Anomalous photoelectronic processes in SrTiO3

Abstract

The photoelectronic properties of nominally pure SrTi${\mathrm{O}}_3$ single crystals were studied in detail. A sharp narrow peak near the fundamental absorption edge was observed in both photoconductivity and photoluminescence excitation spectra, although it was not detected in the absorption spectrum. The temperature dependence of the half-width of this anomalous peak was found to have a behavior similar to that of the absorption bands of $F$ centers in alkali halides. The photocurrent associated with this peak was found to depend strongly on temperature. The photoconductive gain at 10 K was estimated to be at least 8×${10}^5$. Marked thermal and optical quenching of the photocurrent were measured. In addition, a supralinearity phenomenon, by which the photocurrent varies as the 5th power of the light intensity, was observed. On the basis of the results of the varied experiments designed to elucidate the nature of the transition responsible for the anomalous peak, we have reasonably established that it is due to the descrete transition by which an electron is raised from the ground state of a highly localized sensitizing center to an excited state located in the conduction band; subsequently a free electron is produced by the process of autoionization. It has been shown that there is a strong correlation between photoconductivity and photoluminescence. The infrared emission, attributed to the electronic transition within a ${\mathrm{Cr}}^{3+}$ ion, can be excited by either the anomalous-peak radiation or band-to-band radiation; however, the visible emission, ascribed to the recombination of a free hole with a level below the conduction band, can be excited only by band-to-band radiation. These and other photoluminescence results are discussed in terms of charge-transfer processes and an association model involving the ${\mathrm{Cr}}^{3+}$ ion and the highly localized center responsible for the anomalous peak.