Luminescence processes at chromium in GaAs

In GaAs:Cr, the ${\mathrm{Cr}}^{2+}$ internal luminescence is not observed when excited with above-gap light, because the ${}_{}{}^5{}_{}{}^{}E$ state of ${\mathrm{Cr}}^{2+}$ is above the conduction-band minimum. We report here on experiments which allow one to observe this luminescence: luminescence in ${\mathrm{Ga}}_{x-1\mathrm{}}{\mathrm{Al}}_x\mathrm{As}$:$\mathrm{Cr}$, luminescence in GaAs under hydrostatic pressure, or luminescence in GaAs excited with a yttrium-aluminum-garnet:neodymium laser at 1.32 μm (0.9 eV). The zero-phonon line of that luminescence is observed for the first time and corresponds well to the absorption line. It is shown that in any case the radiative transitions are the results of a balance between the internal transition ($A$) and the band-to-level transition ($B$). Photoluminescence excitation spectra of the various luminescence bands involving ${\mathrm{Cr}}^{2+}$ are reported, and they allow one to confirm the previously proposed schemes. A model is developed, in terms of one-electron orbitals, to explain the characteristics of the photoluminescence excitation processes for ${\mathrm{Cr}}^{2+}$ as well as for other transition-metal ions in III-V materials. Internal luminescence at ${\mathrm{Cr}}^{2+}$ is shown to be mainly excited through the capture of electronhole pairs by ${\mathrm{Cr}}^{2+}$ centers. This model allows one to propose some kind of Auger effect to explain the ${\mathrm{Cr}}^{2+}$-related photoconductivity in III-V compounds. Finally, various band-shape calculations are performed that lead to the conclusion that spin-orbit effects on the band shape are negligible and that the main effect is due to a quadratic Jahn-Teller effect in the ${}_{}{}^5{}_{}{}^{}E$ excited state.