Refractive index dependence on optical gap in amorphous silicon—Part I. Si prepared by glow discharge

Abstract

We present here our studies concerning the variation of refractive index with the optical gap in amorphous silicon prepared by glow discharge decomposition of silane. The study has been carried out in the light of the models of Penn, Wemple-Didomenico, Ravindra et al., Moss and Bahl-Bhagat. It is essentially seen that the model of Bahl-Bhagat is good enough to explain the relative shifts in the refractive indices in terms of the changes in the gaps on introduction of hydrogen into amorphous silicon. However, because of weaknesses associated with the fitting parameters, we propose here an alternate model which explains fairly well the dependence of the index of refraction on the optical gap. Furthermore, to explain the gradient of the refractive index vs optical gap plots, we see that a simple model like that of Moss would suffice. This is all the more interesting by virtue of the fact that the Moss formula is basically representative of the atomic picture. Of course, the constant depends on conditions during formation of the sample. Under some limiting conditions, the Bahl-Bhagat relation is shown to reduce to the linear form like that of Ravindra et al. We also attempt to analyse qualitatively the dependence of dispersion energy and the average excitation energy on temperature in the light of the Wemple-Didomenico model. The present study has been carried out for samples prepared by glow discharge at different substrate temperatures and with different hydrogen concentrations.