Abstract
Effect of secondary-light illuminations of $h\ensuremath{\nu}=0.9$ and 1.45 eV on photoquenched and photoenhanced states in semi-insulating GaAs are investigated using piezoelectric photoacoustic (PPA) measurements at 80 K. It is found that the secondary-light illuminations of 0.9 and 1.45 eV cause an optical recovery of the PPA signal from $\mathrm{EL}{2}^{*}$ to ${\mathrm{EL}2}^{0}.$ We observed an appearance of a broad band at 0.8--1.0 eV in the PPA spectrum after the secondary-light illumination of 0.9 eV. The most important finding is that the PPA spectra after the secondary-light illumination of 0.9 eV both on the photoquenched and on the photoenhanced states are the same in the whole photon-energy region. We conclude that the secondary-light of $h\ensuremath{\nu}=0.9\mathrm{eV}$ induces both an optical recovery and a generation of metastable state of the EL6 level at the same time. The different rates for the transformation of these two processes explain well the observed complex natures of the PPA signal under the secondary-light illumination.
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