We report electroreflectance and photoluminescence studies of the chalcopyrite compounds AgIn${\mathrm{Se}}_2$ and CuIn${\mathrm{Se}}_2$. Observation of photoluminescence at low temperatures at the same energy as the direct energy gaps located by electroreflectance measurements confirms that both compounds have direct band gaps. At 300 °K, the values for the energy gaps are 1.24 and 0.96 eV, respectively. The spin-orbit splittings of the uppermost valence bands as observed in electroreflectance measurements are considerably less than expected for $p$ levels, a result which we attribute to ∼ 17% hybridization of Ag $4d$ levels, and ∼ 34% hybridization of Cu $3d$ levels, with the otherwise $p$-like valence bands. An ultraviolet electroreflectance structure observed in CuIn${\mathrm{Se}}_2$ may result from transitions from the $d$ levels themselves to the lowest conduction-band minimum. The crystal-field and spin-orbit parameters for the uppermost valence bands of CuIn${\mathrm{Se}}_2$ disagree with values found in a recent energy-band calculation ignoring $d$ bands, a calculation which also predicted that CuIn${\mathrm{Se}}_2$ has an indirect energy gap. We also observe an anomalous temperature dependence of the energy gap in AgIn${\mathrm{Se}}_2$. Whereas the energy gap in CdSe (the binary analog of AgIn${\mathrm{Se}}_2$) decreases by approximately 80 meV as the temperature increases from 77 to 300 °K, the energy gap of AgIn${\mathrm{Se}}_2$ is independent of temperature over this range within experimental error (± 5 meV).

• Received 1 December 1972

DOI:https://doi.org/10.1103/PhysRevB.7.4485