Photoluminescence in SrTiO3

At 77 K photoluminescence from nominally pure, 0.05% Sm-doped and 0.005% Cr-doped SrTi${\mathrm{O}}_3$ gives identical spectra in the infrared (I.R.) with an intensity ratio of 1:20:400, respectively. In undoped and Sm-doped specimens, the intensity of emission is vanishingly weak at 4 K, in contrast to Cr-doped SrTi${\mathrm{O}}_3$, in which it is practically unchanged relative to that at 77 K. At 77 K, the spectrum consists of a narrow line at 1.5625 eV of half-width 0.5×${10}^3$ eV, which we identify as the zero-phonon line, and 31 satellite lines, which are shown to be vibronics due to lattice phonons from the center and edge of the Brillouin zone. At 4 K the zero-phonon line is a doublet. Comparison with the reported lattice phonon spectrum is satisfactory. In addition, the data suggest the energies of several zone boundary phonons which have not been reported. In spite of identical emission spectra, undoped, Sm-doped, and Cr-doped specimens have different excitation spectra. This suggests energy transfer from Sm or Cr to a tightly bound center, isolated from the electronic energy bands, such as typically arise from rare-earth ions in ionic crystals. It is suggested that the center is an intrinsic defect or an exciton. In undoped SrTi${\mathrm{O}}_3$ an emission band in the visible—2.175 eV (77 K), 2.475 eV (4 K)—and its excitation spectrum are reported. The temperature dependence of both are anomalous. In contrast to the infrared emission, it is broad (half-width 0.63 eV) and structureless. Application of a dc electric field ∼100 V/cm, at 4 K, induces oscillations in time of the visible emission, a property not shared by the I.R. emission.