Abstract
Surface acoustic waves (SAW's) were used to study the influence of defects on the elastic properties of epitaxial films of semiconductors. The object of this study was As-rich GaAs grown by molecular-beam-epitaxy at 220 \ifmmode^\circ\else\textdegree\fi{}C. The SAW velocity on 0.3-wavelength-thick epilayers was 1.2% smaller than on the substrate alone. That velocity difference decreased after loss of some excess As as a result of 350 \ifmmode^\circ\else\textdegree\fi{}C--435 \ifmmode^\circ\else\textdegree\fi{}C anneals. A persistent increase as much as 0.4% of the SAW velocity at low temperatures was observed after illumination; this increase could be quenched by annealing at 120--130 K. This behavior is caused by the metastable transition of EL2-like ${\mathrm{As}}_{\mathrm{Ga}}$ defects and constitutes the direct experimental proof of the illumination-induced large lattice relaxation of this defect. The SAW velocity increase was correlated with the persistent bleaching of EL2-related optical absorption. The spectral dependence of rate of illumination-induced SAW velocity increase was measured.
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