UV Raman study of ferroelectric BaTiO3/SrTiO3 superlattices

The results of the first experimental study of ferroelectric short-period superlattices (SLs) by ultraviolet (UV) Raman spectroscopy will be presented. The high quality (BaTiO$_{3})_{m}$/(SrTiO$_{3})_{n}$ SLs (m, n are between 4 and 10 unit cells) with atomically smooth interfaces were grown by molecular beam epitaxy on SrTiO$_{3}$ substrates. Raman spectroscopy with UV excitation (351.1 nm) made possible the observation of superlattice phonons without overwhelming substrate signal in the spectra. Raman data on the BaTiO$_{3}$ phonons show that BaTiO$_{3}$ layers in SLs remain in the tetragonal phase in the entire temperature range studied (80-400K), and the low-temperature phases characteristic for bulk BaTiO$_{3}$, are suppressed. Biaxial compression of BaTiO$_{3}$ layers in SLs is likely cause for such a behavior, according to the calculated phase diagram for BaTiO$_{3}$ as a function of temperature and strain. Features attributed to the first-order Raman scattering in SrTiO$_{3}$ layers indicate that the inversion symmetry is broken, and the SrTiO$_{3}$ layers in the SLs are polar. Supported in part by DOE (Grant {\#} DE-FG02-01ER45907).