs for as-deposited films: ((7) x,=O.O5, (h) s,=O.5; Td= 580 "C; P, =65 Pa representative SEMs for layers grown from low (x, =0.05) and high (x,=0.5) silane mole fractions, and the changeover from a surface with many small crystallites to one which, apart from impurity particles, is smooth is apparent. The work of Joubert et suggests a sharp transition between amorphous and crystalline material, but our results (Fig.1) show that there is more likely to be a gradual transition between the two states. One factor which compli- cates comparisons of degrees of crystallinity of polysilicon layers grown by different groups of workers is the effect of deposition time. This has been discussed elsewhere,2." but essentially if long growth runs are used then unintentional annealing with partial crystalline growth, during deposition. can occur leading to apparent differences in crystallinity. These differences can be exacerbated by leaving deposited films in the reactor for varying lengths of time after deposition. Unfortunately, while it is possible to minimise the effect of unintentional post-deposition annealing by removing the samples from the reactor immediately after the end of a growth run, the actual deposition time often needs to be varied to obtain layers of a desired thickness, as growth rates change with deposition parameters.In our studies deposition times varied between 60 and 240min without NF, and between 60 and 360min with NF,, and no samples were allowed to remain in the reactor after the end of growth runs. The slow transition from amorphous through mixed crystal- line to almost fully crystalline material is what one might expect for a solid-state process, and is comparable to the gradual transition found by Harbeke et aL3 and ourselves' as the deposition temperature has been varied. The reason why the changeover with silane partial pressure occurs is, of course, that at high values of x, the rate of arrival of SiH4 at the surface, and its subsequent adsorption and decomposition by approximately first-order kinetics' will be high and there will be little time for surface diffusion and ordering, especially at low temperatures.At low x,values, on the other hand, surface species have more time to find crystalline growth sites before the arrival of further adsorbed species. The presence of J. MATER. CHEM.. 1994, VOL. 4 1829 adsorbed hydrogen from the decomposition of SiH, will also hinder surface migration and this effect will be greater the higher the initial partial pressure of silane. Harbeke et aL3 examined with Raman scattering the effect of deposition temperature on the crystalline perfection of silicon films after annealing.Joubert et d4did not report on crystalline quality for films grown with different silane partial pressures. Fig. 3 shows typical examples of Raman spectra obtained after annealing a mainly amorphous (x, =0.4), as-deposited film (Fig. 3A), and after annealing a partially crystal- line (x, =0.3), as-deposited film (Fig. 3B). The complete con- version in both cases to crystalline material after annealing is apparent. Fig. 4 summarizes values of peak intensity as a function of x, for as-deposited and annealed films. For low values of x, (0.05 to 0.2) where films grow as practically fully crystalline layers, there is, not unexpectedly, a large change in Raman intensity, and hence crystallinity, on annealing.For a mixed crystalline film (x,=O.3) there is a slight increase in line intensity on annealing [cf. Fig. 3B] corresponding to a greater degree of crystallinity. For the amorphous, as- deposited films (x,=O.4 to 0.6) the line intensity and the extent of crystallinity have increased significantly; SEM pic-tures essentially confirmed these observations. The maxima in Fig. 4 at M, ~0.3may correspond to changes in the texture of the crystalline grains with silane mole A measure of the crystalline perfection of a film which has been annealed is the Raman line~hape.~ Annealed materials of low distortion and internal strain will give a lineshape resembling that of bulk single-crystal silicon, while strong and extended tails of the Raman line indicate highly distorted or strained material which is undesirable for critical device applications. Fig.5 shows the Raman full linewidth at 1/10 height as a function of silane mole fraction as well as the 8000 A 6000 4000 yv) 2000 u)c) cr3-8 Y.-v) 30300 400 500 600 g 10000 .-C a5 8000U 6000 4000 2000 300 400 500 600 wavenum ber/cm-' Fig. 3 Comparison of Raman spectra for (a)as-deposited and (6)anne-aled films: A, x, =0.4; B, x, =0.3; Td= 580 "C;P, =65 Pa; T,=950 "C 10000I -8000 7 I v) v)YC0' 6000-<x c.-v)C +C.z 4000-a I5CT \ 2000 -Ll1 OJ 0.0 0.2 0.4 0.6 XS Fig. 4 Raman peak intensity as a function of silane mole fracti In for as-deposited (A)and annealed (0)films.Deposition and annealing conditions as for Fig. 3 ............................................................................. singlecrystal silicon 0.0 0.2 0.4 0.6 XS Fig.5 Raman linewidth as a function of silane mole fracticin for annealed films. Deposition and annealing conditions as for Fig 3. corresponding linewidth for single-crystal silicon. The differ- ence between the high silane mole fraction group, with I alues close to the bulk value, and the low mole fraction group is notable. This indicates that whilst there is little disturhance of the crystal lattice for the annealed high x, group, fcllr the low x, group the material after annealing remains in a highly disturbed state with some poor and some good crystal1is;ition. Clearly for good device quality polysilicon a high partial pressure of silane should be used for deposition.Effect of Deposition Temperature on Crystallinity .~As has already been mentioned, Harbeke et ~1 have shown that deposition temperature is also an important parameter which influences film growth and crystalline structure. Fig 6(a) and (b) show Raman spectra for films grown froni gas compositions with x,=0.5 and 1, respectively, at diflerent deposition temperatures. Because of the high values of .Y,, at Td < 580 "C amorphous material is obtained in both cases (c$ Fig. 1). As Td increases, more crystalline material is grown; J. MATER. CHEM., 1994, VOL. 4 1’l’”l’” (a1 the representative SEM pictures in Fig.7 show this effect as well. This is consistent with results obtained by Harbeke et ~1.~ 3000 counts s-’ After annealing, all of the films were shown to be fully crystalline from the Raman spectra. Fig. 8 \bows the Raman Td 1°C full linewidth at 1/10 height, and again amorphous. as-deposited material shows in both cases little strain with linewidths being close to that of single-crystal silicon. For higher-temperature polycrystalline as-deposited materials there are considerable distortions and strains in the annealed layers, particularly for those grown from [he diluted silane. The SEM pictures support this (Fig. 9)-where the very poor crystalline quality for an as-deposited film grown at 670 C is 620 in strong contrast to the rather better quality material grown at, for example, 600°C.The results with pure silane are very similar to those obtained by Harbeke et 01,~ who also used undiluted silane. The better results for the annealed material C 580,-grown at 600°C with pure silane than those for the corre- sponding material produced using diluted silane may be Q,c.E 300 400 500 600 C a associated with the higher amount of adyorbed hydrogen, resulting from the higher partial pressure of silane, which inhibits the surface migration of silicon kpecies leading to fewer opportunities for crystallisation. This could also be the reason why at higher temperatures (T,>620 C) the pure silane still gives less strain and distortion than diluted silane.The reason why a deposition temperature in the region of 500-600 ’C appears to be critical in terms of quality has been discussed el~ewhere.~.~ The conclusion thus far is that in order to obtain good quality crystalline polysilicon for device applications it is necessary to grow amorphous material for subsequent annealing. Harbeke et d3have previously claimed that a deposition temperature of T,<580 C is required to achieve this. The results presented in this section confirm this, but in addition we have shown that it is necessary to have a high partial pressure of silane as well. We now examine the effect on crystallinity of adding NF, to the reactant gases. [r (b: 5000 counts s-’ T,I’C- , .600 560 300 400 500 600 wavenumber/cm-’ Fig. 6 Effect of deposition temperature (Td)on the Raman spectra of as-deposited silicon films: (a)x, =0.5 (SiH,-He), (h)x,= 1 (pure SiH,) -1pm Fig.7 SEMs for as-deposited films showing the effect of deposition temperature on crystalline growth: (a) 600 C, (h) 620 C. (c) 650 C, (d) 670 C; s,=0.5; P, =65 Pa J. MATER. CHEM., 1994, VOL. 4 for growth at 670°C. These observations are consistent with .-I .-0) A= 6ol zt singlecrystal silicon 20I I I I I 1 540 580 620 660 T,j 1°C Fig. 8 Ranian linewidth as a function of deposition temperature (T,) for annealed films: A,x, =0.5 (SiH,-He); ,x, = 1 (pure SiH,) Effect of NF, on Crystallinity Fig. 10(a) and (b)show Raman spectra for films grown from gas compositions with x,=O.5 and 1, respectively, at different deposition temperatures but in each case with NF, (1 sccm) added to the reactant gas mixture.From these spectra it can be seen that the NF, has a marked effect on the crystallinity of the as-deposited films. Whereas in the absence of NF, amorphous films could be grown only at temperatures less than ca. 580 "C with x, 30.5 (cf. Fig. 6), here with a mole fraction of NF, (xN)of only 0.01, amorphous films can be obtained at temperatures as high as 640 "C.The SEM pictures in Fig. 11 show the changeover from a microcrystalline surface obtained at 620 "C to a distinctively polycrystalline surface the model of NF, etching and adsorption presented in the previous paper' since a gas which both etches and is strongly adsorbed would reduce the possibilities for crystalli.;ation, although at high temperatures the adsorption effect would be offset by weakening of the bond between NF, and the surface.The significant effect of NF, is shown in a different manner in Fig. 12, where increasing the mole fraction of NF, allows amorphous films to be obtained at 650'C. Although the signals are very noisy, there is an indication that they are becoming broader and shallower as xN increases, suggesting that the extent of amorphous growth increases with addition of NF,. All samples grown in the presence of NF, were annealed under exactly the same conditions as those grown uithout NF,. Fig. 13 shows Raman spectra obtained after annealing the samples, whose spectra for the as-deposited state are shown in Fig.12. In all cases the material has undcrgone crystallisation, but there are still strong indications of mixed crystallinity in practically all cases, with the shoulders on the low-wavenumber side of the peaks suggesting strain and distortion in the layers. This is confirmed by Fig. 14 and 15 which show Raman full linewidths at 1/10 height as a function of % for x,=O.S and 0.1 (Fig. 14) and as a function Ctf NF, mole fraction (Fig. 15). With an NF, mole fraction of xN= 0.01 the best results are obtained with diluted silane at high deposition temperatures (7'' 3 650 "C). This is also shown in Fig. 15 for results obtained at %=650'C where the best quality layer is found for xN=0.01.From both plots, though, the deviation from the single-crystal linewidth is ca. 15 cm-' which is higher than the ca. 10 cm-' determined from Fig. 8 and the value of ca. 3 cm-' from Fig. 5, both cases uithout NF, addition. Thus although the addition of NF, to the reactant gas mixture can give rise to amorphous filn~s and subsequent annealing of these films can, in accord with the corresponding effects of silane partial pressure and depc sition Fig. 9 SEMs for annealed films showing the effect of deposition temperature on crystalline growth: (a) 600 "C,(b)620 "C, (c) 650 "C, (d) 670 "C; x, =0.5;PT=65 Pa J. MATER. CHI,M., 1994, VOL. 4 2000 counts s-’ 620 I 600 ;c”””””””””” I 400 500 600 1 Td 1°C 640 620 600 580 560 300 400 500 600 wavenumber/cm-’ Fig.10 Effect of deposition temperature (G)on the Raman spectra of as-deposited silicon films grown in the presence of NF, (1 sccm): (a)x,=O.5 (SiH,-He), (b)x,= 1 (pure SiH,) temperature, lead to good quality crystalline layers, there is slightly more strain in these layers than in those grown at low temperature and high silane mole fraction. This difference could arise from incorporation of small amounts of N and F into the layers as a result of the strong adsorption of NF,. In the next paper we show that this is, in fact, the case and that although high-temperature annealing causes most of the flu- orine to diffuse out from the layer, some nitrogen is retained as an impurity. Recry stallisation will not occur readily in impurity regions and consequently poorer quality layers will be obtained on annealing.This effect is expected to be more marked the higher the NF, mole fraction and the lower the deposition temperature, since both these parameters will give higher NF, adsorption. This is indeed observed (see Fig. 14 and 15). Conclusions It has been demonstrated that an additive gas can be used to affect the degree of crystallinity of LPCVD polysilicon films, although not only for the reasons originally expected. The -1pm Fig. 11 SEMs for annealed films showing the etfect of deposition temperature on crystalline growth: (a) 620 C, (h) 650 C. (c) 670 C: x,=0.5; P, =65 Pa use of NF, can produce amorphous films from which low- strain, annealed good quality crystalline polysilicon can be obtained upon annealing.However, amorphous growth results not only because of in situ etching but also because of strong adsorption of NF,, which inhibits the surface diffusion required for crystalline growth on the growth surface. The best quality annealed films produced with NF, are obtained at low NF, mole fraction (x, GO.01) and high deposition temperature (G3650 “C),which is of interest if it is necessary to deposit polysilicon at temperatures greater than 650 C in order to be compatible with other stages of device production. In the absence of NF,, the work has shown that it is necessary not only to use low deposition temperatures to achieve amorphous films from which good quality crystalline material can be obtained upon annealing, as recommended by Harbeke et but also to have a high silane mole fraction.Fig. 16 summarises the results obtained for the degree of crystallinity of as-deposited films as a function of x, and T, both in the absence and presence of NF, (xN=0.01). We gratefully acknowledge the award of an SERC grant for equipment, and financial support from Taiyuan University of Technology, Air Products plc and Epichem Ltd. for J.F.Z. and from the University of Strathclyde for J.F.Z. and S.H.S. Air Products plc and Epichem Ltd. are also thanked for the J. MATER. CHEM., 1994, VOL. 4 T II 1000 counts s-1 I E 0.047 0.041 0.034 0.028 0.022 0.016 300 400 500 600 wavenumber/cm-' Fig.12 Effect of NF, mole fraction (xN)on the Raman spectra of as- deposited silicon films: x,=0.5; & =650 T;P, =65 Pa 2000 counts s-' I 300 400 500 600 waven um berlcm-' Fig. 13 Effect of NF, mole fraction (xN)on the Raman spectra of annealed silicon films. Deposition conditions as for Fig. 12; T,= 950 "C. 80 -60-40 -Fig. 14 Raman linewidth as a function of deposition temperature (Td) for annealed films: A,x, =0.5 (SiH,-He); , x, = 1 (pure Si H4) Fig. 15 Raman linewidth as a function of NF, mole fracllion (xN). Deposition and annealing conditions as for Fig. 13. 1.o 0.8 0.6 0.4 0.2 m rn 0.0 I I I 7I Fig. 16 Dependence of degree of crystallinity of as-deposited films on silane mole fraction (x,) and deposition temperature (T,).Without NF,: ,films totally crystalline; m,films totally amorphous; m, films partially crystalline.With NF, (xN=0.01): 0,films totally amorph- ous; 8,films partially crystalline. provision of gases for this research. We also acknowledge helpful discussions with Prof. W. E. Smith and Dr. B. Rospendowski. References M. L. Hitchman, J. F. Zhao and S. H. Shamlian, J. Muter. Chem., 1994, 4, 1821. M. L. Hitchman and K. F. Jensen, in Chemical Vapor Deposition- Principles and Applications, ed. M. L. Hitchman and K. F. Jensen, Academic Press, London, 1993,p. 159. G. Harbeke, L. Krausbauer, E. F. Steigmeier, A. E. Widmer, H. F. Kappert and G. Neugebauer, RCA Rev., 1983,44,187. P. Joubert, B. Loisel, Y. Chouan and L. Haji, J. Electrochem. Soc., 1987,134,2541. J. MATER. CHl M., 1994, VOL. 4 E. Kinsbron, M. Sternheim and R. Knoell, Appi. Phys. Lett., 1983, 42, 835. M. L. Hitchman, C. W. Jones, J. F. Zhao and S. H. Shamlian, Adz;. Muter. Opt. Electron., 1993,2, 123. M. L. Hitchman, W. Ahmed, S. Shamlian and M. Trainor, Chemtronics, 1987, 2, 147. K. F. Jensen, M. L. Hitchman and W. Ahmed. in Proc. 5th Eur. Conf. on CVD, ed. J. 0.Carlsson and J. Lindstrom, University of Uppsala, Uppsala, 1985, p. 144. M. L. Hitchman, J. Kane and A. E. Widmer. Thin Solid Films, 1979,59,231. W. Lang, in Proc. 12th. Int. Conf on Raman Spectroscopy, ed. J. R. During and J. F. Sulkian, Wiley, Chichester, 1991. p. 816. Paper 4/02725F; Received 9th Muy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401827

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1835-1842 Studies of the Effects of NF, on the Growth of Polysilicon Films by Low-pressure CVD Part 3.-Effect on Composition Michael L. Hitchman,*" Junfu Zhao,+" Sarkis H. Shamlian," Stanley Affrossman," Mark Hartshorne," Ewa A. Maydellb and Hamid Kheyrandish*" " Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G11XL b Department of Physics and Applied Physics, University of Strathclyde, Glasgow, UK G4 ONG Centre for Thin Film and Surface Research, University of Saiford, Salford, UK M5 4WT A range of analytical techniques (FTIR, SIMS, AES, SNMS, XPS) has been used to study the effect of adding NF, to an LPCVD gas mixture of SiH,-He on the composition of the resulting as-deposited and annealed polysilicon.Comparisons have been made with CVD silicon nitride and with polysilicon deposited in the absence of NF,. It has been shown by all the techniques used that for LPCVD in the presence of NF,, nitrogen is incorporated into the layer, but quantitative analysis with SNMS and XPS shows that such films are very similar to polysilicon and contain less than ca. 5 atom% of nitrogen. FTIR and SIMS also have revealed the presence of fluorine in as-deposited layers from LPCVD with NF,, but results from SIMS suggest that its concentration is very low and decreases by a factor of about two on annealing. It is concluded that while polysilicon deposited in the presence of NF, does contain small quantities of N and F, nevertheless the material may have interesting properties, with the use of NF, improving the surface quality.In the first paper in this series' we outlined the uses of polycrystalline silicon in VLSI fabrication and we pointed out that annealing of as-deposited amorphous films gives the highly desirable features of good structural perfection and low strain necessary for the good quality crystalline material required for device applications. The strategies for achieving this have been discussed',2 and we have reported on an alternative approach involving trying to etch the polysilicon partially as it deposits with the aim of reducing the crystallite size, leading to an effective decrease in the crystalline growth rate, and thus obtaining a less crystalline and more amorphous material.The gas we used as a potential etchant was NF,. However, although this did lead to a net decrease in growth rate,' as would be expected if etching were occurring, we have shown that, in addition to in situ etching of the deposited layer, adsorption of this reactant and the blocking of adsorp- tion of the growth precursor SiH4 could also occur. Nevertheless, we have also demonstrated2 that the use of NF, can produce amorphous films from which low strain and undistorted good quality crystalline polysilicon can be obtained upon annealing. The best quality annealed films produced with NF, are obtained with a low mole fraction of NF, (xNGO.01) and high deposition temperature (&3 650 "C), in contrast to results without NF, where typically it is necessary to use Td,<580"C.Also, in comparison with the growth of amorphous layers without NF,, there is slightly more strain after annealing the NF, treated layers. This we have suggested could be due to the incorporation of small amounts of N and F into the layers as a result of the strong adsorption