Effects of Electrode Spacing and Hydrogen Dilution on a -SiC:H and a -Si:H Layers
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EFFECTS OF ELECTRODE SPACING AND HYDROGEN DILUTION ON a-SiC:H AND a-Si:H LAYERS
J. DAEY OUWENS*, R.E.I. SCHROPP*, C.H.M. VAN DER WERF*, M.B. VON DER LINDEN*, C.H.M. MAREE*, W.F. VAN DER WEG*, P. RAVA**, F. DEMICHELIS* *
C.F. PIRRI* * * AND E. TRESSO* * * *Debye Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands **Elettrorava S.p.A., Savonera Torino, Italy * * *Dipartimento di Fisica Politecnico, Torino, Italy ABSTRACT A series of hydrogenated amorphous silicon carbide (a-Sil.CG:H) films was deposited by rf glow discharge deposition using various pressures, electrode spacings and hydrogen dilution ratios. We found that improvement of the structure by hydrogen dilution is more effective when a large electrode spacing is applied. In the case of undiluted a-SiC:H, the product of pressure and electrode spacing appears to be the important parameter. Dilution causes an increase of the photoconductivity. The band gap decreases but increases again for highly diluted samples. A striking result is that the Fourier transform infra-red spectroscopy (FTIR) bands assigned to CH. and SiH,, increase upon dilution when a small electrode spacing is applied, although the hydrogen content is reduced. It is shown that this is due to an increase of the density of the films and to an increase of the amount of carbon built into the bulk instead of into voids. The combination of decreasing hydrogen content, void fraction and increasing amount of carbon atoms into the bulk explains the behaviour of the photoconductivity and band gap as a function of H2 dilution. INTRODUCTION The addition of hydrogen to silane and methane during rf-deposition of a-Si:H and a-SiC:H carbide layers is suggested to have a positive effect on the quality of the materials [1,2,3]. The average energy of the bombarding ions is higher because of a higher plasma potential causing an increase of the mobility of the radicals on the surface. The void fraction and hydrogen content of the films are reduced. The amount of microstructure (Sill,, CH, where x > 1) is also reduced. This will lead to an increase of the photoconductivity and atomic density, and a decrease of the band gap. However, we found that dilution did not increase the quality of intrinsic a-Si:H films, deposited at a small electrode spacing. Therefore, we studied the effects of dilution at several spacings using various deposition conditions. EXPERIMENTAL We deposited two a-SiC:H series at Utrecht University with different pressures p and electrode spacings d, keeping the p x d product constant. Another a-SiC:H series where the p x d product was varied systematically was deposited by Politecnico di Torino. The PECVD deposition systems used are virtually identical and consist of three UHV chambers located around a central isolation and transportation chamber. The systems were designed by MVSystems Inc., Golden, Colorado and constructed jointly with Elettrorava S.p.A., Torino. A careful optimization of the p/i/n structure at Utrecht University has recently led to solar cells with
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