(The) influence of residual gases on electricaland optical properties of a-Si and a-Si:H thin films

Abstract

In this thesis, we have investigated the influence of residual gases including oxygen, nitrogen and air on the electrical and optical properties of electron beam evaporated a-Si and r.f. sputtered a-Si:H. The properties of films have been characterized by dark conductivity, photoconductivity, optical absorption, optical gap, ESR and infrared absorption. It is shown that residual impurities can be incorporated into a-Si during vacuum evaporation in conventional high vacuum of $\sim3\times10^{-7}$ torr. The principal contaminant during vacuum evaporation is found to be oxygen impurity. The electrical and optical properties of a-Si under various deposition conditions could be explained by the degree of contamination which decreases with increasing either substrate temperature or deposition rate. From the annealing studies of spin density and optical gap for a-Si films, it is shown that the generation of dangling bond by high temperature annealing is closely related to the residual impurity incorporation into a-Si during sample preparation. The possible mechanisms of defect generation in conjunction with annealing measurements of infrared absorption were also discussed. The properties of a-Si:H films have been investigated by varying deposition parameters including r.f. sputtering power, total reactive gas pressure and hydrogen partial pressure. It is shown that the changes of electrical and optical properties with decreasing the total gas pressure ($P_T$) at constant \% hydrogen partial pressure from 20 m-torr to 5 m-torr is very similar to those produced by increasing r.f. power above 500 W at constant $P_T$. This result could be explained in terms of energetic positive ion bombardment. In connection to this the influence of hydrogen on optical band gap was discussed in terms of modification of silicon matrix rather than the simple elimination of localized states by hydrogen. The role of oxygen impurity in a-Si:H is found to be quite different from that of nitrog...