Transient photoconductivity and thermalization in amorphous semiconductors

Abstract

In a number of recent publications, transient photoconductivity data for amorphous semiconductors have been interpreted in terms of the thermalization of excess charge carriers with localized states which are distributed over a range of energy. A central feature of the ‘intuitive thermalization’ model, which has been applied in some such analyses, concerns the introduction of a demarcation energy separating ‘shallow’ from ‘deep’ traps. The position of this demarcation energy is defined solely in terms of localized state release time constants, and it is assumed that ‘shallow’ centres are in quasi-thermal equilibrium with extended states.In the present paper, we examine the validity of this approach, and the consequences of application of such a model in the analysis of localized state distributions. It is demonstrated that the thermalization process is a complex function of trapping and release time constants, and that the influence of deep traps prevents the establishment of quasi-thermal equilibrium for shallower centres. We show that the model will, in the general case, yield erroneous results for the energy distribution of localized states.