Ultraviolet Raman spectroscopy characterization of (Ba,Sr)TiO₃ Ruddlesden-Popper films

Variable-temperature ultraviolet Raman spectroscopy was applied to study barium strontium titanate-based, (Ba_xSr_1-xTiO₃)_nBa_xSr_1-xO Ruddlesden-Popper (R-P) thin films with grown by molecular beam epitaxy on SrTiO₃ and TbScO₃ substrates. Pure SrTiO₃ (x = 0) RP series with n varied from 10 to 50, as well as Ba_xSr_1-xTiO₃ – based series with varied x and n = 6, 10 and 20 were investigated. Raman spectra demonstrated that all samples were ferroelectric at low temperatures, with out-of-plane polarization (normal to the film surfaces). Ferroelectric phase transition temperatures, T_c, were determined from the analysis of temperature dependence of Raman intensities of phonon modes. All films studied had T_c values below room temperature, in the approximate range 220-260 K, making them suitable candidates as tunable dielectrics for microwave device applications. Pure (barium-free) (SrTiO3)_nSrO RP films showed weak T_c dependence on the series number n, (230 and 240 K for n = 10 and 20, respectively). In Ba-containing RP series (Ba_xSr_1-xTiO₃)_nBa_xSr_1-xO, T_c varied between 225 and 255 K, depending on Ba composition x and series number n.