J. MATER. CHEM., 1994, 4(1), 51-54 Alternative Single-source Precursor for Growth of Indium Pnictide Thin Layers Rydki Nomura,* Takayuki Shimokawatoko, Haruo Matsudat and Akio Baba Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-Oka, Suita, Osaka 565, Japan Novel organometallic precursors for the growth of indium pnictide thin layers are reported. Tributylindium, n -Bu,ln, reacts readily with pnictogen sulfides, Asps3, P,S, and Sb,S, to give sulfur-bridged hetero-binuclear organometallics such as [Bu,ln(SAsBu),],S (l),[Bu,lnSP(S)Bu],S (2), and [Bu,lnSSb(S)Bu],S (3).They are distil- lable oils which can be pyrolysed to give the corresponding indium pnictides without sulfur contamination, under an N, atmosphere at 500-600 "C.Dip-dry pyrolysis of these complexes on an Si(ll1) substrate gave high- quality indium pnictide thin films: lnAs (p300, 1640 cm2 V-' s-'; n300, 1.3 x lOl4 ~rn-~; 600 "C), InPtype, p; Tsub, (p300,1100 cm2 V-' S-'; nsm, 1.2 x 1019cm-3; type, p; Tsub, 600 "C), and lnSb (p300,18400 cm2 V -'S-'; n300, 6.8~10'6cm-3; type, p; Tsub,450 "C).Organometallic precursors provide excellent routes for the fabrication of advanced electronic devices by metal-organic chemical vapour deposition (MOCVD), metal-organic vapour phase epitaxy (MOVPE) and metal-organic molecular beam epitaxy (MOMBE).' Efforts continue towards the improve- ment of organometallic precursors involving production of devices based on 13-15 semiconductor epilayers of high performance, durability and reliability via a reproducible and safe process.2-6 Single-source systems, in which one molecule contains the two or more elements necessary to form the desired semiconductor, represent a rational chemical approach.7-9 Several arsinogallanes, phosphinoindanes and related binuclear organometallics have been used to prepare thin, epitaxial layers of GaAs, InP and other 13-15 com-pound~.~There are some disadvantages in the use of a straightforward single-source ~ystem.~ First, the precursor molecules exist as aggregates and have low vapour pressures and secondly, the requirement for direct bonding between group 13 and 15 elements limits severely the number of organometallic species available.The presence of a direct bond limits the diversification of the precursor molecules.In this paper we propose alternative single-source precursors containing a metal( 13)-sulfur-metal( 15) bridge for 13- 15 semiconductor layers. Experimental Materials Tri-n-butylindium was prepared via literature methods lo and distilled under reduced pressure before use. Phosphorus sulfide (P2Ss)was used after purification by Soxhlet extraction with CS2.11Other metal sulfides (As2S3, Sb2S3, Sb,S5 and Bi2S3, Aldrich or Wako, pure grade) were used as received. A piece of Si( I 11) (10 mm x 10 mm) was cut from a 5 in$ wafer. The surface of the wafer was covered by a thin oxide layer (p-type, Shin-Etsu Semiconductor Co. Ltd., Gunma, Japan) and washed with deionized water; it was then immersed in a synthetic neutral detergent solution and washed with distilled water in an ultrasound bath.Finally the piece of Si was exposed to the vapour above boiling acetone (electronics grade) and dried. t Present address, Department of Applied Chemistry, Osaka Institute of Technology, 0-Miya, Asahi, Osaka 532, Japan. $ 1 in=2.54~10-~m. Analysis Thermal analysis was