Gin-ichiro Oya

Transition temperatures and crystal structures of single‐crystal and polycrystalline NbNx films

Single‐crystal cubic NbNx films with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of single‐crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH3+H2 or in H2 alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5 were prepared. The maximum Tc was observed in cubic NbNx. Both Nb4N3 and Nb4N5, the tetragonal phases with long‐range‐ordered arrangement of vacancies, exhibited superconductivity. NbN1.0 and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNx became close to stoichiometric, its Tc increased. However, the maximum Tc of cubic NbNx with nearly stoichiometric composition was limited by the lattice instability of cubic NbNx and by the resulting cubic NbNx to hexagonal NbN transformation. It might be expected that a higher Tc for compounds of the NbN family could be obtained if a NaCl‐type crystal with less vacancies could exi...

High-quality single-crystal Nb films and influences of substrates on the epitaxial growth

Highly pure single‐crystal Nb films (thickness, ∼2000 A) with high superconducting transition temperatures Tc of ∼9.3 K and high resistance ratios R300/R10 up to ∼200 are successfully grown epitaxially on single‐crystal sapphire (α‐Al2O3) and MgO substrates at ∼500–∼700 °C by using an electron‐beam evaporation technique. The most high‐quality single‐crystal Nb film (with the maximum Tc of 9.45 K and the maximum R300/R10 of 199) is obtained on a sapphire (1102) substrate, which has a thermal expansion coefficient very close to that of Nb as well as a small lattice misfit to Nb. The quality of obtained single‐crystal Nb films is found to be lowered by the deposition of the films on substrates with thermal expansion coefficients different far from that of Nb because of the production of internal strains and lattice defects in the films cooled down.

Phase transformations in nearly stoichiometric NbNx

Processes of the phase transformation in nearly stoichiometric niobium nitride, from cubic δ‐NbN to hexagonal e‐NbN, are studied by selected‐area electron diffraction, by using NbNx films of the transition state prepared by annealing δ‐NbNx films at about 1000 °C in an atmosphere of H2 gas. The diffraction patterns taken from the annealed NbNx films suggest that the phase transformations (vacancy‐poor‐) δ‐NbNx (cub.) → (N‐vacancy‐) ordered δ‐NbN (tet.)  → ordered ζ‐NbN (hex.)  → ordered γ′‐NbN (mon.)  → δ′‐NbN (hex.)  → e‐NbN (hex.) occur. Ordered δ‐NbN, ordered ζ‐NbN, ordered γ′‐NbN, and δ′‐NbN are transient phases which occur during the transformation δ‐NbNx → e‐NbN, and the superlattice structures of the three ordered phases are described by the ordering of N vacancies with random Nb vacancies in their fundamental lattices. Mechanisms of the transformations from δ‐NbNx to e‐NbN at about 1000 °C are discussed on the basis of the transient phases observed during the phase transformations.

Growth of α‐Al2O3 films by molecular layer epitaxy

Single‐crystal α‐Al2O3 films are, for the first time, successfully grown on sapphire wafers above ∼600 °C by the molecular layer epitaxy (MLE) method using AlCl3 vapor and a He 15%O2 gas mixture. The average growth rate observed barely depends on the substrate temperature, being ∼0.09 nm per cycle of gas transport, under the used growth conditions. The epitaxial growth of α‐Al2O3 films on single‐crystal Nb films at ∼500 °C by the MLE method is also confirmed for the first time.

Superconducting transition temperatures of thin V3Si layers formed by the interaction of V films with thinly oxidized Si wafers

Superconducting thin V3Si layers with overlying VOx layers have been formed by the interaction of thin evaporated V films (300–3500 A thick) with thinly (∼100 A thick) oxidized Si wafers between ∼650 and ∼1000 °C. V3Si layers having thicknesses up to ∼800 A have been obtained. The growth of the V3Si layers has been observed to depend on the annealing temperature and time, and to be affected by the oxygen concentration in VOx layers formed on the V3Si layers, and by the disappearance of SiOx layers on the Si wafers. The transition temperatures of the V films annealed on the oxidized Si wafers can be interpreted with regard to proximity effects between the V3Si layers formed and overlying VOx layers simultaneously formed, and between the V3Si layers and underlying Si‐richer VSix layers additionally formed in the films.

