Optical Probing of Inhomogeneities in n-GaAs with Applications to the Acoustoelectric Instabilities

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Abstract
A study is presented of the intimate relationship between current instabilities of acoustoelectric origin in n‐GaAs at 77°K and the inhomogeneity in the Ohmic resistivity of the samples. In order to correlate various aspects of the acoustoelectric instabilities with the sample inhomogeneities, two optical probing techniques were developed for determining the Ohmic‐resistivity profile with good spatial resolution, ⪝0.1 mm. One involves the measurement of the local photoconductance in the sample, which is related to the local dark resistivity; the other method involves the measurement of the change in optical transmission produced by the local thermal shift in the intrinsic absorption edge when the sample is heated by a high current pulse. The change in optical transmission is related to the local resistivity. The acoustic flux distribution in the sample is determined by yet another optical probe, utilizing the modulation in transmission produced by the high acoustic‐energy density in the domain. Several aspects of the acoustoelectric effects are themselves useful for gauging the homogeneity of the samples. In particular, the propagating acoustoelectric domain serves as a convenient probe of the carrier concentration. These techniques are applied to the analysis of the form of the current instabilities, the distribution of the acoustic flux in the sample, and the domain‐formation process. The presence of a somewhat higher resistance, hence higher acoustoelectricgain region, at the upstream end near the cathode is shown to contribute to the formation of propagating acoustoelectric domains.