Nobuaki Shohata

High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers

As‐deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X‐ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as‐deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.

Epitaxial Y-Ba-Cu-O Films on Si with Intermediate Layer by RF Magnetron Sputtering

Epitaxial films of Y-Ba-Cu-O were obtained on Si substrate using epitaxial intermediate layer consisting of SrTiO3(or BaTiO3)/MgAl2O4. MgAl2O4 was epitaxially grown on Si(100) substrate by chemical vapor deposition, and then SrTiO3 or BaTiO3 was also epitaxially grown on MgAl2O4 layer by means of RF magnetron sputtering. Y-Ba-Cu-O films were prepared on SrTiO3(BaTiO3)/MgAl2O4/Si substrates by RF magnetron sputtering and their epitaxial growth was confirmed by RHEED observation and X-ray diffraction measurements. Epitaxial orientations of Y-Ba-Cu-O films varied in dependence on RF input power; lower RF power resulted in c-axis oriented film and higher RF power resulted in a-axis oriented film. Preparation of Y-Ba-Cu-O directly on MgAl2O4/Si was also studied, but only randomly oriented polycrystal film has been obtained so far. In sputter Auger depth measurement, any notable diffusion between Y-Ba-CuO film and the substrates was not observed. Resistive superconducting transitions with zero resistance at 65K on SrTiO3/MgAl2O4/Si and at 70K on BaTiO3/MgAl2O4/Si were observed.

Preparation of epitaxial ABO3 perovskite-type oxide thin films on a (100)MgAl2O4/Si substrate

Epitaxial thin films of ABO3 perovskite‐type oxides, including PbTiO3, (Pb0.90La0.10) (Zr0.65Ti0.35)0.975O3,BaTiO3, and SrTiO3, have been successfully obtained by rf magnetron sputtering on (100)Si substrate with an intermediate epitaxial layer of MgAl2O4. Only PbTiO3 grew in the tetragonal crystal structure and other materials grew in the cubic structure. The unit axis direction of the perovskite‐type oxide films was coincident with that of the underlying MgAl2O4 films. The tetragonal PbTiO3 films were a mixture of c and a domain. The preferred orientation of the tetragonal PbTiO3 film, that is c to a domain volume ratio, could be controlled by the conditions of sample cooling after the deposition. Highly c‐axis oriented films, which consisted of more than 90% c domains, were produced by cooling the sample at a high cooling rate, typically 30 °C/min, and by maintaining an rf plasma during cooling. The mechanism of the preferred orientation of PbTiO3 film has been explained by a balance of compressive str...

Synthesis and properties of ultrafine AlN powder by rf plasma

High‐purity ultrafine AlN powder was synthesized by rf plasma through direct nitridation of Al. The specific surface area was about 30 m2 /g under typical experimental conditions. Ultrafine AlN powder showed excellent sinterability, compared with conventional AlN powders, whose particle sizes were more than 0.5 μm. The thermal conductivity for the sintered body reached 220 W/m K when sintered at 1900 °C, and 110 W/m K when sintered at 1400 °C, by using YF3 as a sintering aid.

Epitaxial Growth of PbTiO3 on MgAl2O4/Si Substrates

Lead titanate (PbTiO3) thin films are successfully sputter-deposited onto (100)MgAl2O4/(100)Si substrates at 480–550°C sputtering temperatures. It is confirmed, from X-ray and electron diffraction analyses, that the PbTiO3 films grow epitaxially on MgAl2O4 films. The crystal properties of PbTiO3 films depend on those of MgAl2O4 films strongly. The PbTiO3 films are composed of c-axis and a-axis oriented domains. The ratio of the c-axis oriented domain to the a-axis oriented domain is controllable by the cooling rate of the substrate. The dielectric constant of the PbTiO3 film is estimated to be about 100, based on C-V measurements with a metal-insulator-semiconductor structure.

Thermal conductivity of diamond films

The thermal diffusivity of diamond films grown on the Si substrate by the hot‐filament chemical vapor deposition technique in gas mixtures of 1% to 5% methane in hydrogen were studied. Thermal conductivity of the film prepared at 1% CH4 concentration reached about 1200W/mK, which is almost equal to that for type I natural diamond, and decreased rapidly to less than 200W/mK with increasing CH4 concentration. Hydrogen contents in the films increased with a decrease in thermal conductivity. This indicates that the thermal conductivity for diamond films is correlated to the amount of incorporated hydrogen impurity.

The grain size dependence of the mobility and lifetime in chemical vapor deposited diamond photoconductive switches

The grain size dependence of the carrier lifetimes and collection distances of chemical vapor deposited (CVD) diamond films of 0.1–10 μm average grain size was measured. The estimated values of the mobilities and lifetimes indicated that the dominant recombination process had occurred inside the grains, not the grain boundaries. This finding was confirmed by measuring the electric field dependence of the lifetime and the collection distance. Under a high electric field, however, a major decrease in decay time is expected for smaller grain sizes, due to the increase in the number of carriers which reach the grain boundaries during their lifetime. Such a decrease was observed in a 0.8-μm grain size sample at E=105 V/cm. The data also showed that a 1-ps kV electrical pulse from a dc bias across a diamond film coated gap can be achieved with a CVD-deposited diamond photoconductive switch of sub-μm grain size.

Mechanical properties of diamondlike carbon films

Diamondlike carbon (DLC) films have been deposited onto room‐temperature silicon substrates, which are placed on a cathode, in a dc plasma of methane (CH4) and hydrogen (H2) gas mixture. Ultramicroindentation hardness and Young’s modulus for the films were measured. The mechanical properties of DLC films change greatly in two different anode positions relative to the cathode. In the case of an upward anode position (so‐called parallel electrode), the maximum ultramicroindentation hardness value is 200 GPa(3700 Hv) and the maximum Young’s modulus value is 480 GPa. At a sideward anode position, the maximum ultramicroindentation hardness value and Young’s modulus value are 480 GPa(7500 Hv) and 850 GPa, respectively, which are much higher values than those at the upward anode position. The contact‐start‐stop test results indicate that DLC films have a possibility for use as a wear‐resistant protective layer.

Low‐temperature annealing effect of as‐grown Bi‐Sr‐Ca‐Cu‐O thin films on superconducting property and the c‐axis lattice constant

As‐grown crystallized Bi2Sr2Ca1Cu2Ox phase films were prepared on MgO(100) substrates by rf magnetron sputtering. The as‐grown film showed the Tc onset of 92 K and the Tc end of 82 K. By annealing in the furnace at 480 °C after deposition, superconducting property and the c‐axis lattice constant varied with atmosphere during annealing. The film after annealing in air at 480 °C exhibited the highest superconducting transition with the Tc onset of 93 K and the Tc end of 85 K. The Tc onset increased when the c‐axis lattice constant was increased, suggesting that the Tc onset increases with decreasing the oxygen content of the film.

Photoconductive properties of chemical vapor deposited diamond switch under high electric field strength

Photoconductive properties of diamond optical switch made by chemical vapor deposition method were investigated. A new configuration of the diamond gap was proposed to reduce the surface leakage current and avoid surface flashover. This technology made it possible to apply static high electric field up to 2×106 V/cm. The dependence of the mobility‐lifetime product (μτ) on the grain size was measured for a wide range of electric field. The μτ value was increased to be linearly proportional to the electric field for every grain size sample, and no saturation was measured even at a high electric field of E=3×105 V/cm. Larger grain size samples had larger μτ values. The grain size dependence was attributed to the decreasing of the mobility or the lifetime inside the grain not due to the increasing recombination ratio at the grain boundary in smaller grain size samples.