performed with a SEIKO TG/DTA 20 in a flowing N2atmosphere (5ml min- '). Surface morphology was observed with a Hitachi S-800 scanning electron microscopy (SEM, acceleration voltage 20 kV). Film crystal- linity was assessed with a Rigaku RotaFlex X-ray diffractometer (XRD, Cu-Kcr, 40 kVj80 mA). Mass spectra were recorded with a JEOL JMS-DX303 spectrometer with a JMA-DA5000 data-processing system.Microanalysis for elements other than CHN was performed with a Rigaku 3270 X-ray fluorescence microanalyser. Syntheses All synthetic reactions were carried out under dry and deoxy- genated N2, and used the normal Schlenk and syringe tech- niques. Tri-n-butylindium (4 mmol, 1.14 g) was added dropwise into a suspension of pnictogen sulfide (2mmol; 0.49 g for AszS3; 0.44 g for P2Ss; 0.68 g for Sb2S3; 0.81 g for Sb2S5; 1.03 g for Bi2S3) in absolute benzene (20 ml). After reaction (80 "C, 3 h), volatiles were evaporated off into another Schlenk tube, cooled by liquid N2, by trap-to-trap distillation in U~CUO(ca. 6 Pa). The product (1-3) was obtained by fractional distillation under reduced pressure. The products obtained from Sb2S3 and Bi2S3 were resinous and could not be purified by distillation.Procedure Static pyrolysis was conducted using 50mg of sample in a porcelain crucible heated in a cold-wall vertical quartz tube under an N2 atmosphere. The temperature of the sample reached the prescribed value from room temperature in ca. 1 h. The composition of the pyrolysates was determined by X-ray fluorescence. Details of the dip-dry pyrolysis method are as follows. A small piece (10 mm x 10 mm) of Si( 1 11) was dipped in and out of a solution of [Bu~I~(SASBU)~]~S (1) in benzene under N2, repeatedly, as shown in Fig. 1. Then the substrate was heated in a quartz tube at the required temperature under an N2 atmosphere. Growth of indium pnictide thin films under CVD conditions was carried out under reduced pressure (base pressure = 0.1 Torr; total pressure= 10 Torr; source temperature = 150 "C; growth period =3 h).The CVD system used in this study is illustrated in Fig. 2. Fig. 1 Dip-dry pyrolysis procedure mass flow meter Pirani gauge 0I -vacuum pump substrate heater thermocouple I heater bubbler (source) thermocouple quartz reactor, Q = 40 mm Fig. 2 Horizontal cold-wall CVD reactor employed in this study Results and Discussion We have reported that trialkylindiums have unusual alkylation behaviour,' 3914 especially tributylindium which readily alkyl- ates inorganic and organometallic oxides to give coupled products containing pox0 bridges between indium and another meta1.'5.16 We expected that such reactions should enable us to realize our objective: the production of new precursor systems.We tried the reaction of tri-n-butylindium with all five pnictogen sulfides. The results are summarized in Table 1. Generally organometallics, such as Grignard reagents, alkylate metal sulfides to give alkylmetal sulfides such as R3AsS.'7-'9 Unlike other organometallics, tributylindium provides coupled products (1-3) on reaction with As2S3,P2S5, and Sb2S5 (Scheme 1). These products 1-3 appear as colour- less and viscous oils and were purified by distillation. The two sulfides, Sb2S3 and Bi2S3 gave resinous products, in which the ratio of indium to pnictogen was estimated to be close to unity; these resinous products