Optical properties of indium tin oxide thin-films at high temperature*

Indium tin oxide (ITO) is one of the most widely used transparent conductors. However, its high-temperature optical properties, which are important for device applications in extreme conditions such as space missions, have not been studied. We have investigated the optical properties of ITO thin-films up to 800 °C using in situ optical spectroscopic ellipsometry equipped within a vacuum chamber. The amorphous ITO films show an abrupt change of optical constants at 300 °C due to crystallization. Above 400 °C, the bandgap energy of the ITO thin-film decreases with increasing temperature, implying that electron-phonon interaction broadens the interband optical transitions. The reduction of bandgap energies due to electron-phonon interaction is also observed in epitaxial ITO thin-films. Nevertheless, both amorphous and epitaxial ITO thin-films remain optically transparent even at high temperatures up to 800 °C. In addition, background gas environments reversibly alter the bandgap energies of the epitaxial ITO thin-films, suggesting that the diffusion of oxygen vacancies occurs vigorously.