TRANSIENT ACOUSTOELECTRIC SPECTROSCOPY MEASUREMENTS FOR THE CHARACTERIZATION OF GAAS EPILAYER STRUCTURES

A new measurement technique, acoustoelectric deep-level transient spec troscopy (AE-DLTS) is used to characterize the GaAs epilayer film grow n on a semi-insulating GaAs substrate. The electric field, generated b y the propagation of a surface acoustic wave (SAW) On a piezoelectric crystal, is used as a probing tool to study the transient behaviour of deep levels. This field interacts with the free carriers present in t he semiconductor, resulting in an alteration of the carrier density at the surface of the semiconductor and the generation of a cc voltage. The rate at which excess charges are induced at the semiconductor surf ace, or diminished after the passage of the SAW pulse, is related to t he position and the cross section of the deep levels at the interface between the epilayer and the substrate. The theoretical analysis of th e fall time of the Dc voltage is presented along with experimental ver ification. In our experiments the voltage transients were recorded as a function of incident photon energy and at different temperatures, an d the thermal and optical cross sections of the EL2 level in GaAs were evaluated from computer fit.