of NF,. In this paper we report the results of qualitative analysis with FTIR, secondary-ion mass spectrometry (SIMS) and Auger electron spectroscopy (AES), and of quantitative analy- sis with secondary neutral-atom mass spectrometry (SNMS) and X-ray photoelectron spectroscopy (XPS) of films grown with and without NF,. t Present address: Department of Applied Chemistry, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P.R.China. j: Present address: MATS UK, Wavertree Boulevard South, Wavertree Technology Park, Liverpool, UK L7 1PG. Experimental The LPCVD reactor, the gas-handling system and rypical deposition procedures and conditions have been described in detail previo~sly~,~ with the particular conditions used for the study being given in the earlier papers in this series 1,2 To allow a ready comparison between the results for the karious techniques all the results reported in this paper were obtained using the same set of samples. The film deposited mithout NF, (labelled sample 11)was grown from an SiH,-He mixture onto a silicon( 111) substrate with the mole fraction (x,) of silane being 0.1. The deposition temperature was 600'C and the total pressure (PT)in the reactor was 65 Pa.The thickness of this film as determined by surface profilometry' was 465 nm. The layer deposited with NF, (sample 111) had x,=O 5 and PT=65 Pa, Td=670 "C and xN=0.0093. This film was 456 nm thick and had a silvery, metallic appearance, exactly the same as that of the polysilicon sample. Both samples I1 and I11 were annealed at 950 "C under 1 atm of nitrogen. In order to have a nitrogen-containing reference material, a Sam pie of silicon nitride (sample I) was also analysed by the karious techniques. This sample was prepared by a standard thermal CVD method and was supplied by Philips Components Ltd., Southampton. The thickness of this layer as determined by ellipsometry was 120nm.The film showed a uniform blue interference colour. FTIR analysis of the layers in the range 390-2000 cm-' was carried out using a Nicolet spectrometer (Model 51: DX), although in all cases significant spectral features were only observed at wavenumbers less than ca. 1400 cm-' and so just the range from 390 to 1380cm-' is reported on here. Static SIMS data were obtained using a VG Quadrupole SIMS LAB3 instrument. A Ga' ion beam was used for monitoring nitrogen and fluorine as well as oxygen in the layers. With a 3.5 keV Ga' beam of current intensity 1 nA, rastered over an area of ca. 110 pm x 110 pm, sputter etch rates were low. However, in order to remove any surface contamination effects the sample was sputter-cleaned for ca.10 min arid it is estimated that material to a depth of 6-12 nm was removed during this time. Both positive and negative SIMS analysis was then carried out. Quantification in SIMS is generally ambiguous, except for low concentrations of impurities in homogeneous matrices. This is because the process of ionisation on the surface is critically dependent on the chemistry of the surface. For this reason the samples were also analysed using electron impact J. MATER. CHE:M., 1994, VOL. 4 annealing occurred, possibly because of leakage of oxygen into the atmospheric-pressure annealing system. Fig. l(a) and (b) show spectra for sample 111, polysilicon deposited in the presence of NF,, for as-deposited and annealed material, respectively.The spectra of both layers SNMS. This is a relatively new surface analysis te~hnique~.~ show similar peaks to those found for sample I1 at and is a valuable, complementary method to SIMS since it is 609-611 cm-' and at 471-473 cm-'. The ax-deposited layer not significantly influenced by matrix effect^.^ In e-beam spectrum has a band at 1094cm-' correhponding to the SNMS the positive and negative SIMS signals are suppressed Si-0 bending mode, but this is not apparent for the annealed and sputtered neutrals are ionised at some distance away from the sample surface using electron impact post-ionisation. Thus the signal is largely independent of any matrix effects and depends only on the electron impact ionisation cross-sections, the sputter yields and the collection efficiencies of the neutrals. The technique can therefore be calibrated by using known standards and so serves as a useful quantitative complement to SIMS.By using a switching facility between a STMS operational mode to a SNMS mode the same instru- ment can be used for both techniques and a wide range of sensitivities from the sub-ppb level upwards can be obtained. A detailed description of the procedures for SNMS has been given previ~usly.~ For the SNMS studies in this work, the system used was a VG Scientific SIMSLAB in the Centre for Thin Film and Surface Research at the University of Salford. The primary analysing beam used was either 10 keV Ar+ or 30 keV Ga' . With the former beam the crater dimension was 370 pm x 490 pm while with the latter it was 100 pm x 80 pm. The etch rates were determined using surface profilometry; typical etch rates were in the range 0.4-0.8 nm s-'.The instrument was also used in the dynamic negative SIMS mode for monitoring of fluorine, oxygen and other impurities. All analyses were carried out with a system pressure of ca. lop8Pa. AES was used for surface compositional analysis. This was performed in the same system as used for the static SIMS. The Ga+ ion beam was again used to remove surface contami- nation prior to electron beam irradiation for AES analysis. XPS data were obtained with a Vacuum Science Workshop X-ray anode, using AI-Ka radiation, and a 100mm hemi- spherical analyser. The photoelectron intensities were meas- ured at a take-off angle normal to the surface. Results FTIR Spectroscopy The main feature of the spectrum for sample I, the silicon nitride layer, was a strong absorption peak at 845 cm-'.This is characteristic of the Si-N asymmetric stretching mode.8 A weak band at 460 cm-' probably arose from an Si breathing mode,9 while a very weak band at 1085cm-' could be associated with an Si-0 bending mode,g arising from the incorporation of small amounts of oxygen impurity in the layer. The absence of any band at ca. 610 cm-' showed there was no Si-Si bonding, while the absence of any band in the region of 1200 cm-' indicated there was no, or very little, hydrogen incorporated into the layer, as would be expected for thermal CVD material; this is in contrast with plasma CVD material where significant amounts of hydrogen incor- poration can occur.1o In the spectra for as-deposited and annealed polysilicon, sample 11, three peaks of interest could be identified in each case.A peak at ca 610 cm-' could be assigned to the Si-Si bending mode," while that at 475 cm-' was again due to the Si breathing mode. Neither of these two peaks showed any significant changes in position or shape on annealing. A third sample, probably because it is masked by the broad band centred around 897cm-'. This broad band could be associ- ated with some Si-N bond stretching in the material. However, the presence of the peak at 611 cm-' shows that if N is present then it does not form stoichiometric Si,N,.This conclusion is supported by the silver, metallic appearance of sample 111; if the sample had had a composition close to that of Si,N, then one would have expected to have seen an interference colour. The situation is complicated, though, by the possible pres- ence of F in the layers. Fig. l(a)for the as-deposited film has a peak at 847cm-' which could be attributed to an Si-N stretching mode, but in addition there are peaks at 964 and 791 cm-' which could be associated with SiF, and SiF, stretching modes, re~pectively.'~-'~ On annealing, the three individual peaks disappear and the region of the spectrum between 750 and 1100 cm-' is occupied by the broad band mentioned above and centred at 897cm-'.The spectrum is 94 88 82 76 h8 v 70 m Y '"1 70 I 1310 1080 850 620 390 wave nu mbe rkm-' peak at 1093 cm-' in the as-deposited layer and 1086 cm-' Fig. 1 FTIR spectra for polysilicon films with NF, additionin the annealed layer was again associated with Si-0 bending, (sample 111). (a) As-deposited: & =670 'C, xS=0.5, xN=0.0093. and a broadening and slight intensification of this peak after (b)Annealed: T,=950 "C J. MATER. CHEM., 1994, VOL. 4 very similar to that reported by Fujita et a1." who have prepared fluorinated silicon nitride films by plasma CVD of a mixture of SiF,, N, and H,. They observed a strong, but broad peak in the region 750-1100 cm-' and centred on 900 cm-', which they also assigned to Si-N stretching and which they pointed out would veil absorptions due to Si-F, and N-F, bonding. An absorption at cu.1032 cm-' would be expected for a N-F,, band (m= 1-3) and this may be related to the small shoulder at 1026 cm-I in Fig. I@). However, on the basis of the relative bond energies of 271 kJ mol-' for N-F and 535 kJ mo1-I for Si-F, it would be expected that the majority of the fluorine would be bonded to Si rather than N, and the spectrum in Fig. 1(a) is indicative of this. Compared with Fig. 1(a),the apparent disappearance of the absorption peaks associated with Si-F, bonds [Fig. l(b)] could be simply due to the diffusion of F out of the layers during the high-temperature anneal. The loss of fluorine from a fluorinated silicon nitride by such a mechanism has been suggested by Fujita and Sasaki." However, they suggested this would occur only in the presence of hydrogen in the layer when the slightly higher H-F bond energy of 570.3 kJ mol-I compared with 552.7 kJ mol-' for the Si-F bond would favour HF formation and desorption to the gaseous phase.This is not likely to be an efficient mechanism in our case because there is little evidence for much hydrogen incorpor- ation in the layer. Peaks in the region 2000-2300 cm-' would be expected'' for Si-H symmetric stretching and in the region 3340-3350 cm-' for N-H symmetric stretching. No structure was observed in either of these regions for the as- deposited material. The small peak at ca. 1144 cm-' in Fig. l(a) could be associated with an N-H stretch,8 but if this is the case, then there is only a very small amount of hydrogen in the film, as would be expected for a film deposited at 670'C.Therefore, it is unlikely that on annealing all of the F will be lost from the layer and we conclude that the peaks due to residual Si-F bonds are not seen because they are masked by the broad band centred on 897 cm-l. The reason why the broad band replaces the individual peaks could be associated with changes in bonding on annealing. Fig. 1 (u) clearly indicates that during deposition there is some Si-N and Si-F bonding. Annealing will lead to molecular rearrangement and crystalline growth with prob- able strengthening of these bonds. The individual absorption bands will therefore become larger leading to a greater overlap and eventually an envelope encompassing the individual absorptions.The central frequency of this envelope at 897 cm-' is consistent with this picture. In pure Si,N, the Si-N absorption maximum is ca. 850 cm-'. An Si-N bond will be polarized (Si6+ -Nap) because of the higher electro- negativity of N (3.0) compared with Si (1.8).17 If fluorine, which has an even higher electronegativity (4.0), is present in the film and is also bonded to Si then the Si will be more positively ionised and the force constants will be enhanced. Hence any absorption due to Si-N bonding will be shifted to a higher frequency, as is found. The FTIR analysis therefore suggests that polysilicon films deposited in the presence of NF, will contain N and F as impurities, although the layers probably do not comprise stoichiometric Si,N,.The observations made in ref. 2 concern-ing the slightly greater strain observed in annealed polysilicon layers grown in the presence of NF, than in those grown in the absence of NF, (resulting from N and F impurities) are thus seen to be justified. Further evidence for these impurities is given by other analysis techniques. SIMS Fig. 2(u) and (h) show positive and negative SIMS data for the silicon nitride layer, sample I. The spectra show a large 1837 10~~ I 1o4 1o3 1o2 10'-'m loo 4.-C 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.C.100.0 m /z Fig. 2 SIMS data for a silicon nitride film (sample I): (a) positive SIMS, (b)negative SIMS number of peaks and only a few have been selected in the discussion which follows for illustrative purposes.No unex-a pectedly, the presence of Si is clearly seen (e.g. 28Si+, "Si,' ) as is the presence of N (e.g. 7oSi2N+, 42SiN-). The appvarance of 0 (e.g. l6O+,45SiOH+, 72Si20+, l6O-, I7OH-, 44SiO-, 60Si02-, 76Si03-) is also not unusual because, even though a surface layer of ca. 20nm had been removed before the analysis, CVD of Si,N, is not normally carried out in ii high-vacuum integrity reactor. Therefore, contaminatioli with oxygen can readily occur. The presence of 0 is coirsistent with the evidence given by FTIR for traces of Si-0 bonding in the layer. The apparent appearance of mass 19, which could correspond to F in the layer, is believed to be an electron-stimulated desorption signal and not a true SIMS signal for this sample.Fig. 3(a) and (b)show the positive and negative STh4S data for polysilicon without NF, addition (sample 11).Again there are a large number of peaks, but the important point to note is the absence of any peaks which could be associated with N; the peak at m/z= 14 can be attributed to 28Si2+. As in the case of Si,N, the presence of 0 is consistent with the type of LPCVD reactor used for film growth and also it agrees with the FTIR spectrum for sample TI. Sample 11 i:b, apart from oxygen contamination, a reasonably pure sample of poly silicon. Fig. 4(u) and (h)show the positive and negative SIRIS data for sample 111, the polysilicon layer with NF, addition.In the positive spectrum there is ample evidence of Si (e.g. 28Si2+, 28Si+, %i2+), but there is also the appearance of a peak at m/z=70 corresponding to Si,N+, which was also present in sample I, but not in sample 11. So the suggestion from FTIR of the presence of N (Fig. 1) is supported by SIMS. Further confirmation is given by negative SIMS which clear1-v shows 42SiNp, as was obtained for the Si3N4sample. The presence 1838 1 SiOH' i 0-lo4j 1.OH-1o2 10' 1.-no 0.0 10.0 20.0 30.0 40.0 50.060.070.0 80.0 90.0 100.0 m Jz Fig. 3 SIMS data for a polysilicon film without NF, addition (sample IT): (a) positive SIMS, (h)negative SIMS 1o7 1o6 1o5 Ga' 1o4 Si+ 1o3 1o2 10' 1o2 10' '*"OO 10.0 20.0 30.0 40.0 50.060.0 70.0 80.0 90.0 100.0 m Jz Fig.4 SIMS data for a polysilicon film with NF, addition (sample 111): (a)positive SIMS, (h)negative SIMS J. MATER. CHI:M., 1994, VOL. 4 of F in this sample is indicated by the large peak at m/z= 19 in the negative SIMS as well as by the peaks at m/z=47 in both spectra. A further indication of F in the layer is given by the small peaks at m/z=66 and 85 in the positive SIMS which could correspond to the appropriatc SiF, + species. Again the evidence given by FTIR is supported by the SIMS, and one can conclude that polysilicon grown in the presence of NF3 does incorporate some N and F into the layer. It is difficult to quantify the amount of N in the film though because of matrix effects and the presence.for example, of 28si2 + in the spectrum as well which swamps the weak N+ signal arising from the low-efficiency nitrogen secondary ion formation. Other techniques are needed to give more infor- mation about the extent and nature of the N and F presence in sample 111. AES Auger electron spectroscopy (AES) is a useful complementary method of surface analysis to static SIMS, sampling as it does the top few nm of a film.'' Fig. 5(a)shows the Auger spectrum for the Si3N, film (sample I). This spectrum was recorded after sputtering ca. 20nm of the layer. Before this was done there was an additional peak at 270 eV corresponding to the C KLL signal. In the sputtered sample the Si LMM peak at 97eV is clearly seen as is the N KLL peak at 380eV.The presence of 0 is again revealed (0KLL) at 510 eV. Sputtering of a further 30-50 nm showed that the presence of 0 persisted 60I t 3 1601 1 " 100 200 300 400 500 600 700 800 kinetic e ne rgy/eV Fig. 5 AES for samples I (a),I1 (h)and I11 (c) J. MATER. CHEM., 1994, VOL. 4 even quite deep into the layer. Very significantly, there is no evidence for F (F KLL) at 650 eV, although, as pointed out below, AES is a much less sensitive technique than SIMS.18 Fig. 5(b) shows the Auger spectrum for sample TI, pure polysilicon, after ca. 30 nm had been removed by sputtering; the only element found is Si at 98 eV (Si LMM). The low sensitivity of AES is probably the reason for not detecting 0 in the layer, although there is possibly a very small peak at 510 eV corresponding to 0 KLL.Fig. 5(c) is the Auger spectrum for sample I11 taken at a depth of ca. 34 nm, and the only element found is Si (Si LMM at 97.1 eV). There is no evidence for 0, N or F which is in contrast to the FTIR and SIMS results. Auger spectroscopy typically has a sensitivity18 of ca. 1 atom% which can make the technique of limited value, but in this case it does strongly suggest that whilst N and F are certainly present in polysilicon grown in the presence of NF,, as indicated by FTIR and SIMS, the layer is by no means 'pure' silicon nitride, which is again consistent with the FTIR results. We now examine this idea with more quantitative methods of analysis.SNMS Fig. 6 shows SNMS data for the silicon nitride sample. As with the static SIMS data [Fig. 2(a)] Si is readily seen (e.g.28si2+ 28si+ , 56Si2+) as is the presence of N (e.g. 42SiN+).7 Oxygen is again found (e.g. l60+, ,%OH+). Fig. 7 shows an SNMS depth profile for sample I. The SIN' profile clearly shows the sharp interface between the film and the substrate, with no evidence of any N in the silicon substrate. The profile for m/z=14 which can arise from both Si2+ and N+ also shows a sharp signal decrease as the interface is crossed and N is no longer present. The increase at the interface in the rn/z=28 profile for Si+ is consistent with the changeover from an Si,N, film to pure Si. Quantitative analysis based on the stoichiometry of Si,N, showed that the relative sensitivity factors for 14Nf and 42SiN+ were 0.06 and 0.02, respectively; both these values agree well with the values of 0.06 and 0.03 obtained by independent analysis by the equipment manufacturer.These relative sensitivity factors can be used to quantify any nitrogen found in the other samples analysed. Fig. 8 is an SNMS depth profile for the polysilicon layer deposited in the absence of NF, (sample 11). This SNMS profile, not unexpectedly, shows no change at the interface region, indicating, as the SIMS did, that the layer is reason- ably pure. Fig.9(a) shows the SNMS depth profile for as-deposited polysilicon in the presence of NF,. The presence of N is lo5k 1o4 Si+ 0.0 6.0 12.0 18.0 24.0 30.0 36.0 42.0 48.0 54.0 60.0 m lr Fig.6 Static SNMS data for a silicon nitride film (sample I) t SIN' \ \ loo-' ' ' ' " ' ' . .0.0 106.3 212.7 319.0 425.3 531.7 t/s Fig. 7 Dynamic SNMS depth profile for sample I t 1 0.0 237.4 474.8 712.2 949.6 1187.0 t /s Fig. 8 Dynamic SNMS depth profile for sample I1 apparent from the 42SiN+ profile, with a sharp step at the interface with the silicon substrate. However, in contrast to the Si,N4 sample (Fig. 7) there is only a small change at the interface for the profile for rn/z=14 corresponding to Si2+ and N+. Although an enlargement of this profile showed the small step more clearly, there was obviously less contribution of N to the m/z= 14 signal for sample 111 than for sample I, again supporting the evidence of FTIR and AES that sample 111 is not pure Si,N,.A calculation of the N content of the film, taking into account the contribution of doubly charged silicon to the signal for m/z =14, showed it to be certainly <5%, which is reasonably consistent with the AES result above. The SNMS depth profiles [Fig. 9(b)] for sample I11 after annealing are very similar to those for the as-deposited layer. A magnification of the scale for the profile for m/z= 14 again illustrated the contributions of both Si2+ and N+ to the signal. Most importantly, analysis of the spectrum showed that the nitrogen content of the film remained essentially J. MATER. CHEM., 1994, VOL. 4 lo4 Si2', N+ :2102 rn Y ,l@L .0.0 180.9 361.8 .542.6 723.5 904.4 tls Fig. 9 Dynamic SNMS depth profile for sample 111: (a)as-deposited, (h)after annealing unchanged after annealing. This strongly suggests that the peak at 897 cm-' in the FTIR spectrum [Fig. l(b)] does arise from the Si-N