Photoluminescence and reversible photo-induced spectral change of SrTiO3

When strontium titanate (SrTiO3) single crystal is irradiated at room temperature with a 325 nm laser light in an evacuated specimen chamber, the luminescence intensity increases, creating a broad visible luminescence centred at about 2.4 eV. Then, introducing oxygen gas into the specimen chamber, the photoluminescence spectrum returns reversibly to the original weak luminescence under the same laser light irradiation. After removing the laser light irradiation, each photoluminescent state is stored for a long time at room temperature under room light, regardless of any changes of atmosphere. Such photo-induced spectral change has been observed also at different temperatures from 13 K to room temperature. The observed phenomenon is explained by means of the photo-induced oxygen defect formation at the surfaces of SrTiO3 crystal. For the same SrTiO3 single crystal, we have studied the photolunimescence properties. Besides the 2.4 eV luminescence band, we have observed new two luminescence bands centred at about 3.2 eV and about 2.9 eV. The energy, 3.2 eV, is close to both the photoluminescence excitation edge energy and the reported band edge energy of SrTi03 crystal. Both the 3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsed photoexcitation, while the 2.4 eV luminescence lasts for several seconds at 13 K. The excitation light intensity dependence of these luminescence bands has been also measured at 13 K. The 2.4 eV luminescence increases in intensity with increasing excitation intensity up to 4 Mj CM-2, and then it becomes decreased with further increase in the excitation intensity. On the other hand, both the 3.2 eV luminescence and the 2.9 eV luminescence increase in intensity with increasing excitation intensity, without any saturation. Although the 2.4 eV luminescence had been assigned to the radiative decay of intrinsic self-trapped excitons in a superparaelectric state by several workers, the present studies have clarified that the luminescence originates mainly from crystal defects (oxygen defects and chemical heterogeneity in the surface region).