Near-band-edge transition in SrTiO3 below 100 K: Role of electron-polarons, defects and sample surface

The ionoluminescence of strontium titanate (SrTiO3) under the intense electronic excitation produced by 3 MeV H, 19 MeV Si, and 19 MeV Cl ions was investigated for temperatures between 30 and 100 K. In addition to previously reported emission bands centered at 2.0 eV, 2.5 eV, and 2.8 eV, an asymmetric, narrow emission band centered at 3.15 eV was observed for the first time under ion irradiation. The 3.15 eV band appeared only under heavy ion irradiation (19 MeV Si and Cl) and at temperatures below similar to 70 K. The absence of the 3.15 eV emission under proton irradiation indicates that impurities and the pre-irradiation defect population likely play little or no role in the emission process, while electronic excitation density does. At the same time, the absence of fluence-dependent growth in the yield suggests that irradiation-induced defects are also unlikely to be the main cause of the emission. Upon comparing the proton induced ionoluminescence, heavy ion induced ionoluminescence, and available literature on low temperature photoluminescence of strontium titanate, a self-consistent interpretation emerges, where the 3.15 eV emission is associated with the recombination of large polarons.