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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 superlattices 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-ferroelectric 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 phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.
Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.
The thermal expansion coefficients of DyScO3 and GdScO3 were determined from298 to 1273 K using x-ray diffraction. The average thermal expansion coefficients of DyScO3 and GdScO3 were 8.4 and 10.9 ppm/K, respectively. No phase transitions were detected over this range, though the orthorhombicity decreased with increasing temperature. These thermal expansion coefficients are similar to other oxide perovskites (e.g., BaTiO3 or SrTiO3), making these rare-earth scandates promising substrates for the growth of epitaxial thin films of many oxide perovskites that have similar lattice spacing and thermal expansion coefficients.
Using a variety of in situ monitors and when possible adsorption-controlled growth conditions, layered oxide heterostructures including new compounds and metastable superlattices have been grown by reactive molecular beam epitaxy (MBE). The heteroepitaxial layers grown include Bi4Ti3 O12—SrTiO3 and Bi4Ti3O12—PbTiO3 Aurivillius phases, Srn+1TinO3n+1 Ruddlesden-Popper phases, and metastable PbTiO3 / SrTiO3 and BaTiO3 / SrTiO3 superlattices. Accurate composition control is key to the controlled growth of such structures, and to this end combinations of reflection high-energy electron diffraction (RHEED), atomic absorption spectroscopy (AA), a quartz crystal microbalance (QCM), and adsorption-controlled growth conditions were employed during growth. The structural perfection of the films has been investigated using in situ RHEED, four-circle x-ray diffraction, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (TEM).
Ferroelectric superlattices have been actively and intensively studied in recent years for their great scientific and technological interest. Superlattice containing Pb-based ferroelectric layers are important among ferroelectric superlattice systems, however, it is difficult to grow such superlattice due to the high volatility of Pb. Recently, great progress has been made in fabricating superlattice structure of PbZrO3/PbTiO3 by multi-ion-beam sputtering’ and molecular beam epitaxy (MBE). In this paper, we report the microstructural investigations of PbTiO3/SrTiO3 superlattice films, which were epitaxially grown on the SrTi03 substrate by MBE, using transmission electron microscopy (TEM).
[(PbTiO3)l0/(SrTiO3)l0]15 superlattice films were stacked on (100) SrTiO3 substrate alternately by MBE. Before growing the superlattice structure, a baffle layer including the 1000Å La-doped SrTi03 and the subsequent 500 Å PbTiO3 thin films was grown on the substrate. Above the PbTi03/SrTi03 superlattices, another PbTi03 thin film (1000 Å) was grown. Cross-section TEM specimens were prepared by standard methods.
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