Hisashi Nagai

SURFACE MORPHOLOGY AND GROWTH MECHANISM OF YBA2CU3O7-Y FILMS BY METALORGANIC CHEMICAL VAPOR DEPOSITION USING LIQUID SOURCES

We have investigated surface morphology of YBa2Cu3O7−y thin films prepared by chemical vapor deposition (CVD) using liquid metalorganic (MO) sources on MgO(100) single crystalline substrates by atomic force microscopy (AFM). An abrupt change in the terrace width was observed at the deposition temperature of around 750 °C. An anomalous decrease in the efficiency of incorporation of the yttrium component into the film was also found above the same temperature. It suggests that the appearance of liquid phase on the growing surface and the growth mode change from the conventional vapor growth to the VLS (vapor–liquid–solid) growth mode at this temperature.

Preparation of films by metal organic chemical vapour deposition using liquid-state nonfluorinated sources

We have prepared superconducting films by the metal organic chemical vapour deposition (MOCVD) technique using liquid-state fluorinated sources and observed the temperature dependence of the morphology of the films. To prevent several problems caused by the existence of fluorine, we prepared (YBCO) films on (100) MgO and (100) (STO) substrates by MOCVD using liquid-state nonfluorinated sources in this work. The reproducibility of the film composition was almost the same as for the liquid-state fluorinated sources, and better than in the case of the solid-state sources. The critical temperature of the film on STO was 89.5 K. A diffusion layer was found between the (100) MgO substrate and the YBCO film from measurements by Rutherford backscattering spectroscopy on the film deposited using the fluorinated sources, whereas such a layer did not appear in the films deposited using the nonfluorinated sources. This revealed that the films prepared from the nonfluorinated sources had higher quality than the films prepared from the fluorinated sources.

Preparation of YBa2Cu3O7-δ Thin Films by Metal-Organic Chemical Vapor Deposition Using Liquid Sources

YBa2Cu3O7-δ (YBCO) thin film was fabricated by metal-organic chemical vapor deposition (MOCVD) using newly developed liquid MO-sources (MO=metal-organic). The melting points of the MO-sources are lower than those of conventional MO-sources, and they have stable vapor pressure even in the liquid state. The MO-sources contain fluorine; however, when water was added to the flowing oxygen gas, the fluorine-to-oxygen ratio of the films was reduced to less than 0.08 at% as determined by secondary ion mass spectroscopy (SIMS). The reproducibility of the composition and the superconducting properties of the prepared films was excellent compared to that of films fabricated using conventional MO-sources.

Surface Morphology of YBa2Cu3O7-y Films Prepared by Metalorganic Chemical Vapor Deposition Using Liquid Sources

We have prepared c-axis oriented YBa2Cu3O7-y thin films by metalorganic chemical vapor deposition (MOCVD) on MgO(100) substrates using newly developed liquid-state sources with and without fluorine and studied the surface morphology of the films using atomic force microscopy (AFM). Spiral steps accompanied by screw dislocations were observed on the surface of films prepared with both fluorinated and nonfluorinated sources in the AFM image. The shape of growth steps for the former is rounded, whereas the latter is polygonal, suggesting that the lateral bonding force between the growth units, which is represented by the Jacksons α factor, varies for each case. The kinetic roughening effect on the spiral shape is a minor effect compared to the α factor effect.

Preparation and surface morphology of YBa2Cu3O7−y films by metalorganic chemical vapor deposition block by block deposition using liquid sources

Abstract We have recently studied the preparation and the characterization of the films deposited by metalorganic chemical vapor deposition (MOCVD) using liquid state sources [Y. Yoshida, Y. Ito, I. Hirabayashi, H. Nagai, Y. Takai, Appl. Phys. Lett. 69 (1996) 845; Y. Ito, Y. Yoshida, Y. Mizushima, I. Hirabayashi, H. Nagai, Y. Takai, Jpn. J. Appl. Phys. 35 (1996) L825.]. In this paper, we describe the CVD process for the ultra-flat YBCO films block-by-block deposition using liquid sources. The high stability and good controllability of the sources enable us to study the early stage of epitaxial growth by depositing c -axis-oriented films with different number of sequence and different kind of substrates. Investigating the films deposited at 800°C, we confirmed that the initial growth of YBCO films on MgO is multinuclear growth, and in the early stage ( t ≤200 A), spiral growth appears, whereas that on SrTiO 3 is two-dimensional growth and spiral growth appears after coalescence of initially grown crystallites having a flat-faceted surface.

Preparation of a-axis oriented YBa2Cu3O7−y films by metalorganic chemical vapor deposition using liquid sources

Abstract We have recently studied the preparation and the characteristic of the YBCO thin films by metalorganic chemical vapor deposition (MOCVD) using liquid state sources. In this study, we describe the control orientation of the films. The films were prepared on SrTiO 3 (100) single crystalline substrate using liquid state sources in order to discuss such issues as the a-axis growth mechanism. At the temperature between 650 and 700°C the films deposited on SrTiO 3 were strongly a-axis oriented even without photo-assistance. The TEM image of the film deposited at 700°C showed that the structure is c-axis oriented grains in the region less than about 200Afrom the substrate, whereas the main structure is a-axis oriented grains. Thus liquid sources enable us to control the crystalline orientation in MOCVD, that has never been attained.

Preparation of A-axis oriented YBa2Cu3O7-y films by metalorganic chemical vapor deposition using liquid sources

Superconducting YBa2Cu3O7-y (YBCO) thin films were prepared on SrTiO3(100), NdGaO3(110) and LaAlO3(100) single crystalline substrates by metalorganic chemical vapor deposition (MOCVD) using various kinds of liquid sources. Films deposited using Ba(DPM)2tetraen on SrTiO3(100) at temperatures between 650° C and 700° C were strongly a-axis oriented. Furthermore, films deposited in the same temperature range, but grown on NdGaO3(110) and LaAlO3(100), showed a-/b-axis mixed orientation and b-axis orientation, respectively. Thus, deposition using liquid sources enables us to control the orientation in MOCVD, which has never been attained.

Reproducibility of Composition and Superconducting Properties for YBa 2 Cu 3 O 7-δ Thin Films Prepared by Metal-Organic Chemical Vapor Deposition Using Liquid Metal-Organic Sources

YBa2Cu3O7-δ (YBCO) thin film was prepared by metal-organic chemical vapor deposition (MOCVD) using liquid MO-sources. The melting points of the MO-sources were lower than those of conventional ones, and they have stable vapor pressure even in the liquid state. The MO-sources used contain fluorine atoms, however, when water was added to the supplying oxygen gas, fluorine compound was removed from the films less than 0.08at.% of [F]/[0] ratio by secondary ion mass spectroscopy(SlMS). The reproducibility of the composition and the superconducting properties of the prepared films was excellent as compared to those of the films made from conventional MO-sources.

Preparation and Surface Morphology of YBa2Cu3O7-y Films by MOCVD using Liquid Sources

The superconducting YBa2Cu307-y thin films were prepared on single-crystalline substrates by metalorganic chemical vapor deposition (MOCVD) with liquid sources. The stability of liquid sources and reproducibility of deposition enable us to observe details of the crystal of the growth in CVD. We investigated surface morphology of the samples by atomic force microscopy (AFM), and found the abrupt change in the terrace width at the deposition temperature of around 750°C. It suggests the appearance of a liquid phase on the growing surface by CVD growth and the change of the growth mode at this temperature. Key words: YBa2CO7-y thin film, MOCVD, surface morphology, liquid source