stretching mode. Any evidence for the role of F in the layer, though, is not given by the SNMS analysis. As has already been indicated, SNMS is not inherently as sensitive as SIMS and so this is why it was not possible to detect any F in sample 111. Therefore, negative SIMS depth profiling was carried out in the same apparatus on sample 111. Fig. lo@)shows the depth profile obtained with this technique; the 19F- is prominent. Unfortunately, it is not possible to quantify the amount of F present, although its normalized yield relative to 28Si-is 13: 1.The oxygen concentration in the film is very low, estimated to be certainly less than 1 part in lo4. SIMS being more sensitive than SNMS shows up the ubiquitous C in the film with, as is often the case, a higher level at the near-surface region. Fig. 10(b) gives the negative SIMS depth profile for sample 111 after annealing. The most significant features of this depth analysis are the very different profile for 19F-and a normalized yield relative to '*Si- of 6: 1. These changes indicate that not only has there been a redistribution of the fluorine but also more than half of it has diffused out during 0- 10' Ll loo2 0.0 478.2 956.5 1434.7 1913.0 2391.2 10' inoIV 0.0 340.8 681.6 1022.4 1363.8 1703.9 tIs Fig.10 Dynamic SIMS depth profile for sample 111: (a)as-deposited, (b)after annealing annealing. The FTIR spectrum [Fig. l(b)] is consistent with this observation since we concluded that it was unlikely that all of the F had been lost from the layer on annealing, with the shift of the Si-N peak to a higher frequency indeed suggesting that there was still some F present in the film. The 0 profile in Fig. lo@)is also consistent with the FTIR results [cj Fig. l(a) and l(b)] since the two to three times higher l60-SIMS signal at the surface than in the as-deposited layer shows that some oxidation has probably occurred during the nitrogen atmospheric-pressure anneal.In summary, the SNMS analysis has confirmed the findings of FTIR and SIMS that for polysilicon grown in the presence of NF, (sample 111) N is present, but only at a level of < 5 atom%; i.e. the film is certainly not stoichiometric Si,N,. The SNMS analysis also shows that after annealing there is essentially no change in the N content. The lack of sensitivity of SNMS does not allow any F to be detected in sample 111, but SIMS depth profiling does reveal it and, furthermore, shows that about 50% of the F is lost from the layer on annealing. The remaining F is, in view of the strong Si-F bond energy, undoubtedly bonded to Si and this probably J. MATER. CHEM., 1994, VOL. 4 E, (exp.)sample lev Ib 101.9IF 99.4 IIId 99.2 a Full width at half height.Si,N,. Table 1 XPS binding energies Si 2p N 1s 0 Is E, (lit.) FWHH" Eb (exp.) Eb (lit.) Eb (exp.) /e v lev lev IeV lev 101.9 2.4 397.3 397.7 532.2 99.4 2.1 --532.6 99.4 2.1 397.2 397.7' 532.3 'Poly Si. Poly Si+NF,. For Si,N,. FTIR spectrum, which can be attributed to Si-N stretching, to a higher frequency than one normally associates with that mode. and escape depths using Wagner's sensitivity factors." Fig. 11(a) and (b)show the peaks for Si 2p and N Is, respect- ively, for silicon nitride, sample I. Data for the binding energies (E,) and compositions are summarised in Tables 1 and 2, respectively. Peaks for 0 Is and C 1s were also found, but they are not shown here, although the data for oxygen are also summarised in the tables.The presence of oxygen is consistent with the results of the other methods of analysis Si 2p N Is (6) Fig. 11 XPS for the silicon nitride film (sample I): (a) Si 2p spectrum, (b)N Is spectrum Si 2p I[111111111~11111l111 100 110 120 130 EdeV .gc I 3 $ EdeV Fig. 13 XPS for the polysilicon film with NF, addition (sample TIT): (a) si 2P spectrum, (b)N 1s spectrum Table 2 XPS composition (atom '30) sample Si 2p" N 1s 0 Is I 0.48 0.36 0.16 I1 0.69 0.00 0.31 111 0.72 0.02 0.26 " Note that silicon compositions include all core contributions. reported above. The rather high oxygen content of 16 atom% probably results, as mentioned earlier, from contamination of the sample during deposition and from the fact that XPS monitors only a few monolayers at the surface.The Si 2p signal shows only one peak at the expected binding energy for silicon nitride and the nitrogen signal also correspmds to that of the nitride. The quantitative amounts of nitrogen and oxygen suggest that the surface silicon is in the form of nitride and oxide in the ratio 3: 1, and the fact that the Si ?p peak half width is greater than that shown by either of the two polysilicon samples (cJ Fig. 12 and 13) is consisterit with contributions from Si-0 bonds and with a separation of only 1.5 eV between the Si 2p peaks for Si,N, and for SiOz (Eb= 103.4 eV19). For the polysilicon film deposited without NF, addition the XP spectrum is shown in Fig.12 and the Si 2p signal consists of a major peak corresponding to elemental silicon with an E, of 99.4 eV and a contribution from oxidised silicon 3.6 eV higher and separated by a distinct valley; both Ebs are in good agreement with literature values." The signal at ca. 17 eV above the core signal is associated with the elemental silicon bulk plasmon. Comparison of the 0 1s and Si 2p oxide signals shows the ratio of the oxygen to oxidised silicon to be 1.8: 1, which suggests that the silicon had an overlayer of SiO,; this is contrary to the AES result, but in that case some 30 nm were sputtered off before the measurement was made. Fig. 13 shows the XPS result for the polysilicon film deposited with NF, addition (sample 111).The peak due to the silicon bulk plasmon is again in evidence, while thc Si 2p peak appears at 99.2eV which is very close to that for pure Fig. 12 XPS for the polysilicon film without NF, addition (sample 11) silicon,-indicating that the environments of the Si atclms in this sample and sample I1 are very similar; in other words, sample 111 is very much like a polysilicon layer. This is in contrast to sample I where clearly the layer is very different from polysilicon. However, N is definitely present in sample 111, and, although the N 1s peak occurs at a binding energy close to that expected for nitride, it can be seen from Table 2 that the layer contains only ca. 2 atom% of N, which is in agreement with the SNMS calculated value of 65atom%.The oxygen content again appears to be high and the Si 2p signal, as for sample 11,also shows a contribution from Si-0 bonds. For sample 111, though, the valley between the two Si 2p peaks is less pronounced than for sample I1 and this is postulated to arise from a small Si 2p nitride signal which reduces the valley depth. However, the contribution from the Si 2p nitride is more marked than expected from the nitrogen to oxygen composition ratio, but it should be remem- bered that these elemental amounts are derived assuming uniform distributions whereas the oxide is probably again an overlayer; the calculated 0 to N ratio is therefore exaggerated. No evidence was found in any of the spectra for F in the films.XPS is a technique which has a detection limit compar- able to that of AES (i.e. at the atom% level) and the earlier evidence for only small amounts of F in sample I11 is supported by its absence from the XPS results. Therefore, once again one sees that growing polysilicon in the presence of NF, does introduce a small amount of N, but the layer is still much more like pure polysilicon than stoichiometric Si,N,. Conclusions Polysilicon deposited by LPCVD in the absence and presence of NF, has been analysed by FTIR, SIMS, AES, SNMS and XPS. All of the techniques have shown that the incorporation of nitrogen occurs during polysilicon deposition in the pres- ence of NF,. However, a comparison with corresponding analyses of CVD silicon nitride has shown that the film is not stoichiometric Si,N,, and quantitative analysis by SNMS and XPS, in fact, shows the nitrogen content to be less than ca.5 atom%. Fluorine is also revealed by FTIR and SIMS to be present in the film deposited in the presence of NF,, and although it was not possible to obtain a quantitative measure of the amount of fluorine present, the fact that other less sensitive techniques, such as AES, SNMS and XPS, did not show any evidence of fluorine indicates that the amount was probably significantly less than 1atom%. SIMS analysis also suggested that annealing of the film caused about 50% of the fluorine to diffuse out of the layers. Earlier work2 has shown that growing polysilicon in the presence of NF, can yield good quality annealed layers even when the deposition temperature is much higher than is normally possible.The results in this paper show that this advantage can be offset by the incorporation of N and F into the layers. However, since the amount of this elemental contamination is small, especially for F, this may not be a significant disadvantage, especially if a material with proper- ties intermediate between those of a true semiconductor and J. MA'TER. CHEM.. 1994, VOL. 4 an insulator is required, (as in the case of using N,O with SiH, to produce SIPOS20). Studies are now needed on the electrical characteristics of polysilicon deposited with the aid of NF, and on its use in simple devices to investigate the potential of the material for microelectronic applications.An investigation of the use of other fluorine-containing species (e.g. F2, HF, ClF, and XeF,) for the control of the crystalline quality of polysilicon might also be interesting, and may also help to gain a better understanding of the chemistry of the deposition process. However, from a practical point of view, these gases would generally be more difficult and dangerous to handle than NF,. We gratefully acknowledge the award of an SERC grant for equipment, and financial support from Taiyuan University of Technology, Air Products plc and Epichem Ltd for J.F.Z. and from the University of Strathclyde for J.F.Z. and S.H.S. Air Products plc and Epichem Ltd. are also thanked for the provision of gases for this research.References 1 M. L. Hitchman, J. F. Zhao and S. H. Shamlian. J. Muter. Chem., 1994,4, 1821. 2 M. L. Hitchman, J. F. Zhao and S. H. Shamlian. J. Muter. Chem., 1994,4, 1827. 3 K. F. Jensen, M. L. Hitchman and W. Ahmed, in Proc. 5th Eur. Conj: on CVD, ed. J. 0.Carlsson and J. Lindstrom, University of Uppsala, Uppsala, 1985, p. 144. 4 M. L. Hitchman, J. Kane and A. E. Widmer, Thin Solid Films, 1979,59,231. 5 J. Tumpner, R. Wilsch and A. Benninghoven, J. Vac. Sci. Technol. A, 1987,5,1186. 6 R. Wilson, J. A. Van den Berg and J. C. Vickerman, Surf Interface Anal., 1989,14, 393. 7 J. S. Colligon, H. Kheyrandish, J. M. Walls and J. Wolstenholme, Thin Solid Films, 1991,200,293. 8 J. Kanicki, MRS Symp. Proc., 1988, 118,671. 9 G. Lucovsky and D. V. Tsu, J. Vuc.Sci. Technoi. A, 1987,5,2231. 10 S. E. Alexandrov, M. L. Hitchman and S. H. Shamlian, Adv. Muter. Opt. Electron., 1993,2, 301. 11 F. A. Johnson and W. Cochran, in Rep. Int. Conf. Phys. Semiconduct., ed. A. C. Stickland, Institute of Physics, London, 1962, p. 498. 12 K. Yamamoto, T. Nakanishi, H. Kasahara and K. Abe, J. Non-Crystal. Solids, 1983,59-60, 213. 13 C. R. Bailey, J. B. Hale and J. W. Thompson, J. ('hem. Phys., 1937, 5,274. 14 E. A. Jones, J. S. Kirby-Smith, P. J. H. Woltz and A. H. Neilsen, J.Chem. Phys., 1951,19,242. 15 S. Fujita, H. Toyoshima and A. Sasaki, J. Appl. Phys., 1988, 64, 3481. 16 S. Fujita and A. Sasaki, Proc. Symp. Silicon Nitride and Silicon Oxide Thin Insulating Films, 1987,87, 535. 17 N. N. Greenwood and A. Earnshaw, Chemistrv of the Elements, Pergamon, Oxford, 1984, pp. 473,431. 18 S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era, Vol.1-Process Technology, Lattice Press, Sunset Beach, CA, 1986, ch. 17. 19 D. Briggs and M. P. Seah, Practicul Surface Analysis, Wiley, Chichester, 1990. 20 M. L. Hitchman, Vacuum, 1984,34,377. Paper 4/02728K; Receiued 9th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401835

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1843-1847 Bonded Hydrogen in Silicon Nitride Films deposited by Remote Plasma-enhanced Chemical Vapour Deposition Sergei E. Alexandrov,+ Michael L. Hitchman* and Sarkis H. Shamlian Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, UK G1 1XL Using FTlR the hydrogen content of silicon nitride films grown by remote plasma-enhanced CVD (RPECVD) has been quantitatively examined. The influences of process parameters on the concentration of bonded hydrogen and on its nature are discussed. It is suggested that the incorporation of Si-H bonds is largely determined by gas-phase processes while that of N-H bonds arises from surface interactions. The preferential type of hydrogen bonding to silicon is found to occur as Si-HH2 whereas for nitrogen N-H appears to be the most likely form of bonding.The total hydrogen content of capacitively coupled RPECVD films is found to be only ca. 9 x lo2' cmP3 at a growth rate ca. 20 times that obtainable by inductively coupled RPCEVD. Therefore there is considerable potential for depositing films with very low total hydrogen content by capacitively coupled RPECVD. Amorphous silicon nitride films deposited at low temperatures have found widespread use as dielectric layers in the technol- ogy of semiconductor devices and integrated circuits.',2 One of the methods which has often been employed for silicon nitride deposition at relatively low temperatures (e.g. in the range 200-400 "C) is that of plasma-enhanced chemical vapour deposition (PECVD)3 based on the use of rf or ECR low-pressure glow discharges.Silane and ammonia or nitrogen are usually used as initial reagents, and because of the considerable amount of hydrogen in the gas phase during deposition it is to be expected that hydrogen will be incorpor- ated into deposited films through adsorption of partially reacted gaseous hydrides. This is confirmed by experimental results which show that silicon nitride films deposited by PECVD can have up to 30 atom% of hydrogen incorporated into the film^,^,^ and that the use of nitrogen in place of ammonia leads to the formation of films with a lower concen- tration of hydrogen. Hydrogen is present in the films in a chemically bonded form, such as Si-Hi (i= 1-3) and N-H, (j=1,2) with the bonds being statistically distributed in the volume of the films.6 The concentrations of these types of bonds as well as the values of i andj depend on the deposition condition^.',^ Thus amorphous silicon nitride deposited by PECVD could be described more generally as SiN,H,, to emphasise the non-stoichiometry of the material. Bonded hydrogen incorporated into deposited SiN,H, films very strongly influences the physical, chemical and mechanical properties and, in particular, it dramatically causes a deterior- ation of the dielectric and insulator properties as well as the chemical and thermal ~tability.~,' Therefore research has been carried out in a search for deposition conditions which provide minimal content of bonded hydrogen in deposited SiN,H, films.The aim of this work has been to extend these investigations to the technique of remote plasma-enhanced chemical vapour deposition ( RPECVD).lO.ll The influence of process param- eters on the concentration of bonded hydrogen and on the configuration of Si-H, and N-H, bonds in SiN,H, films deposited by capacitively coupled RPECVD is described. Experimental A detailed description of the reaction chamber with capaci- tively coupled rf power, as used in this work, as well as 7 Permanent Address: Department of Electronic Material Technology, Faculty of Physical Chemistry and Metallurgy, St. Petersburg State Technical University, Polytechnical Str. 29, St. Petersburg, Russia 195 251. information of the deposition conditions have been given We briefly review the experimental conditions here.The glow discharge was generated between two circular parallel-plate electrodes positioned in the upper part of a cylindrical stainless-steel reactor chamber (Fig. 1). The lower rf (generally 13.56 MHz) powered electrode was 16 cm above the substrate which was placed on the heated susceptor. The electrodes were separated by a 4cm gap. The reactant gases were electronic-grade nitrogen and silane (Air Products). A flow of nitrogen (QNt= 150 sccmt) introduced from the top of the reaction chamber was directed through the discharge region for RF excitation and then mixed with a silane-nitrogen mixture (Q, =0.3 sccm, QNS =9.7 sccm), which was introduced downstream from the plasma generation region through an injection nozzle positioned 6cm above the sub- strate.The gas flow conditions used minimised backstreaming of the silane in the direction of the powered electrode and also provided a high uniformity of layer growth, A typical thickness variation was < k2Oh over the entire surface of a 76 mm substrate. The full range of deposition parameters has been given previously." For the studies reported here the substrate temperature was varied from 150 to 350 'C, the silane flow from 0.3 to 5 sccm, the rf power from 50 to 100 W, and rf frequency from 6 to 20 MHz. t Standard cm3 min-l. nitrogen dh ____________-_-_____----------PERC tomass I -1 Ispectrometer sp mixture-Fig. 1 Schematic diagram of the remote PECVD reactor with capaci- tively coupled rf power for the deposition of SiN,H,: GD, gas distributor; GE, grounded electrode; PE, powered electrode; RC, reactor chamber; SP, substrate platform; SU, substrate; TC, thermocouple.J. MATER. CHEM.. 1994, VOL. 4 Film thickness and refractive indices were measured using an automatic ellipsometer (Gaertner L116B) with an He-Ne laser. Typical values of the film thickness were in the range 420-500 nm. FTIR transmission spectra of the deposited layers were obtained with a Galaxy Series FTIR 3000 spec- trometer, which had a maximum resolution of 2 cm-'. Films used for measuring the IR transmission spectra were deposited on high ohmic resistance silicon substrates polished on both sides.Background absorption from the substrate was sub- tracted from the spectra to obtain the film spectra. The number density of Si-Hi and N-H, bonds in the SiN,H, films was calculated from the FTIR spectra using a method given by Lanford and Rand.4 Note that in that paper no errors are given for the values of the coefficients which are used for the conversion of N-H and Si-H absorption band intensities to bonded hydrogen concentrations. Therefore no estimate of absolute errors in concentration levels can be made here. An error estimate was made, though, on the basis of results from five experiments carried out under the same conditions. This is discussed below. Thermal annealing of deposited films was carried out at 500°C in a nitrogen atmosphere for 30 min in order to evaluate their thermal stability.Results and Discussion A typical IR transmission spectrum for the region 500-4000 cm-of a near-stoichiometric RPECVD silicon nitride film, as shown by a refractive index of 2.05, is shown in Fig. 2. Apart from the main absorption band (at ca. 830 cm-'), related to the stretching mode of the Si-N matrix, several other absorption peaks with lower intensities can be identified. A weak peak at about 3390cm-' corresponds to the N-H, stretching modes' and the one with the maximum at ca. 2165 cm-' is related to the Si-Hi stretching modes.' The shoulder at ca. 1190 cm-' is related to N-H, bending modes.I3 The N-H, and Si-Hi absorption bands corresponding to stretching vibrations are well isolated from other peaks and this allowed measurement of their intensities to be used for the calculation of the bonded hydrogen content of deposited films.The absorption band related to the bending vibration of N-H, bonds could not be used for the evaluation of bonded hydrogen content because of its superposition with the peak related to the vibrational modes of the Si-N matrix. It is commonly found for plasma-activated processes that the temperature of the substrate very strongly influences the composition of the deposited films.3 Fig. 3 shows that as the r 0.30i d z 0.204 gl I' 0.10 1 4000 3000 2000 1000 waven um berkm-' Fig. 2 Typical FTIR transmission spectra for SiN,H,.Deposition conditions: Td=300 "C, P,,, =22 Pa, QN'= 150 sccm, QNS=9.7 sccm, Qs =0.3 sccm, rf power =80 W. 'OI "? 30t .