Density Of States In Amorphous Silicon

Abstract

Density of midgap defect states (DOS) in amorphous silicon (a-Si:H) is a fundamental material parameter in determining the transport properties. It is shown in this paper that the commonly believed model DOS, consisting of one major mid-gap defect, namely a Si dangling bond, is probably not correct, and is logically inconsistent with the position of the Fermi level. The dangling-bond DOS is also inconsistent with data from space-charge-limited-current, with experimentally determined device field profiles, and with some data on photo-degradation. In contrast, we propose that a DOS model proposed by Adler, based on negative corelation energy and consisting of primarily doubly charged and empty dangling bonds (T.- and TaAƒÂ· states), is a more accurate representation of actual DOS in undoped, high quality a-Si:H. This model is shown to be consistent with all the experimental data on material and device properties, including photo-degradation and doping.