Chem. Mater. 1993,5, 1344-1351
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Chemical Vapor Deposition of Gallium Sulfide: Phase Control by Molecular Design Andrew N. MacInnes,? Michael B. Power,? and Andrew R. Barron' Department of Chemistry, Haruard University, Cambridge, Massachusetts 02138 Received April 30, 1993. Revised Manuscript Received July 8, 1 9 9 P
Gallium sulfide (Gas) thin films have been grown at 380-420 "C by atmospheric pressure metal-organic chemical vapor deposition (MOCVD) using the single-source precursors [(tB~)2Ga(StBu)]2, [ (tBu)GaS]4,and [(tBu)GaS17. Characterization of the films by X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), and energy-dispersive X-ray (EDX) analysis shows all the films t o be of chemical composition Ga:S (1:l). However, from transmission electron microscopy (TEM) and X-ray diffraction (XRD) the film structure was found to be dependent on the molecular precursor. In the case of films grown from [(tBu)2Ga(StBu)12 deposition results in the formation of the thermodynamic hexagonal phase of Gas. Deposition using [(tBu)GaS14 as the precursor gives a novel metastable face-centered cubic phase of Gas. Use of [(tBu)GaS]7as the precursor results in amorphous films. The relationship between the molecular precursor and the deposited films is discussed in terms of the possibility of molecular control over solid-state phase synthesis.
Introduction "Form follows function"is an axiom traditionallyapplied to the ergonomic design of consumer goods in the macroscopic world. It has become chic, however, to apply this concept to the microscopic world. In particular, many workers have recently expounded the idea of "molecules to materials", in which molecular species are converted, by either thermal or photochemical decomposition, to simple inorganic materials such as oxides, carbides, phosphides, etc. The vast majority of chemical studies have employed molecules which contain all the elements required in the final material, hence the term "singlesource precursorn,and have often consisted of a molecular structure related (in part) to the structure of required phase. Thus, in the earliest studies, for a material ME, precursors with M-E bonds were investigated.' More recently,researchers have synthesizedmolecules with core structures designed to mimic a fragment of the target solidstate phase. While this concept has led to the elegant synthesis of known materials,2 especially in the field of 111-V semiconductor c o m p o ~ n d sit, ~is only recently that previously unreported (or unstable) phases (and materials) have been prepared. The groups of Gladfelter4 and Wolczanski5 have reported the formation of cubic GaN and cubic TaN respectively, by the solid-state pyrolysis of molecular precursors whose metal-nitrogen cores are subunits of the resulting solid-state phase. In a recent publication6 we have expanded these results to the vapor + Present address: Gallia, Inc., Weston, MA 02193.
* To whom correspondence should be addressed.
*Abstract published in Advance ACS Abstracts, August 15, 1993. (1) For recent review articles see: (a) Bradley, D. C. Chem. Rev. 1989, 89,1317. (b) Girolami, G. S.; Gozum, J. E. Mater. Res. SOC.,Symp. Proc. 1990,168, 319. (2) See for example: (a) Interrante, L. V.; Sigel, G.; Garbauskas, M.; Hejna, C.; Slack, G. A. Inorg. Chem. 1989, 28, 252. (b) Sula, F. C.: Interrante, L. V.; Jiang, 2.Znorg. Chem. 1990,29,2899. (3) See for example, Cowley, A. H.; Harris, P. R.; Jones, R. A.; Nunn, C. M. Organometallics 1991,10, 652. (4) Hwang, J.-W.; Hanson, S. A.; Britton, D.; Evans, J. F.; Jensen, K. F.; Gladfelter, W. L. Chem. Mater. 1990,2,342. (5) BanaszakHoll, M. M.; Wolczanski,P. T.; VanDuyne, G. D. J.Am. Chem. SOC. 1990,112,7989.
phase. The metal-organic chemical vapor deposition (MOCVD) of InS using the dimeric indium thiolate precursor [(tBu)21n(StBu)l~ does not yield the thermodynamic orthorhombic phase but rather the metastable high-pressure tetragonal phase.6 This result prompted us to further study the possibility of controlling the deposited phase by the molecular design of the precursor. Our recent studies of the chemistry of gallium thiolate and sulfide compounds7-gprovided us with a sequence of possible precursor compounds with distinctive structures. In a preliminary communication,10 we reported the MOCVD of a new cubic phase of GaS using the cubane precursor [(tBu)GaS14. We now describe full details of the MOCVD of GaS thin films from three structurally distinct precursors,dimeric (tBu)2Ga(StBu)12,tetrameric [(tBu)GaS14,and heptameric [(tBu)GaS17and discuss the control they exert on the structure of the resulting films.
Results and Discussion The majority of previous work on single-source precursors has been related to the growth of 111-V semiconductor materials and has concentrated on the dimeric and trimeric pnictides I and 11.11 It is unsurprising, therefore, that in the area of group 111-chalcogenide CVD, analogous thiolate6J2J3 and sele(6) MacInnes, A. N.; Cleaver, W. M.; Barron, A. R.; Power, M. B.; Hepp, A. F. Adv. Mater. Opt. Electron. 1992,1,229. (7) Power, M. B.; Barron, A. R. J. Chem. Soc., Chem. Commun. 1991, 1315. (8) Power, M. B.; Ziller, J. W.; Tyler, A. N.; Barron, A. R. Organometallics 1992,11, 1055. (9) Power, M. B.; Ziller, J. W.; Barron, A. R. Organometallics 1992, 11, 2783. (10) MacInnes, A. N.;Power, M. B.;Barron, A. R. Chem.Mater. 1992, 4, 11. (11) For a recent review article see: Jones, A. C. Chemtronics, 1989, 4, 15.
(12) (a) Nomura, R.; Konishi, K.; Matsuda, H. Thin Solid Film 1991, 198,339. (b) Nomura, R.; Konishi, K.; Futenma, S.; Matsuda, H. Appl. Organomet. Chem. 1990,4,607. (c) Nomura, R.; Fujii, S.; Kanaya, K.; Matsuda, H. Polyhedron 1990,9,361. (13) MacInnes, A. N.; Barron, A. R.; Power, M. B.; Hepp, A. F. J. Organomet. Chem. 1993,449,95.
Q891-4756/93/28Q5-1344$Q4.QQ/Q0 1993 American Chemical Society
Chem. Mater., Vol. 5 , No. 9, 1993 1345
CVD of Gallium Sulfide
M = AI. Ca. In: E = N. P. As. Sh (1)
111)
M = Ga. I": E =
s. sc
1111)
nolate" compounds 111 have been employed as singlesource precursors for indium sulfide and selenide thin films, respectively. On the basis of precedent, it is reasonable to postulate that the gallium thiolate dimer [('Bu)&a(S'Bu)l~ (IV)*should be a suitable precursor '0"
I
F G
ai
IlVI
(VI
(VI)
for Gas. We have previously noteds that deposition using [('BU)ZI~(S~BU)IZ below 380 O C results in the formation
of sulfur-deficient films with 'BuSS'Bu as a volatile coproduct and have proposed that this is due to the higher relative bond strength of the S-C bond than the In-S bond.6 While such an effect may not be an issue for the (14) Gysling. 1992.4,m.
H.J.: Wernberg, A. A,: Blanton. T.N. Chem. Mater.
1346 Chem. Mater., Vol. 5, No. 9,1993
MacInnes et al.
Table I. Summary of the Dependence of the Gas Phase with Precursor and Deposition Temperature precursor
deposition temp ( O C )
400 475
400
