Properties of a-Si:H and a-SiGe:H Films Deposited by Photo-Assisted CVD
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PROPERTIES OF a-Si:H AND a-SiGe:H FILMS DEPOSITED BY PHOTO-ASSISTED CVD R.E.
Rocheleau,
S.C. Jackson.
Institute of Energy Conversion,
S.S. Hegedus and B.N.
Baron
University of Delaware, Newark.
DE 19716
INTRODUCTION Chemical vapor deposition techniques, in particular plasma enhanced CVD, have been used to produce high quality a-Si:H materials. Continuing research is directed toward increased device performance, improved stability, and translation of scale to commercial production. A part of this effort is the evaluation of alternate CVD techniques which in addition to providing technical options for high efficiency and long term stability are likely to lead to improved understanding of the relationships between deposition processes and material properties. A relatively new technique for depositing a-Si:H is photo-CVD which utilizes ultraviolet light to initiate the decomposition of silane or disilane. The best results from both materials properties and device efficiency points of view have been achieved using mercury sensitized photo-CVD. Recently, a 10.5% efficient a-Si:H p-i-n photovoltaic cell, fabricated by photo-CVD, was reported [1]. A limitation in photo-CVD has been preventing deposition on the UV transparent window. In this paper we describe a new photo-CVD reactor with a moveable UV-transparent Teflon film and secondary gas flows to eliminate window fouling. The deposition and opto-electronic characterization of intrinsic a-Si:H and a-SiGe:H and p-type a-SiC:H are described. Finally, preliminary results of p-i-n solar cells are presented. PHOTO-CVD REACTOR AND FILM GROWTH The photo-CVD reactor used in this work is shown schematically in Figure 1. A flexible curtain (Teflon PFA, 1 mil) separates the reactor into two chambers to minimize the transport of
reactants to the windows. Deposition on the window
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Films of a-Si:H, aSiGe:H and a-SiC:H have been deposited by Hgsensitized photolysis using a low pressure mercury vapor lamp (BHK Inc., 2 Model #88-9101-02) rated at 12 mw/cm with 80% of the light output at 254 nm. Silane or disilane were used for a-Si:H; germane with disilane or silane for a-SiGe:H, and methylsilane with silane for a-SiC:H. The range of reaction conditions used included: reactor pressure - 5 to 15 torr; reaction zone flow - 2 to 30 sccm; 10 to 30% reactant gases in reaction zone flow; substrate temperature-165 to 280oC; diluent gases - Ar,He,or H2 . Figure 1:
Photo-CVD Reactor
Mat. Res. Soc. Symp. Proc. Vol. 70. 1986 Materials Research Society
38
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