Molecular layer epitaxy of α-Al2O3 films

Abstract α-Al 2 O 3 films are grown epitaxially on single-crystal sapphire (1 1 02) and (0001) wafers at ≈ 660° C and Nb (100) films at ≈ 450° C by the molecular layer epitaxy (MLE) method using AlCl 3 vapor and a He-15%O 2 gas mixture, and are characterized systematically. The epitaxial films are observed to have surface flatness depending on that of the substrates in the early stage of the growth. Their growth rates are observed to be 0.09-0.1 nm per cycle of gas transport, depending on the stacking sequences of Al and O atomic layers and on the geometrical limitation of the amount of aluminium chloride molecules produced by AlCl 3 dissociation and adsorbed on the wafer. The low-temperature growth of α-Al 2 O 3 films on Nb films is confirmed to occur favorably without alloying.

Enhanced growth mechanism and superconducting properties of V3Si layers formed by sputter‐deposited V films on SiO2 substrates

An enhanced growth of V3Si layers by rf‐sputter deposition of V films on fused silica substrates held at temperatures between ∼350 and ∼850 °C has been found. The growth rate at 500 °C shows a (time)1/2 dependence and is as high as ∼1.8×10−14 cm2/sec. The activation energy is estimated to be as low as ∼1 eV. The enhanced growth is attributed to the formation of intermediate V‐Si‐O layers and to the presence of lattice defects induced in the V films by the sputter‐deposition process. The superconducting transition onset temperatures of the V3Si increases with increasing thickness up to 16.9 K. The layers formed at 500 and at 600 °C have critical current densities of (2–7)×105 A/cm2 at 4.2 K in applied magnetic fields lower than 35 kOe. These layers show different angular dependences of critical currents with respect to field direction. This difference is related to the different growth morphologies of the V3Si layers.

SUPERCONDUCTING TRANSITION TEMPERATURES OF VAPOR-DEPOSITED NIOBIUM NITRIDE.

Thin films of the superconducting compound niobium nitride were deposited at atmospheric pressure on fused silica substrates by reaction of gaseous niobium pentachloride with ammonia and hydrogen gases in a fused silica apparatus. Suitable conditions for the preparation of NbN were found by calculating the free energies of reaction for several possible reactions involving niobium chlorides, ammonia, and hydrogen. Niobium nitride films could be formed at substrate temperature ranging from 900°–1000°C. The deposition rate varied with the reaction conditions from 20 to 1500 A/sec, and deposits ranged from smooth films of polycrystals in which the individual crystal faces were 20 μ in length. The x-ray diffraction data for the NbN films deposited at 1000°C revealed face-centered cubic structure with cube edge of about 4.358 A. Niobium nitride films prepared by this method had a wide range of resistivities, temperature coefficients of resistance, and superconducting transition temperatures depending on the dep...

Influences of gas flow on chemical vapor deposition of superconducting Nb‐Ge films

Nb‐Ge films, which have been prepared by chemical vapor deposition, have been studied for superconducting transition temperatures, crystal structures, and growth morphology. In the films a relationship was obtained among positions at which the highest Tc films are deposited, the degree of orientation for A15 crystallites is lowered, and the microstructures for film surface change has been found. This can be explained in terms of a higher deposition rate of Ge than that of Nb.

An anomaly in the angular dependence of critical current densities of niobium films

The angular dependence of critical current density J c of a highly pure single-crystal Nb film in the mixed state near its upper critical field B c2 is measured and compared with that of a less pure polycrystalline Nb one. An anomalous behavior of J c is found in addition to a high J c due to the pinning of flux lines at the surface for the single-crystal film. The anomaly is discussed with respect to an interaction between the flux-line lattice and Nb crystal lattice.