Indium tin oxide (ITO) is one of the most widely used transparent conductors in optoelectronic device applications. We investigated the optical properties of ITO thin films at high temperatures up to 800 °C using spectroscopic ellipsometry. As temperature increases, amorphous ITO thin films undergo a phase transition at ~ 200 °C and develop polycrystalline phases with increased optical gap energies. The optical gap energies of both polycrystalline and epitaxial ITO thin films decrease with increasing temperature due to electron-phonon interactions. Depending on the background oxygen partial pressure, however, we observed that the optical gap energies exhibit reversible changes, implying that the oxidation and reduction processes occur vigorously due to the low oxidation and reduction potential energies of the ITO thin films at high temperatures. This result suggests that the electronic structure of ITO thin films strongly depends on temperature and oxygen partial pressure while they remain optically transparent, i.e., optical gap energies > 3.6 eV.
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We acknowledge the support of NASA Kentucky under NASA award number NNX15AR69H (for high-temperature optical measurements) and a National Science Foundation grant DMR-1454200 (for thin-film synthesis and characterization). J.K. acknowledges the support from the Global Ph.D. Fellowship Program (2015H1A2A1034200) through the NRF Korea funded by the Ministry of Education.
Kim, Jiwoong; Shrestha, Sujan; Souri, Maryam; Connell, John G.; Park, Sungkyun; and Seo, Ambrose, "High-Temperature Optical Properties of Indium Tin Oxide Thin-Films" (2020). Physics and Astronomy Faculty Publications. 667.