摘要:

Electrical switching in VOz sol-gel films Guillermo Guzman, Fabien Beteille, Roger Morineau and Jacques Livage" Chimie de la Mati2re Condenst!e, Universitt! Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France Vanadium dioxide thin films have been made from vanadium oxoalkoxides. The precursor solution is spin-coated onto a silica substrate and heated under a reducing atmosphere. These V02 films exhibit well known insulator-metal transition at around 67°C. Electrical switches have been made from such films that reversibly switch from an OFF state to an ON state upon applying a voltage of ca. 50 V. This thermally driven process is fast and highly reversible. More than lo8 cycles have been performed without failure. Two-terminal electrical switching devices based on V205 l.8H20 xerogels have been reported by Bullot et a2.l Such devices can be easily formed via the deposition of two gold electrodes, about 100 pm apart, at the surface of a xerogel thin film.The reversible switching between ON and OFF states occurs within a few ms and involves a change in conductivity of about two orders of magnitude. Later, Zhang and Eklund showed that the switching was not observed unless the device was first 'over-biased' to a voltage around the switching voltage.' They observed the formation of a VO, filament between the electrodes and suggested that the switch- ing phenomenon was associated with an electrothermally driven metal-insulator transition of the filament.3 The forma- tion of V02 is probably due to the electrochemical reduction of Vv in the V,O, l.8H20 film.4 Such devices are easy and cheap to make but their current-voltage characteristic (I-V) is not reproducible.This arises mainly from the fact that the diameter of the VO, filament is difficult to control. All these results have been obtained with V205 * l.8H20 gels arising from the polymerization of vanadic acids in aqueous solutions. This paper shows that switching devices can be made directly with VOz thin films synthesized from vanadium alkoxides. Vanadium dioxide thin films were synthesized via the hydrolysis and condensation of vanadium alk~xides.~.~ A solu-tion (0.5 mol dm-3) of vanadium oxoalkoxide, VO(OR)3, in its parent alcohol ROH (R =Pr', tert-amyl) is spin-coated onto a silica substrate about 2cm2 in size.Spin-coating is performed in air so that the alkoxide film is partially hydrolysed by ambient moisture. Not all of the alkoxy groups are hydro- lysed. They remain bonded to the oxide network and amorph- ous 0x0-polymers [V205 -x(OR)x]n rather than hydrated vanadium oxides are formed. The coating is then dried at around 60°C in order to remove excess alcohol. The coating becomes slightly green owing to some reduction of Vv to VrV. Freshly deposited films do not exhibit the layered structure shown by V,O, * 1.8H20 films synthesized from vanadic acid in aqueous sol~tions.~ They appear to be amorphous by X-ray diffraction [Fig. l(a)]. Crystalline VO, films are formed upon heating for 2 h at 500°C under a reducing atmosphere (Ar-H,, 5%) [Fig.l(b)]. Optically transparent dense films are then obtained. They are made of VOz particles about 100 nm in size. They exhibit a homogeneous flat surface and their thickness is about 0.05 pm. Several coatings have to be made in order to obtain thicker films. The successive spin-coatings are performed after drying the previous film at 60°C. Fig. 2 I-I/ characteristic of a switching device based on VO, thin films I II I I 10 20 30 40 50 60 20/degrees Fig. 1 XRD patterns of (a) amorphous freshly deposited [V,O,-,(OR),], film, (b) crystalline VO, thin film after thermal treatment (the broad peak corresponds to the silica glass substrate) The electrical resistance of the films was measured in the 20-100°C temperature range using a two-probe device.A typical hysteresis loop is observed. Upon heating, the resistance drops rapidly by about three orders of magnitude at 70°C. Upon cooling, the insulating state is recovered around 60°C. Electrical switching devices are simply made via the depos- ition of gold electrodes, 0.4mm apart, at the surface of