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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.
Thin films of GaN have been deposited at relatively low growth temperatures by remote plasma-enhanced chemical-vapor deposition (RPECVD), using a plasma excited NH3, and trimethylgallium (TMG), injected downstream from the plasma. The activation energy for GaN growth has been tentatively assigned to the dissociation of NH groups as the primary N-atom precursors in the surface reaction with adsorbed TMG, or TMG fragments. At high He flow rates, an abrupt increase in the growth rate is observed and corresponds to a change in the reaction mechanism attributed to the formation of atomic N. X-ray diffraction reveals an increased tendency to ordered growth in the 〈0001〉 direction with increasing growth temperature, He flow rate, and rf plasma power. Infrared spectra show the fundamental lattice mode of GaN at 530 cm−1 without evidence for vibrational modes of hydrocarbon groups.
Films of GaN have been grown on silicon by remote plasma-enhanced chemical-vapor deposition using trimethyl gallium and ammonia. The ammonia is rf plasma excited along with He, and the trimethyl gallium is introduced downstream from the plasma generation zone. The activation energy for the growth of GaN is 0.95±0.05 eV. This is tentatively assigned to the activation of NH groups, extracted from the plasma as primary precursors for the surface reaction with trimethyl gallium (TMG), or adsorbed fragments of this molecule. At high He flow rates, an abrupt increase in the growth rate is observed corresponding to a change in the reaction mechanism which is attributed to the formation of atomic nitrogen.
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