Yasuhiro Yoneda

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 600u2009°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.

Ferroelectric phase transition in BaTiO3 films

Abstract Epitaxial BaTiO 3 single crystals were grown on Pt/MgO (Pt for electrodes, MgO for substrates) by means of the activated reactive evaporation technique. X-ray diffraction patterns were measured to clarify the properties such as the temperature dependence of lattice parameters. It was found that the temperature dependence of lattice parameters and integrated intensities of Bragg reflections showed different behavior depending on the film thickness. The relative dielectric constant ϵ r was measured in order to investigate the dielectric properties of the films. Film crystals of BaTiO 3 showed the phase transitions which are different from those in bulk state.

Growth of ultrathin BaTiO3 films on SrTiO3 and MgO substrates

Abstract Epitaxial BaTiO 3 thin films were successfully grown on SrTiO 3 (001) and MgO (001) substrates by molecular beam epitaxy. These 50xa0ML thickness BaTiO 3 films were prepared under the same growth conditions on both substrates. The films were characterized by in situ reflection high-energy-electron diffraction observations and ex situ X-ray diffraction measurements. The epitaxial relationship between the BaTiO 3 films and both substrates was cube-on-cube, though the cubic lattice parameter of BaTiO 3 was larger than that of SrTiO 3 , and smaller than that of MgO at the growth temperature (700°C). In addition, the films on both substrates were found to have a highly c -axis-oriented tetragonal phase at room temperature. However, despite the finding that the lattice parameters of these films were very close to the bulk value of the tetragonal phase, the crystal quality as found to depend strongly on the magnitude of the lattice mismatch.

Magnetic properties of SrRuO3/SrTiO3 superlattices

Abstract We have fabricated n-UCT SrRuO3/SrTiO3 superlattices, [SrTiO3 5-UCT/SrRuO3 n-UCT]m (n=1∼20), where UCT is a unit-cell thickness, on SrTiO3(001) substrates to study a low-dimensional structural effect on the magnetic properties of SrRuO3. We measured temperature-dependent and field-dependent magnetization. The ferromagnetic transition temperature decreased with decreasing n, and the transition vanished for n=1. A strong anisotropy of magnetization with an easy axis perpendicular to the film plane was observed for all the samples, as seen in SrRuO3 thin films on SrTiO3(001) substrate. The in-plane magnetization was extremely suppressed as n decreased. For 3-UCT superlattice, the in-plane magnetization became 1/10 of the out of plane magnetization at T=5 K and H=1 T. The role of low-dimensionality between SrRuO3 and SrTiO3 on magnetism of SrRuO3 is discussed.

Epitaxial BaTiO3 crystals

Abstract Epitaxial crystals of BaTiO3 have been grown by activated reactive evaporation. The ultra-thin crystal less than 100A thickness shows an elongation along the c-direction. The ferroelectric phase transition of the thin films is discussed in terms of the epitaxial effect and the surface layer.

Local Distortion of AsO4 and PO4 Molecules in KDP-Family Crystals

XAFS experiments around the K-absorption edge of P atoms in KDP and As atoms in KDA were carried out in order to study the existence of local distortion in PO4 and AsO4 molecules. Well defined XAFS signals were obtained up to 18A-1 in KDA and 14A-1 in KDP and DKDP. Extraordinary temperature dependence of the higher frequencies in k3 χ (k) of KDA was found, but nothing happened at Tc. Obtained bond lengths P-O and As-O are consistent with structural analyses. Present experiments suggest that the AsO4 molecule already distorts at the high temperature phase, while it is hardly to conclude that a PO4 molecule is in a disordered state because of the insufficient resolution.

Lattice distortion of an In0.2Ga0.8AsGaAs superlattice

Abstract We investigated the elastic behavior of an In 0.2 Ga 0.8 As GaAs strained superlattice grown on a (001)-oriented GaAs substrate by molecular beam epitaxy. Using high-resolution X-ray diffraction, we found that the strained superlattice has monoclinic, and no longer tetragonal crystal symmetry. The measurements of the peak shift of X-ray Bragg reflections were performed using a diamondanvil cell. The behavior of the system under hydrostatic pressure gave the magnitude of the stress from the monoclinic distortion. The experimental value of the stress was about 5 kbar. We propose the microscopic aspects of the In 0.2 Ga 0.8 As GaAs superlattice including elastic and structural aspects.

Growth and phase transition of [(C60)5(Cu)5]52 multilayer film

Abstract [(C 60 ) 5 (Cu) 5 ] 52 multilayer film was grown on a GaAs(111) substrate with a C 60 buffer. The [(C 60 ) 5 (Cu) 5 ] 52 multilayer film was investigated by in situ observation of RHEED. In the growth plane, the [(C 60 ) 5 (Cu) 5 ] 52 multilayer film grew into a textured film within the coherence length of the probed electron beam. The multilayer film was confirmed by X-ray diffraction study to be of (111)-oriented crystal in the growth direction. The transition temperature of the C 60 layers of the multilayer film was 260 K. We also observed the glassy behavior of the C 60 layers of the [(C 60 ) 5 (Cu) 5 ] 52 multilayer film. The results may put multilayer film, which consists of metal and fullerene, into use for many applications.