Ken Sakuta

Hydrogenating Effect of Single-Crystal Diamond Surface

Hydrogenation of diamond has been carried out using the electron-cyclotron-resonance microwave plasma chemical-vapor deposition apparatus. According to reflection high-energy and low-energy electron diffraction and X-ray photoelectron spectroscopy measurements, the natural- and synthetic-diamond surfaces maintained their crystallinity even after the hydrogenation. Seebeck effect measurement and the temperature dependence of the resistance revealed an appearance of deep acceptor levels in the hydrogenated diamond layer. The diffusion depth of the hydrogen by the plasma treatment (2 h, 830°C) was roughly estimated to be ~0.6 µm from the drain current-voltage characteristics of a rudimentary MISFET using the hydrogenated diamond.

Metal-Insulator-Superconductor Field-Effect-Transistor Using SrTiO3/YBa2Cu3Oy Heteroepitaxial Films

Planar-type metal-insulator-superconductor field-effect-transistors (MISFET) were fabricated and their current modulation characteristics were investigated. The FET had a structure of Al-gate metal/(100)SrTiO3-gate insulator/(001)YBa2Cu3Oy-channel, where the oxide layers were grown by selective heteroepitaxy employing pulsed ArF excimer laser deposition. The FET gate was 10 µm long and 100 µm wide. Apparent field-effect modulation of drain current was seen in both the normal (T>Tc) and superconducting (T<Tc) states. In the superconducting state, not only the critical current but also flux-flow resistance were appreciably changed according to the applied gate voltage.

Dielectric properties of SrTiO3 epitaxial film and their application to measurement of work function of YBa2Cu3Oy epitaxial film

The dielectric properties of SrTiO3 epitaxial film were studied using film grown on oxide superconductor YBa2Cu3Oy. They were, to a certain extent, similar to those of the single-crystal bulk in terms of both temperature and electric field dependence, though the value of the dielectric constant was low, particularly at cryogenic temperatures. The observed dielectric properties were analytically expressed. Using the dramatic field dependence of the dielectric constant of the SrTiO3 film in the Au-SrTiO3-YBa2Cu3Oy metal-insulator-superconductor (MIS) structure, we determined the work function of (001)YBa2Cu3Oy epitaxial film to be ~6.1 eV.

Improved Surface Crystallinity of MgO Crystal Substrate through Annealing in Oxygen Atmosphere

The oxygen annealing (OA) effects on MgO substrates commercially available were closely investigated. The smeared low-energy electron diffraction spot pattern and transmissionlike spot pattern of reflection high-energy electron diffraction were dramatically changed to those of almost ideal MgO crystals through OA at 1000°C. After OA, the spotty cathodeluminescence (CL) pattern disappeared almost completely, and the peak wave-length of CL spectra shifted from blue (~400 nm) to red (~715 nm). The 5-nm-thick epitaxial YBaCuO films grown on (100)MgO substrates with OA exhibited good superconductivity. Otherwise, superconductivity was not evident. However, the macroscopic subgrain structure seen in the X-ray topographs showed no meaningful changes between before and after OA.

Epitaxial SrTiO3 thin films grown by ArF excimer laser deposition

SrTiO3 (STO) thin films were prepared by an ArF excimer laser deposition method. At the growth temperature of 630°C, the natural orientation of STO was significantly changed from (111) to (100) and (110). The ratio of X-ray diffraction intensities (100)/(200) was closest to the ideal value when grown at 660°C, resulting in minimized disorder of atomic arrangement. Epitaxial STO films were obtained on STO(100) and MgO(100) substrates, and on an YBaCuO(001) epitaxial film. They showed resistivity in excess of 5×1010 Ωcm and dielectric constant as high as 140e0 at room temperature. The maximum dielectric constant of 440e0 was obtained at 40 K.

Effects of Oxygen Addition on Diamond Film Growth by Electron-Cyclotron-Resonance Microwave Plasma CVD Apparatus

Diamond thin films have been grown by an electron-cyclotron-resonance (ECR) microwave plasma chemical vapor deposition (CVD) apparatus under the addition of oxygen to the host ambient gas of a mixture of CO and H2 gas. Although the amount of oxygen was very minute (0-6% vol. fraction), significant changes due to the oxygen were brought about in the growth properties and the film crystallinity. The Raman spectroscopy revealed a dramatically improved film crystallinity and the film was dominated by the grain growth, resulting in the faceted morphology. With increase in the oxygen up to 4.55%, the visible cathodeluminescence (CL) intensity grew by more than ten times. However, the CL spectrum indicated an appearance of one or two extra recombination centers depending on the amount of oxygen.

YBaCuO Thin-Film Growth on Electrooptic LiNbO3 Substrate with Buffer Layer

Oxide superconducting YBaCuO thin films were grown on Y- and Z-cut LiNbO3 (LN) electrooptic crystal substrates with and without buffer layers, and their film crystallinity was closely investigated by X-ray diffraction, transmission electron microscopy, reflection high-energy electron diffraction and Auger electron spectroscopy. Due to the intensive reaction, direct deposition of films resulted in poor quality. When sputtered (111)-MgO film was introduced as a buffer layer, significant diffusion of constituent atoms was unavoidable among YBaCuO, MgO and LN. As a result, the superconductivity of the YBaCuO layer was substantially degraded. The Y2O3- stabilized ZrO2 (YSZ) grown by sputtering, followed by wet O2 anneal, served as a good buffer layer for Y1Ba2Cu3Oy films on LN substrates, and epitaxial growth of (00l)-Y1Ba2Cu3Oy with Tc0=89 K was reproducibly obtained on Y-cut LN substrates.

Atmospheric Deterioration of Clean Surface of Epitaxial (001)-YBaCuO Films Studied by Low-Energy Electron Diffraction

The effects of gas exposure on the clean surface of the epitaxial YBaCuO thin films were closely investigated using the low-energy electron diffraction (LEED) method. The clean surface was obtained by in-vacuum annealing at 500°C. Once the clean surface was exposed to air, even at room temperature, the LEED spots disappeared or sometimes became faint. To ensure the degradation mechanism of the YBaCuO clean surface, the specimens were exposed to pure O2 and N2 gases separately and measured by LEED. As a result, it was found that O2 is very safe but N2 serves as a poisonous gas for the YBaCuO clean surface.

Homoepitaxial growth of diamond thin films by electron cyclotron‐resonance microwave plasma chemical‐vapor‐deposition apparatus with CO/H2 gaseous source

Homoepitaxy of the diamond thin films has been carried out by employing the ECR microwave plasma CVD apparatus with a CO/H2 gaseous source. From RHEED observation, it appeared that the surface morphology of (100) oriented film was much better than that of the (110) oriented. The (2×1) surface reconstruction remained unchanged even when the film was cooled down in H2 ambient after the epitaxy was completed. Although the synthetic diamond substrates had nonuniformity in their cathodoluminescence patterns, epitaxial films grown onto them offered uniform ones.

Interfacial properties of Al2O3‐InP metal‐insulator‐semiconductor structure prepared in excess organophosphorus atmosphere

The effects of the excess phosphorus vapor added in the chemical vapor deposition (CVD) ambient on Al2O3‐InP metal‐insulator‐semiconductor (MIS) interfacial properties were investigated. The triethylphosphorus (TEP) was used as an excess phosphorus source gas suitable for the low temperature CVD. The change in the surface state density distribution (Nss) curves of the MIS diodes due to TEP introduction was clear: the Nss value of the broad peak at 1 eV, superimposed on the U‐shaped background, was reduced to one order of magnitude lower than that of the TEP‐free device.