Differential Optical Spectra and Band Structure of SrTiO3

The results of electroabsorption measurements at 80 °K confirm the indirect nature of the absorption edge of SrTi${\mathrm{O}}_3$, but suggests that the absorption observed between 3.27 and 3.32 eV is mainly a result of a direct unallowed transition. The threshold for this process is at 3.292 ± 0.002 eV. The thermoreflectance spectrum of SrTi${\mathrm{O}}_3$ near room temperature has been measured in the spectral range 3.0-5.8 eV. The derivative of the reflection coefficient with respect to photon energy in the same range was also measured. The Kramers-Kronig transforms of these two spectra and of the electroreflectance spectrum are calculated. Comparison of the results of these spectra show good correspondence in peak positions among the three different measurements. Based on this comparison the appropriate critical energies are deduced. The number of the observed critical points is, however, much less than expected from the theoretical band structure of SrTi${\mathrm{O}}_3$. A qualitative method is suggested to eliminate weak transitions. The experimental critical energies are found to correspond to theoretical transitions classified as strong to within 0.2 eV from the energies predicted by Kahn and Leyendecker. Positive peaks correspond to transitions at $\chi$ or $\Gamma$, while negative peaks correspond to transitions at $M$. This again is in agreement with the theoretical model.