The demarcation energy concept in the analysis of charge carrier emission kinetics from deep mobility gap states in amorphous semiconductors

Abstract

The dark discharge of surface potential on corona or capacitively charged amorphous semiconductors via charge carrier emission from an energy distribution of deep bulk or surface localised states in the mobility gap has been recently studied by a number of authors. The demarcation energy concept has been widely used to extract the necessary theoretical expressions to analyse the surface potential decay experiments. The demarcation energy concept is analytically examined by the author to derive the conditions under which it remains valid. It is shown that for an arbitrary distribution of localised states, the fractional rate of change of the density of states function N(E) with respect to energy at the demarcation energy E'=kTIn vt, over approximately 1/2kT, must be less than unity. In the case of an exponentially decaying distribution of mid-gap states the demarcation energy approach predicts the correct functional time dependence of the rate of charge carrier emission. As an example, the decay of surface potential on an amorphous semiconductor due to field-enhanced surface generation from an exponential distribution of surface mid-gap states is considered and a theoretical expression is derived for the time evolution of the surface potential.