Optical spectroscopy of erbium-doped SrTiO3 crystals

Results of a detailed study of optical absorption and photoluminescence of slightly erbium-doped (~50 ppm) strontium titanate (SrTiO3) single crystals are presented. This study allowed us to specify the structure of energy levels that determine the main internal optical transitions within the 4f11 electron configuration of Er3+ ions in SrTiO3:Er crystals at temperatures below the transition from cubic to tetragonal phase. The numbers of energy levels of the studied multiplets obtained at 2 K correspond exactly to the theoretically expected numbers of crystal field levels for multiplets of only one type of Er3+ centers with non-cubic symmetry. Experimental results indicate that Er3+ ions most probably substitute for Sr2+ ions acting in SrTiO3 as ionized donors. Such location of Er3+ ions is consistent with both n-type conductivity and substantial free charge carrier absorption in the infrared spectral region observed in the studied SrTiO3:Er single crystals at room temperature.