Structural and thermal properties of ternary narrow-gap oxide semiconductor; Wurtzite-derived β-CuGaO₂

The crystal structure of the wurtzite-derived β-CuGaO₂ was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I-III-VI₂ and II-VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the β-CuGaO₂ from an ideal wurtzite structure are small. The direct band-gap nature of the β-CuGaO₂, unlike β-Ag(Ga,Al)O₂, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the β-CuGaO₂ irreversibly transforms into delafossite α-CuGaO₂ at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately -32 kJ mol^-1, from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in β-CuGaO₂ was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa₂O₄ spinel and CuO at temperatures >350°C. These thermal properties indicate that β-CuGaO₂ is stable at ≤300°C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300°C.