R. Takahashi

Ferromagnetism in Co-Doped TiO2 Rutile Thin Films Grown by Laser Molecular Beam Epitaxy

Epitaxial TiO2 rutile films were fabricated on α-Al2O3 (1012) substrate in the layer-by-layer fashion by laser molecular beam epitaxy. Ablation with a Co-doped TiO2 target produced single phase of rutile film with the concentration of Co between 0 and 5%. Some ferromagnetic domain structures were observed in CoxTi1-xO2 rutile films by a scanning superconducting quantum interference device microscope at 3 to 90 K. The magnetic hysteresis could also be observed even at room temperature with a magnetic moment of ~ 1 µB/Co atom.

Concurrent x-ray diffractometer for high throughput structural diagnosis of epitaxial thin films

We have developed a concurrent x-ray diffractometer that concurrently measures spatially resolved x-ray diffraction (XRD) spectra of epitaxial thin films integrated on a substrate. A convergent x-ray is focused into stripe on a substrate and the diffracted beam is detected with a two-dimensional x-ray detector. The obtained snapshot image represents a mapping of XRD intensity with the axes of the diffraction angle and the position in the sample. In addition to the parallel XRD measurements of thin films with various compositions and structures, two-dimensional spatial mapping of XRD peak with a resolution of ∼100u200aμm is demonstrated. This technique will provide us a high throughput characterization method of various devices composed of epitaxial films.

Growth dynamics of the epitaxial SrO film on SrTiO3(001)

Abstract The initial growth of SrO thin films was investigated by in-situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscope (STM). When SrO was deposited under epitaxial growth conditions on B-site terminated SrTiO3(0xa00xa01) substrate, the first layer reacted with the B-site layer to grow as A site layer of the perovskite. Once this A-site layer was completed, the subsequent SrO layers were grown in a layer-by-layer manner. The basic unit for layer-by-layer growth is a rock-salt unit cell. The growth mode of SrO on SrTiO3(0xa00xa01) is discussed in relation to the crystal lattice habit.

Development of a new combinatorial mask for addressable ternary phase diagramming: application to rare earth doped phosphors

Abstract We report on the computer-aided design of a novel mask for the addressable ternary phase diagram to serve a quick screening of multi-component functional materials. Masking patterns were optimized to make a triangular ternary diagram with each composition changed from 0 to 100% by simulating the deposition process under the condition of synchronous control of the mask motion, target exchange, and laser pulses. Fabrication of a ternary M0.01Y1.99O3 (M=Eu, Tm, Tb) phosphor material demonstrates of the direct mapping relationship between the film composition and its cathode luminescence (CL) property.

Nanoskyscrapers of ferroelectric Bi4Ti3O12

Self-assembled nanoskyscrapers of ferroelectric Bi4Ti3O12 were fabricated by a bottom-up process: flux-mediated epitaxy using an admixture flux. By combinatorial technique, we have discovered a novel flux, VOx, to assist the unique crystal growth for the in-plane alignment of the long c axis of Bi4Ti3O12 on a SrTiO3(001) substrate. We report on the self-assembly of an isolated epitaxial (110)-oriented Bi4Ti3O12 nanoplate and the direct measurement of the piezoelectric properties by a scanning probe microscope technique. Piezoelectric lattice displacement showed a typical butterfly curve, indicating good ferroelectric property.

Ceramic liquid droplets stabilized in vacuum

We have studied the ceramic liquid droplet of CuOx-added BiOx at high temperature in vacuum. CuOx always floated on the BiOx as a surfactant and suppressed the evaporation of volatile BiOx liquid droplets. A clear liquid behavior of the BiOx droplets was directly observed by in situ laser microscope, with numerous liquid droplets growing by the coalescences in accordance with Marangoni’s [Nuovo Cimento Ser. 2, 239 (1872)] effect involved with the precursor film. We have also found a unique absorption of CaO into the BiOx liquid droplets, based on which a reliable process has been established for an atomically flat surface of MgO(001). These results open a broad window of opportunity to tailor not only a chemical interaction on oxide surface but also a liquid droplet dynamics in vacuum.

Combinatorial optimization of atomically controlled growth for oxide films by the carrousel type laser molecular beam epitaxy

Abstract We have developed a new combinatorial technique, “carrousel deposition” for the atomically controlled growth of oxide films under in situ diagnostics of surface reaction with reflection high energy electron diffraction (RHEED). In this technique, a sequential fabrication of eight different films is performed in one experiment, enabling quick optimization of the growth conditions with the assistance of RHEED monitoring. Thus, we succeeded in the one lot optimization of the layer-by-layer SrO and BaO film growth on the B-site (TiO 2 ) terminated SrTiO 3 (0xa00xa01) substrate. Furthermore, the characteristic crystal habit in the initial growth of MO atomic layers on the SrTiO 3 substrate was elucidated to be dominated by the topmost atomic layer of the substrate or film and the atomic layer to be formed on it.

Epitaxial ScAlMgO4(0001) films grown on sapphire substrates by flux-mediated epitaxy

ScAlMgO4(0001) epitaxial films have been synthesized on sapphire (0001) substrates by flux-mediated epitaxy. The key points were high temperature deposition and the use of additive BiOx, which was simultaneously supplied during the deposition; otherwise the growth of spinel phase such as MgAl2O4 was dominant. The BiOx is thus considered to promote the growth of ScAlMgO4 like a flux. This result indicates that the flux-mediated epitaxy is a promising way to high quality single crystal ScAlMgO4 films as a lattice-matched substrate for p-type ZnO and GaN films.

Recognition of the Atomic Terminating Layer in Perovskite Oxide Substrates by Reflection High Energy Electron Diffraction

The chemical surface structure in perovskite oxides has been identified by monitoring the oscillation intensity of reflection high energy electron diffraction patterns during initial growth of MO (M=Sr, Ba) films on the perovskite substrate. This successful analysis technique is demonstrated for the film growth on A and B-site oxides terminated ABO3 substrates. Epitaxial growth of MO thin films is dominated by chemical interaction between the growing lattice and the underlying atomic layer to follow the crystal habit of forming a more stable layer unit cell. This simple technique can be used as a substitute to relevant physical techniques such as coaxial impact-collision ion scattering spectroscopy and friction force microscopy that are currently used techniques to determine the termination layer.

Computer design of combinatorial shadow mask for ternary composition spread library

A simple rotational shadow masking system has been designed by computer simulation for application to the fabrication of a ternary composition spread material library. A mask plate is stencilled off a right triangle pattern and a trapezoid pattern. This can define the thickness gradient of films deposited from three different sources along the axes between the tops and the mid-points of bottom lines of a triangle on a substrate surface. Optimizing the angles of the triangle and trapezoid patterns and regulating the rates of mask moving, the computer simulation predicts that the film thickness of each component varies linearly from one mono-layer at the top to zero at the bottom. This process enables the construction of a ternary phase diagram by repeating the film deposition sequence. This prediction has been mathematically derived in this study.