Large grain polycrystalline silicon via chemical vapor deposition
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Large grain polycrystalline silicon via chemical vapor deposition Bruce N. Beckloff School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
W. Jack Lackey George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Elliott M. Pickering School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 25 September 1997; accepted 21 August 1998)
Large grain polycrystalline Si films were grown by chemical vapor deposition (CVD) onto TiB2 substrates using the SiCl4 –H2 reagent system. A statistically designed processing study was used to correlate the film growth rate, crystallographic orientation, and grain size with deposition temperature, the SiCl4 : H2 ratio, and the level of B doping. Each process variable influenced grain size with temperature having the dominant effect. Grains as large as 15 to 20 mm were achieved for a coating thickness of about 50 mm.
I. INTRODUCTION
Polycrystalline Si films are desired for a number of optical and electronic applications including photovoltaic cells. For most applications, large grained material is preferred since grain boundaries act as defects. In photovoltaic cells it is desired that any grain boundaries be oriented parallel to the transport direction; i.e., columnar grains are preferred. There is a large body of information on the growth of polycrystalline Si. Fabrication methods include chemical vapor deposition (CVD),1–3 liquid phase epitaxy (LPE),4,5 evaporation,6 and these techniques combined with recrystallization.3,7 In order to conserve material and lower costs, there is renewed interest, particularly for photovoltaics, to prepare thinner films (,50 mm) yet retain large grain size. This is the thrust of the research presented here. The CVD process offers the potential of achieving this goal plus it is amenable to the fabrication of large area devices. There has been extensive research on the CVD of polycrystalline Si.8–26 Often the objective was to increase grain size. Unfortunately, most of these efforts were necessarily limited to low deposition or annealing temperatures (,600 ±C) and thin films (,1 mm) because of other constraints. Nevertheless, this body of information has identified critical processing parameters that influence the grain size of CVD polycrystalline Si. Of all the CVD processes, that for preparing Si is the most widely used and best understood because of the use of CVD Si in numerous electronic devices. There are a number of reagent gases that are used for the deposition of polycrystalline Si. The most common of the precursors employed is silane (SiH4 ) diluted with hydrogen, nitrogen, or an inert gas. Seto21 showed that higher temperatures over the range 650–800 ±C and higher SiH4 concentrations increase the Si deposition 672
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 3, Mar 1999
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rate. Also, SiH4 is expensive and so
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