Osamu Michikami

Surface Damage on Si Substrates Caused by Reactive Sputter Etching

Surface damage on silicon substrates caused by RSE has been investigated. Defects are classified into four categories in order of destruction, namely, precipitates surrounded by elastic strain field, surface roughness pattern, polycrystalline and amorphous silicon, and no defects, based on the results of TEM and RHEED observation. In particular, precipitates which reach a depth more than 500 A were found to cause OSF. The surface damage consists of contamination (C, F, O) layer, C and defect mixed layer, and defect layer in order from the top. The degree of these defects and contaminations expand with increasing power density and etching duration. RSE conditions, where no defects are formed, were determined, e.g., within 1 minute at 0.4 W cm-2.

Synthesis of Y-Ba-Cu-O Thin Films on Sapphire Substrates by RF Magnetron Sputtering

Y-Ba-Cu-O thin films have been prepared on sapphire substrates by RF magnetron sputtering from sintered targets. The substrate temperature dependence of Ba and Cu concentrations against Y of as-sputtered films has been clarified. Film composition deviated from target composition. Cu concentration decreased significantly with increasing Ts, while Ba concentration was independent of the substrate temperature. Using the CuO-compensated Y1Ba6Cu10Ox target, assputtered high-Tc films (Tc onset=87 K, Tc endpoint=40 K) have been successfully obtained as a Ts of about 580°C.

Superconducting and Structural Properties of EuBa2Cu3O7-δ Ultrathin Films Deposited on MgO(100) Substrates Using Magnetron Sputtering

EuBa2Cu3O7-δ thin films were produced on MgO(100) substrates at 580°C using planar-type magnetron sputtering and were studied by X-ray diffraction and RHEED. The c-oriented films were epitaxially grown with a crystal orientation of [110]EuBaCuO//[010]MgO. Films more than 1000 A thick exhibited Tc endpoints of ~90 K. As film thickness decreased, superconducting properties deteriorated. However, even films with 2-unit-cell thickness became superconducting. The cause of the deterioration of superconducting properties in ultrathin films is discussed on the basis of the relationship between lattice matching and structural properties.

Epitaxy and orientation of Eu1Ba2Cu3O7−y films grown in situ by magnetron sputtering

Superconducting films of Eu1Ba2Cu3O7-y have been grown in situ onto SrTiO3 and MgO (100) substrates by magnetron sputtering from a stoichiometric oxide target. The sputtering process results in the growth of epitaxial films with perfect a-axis orientation as well as perfect c-axis orientation. The orientation of the films can be controlled by the proper choices of substrate temperature, oxygen pressure, and film growth rate. The routine production of films showing Tc zero of 88–90 K is accomplished. In situ growth of the films with the particular orientation is of importance in the fabrication of multilayer structures such as tunnel junctions.

EuBa 2Cu 3O 7-δ Thin Films Grown on Sapphires with Epitaxial CeO 2 Buffer Layers

Epitaxial growth of CeO2 films on Al2O3 (102) substrates and the effect of CeO2 buffer layers on the growth orientation and superconducting properties of EuBa2Cu3O7-δ (EBCO) were investigated. CeO2 and EBCO films were prepared by rf and dc magnetron sputtering, respectively, and were characterized by X-ray diffraction (θ-2θ scan and scan), atomic force microscopy (AFM) and high resolution scanning electron microscopy. Epitaxial (001) CeO2 films were obtained at an off-center distance (D on-off) of 3.5 cm and a substrate temperature of 660°C. The structural and superconducting properties of EBCO films deposited at 650°C depended on the thickness of the CeO2 buffer layer. The EBCO films deposited on CeO2 50-400-A-thick buffer layers had T ces of 90 K or above. The high-T c EBCO films had in-plane epitaxial orientation relationships of Al2O3 [110]\varparallelCeO2 [100]\varparallelEBCO[110]. The EBCO films on the thin CeO2 buffer layers had rectangular grains similar to those on MgO(001) substrates. The critical current density of the EBCO films with T ce=90 K was about 6 ×105 A/cm2 in zero field at 77.3 K. The T ce varied largely and decreased with increasing CeO2 buffer layer thickness above 500 A. AFM observation of a 1000-A-thick CeO2 film showed growth of bamboo-like crystal grains 1700 A long and 300 A wide along the direction of CeO2 [110]. The ravine depths were about 100 A. The EBCO films on the thick CeO2 buffer layer (>500 A) exhibited poor superconducting behavior and gave (103) or (110) diffraction peaks.

