Reactive Deposition of a-Silicon and Si-Based Alloys
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REACTIVE DEPOSITION OF a-SILICON AND Si-BASED ALLOYS J.HANNA, S.ODA, H.SHIBATA, H.SHIRAI, A.MIYAUCHI, A.TANABE, K.FUKUDA, T.OHTOSHI, O.TOKUHIRO, H.NGUYEN, AND I.SHIMIZU The Graduate School at Nagatsuta, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama, Japan 227 ABSTRACT High photoconductive a-Si:H(F) and St-based alloys were prepared from "precursors" made by oxidation of silane with F2 or reduction of SiFn (n=1,2,) with atomic hydrogen. Both crystalline and amorphous silicon were prepared at will, in the latter case, by controlling the flow of hydrogen. A marked reduction in the localized states in the vicinity of valence band was eatablished in the a-Si:H(F) prepared under optimal condition. A novel photoconductive film with high photoconductive gain for near-tr light was successfully made by multiplying a-Si:H/a-SiGe:H(F) periodically by a new preparation technique without a rise in the dark conductivity. INTRODUCTION Although generally used as the preparation technique of hydrogenated amorphous silicon (a-Si:H) or Si-based alloys"), the glow discharge of silane or mixtures of hydrides has many problems caused by complicated chemical reactions in plasma which are expected to be solved from technical point of view, i.e., acceleration of the growth rate and improvement of the qualities of Si-based alloys. We have proposed novel preparation methods termed "Hydrogen Radical assisted CVD(HR-CVD)" 2 ) and "Chemical Deposition(CD" 3' based on reduction in silicon fluorides with atomic hydrogen and oxidation of silicon hydrides with F2, respectively. With regard to the total chemical reactions in forming silicon networks, simple "redox" reactions would play a mojour role in these techniques. Apart from the complicated details of the mechanisms, atomic hydrogens and F2 are key species in controlling reactions to form precursors, which leads to tools to control chemical reactions directly in these methods. By virture of these tools, Si-based 5 alloys, i.e., a-SiGex:H(F) 4 1, ) and a-SiCx:H:F, 6).7) exhibiting high photoconductivity had been made from the mixture of fluorides under control of the flow of atomic hydrogen. In this study, some distinctive features will be described of films made by these methods, HR-CVD and CD, in respect of their optoelectric properties. EXPERIMENTAL
Preparation
Mat. Res. Soc. Symp. Proc. Vol. 70. c1986 Materials Research Society
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The prepartlon apparatus are schematically Illusterated in Fig. 1. Type (a) is for "CD" consisting of a mixing nozzle and a pedestal to support the substrate. Silane and F2 are mixed homogeneously by passage through the numerous nozzles made separately for each gas. Both (b) and (c) are for "HR-CVD", where atomic hydrogen is prepared by passing H2 through microwave plasma. In the coaxial type (b), fluorides (51F 4, 6eF4 or C2F6) are carried through the inside tube and are exposed to the plasma at its end. Consequently, precursors are made by mixing fragments resulted from plasma-induced decomposition of fluorides and atomic hydrogens. Films
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