the VO, film (thickness ca. 0.2 pm). These electrodes are evapor- ated through a mask so that the geometry of the interelectrode space can be accurately controlled. An ac voltage is applied to the device and the I-V curve is recorded on an oscilloscope. A typical curve is shown in Fig. 2. Along the OA line, the device follows Ohm's law, V= RI. The film is in the insulating state and its resistance is quite high: this is the OFF state.However, as the current increases, the film is heated via the Joule effect. At a given threshold voltage (& M 50 V) the temperature of the film becomes higher than its transition temperature and the vanadium dioxide becomes metallic. Its resistance drops by a factor of about lo3:this is the ON state. The voltage decreases while the current increases (line AB). The device remains in the ON state as long as its temperature 0 t H VN A 4 Volts J. Muter. Chem., 1996, 6(3), 505-506 505 8 6 a 4 *4 2 '0 10 20 30 40 50 60 VN Fig. 3 Variation of the threshold voltage, Tih, with temperature is higher than the transition temperature, ie as long as the current is larger than the holding current, IhZO6mA (point C) Beyond this point, the film switches back to the OFF state (line CO) This electrical switching is due to the well known insulator- metal transition of VO, It is thermally driven and the threshold voltage depends on the amount of heat required to reach the transition temperature This amount depends on the difference between the ambient temperature and the transition temperature, T, Therefore V,, decreases regularly when the temperature increases (Fig 3) The switching behaviour disap- pears when the room temperature reaches T, The whole film is then in the metallic state Electrical switching devices can be made easily with VOz thin films deposited from vanadium alkoxides The switching mechanism is the same as previously reported for devices based on aqueous V,O, * 1 8H20 xerogels In this case, a VO, filament was electrochemically formed through the xerogel upon applying an over-voltage to the device The diameter of this filament was difficult to control as it was not formed homogeneously between the gold electrodes It was therefore difficult to build devices showing a given threshold voltage In this paper, the whole film is made of VO, The size of the active materials and the threshold voltage, T/h, depend on the geometric characteristic of the VO, film between the electrodes, R = ps/A, where R is the resistance, p the bulk resistivity, s the interelectrode separation and A the electrode contact area They are defined by the geometnc characteristics of the mask used for the deposition of the gold electrodes Reproducible results can then be obtained when these charac- teristics are carefully controlled Moreover, the transition temperature can be easily modified when doped VO, films are used Such doped thin films can be made easily by mixing molecular precursors with the alkoxide solution We have shown, for instance, that T, decreases with high-valent cations (NbV, Wv') and increases with low-valent cations such asA11116 8 The switching behaviour of VO, thin films, recorded on an oscilloscope, can be observed continuously for several weeks or even months (Fig 2) This points out the high reversibility of such devices based on VOz thin films Bulk materials do not withstand many cycles The structural distortion associated with the insulator-metal transition rapidly leads to a degra- dation of the materials Large crystals break after few cycles only References 1 J Bullot, 0 Gallais, M Gauthier and J Livage, Phys Status Solzdz A, 1982,71, kl 2 J G Zhang and P C Eklund, J Appl Phys, 1988,64,729 3 J G Zhang and P C Eklund, J Mater Res, 1993,8,558 4 J Livage, Chem Mater, 1991,3,681 5 G Guzman, R Morineau and J Livage, Muter Res Bull, 1994, 29,509 6 G Guzman, F Beteille, R Monneau and J Livage, Eur J Solid State Inorg Chem , 1995,32,851 7 F J Mom, Phys Rev Lett, 1959,3,34 8 J Livage, G Guzman, F Beteille and P Davidson, J Sol-Gel Scz Techn , in press Communication 5/073491, Received 8th November, 1995 506 J Mater Chem, 1995, VOL 6