\ I L lo/ 0 100 200 300 400 deposition temperature, TdPC Fig. 3 Dependence of hydrogen content of films on deposition tem-perature (T',): (m)[Si-H,], (0)[N-Hj]. P,,,=22 Pa. QNt= 150 sccm, QNS=9.7 sccm, Qs= 0.3 sccm, rf power = 80 W. deposition temperature decreases there is a marked increase in the bonded hydrogen content in the deposited films. Also one can see that the content of hydrogen bonded with silicon in the films is about one order of magnitude higher than that of hydrogen bonded with nitrogen. As noted above, an estimate of the errors in the N-H, and Si-Hi concentrations was made.This gave an error at the 90% confidence level of 0.4 x lo2' cm-3 for the concentration of N-H, groups with the corresponding value for the Si-Hi groups being 1.7x lo2' cmP3. By comparing these values with average values of the N-H, and Si-Hi group concentrations in the deposited films (ca. 1x 1021 cm-3 and 2 x cmP3, respect- ively) it is clear that while there is a high degree of inaccuracy for estimates of the N-H, group concentration, the error in the %-Hi concentrations is not unreasonable. Also it can be seen that because the amount of hydrogen bonded to silicon in the films is approximately one order of magnitude higher than that of hydrogen bonded to nitrogen, inaccuracies in the estimation of the total hydrogen concentration will be mainly determined by errors in the estimates of the concentration of Si-Hi groups.The considerable difference in the concen-trations of Si-Hi and N-H, bonds can be understood on the basis of the results of mass-spectrometric analysis of the gas phase during deposition, which have been described previously.12 In that work it was shown that ca. 80% of the original silane introduced into the reaction chamber was dissociated into various SiH, (x=1-3) species, and there was no evidence of species with Si-N bonds, such as aminosilanes which are formed in the gas phase in the SiH,-NH, system.14 The presence of species with Si--H, bonds in the gas phase apparently leads to a higher content of Si-Hi bonds in the deposited films with N-H, groups being formed only by heterogeneous reactions occurring on the surface of the grow- ing film.The steeper increase in the content of Si-Hi groups in the films with increasing silane flow rate (see Fig. 4) probably supports this idea. Note that although both types of bond are thermally unstable, with increasing deposition temperature leading to a decrease in the probability of their existence in the films (Fig. 3), the rate of decrease in content of Si-Hi bonds with the deposition temperature was more rapid than for N-H, bonds, suggesting a higher thermal stability for N-H, bonds. The position of the maximum in the range 2150-2170 cm-' for the absorption band related to the Si-Hi stretching mode (Fig. 5) probably indicates that Si-HH2 is the preferential form for silicon-hydrogen bonds, although the considerable width J.MATER. CHEM., 1994, VOL. 4 Od 0123456 silane flow rate, Q&m3 min-‘ Fig.4 Dependence of hydrogen content of films on silane flow rate (Qs): (.)[Si-H,], (m)[N-H,]. Td=300”C, PT=22 Pa, QNt=150 sccm, Qhs=9.7 sccm, Qs=0.3 sccm, rf power=80 W. 2oor wavenum be r/cm-’ Fig. 5 Spectral dependence of absorption band related to the Si-Hi stretching mode on deposition temperature. Td/oc:(a) 150, (b) 200, (c) 250, (d)300,(e) 350. PT=22 Pa, QNf= 150 sccm, Q~~~9.7 sccm, Qs=0.3 sccm, rf power=80 W. of this peak also probably reflects the result of superposition of vibrations of all forms of Si-Hi bonds in the films. For silicon nitride films prepared by PECVD it has been shown6 that absorption related to the stretching modes of Si-Hi bonds (i= 1-3) is a superposition of three absorption bands near 2120,2180 and 2255 cm-’, which can be assigned to the stretching modes of Si-H, Si-H, and/or (Si-H2), chains, and Si-HH,, respectively; the centering of the maximum for our films around 2160cm-1 suggests a predominance of Si-H2.In addition one should notice the steady shift in the position of the absorption band maximum to higher wave- numbers with increasing deposition temperature, which indi- cates an increasing proportion of Si-Hi with a higher value of i as the total amount of bonded hydrogen simultaneously decreases. However, there is another possible reason for this shift.It could also be explained on the basis of a change in the Si-N matrix composition. We have found previously’’ that an increase of deposition temperature leads to a gradual change in film composition from silicon-rich SiN,H, to near- stoichiometric silicon nitride. It has also been shown for silicon nitride films deposited by PECVD” that an increase in content of more electronegative nitrogen atoms in an Si-N 1a45 matrix causes a considerable change in the oscillator strength for silicon-hydrogen bonds, which will lead to a corresponding shift in the absorption band for Si-Hi to higher wavenumbers. Both the above mentioned factors could therefore result in the observed shift with the deposition temperature of the absorption peak corresponding to the silicon-hydrogen stretching mode.The level of rf power coupled into the system also has a very strong influcnce on the concentration of bonded hydro- gen (Fig. 6). We have shown from results of mass-spectro- metric analysis that an increase in rf power density causes an increase in degree of dissociation of the original silane intro- duced into the reaction chamber.I2 That means that there is a decrease in the average number of hydrogen atoms bonded to a specific Si atom in the gas phase. So it is quite reasonable to expect a decrease in the average number of hydrogen atoms bonded to a specific Si atom in the film, leading to a concomitant decrease in concentration of Si-Hi bonds in the deposited films with increasing rf power.Following on from this, one would then expect a shift of absorption band related to the Si-Hi stretching modes to lower wavenumbers with an increase of plasma power density. However, there is a shift in opposite direction (Fig. 7). This observed shift of the absorption band at ca. 2150 cm-’ to higher wavenumbers 1 40 80 120 ri power/W Fig.6 Dependence of hydrogen content of films on rf power: (HjCSi-H,], (Oj[N-H,], Td=3OO0c, PT=22 Pa, QNt=150 SCCm, QNS=9.7 sccm, Qs= 0.3 sccm. 2oo r 0 1900 2000 2100 2200 2300 wavenu m ber/cm-’ Fig.7 Spectral dependence of absorption band related to Si-H, stretching mode on plasma power. Rf power/W: (a) 60, (b)75. (c) 80, (d) 100. Td =300 “C, PT= 22 Pa, QNt= 150 SCCm, Q~~z9.7SCCD1, Qs= 0.3 sccm, rf power =80 W. with an increase in plasma power density is probably caused by a corresponding increase in content of nitrogen atoms in the Si-N matrix,'* leading to an increase of the oscillator strengths of any kind of silicon-containing bonds, including Si-Hi.The dependence of N-H, bond concentration on plasma power is in the opposite sense to that for Si-H, bonds. It can be seen from Fig. 6 that there is a gradual increase in content of hydrogen bonded to nitrogen with increased RF power density. In accordance with the qualitative model of the RPECVD process mentioned in ref. 11, the formation of N-H, bonds probably takes place on the growing surface so that the variation of N-H, bond concentration with process parameters has to be explained in terms of conditions for surface rearrangement.As the level of plasma power intro- duced into the system is increased, the amount of energy delivered to the surface of the growing film through excited particles will increase as well. This will, in turn, promote a higher mobility and activity of reacting particles on the growing film surface giving an increase in the number of N-H, bonds and an enhanced probability of incorporation into the layer. Besides plasma power density, the frequency of the rf generator used for the ignition of the glow discharge affects the bonded hydrogen content of SiH,N, films (Fig. 8). For conventional PECVD processes the influence of frequency of applied field is connected with the plasma bombardment of the growing films,5 and it has been shown that the effect of plasma bombardment on surface processes increases with decreasing discharge frequency.16 However, for PECVD the probability of plasma bombardment of the growing film is considerably reduced because substrates are positioned out- with the discharge generation region.Thus the negligible variation in N-H, bond content with rf frequency (Fig. 8) is not unexpected. On the other hand, the observed increase with rf frequency in the content of hydrogen bonded to silicon in the deposited films is probably caused by a decrease in the degree of dissociation in the gas phase of the original silane introduced into the reactor. Thus the variation of overall hydrogen concentration with RF frequency reflects changes in glow discharge parameters (e.g.concentrations and mean energies of electrons and ions) and accordingly the efficiencies of ionisation, excitation and dissociation processes occurring in the plasma. When the film was annealed it was found that for all the samples the absorption bands related to the vibration modes 30r1 0 0 10 20 30 rf frequency/MHz Fig. 8 Dependence of hydrogen content of films on plasma frequency: (B)[Si-Hi], (O)[N-H,]. Td=3OO0C,Pt,,=22 Pa, QNt=150 sccm, QNs =9.7 sccm, Q, =0.3 sccm, rf power =80 W. J. MATER. CHEM.. 1994, VOL. 4 of N-H, bonds disappeared. The disappearance in the spectra of the absorption peaks due to N-H, vibration modes indicates that N-H (j=1) is the preferential type of N-H, bond in the deposited films since dissociation of N-H2 bonds is only observed at temperatures >700 0C.6317The mean value of the total bonded hydrogen concentration (i.e.Si-Hi and N-H,) in annealed films was ca, 1x 1021 cm-3 and did not depend on the hydrogen content of films before annealing. Note that annealing of the films containing total hydrogen concentrations in excess of ca. 2.5 x cm-3 caused bubbles in and cracking of the layers, as well as variation in their colour. It is interesting to compare our results for values of bonded hydrogen concentration in the films with those obtained for the SiN,H, films deposited by inductively coupled RPECVD under similar conditions." Films deposited by inductively coupled RPECVD with a growth rate of ca. 0.18 nm min-' contained ca.3 x lo2' ~rn-~bonded hydrogen. For films deposited in our system with a growth rate of ca. 3.5 nm min-' we have obtained a mean value of total hydrogen concentration of ca. 9 x lo2' cm-3. These results suggest that capacitively coupled RPECVD could provide reasonably low concentrations of bonded hydrogen in films but at a much higher growth rate than for films deposited by inductively coupled RPECVD. Conclusions The experimental results reported in this work show that the concentration and configuration of bonded hydrogen in SiN,H, films deposited by a new variation of RPECVD is determined by gas-phase interactions as well as by hetero- geneous reactions occurring on the surface of growing films.The gas-phase processes probably involve the initial excitation of the nitrogen which is passed through the discharge region, and interaction of this excited nitrogen (as well as electrons) with silane molecules, leading to the formation of Si-H, species and their transport to the substrate. The nature and concentration of the Si-H, species are probably the main factors determining the configuration and content of hydrogen bonded with silicon in the films. In our case Si-H2 has been found to be the preferential form for the silicon-hydrogen bond in the deposited SiN,H, films, and this perhaps suggests that SiH2 is the dominant silane species in the gas phase during PECVD.The intensity of gas-phase interactions deter- mines not only concentrations of excited particles in the plasma, but also the amount of energy delivered to the surface of the growing film. Surface processes, dependent on this plasma energy, will control the configuration and content of hydrogen bonded to nitrogen in the films. An increase in the plasma power density has been found to cause a slight increase in the N-H, bond concentration. From annealing experi- ments it has been shown that N-H is the most likely type of bonding occurring in the films. In general, it has been shown that the concentration of bonded hydrogen in SiN,H, films could be adjusted by variation of the deposition tempera- ture, gas-phase composition, plasma power density and rf frequency. The lowest content of bonded hydrogen of ca.9 x 10,' ~171~~was found in SiN,H,, layers deposited in the temperature range 300-350 "C, at an N, :SiH, ratio of >350 and an rf power level of 100 W. These conditions also provided deposition of stoichiometric silicon nitride films without excess silicon. Films deposited under these conditions have pre- viously been shown to have good dielectric properties and chemical and thermal stability. Therefore they are of interest for electronic applications. In summary, it is clear that with capacitively coupled RPECVD there is the possibility of having a good control J. MATER. CHEM., 1994, VOL. 4 over reactions occurring in the gas phase and on the surfaces of growing films and, hence, over the hydrogen concentration in deposited SiN,H, films.References 1 C. R. M. Grosvenor, Microelectronic Materials, Adam Hilger, Bristol, 1981. 2 M. Gupta, V. K. Rathi, R. Thangaraj, 0. P. Agnihotri and K. S. Chari, Thin Solid Films, 1991,204, 77. 3 D. W. Hess and P. B. Graves, in Chemical Vupor Deposition- Principles and Applications, ed. M. L. Hitchman and K. F. Jensen, Academic Press, London, 1993, ch. 7. 4 W. A. Lanford and M. J. Rand, J. Appl. Phys., 1978,49,2473. 5 W. A. P. Claasen, W. G. J. N. Valkenburg, M. F. C. Willemsen and W. M. v. d. Wijgert, J. Electrochem. Soc., 1985, 132, 893. 6 M. Maeda and H. Nakamura, J. Appl. Phys., 1985,58,484. 7 G. L. Valco and V. J. Kapoor, J. Electrochem. Soc., 1987,134,685. 8 D. Schalch, A. Scharmann and R. Wolfrat, Thin Solid Films, 1985, 124, 301. 9 A. Piccirillo and A. L. Gobbi, J. Electrochem. Soc., 1990, 137, 3910. 10 G. Lucovsky, D. V. Tsu, R. A. Rudder and R. J. M'irkunas, in Thin Film Processess II,ed. J. L. Vossen and W. Kern Academic Press, London, 1991, ch. IV-2. 11 S. E. Alexandrov, M. L. Hitchman and S. H. Shamlian, Adc. Muter. Opt. Electron., 1993,2, 301. 12 S. E. Alexandrov, M. L. Hitchman and S. H. Shamlian, J. Phys. (Paris) IV, 1993,3, 233. 13 J. Kanicki, MRS Symp. Proc., 1988,118,671. 14 D. L. Smith, A. S. Alimonda, C. C. Chen, S. E. Ready and B. Wacker, J. Electrochem. Soc., 1990,137,614. 15 C. Chaussat, E. Bustarret, J. C. Bruyere and R. Grolfau, Physica, 1985,129,215. 16 R. H. Bruce, J. Appl. Phps., 1981,52, 7064. 17 T. Kamada, T. Hirao, M. Kitagawa, K. Setsune and K. Wasa, Appl. Surf. Sci., 1988,33134, 1094. 18 S. V. Hattangady, G. G. Fountain, R. A. Rudder and R. J. Markunas, J. Vuc. Sci. Technol., 1989,7, 570. Paper 4/028146; Received 12th Afay, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401843

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1849-1850 A New Synthetic Route to Hydroxyapatite Coatings G. Spoto,aE. Ciliberto" and G.C. Allenb " Dipartimento di Scienze Chimiche, Universita di Catania, Italy Interface Analysis Centre, University of Bristol, Oldbury House, 121 St. Michael's Hill, Bristol, UK BS2 8BS Metal-organic chemical vapour deposition (MOCVD) has been used to produce stoichiometric hydroxyapatite coatings using as sources calcium dipivaloylmethane chelate and P,05. In the past calcium phosphate ceramic coatings have been produced in different ways. Over the last few years much effort has been made to develop ceramic materials containing calcium and phosphorus because of their chemical similarity to many biological minerals.' Octacalcium phosphate (OCP) tricalcium phosphate (TCP) with amorphous calcium phos- phate (ACP) and dicalcium phosphate dihydrate (DCPD) have been suggested as the most important intermediate phases in the formation of the thermodynamically more stable hydroxyapatite (HAP) during biological mineralisation pro- cesses., These synthetic ceramic materials have been shown to induce cellular attachment when tested in vivo, but poor mechanical properties prevent their use as load-bearing implant^.^ Thus, stronger implants coated with bioactive ceramic materials have been used to enhance the performance.Several methods have been proposed for the production of calcium phosphate coatings including plasma ~praying,~ ion beam sputter dep~sition,~ laser ablation6 and electrodepos- ition.' Unfortunately though, the characteristics of the coat- ings so obtained and difficulties encountered with these preparative techniques has prompted the search for new methods of deposition.Of the many ways of producing HAP coatings, the plasma spraying technique first proposed by de Groot4 has probably been the most thoroughly investigated. By this means calcium phosphate coatings 40-60 pm in thickness have been obtained but they are of variable stoichiometry and non-uniform thick- ness when applied to porous surfaces. MOCVD techniques have found an increasing application in a wide variety of research fields.8 The ability to control kinetically the depos- ition process and to pattern highly uniform coatings on complex shaped objects allied to the facility for large-scale production makes this technique potentially very powerful.Here we report results from experiments using the MOCVD method to prepare hydroxyapatite coatings. Such coatings were deposited on an a-A1,03 substrate in a horizontal quartz hot-wall CVD reactor using a single inlet tube, carrier-gas controller and thermocouple-heater assembly for each of two volatile sources calcium dipivaloylmethane chelate, Ca( DPM),9 and P205.The deposition was carried out using a total pressure of 10 Torr and an oxygen carrier gas flowing at 150 ml min-'. Source temperatures of 210 and 270 "C were used for the precursors Ca( PDM)2 and P20,, respectively. The external heat source surrounding the deposition region of the reactor was maintained at 850°C.A scanning electron micrograph of the initial coating formed by the CVD process is shown in Fig. l(a). It can be seen that this is highly homogeneous with an average particle size of 5-10 pm. X-Ray diffraction (XRD) (Fig. 2) shows this film to consist mainly of P-Ca,P,O, and a small amount of A1P04. This biomaterial in itself has interesting properties," but after it had been subjected to two hourly heating cycles at 1000, 1100, 1200 and ultimately 1350°C in air it was found to transform to b-Ca,(PO,),. During the heating process the Fig. 1 Scanning electron micrographs of (a) the as-deposited sample and (b) the sample after heating at 1350 *C bulk concentration of AlPO, was found to increase progress- ively up to 12OOcC, but the transformation of P-Ca2P207 to P-Ca3(PO4), appears complete at 1100"C.At 1350 "C features characteristic of AlPO, are absent. The spectacular change in crystallinity produced after heating the sample to this tempera- ture is illustrated in the scanning electron micrograph of Fig. l(b). This deposit is identified as /?-Ca,(P04)2 by XRD. Changes in the surface composition after the deposit was heated to various temperatures were investigated using X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS). These techniques, which specifically analyse only the outermost layers of the deposit, revealed interesting differences between the surface and bulk cclmposi- tion of the deposited coating.The Ca :P ratio measurcd from fo 00 0 o on oom 0 20 22 24 26 28 30 32 34 2B/degrees Fig. 2 X-Ray diffraction pattern showing the change in composition of the 'bulk' of the ceramic coating after each heating cycle. Compounds are identified as: 0,P-Ca,P,O,; @, p-Ca,(PO,),; +,AlPO,; and *, x-A1,O3. TC:(a) 850, (b) 1000, (c) 1100, (d) 1200, (e) 1350. 0.5I 850 1000 1100 1200 1350 TI'C Fig. 3 Surface Ca: P ratio after each heating cycle 01 I 535 533 531 529 527 binding energy/eV Fig. 4 The 0 Is XP spectrum recorded from the sample surface after heating it to 1350°C the Ca2p and P2p peak intensities in the XP spectrum recorded after each step in the heating process are shown in Fig.3. It can be seen that the Ca:P ratio is characteristic of OCP [Ca,( HP04)(P04)45H20]" for temperatures below J. MATER. CHEM.. 1994, VOL. 4 1200"C, but at 1350°C it reaches the value of 1.67 expected for HAP [Cal,(P0,),(OH)2]. Moreover the P : 0 and Ca: 0 ratios, 0.22 and 0.37, respectively, obtained from the XP spectrum after the sample has been heated to 1350°C are very close to those for HAP (0.23 and 0.38, respectively). The 0 1s peak recorded in this spectrum is shown in Fig. 4. Noting the C 1s peak at 284.5 eV, two peaks may be identified in the 0 1s region at 530.6 eV (FWHM 2.07 eV) and 532.2 eV (FWHM 2.07 eV) and may be assigned to two chemically different oxygen environments in HAP for the P043-and OH-groups. The ratio of the peak areas obtained after curve fitting is 11.5. The SIMS results appear to confirm the existence of a hydrated calcium phosphate compound at the surface of the coating after it is heated to 1350 "C.The negative-ion spectrum shows OH-(17 u), PO- (47 u), PO,- (63 u) and PO,- (79 u), cluster species and the positive-ion spectrum identifies Ca + (40 u) and CaH' (41 u). Unlike the XPS results, it is not possible to use these spectra confidently to characterise a particular compound from intensity data, but such clusters were identified in the spectra acquired after each heating cycle and as such are entirely consistent with the presence of the same type of calcium compound containing both hydroxy and phosphate groups.12 The formation of the hydrated calcium phosphate phase at the surface of the coating occurs after the coating is heated to 1350"C and subsequently rehydrated during the final stages of the slow cooling process carried out at a rate of 1.17"C min-'.Bulk analysis of the coating as initially laid down in the MOCVD process was carried out using energy-dispersive X-ray analysis (EDX) in the scanning electron microscope. This gave a Ca :P ratio of 0.66, much lower than that obtained from EDX analysis after the sample was heated to 1350°C. This observation may be attributed to the loss of phosphorus that is presumably present in an acidic form, during each stage of the heating cycle. Such behaviour does not appear to alter the Ca :P ratio at the surface of the coatings. References 1 L. L. Hench and J. Wilson, MRS Bull., 1991, 16, 62. 2 G. H. Nancollas, Pure Appl. Chem., 1992,64, 1673. 3 R. M. Pilliar, J. E. Davies and D. C. Smith, MRS Bull., 1991, 16, 55. 4 K. de Groot, R. Geesink, C. P. A. T. Klein and P. Serekain, J. Biomed. Muter. Rex, 1987,21, 1375. 5 J. L. Ong, L. A. Harris, L. C. Lucas, W. R. Lacefield and D. Rigney, J. Am. Ceram. Soc., 1991,74,2301. 6 P. Baeri, L. Torrisi, N. Marino and G. Foti, Appf. Su~fSci., 1992, 54, 210. 7 M. Shirkhanzadeh, J. Muter. Sci. Lett., 1991, 10. 1415. 8 J. T. Spencer, Prog. Znorg. Chem., 1994,41, 145. 9 E. W. Berg and N. M. Herrera, Anal. Chim. Am. 1992,60, 117. 10 H. Fleish, J. Crystal Growth, 1981,53, 120. 11 W. E. Brown, Nature (London), 1962,196, 1048. 12 D. Briggs, A. Brown and J. C. Vickerman, Handbook of Static Secondary Ion Mass Spectrometry, J. Wiley, Chichester, 1989. Paper 4/04932B; Received 1 1 th August, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401849

