Kousei Kamigaki

Atomic layer growth of oxide thin films with perovskite‐type structure by reactive evaporation

Epitaxial growth of BaTiO3 and SrTiO3 films by the reactive evaporation method was investigated using reflection high‐energy electron diffraction (RHEED). The investigations were carried out using two growth methods: coevaporation and alternate evaporation of the metal elements in an oxygen atmosphere. Atomic layer growth was achieved by the alternate supply of Ba or Sr and Ti on the growing surface. In the case of coevaporation, epitaxial growth occurred in a two‐dimensional unit‐cell‐by‐unit‐cell mode. The surface of each unit cell is terminated by a (TiO2) layer. Artificial superlattices of BaTiO3/SrTiO3 were fabricated by monitoring the film thickness with the RHEED oscillations.

X‐ray study of misfit strain relaxation in lattice‐mismatched heterojunctions

High‐resolution x‐ray diffraction measurements have been carried out in AlxGa1−xAs and InxGa1−xAs grown by the molecular beam epitaxy method on (001) GaAs substrates. The thin epitaxial layers in these lattice‐mismatched semiconductor single heterojunctions are uniformly distorted and there is an elastic limit for large x. The epitaxial layer is affected by a thick substrate even over the elastic limit, i.e., the epitaxial layer still shows a strained state beyond the elastic limit. The relationship between the misfit strain and the lattice distortion is discussed.

Anomalous misfit strain relaxation in ultrathin YBa2Cu3O7−δ epitaxial films

Ultrathin YBa2Cu3O7−δ epitaxial films were successfully grown in situ on (001) SrTiO3 and MgO substrates by means of ozone‐incorporating activated reactive evaporation. The x‐ray‐diffraction study was carefully examined to determine the structural properties of the grown films. Excellent crystallinity with no interfacial disorders was revealed by the appearance of the Laue oscillations. It was found that in a well lattice‐matched YBa2Cu3O7−δ/SrTiO3 system, the crystallinity was deteriorated due to defect introduction at the critical layer thickness hc ( ∼ 130 A). Interestingly, also in a poorly lattice‐matched YBa2Cu3O7−δ/MgO system, excellent crystallinity was revealed even at above hc ( < 24 A). This implies that an anomalous misfit relaxation process exists in the YBa2Cu3O7−δ/MgO system. In such a system, no crystal imperfection of the MgO substrate caused by defect introduction was elucidated by the grazing incidence x‐ray scattering, which indicated that the MgO substrate did not contribute to the ...

Dielectric Properties of Sol-Gel Derived Pb(Mg1/3Nb2/3)O3?PbTiO3 Thin Films

Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) thin films were fabricated using a sol-gel method. The dielectric constant was found to be strongly dependent on the number of strong Mg-O-Nb bonds in the precursor solution. A large number of strong Mg-O-Nb bonds were formed after a long reflux time. This process resulted in PMN thin films with a large dielectric constants sometimes greater than 3000, with a loss tangent of 0.5%. Relaxor characteristics such as weak temperature dependence and weak dc bias dependence were also observed in the thin films. These results suggest that PMN-PT is an important material for thin film capacitor applications.

X-ray studies of semiconductor superlattices grown by molecular beam epitaxy

X-ray diffraction measurements of semiconductor superlattices grown by molecular beam epitaxy (MBE) have been carried out. Alternating monolayer growth of GaAs and AlAs has been confirmed by detecting the reflections at Brillouin zone boundaries in the reciprocal space. In strained GaAs–Ga 1- x In x As superlattices the tetragonal distortion is found and the tetragonality is independent of the thickness of the strained layer but increases with decreasing thickness of the GaAs layer. The X-ray study of the interdiffusion in the superlattice has been also carried out.

X-Ray Diffraction Patterns of Configurational Fibonacci Lattices

Four types of the configurational Fibonacci lattices have been grown by molecular beam epitaxy. The golden mean τ in a present Fibonacci lattice is not associated with the spacing between the alternating layer but is associated with the species of atomic layers or the hexagonal-cubic packing sequence. Many satellite peaks are observed having a self-similarity feature when the instrumental resolution of the X-ray diffraction is extremely high. Special attention was paid for the ABC stacking Fibonacci sequence to compare with the structure of stacking fault observed in a metal alloy such as MgCuAl.

X-ray study on a 100 Å-thick YBa2Cu3O7−δ epitaxial film: The relationship between the orthorhombic symmetry and superconductivity

A 100 A-thick YBa2Cu3O7−δ single crystal film with Tc=81 K was investigated by means of X-ray diffraction. No orthorhombic distortion was found in spite of the existence of the Cu-O chain structures. This implies that the two-dimensional Cu sublattice is isotropic and the oxygen configuration is anisotropic with the displacements along the c-direction in the Cu-O plane. It is concluded that the anisotropic oxygen configuration strongly correlates to the origin of the high-Tc superconductivity.

Synchrotron X-Ray Diffraction Study of Third-Order Fibonacci Lattices

Third-order Fibonacci lattices have been grown by means of molecular beam epitaxy. Many satellite peaks are observed by using synchrotron X-rays. A broad peak observed by a conventional diffraction method is found, at high resolution, to be a gathering of many sharp peaks. The satellite peaks are assigned by a power law, implying that the third-order Fibonacci lattices have a self-similar feature.

RHEED study of crystal growth of high temperature superconducting oxides in reactive coevaporation

Abstract We have found oscillations in the reflection high energy electron diffraction (RHEED) specular intensity during growth of the superconducting oxides, indicating that the growth of the oxides occurs in monomolecular layer by monomolecular layer mode. The film thickness has been exactly controlled by monitoring the RHEED oscillations which make it possible to form a one unit-cell layer and artificial superlattices as designed. The isolated ultrathin YBa 2 Cu 3 O 7−x (YBCO) films sandwiched in between PrBa 2 Cu 3 O 7−x (PrBCO) layers were prepared by the growth interrupt technique using the RHEED intensity oscillations. A one unit-cell YBCO layer has a superconducting transition with onset temperature at 70K and zero resistance at 30K. The overlayer of PrBCO was required to give the underlying one unit-cell YBCO superconductivity. The resistive transition for one unit-cell YBCO layer and the YBCO (one unit cell) / PrBCO (one unit cell) superlattice was independent of the magnetic field up to 10T applied parallel to the CuO 2 layers.

X-Ray Study on Low-Temperature Behavior of an Incommensurate Superstructure in the New High-Tc Superconductor Bi2Sr2Ca1Cu2O8+δ

Low-temperature behaviors of the incommensurate superstructure in the recently discovered new oxide superconductor, Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ , are investigated by single-crystal X-ray diffraction. The amplitude of the modulation wave, peak intensities, and widths of the superstructure reflections are independent of temperature in the range from 10 K to room temperature. This implies that the superstructure is not directly associated with the superconductivity.