ISSN:0959-9428    

DOI:10.1039/JM9960600505

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Effect of pyridine upon gas-phase reactions between H2Sand Me2Cd; control of nanoparticle growth Nigel L. Pickett, Douglas F. Foster and David J. Cole-Hamilton" School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK K Y16 9ST The effect of pyridine upon the gas-phase reaction of dimethyl- cadmium (Me,Cd) with hydrogen sulfide (H2S) has been investigated. Analysis of the solid reaction product by powder X-ray diffraction (PXRD), elemental analysis (EA) and trans- mission electron microscopy (TEM) reveals the deposit to consist of crystalline CdS particles in the nanometre size range of the Greenockite (hexagonal) phase, with pyridine molecules bound to surface cadmium atoms. The average particle size within the deposit is dependent upon both the concentration of pyridine in the gas phase and the temperature at which the gas-phase reactants are mixed.When wide bandgap Group 12/16 (11-VI) compound semi- conductors (ZnS, ZnSe, CdS and CdSe) are grown by metal- organic vapour phase epitaxy (MOVPE) from the Group 12 alkyls Me,Zn, Me,Cd and the Group 16 hydrides H2S, H,Se, a low-temperature premature reaction (prereaction) occurs.' This prereaction results in the formation of a solid deposit upstream of the heated substrate in the cold zone of the reactor cell, and as well as affecting growth rates it can also drastically affect layer thickness uniformity and subsequent optoelectronic properties of grown epilayers. This prereaction has been attri- buted to the proficient elimination of the alkyl groups from the Zn and Cd precursors in the presence of the Group 16 hydrides.The addition of a nitrogen o-donor compound (tertiary or aromatic amine) into the growth system, either in the form of an adduct with the Group 12 alkyl precursor, which can have the added advantage of a more convenient vapour pressure, or by direct addition of the amine into the carrier-gas flow, leads to a reduction or even complete elimination of prereac- tion. Amines which have been employed include triethyl- amine,2*3 tria~ine,~N,N,N',N'-te trame thyldiaminome t hane, 4*5 N,N,N',N'-tetramethyl- 1,Zdiaminoethane and ~yridine.~.' Introduction of pyridine into the gas phase during MOVPE growth of ZnSe from Me,Zn and H,Se, even at very low concentrations, eradicates prereaction while giving concomi- tant improvements in the growth rate and optoelectronic properties of the grown layer^.^ In contrast, similar addition of pyridine fails to inhibit prereaction between Me,Cd and H,Se, although the colour of the solid deposit changes from black to red and an improvement is observed in layer thickness uniformity of the grown CdSe.7 The introduction of pyridine into ZnS and CdS growth systems has yet to be reported.The role that these amines play in reducing or eliminating prereaction during growth of Group 12/16 materials, and the reason for improved growth rates and layer uniformity, are of considerable debate. Several mechanisms for the inhibition of prereaction have previously been proposed.These include a mechanism by which stoichiometric adduct formation between a Group 12 alkyl and an amine simply prevents the weaker 0-donors H,S or H2Se from coordinating to the Group 12 metal Against this is the small ratio of pyridine :Me,Zn (0.3 :1)that is required in order virtually to eliminate prereac- tion during growth of ZnSe,7 and gas-phase spectroscopic studies which clearly show such adducts to be fully dissociated into their substituent parts when in the gas phase, even at room temperature.' Another proposed mechanism involves amine stabilisation of highly reactive radical intermediates which might form in the cold zone of the reactor and which may otherwise induce a chain reaction leading to the eventual formation of a solid The change in colour of the deposit obtained when pyridine is added to the gas stream containing Me2Cd and H,Se has led uslo and others' to propose an alternative explanation, since it is known that the colour of a semiconductor is dependent upon its particle size, if the size is in the nanometre range.For example, Cd,P, changes from