Suppression of oxygen diffusion by thin Al 2 O 3 films grown on Sr Ti O 3 studied using a monoenergetic positron beam J. Appl. Phys. 97, 033508 (2005); 10.1063/1.1836010 Defects in CeO 2 / SrTiO 3 fabricated by automatic feeding epitaxy probed using positron annihilation J. Appl. Phys. 94, 5193 (2003); 10.1063/1.1606112Vacancy-type defects in BaTiO 3 / SrTiO 3 structures probed by monoenergetic positron beams Oxygen vacancies introduced by homoepitaxial growth of thin films on strontium titanate (SrTiO 3 ) substrates were studied by means of positron annihilation. The SrTiO 3 films were grown by molecular-beam epitaxy without using an oxidant. The Doppler broadening spectra of the annihilation radiation were measured as a function of incident positron energy for the SrTiO 3 /SrTiO 3 samples fabricated by various growth conditions. The line shape parameter S, corresponding to the annihilation of positrons in the substrate, was found to be increased by the growth of the film. This increase was attributed to the diffusion of oxygen from the substrate into the film, and the resultant introduction of oxygen vacancies in the substrate. Two different types of defects in the substrate were identified: one is oxygen multivacancies, such as oxygen divacancies, and the other is their complexes with Sr vacancies. The concentration of oxygen vacancies in the subsurface region increased as the substrate temperature during the growth decreased (р320°C). This fact was attributed to the decrease in the diffusion length of oxygen at low temperatures, and the resultant accumulation of oxygen vacancies in the subsurface region.
Thin BaTiO3 films grown on SrTiO3 substrates were characterized by means of positron annihilation. The films were deposited by molecular-beam epitaxy without using oxygen source. We measured the Doppler broadening spectra of annihilation radiation and x-ray diffraction of the films and found that vacancy-type defects such as oxygen vacancies and other related defects caused lattice relaxation in the films. These defects disappeared after the films were annealed at 600 °C in an O2 atmosphere. Lattice relaxation in the films was also observed when the films were annealed at 1050 °C, but there was no direct relationship between the lattice relaxation that occurred at this temperature and vacancy-type defects. Vacancy-type defects were introduced into the SrTiO3 substrates by the growth of the BaTiO3 films. The species of these defects were identified as oxygen vacancies or defects related to oxygen vacancies, and they appeared as a result of diffusion of oxygen toward the BaTiO3 films. Almost all oxygen vacancies in the SrTiO3 substrates were annealed out at 500 °C.
Thin films of BaTiO3 and SrTiO3 and their superlattice were grown on SrTiO3 substrates using oxygen from the substrates as an oxygen source. Epitaxial growths were carried out by coevaporations of the component metals under ultrahigh vacuum without introducing any oxidants. The growth mechanism in the extremely low oxidation ambient was studied. Oxygen was automatically fed from the substrate to the growing film surface. As a result, oxygen vacancies were incorporated deep into the inside of the SrTiO3 substrate, confirmed by means of positron annihilation. The local oxygen deficiency at the growing surface is buffered by the fast diffusion of incorporated oxygen vacancies into deep inside of the substrate, avoiding decomposition or nucleation of the metallic phases on the surface. The BaTiO3/SrTiO3 interfaces were formed without intermixing, despite considerable amounts of oxygen having moved through the interfaces.
A focused ion beam (FIB) system is process equipment used to make a wide variety of small structures of various materials by irradiating a focused gallium ion beam of nanometer-order diameter to a surface of specimens and by utilizing spattering etching and ion beam induced deposition. In order to realize greater diversity of structures with use of the FIB system, we developed a technology of making three-dimensional structures by using gas assist etching, and a precision wheel stage to be used in the focused ion beam (FIB) system. Using these technologies with the FIB system, we achieved a nano processing machine which can be called a nano milling machine or a nano lathe.
Homoepitaxial growth of SrTiO 3 without introducing any oxidants has been achieved at low temperatures. The growth was carried out by coevaporation of Sr and Ti metals under extremely low oxygen partial pressure ( pO 2 < 1 × 10 −8 Pa). A clear reflection high-energy electron diffraction (RHEED) intensity oscillation from the layer-by-layer growth of SrTiO 3 was observed during the growth at a substrate temperature of 370 • C. The deposited film was found to have an approximately stoichiometric composition and a single-phase of SrTiO 3 , from the analyses of Auger electron spectroscopy (AES) and RHEED. Oxygen was automatically fed from the substrate to the growing surface. Instead, oxygen vacancies were incorporated into the bulk of the substrate. The incorporated oxygen vacancies were evaluated by positron annihilation.
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