Phonon scattering from residual defects in GaAs: Observation of optically induced metastability by phonon imaging

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Abstract
Phonon imaging with frequency-selective detectors is used to probe the scattering from residual defects in semi-insulating GaAs. As in previous heat-pulse studies, we find that the transmission of non- equilibrium phonons with longitudinal and transverse polarizations can be modified by near-infrared excitation of the crystal at low temperatures. These effects have been attributed to metastable states of the ubiquitous EL2 defect in liquid-encapsulated-Czochralski-grown GaAs. In the present work we use the dispersive shifts in the phonon-focusing pattern to gain information about the frequency dependence of these optically induced changes. For particular polarizations and propagation directions, near-infrared excitation causes a huge decrease in the scattering of phonons with frequencies above 700 GHz. Our results show that, unlike the elastic scattering of simple mass defects, the residual defects are effective inelastic scatterers of phonons. An analysis of our data based on the theory of Nowick and Berry indicates that the change in the transmission of phonons following photoexcitation cannot be attributed to a unique symmetry change of a single defect.