Optical Transient Current Spectroscopy for Trapping Levels in Semi‐Insulating LEC Gallium Arsenide

Abstract

Optical transient current spectroscopy (OTCS) is a member of the class of deep level transient spectroscopy (DLTS) techniques. Its advantage is that, unlike other DLTS techniques, it can be applied to semi‐insulating (which is the starting material for integrated circuit (IC) fabrication). The aim is to obtain information on deep trapping levels, which may be of fundamental interest and is certainly of practical interest since it can help in diagnosing the suitability of the material for IC fabrication. We have investigated the method using different device geometries, electrode materials, surface preparations, and crystals. Various “peaks” in the OTCS spectrum were identified, including EL2, the center responsible for the semi‐insulating property in undoped LEC . One is of particular interest, since it is a negative peak, i.e., the current increases (instead of decreasing) with time (after the sudden drop when the light ceases) until it settles at its steady‐state dark value. The amplitude of this peak was found to be much bigger with thinned sandwich structures than with planar electrode structures. Evidence was obtained that this peak is associated with crystal damage produced by abrasion or polishing. The current theory of OTCS due to Hurtes et al. and Martin et al. is based on the Sah et al. treatment of depletion regions. It is pointed out that it may not necessarily apply with typical electrode geometries. In particular, the model that this theory provides for negative peaks is not the only possible one. Two further models are proposed. Negative peaks could be due to an electrically neutral semiconductor model in which more majority carriers are trapped due to illumination. They could also be given by an insulator model in which the current is due to thermal release of carriers from traps that then travel a certain way before recapture.