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

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.