Control of a-Si:H Film Properties by Phoyo-Assisted Plasma CVD
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CONTROL OF A-Si:H FILM PROPERTIES BY PHOYO-ASSISTED PLASMA CVD NOBUHIRO FKUDA, KENJI MIYACHI, HIROFUMI TANAKA, TAKASHI IGARASHI AND SADAAKI YAMAMYTO Central Research Institute, Mitsui Toatsu Chemicals Inc., 1190 Kasama-cho, Totsuka-ku, Yokohama, Kanagawa, 247 Japan ABSTRACT 002 laser irradiation onto growing a-Si:H surface caused a decrease of the optical bandgap down to 1.63 eV and a significant increase of the photosensivity. Primary effect of the laser irradiation was a conventional substrate heating, while the optical bandgap narrowing could not be explained by heating effect alone. INTRODUCTION Recent developments in a-Si:H applications to low cost solar cells and photoreceptors raised a strong demand for realizing the high rate deposition. We have recently reported to achieve high quality films at high deposition rate by decomposing disilane in the region exceeding a threshold of the supplied energy by using a conventional glow discharge apparatus[ (]. However, higher electric power density and higher substrate temperature were required. These growth conditions ýere not desirable for fabrications of a-Si devices. In this paper, it is described that the film fabrication conditions were controlled by using light irradiation without changing the plasma conditions and that the resulting film properties were improved. This method was named photo-assisted plasma CVD. In this study, C)2 laser was chosen as such a light not absorbed by the source gas material Si2H6 utilized for high rate deposition. The optical bandgap, conductivity, deposition rate, and hydrogen content were examined. EXPERIMENTAL A-Si:H films were prepared by the plasma deposition technique with OD)2 laser irradiation on the growing surface. A schematic diagram of an apparatus used in this study is shown in Fig.1. Disilane was used as a source gas and decomposed by the glow discharge under such conditions that RF power density was 0.1 to 0.3 W/cm2, gas flow rate 10 to 30 sccm, pressure 0.25 to 1.0 Torr and 00O2 laser power at 10.6 pm band 0.15 to 3.0 W. The laser beam size was about 8 mm in diameter. The CO2 laser beam was introduced into the stainless steel chamber through a ZnSe window. Corning 7059 substrates were set as shown in Fig.1 (a),(b),(c) and not heated by any external heater. The main results in this paper are concerned with the substrate configuration of Fig.1 (b). RESULTS AND DISCUSSION Laser Power Dependence of Film Properties The conductivity and optical bandgap of the films as a function of the
Mat. Res. Soc. Symp. Proc. Vol 70. 1986 Materials Research Society
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Fig.2 Photo and darkconductivity and optical bandgap as a function of 00 2 laser power.
laser power are shown in Fig.2. These films were grown under the laser irradiation no
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