were not studied in detail.It is interesting that all of the compounds possess In-S-pnictogen linkages, and it should be emphasized that 1-3 can be purified by distillation [boiling points, InBu, + As2S3 --InBu, + P,S, InBu, t Sb2S5 -t I. .zt 0 10 20 30 J. MATER. CHEM., 1994, VOL. 4 &In-(SAsBu),-SlnBu, (1) Bu Bu Bu,ln-S-P-S-P-S-InBu, I (2) : E Bu2h-S-kb-S-Sb-S-tnB~2 (3): : Scheme 1 II (a) 40 50 60 70 80 29fdegrees Fig. 3 XRD patterns of indium pnictide thin films obtained from 1-3 by dip-dry pyrolysis. (a) InAs film obtained by dip-dry pyrolysis at 600 "C for 3 h; (b) InAs film obtained by CVD (T,,,=400 "C);(c) InP film obtained by dip-dry pyrolysis at 650 "C for 5 h; (d) InSb film obtained by dip-dry pyrolysis at 450 "C for 5 h.100 "C/0.5 Torr (l),130 "C/0.6 Torr (2), and 100 "C/0.3 Torr (3)]. Thus, 1-3 have relatively high vapour pressure compared to conventional single-source precursors such as arsinogal- lanes and their analogues.20 These are generally solids at room temperature and have low vapour pressure, which must result in low growth rate and require high vacuum conditions to prepare 13-1 5 semiconductor layer^.^,'^ These new sulfur- bridged organometallics are potentially useful sources if they give indium arsenide free from sulfur contamination. Thermal analysis of 1 showed that the pyrolysis started at 160 "C (onset of weight loss in TG) associated with an exotherm peak which appeared at ca.230 "C(in DTA). InAs was eventually formed with a successive release of the butyl groups and sulfur (up to 600 "C). Static pyrolysis (up to 600 "C)of 1-3 was then investigated further and the pyrolys- ates were analysed by XRD and X-ray fluorescence microanal- Table 1 Preparation of new sulfur-bridged binuclear organometallics pnictogen sulfide product" YO yield' b.p./"C (Torr) [Bu,In(SAsBu),],S (1)' [Bu,lnSP(S)Bu],S (2)d not identified" [Bu,InSSb(S)Bu] ,S (3)j not identified' 98 95 56 100 (0.5) 130 (0.6) 100 (0.3) "For 1-3, satisfactory microanalysis data were obtained. bYields were estimated by the formulae based on the amounts of the sulfides used. '1: 13C{'H} NMR 6, (22.6 MHz; solvent CDCI,; standard TMS) 13.5, 13.7 (CH,), 23.9, 24.0, 24.2, 24.6, 24.7, 25.0, 25.3, 28.4, 29.1, 29.5, 29.7, 30.7 (CH,); MS (EI, 70eV) m/z 1474 (M+,O.l%), 821 (M-Bu,InS-Bu,, 21Y0), 425 (Bu,InSAsBuS+, 55%), 132 (AsBu', 100%).d2:13C(1H} NMR, 6, (22.6 MHz; solvent CDCI,; standard TMS) 13.7, 28.2, 29.4, 23.0 (CH,CH,CH,CH,-In), 13.7, 27.2, 29.2, 23.5 (CH,CH,CH,CH,-P); IR (KRS- 5) vmax/cm-* 590 (P=S); MS (EI, 70 eV) 533 [Bu,InSP(S)BuSP(S)Bu+, 10Y01,38 1 [Bu,InSP(S)Bu+, loo%]. "Resinous products were obtained. '3: 13C{'HJ NMR, 6, (22.6 MHz; solvent CDCl,; standard TMS) 13.3, 25.2, 26.5, 21.8 (CH,CH,CH,CH,-In), 13.5, 24.8, 26.7, 21.5 (CH3CH2CH2CH,-Sb); IR (KRS-5) v,,,/cm-' 435 (Sb=S); MS (EI, 70 eV) 503 [Bu,InSSb(S)BuS', 1%], 471 [Bu,InSSb(S)Bu+, 5"/,], 178 (BuSb', 100%). J. MATER. CHEM., 1994, VOL.4 ysis (Table 2). Formation of a mixed sulfide was detected at temperatures lower than 450 "C. For example, the pyrolysis of 1 at 450 "C for 2 h gave greyish powders that showed the characteristic X-ray diffraction patterns of In2S3 and As2S3. The pyrolysates obtained at 500 "C showed the characteristic X-ray diffraction patterns of InAs. However, they still con- Table 2 Static pyrolysis of 1-3" compound T/"C t/h