Ultraviolet Raman spectroscopy characterization of epitaxial (Ba,Sr)TiO₃ films grown under adsorption-controlled conditions

Variable temperature ultraviolet Raman spectroscopy was applied to characterize (Ba,Sr)TiO₃ films grown by molecular-beam epitaxy and pulsed-laser deposition under adsorption-controlled conditions at very high substrate temperatures (≥ 1400 °C). Bulk SrTiO₃ has a cubic perovskite structure with all phonon modes being symmetry forbidden in the 1^st order Raman spectra. Therefore, the defect-induced activation of the Raman peaks can be used to characterize the structural quality of the films. Normalized Raman intensity ratio of the 1^st order SrTiO₃ phonon peaks to the substrate features has been used to characterize the homoepitaxial SrTiO₃ films grown by different techniques at various conditions. Raman spectroscopy results show that films grown by adsorption-controlled epitaxy at very high substrate temperatures and higher oxygen pressure have the best crystalline quality in terms of the lowest concentration of defects, such as oxygen vacancies and cation non-stoichiometry. Strained Ba_xSr_1-xTiO₃ films on grown SrTiO₃ show increased ferroelectric phase transition temperature compared to bulk Ba_xSr_1-xTiO₃ of corresponding compositions, and the films show behavior different from bulk materials: the low-temperature transitions from tetragonal to orthorhombic and from orthorhombic to rhombohedral phases, which are characteristic for bulk Ba_xSr_1-xTiO₃ have not been observed in the strained films; the tetragonal phase is stabilized by epitaxial strain.