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12), 1851-1854 2-Amino-5-nitropyridinium Acetophosphonate: A Engineered Non-linear Optical Crystal J. Pecaut and R. Masse* Laboratoire de Cristallographie, associe a I'llniversite Joseph Fourier, CNRS, Cedex 09, France Deliberately B.P. 766, 38042 Grenoble The crystal structure of 2-amino-5-nitropyridinium acetophosphonate confirms the possibility to design at request non- centrosymmetric structures based on the 2-amino-5-nitropyridinium polar chromophore. The non-centrosymmetry in this class of materials is mainly dependent on the structure of the associated counter-anion. Layered anionic aggregates always favour non-centrosymmetrical organization of non-linear cations as evidenced by many crystal structure investigations. During the past 15 years a great variety of organic crystalline and polymeric materials with enhanced quadratic non-linear optical properties have been proposed for three-wave-mixing optical devices.* Their design was guided both by the measure- ment of non-linear efficiency at the molecular level based on EFISH by the shape of materials resulting from the geometry of non-linear chromophores and by mechanical, thermal and chemical requirements. The chromo- phores used in this engineering were mainly derivatives of nitroanilines, stilbenes and polyenes, modified through adequate chemical substitutions with a view to optimizing the tensorial components Bijk (zijk)and the transparency band- width.The enhancement of the macroscopic second-order susceptibility coefficients in uniaxial or biaxial crystals built up from one- or two-dimensional molecular units (nitroanil- ines) and related phase-matching configurations are dependent on the orientation6 of the optimal chromophore in the crystal.Such ideal packings are extremely difficult to engineer: the crystal structure of N-(4-nitrophenyl)-~-prolinolillustrates an exceptional arrangement of chromophores7 in the point-group 2. A remarkable family of molecular crystals exhibiting large quadratic macroscopic susceptibilities designed from 2-amino- 5-nitropyridine has been intensely studied: 2-adamantyl-amino-Snitropyridine (AANP),8,9 2-cyclooctylamino-5-nitro-pyridine (COANP),lo-l' 2 [(S)-phenylethyl] amino-5-nitro- pyridine ( MBANP),I2-l4(4-nitro-2-pyridinyl)-(S)-phenylalani-no1 (NPPA)'5,16 and N-(5-nitro-2-pyridy1)leucinol (NPLO)." These crystals exhibit structural organization of the non-linear chromophores in herringbone motifs with the slipped conformation, which favours large second-order optical non- linearities, as established by Di Bella et A crystal engineer- ing strategy based on the encapsulation of the 2-amino-5- nitropyridinium cations in inorganic and organic layered host matrices has been developed with a view to obtaining crystals with a large transparency range (IR+UV), high packing cohesion of chromophores and improved mechanical and thermal resistances compared to molecular crystals built up with the 2-amino-5-nitropyridine as chromophore." This approach, which combines the cohesion of ionic polymeric inorganic (organic) lattices with the enhanced polarizability and flexibility of organic molecules, has been proposed by several authors and clearly illustrated with (H,PO,-),, (H,AsO,-), polyanions.20-22 Encouraged by the first physical investigation^,^^ it was fascinating to determine the parameters which control the building of acentric frameworks in the special case of crystals including 2-amino-5-nitropyridinium cations (2A5NP + ).Furthermore, some centric and acentric crystals structures combining 2ASNP+ cations with inorganic anions of various shapes and charges have been solved, showing the role played by the layered anionic mat rice^.^^-^^ A quasi-polar alignment of 2A5NP' cations has been observed in 2-amino-5-nitropyridinium L-monohydrogen-tartrate structure,27 evidencing the role of the two-dimensional hydrogen-bonded aggregate (C4H506-),. The use of (C4H,06 -), in crystal engineering has been clearly e Kplained and systematized by Aekeroy et Analogues of tartrate, dihydrogenmonophosphate and dihydrogenmonoarsenate layered host matrices are capable of inducing stable acentric structures with 2A5NP' as guests if the requirement pH<2 is respected and a short 2D hydrogen-bonded anionic network is formed. The acetophosphonic acid has been selected because the first acidic function of the phosphonate group is relatively strong and allows proton transfer towards the 2-amino-5- nitropyridine (pH 2, 20 "C) following eqn.( 1): C5H5N302+H+*(P03H)-CH2-C02H-The acetophosphonate anion has two hydrogen-donor groups, -P-OH and -C02H, and sufficient hydrogen acceptors to frame a 2D anionic network based on two strong hydrogen bonds 0-He. .O; such a layer is capable of favouring herring- bone assemblies as a consequence of avoiding the formation of local inversion centres in the structure. The observed crystal morphology (point group 222), the positive signal in second- harmonic generation (Nd3+: YAG laser, 1.06 pm) md the crystal structure investigation of 2-amino-5-nitropyridinium acetophosphonate confirm our assumption. Experimental Crystals of 2-amino-5-nitropyridinium acetophosphonate were prepared by dissolving 0.001 mol of purified 2-amino-5- nitropyridine (2A5NP) in 20 ml water containing 0.002 mol acetophosphonic acid at 30°C.Slow evaporation of the solution yielded transparent crystalline needles up to 4mm x 2 mm x 2 mm in size. The chemical formula was estab- lished via the crystal-structure investigation. The cell param- eters and space group mentioned in Table 1 were determined by traditional X-ray methods using four-circle diffractometer data. The P2,2121 space group was confirmed both by the last reliability factor (Table 1)and a positive second-harmonic generation2' powder test (2.5 x urea) from an Nd3+: YAG laser fundamental beam (1.06 pm). The crystal structure was solved by direct methods using Multan 7730 and difference Fourier syntheses (Table 2).Full-matrix least-squares refine- ments were performed on F, using a unitary weighting scheme. Scattering factors for neutral atoms andf,f' have been taken from International Tables for X-ray Crystallography ,31 The J. MATER. CHEM., 1994, VOL. 4 Table 1 Crystal data, intensity measurements and structural refine- Table 3 Interatomic distances (A), bond angles (') and their esds ment parameters observed in the cation-anion packing C5H6N302+C2H405P-hydrogen bonds connecting the cations to the anionic framework formula wt. 279.15 and between them space group p2 12 12, cell parameters 5.003(1)A; b=8.090(1) A, H(5)--0( 1) 1.90( 3) ~=27.65( 1) A H(5)--N(1) 0.78(3) 0(1)-H(5)-N( 1) 160( 3) N(1)--0(1) 2.655( 2) diffractometer CAD4 Nonius radiation, monochr.Ag-Ka, graphite scan mode o scan H(6)-C(3) 0.94(3) data collection limits 2"<0<30"; O<h<7; O<k<12; H(6)--0(2) 2.48( 3) O(2)-H (6)- C( 3) 134( 2) 0<1<41 H(6)--0(3) 3.00( 3) 0(3)-H(6)-C( 3) 117(2) number of reflections total =3872, independent =2372, C(3)--0(2) 3.212( 3) with 12341) C(3)--0(3) 3.543( 3) number of variables, R, RM. 203, 0.035, 0.038 p in final AF synthesis/e A-3 pmax=0.55; pmin= -0.39 W7)-C(5) 0.87(4) Z 4 H(7)--0(7) 2.53(4) C(S)-H(7)-0( 7) 156( 3) D,/g cmp3 1.656 C(5)--0(7) 3.3 52 (4) F(000) 576 T/K 293 H( 10)-0(4) 2.08(3) crystal size/mm 0.19 x 0.19 x 0.32 H( 10)-N(2) 0.80(3) O(4)-H( 10)-N(2) 154(3) p/cm-' 1.586 N(2)--0(4) 2.833( 3) H(9)--0(2) 1.94(3) H(9)-"2) 0.92( 3) 0(2)-H(9) -NI 2) 166(2) Tableo2 Positional parameters, B,,/A2 for non-hydrogen atoms" and N(2)--0(2) 2.847( 2) Bi,,/A2 for hydrogen atoms refined isotropically (estimated standard deviations in parentheses) hydrogen bonds in the acetophosphonate layer ensuring the anionic aggregation atom Y 4' Z B/A2 H(l)-C(1) 0.91( 3) 0.06391 (9) -0.01 137( 6) 0.42399( 2) 1.791( 6) H(1)--0(4) 2.42( 3) C(1)-H( 1)-O(3) 152(2)0.1110(3) 0.1705(2) 0.4 1467 (5) 2.43(2) C( 1)--0(4) 3.255( 3) -0.1994( 3) -0.0767(2) 0.40645 (6) 2.52(2) 0.2868( 3) -0.1182( 2) 0.39905( 6) 2.64( 3) H(2)--C(1) 0.93( 3) 0.1105( 4) -0.0510(2) 0.48763(8) 2.20( 3) H(2)--0(4) 2.57( 3) C( l)-H(2)-0(4) 152( 2) -0.0886( 4) 0.0318( 2) 0.5201 5( 7) 2.05(3) C(1 )--0(4) 3.429( 3) -0.2917( 3) -0.03 59 (2) 0.53384( 6) 3.21 (3) -0.0218(3) 0.1826( 2) 0.53257(6) 2.87( 3) H(4)--0(2) 2.21 (3) 0.275 (7) -0.010(4) 0.495( 1) 2.1 (6)" H(4)--0(3) 0.40(3) 0(2)-H(4)-0( 3) 160(5)0.098 (6) -0.166( 3) 0.491 3( 9) 1.3(6)* 0(2)--0(3) 2.600( 2) 0.895(7) 0.228 (4) 0.554( 1) 2.8(8)* 0.361 (7) 0.893 (4) 0.404( 1) 3.5( 8)* H(3)-0(1) 1.86(3)0.5960(5) 0.2088 (3) 0.33472(8) 2.80( 4) H(3)--0(5) 0.80(3) O(1)-H(3)-0( 5) 159( 3) 0.3967(5) 0.2653( 3) 0.30624(8) 3.01(4) 0(1)--0(5) 2.630( 2) 0.3285(5) 0.4331(4) 0.30597( 9) 3.24(4) 0.4658( 5) 0.5395(3) 0.33451(8) 2.96( 4) 0.6795(4) 0.479S( 3) 0.36355( 7) 2.29(3) 0.7327(4) 0.3159( 2) 0.36247( 7) 2.49 (3) cohesion through a multiple hydrogen-bond network.Several 0.8240( 4) 0.5753( 3) 0.39182( 7) 2.97(3) types of hydrogen bonds occur in the anion aggregation 0.2481(5) 0.1472(4) 0.27716( 9) 4.59( 5) 0.3072( 6) O.OOlO( 3) 0.28007(8) 6.27(6) (Table 3).Two reladively short O-H---0 contacts [H---O= 0.0777(5) 0.2010( 4) 0.2502( 1 ) 8.38(7) 1.86(3), 2.21(3) A] a,nd a long C(l)-H(2)---0(4) 0.8 15( 7) 0.269( 4) 0.382( 1) 2.2(7)* [H(2)---0(4)=2.57(3) A] ensure the cohesion between the 0.643( 6) 0.096( 3) 0.3380( 9) 1.3 (6)* anions, building an infinite double chain in the a direction. 0.220(8) 0.478(5) 0.285(1) 3.9(9)* These chains are assembled in the b direction throuFh 0.418(6) 0.656(4) 0.33S( 1) 1.8(6)* C(1)-H( 1)---0(4) bonds [H( 1)---0(4)=2.42(3) A].0.808(6) 0.688(3) 0.391 3( 9) 1.2(6)* 0.920( 7) 0.533(4) 0.412(1) 3.3(8)* Taking into account the main works on the structural evidence of C-H---0 bonds in molecular ~rystals,~'-~' we assume a B,,=4/3 XiCj pij a, aj.that the existence of such interactions in this crystal is due to the observed hydrogen-bond geometry (Table 3). Furthermore, in this case the polar nature of the anion Enraf-Nonius SDP program32 operating on a micro-Vax I1 increases the possibility of multiple hydrogen bonds between computer was applied for all the calculations. The structure the anion and its neighbours. Each cation is anchored onto was drawn using the Molview program.33 the acetophosphonate layer through three N---H ---0 bonds and the three-centre hydrogen bond C(3) -H(3)---0(2), Results O(3). The twisting angle between the planes of the NO, group and the pyridinium ring is 3.6" due to the C(5)-H(7)---0(7) The 2-amino-5-nitropyridinium acetophosphonate contact interconnecting the cations.The interatomic distances, (2A5NPAP) crystal structure is built with 2A5NP+ cations bond angles and anisotropic thermal parameters observed in and acetophosphonate anions interconnected through a three- the cation and anion structures have been deposited with the dimensional hydrogen-bonding network (Fig. 1). Half-Cambridge Crystallographic Data Centre. The second-har- herringbone motifs of cations organized in a double layer monk generation signal29 from a powder sample of 2A5NPAP are sandwiched between acetophosphonate aggregates. The at 530 nm is weak: 2.5 x urea (Nd3+: YAG laser, 1.06 pm). anionic layer parallel to the (ab) plane (Fig.2) gains its This low efficiency should be interpreted through the evalu- J. MATER. CHEM., 1994, VOL. 4 Fig. 1 Projection of 2A5NPAP crystal structure along the a axis. Hydrogen bonds are indicated in dotted lines. Fig.2 Layer of acetophosphonate anions parallel to the (ab)plane. The anionic aggregation is based on 0-H---0 and C-H---0 bonds. The P0,C tetrahedra are hatched. ation of tensorial coefficients and the density of chromophores in the crystal structure: (a) the cations probably do not have the optimal orientation required in the point group 222. The assumption of a simplified one-dimensional description of the molecular non-linear tensor6 with by,, along the molecular axis C(4)-C(7) cannot be applied. The optimal values of angular parameters in this model are a=54.74", $=45".In the case of 2A5NPAP we have calculated a= [b,C(4)C(7)]= 50.7", $=42". i,$ is the angle between the c axis and the projection of the C(4)C(7) vector on the (ac) plane. Although these angles are close to the optimal values, the SHG signal is low, implying that the one-dimensional model is not appro- priate for explaining the weak non-linear optical efficiency of this crystal. The assumption of a two-dimensional description of the molecular tensor would be more appropriate in account- ing for the direction of the ground-state dipole moment which is not along C(4)C(7) in the case of the 2-amino-5-nitropyridi- nium cation and the local-field factors in this ionic structure which can considerably modify the contribution of the Pijk coefficients to the macroscopic susceptibilities.(b) Another important parameter is the density of chromophores (or oscillators) per unit volume of matter. The ratio z= Zl$A5NP+ indicates the proportion of the cell yolume occupied by the chromophores. < T/2A5NP+ > =146.6 A3 is an average volume defined in a previous Various values of z observed in molecular and ionic crystals containing 2A5NP or 2A5NPf entities are compared in Table4. This ratio is a parameter which can explain the enhancement of the macroscopic susceptibilities x( 2) when the required optimal orientation of chromophores in the structure is already reached or the situation is close to optimal. In the structures of 2A5NPC1 and 2A5NPDP the cation axis 0(2)N-C---C-NH2 is tilted at 31" towards the anionic network (Cl-),, or (H,PO,-),,.The non-optimal orientation of chromophores is balanced in 2A5NPCl by the high value of z, explaining why the SHG response is higher than that observed in 2A5NPDP. Conclusion If we wish to design new non-linear optical crystals including the 2-amino-5-nitropyridinium cation as the chromophore, two conditions are presently required: (a) a pH specification allowing the formation of cation and ensuring its stability in the crystal structure, (b)the formation of a two-dimensional anionic layer that is cap%ble of removing the cations to distances dNHzPNHz>4.702 A (Table 5), thus favouring the organization of herringbone assemblies that are inconsistent with the introduction of local inversion centres in the structure.The formation of an anionic layer can be realized from anions Table 4 Occupancy ratio of non-linear optical cations (oscillators) in molecular and ionic structures containing the same geometrical entity, 2A5NP or 2A5NP' crystal 7 (Yo) ref. COANP 43.9 10,11 AANP NPPA 42.2 42.4 8,9 15,16 MBANP 47.6 12-14 2A5N PA P 52.3 this work 2A5NPDP 64.7 22 2ASNPBr 75.7 38 2A5NPC1 79.5 38 1854 J. MATER. CHEM.. 1994, VOL. 4 Table 5 Main intercationic distances in relation with their centric or 5 J. L. Oudar, J. Chem. Phys., 1977,67,446. acentric packing in structures designed with 2A5NP+ non-linear optical cations [t is a distance corresponding to a cell parameter translation] 6 7 8 J.Zyss and J. L. Oudar, Phys. Rec. A, 1982,26,2028. J. Zyss, J. F. Nicoud and M. Coquillay, J. Chem. Phys., 1984, 81,4160. J. F. Nicoud, Mol. Cryst. Liq. Cryst., 1988, 156,257. anions dNn,-w,IA dNH2-NH21A ref. 9 S. Tomaru, T. Kurihava, H. Suzuki, N. Ooba, T. Kaino and 3.454 25 10 S. Matsumoto, Appl. Phys. Lett., 1991,58, 2583. P. Gunter, C. Bosshard, K. Sutter, H. Arend, G. Chapuis, R. J. Twieg and D. Dobrowolski, Appl. Phys. Letr., 1987,50,486. 4.029 6.390 38 11 C. Bosshard, K. Sutter, P. Gunter and G. Chapuis, J. Opt. Soc. Am. B, 1989,6,721. 3.896 5.189(t) 26 12 T. Kondo, N. Ogasawara, R. Ito, K. Ishida, T. Tanase, T. Murata and M. Hidai, Acta. Crystallogr.Sect. C, 1988,44. 102. 4.702 4.8 13(t) 38 13 R. T. Bailey, F. R. Cruickshank, S. M. G. Gurthrie, B. J. McArdle, H. Morrison, D. Pugh, E. A. Shepherd, J. N. Sherwood, 4.807 4.949 (t) 38 C. S. Yoon, R. Kashyap, B. K. Nayar and K. I. White, Opt. Commun., 1988,65,229. 6.717 5.675(t) 22 14 T. Kondo, R. Morita, N. Ogasawara, S. Umegaki and R. Ito, Jpn. J. Appl. Phys., 1989,28, 1622. 6.941 5.814(t) 25 15 T. Uemiya, N. Uenishi, Y. Shimizu, T. Yoneyama and K. Nakatsu, Mol. Cryst. Liq. Cryst., 1990, 182A. 51. 16 K. Sutter, G. Knopfle, N. Saupper, J. Hulliger, P. Gunter and 5.05(t) this work 7.07 17 W. Peter, J. Opt. SOC.Am. B, 1991, 8, 1483. T. Sugimaya, T. Shigemoto, H. Komatsu, Y. Sakagushi and 8.248(t) 7.611(t) 27 18 T.Ukachi, Mol. Cryst. Liq. Cryst., 1993,224,45. S. Di Bella, M. A. Ratner and T. J. Marks, J. .4m. Chem. SOC., 1992,114,5842. 19 R. Masse, M. Bagieu-Beucher, J. Pecaut, J. P. Levy and J. Zyss, that can aggregate through two short hydrogen bonds as in the phosphate salt. Three hydrogen bonds occur in the anion aggregation process of 2-amino-5-nitropyridinium L-tartrate. In the acetophosphonate salt we were waiting for the forma- tion of two short O-H---0 bonds, as was effectively observed; however, two further C-H ---0 bonds appear, 20 21 22 23 Nonlinear Opt, 1993,5413. C. B. Aekeroy, P. B. Hitchcock, B. D. Moyle and K. R. Seddon, J. Chem. Soc., Chem. Commun., 1989,23,1856. R. Masse and A. Durif, 2.Kristallogr., 1990, 190. 19. R. Masse and J. Zyss, Mol.Eng., 1991, 1, 141. Z. Kotler, R. Hierle, D. Josse, J. Zyss and R. Masse, J. Opt. Soc. Am. B, 1992,9,534. building the anionic layer along the u and 6 directions. The structures of 2-amino-5-nitropyridinium chloride and bromide (P2,) reveal that anionic aggregation in layers without the occurence of hydrogen bonds is also possible. Until now our engineering has been based on using numerous crystal struc- ture investigations and chemical observations to predict a 24 25 26 27 M. Bagieu-Beucher, R. Masse and D. Tranqui, 2. Anorg. Allg. Chem., 1991,606,59. J. Pecaut, Y. Lefur and R. Masse, Acta Crystallogr., Sect. B, 1993, 49, 535. J. Pecaut and R. Masse, Acta Crystallogr., Sect. B,1993,49,277. J. Zyss, R. Masse, M. Bagieu-Beucher and J. P. Levy, Adv. Muter., 1993,5, 120; 0.Watanabe, T.Noritake, Y. Hirose, A. Okada and non-centrosymmetric framework in the acetophosphonate salt. At this stage, the use of molecular simulation programs could be very fruitful for designing new acentric structures containing the 2-amino-5-nitropyridinium chromophores and increasing this large family of stable non-linear optical crystals. 28 29 30 T. Kurauchi, J. Muter. Chem., 1993,3, 1053. C. B. Aekeroy and P. B. Hitchcock, J. Muter. Chem., 1993,3,1129. S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968,39, 3798. P. Main, L. Lessinger, M. M. Woolfson, G. Germain and J. P. Declercq, MULTAN 77, User guide, University of York, England, and Louvain La Neuve, Belgium, 1977. We are very grateful to Dr. Rolland Hierle for SHG powder tests performed at the Departement d’klectronique quantique et moleculaire, CNET, B.P. 107, 92225 Bagneux Cedex, France. 31 32 33 R. Steward, E. R. Davidson and W. T. Simpson, International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, Structure Determination Package, Version RSX 1lM, 1977, Enraf-Nonius, Delft. J. M. Cense, Tetrahedron Comput. Methodol., 1989,2,65. 1974, VO~.IV, 2-2~. 34 0.Kennard and R. Taylor, J. Am. Chem. SOC.,1982,104,5063. References 35 36 G. R. Desiraju, Acc. Chem. Res., 1991,24, 290. T. Steiner and W. Saenger, J. Am. Chem. SOC.,1993,115,4540. 1 D. J. Williams, Angew. Chem., Int. Ed. Engl., 1984,23, 690. 2 B. F. Levine and C. G. Bethea, Appl. Phys. Lett., 1974,24,445. 3 B. F. Levine and C. G. Bethea, J. Chem. Phys., 1975,63,2666. 4 J. L. Oudar and H. Le Person, Opt. Commun., 1975,15,258; 1976, 37 38 T. Steiner and W. Saenger, Acta Crystallogr., Sect. B, 1994, 50, 348. J. Pecaut, J. P. Levy and R. Masse, J. Muter. Chom., 1993,3, 999. 18,410. Paper 4/04052J; Received 4th July, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401851

