Effects of electrode geometry and polarity on the occurrence of negative peaks in optical transient current spectroscopy applied to semi-insulating gallium arsenide

Abstract

Optical transient current spectroscopy (OTCS) involves observing the transient decay of “photoconductivity” and is of interest as a means of characterizing semi-insulating GaAs wafers. We report experimental results which show that the OTCS spectrum and, in particular, the occurrence of negative peaks is strongly dependent on the shape, size and electrode configuration of the specimen, as well as on the magnitude of the applied voltage and (with asymmetric electrodes) its polarity. A negative peak sometimes changes to a current oscillation with increasing voltage, probably indicating field enhanced trapping. Negative peaks may be observed with sandwich structures with a transparent electrode covering the illuminated surface. This shows that a recent proposal that surface states are involved is at least not always correct. It is suggested that several mechanisms can give negative peaks, and probably more than one has done so under the various experimental conditions which have been used. The present work shows that any practical application of OTCS for ingot qualification will be dependent on standardization of specimen geometry, electrodes and operating parameters.