Hikaru Terauchi

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.

Epitaxial growth and dielectric properties of BaTiO3 films on Pt electrodes by reactive evaporation

Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high‐energy electron diffraction and cross‐sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two‐dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant er. Good agreement between the experimental results and the theoretical calculations leads to ...

STRUCTURAL CHARACTERIZATION OF BATIO3 THIN FILMS GROWN BY MOLECULAR BEAM EPITAXY

Epitaxial ultrathin films of BaTiO3 were prepared using molecular beam epitaxy. For the substrate, (001)-oriented SrTiO3 single crystals were used. Controlling the growth conditions of these films as well as the semiconductor thin films, led to the successful growth of the BaTiO3 films as single crystals, characterized by x-ray diffraction even in the ultrathin range. The ultrathin BaTiO3 films are highly c-axis-oriented tetragonal phaselike bulk BaTiO3 crystals. The tetragonality of the thin film crystals is much larger than bulk crystal’s. We also measured the saturated polarization (Ps) of the BaTiO3 films at temperatures ranging from room temperature to 600 °C. The results confirmed again that the films are ferroelectric tetragonal phase crystals. Moreover, they showed that the transition temperature for the ferroelectric-paraelectric phase transition of the films is higher than bulk crystal’s.

Structure Analysis and Hydrogen Bond Character of K3H(SO4)2

Structure analysis of K 3 H(SO 4 ) 2 was performed by X-ray diffraction experiments in order to study the hydrogen bond character in connection with the geometrical isotope effect. Crystal data at room temperature were obtained as a=9.777(1)A, b=5.674(1)A, c=14.667(4)A, β=102.97(2)° and the space group A 2/ a , Z =4. The hydrogen bond length R OO determined from the positional parameters was 2.493(1)A which was close to but still longer than the so called critical bond length r c . The electron density of split hydrogen atoms was observed on a differential Fourier map. It is concluded that the hydrogen atoms take the disordered state at room temperature and the distance between hydrogen atoms R HH is approximately 0.65 A. The possibilities for the existence both of the low temperature phase and the high temperature prototype phase are discussed.

X-Ray Diffraction Study on Discotic Lamellar Phase in Bis(1,3-di(p-n-alkoxyphenyl)propane-1,3-dionato)copper(II)

Abstract X-ray diffraction measurements have been carried out on bis(1,3-di(p-n-alkoxyphenyl)propane-1,3-dionato)copper(II) to clarify the detailed structural properties on the discotic mesophase. It is confirmed that the lamellar structure is realized in the discotic phase and the tilt angle of molecules to the layer is about 5°. The crystal-to-discotic phase transition occurs involving a large fluctuation of the inter-molecular distance along normal to the molecular plane within the layer. On the other hand, the transition from the discotic-to-isotropic liquid phase is characterized by the destabilization of the layer structure due to the randomizing of both the position and the orientation of molecules.

Temperature dependence of hydrogen bond nature in K3H(SO4)2

Structure analysis of K 3 H(SO 4 ) 2 was performed by X-ray diffraction experiments at various temperatures down to 27 K in order to investigate the hydrogen bond character in connection with the geometrical isotope effect. The hydrogen bond length R OO monotonically decreases with decreasing temperature, accompanied by the contraction of lattice parameters, and it crosses the critical bond length r c ( H) at about 100 K. Simultaneously, the double site feature of hydrogen atoms changes into the single site state at the center of the hydrogen bond. This behavior explains why K 3 H(SO 4 ) 2 has no phase transition in spite of the existence of the disordered character at room temperature.

Structural Study of Epitaxial BaTiO3 Crystals

Epitaxial crystals of BaTiO 3 with the thicknesses of 67 A, 230 A, 600 A and 1500 A were grown on SrTiO 3 substrates by activated reactive evaporation. The tetragonality of the crystal decreases with increasing thickness and remains in the temperature range of 15 K to 670 K. The epitaxial effect is discussed in terms of a critical thickness, as well as of a surface layer of fine particles.

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.

The Structural, Dielectric, Raman-Spectral and Low-Temperature Properties of Amorphous PbTiO3

Amorphous ribbons of pure PbTiO3 were obtained by the rapid solidification technique and were confirmed to be in the amorphous state by several different experimental techniques. Structural studies revealed clusters with radii up to about 150 A, larger than those so far found in other materials such as metals or semiconductors in the amorphous state. Dielectric and Raman spectral studies confirmed that the material remains in the amorphous state indefinitely. Low-temperature studies showed that not only the amorphous state but also the crystalline state of PbTiO3 has a dielectric constant minimum in the mK region, a characteristic of the amorphous state.

Dipole-Glass Phase in Random Mixture of Ferroelectric and Antiferroelectric: Rb1−x(NH4)xH2PO4

From the measurements of dielectric constants and X-ray diffraction profiles on a random mixture of a ferroelectric and an antiferroelectric, Rb 1- x (NH 4 ) x H 2 PO 4 , for \(0.3{\lesssim}x{\lesssim}1.0\), the phase diagram was determined. For \(0.8{\lesssim}x{\lesssim}1.0\) the long range antiferroelectric phase was observed as like in pure NH 4 H 2 PO 4 . A dipole-glass phase was found in the intermediate concentration range (\(x{\lesssim}0.7\)).