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(12), 1855-1860 Relaxation Behaviour of NLO Chromophores Grafted in Hybrid Sol-Gel Matrices 6. Lebeau,aJ. Maquet,” C. Sanchez,*a E. Toussaere,b R. Hierleband J. Zyss a Laboratoire de Chimie de la Matiere Condensee, URA CNRS7466, Uniwersite Pierre et Marie Curie, 4 place Jussieu 75252 Paris Cedex 05, France Centre National d’Etudes des Telecommunications, France TELECOM, 796 Avenue Henri Rawera, BP 707, 92 225 Bagneux Cedex, France Poled hybrid siloxane-oxide coatings synthesized through hydrolysis and condensation from tetramethoxysilane (TMOS) and N-[3-triethoxysilylpropyl]-2,4-dinitrophenylamine (TSDP) precursors have been characterized via second-harmonic generation (SHG), differential scanning calorimetry (DSC) and 29Simagic-angle spinning nuclear magnetic resonance (MAS NMR) measurements.Depending on chemical composition the SHG values range between 2.5 and 10 pm V-’. The matrix rigidity seems to have a strong influence on the relaxation behaviour of the optically non-linear chromophores grafted in these hybrid networks. The glass-transition temperatures of the hybrid coatings increase with the TMOS content and the thermal curing time. These modifications can be explained by an increase in the condensation and degree of crosslinking of siloxane and silica species. The sol -gel process offers new approaches to the synthesis of hybrid materials in the field of optics.’-3 The chemistry involved in the sol-gel process is based on hydrolysis and condensation of metal alkoxides, leading to the formation of metal-oxo based macromolecular The sol-gel process presents two main advantages.First, rheological properties of sols allow the easy deposition of coatings onto glass, semiconductors, ceramics or polymeric substrate^.^ The second advantage results from the various characteristics of sol-gel process (metallo-organic precursors, organic solvents, low processing temperatures) that allow the introduction of ‘fragile’ organic molecules into an inorganic network.’-12 Inorganic and organic components can be mixed in virtually any ratio to obtain the targeted properties, making these hybrid nanocomposites extremely versatile in their composi- tion, processing, and optical and mechanical properties. 11,12 As shown in the pioneering work performed by Avnir et organic dyes can be incorporated into an oxide gel matrix without any risk of thermal damage.Extensive work on dyes embedded in silica, aluminosilicates or transition-metal oxide based matrices made by sol-gel procedures has been per- formed over the past ten years.8,10312-21 Th ese dyes can play an important role in applications such as tunable lasers, luminescent solar concentrators or materials for non-linear optics12 and related devices. Most of the sol-gel optics research devoted to NLO materials was first related to third-order processes which are compatible with the isotropy of amorph- ous sol-gel matrices.’.2 Organic molecules inside amorphous sol-gel matrices are in general randomly oriented, thus ruling out the emission of the second harmonic.As the second-order non-linearities are achieved only in a non-centrosymmetric environment, we first demonstrated that orientation of the organic chromophores could be performed in hybrid sol-gel by using electrical field induced second harmonic (EFISH) or Corona electrical field poling techniques. Organic molecules such N-[3-triethoxysilylpropyl]-2,4-dinitrophe-nylamine (TSDP) were chemically bonded to the oxide back- bone of gels. The chemical bonding of the dye to the sol-gel matrix allowed the dye concentration to be increased without any crystallization o~curring.~~-~~ The sol-gel matrices were synthesized by copolymerization of silicon alkoxysilanes [TSDP and SiHCH,(OEt),] and zirconium propoxide pre- cursors.2J The sols were deposited as transparent coatings and exhibited (after corona poling) an SHG response of 1.6 pm V-’.22-24 Even though in this first generation of sol-gel matrices relaxation of the organic chromophores occurred over several hours, these results indicated the feasibility of using poling techniques with hybrid inorganic sol-gel matrices that are more ionic than classical polymers.Consequently a wealth of opportunity for the synthesis of optical sol-gel devices with efficient second-harmonic properties was exposed. Over the past two years there has been increasing interest in second-order NLO materials synthesized cia sol-gel chemis-try.26-33 The optimization of the second-order NLO rcsponse of hybrid sol-gel matrices with grafted chromophores is currently under investigation by several research groups.Several strategies are used to improve the NLO response of the hybrid coatings. (i) The intrinsic NLO response of the dye can be increase by using chromophores such as N-(4-nitropheny1)-~-prolinol (NPP) or ‘disperse red one’ (DR1) derivative^^'-^^ which exhibit higher non-linearities than nitro- aniline derivatives. (ii) The chromophore relaxation can be controlled by increasing the matrix rigidity. This point is, without doubt, the most important to be solved in order to be able to make efficient NLO devices. The modification of the binary composition (siloxane-crosslinker), the nature of the M(OR)4 crosslinking alkoxide [SiR,(OR),-,/M (OR),: R’=any NLO chromophore; M=Zr, Si, Ti.,.], and the pro- cessing of these hybrid materials in the presence of pdymers with well known mechanical properties such as methyl metha- crylates or polyimides, are the most commonly used st1 ategies to minimize dye rela~ation.~~-~’ This paper describes the optical, chemical and thermochem- ical characterizations of hybrid sol-gel coatings with second- order NLO properties of a simple binary (siloxane-silica) hybrid sol-gel system in which the organic chromophores (TSDP) are incorporated inside hybrid coatings through hydrolysis and co-condensation between TSDP and Si( OMe), (TMOS).Recently the effect of the processing parameters on the NLO behaviour of a similar system [TSDP/Si(OEt),] were reported.26 29Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the initial stages of poljmeriz- ation.26 It was postulated that after thin-film deposition no further cross-linking could occur.26 We expected of course that the degree of condensation and crosslinking observed in the sol state could be very different from those of thc.dried coating which is the NLO material. The present work aims to shed some light on the relationship between the chemistry, thermal treatment and NLO properties of these hybrids by coupling in situ temperature-dependent NLO measurements, DSC calorimetry measurements and 29Si MAS NMR per- formed in the solid state on the different hybrid NMR liquid-state experiments were also performed on the sols before film deposition.Experimental Synthesis of Hybrid NLO Materials TSDP was used as the siloxane network precursor carrying the NLO chromophore (Fig. 1) while TMOS was used as a crosslinking reagent to increase the network rigidity. Sol-gel coatings with different TSDP :TMOS molar ratios (1, 0.8, 0.6, 0.4, 0.2) were prepared as follows. The precursors, TSDP and TMOS, were mixed in THF and co-hydrolysed with acidic water (HC1; pH 1). The H20 :Si molar ratio was 2: 1. The solution was stirred for 30 min and the resulting sols were aged for several hours. From these sols, hydrophobic transparent films several micrometres thick, were produced without cracks or failure. Two procedures were used to process the coatings.They were prepared on ordinary soda-lime glass sheets previously cleaned and dried by simple deposition (for NMR and DSC experiments) or by spin-coating (for NLO, DSC and some NMR experiments). In the first process an appropriate amount of solution was poured onto the glass sheet and allowed to gel before being dried at room tempera- ture. Thin films were also spin-coated onto clean glass sub- strates. The spinning rate was 3000 rpm and the deposition took 30 s. The resulting hybrid xerogels were then charac- terized. In this paper, samples are labelled Tx-Qy, where T stands for TSDP, Q for the silicon added from TMOS. x and y are the molar percentage (x+y=lOO) of the different precursors. These hybrids can be described as block copoly- mers" made from siloxane and silica units which can be labelled by (T -T),-(Q -Q)bwhere: PP PPT-T = -gi-O-Si-0-and Q-Q = -Si-O-Si-0-Rh bb Optical Characterization A standard Corona34 poling technique was used to orient the chromophores in the hybrid sol-gel matrices.24 The samples were placed on a temperature-regulated heating substrate 2 cm from a tungsten needle and a high voltage was applied between the needle and the plate (10 kV).In a typical experi- ment a poling temperature of 120°C and a poling time (at this temperature) of 30 min were used. The current reached typical values in the range of several PA. The optical set-up was modified to measure in situ the time and temperature dependence of the second-harmonic response.Second-harmonic measurements were performed by the Maker fringe method?' A Y-cut quartz crystal was used as the reference, allowing for calibration of the measurements. 0.46 pm V-' was taken as a reference value for the optical non-linear coefficient of the quartz. The measurements were Fig. 1 Structure of the N-[3-triethoxysilylpropyl]-2,4-dinitrophenyl-amine (TSDP) NLO chromophore J. M14TER.CHEhI., 1994, VOL. 4 performed by using an irradiating pulsed Nd3' :YAG laser beam operating at 1.34 pm (10 ns per pulse, 10 mJ per pulse, 10 Hz) with both fundamental and harmonic wavelengths removed from the resonances of the non-linear chromophores. The thicknesses of the spin-coated films meavured by trans- mission spectroscopy ranged from 2 to 3 pm.The refractive indices were measured by spectroscopic ellipsometry (using the Sellmeier method36) and by a simple method based on the fringe pattern of the transmission spectrum.37 The mean refractive index for the hybrid coatings obtained from hydrolysis and co-condensation of TSDP and TMOS was 1.62. Chemical Characterization Elemental analysis (Si, N, C, H) was performed by the Service d'Analyse du CNRS on each sample in order to check if the initial chromophore :silicon ratio was conserved in each of the hybrid xerogels after drying and curing. '9Si liquid-state NMR data were recorded on a Bruker AM 250 spectrometer working at 49.69 MHz. MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer.The frequency was adjusted to 79.5 MHz for 29Si nuclei. The pulse width and relaxation delays were 2.5 ps and 60 s for "Si MAS NMR. The solid samples were spun at 5 kHz. Tetramethylsilane (TMS) was used as the reference. For silicon NMR data T,, (m=0-3) and Q, (n=0-4) notations have their usual meaning. In T, notation38 T refers to the 0x0 trifunctional R-SiO, central unit and m represents the number of Si-0"-Si bridging oxygens attached to the central unit. The Q, notation rep- resents the silicon-oxygen tetrahedron; Q refers to the 0x0 quadrifunctional central Si04 unit and n indicates the number of other silicate structures attached to the central unit. DSC experiments were performed on a Perkin-Elmer 7 series instrument, using ca.15 mg of each xerogel. The experi- ments were carried out under cyclic conditions. First the temperature was increased from -20 to 145 -C to clear the sample memory. Then the sample temperature was set to -20 "C. Heating and cooling rates for each run were fixed at 10°C min-'. The data were collected during the third run (the second temperature increase) and were treated using the Perkin-Elmer thermal analysis software. The determination of the glass-transition temperatures, q,was performed using an integration area as large as 40°C for each measurement. The accuracy of the Tg values is f2"C depending on the capability to adjust the tangents of the curve\' steps. Results and Discussion NLO Measurements Second-harmonic measurements (d33 valuesI performed on poled coatings for several TSDP/TMOS compositions are listed in Table 1.The d,, values are higher than those for TSDP-SiHCH,(OEt,)-Zr(OPr"), and close to those reported for the TSDP-TEOS2s hybrid systems. The d,, values decreases with the initial TSDP-TMOS ratio, except for the sample T60-Q40. This singularity is assigned to differences in the sequencing of the (T-T), and (Q-Q),,units Table 1 Second-harmonic measurements values) measured on poled coatings for several TSDP-TMOS compositions TSDP-TMOS d,,,'pm V-' 1oo:o 10 80:20 5.63 60 :40 2.5 40:60 3 J. MATER. CHEM., 1994, VOL. 4 in this hybrid coating. Optimisation of the curing time and poling conditions should lead to a better sequencing. The room-temperature decay of the SHG intensity was studied as the poling voltage was turn off, for pure TSDP and for T80-Q20 binary systems.In both cases the decay occurred over several months. After 250 days the measured d,, values decreased, respectively, to 18% (T100) and 30% (T80-Q20) of the initial d3, values. This shows that the crosslinking of the siloxane network by silica units improves but does not inhibit the chromophore relaxation. Fitting the time depen- dence of the d,, value to an exponential law [d3,(t)=d3, (0)exp(-t/z)] leads to a relaxation time z of ca. 3100 h for the T80-Q20 system. This shows that hybrid organic-inorganic systems (TSDP-TMOS) have a much longer NLO chromophore relaxation time than grafted organic polymers with a similar glass-transition temperature (q=30-70 "C, uide infra) such as butoxymethacrylate (z=500 h).39 The in situ temperature dependence of the SHG values were measured as follows. The hybrid coatings were first poled at 120°C for 30min, then the samples were cooled and the electric field was turn off when room temperature was reached.The d,, values were then measured and monitored us. tempera-ture. The heating rate was 10 'C min-'. Fig. 2 shows the temperature dependence of the SHG signal for Tx-Qy hybrid coatings. The relaxation of the NLO chromophores is favoured by short- and long-range order polymeric chain motions. These degrees of freedom are gener- ally allowed by increasing temperature. Thus chromophore relaxation is characterized by the strong decay observed in the SHG response as temperature is increased (Fig.2). The temperature at which the SHG decreases increases with the amount of crosslinking reagent Q. For a given composition a sample poled and cured for longer times (several hours) exhibits a displacement of this transition towards higher temperatures, e.g. this transition occurs at 20-35°C for a freshly deposited and poled TlOO coating while it reaches 60-65 -C for a TlOO sample poled and cured for 2 h. DSC Experiments DSC studies were performed on the different Tx-Qy hybrid films. Effect of TMOS Content Fig. 3 shows DSC traces of air-dried hybrid coatings with different TSDP :TMOS molar ratios. A relaxation phenom- enon occurs between 20 and 100°C and corresponds to the glass transition of the gel.A plot of Tg us. TMOS molar concentration is shown in Fig. 4.Tgis shifted to higher values n20 r I 20 40 60 80 T/"C Fig.2 In situ evolution of the SHG response uersus temperature for coatings with different Tx :Qy compositions: (a) 40 :60, (b) 60 :40, (c) SO : 20 and (d) 100:0 0.0 1 I I I I I ! 1 I1 0 20 40 60 80 100 T/"C Fig. 3 DSC traces of air dryed hybrid coatings with different TSDP: TMOS molar ratio: (a) 100: 0, (b)90: 10, (c) 80: 20, (ti) 60: 40. (e)40 :60 and (.f)20 :80. 'O0I80 *,t Fig. 4 Glass-transition temperature (T,)as a function of the TMOS molar concentration with increasing TMOS concentration. This is due to the increased molecular weight and restricted segment niobility of Tx-Qy hybrid systems as a consequence of the cross-linking reaction.This point will be discussed in the NMR section. The inflection point of the curves in Fig. 2 yields the temperature of depolarization (the temperature for which the SHG decreases) The values of the depolarization temperature and the Tgmeasured by DSC for several coatings with different Tx-Qy compositions are listed in Table 2. For a given Tx-QjI composition both temperature values are in good agreement. Effect of Thermal Curing DSC studies were also performed in order to characterize the evolution of the mechanical properties the Tx-Qy film as a function of temperature. For an air-dried TlOO sample the Tg is of ca. 18-25'C. After thermal curing, without and with poling, increases up to 32 and 59"C, respectively.Strong variations in 7; were also observed for the Tx-Qy xerogel coatings depending on their thermal history. Systematic DSC experiments were car- ried out to study the influence of the thermal treatment on Tg. Table 2 Depolarization temperature and T, values of air-dried hybrid coatings with different TSDP-TMOS molar ratios TSDP :TMOS GI, c depolarisation temperature/-C 1oo:o 25 27 80 :20 38.5 38 60 :40 59 56 40 :60 61 60 J. MATER. CHEM., 1994. VOL. 4 The evolution of the glass transition for T80-Q20, T60-Q40 for TlOO and QlOO. This indicates a higher degree of conden- and T40-Q60 samples has been carefully studied after different sation in binary systems than in pure sol-gel polymers.This successive curings. Before each measurement the sample was phenomenon is particularly marked in the 29Si MAS NMR cured for 30min at 120°C. The glass transition is clearly spectrum of a T40-Q60 film which only shows T2(-58 ppm), shifted to higher temperatures with increasing curing time. T3 (-65 ppm), Q3 (-100 ppm) and Q4 (-108 ppm) reson- After curing at 120"C,Tg for each sample reached a maximum. ances. Table 3 reports the evolution of the concentration of T The value of this maximum increased with the TMOS content, and Q units us. sample composition. From the relative inten- i.e. values of 45, 73 and 84°C are, respectively, reached with sity of each T and Q resonance, the concentration of each T80-Q20, T60-Q40 and T40-Q60 samples.Moreover, the species can be measured. Fig. 6 represents a plot of the curing time at which the maximum is reached decreases with evolution of T-unit concentration with the TMOS content the T: Q ratio. This time is ca. 75 h for a T80-Q20 coating and shows that the number of T3 species (fully condensed) and decreases to 6 h and 3 h, respectively, for T60-Q40 and increases with the addition of TMOS. The mean degree of T40-Q60. The evolution of the rigidity of these hybrid condensation, C, can be calculated for each set of T or Q matrices could be described by time-temperature transform-components by taking C(T)=Ci iti/3 and C(Q)=Gj jqj/4. ti ation models. However, in order to complete the relationship and qj are the concentration measured by NMR for Ti and the chemical composition Qj species (i andj the number of Si-0-Si bridging oxygen between the matrix rigidity (q),and the degree of condensation, the different hybrid Tx-Qy i=O to 3; j=O to 4).The degree of condensation for T and coatings were characterized by high-resolution solid-state Q units us. TMOS concentration are plotted in Fig. 7. For (MAS) NMR. the binary Tx-Qy systems the degree of condensation increases with the TMOS content. 