Optical Properties of SrTiO3in the Region of the Absorption Edge

The absorption coefficient, reflectivity, and electroreflectance of SrTi${\mathrm{O}}_3$ in the neighborhood of the fundamental absorption edge are reported. The absorption coefficient shows an exponential rise with incident energy (Urbach rule) over five orders of magnitude. Its logarithm has a slope of $\frac{1}{\mathrm{kT}}$. The data indicate that the band gap of SrTi${\mathrm{O}}_3$ is about 3.4 eV. Additional results for reduced SrTi${\mathrm{O}}_3$ show that Redfield's model for an exponential edge is not applicable to this material. The reflectivity indicates structure in the region of 3.4 eV and the electroreflectance shows an appreciable signal from 2.5 to 5.0 eV with some structure near 3.4 eV. The electroreflectance signal appears to be due to some process other than the Franz-Keldysh effect. It could be due to a shift or splitting in the major oscillator (4.4 eV) responsible for the refractive index, with induced lattice polarization.