Control of plasma chemistry for preparing highly stabilized amorphous silicon at high growth rate

Recent progress in the growth process of hydrogenated amorphous silicon (a-Si:H) from a silane glow-discharge plasma is reviewed, being focused on the improvement of photo-induced degradation properties in a-Si:H deposited at high growth rate. Contribution of higher-silane related reactive species (HSRS) during film growth is suggested as a key event to increase the degree of photo-induced degradation in the resulting a-Si:H through an incorporation of excess Si–H2 bonds in the network as far as the substrate temperature is kept constant. The contribution ratio of HSRS to film growth is derived using a couple of gas-phase reaction-rate equations, which shows strong dependence on electron temperature, hydrogen-dilution ratio, gas-flow rate, and gas temperature in the plasma. Various methods are adopted to reduce the contribution ratio of HSRS during film growth and the validity of theoretical approach is confirmed experimentally. Finally, stabilized conversion efficiency of 8.2% is demonstrated in the a-Si:H-based single junction solar cell with high deposition rate of 2.0 nm/s.