Distortions in optical deep level transient spectroscopy on semi-insulating GaAs with a conducting layer of finite thickness

Abstract

The use of optical deep level transient spectroscopy with intrinsic illumination to characterize deep acceptor levels in thin conducting layers on semi-insulating GaAs is shown to generate a majority-carrierlike peak that is, in fact, erroneous. A quantitative model is presented which explains this peak in terms of conversion of the semi-insulating bulk GaAs below the active layer to p type due to the filling of unoccupied EL2 traps. This type of conversion induces a space-charge region at the active layer–bulk interface which increases the resistance in series with the Schottky-barrier depletion-layer capacitance. The relaxation of the series resistance to its unperturbed value appears as a change in the measured capacitance, thereby creating a false majority-carrier peak. It is emphasized that the false peak shifts the apparent position of nearby minority-carrier peaks, thus seriously distorting the true ODLTS spectrum and making the identification of key minority-carrier traps much more difficult.