Doping and temperature-dependent UV-Vis optical constants of cubic SrTiO3: a combined spectroscopic ellipsometry and first-principles study

SrTiO3 is the model perovskite compound and has demonstrated technological relevance from being the foundation of oxide electronics to a promising candidate as a high-temperature hydrogen sensor. Here, we fully investigate the temperature-dependent UV-Vis optical constants of pristine and doped SrTiO3 in the spectral range 0.73-5.90 eV using spectroscopic ellipsometry and first-principles. The oscillator model is applied to analyze and quantify the temperature and doping effect. After heating SrTiO3 from 300 to 873 K, the amplitude of the dominant absorption peak at 4.752 eV is reduced by 11% and the shoulder of optical spectrum at 3.708 eV gradually disappears. By doping SrTiO3 with Nb element from 0.05 to 0.7% wt, extra momentum is provided to enable indirect absorption and an unconventional Urbach tail is observed due to the increased crystalline disorder. Moreover, the first-principles calculations considering the many-body excitonic effect and phonon-assisted indirect absorption are performed, which help to determine the physics origin of absorption peaks and explain the temperature influence. This work sheds light on temperature and doping dependence of UV-Vis dielectric functions of cubic SrTiO3, aiming to elucidate the novel fundamental optical phenomena of SrTiO3-based heterostructures or devices.