Luminescence and possible phase transformations in Cr-doped SrTiO₃ and Sr_0.998Ca_0.002TiO₃ thin films deposited by the excimer laser ablation method

Abstract

The dielectric permittivity and photoluminescence of the title films were studied in detail. SrTiO3:Cr films show a temperature maximum of ε′ (T) and a saturation of the TO-mode softening near 100 K. In (SrCa)TiO3:Cr films a temperature maximum of ε′ occurs at 28 K; we suggest this is due to an onset of XY, n = 2 quantum ferroelectricity. A ‘blue shift’ of the zerophonon R-luminescence line of octahedral Cr³⁺ in SrTiO3 films, and an increase of vibronic sideband intensities in comparison with Cr³⁺ luminescence in the same crystals are reported and discussed. Attention was focussed on SrTiO3:Cr films in order to explain a large and unusual temperature shift of the R-luminescence line position. It is shown that this shift is a consequence of local configurational instability and the off-center position of Cr³⁺ in the ground 4A2-state and the excited 2E-state, which occurs at least for T < 77 K.