Investigation of the structure of barium titanate thin films by Raman spectroscopy

Abstract

Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal-organic chemical vapor deposition (MOCVD) and laser-assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non-BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. The film-to-film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity-phase features, was qualitatively consistent with the x-ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser-deposited film varies significantly over the deposited area. The temperature dependencies of the Raman spectra of two MOCVD films were examined in the 25–175 °C range. Raman peaks due to the tetragonal phase of BaTiO3 were observed at temperatures well above the Curie temperature of bulk single-crystal BaTiO3 (132 °C). This observation suggests that the tetragonal ferroelectric phase is stabilized by an anisotropic film-substrate interaction that gives rise to a two-dimensional stress in the plane of the film.