Hideaki Zama

Properties of metalorganic precursors for chemical vapor deposition of oxide superconductors

In an attempt to improve the reproducibility of chemical vapor deposition of YBaCuO films from metalorganic compounds, we have investigated some properties of several metal β-diketonates by means of thermogravimetric analysis and mass spectroscopy. The results suggest that Ba-compounds should be vaporized under a reduced pressure of as low as 1 Torr. The vaporizing temperature becomes 180°C, being separated from the decomposition temperature of 250°C. Mass spectra of several metal β-diketonates have been obtained for the first time by paying much attention to the vaporizing temperature and ionization voltage.

Epitaxial Growth of YBaCuO Films on Sapphire at 500°C by Metalorganie Chemical Vapor Deposition

Single crystalline films of YBaCuO have been epitaxially prepared for the first time on sapphire substrates at temperatures as low as 500°C by the metalorganic chemical vapor deposition method. The starting materials were Y(HFA)3, Ba(HFA)2, and Cu(AcAc)2. Deposition was carried out with an oxygen pressure of 10 Torr. Electron probe measurements and Auger electron spectroscopy suggest that the constituent of the film is YBa2Cu3Ox. Carbon has not been detected in the film. Spots and streak patterns obtained in the electron diffraction measurements suggest high crystalline quality and smoothness of the surface.

Atomically Flat MgO Single-Crystal Surface Prepared by Oxygen Thermal Annealing

Single-crystal MgO is a good candidate substrate for growing metal oxide films, even though it has a few drawbacks, such as surface degradation caused by its easy reaction with H2O and CO2 and high concentration of impurities originating from the method by which it is grown. We propose a thermal annealing carried out under novel conditions, 700°C and 1 atm O2 atmosphere, for resolving these drawbacks. An atomically flat surface with a half-unit-cell-high step-and-terrace feature was realized on a MgO(100) substrate with a reduced surface segregation of impurities.

Preparation of YBa2Cu3Ox thin films by layer-by-layer metalorganic chemical vapor deposition

We have investigated the preparation of YBa2Cu3Ox thin films on MgO (100) substrates by sequential metalorganic source supply with the metal configuration along the c-axis at low substrate temperature (560°C). Oxygen is also sequentially supplied to the substrate. Four types of films are prepared by various sequential supply patterns of metalorganic precursors and oxygen. Our results show that layer-by-layer oxidation is important for c-axis orientation of the films and that Ba is more suitable than Cu as the first layer with respect to preventing the growth of CuYO2.

In situ monitoring of optical reflectance oscillation in layer-by-layer chemical vapor deposition of oxide superconductor films

We have applied in situ optical diagnostics for the first time in layer-by-layer chemical vapor deposition (CVD) of oxide superconductor films. A cw diode laser of 780 nm emission is used for the light source of the optical experimental system. We have observed the variation of the optical reflection from the growing surface during the layer-by-layer CVD of YBaCuO and related oxides. This method can be used as a diagnostic tool to monitor (i) the crystal growth with an atomic-scale accuracy similar to RHEED oscillation and (ii) the surface reactions of the decomposition of precursors, the oxidization of metals, and so forth.

Atomic layer controlled metalorganic chemical vapor deposition of superconducting YBa2Cu3Ox films

Abstract Thin films of YBa2Cu3Ox(YBCO) have been prepared by atomic layer-by-layer metalorganic chemical vapor deposition (MOCVD) using β-diketonate complexes as precursors and N2O as an oxidizing agent. A nearly monomolecular layer can be deposited by a combination of one cycle of precursor supply. Tc = 78 K and 85 K has been obtained for a 12 nm thick film and a 24 nm thick film, respectively. Atomic force micrographs have clarified that surfaces of YBCO films consist of steps of one unit cell height. The extended width of terrace from step flow growth compared to the co-deposition method may be evidence for migration enhancement due to layer-by-layer growth. A very smooth surface with roughness of less than monomolecular layer and free of precipitates over 10 μm×10 μm has been obtained.

Superconducting Properties of Ultrathin Films of YBa2Cu3Ox Prepared by Metalorganic Chemical Vapor Deposition at 500°C

Low-temperature ( 500°C) growth of YBa2Cu3Ox films by metaloganic chemical vapor deposition on (100)MgO substrates using N2O as an oxidizing agent has been investigated. Films 12 unit cells thick show superconducting onset critical temperature of 89 K and zero-resistivity critical temperature of 80 K. Films 4 unit cells thick show T c (zero) of 30 K. Films as thin as 3 unit cells thick reveal superconducting onset characteristics. If the topmost (cap) layer and the bottom (buffer) layer are nonsuperconducting, this result suggests that superconductivity arises even from monomolecular layer of YBa2Cu3Ox .

Preparation of Sr/sub 2/AlTaO/sub 6/ thin films by metalorganic chemical vapor deposition

Sr/sub 2/AlTaO/sub 6/ (SAT) thin films were prepared by metalorganic chemical vapor deposition (MOCVD) using a double-metal alkoxide TaAl(O-iC/sub 3/H/sub 7/)/sub 8/ and Sr(DPM)/sub 2/-2tetraene as precursors. The former precursor was found to be stable for more than 300 h to prepare nearly stoichiometric thin films with the Ta/Al ratio of 1.0-1.3. High crystallinity of the films on SrTiO/sub 3/ substrates with the Sr/Al ratio slightly smaller than 2.0 was confirmed by X-ray diffraction. Moreover, 200 nm-thick SAT thin films were deposited on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thick films grown by liquid phase epitaxy. Measurements of dielectric properties using a parallel capacitor structure 200 /spl mu/m in diameter confirmed the conductance of the film lower than 10/sup -8/ S as well as an almost temperature-independent dielectric constant of approximately 24. These results indicate that SAT films grown by MOCVD are promising to be incorporated in high-T/sub c/ multilayer structures for electronic devices.

Investigation of YBCO Single Crystals Grown under 1 atm Oxygen Pressure and Air Atmosphere

The effects of oxygen partial pressure on Y123 single-crystal growth, morphology and properties were investigated. YBCO (Y123) single crystals were fabricated using the crystal pulling method under 1 atm oxygen pressure. Compared with crystals fabricated under air atmosphere, it was found that this high oxygen partial pressure played a significant role in increasing growth rate, which was 1.5–2.5 times higher than that of crystals fabricated under air atmosphere. A large crystal with size of 19.8×19.5 mm2 in the a–b plane and 16.5 mm in the c-axis direction was successfully obtained using a crucible 50 mm in diameter. In the YBCO system, critical temperature T c was not sensitive to the growth atmosphere.

New Growth Conditions for the c-Axis Oriented NdBa2Cu3Ox Films by Pulsed Laser Deposition: Lower Substrate Temperature and Higher Oxygen Pressure

The substrate temperature at which the c-axis oriented NdBa2Cu3Ox (NdBCO) film grows is 80–100°C higher than that for the c-axis oriented YBa2Cu3Ox (YBCO). This is a drawback of composing multilayers for device applications although NdBCO has crystalline and superconductive properties superior to those of YBCO. We successfully grew the c-axis oriented NdBCO films in the temperature range lower than that for the a-axis preferred oriented film at a high oxygen pressure by pulsed laser deposition. The optimum c-axis oriented growth condition was determined as follows: substrate temperature is 790°C, oxygen pressure 900 mTorr, and laser energy density 0.5 J/cm2.