New EPR data and photoinduced changes in GaAs: Cr. Reinterpretation of the "second-acceptor" state asCr4+

Several samples with Fermi levels ranging from the valence to the conduction band show that the resonance previously attributed to ${\mathrm{Cr}}^{1+}$ is due to ${\mathrm{Cr}}^{4+}$, and no additional signal is observed under conditions where ${\mathrm{Cr}}^{1+}$ would be expected to exist. The double-acceptor property postulated for Cr in GaAs is thus questionable. Depending on the Fermi-level location, two different spectra of light-induced changes in the Cr charge states are observed.