Ultraviolet Raman Spectroscopy Characterization of (Ba,Sr)TiO₃ Thin Film Heterostrucrures

Ferroelectrics based on BaxSr1-xTiO3 are materials are studied for various device applications, such as non-volatile memories and microwave devices with high dielectric permittivity at GHz frequencies and high tunability. We applied variable-temperature ultraviolet Raman spectroscopy to study barium strontium titanate-based, (BaxSr1-xTiO3)nBaxSr1-xO Ruddlesden-Popper (R-P) thin films grown by molecular beam epitaxy on SrTiO3 substrates. Raman spectra demonstrated that R-P samples were ferroelectric at low temperatures, and the ferroelectric phase transition temperatures, Tc, were determined from the analysis of temperature dependence of Raman intensities of phonon modes. We have also characterized homoepitaxial SrTiO3 films grown in various conditions by low temperature ultraviolet Raman spectroscopy. Bulk SrTiO3 has a cubic perovskite structure with all phonon modes being symmetry forbidden in the 1st order Raman spectra. Therefore, the defect-induced activation of the Raman peaks can be used to characterize the structural quality of the films. Our Raman results combined with the data from other characterization techniques show that the films grown by adsorption-controlled epitaxy at very high substrate temperatures (1450–1475℃) have the best crystalline quality in terms of the lowest amount of defects, such as non-stoichiometry.