29SiMAS NMR Fig. 5 shows 29Si NMR MAS spectra for air-dried hybrid coatings prepared with different TSDP :TMOS ratios from sols aged for several hours. The isotropic chemical shifts observed in 29Si spectra are dependent on the degree of h Their assignment has z.condensation of silicate stru~tures.~~-~~ 601-/been made in agreement with the literature. The 29Si MAS NMR spectrum of a TlOO coating shows three components due to T, (-51 ppm), T, (-57.4 ppm) and T, (-64.9 ppm) units.Those of a QlOO coating shows three components corresponding to Qz (-91.6 ppm), Q3 (-100.7 ppm) and Q4 (-108.7 ppm) units. The 29Si NMR spectra of the Binary T-Q hybrid samples show both T and Q components. In the (a12ollzlIzLTx-Qy binary samples, the relative intensities of the reson- 00 20 40 60 80 ances due to TI and Q2 units are lower than those observed TMOS content (%) Fig.6 Evolution of T units concentration with TMOS content. (a) Q TI, (b)Tz and (c) T3 I\ 1OOr -20 -60 -1 00 -1 40 6 0 20 TMOS40 content60(%) 100 Fig. 5 29Si NMR MAS spectra for air dryed coatings prepared with different TSDP :TMOS ratios: (a) 0: 100, (b) 20: 80, (c) 40 :60, (d) Fig.7 Degrees of condensation (C)for T and Q units wrstls TMOS 60:40, (e)80:20, (f) 90: 10, (g)1OO:O concentration Table 3 Evolution of the concentration of T and Q units measured by 29Si NMR for several TSDP-TMOS compositions 100:o 9.7 54.2 36.1 0 0 0 75.5 0 90: 10 0 20.3 63.2 1.4 8.2 6.9 91.9 83.3 80: 20 0 17.3 59.7 0.9 14.2 7.9 92.5 82.6 60 :40 0 9.1 52.6 0.4 23.5 14.4 95.1 84.1 40 : 60 0 5 46.5 0.7 30.5 17.3 96.8 83.6 20: 80 0 3.3 34 0.9 31.1 30.7 97.1 86.8 0:100 0 0 0 5.2 67.6 27.2 0 80.5 J. MATER. CHEM., 1994, VOL.4 The T40-Q60 hybrid network reached C(T) =97%, much higher than those of the pure TlOO hybrid xerogel [C(T)= 75%]. Obviously the addition of TMOS greatly increases the degree of condensation of the siloxane network. This pheno- menom is related to the increase of Tg observed with increasing TMOS content. Efect of Sol Ageing Time bcfore Coating Deposition When TSDP and TMOS precursors were co-hydrolysed in acidic conditions, the second one hydrolysed more rapidly than TSDP.26b,43 The hydrolysis rate of the latter is known to be slowed down by the steric hindrance provided by the propyldinitrophenylamino s~bstituent.~~,~~However, as soon as silanol groups are formed from the TMOS precursors, condensation occurs between these silanols and other TMOS or TSDP precursors. On the course of the reactions both precursors hydrolyse and condense, leading to different T and Q species, as evidenced by the 29Si NMR study performed in the sol state.As an example, for a T60-Q40 sol aged for 8 h the 29Si NMR spectra exhibit the characteristic resonances of TI (6= -49.5 ppm), cyclic T, (6= -51.1 ppm), linear T2 (6=-58.5 ppm), T, (6 =-67 ppm), Q2 (6=-92 ppm), Q, (6= -99.7 ppm) and Q4 (6= -109 ppm). In the aged T60-Q40 sol Toand Qounits are not present and T,, Q3 and Q4 resonances are weak. This indicates that a complete hydrolysis but a low extent of condensation reaction occurs in the sol state even after long ageing times. After coating deposition, as shown in Fig. 6 and Table 3, the structure of the xerogels (T60-Q40 is taken as an example) is mainly composed of T2, T,, 4, and Q4 silicon units. This difference between the sol state and xerogel state demonstrates that the main condensation and crosslinking reactions occur upon solvent removal. However, even if condensation reac- tions are mostly complete upon drying, the extent of reaction in the sol state (which depends on ageing time) has an influence on the final structure of the coating.29Si NMR data recorded from air-dried hybrid coatings (T60-Q40) deposited after short (sample A) or long (sample B) ageing times are gathered in Table 4. The extent of reaction is clearly different for both samples. Some T, units are present in sample A and the T, :T, and Q4 :Q3 ratios are smaller in this sample than in sample B.C(T) is lower in coatings deposited after a short ageing time (sample A) than for those deposited after a long ageing time (sample B), while their degrees of condensation C(Q) are quite close. For other Tx-Qy compositions the same tendency is observed. All coatings processed from freshly made sols are made with T-Q copolymers that have a lower degree of condensation than those present in coatings deposited from aged sols. For a set of coatings deposited from freshly made sols, Tg increases with the TMOS content. However, note that for a given Tx-Qy composition, the difference between Tgvalues measured for coatings A and B is quite small (Table 4) even if both samples exhibit a different C(T).This result raises several questions: Is Tg mainly gov- erned by the amount of Q species, their degree of condensation and their degree of crosslinking with T-species units? As 29Si MAS NMR do not provide enough resolution to estimate the ratio between linear and cyclic T species, the C(T) values of two samples can give numerically the same number but could represent a different distribution between linear and cyclic units. The crosslinking behaviour of these species with silica units is of course different and will lead to different thermo- mechanic behaviour. The size of the polymeric (T-T), and (Q-Q)b segments and their sequencing should be an important parameter that probably governs the mobility of the segments and consequently Tg and the relaxation properties of such hybrid materials. Obtaining hybrid copolymers made from (T-T),-(Q-Q)b segments with low a and b values should lead to behaviour closer to the glass state in terms of mechanical properties (ix.less chromophore rela~ation).~' "Si CP MAS NMR and NMR relaxation measurements are being carried out in order to shed some light on this point.Conclusion Compared to the tremendous amount of work and time devoted to polymeric NLO materials, NLO materials made by sol-gel processes are still in their infancy. As a consequence, the sol-gel materials described in this work have Tgvalues in the range 30-70"C, well below the state of the art obtained with pure organic polymeric materials based on p~lyimides~~ which are highly non-linear and stable for hundreds of hours at temperatures >100"C.However, the results obtained with the sol-gel materials can be improved by carrying out a multiple covalent bonding of azo dye derivatives which have a high non-linear response.46 Polyimide-inorganic sol-gel composites recently showed an excellent temporal stahility at 120OC31 Moreover the use of transition-metal 0x0-pcllymers (M=Zr, Ti, V, Fe) as crosslinking agents of higher func- tionality should modify the mechanical properties of these hybrid materials. The presence of transition-metal 0x0 species may also offer the possibility of modifying the refractive index of the coating, or creating materials that carry both optical and redox or magnetic properties.Sol-gel chemistry is versa-tile, but the quality and reproductibility of the resulting materials are strongly linked to the chemical and processing conditions of preparation. Therefore a fundamental under- standing of the relationship between chemistry and 21 given property such as NLO chromophore relaxation is crucial for the improvement of these systems. This study concerns the synthesis and characterization of hybrid siloxane-oxide compounds with second-order NLO properties. Depending on the chemical composition the SHG values ranged between 2.5 and 10 pm V-'. The thermal treatment of the hybrid coatings has a strong influence on the relaxation behaviour of the NLO chromophores grafted into these hybrid networks. These compounds are made uia hydrolysis and co-condensation of TSDP (T units) and 7'MOS (Q units) precursors.In such systems gelation most likely occurs through the crosslinking of linear or branched T siloxane polymers with Q silica species. These hybrid net works are made of (T-T), units co-condensed with (Q-Q)bspecies as evidenced by 29Si MAS NMR. Depending on a and h they can be described as copolymers between T and Q species (low a and b) or as nanocomposites made of silica domains Linked to siloxane polymers (higher a and b)." The TSDP :TMOS ratio, proton concentration, hydrolysis ratio, sequence of mixing the reagents and ageing time of the sol are the chemical parameters that should directly influence a and b values. For Table 4 29Si NMR data and T, values for two air-dried hybrid (T60-Q40) coatings deposited after short (sample A) or long (sample B) ageing times T60-Q40 8 (T2) 6 (T3) 8 (Q3) 6 (44) C(Ti ("/.I C(Q) T,/'C sample A sample B 23.1 9.1 34.4 52.6 27.8 23.9 14.7 14.4 86.6 95.1 83.6 84.1 41.5 59 1860 J.MATER. CHEM., 1994, VOL. 4 a given set of chemical parameters this study shows that Tg increases with TMOS content, as evidenced by DSC experi- ments. To a first approximation this behaviour can be related to the increase in the degree of condensation and crosslinking 9 10 11 12 H. Schmidt and B. Seiferling, Muter. Res. Soc. Sy'np., 1986,73, 10. B. Dunn and J. I. Zink, J. Mater. Chem., 1991, 1, 003. C. Sanchez and F. Ribot, Proc. 1st Eur. Worhshop on Hybrid Organic-Inorganic Materials, New J.Chem., 1994. 18, 1007. R. Reisfeld, SPIE Proc. Sol-Gel Optics, ed. J. D Mackenzie and of the polymers as shown by 29Si MAS NMR. However, the mechanical properties of hybrid siloxane-oxide materials and thus the relaxation behaviour of chromophores grafted into these matrices is strongly dependent on their thermal history. The physical behaviour of these organic-inorganic hybrid 13 14 15 D. R. Ulrich, 1990, 1328,29. J. C. Pouxviel, S. Parvaneh, E. T. Knobbe and B. Dunn, Solid State Ionics, 1989,32,646. D. Avnir, V. R. Kaufman and R. Reisfeld, J. Ann-Crjst. Solids, 1985,14, 395. C. Sanchez, SPlE Proc., Sol-Gel Optics, ed. J. D Mackenzie and T-Q copolymers could be described by time-temperature transformation (TTT) models, such as those proposed for thermoset^.^^ Chemical crosslinking is not complete at the gelation or even after room-temperature air drying as shown by 29Si NMR experiments. When the samples are aged and cured the chemical reactions continue closer towards com- 16 17 18 D.R. Ulrich, 1990, 1328,41. C. Guizard, J. C. Achdou, A. Larbot, L. Cot, G. Le Flem, C. Parent and C. Lurin, SPIE Proc., Sol Gel Optics, ed. J. D. Mackenzie and D. R. Ulrich, 1990, 1328,208. P. N. Prasad and B. A. Reinhardt, Chem. Muter. 1990,2,660. S. Dirk, F. Babonneau, C. Sanchez and J. Livage. J. Muter. Chem., 1992,2,239. pletion. Consequently, given sufficient time and temperature to allow the mobility of the species, a network-forming system continues to crosslink far after gelation.The increase in the density of crosslinks modifies the thermomechanical properties of the hybrid, as illustrated by the changes observed in Tp 19 20 21 B. Dunn, E. Knobbe, J. M. Mc. Kiernan, J. C. Pouxviel and J. I. Zink, Muter. Res Soc. Symp. Proc., 1988, 121, 331. J. C. Pouxviel, B. Dunn and J. I. Zink, J. Phys. Chem., 1989, 93,2134. J. M. Boulton, J. Thompson, H. H. Fox, I. Gorodisher, G. Teowee, P. D. Calvert and D. R. Uhlmann, Muter. Res. Soc. Sjwp., 1990, upon thermal curing. Consequently, care must be taken when comparing physical properties of hybrid organic compounds. Even if such hybrid coatings have been synthesized following the same route (i.e. same precursors, same chemical conditions) small differences in the ageing time of the sols and in the thermal treatment may lead to large modifications of their chemical structure and consequently different relaxation behaviour.Another processing parameter that should have a high importance is the electrical field used to poled the NLO chromophores. 22 23 24 25 26 180,987. G. Pucetti, 1.Ledoux, J. Zyss, P. Griesmar and <'.Sanchez, Polym. Prepr., 1991,32, 61. J. Zyss, G. Pucetti, J. Ledoux, P. Griesmar, J. Livage and C. Sanchez, Eur. Pat. 11 52, March 1991. (a)E. Toussaere, J. Zyss, P. Griesmar and C. Sachez, Nonlinear Opt., 1991, 1, 349; (b) C. Sanchez, P. Griesmar, E. Toussaere, G. Puccetti, 1. Ledoux and J. Zyss., Nonlineur Opt., 1992,4, 245. P. Griesmar, C. Sanchez, G. Pucetti, I. Ledoux and J. Zyss, Mol. Eng., 1991, I, 205. (a) J. Kim, J. L. Plawsky, R.LaPeruta and G M. Korenowski, Chem. Muter., 1992, 4, 249; (b) J. Kim, J. L. Plawsky, E. Van Wagenen and G. M. Korenowski, Chem. Muter., 1993,5.1118. Accelerated field-induced curing must likely occur in these hybrid TSDP-TMOS materials. The high electrical field pro- vided during poling must favour crosslinking and interpen- etration of both polymeric T and Q networks. As mentioned by Haruvy and Webber,27*4g this dramatic curing effect should 27 28 29 Y. Haruvy and S. E. Weber, Muter. Res. Soc. Symp. Proc., 1992, 271, 297. F. Chaput, D. Riehl, Y. Levy and J. P. Boilot, C'hem. Mater., 1993, 5, 589. Y. Zhang, P. N. Prasad and R. Burzynski, Chem. Muter., 1992. 4, 851. involve bond and chain segment orientations, ion migration and surface corona electrolysis.Consequently, this effect should depend on the siloxane :water ratio, the polymer length and the initial crosslinking and degree of polycondensation reached in the sol state. Thus, for a given temperature, voltage 30 31 32 Y. Nosaka, N. Tohriiwa, T. Kobayashi and N. Fuji, Chem. Mater., 1993,5,930. S. Marturunkakul, J. I. Chen, R. J. Jeng, S. Sengupta, J. Kumar and S. K. Tripathy, Chem. Mater., 1993,5, 743. K. Izawa, N. Okamoto and 0.Sugihara, Jpn. J. Appl. Phys., 1993, 32, 807. and substrate, the efficiency of the field-induced curing must 33 L. Kador, R. Fischer, D. Haarer, R. Kaieman, S. Briick, depend on the ageing time of the polymeric precursor solution and on the ageing time of the hybrid coatings before poling occurs. Further studies are underway to shed some light on this difficult phenomenon. 34 35 H.Schmidt and H. Durr, Adv. Muter., 1993,5,270. K. D. Singer, M. G. Kuzyk, W. R. Holland, J. E. Sohn, S. J. Lalama, R. B. Comizzolli, H. E. Katz and M. L. Schilling, Appl. Phys. Lett., 1988,53, 19. P. D. Maker, R. W. Terhune, N. Nisennoff and C. M. Savage, Phys. Rev. Lett., 1962, 8, 21. DRET is gratefully acknowledged for financial support. 36 J. A. Dobrowski, F. C. Ho and A. Waldorf. Appl. Opt.. 1983, 22, 3191. 37 J. C. Manifacier, J. Gasiot, J. P. Fillard, J. Php. E, 1976,9, 1002. References 38 R. H. Glaser, G. L. Wilkes and C. E. Bronnimann, J. Non-Cryst. Solids, 1989,113, 73. Sol-Gel Optics I, ed. J. D. Mackenzie and D. R. Ulrich, Proc. SPIE 1328, Washington, 1990. Sol-Gel Optics Il, ed.J. D. Mackenzie, Proc. SPIE 1758, Washington, 1992. Sol-gel Optics, Processing and Applications, ed. L. C. Klein, Kluwer, Boston, 1993. C. J. Brinker and G. Scherrer, Sol-Gel Science: the Physics and Chemistrjl of Sol-Gel Processing, Academic Press, San Diego, 1989. (a)J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18, 259; (b)C. Sanchez and J. Livage, New J. Chewi., 1990, 14, 513. 39 40 41 42 43 44 45 H. T. Man and H. N. Yoon, Adv. Muter., 1992.4, 159. E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A. R. Grimmer, J. Am. Chem. Soc., 1980,102,4889. G. Engelhardt, H. Jancke, E. Lippmaa and A. Samoson, J. Orgunomet. Chem., 1981,210,295. M. Magi, E. Lippmaa, A. Samoson, G. Engelhardt and A. R. Grimmer, J. Phys. Chem., 1984,88, 1518. B. Lebeau, personal communication. Silune Coupling Agents, Plenum Press, Edwin P. New York, 2nd edn., 1991. R. F. Shi, M. H. Wu, S. Yamada, Y. M. Cai and A. F. Garito, Appl. Phys. Lett., 1993, 63, 1173; J. Appl. Polym. SCL, 1983. 28, 2567. C. Sanchez, F. Ribot, S. Doeuff, in Organometallic Polymers with Special Properties, ed. R. M. Laine, Kluwer, Dordrecht, 1992, p. 267. The Sol-Gel Process in Sol-Gel Technolog!! for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, ed L. C. Klein, Noyes, 46 47 48 B. Lebeau, C. Guermeur and C. Sanchez, Muter. Res. Soc. Symp. Proc., 1994,346, 3 15. B. Enns and J. K. Gillham, J. Appl. Polym. Sci., 1983,28, 2567. Q; Hibben, E. Lu, Y. Haruvy and S. E. Webber, Chern. Muter., 1994,6, 76 1. 1988. D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem., 1984,88, 5956. Paper 4/02634I; Receiced 3rd May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401855

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 12). 1861-1 866 Synthesis, Physical Properties and X-Ray Crystal Structures of a Series of Nickel Complexes based on n-Alkylthio-substituted Ethylene-I,2=dithiolene Ligands A. Charlton," C.A.S. Hill," A.E. Underhill," K.M.A. Malik,b M.B. Hursthouseb A.I. Karaulovb and J. Meller" a Department of Chemistry, University of Wales, Bangor, Gwynedd, UK LL572UW Department of Chemistry, University of Wales, Cardiff, UK CFl3TB " Kemisk Institut, Odense Universetet, Campusvej 55, DK5230 Odense M, Denmark The synthesis and physical properties of a series of soluble, tetrakis(n-alky1thio)nickel bis(ethylene-l,2-dithiolene), rTC,-Ni(edt),] complexes (n=4-1 1) is described. The room-temperature electrical conductivities of these materials in the undoped state are reported and the X-ray crystal structures of the n =4 and 6 derivatives have been determined.In the search for compounds with enhanced electrical and optical properties for potential uses in molecular electronics and advanced telecommunications systems, organometallic species, particularly transition-metal complexes of substituted 1,2-dithiolene ligands appear to be emerging as a class of materials with considerable potential.' The highly delocalised metal/sulfur core gives rise to the novel physical properties observed in these materials, includ- ing low-energy near-infrared transitions associated with the neutral and monoanionic compounds,2 reversible redox behaviour3 and the high electrical conductivities of some derivative^.^ One of the major problems associated with these materials, particularly from the viewpoint of non-linear optical appli- cation~,~is their low solubility in organic solvents and suitable polymer hosts,6 making the fabrication of prototype devices very difficult.We have approached this problem in two ways: (1)the development of nickel dithiolene based polymers which are soluble in poly(methy1 methacrylate) (PMMA) at high loading levels' and (2) the synthesis of monomeric neutral nickel dithiolenes with extended alkyl chains as substituents to improve solubility in organic solvents and PMMA. As part of this latter programme we describe the synthesis, characterisation and structural properties of a series of soluble, monomeric nickel complexes based on alkylthio-substituted ethylene-172-dithiolene ligands. The general structure of the [TTC,-Ni(edt),] complexes synthesized is shown in Fig.1. Previous studies on this type of compound have been restricted to the tetramethyl (n= 1) derivative,8 and salts of complexes of the related capped ligand dddt (H,dddt =5,6-dihydro-l,4-dithiin-2J-dithiol) such as [Ni(dddt),], studied extensively because of their novel electrical properties.' The synthesis of compounds with long alkylthio chains to confer increased solubility is seen as an essential step in the development of metal dithiolenes for all optical device use. Results and Discussion The complex where n= 1 (Me) has been previously pre- pared by Fanghanel and Poleschner' by photolysis of the corresponding substituted 1,3-dithiole-2-one derivative.The resultant unstable dit hiete intermediate reacted with nickel tetracarbonyl to give the neutral metal dithiolene complexes in cu. 60% yield. In the present work a much easier and more generally applicable synthetic route has been developed. The synthesis of the [TTC,-Ni(edt),] complexes is outlined in Scheme 1. Starting from the 4,5-bis( benzoy1thio)-1, l-dithi- ole-2-thione (l),in situ generation of the air-sensitive dit hiolate dianion 2 was achieved by treatment with sodium methoxide in dry methanol, under argon. The dianion was then allowed to react with a suitable alkyl bromide, in a similar way to the method of Schultz et d.,"to form the bis(n-alky1thio)- substituted 1,3-dithiole-2-thione derivatives (3).Preparation of the metal complexes 4 was then achieved by treatment of 3 with sodium methoxide, to form the disodium salt of the ditholate dianion. To this was added tetrabutylammonium bromide, followed by a solution of nickel(I1) chloride hexahydrate. Oxidation of the product with s4;xscophSCOPh t s-<"Is-Na+SNa+ (ii)1 sdsf-" (3) S-R (iii) (4) R-S s' 's S-R Scheme 1 Reagents and conditions: (i) NaOMe/MeOH room tem-Fig. 1 General structure of the TTC,-Ni(edt), complexes prepared, perature (rt); (ii)RBr (R =C4H,-C,,H23), reflux, (iii)NaOMe/hIeOH, (n=4-11) reflux, NBu,Br then NiCl, * 6H20,followed by iodine/CH,CO(:H, rt J. MATER. CHEM., 1994, VOL.4 Table 1 Analytical and physical data for the [TTC,-Ni(edt),] complexes analysis (%)a FAB-MS~ I7 colour yield (YO) mpi 'C C H [M-HI-4 15 103 40.46 (40.62 j 6.03 (6.08) 590 5 15 97-98 44.12 (44.52) 6.23 (6.79) 646 6 dark green needles 20 71 47.21 (47.80) 6.53 (7.39) 702 7 27 82-83 50.39 (50.60) 8.35 (7.90) 758 8 31 67-68 52.82 (53.01) 7.84 (8.34) 814 9 30 82 54.89 (55.22) 8.56 (8.73) 8 70 10 olive green wax 25 71-72 56.88 (57.07 j 8.93 (9.07) 926 11 23 84-85 58.87 (58.62) 9.57 (9.35) 98 1 'Calculated values in parentheses. Recorded in negative mode; correct isotopic patterns corresponding to the elemental formulae. iodine yielded the neutral complex 4. In all cases, recrystallis- 4.01 ation yielded the pure compounds as either dark green crystalline solids, or olive green waxes (see Experimental).The analytical and some physical data for the complexes are presented in Table 1. The solution near-infrared spectra of all the derivatives were recorded in dichloromethane and exhibited an intense band at 1000 nm [ln(€/dm3 mol-I cm-')z 10.61 (see Fig. 2), corresponding to a n+n* transition characteristic of neutral and monoanionic nickel 1,2-dithiolene~.