black (macrocrystal- line) throuFh brown (30 A particles), red, orange, yellow to white (15 with similar changes occurring for CdSe.'' Welo and others' have proposed, therefore, that nitrogen donors might bind to the surfaces of growing particles, inhibiting particle growth and, in some cases where the inter- action is sufficiently strong, preventing gas-phase nucleation.Herein, we report the analysis, with respect to composition and structure, of the solid deposit formed during the gas-phase mixing of Me2Cd and H,S in the presence of varying concen- trations of pyridine, and also with a fixed concentration of pyridine at varying temperature. Helium gas streams of Me,Cd, H,S and pyridine were allowed to meet at the same point along a quartz tube which was heated, when required, by means of a ceramic tube furnace. With a total helium flow in the region of 600 cm3 min-', the gas-phase concentration of H2S was maintained in a 30-fold excess to that of Me,Cd [range (1.2-9.2) x mol dm-3 at the mixing point, depending on the pyridine : Me,Cd ratio used], while the pyridine :Me,Cd ratio was varied from 1 :20 to a maximum of 2: 1.At ambient temperature, a solid deposit formed along the entire length of the quartz tube. The colour of the deposit underwent a gradual change from bright orange to pale yellow as the pyridine:Me,Cd ratio was increased from 1 :20 to 2 : 1. A similar lightening in colour, as mentioned above, also occurs in the gas-phase reaction of Me,Cd and H,Se, with the prereaction deposit changing from black to red when pyridine is added to the ~ystem.~ Powder X-ray diffraction (PXRD) patterns of material formed at ambient temperature are characteristic of the Greenockite (hexagonal) form of CdS when pyridine is present during the deposition, and a mixture of both Greenockite and Hawleyite when pyridine is omitted (Fig. 1). The PXRD pat- terns show a broadening in peak width as the pyridine :Me,Cd ratio increases, indicative of the particle size decreasing as the concentration of pyridine is increased.The average particle size, determined from the half-width of t!e diffraction peaks using Scherrer's formula,I3 falls from 160 4 in the presence of a 1:20 ratio of pyridine: Me,Cd, to 40 A when pyridine is present in a 1 :1ratio with Me,Cd (Table 1). Further increasing the concentration of pyridine above a 1:1 ratio has little effect on !he average particle size, measured by PXRD at 44 and 46 A for 1 :1 and 2 : 1 pyridine : Me,Cd ratios respectively. A sample of the solid obtained with pyridine :Me,Cd =1:20 has also been analysed by transmission electron microscopy (TEM).It can be seen that the particles are rel$ively spherical (Fig. 2), with sizes ranging from 140 to T60 A, compared to the average obtained from PXRD of 160 A. J. Muter. Chern., 1996, 6(3),507-509 507 50- x-0- I Ill 1 . 7s- (f) 90- 25- o . v , . . . . , . . . . , . . . . , . I " " 11 ' I Fig. 1 PXRD pattern of the CdS prereaction deposit formed during gas-phase mixing at ambient temperature of a 30 1 mole ratio of H,S and Me,Cd when (a)in the absence of pyridine, (b)with a pyndine Me,Cd ratio of 1 20, (c) with a pyndine Me,Cd ratio of 3 5 and (d) with a pyndine Me,Cd ratio of 2 1 (peaks at 20=21,24" are due to the Vaseline support) Standard PXRD patterns of the Greenockite (e) and Hawleyite (f)phases of CdS Table1 Dependence of CdS particle size on the concentration of pyndine and the temperature at which the gas-phase reactants are mixed ~ Py Cd" 0 05 0 16 0 30 0 60 100 200 2 00 2 00 2 00 2 00 a Py =Pyridine * calculated diameter/A T/"C PXRD EA~ 25 160 147 25 100 105 25 70 84 25 69 95 25 44 62 25 46 67 50 129 - 100 231 - 150 376 - 200 383 - EA =elemental analysis These results strongly support our proposal that the particu- late matter forms through a process of nucleation and growth, with pyridine acting to inhibit the growth by binding to cadmium atoms on the surface of the growing particles These surface cadmium sites are likely to be Lewis acidic in nature 508 J Muter Chew, 1996, 6(3), 507-509 so that H2S molecules (Lewis base) will adsorb These may now react with surface-bound Me-Cd fragments or with Me,Cd molecules from the gas phase, in either case