pyrolysisb,' 1 500 9 InAsSo.ld 600 9 InAs' 2 600 9 InPSo,2dJ 3 500 5 InSbd "Conditions of the static pyrolysis are described in the Experimental; bcomposition was determined with X-ray fluorescence microanalysis and phase was analysed by XRD; 'hydrocarbons, butane and butene, were detected in the exhaust and the corresponding pnictogen sulfides were deposited on the inner surface of the reactor; dno XRD peak was detected; esharp XRD pattern assignable to InAs was observed (JCPDS no.15-869); fbroad XRD pattern assignable to InP was observed (JCPDS no. 32-453); sharp XRD pattern assignable to InSb was observed (JCPDS no. 6-208). tained considerable amounts of both indium sulfide and arsenic sulfide. To obtain sulfur-free indium arsenide, the pyrolysis must be conducted at 600 "C and long pyrolysis times >9 h are needed. In the case of 2, some different features appeared. A significant sulfur contamination remained in InP obtained at 500 "C, and it was difficult to exclude sulfur contamination even at 600 "C.InP thin films contaminated with sulfur had poor morphology.In contrast, indium anti- monide without sulfur contamination was readily available at 500 "C. This may be a result of the low melting point of indium antimonide itself (535 "C). The static pyrolysis results suggest that 1-3 can be readily converted into the corresponding indium pnictides under sufficiently extreme pyrolysis conditions. We attempted to prepare indium pnictide thin layers on an Si(ll1) wafer by the dip-dry pyrolysis technique. InAs layers from 1 with a preferential orientation of (1 11) (Fig. 3), on Si(ll1) at 600 "C, for 3 h (film thickness 0.6 pm) without sulfur contamination. Hall measurements (Van der Pauw) at 300 K indicated that this InAs thin film had p-type conduction and a relatively high carrier mobility ~300= 1640 cm2 V-' s-and low carrier concentration n300 = 1.3 x lOI4 cm-3.Similarly the preparations of InP and InSb thin films from Fig.4 SEM photomicrographs of indium pnictide thin films obtained from 1-3 by dip-dry pyrolysis. (a) InAs film obtained by dip-dry pyrolysis of 1at 600 "Cfor 3 h; (b)InAs film obtained by CVD from 1 (Tub400 "C);(c)InP film obtained by dip-dry pyrolysis of 2 at 650 "C for 5 h; (d) InSb film obtained by dip-dry pyrolysis of 3 at 450 "Cfor 5 h J. MATER. CHEM., 1994, VOL. 4 Table 3 Dip-dry preparation of indium pnictide thin films" source thin film n,,,/cm- ~ ~~~ 1 600 3 InAs 1640 1.3 x 1014 2 650 5 In P 1100 1.2 x 1019 3 450 5 InSb 18400 6.8 x 10l6 "Dip-dry cycles were repeated 10 times and the film thickness was estimated by SEM as ca.0.5 pm; bmobility measured at 300 K; 'carrier concentration at 300 K. 2 and 3 were carried out and the results obtained are summarized in Table 3. Films of InP and InSb obtained here also possessed relatively high mobility and low carrier concen- tration. These results indicate that the thin films obtained from the new type of precursor of this study suffered from a very low level of sulfur contamination. The pnictide films thus prepared by dip-dry pyrolysis showed a smooth surface morphology without any cracks, as shown in Fig. 4.However, only for 1 could the molecular ion peak be detected (rn/z= 1474). Similar stable binuclear fragment ions were detected in all cases: e.g.