Ellipsometric and optical reflectivity studies of reactively sputtered NbN thin films

The optical parameters of NbN thin films reactively sputtered mainly at room temperature are investigated by in situ ellipsometry using a He‐Ne laser source. Systematic correlations between the ellipsometric parameters and the film properties are observed. In particular, the Tc film shows an approximately linear relation with the parameter ψ over the wide range of 7–17 K. Optical reflectivity measurements are carried out in the wavelength range of 0.2–2.5 μm for these films. Each spectrum is well fitted by the curve calculated using the Drude model with a screened plasma frequency of 25 000–30 000 cm−1. The Tc film also shows a stronger correlation with the damping constant of plasma excitation than with the reflectivity. This is considered to be a universal degradation curve for this system, similar to those observed in high ‐Tc A15 compounds. The relation of Tc to ψ is found to be equivalent to this correlation.

High-Tc Y-Ba-Cu-O Thin Films Prepared by Dual Magnetron Sputtering

Y-Ba-Cu-O thin films (200-300 nm thick) have been magnetron-sputtered on heated sapphire substrates. By means of dual sputtering configuration with a Cu and sintered Y-Ba-Cu-O target, films having compositions closer to Y1Ba2Cu3Oy are reproducibly obtained at a substrate temperature of about 600°C. They have a nearly single-phase perovskite-like structure with highly c-axis preferred orientation and show a Tc onset of 90 K. Remarkable improvement in the zero resistivity temperature (up to 73 K) is achieved by subsequent annealing of these films at 550-650°C, while annealing at higher temperatures leads to Tc degradation.

YBaCuO Epitaxial Film Formation by Magnetron Sputtering with Facing Targets : I. Effects of Target and Substrate Positions

YBa2Cu3O7-δ (YBCO) thin films were prepared on non-rotating MgO(100) substrates at 630°C using dc magnetron sputtering with facing targets (dc-MSFT) and the influence of target and substrate positions on film characteristics, such as superconducting properties, composition (CCu/Y, CBa/Y), crystallinity and uniformity was investigated. Spatial positions at which as-grown epitaxial films with Tce=90 K were deposited were consequently found. Away from the center position, the composition of thin film deviated from a stoichiometric composition, that is, both CCu/Y and CBa/Y had a tendency to decrease. As-grown films with of Tce over 88 K were obtained in a 5×5 cm2 area of the substrate. The deposition rate was about 20 A/min at the center position and decreased by 22% at a position 2.5 cm away from the center.

Epitaxial Growth of SrxTiOy and Fabrication of EuBa2Cu3O7-δ/SrxTiOy/Pb Tunnel Junctions

Thin films deposited from a SrTiO3 (STO) target using rf magnetron sputtering were examined. The Sr1.6TiOy films with perovskitelike structure grew epitaxially at substrate temperatures above 500°C. The trilayered films of EuBa2Cu3O7(EBCO)/STO/EBCO were deposited eptaxially on STO(110) substrates, but partial polycrystal growth in the EBCO film of the third layer was observed. In view of this result, EBCO(110)/STO(110)/Pb tunnel junctions were produced on STO(110) substrates. The I-V characteristics of junctions with an Rnn of 10 Ω showed a gap opening at a bias voltage of about 10 mV, a clear gap structure at 2.5 mV and Rj/Rnn=12.4 below 2.5 mV. The low-energy gap below 2.5 mV was caused by the deterioration of the EBCO base electrodes due to the junction fabrication process. The large Rj/Rnn suggests that an STO epitaxial ultrathin film can be a good low-leakage barrier. On the other hand, lower resistance junctions showed the development of a supercurrent at zero bias.

Epitaxial growth of EuBa2Cu3O7-y films on YAlO3 single crystals

Epitaxial growth of 1-2-3 superconductor films is reported on a new substrate material that possesses desirable dielectric properties. Magnetron-sputtered EuBa2Cu3O7-y films on YAlO3(001) show excellent properties with a Tc of 91 K. Films are epitaxially grown with the c-axis oriented normal to the surface, and the a- and b-axes ordered in the plane. The high-frequency (50 GHz) surface resistance of a film is 2 mΩ at 30 K. The dielectric constant of YAlO3 is 16. The dielectric loss tangent at 10 GHz is as low as 7.3×10-5 at 300 K and 2×10-6 at 40 K. YAlO3-based films have considerable potential for high-frequency application.