Optical and Electrical Properties of AgGaS2and AgGaSe2

From the results of low-temperature luminescence and reflectivity, both AgGa${\mathrm{S}}_2$ and AgGa${\mathrm{Se}}_2$ are determined to have a direct energy band gap. The values are 2.727 and 1.830 eV at 2 °K, respectively. The gap shifts to slightly higher energy at 77 °K, which is opposite to that observed in most semiconductors. Both crystals appear to contain shallow impurities or defects. However, the crystals are semi-insulating as-grown, and various annealing and diffusion procedures have failed to produce useful conductivity.