Single crystals of ${\mathrm{ZnGa}}_2$${\mathrm{Se}}_4$ and ${\mathrm{ZnGa}}_2$${\mathrm{Se}}_4$:${\mathrm{Co}}^{2+}$ were grown by the chemical transport reaction method. The photoconductivity spectra of these compounds were closely investigated over the temperature range 30–300 K. We observed photoconductivity peaks due to the cobalt impurity at 729 (1.70), 770 (1.61), 808 (1.53), 902 (1.37), and 1021 nm (1.21 eV) at 289 K. The photoconductivity peaks are explained by the electronic transitions from ${\mathrm{Co}}^{2+}$ ions between the valence band, the conduction band, and the split energy levels of the ${}_{}{}^4{}_{}{}^{}\mathrm{T}_1^{}{}_{}{}^{}$ state in $T_d$ point-group symmetry due to spin-orbit coupling effects. The peak at 676 nm (1.83 eV) is attributed to the electronic transitions between the valence band and the defect level of 0.23 eV below the conduction band of ${\mathrm{ZnGa}}_2$${\mathrm{Se}}_4$:${\mathrm{Co}}^{2+}$ single crystals.

• Received 1 December 1988

DOI:https://doi.org/10.1103/PhysRevB.39.8747