Electrical Properties, Optical Properties, and Band Structure of CuGaS2and CuInS2

Various optical and electrical properties of the I-III-V${\mathrm{I}}_2$ compounds CuGa${\mathrm{S}}_2$ and CuIn${\mathrm{S}}_2$ have been studied. From the results of low-temperature luminescence and reflectivity, both crystals are determined to have a direct band gap. The band gaps at 2 °K are 2.53 eV for CuGa${\mathrm{S}}_2$ and 1.55 eV for CuIn${\mathrm{S}}_2$. CuIn${\mathrm{S}}_2$ has been made conducting both $n$ and $p$ type, while CuGa${\mathrm{S}}_2$ has been made $p$ type only. Electroreflectance measurements have been performed in an attempt to determine the band structure. The highest valence band appears to be a doublet with a large admixture of Cu $3d$ wave functions.