Ryuta Tsuchiya

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO3 (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH4F-HF solution. The homoepitaxy of SrTiO3 film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO2 atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa2Cu3O7-δ.

YBa2Cu3O7−δ trilayer junction with nm thick PrGaO3 barrier

We have established a deposition process of high quality a axis oriented YBa2Cu3O7−δ (a-YBCO) and insulating epitaxial PrGaO3 (PGO) films to fabricate a-YBCO/PGO(2.0–3.2 nm)/a-YBCO trilayer junction. The precipitate formation on the bottom a-YBCO was greatly suppressed by the atomic layer modification of the substrate surface with a wet etching and successive atomic layer epitaxy of SrO and BaO atomic layers prior to the YBCO deposition. Crack formation and residual stress in the film due to the thermal expansion mismatch along c axis of YBCO could be eliminated by inserting a buffer layer of a-YBCO deposited with changing the substrate temperature from 580 to 735u2009°C. The junctions showed a clear hysteresis with its current jump as large as 30%, together with the Fraunhofer diffraction.

Atomically regulated layer-by-layer growth of SrTiO3 on a-axis oriented YBa2Cu3O7−δ thin films☆

Abstract The intensity oscillation of reflection high energy electron diffraction (RHEED) was observed during the pulsed laser deposition of ultra-thin SrTiO 3 films on a-axis oriented YBa 2 Cu 3 O 7−δ (YBCO) thin films. This enabled us to control digitally the overlayer thickness on an atomic scale. In the same deposition chamber, we could fabricate high quality epitaxial YBCO thin films on SrTiO 3 with Tc exceeding 80K and 90K for a-and c-axis oriented films, respectively. Reducing the particle density below 10 5 cm −2 on the bottom YBCO films should be promising to construct Josephson tunnel junctions.

Suppression of Precipitate Formation in Heteroepitaxial Growth of YBa2Cu3O7-δ Film on Misoriented SrTiO3 Substrates

We have investigated the effect of misoriented substrates on precipitate formation in c-axis-oriented YBa2Cu3O7-δ films grown by the pulsed laser deposition technique. The films grown on exact SrTiO3(100) substrates had precipitate densities as high as 107–108/cm2, whereas the films on substrates with surfaces misoriented from (100) by 0.5°–1° had fewer precipitates by two to three orders of magnitude. A film surface of 105/cm2 precipitates can give 90% probability of a precipitate-free practical device size area (10 µ m×10 µ m), providing a high possibility of successful fabrication of a Josephson tunnel junction.

Crystal engineering of oxide films in the fabrication of high-Tc Josephson tunnel junction

In relation to our research for the fabrication of high-Tc Josephson tunnel junction composed of YBCO(S)/oxide insulator(I)/YBCO layers, two crystal engineering issues are presented and discussed on pulsed laser processing of oxide thin films. One is the epitaxial growth of highly crystalline and orientation-controlled YBCO films and the other is the molecular layer epitaxy of perovskite and rock salt oxides films. Quantitative results are presented on the crystal quality, surface atomic layers and morphology, and electronic properties of the films and junctions. Discussion will be made on such problems as the thermodynamics versus kinetics in the film growth, identification and control of the topmost atomic layers of substrates and growing films, and electronic state of high-Tc films based on the scanning tunneling and photoelectron yield spectra.

Dimension controlled epitaxy of quantum functional oxide thin films by pulsed-laser processes

Abstract Laser MBE has been demonstrated to be a process especially useful for epitaxial layer-by-layer growth of ceramics thin films directly from sintered ceramics targets. Using the laser MBE system designed for oxide film growth, we have convincingly proved the atomically controlled epitaxy of various oxide films with such lattice structures as perovskite, rock salt, fluorite, and corundum, by the in situ observation of clear RHEED intensity oscillations. Due to the non-steady state nature of film growth by the laser MBE, as well as by the conventional pulsed-laser deposition, we could find a further advantage of the method; the independent optimization of nucleation and growth processes by pulse sequence and laser energy density. This facilitates the control of dimensionality of films. Some of our recent results on the fabrication of dimension controlled epitaxial oxide films are presented and discussed in relation to the atomic scale characterization and exploration of quantum structures and properties in oxides.

High-resolution photoelectron yield spectroscopy of oxide superconductors

We have designed and constructed a cryogenic photoelectron yield spectroscopy (PYS) and demonstrated a high energy resolution of 10 meV and a wide dynamic range of 7 orders of magnitude. This system enabled us to measure the absolute value of work function and analyze the electronic structure around the Fermi level (EF) for a Bi2Sr2CaCu2Ox(BSCCO) single crystal and YBa2Cu3O7-(delta ) (YBCO) epitaxial films. When the photoelectron was collected along <001> axis, semiconductive spectrum having a broad tail was observed for both materials. When the measurement was performed from the edge of CuO2 plane, we observed metallic spectra with sharp threshold, of which slope depended on temperature in a way consistent with the Fermi-Dirac distribution function. For BSCCO, a metallic spectrum superimposed a dip structure at 10 K in the vicinity of EF. The data could be well fitted with a curve numerically simulated from the BCS function with a superconducting gap value ((Delta) ) of 20 meV.

Thin Film Technology Directed towards Josephson Tunnel Junction

An overview is given on the high Tc and related oxide epitaxy directed towards Josephson tunnel junction with emphasis on atomic scale control of film deposition by the laser MBE process. We show how the control of the topmost atomic layer is important for ideal epitaxy of complex oxide. The development of wet process for obtaining atomically flat SrTiO3 surface remarkably improved the controllability of oxide epitaxy. Persistent RHEED oscillation demonstrated that the epitaxy of oxide reached such a level as perfect as that of semiconductors.

Crack formation and electrical properties of [100] and [110] oriented YBa2Cu3O7-δ thin films

We have grown [100] and [110] oriented YBa2Cu3O7−δ thin films on SrTiO3(100), (110) and LaSrGaO4(100) substrates using the PrBa2Cu3O7−δ template method. An array of cracks running perpendicular to the c-axis was observed on c-axis aligned YBa2Cu3O7−δ films apparently due to the uniaxial stress caused by the large thermal expansion mismatch along the c-axis. The [100] oriented YBa2Cu3O7−δ films deposited on SrTiO3(100) substrates contain two epitaxial domains with a different in-plane c-axis orientation by 90°. They show orthogonally crossing cracks developed due to the biaxial stress. The residual stress along the c-axis of YBa2Cu3 dramatically degrades the electrical properties.

Cryogenic STM/STS of Sub-Nanometer Superstructure in High-Tc YBa2Cu3O7-δ Thin Films

Atomic resolution STM images and tunneling spectra with superconducting energy gap structure were simultaneously obtained on (001) and (110) surfaces of superconducting YBa2Cu3O7-δ (YBCO) thin films, for the first time. S-wave pairing mechanism is strongly supported by our spectrum with zero density of states inside the superconducting gap. Scanning tunneling spectroscopy clearly demonstrates the modulation of local electronic structure with a period of 1.2nm, which corresponds to the superstructure consisting of six atomic planes in YBCO.