~ The electrochemical properties of the [TTC,-Ni(edt),] com-plexes were investigated using cyclic voltammetry. Previous studies concerning the solution redox behaviour of the tetra- methyl (n =1) derivative have been reported by Keller et d.," in which two reversible peaks were observed in the cyclic voltammogram (El = -0.0875 V, E2= t-0.775 V; CH2CI2, 0.0 0.6 1.2 ENTBAP).These one-electron processes were assigned to the [M(dithiolene),]'--+ [M(dithiolene),] -(E,) and Fig. 3 Cyclic voltammogram of the TTC,,-Nl(edt)2 derivative [M (dithiolene),] -+[M(dithiolene),]O (E2) redox couples. recorded in CH,Cl, ljersus SCE, 0.1 mol dm-3 NBu,PF,. 100 mV s-' The results obtained in the present study are in close agree- ment with these values (see Table2), but a third quasi-assigned to the formation of a cationic species, similar to the reversible peak not previously discussed (see Fig. 3) was also anionic [Ni(dddt),ld+ species observed in the voltammogram observed at more positive potentials (E3= + 1.11V). This is of [Ni(dddt),] (E, >0.80 V),', from which electrically con- ducting [Ni(dddt),],X, (X =C104-, BF,-) complexes have been prepared.13 The melting-point behaviour of the [TTC',-Ni(edt),] was examined, and Fig.4 shows a plot of the melting point us. the alkyl chain length attached to the nickel/sulfur core. It can be seen that the melting point decreases rapidly with increasing chain length from n= 1'' to n =6, presumably reflecting the greater motional freedom of the alkyl chains in the latter 0JLl,,, , , , , , , , , \, , , , ,+ 20030k600 700 800 900 1000 1100 1200 1300 Unm Fig. 2 Near-infrared spectrum of the TTC,,-Ni(edt), derivative recorded in dichloromethane (2 x lo-' mol dm-3j Table 2 Cyclic voltammetric data' for the [TTC,-Ni(edt),] com-plexes (4) too 4 -0.08 +0.77 + 1.12 6 -0.08 +0.76 + 1.11 50 1 I I 1 , I11 -0.11 +0.76 + 1.12 Measured L'S.SCE in CH,C12; 0.1 mol I-' NBu4PF,, Pt button working electrode, Pt spade counter electrode, 100 mV s-', Defined Fig. 4 Melting-point cersu.s alkyl chain length for the TTC,-Ni(edt)2 as peak oxidation potentials. derivatives J. MATER. CHEM., 1994, VOL. 4 compounds. This type of behaviour has been observed before in a series of n-alkylthio-substituted tetrathiafulvalene (TTC,- TTF) derivative^'^ and in alkyl-substituted napthalene compound^.'^ The melting points of the n=4 to n= 11 derivatives fluctuate over a 20 "C temperature range, with the odd-numbered alkyl chain length derivatives possessing slightly higher melting points. It is postulated that in these derivatives a different conformation adopted by the alkyl chains enables the NiC4S8 cores to interact more closely through either Ni...S, or S-..S contacts, thereby elevating the melting point.It was not possible to obtain the X-ray crystal structures of any of the odd-numbered chain length derivatives to confirm this, but the structures of the n=4 and 6 derivatives were determined. Structural Studies X-Ray Structuresof [Ni{S2C2S2(C4H,),)2](A) and "i (S*C*S2 (CsH13)212 1 (B) The molecular structures of (A) and (B), both of which are centrosymmetric, are shown in Fig. 5 and 6, respectively, and the bond lengths and angles are presented in Tables 3 and 4. In both cases, the Ni atoms are chelated to two pairs of S atoms from two n-alkyldithio-substituted ethylene-l,2-dithio-lene ligands in square-planar environments.The unique Ni- S Fig. 5 Molecular structure of Ni[S2C2S2(C4H,)2]2showing the atom labelling Fig. 6 Molecular structure of Ni [S2C2S2(C6H13)2]2showing the atom labelling 1863 Table 3 Bond lengths (A) and angles ( ) for [Ni{S2C2S2(C,H,),},] 2.1283(7) Ni-S(l) L.1332( 7) 1.708( 2) S(2)-C(6) 1.706 (2) 1.747(2) S(3)-C( 2) 1.806( 3) 1.752(2) S(4)-C( 7) 1.810(3) 1.381 (4) C(2)-C( 3) 1.524(3) 1.512(4) C(4)-C(5) 1.521 (4) 1.512(4) C(8)-C(9) 1.495( 4) 1.515(5) S(2)-Ni -S( 1) 91.60(3) S(2)- Ni-S( l'y 1(8.40(3)C( 1)-S( 1)-Ni 104.82(9) C(6)--S( 2)- Ni 104.86( 9) C(l)-S(3)-~(2) 103.8( 1) C(6)-S( 4)-C( 7) 105.2(1)C(6)-C( 1)-S( 1) 119.2(2) C(6)-C( 1)-S(3) 119.1(2)S(1)-C( 1)-s(3) 121.7(2) C( 3)-c(2)-s(3) 109.6( 2) C(4)-C( 3)-c(2) 111.1(2) C(3) -C(4)-C(5) 112.9(3)C(1)-C(6)-S(2) 119.32) C(l)--C(6)--S(4) 113.7(2)S(2)-C(6)-S(4) 121.8(2) C(8)-C( 7)-S(4) 114.4(2)C(9)-C( 8)-C(7) 115.1(3) C(S)-C(9)-C(lO) 113.1(3) a The primed atoms belong to the same molecule and are generated by the symmetry (-x, -4'.-z). Table 4 Bond lengths (A)and angles ( ) for [NifS2C2S2(C6H13)2j2] 2.113(2) Ni -S( 1) 2.132(2) 1.698( 6) S(2)-C(8) 1 700(6) 1.748(5) S(3)-C(2) 1 796(6) 1.750(6) S(4)-C(9) 1817(6) 1.376(9) C(2)-C(3) 1529(8) 1.520(9) C(4)-C(5) 1534(9) 1.517(9) C(6)-C( 7) 1.524(9) 1.5O2(8) C(10)-C( 11) 1512(8) 1.497(9) C(12)-C( 13) 1.522( 10) 1.506( 12) S(2)-Ni-S( 1) 92.16(6) S(2)-Ni-S( 1')n 87.84( 6) C( 1)-S( 1 )-Ni 104.1(2) C(8)-S(2)-Ni 104.4(2) C(l)-S(3)-C(2) 104.0(3) C(8)-S(4)-C(9) 102.7(3 ) C(8)-C( 1)-S( 1) 119.5(4) C(8)-C(l)-S(3) 119 l(4) S( 1)-C( 1)-S(3) 121.3(4) C(3)-C(2)-S(3) 108 6(4) C(4)-C(3)-C(2) 113.1(5) C(3)-C(4)-C(5) 112 3(5) C(6)-C(5)-C(4) 114.3(6) C(5)-C(6)-C(7) 112 3(6) C(l)-C(S)-S(2) 119.8(4) C(l)-C(S)--S(4) 119 l(4) S(2)--C( 8)-S( 4) 121.0(4) C(10)-C(9)-S(4) 1089(4) C(9)-C(lO)-C(11) 112.1(5) C(l2)-C(ll)-C(lO) 115 l(6) C(11)-C( 12)-C( 13)113.5(6) C( 14)-C(13)-C(12) 112.3(8) a The primed atoms belong to the same molecule and are generated by the symmetry (-x, 1 -y, 1-z). distances$re 2.128( 1) and 2.133(1) A in (A) and 2.113( 2) and 2.132(2) A in (B);these are comparable wit! one another and also with those [2.119(1) and 2.125(1) A] in the related complex [Ni(d~dt)~].l~ The S-Ni-S cis angles are 91.60( 3) and 88.40(3)" in (A) and 92.16(6) and S7.84(6)' in (B); these show only minor deviations from the ideal value (90.0") for square-planar geometry, but the fact that the larger angles are always associated with the pair of S atoms from the same ligand suggests that the variations are significant and deter- mined by the requirement of the bite angles of the dithiolene ligands.Similar variations in the S-Ni-S angles [91.76(5) and 88.24(5)'] were also observed in [Ni(dpdt),] .I6 An interesting point to note is the different orientations of the two alkyl chains in each ligand relative to the Ni(S,C,S,) mean plane. In (A) the alkyl carbon atoms in one chain lie very close (within 0.042A) to this plane, whilst the other chii? is considerably twisted with atomic deviations up to 4.02 1 A.A similar situation is also observed ip (B), where the carbon atoms in one chain lie within 0.454 A of the Ni(S2C2S,) mefn plane, but those in the other show deviations up to 2.748 A. The adoption of different conformations by the two alkyl chains in each ligand in both compounds reflects the flexibility of such chains, and the particular orientations are determined, most probably, by packing requirements. The chemically equivalent bond lengths within each S,C,S, moiety are the same within experimental error and are mutually comparable in the two complexes. The average C(sp2)-S (chelated), C(sp2)-S (free) and C(sp3):S(free) dis-tances are, respectively, 1.707, 1.750 and 1.808 A in (A) and 1.704, 1.749 and 1.807 A in (B), following the trend expected for such chemically inequivalent bonds.The C-C bonds have normal values. The packing of molecules of (A) and (B) is shown in Fig. 7 and 8, respectively. In each case, the molecules are packed parallel to one another and held together only by van der Waals' forces. The shortest intermolecu!ar contacts between a pair of non-hydrogen atoms are 3.565 A [C(S)..C(S) at (-x, b Fig. 7 Packing of Ni[S2C2S2(C,H,),], molecules in the crystal, viewed along c Fig. 8 Packing of Ni[S2C2S2(C6H,,),1, molecules in the crystal,viewed along c J. M.4TER. CHEM., 1994, VOL. 4 -y, 1 -z)] in (A) and 3.580 A [C( lO..-C(8) at (1 +x, y, z) in (B).There are no short S...S contacts. Electrical Conduction Studies The electrical conductivities of the [TTC,-Ni(edt),] complexes were investigated using the four-probe met hod on compressed pellets of the n =5 and n =11 derivatives. The room-tempera- ture conductivities of these compounds were 1.14 x lop6 and 4.3 x lop7S cm-', respectively. The low conductivities of these compounds are not surpris- ing, considering the absence of the short intermolecular S...S contacts generally considered as vital for the formation of conducting species" and the fact that these materials are in an undoped, neutral state, in which no mobile charge carriers are present. Inokuchi et d.ls reported a similarly low room-temperature conductivity of lo-' S cm-' for an n-alkylthio-substituted TTF derivative, but pointed out that this value was high compared with those of other organic-based semiconductors composed of a single component.The comparatively high conductivity of this compound was attributed to the linking of the central TTF cores uiu the attached alkyl chains, which was described as a 'molecular fastener effect'. Whilst it is apparent that the undoped nickel complexes described in this study possess low electrical conductivities, it is anticipated that the preparation of partially oxidised [TTC,-Ni(edt),] compounds currently under investigation will yield conducting materials. The results of these studies will be reported in due course.Experimental 4,5-Bis(benzoy1thio)- 1,3-dithiole-2-thione was prepared according to literature methods." Methanol was dried by refluxing it over magnesium tyrnings followed by distillation, and was then stored over 4A molecular sieves under argon prior to use. Elemental analyses were obtained using a Carlo Erba 1106 elemental analyser, whilst FAB mass spectra were recorded on a Kratos MS50 RF machine, using 9 kV xenon atoms and 3-nitrobenzyl alcohol as the matrix. UV-VIS and near-infra- red spectra were recorded on Pye Unicam PU8800 and Perkin-Elmer 19 spectrophotometers, respectively. Melting points were determined using a Linkam hotstage (model TH600) with a ramprate of 3°C min-', in conjunction with an Olympus optical microscope (model BH2) fitted with polarising filters.Cyclic voltammetry measurements were recorded on an EG&G Princeton Applied Research Model 264A Polarographic Analyzer/Stripping Voltammeter, connected to an EG&G Condecon 300 controller. All measurements were carried out in dichloromethane which had been dried over P20, prior to use. [Bu",N][PF,] (0.1 mol dm-3) was used as the background electrolyte. All potentials are quoted in V relative to a saturated calomel electrode (SCE), using a Pt button working electrode and Pt wire counter-electrode. Typically, scan rates of 100 mV s-l were used. Preparation of the Bis(n-alky1thio)-substituted 1,3-Dithole-2-thione Derivative (3) This was prepared according to the general method of Schultz et a1.l' with minor modifications.The II =4-9 derivatives were obtained as dark orange oils, whilst the M =10, 1 1 compounds were isolated as waxy solids. The synthesis of the n = 11 derivative is described. To a slurry of 4,5-bis( benzoy1thio)- 1,3-dithiole-2-thione J. MATER. CHEM., 1994, VOL. 4 1865 Table 5 Crystal data and details of data collection and structure refinement for [Ni{S,C,S,(C,H,),},] (A) and [Ni{S2C2S,(C,H,3),}2] (B) complex (A) complex (B) ~ ~-empirical formula C,OH,,NiS, C,,HS,NiS,formula weight 591.68 703.89 TIK 150(2) 293 (2) crystal system triclinic triclinicsp!ce group pi pi44 7.753(1) 5.254(4)bI+ 9.368( 2) 12.523(3)CIA 10.399(2) 14.017(3)x/degrees 84.39( 1) 103.46(1)Pldegrees 82.80( 1) 96.08( 1 )?/degrees 67.08( 1) 91.85( 1) VIA3 689.2( 2) 890.3(8)1 1 DJg cmP3 1.426 1.313 p( Mo-Kx)/cm-' 13.1 7 10.31 F(000)/r 312 376 crystal size/mm 0.45 x 0.10 x 0.08 0.35 x 0.15 x 0.10 8 range for data collection/degrees 2.36-29.73 1.97-25 45 4nin9 hmax -10, 10 -4, 5 kin.kmax -12,9 -13, 13 Ln 9 Lax -14, 5 -15, 11 reflections collected 3037 2715 independent reflections 2615 2402 R,,, 0.0308 0.0449 absorption corr. factors 0.904 to 1.078 0.929 to 1.118 no. of data in the refinement 2615 2402 no. of parameters 153 197 final R" indices (all data) R1= 0.0360(0.0303)* 0.0775(0.0352jh IVR,= 0.0872( 0.075qb 0.1589( 0.0793)b largest diff. peak and holeje AP3 0.341 and -0.270 0.471 and -0.366 w= l/a2(Fo2). R1 and wR, values for data with I >20(Ij are given in" R,=Z(F,-F,)/C(F,); wR,=[~{w(F,2-Fc2))"}/~{~(F,2)2}]1~2; parentheses.(4.00 g, 9.8 mmol) in methanol (20 ml), under a dry argon product (0.11 g, 15%); UV-VIS(CH2C12),~b,,,/nm [ln(e/dm3 atmosphere, was added hexane-washed sodium (0.46 g, mol cm-l)]: 361(8.12), 292(9.28), 258(9.02). 20 mmol) dissolved in methanol (15 ml). The mixture was stirred for 1 h and 1-bromoundecane (4.62 g, 0.02 mmol) was X-Ray Crystallography for the Complexes added to the dark purple solution, which was then refluxed for 2 h. The dark orange solution was concentrated in uucuo "W2C2S2 (C4Hdd21 (A) and "i{S2C2S2(C6H13)2}2 1(B) and the solid residue was recrystallised from CHC1,-MeOH Crystallographic measurements for both complexes were made to yield a red waxy solid (5.56 g, 89%), mp 40-43 "C(Found: using a Delft Instruments FAST areao detector diffrac*.ometer C, 59.06; H, 9.69%. CZ5H4& requires: C, 59.27; H, 9.08%); and Mo-Ka radiation (2=0.71069 A) following previously m/z (EI) 506 (M', 75%); IR (CHC13) vjcm-': 1465 (C=C), described procedures.20 The structures were solved by direct 1064 (C=S). methods and difference synthesis, and refined on F2 by full- matrix least-squares using all unique data, corrected for Preparation of the [TTC,-Ni (edt),] Complexes (4) Lorentz and polarisation factors and also for absorption.The The method used for the preparation of all the derivatives Table 6 Atomic co9rdinates (x lo4)and equivalent isotropic displace- was the same, including the relative reagent quantities and ment parameters (A2 x lo3)for [Ni{ S2C2S2(C4H9)2}2] reaction conditions.The preparation of the n=4 derivative is Ydescribed. 4' Z ue, To a slurry of the 4,5-bis( butylthio)-1,3-dithiole-2-thione 0" 0" 0" (1.00 g, 3 rnmol) in methanol (30ml), under a dry argon 1314( 1) -2345( 1) -552( 1)atmosphere was added hexane-washed sodium (0.14 g, -422( 1) -727(1) 1977(I) 6 mmol) dissolved in methanol (10 ml). The mixture was 2307( 1) -5466(1) 903( 1) refluxed for 1.5 h and to the clear orange-brown solution was 410( 1) -3843(1) 3428( 1 ) added [Bu,N][Br] (2.07 g, 6 mmol) in methanol (10 ml), 1317(3) -3440( 3) 861(2) 3193( 4) -5900( 3) -764( 3) followed by the dropwise addition of a solution of 4104 (4) -7653(3) -878(3)NiC1, -6H,O (0.38 g, 1.5 mmol) in methanol (30 ml) with 4868 (4) -8060(3) -2262( 3) stirring.The solvent was removed under vacuum to yield a 5777(5 j -9801(3 j -2424( 4 j brown oily solid which was redissolved in acetone (30 ml) 512(3) -2705( 3) 2001(2) -407(4) -2499( 3) 4715(3)and filtered. To the dark brown solution was added iodine (2 equiv.) dissolved in acetone (20ml), resulting in an 1099 (4) -2062( 3 ,) 5157(3) 2649( 5) -3371 (4) 5763(4)instantaneous colour change to dark green. Reduction of the 4171(5) -2892( 5) 6129(4)solvent volume induced a dark coloured solid to precipitate, which was filtered off, washed with methanol and dried. U,, is defined as one third of the trace of the orthogonalized Uij Recrystallisation from CH,Cl,-MeOH afforded the pure tensor." Invariant parameters. Table 7 Atomic coqrdinates (x lo4)and equivalent isotropic displace- ment parameters (A2x lo3)for [Ni{S2C2S2(C,H1,)2)21 Y Z 0" 5000" 5000" 2701(3) 1436(3) 4871(3) 1362( 11) -582( 3) -860(12) -1012( 14) -2806( 13) -2928( 14) -4732( 15) -4976( 19) 4633( 1 ) 3785( 1) 2955( 1) 2151(1) 3591(5) 3653(5) 3 138(6) 3706(5) 3178(6) 3706(7) 3089(8) 6374( 1) 4712( 1) 7436( 1) 6450(4) 8183(4) 9065(5) 9780(5) 10658( 5) 11384(5) 12191(7) 5794( 1) 2823( 11) 6348( 11 ) 8107( 12) 3205(5) 1938(5) 1010(5) 5697(4) 4655(4) 4613(5) 9402( 13) 778(5) 3679(5) 7687( 14) 250(6) 2755(5) 9059( 17) 7315(22) 34(7) -542( 10) 1832(6) 922(7) Ue, is defined as one third of the trace of the orthogonalized U, tensor. "Invariant parameters.non-hydrogen atoms were anisotropic; the hydrogens were included in idealised positions with individual Uisos freely refined. The weighting scheme used was w= l/a2(FO2)which gave satisfactory agreement analyses. Final wR, (on F2) and R, (on F) values were 0.0872 and 0.0360, respectively, for A (153 parameters and all 2615 data), and 0.1589 and 0.0775 for B (197 parameters and all 2402 data). All calculations were done on a 486DX2/66 personal computer using the programs SHELX-S (structure solution),21 SHELXL-93 (refinement),22 DIFABS (absorption c~rrection)~~and SNOOPI (diagrarn~).~~Sources of scattering factor data are given in ref. 21. The crystal data and details of data collection and structure refinement for both complexes are presented in Table 5.The fractional atomic coordinates are given in Tables 6 and 7. Anisotropic displacement coefficients, hydrogen-atom parameters and tables of FJF, have been deposited as supplementary material. We thank SERC for the provision of a studentship (to A.C.), and British Telecom for support. References 1 A. E. Underhill, J. Muter. Chem., 1992, 2, 1; C. S. Winter, S. N. Oliver, R. J. Manning, J. D. Rush, C. A. S. Hill and A. E. Underhill, J. Muter. Chem., 1992, 2,443. J. MATER. CHEM., 1994, VOL. 4 U. T. Mueller-Westerhoff and B. Vance, Comp Coord. Chem., 1987, 2, 595. G. A. Bowmaker, P. D. W. Boyd and G. K. Campbell, lnorg. Chem., 1983,22,1208. P. Cassoux, L.Valade, H. Kobayashi, A. Kobayashi, R. A. Clark and A. E. Underhill, Coord. Chem. Rev., 1991,110. 115. P. N. Prasad and D. J. Williams, in Introduction to NLO effects in Molecules and Polymers, J. Wiley, Chichester, 1992. C. S. Winter, R. J. Manning, S. N. Oliver and C A. S. Hill, Opt. Commun., 1992,90, 139. C. A. S. Hill, A. Charlton, A. E. Underhill. S. N. Oliver, S. V. Kershaw, R. J. Manning and B. J. Ainslie, J. Muter. Chem., 1994,4, 1233. 8 E. Fanghanel and H. Poleschner, J. Prakt. Chem.. 1981,323, 1. 9 S. S. Nagapetyan, V. E. Shklover, L. V. Vetoshkina, A. I. Kotov, L. Yu. Ukhin, Y. T. Struchkov and E. B. Yagubskii, Mater. Sci., 1988, 14, 5. R. Schultz, A. Schweig, K. Hartke and J. Kostrr, J. Am. Chern. Soc., 1983, 105,4519.C. Keller, D. Walther, J. Reinhold and E. Hoyer, Z. Chem., 1988, 11,410. C. Faulmann, A. Errami, J.-P. Legros, P. Cassoux, E. B. Yagubskii and A. I. Kotov, Synth. Met., 1993.56,2057. E. B. Yagubskii, A. I. Kotov, L. 1. Buravov, A. G. Khomenko, V. E. Shklover, S. S. Nagapetyan and Y. T. Struchkov, Synth. Met., 1990, 35, 271. 14 P. Wu, G. Saito, K. Imaeda, Z. Shi, T. Mor, T. Enoki and H. Inokuchi, Chem. Lett., 1986,441. 15 D. G. Anderson, J. C. Smith and R. J. Rallings, J. Chem. Soc., 1953,443. 16 A. Charlton, A. E. Underhill, K. M. A. Malik, M. B. Hursthouse, T. Jrargensen and J. Becher, Synth. Met., 1994, in the press. 17 P. Cassoux and L. Valade, in Inorganic Materials, ed. D. W. Bruce and D. O'Hare, J. Wiley, Chichester, 1992. 18 H. Inokuchi, G. Saito, P. Wu, K. Seki, T. B Tang, T. Mori, K. Imaeda, T. Enoki, Y. Higuchi, K. Inaka and N. Yasuoka, Chem. Lett., 1986, 1263. 19 G. Steimecke, H. J. Sieler, R. Kirmse and E. Hoyer, Phosphorus and Sulphur, 1979, 7, 49; K. S. Varma, A. Bury. N. J. Harris and A. E. Underhill, Synthesis, 1987, 837. 20 J. A. Darr, S. R. Drake, M. B. Hursthouse and K. M. A. Malik, lnorg. Chem., 1993,32, 5704. 21 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46,467. 22 G. M. Sheldrick, SHELXL-93 Program System, J. Appl. Crystullogr., 1994, in preparation. 23 N. P. C. Walker and D. Stuart, Acta Crystallo,qr., Sect. A, 1983, 39, 158. 24 K. Davies, SNOOPI Program for Crystal Structure Drawing, University of Oxford, 1983. Paper 4/03982C; Receiurd 1st July, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401861

出版商:RSC    

年代:1994

数据来源: RSC