Photoluminescence and reversible photo-induced spectral change ofSrTiO3

Abstract

When strontium titanate (SrTiO3) single crystal is irradiated at room temperature with a 325 nm laser light in anevacuated specimen chamber, the luminescence intensity increases, creating abroad visible luminescence centred at about 2.4 eV. Then, introducing oxygen gasinto the specimen chamber, the photoluminescence spectrum returns reversiblyto the original weak luminescence under the same laser light irradiation. Afterremoving the laser light irradiation, each photoluminescent state is stored fora long time at room temperature under room light, regardless of any changesof atmosphere. Such photo-induced spectral change has been observed also atdifferent temperatures from 13 K to room temperature. The observed phenomenon isexplained by means of the photo-induced oxygen defect formation at the surfaces ofSrTiO3 crystal. Forthe same SrTiO3 single crystal, we have studied the photoluminescence properties. Besides the2.4 eV luminescence band, we have observed new two luminescence bandscentred at about 3.2 eV and about 2.9 eV. The energy, 3.2 eV, is close to both thephotoluminescence excitation edge energy and the reported band edge energy ofSrTiO3 crystal. Both the 3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsedphotoexcitation, while the 2.4 eV luminescence lasts for several seconds at 13 K. The excitationlight intensity dependence of these luminescence bands has been also measured at 13 K. The2.4 eV luminescence increases in intensity with increasing excitation intensity up to4 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 luminescenceincrease in intensity with increasing excitation intensity, without any saturation.Although the 2.4 eV luminescence had been assigned to the radiative decay ofintrinsic self-trapped excitons in a superparaelectric state by several workers, thepresent studies have clarified that the luminescence originates mainly from crystaldefects (oxygen defects and chemical heterogeneity in the surface region). Boththe 3.2 eV luminescence and the 2.9 eV luminescence are discussed qualitatively.