[Bu,InSAs(S)Bu] + for 1 (rn/z=425, intensity 55%); [Bu21nSP(S)Bu]+ for 2 (rn/z= 381, intensity 100%); +[BuJnSSb(S)Bu] for 3 (m/z=471, intensity 5%) (Table 1). These fragments contain both indium and pnictogen atoms in a 1 : 1 ratio and strongly suggest the formation of the compounds. Furthermore, the release of the sulfur moiety might occur from the immature pyrolysates during the forma- tion of the indium pnictide lattice, because the metal sulfide will readily lose sulfur under reduced pressure or at tempera- tures below the melting point.'2,21,22 The new single-source precursor containing sulfur bridges between 13 and 15 elements are excellent candidates for the deposition of 13- 15 semiconductor layers.These precursors may offer easy access to sulfur-doped semiconductor layers.23 Preliminary experiments showed that growth of InAs thin layers from 1 by CVD is possible but the InAs films thus obtained have a rough grain geometry and poor crystallinity [Fig. 3 (b) and 4 (d), respectively]. Consequently, further optimization of growth parameters is still necessary. Prep- aration of other 13-15 semiconductor layers by MOCVD is also being developed. Part of this work was supported by Grant-in-Aid for Scientific Research, C-04806085 (RN) and the Priority Area of Reactive Organometallics No. 05236102 (AB). R.N. is grateful for the Miyashita Research Grant. We would like to thank Mr. H. Moriguchi of the Analytical Centre of the Faculty for recording the mass spectra.References 1 H. M. Manasevit, Appl. Phys. Lett., 1968, 12, 156; J. Cryst. Growth, 1981,55, 1. 2 W. T. Tsang, J. Cryst. Growth, 1992, 120, 1. 3 K. Adomi, J-I. Chyi, S. F. Fang, T. C. Shen, S. Strite and H. Morkocq, Thin Solid Films, 1991,205, 182. 4 H. Sato, T. Yamada and H. Sugiura, J. Cryst. Growth, 1991, 115,241. 5 M. Kamp, F. Konig, G. Morsch and H. Luth, J. Cryst. Growth, 1992,120,124. 6 H. Sato, Appl. Organomet. Chem., 1991,5,207. 7 A. H. Cowley and R. A. Jones, Angew. Chem. Int. Ed. Engl., 1989, 28,1208. 8 D. C. Bradley, M. M. Faktor, M. Scott and E. A. D. White, J. Cryst. Growth, 1986,75, 101. 9 R. Nomura, H. Matsuda, Invited lecture, 65th Annual Meeting of Chemical Society of Japan, Tokyo, March, 1993, Abstract, p.183. 10 R. Nomura, S-J. Inazawa, K. Kanaya and H. Matsuda, Polyhedron, 1989,8,763. 11 R. Nomura, S-I. Miyazaki, T. Nakano and H. Matsuda, Chem. Ber., 1990,123,2081. 12 R. Nomura, K. Konishi and H. Matsuda, Thin Solid Films, 199I, 198,339;J. Electrochem. Soc., 1991, 138,631. 13 R. Nomura, S-I. Miyazaki and H. Matsuda, J. Am. Chem. Soc., 1992,114,2738. 14 R. Nomura, S-I. Miyazaki and H. Matsuda, Organometallics, 1992, 10,2. 15 R. Nomura, S. Fujii and H. Matsuda, Inorg. Chem., 1990, 28, 4586. 16 R. Nomura, S. Fujii, T. Shimokawatoko and H. Matsuda, J. Polym. Sci., Part A, 1992,30, 153. 17 F. F. Blicke and F. D. Smith, J. Am. Chem. Soc., 1929,51, 1558. 18 F. F. Blicke and E. L. Cataline, J. Am. Chem. SOC., 1938,60,423. 19 N. A. Chadaeva, K. A. Mamakov and G. Kh. Kamai, J. Gen. Chem. USSR, 1966,43,821. 20 A. M. Arif, €3. L. Benac, A. H. Cowley, R. J. Jones, K. B. Kidd and C. M. Nunn, New J. Chem., 1988,12,553. 21 G. B. Samsonov and S. B. Drozdova, ed. Handbook ofSulJdes, Metallurgy Press, Moscow, 1972. 22 R. Diehl and R. Nitsche, J. Cryst. Growth, 1975,28, 306. 23 B. StEpanek, V. Sestakova, V. $mid and V. Charvat, J. Cryst. Growth, 1993,126,617. Paper 3/04236G; Received 20th July, 1993