Effect of Doping SrTiO3 with Nb Studied with Wide-Range Spectroscopic Ellipsometry

The low-temperature optical properties of a SrTiO3 crystal doped with 0.7% wt. Nb and a pure SrTiO3 reference crystal were studied using spectroscopic ellipsometry. The optical constants and dielectric functions were obtained in the spectral range of 0.8-8.8 eV at temperatures from 10 to 300 K, and the optical conductivity was obtained in the spectral range of 0.03-1 eV at 300 K. Analysis of the optical conductivity spectra in the infrared spectral range confirmed the presence of free electrons and additional absorption hump between 0.1 and 0.4 eV (806 and 3226 cm(-1)) in doped SrTiO3 and optical phonons in both doped and undoped SrTiO3. The performed analysis of optical absorption spectra in the range 0.8-8.8 eV revealed frustration of the indirect bandgap and an increase in the direct bandgap energy by similar to 0.03 eV in Nb-doped SrTiO3, compared to undoped SrTiO3 over the whole temperature range. The energies of the peaks' maxima of the dielectric function spectra did not significantly differ for doped and pure SrTiO3. For both Nb-doped and pure SrTiO3, temperature dependence of the direct bandgap energy and that of the index of refraction showed inflection between 100 and 150 K, which may be considered evidence of an antiferrodistortive phase transition from a cubic to a tetragonal structure.