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. - Highlights: • For the first time observed absorption lines of Er³⁺ in SrTiO₃:Er single crystals. • Only one type of Er³⁺ centers of non-cubic symmetry was found in the crystals. • Structure of 4f electron energy levels at 2 K was determined for the Er³⁺ center. • Er-doping results in pronounced free charge carrier absorption in the IR region