INFLUENCE OF DEPOSITION RATE AND HYDROGEN HELIUM DILUTION ON THE STRUCTURAL RELAXATION OF A-SIGE-H NETWORK PREPARED AT LOW SUBSTRATE-TEMPERATURE

Device quality a-SiGe:H alloy has been developed at low substrate temp erature (T(s) approximately 180-degrees-C), suitable for solar-cell fa brication using low flow rate of silane and germane and strong hydroge n dilution. The optoelectronic properties of a-SiGe:H films prepared w ith different substrate temperature and under different deposition con ditions, i.e., low flow rate of source gases and with/without hydrogen dilution, high flow rate and with hydrogen dilution, low flow rate an d helium dilution, are compared. It is found that both low deposition rate and strong hydrogen/helium dilution are essential to develop a-Si Ge:H material with low defect density. Structural properties, midgap d efect densities, and Urbach energies of the optimized and unoptimized materials are correlated with their optoelectronic properties. The res ults very well support the idea that there should be a fine balance be tween deposition rate and substrate temperature in developing low defe ct density materials. Quantum efficiencies of three Schottky battier d evices were compared where a-SiGe:H layers have been deposited under d ifferent deposition conditions. The results show that both the hole an d electron transport remains almost unaffected as T(s) decreases from 250 to 180-degrees-C.