Mechanisms of temperature- and field-dependent effective drift mobilities and impact ionization coefficients in amorphous selenium

Abstract

The mechanisms of electric-field- and temperature-dependent effective drift mobility and impact ionization coefficient of both holes and electrons in amorphous selenium (a-Se) are investigated in this paper. An analytical model for the microscopic mobility, momentum relaxation mean free path, and hence the effective drift mobility and impact ionization coefficient of carriers, is proposed in this paper by considering the density of states distribution, field enhancement release rate from the shallow traps, and carrier heating. The results of the model are fitted with the published experimental results on effective mobility and impact ionization coefficient with wide variations of the applied electric field and temperature. A better fitting considering thermally activated tunneling for the field-enhancement release rate indicates that the effective drift mobility at extremely high fields is mainly controlled by the neutral defect states near the band edges. The density of state function near the band edges, consisting of an exponential tail and a Gaussian peak, can successfully describe the electric-field- and temperature-dependent effective drift mobility characteristics in a-Se. The momentum relaxation mean free path decreases with increasing field and decreasing temperature, which is required to describe the electric-field- and temperature-dependent behaviors of impact ionization coefficient in a-Se.