Film Deposition Process in Pulse Discharge CVD
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FILM DEPOSITION PROCESS IN PULSE DISCHARGE CVD T.Yoshida, H.Fujisawa, T.Hokaya, Y.Ichikawa and H. Sakai Fuji Electric Corporate Research and Development, Ltd. Yokosuka, Kanagawa 240-01, Japan
Nagasaka2-2-1,
ABSTRACT To understand the deposition process of pulse discharge (PD) CVD, we studied spatial distribution and decay of plasma parameters and those of SiH LIF signals. In addition, we studied film properties of a-SiC:H and a-SiGe:H deposited by the PD-CVD. It was found that the quality of p-type a-SiC:H films deposited by the PD-CVD from SiH4+CH 4 +BF 3 was better than those of conventional p-type a-SiC:H films. The p-type a-SiC:H films were applied to the p-layer of p-i-n a-Si solar cells, and a conversion efficiency of 12.3% was obtained for a lcd cell. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) based solar cells have made remarkable progress last several years. Recently we reported that the total area efficiency exceeded 10% for 30cm x 40cm double-junction tandem submodules[1]. Moreover, the light-induced degradation of the double-junction tandem modules was found to be less than 15% after a 1000 hours light exposure test under 100mW/crd simulated sunlight [1]. To improve the conversion efficiency further, we focussed on improving both film quality of a-Si based alloys and the device structure of a-Si solar cells[2]. We believe that improvement in quality of a-Si:H based alloys is directly connected to the increase in conversion efficiency of a-Si solar cells. Film deposition technique is a key to improve the quality of the alloys. Recently we demonstrated a new CVD technique employing pulse discharge, which enables us to deposit high quality intrinsic a-SiC:H and p-type a-SiC:H films [3]. In this technique, we supply rectangular pulse voltage with a small duty-cycle to the cathode periodically. This generates a pulse discharge, in which a gas with higher dissociation energy than SiH4 is easily decomposed in gas mixture with SiH 4 , such as CF4 (or BF3 ) in SiH 4 . Moreover, the quality of these films deposited by PD-CVD exceeds that of the conventional: p-CVD films. These features result from radical generation, transport and surface reaction processes. In this paper, to understand the deposition process of the PD-CVD, we studied the spatial distribution and decay of the plasma parameters and those of short-lifetime radicals such as SiH. From these results, we discuss the advantages of the PD-CVD. In addition, we present and discuss the film properties of a-SiC:H and a-SiGe:H deposited by this technique. Performance of a-Si:H solar cells with a-SiC:H p-layer deposited by the PD-CVD is also presented. EXPERIMENTAL We used a reactor with conventional parallel plate electrodes with 12cm diameter; one electrode was used as a cathode to which we applied rectangular voltage pulses and the other was used as an anode to set Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
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Table I Typical discharge conditions.
Pressure Pulse period Pulse width Pulse voltage
(torr) T W V
(mis
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