A method for determining band parameters from the optical absorption edge of amorphous semiconductor: Application to a-Si:H

The dispersion model for fitting the absorption spectrum of amorphous semiconductors is an important method for probing into the electronic structure near the mobility edge. In this paper, a model for describing the absorption coefficient is developed based on optical transitions with a normalized average dipole matrix element. This developed model is employed for fitting the optical absorption edges of annealed a-Si:H films and the values of the optical gap, tail breadth, and mobility gap are determined accordingly. These determined parameters and their corresponding characteristics with structural disorder are consistent with the experimental observations in the literature. A further analysis of the relationship between the optical gap and the band tail breadth reveals that the tail breadth depends linearly on the structural disorder.