Anomalous temperature dependence of optical properties of cubic MgZnO: Effect of carrier localization

We investigated the temperature-dependent optical transmission spectra in reactive-electron-beam-evaporation (REBE)-grown cubic MgZnO thin films and extracted the band gap Eg, broadening factor and refractive index. Incomplete and complete “S-shaped” thermal dependence of Eg are observed in Mg0.77Zn0.23O and Mg0.55Zn0.45O, respectively. This behavior is caused by carrier localization induced by potential fluctuation or possible other disorder mechanisms, such as phase separation and phase transition. Also, the broadening factor and refractive index exhibit similar anomalous temperature dependence, supporting the occurrence of carrier localization. Furthermore, the magnitude of carrier localization was estimated by simulating the anomalous behaviors of Eg with different models.