leading to methane elimination and further cadmium sites When added to the growth system, pyridine molecules simply adsorb, in competition with H2S molecules, at some of the Lewis-acceptor sites of the surface-bound cadmium atoms, as schematically represented in Fig 3, inhibiting further growth Additional evidence for this proposal comes from elemental analysis (EA) data, where the carbon nitrogen ratio within all the samples obtained at ambient temperature, ranging from 4 4 to 4 8, is consistent with pyridine (C N= 4 3) as the only organic residue within the samples Further, by assuming that a pyridine molecule is bound to every surface cadmium atom of the spherical particles, calculated values for the average particle sizes are obtained which are in good agreement with those obtained experimentally from PXRD (Table 1) The increase in particle size with temperature (Table 1) for a fixed pyridine Me2Cd ratio (2 1) simply reflects the known lability of the Cd-N donor-acceptor bond Since the dis- sociation of pyridine has a positive ASo value, the equilibrium will lie further towards the dissociated state at higher tempera- tures Thus, increasing temperature leads to increasing numbers of vacant Lewis-acid sites on surface-bound cadmium atoms Fig.2 TEM image (x 87 000) of the CdS prereaction deposit formed during the gas-phase mixing of H2S, Me2Cd and pyridine (mole ratio 600: 20.1) at ambient temperature Fig. 3 Schematic illustration of a CdS particle with pyridine molecules bound to surface Cd atoms at which adsorption of H2S and rapid elimination of methane will occpr.Average partitle sizes obtained from TEM (FiG. 4), at 404 A (range 90-710 A), and PXRD analysis, at 380 A, of the deposit formed at a 2 :1 pyridine :Me,Cd ratio and 200 “C, are again in good agreement. The ability of pyridine to prevent prereaction during growth of ZnSe from MezZn and H2Se, as opposed to its failure to prevent prereaction during CdSe growth from Me2Cd and H,Se, almost certainly reflects the higher strength of the donor- acceptor bonds expected between surface-bound zinc atoms and pyridine. Here, pyridine inhibits the growth of ZnSe microcrystals at a sufficiently early stage in the growth process, or may even inhibit the nucleation step, so that particulate matter does not form before the reactive species have reached the heated substrate.Further work is in progress on the analysis of prereaction Fig. 4 TEM image (x 135000) of the CdS prereaction deposit formed during the gas-phase mixing of H2S, Me,Cd and pyridine (mole ratio 30:1:2) at 200°C deposits formed during the growth of CdSe, ZnS and ZnSe and will be reported in the near future. We thank the EPSRC for financial support (N.L.P., via a ROPA award, and D.F.F.) and Dr. C. Glidewell for helpful discussions. References 1 A.C. Jones, J. Phys. (Paris), 1991, 1,C2-253. 2 P J. Wright, P. J. Parbrook, B. Cockayne, A. C. Jones, E. D. Orrell, K. P. O’Donnell and B. Henderson, J. Cryst. Growth, 1989,94,441. 3 P. J. Wright, P. J. Parbrook, B. Cockayne, A. C. Jones, P O’Brien and J. R. Walsh, J. Cryst. Growth, 1990,104, 601. 4 0.Briot, M. DiBlasio, T. Cloitre, N. Bnot, P. Bigenwald, B. Gil, M. Averous, R. L. Aulombard, L. M. Smith, S. A. Rushworth and A. C. Jones, Muter. Res. SOC.Symp. Proc., 1994,340,515 5 A. C. Jones, J. Cryst. Growth, 1994,145,505. 6 M. J. Almond, M. P. Beer, K. Hagan, D. A. Rice and P. J Wright, J. Muter. Chem., 1991, 1, 1065. 7 P. J. Wright, P. J. Parbrook, B. Cockayne, A. C. Jones and P. E. Oliver, J. Cryst. Growth, 1991, 108, 525. 8 M. A. Malik, M. Motevalli, J. R. Walsh, P. O’Brien and A C. Jones, J. Muter. Chem., 1995,5, 731. 9 0. F. Z. Khan, P. O’Brien, P. A. Hamilton, J. R. Walsh and A. C. Jones, Chemtronics, 1989,4,244. 10 X. Li, J. R. Fryer and D. J. Cole-Hamilton, J. Chem. Soc., Chem. Commun., 1994,1715. 11 H. Weller, Angew. Chem., Int. Ed. Engl., 1993,32,41 12 H. Weller, Adv. Muter., 1993,5, 88. 13 S. F. Bertram, in Handbook ofX-rays, ed. E F. Kaelble, McGraw- Hill, New York, 1967, pp. 17-19. Communication 5/06621B; Received 6th October, 1995 J. Muter. Chem., 1996, 6(3), 507-509 509

ISSN:0959-9428    

DOI:10.1039/JM9960600507

出版商:RSC    

年代:1996

数据来源: RSC