Free-carrier and defect absorption in gallium oxide

Transparent conducting oxides (TCOs) are a technologically important class of materials used in optoelectronic devices, as TCOs balance two conflicting properties: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higher-lying conduction band states, or by an indirect process mediated by phonons or charged impurities. Defects can also lead to additional absorption.

We performed a detailed first-principles study of these absorption processes in Ga₂O₃ [1,2], a material with promising applications in high-power devices and UV photodetectors. Our results elucidate the fundamental limitations of optical absorption in Ga₂O₃, which have subsequently been confirmed experimentally [3], and explain observed defect-related absorption [4].