Ferroelectricity in nonstoichiometric SrTiO3 films studied by ultraviolet Raman spectroscopy

Homoepitaxial Sr1+xTiO3+delta films with -0.2 <= x <= 0.25 grown by reactive molecular-beam epitaxy on SrTiO3 (001) substrates have been studied by ultraviolet Raman spectroscopy. Nonstoichiometry for strontium-deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of a spontaneous polarization with a paraelectric-to-ferroelectric transition temperature above room temperature. Strontium-rich samples also show a strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution of the disorder-activated mechanism in Raman scattering. (C) 2010 American Institute of Physics. [doi:10.1063/1.3499273

Growth of nanoscale BaTiO3/SMO3 superlattices by molecular-beam epitaxy

Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/ SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the soperlattices was as narrow as 7 arc s (0.002??). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroeleotric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/ SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant: for novel phooon devices, including mirrors, filters, and cavities for coherent phonon generation and control.close171