Optical Transitions in Amorphous Semiconductors

Abstract

In this thesis, a quantitative analysis of the optical response of an amorphous semiconductor is presented. The entire analysis is cast within the framework of an empirical model for the valence band and conduction band density of states functions, that captures the basic features expected of these functions. A novel aspect of this analysis is the introduction of the density of localized valence band and conduction band electronic states and the establishment of a means of evaluating these densities from knowledge of the density of states functions coupled with the locations of the valence band and conduction band mobility edges. The determination of the contributions to the joint density of states function attributable to the various types of optical transitions, as a function of the location of these mobility edges, is another novel feature of this analysis. This formalism is then applied in order to determine the spectral dependence of the normalized dipole matrix element squared average corresponding to such a semiconductor. A means of determining the spectral dependence of the optical absorption coefficient is also provided. Finally, this formalism is applied to the specific case of plasma enhanced chemical vapor deposition deposited hydrogenated amorphous silicon, this being the most widely used amorphous semiconductor at present. It is found that the mobility gap value suggested by Jackson et al. [Physical Review B, vol. 31, pp. 5187-5198, 1985] is discordant with the experimentally measured optical response. It is also found that the effective masses associated with the electrons and holes within plasma enhanced chemical vapor deposition are greater than those that occur in crystalline silicon. The prospects for future work in this field, that builds upon the results presented herein, are commented upon.