Reactive coevaporation of YBaCuO superconducting films
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Temperature (°C) FIG. 2. The regions of phase space where films were made, as described in this paper. In Region I superconducting films were only obtained with activated oxygen species, O and O3. Superconducting films were obtained across all the other regions. The different shading of the regions represents the various oxygen species used. The line on the left is the thermodynamic stability line for YBa2Cu3O,,, the solid part is taken from the data as presented in Ref. 19, and the dashed part is an extrapolation. The dashed line on the right represents the tetragonal to orthorhombic transition as presented in Ref. 30.
J. Mater. Res., Vol. 6, No. 4, Apr 1991
685
V. Matijasevic ef al.: Reactive coevaporation of YBaCuO superconducting films
oxygen were used in different regimes for purely historical reasons. The dimensions of each region correspond to both the extent of explored parameters and their uncertainty. Region IV in Fig. 2 is where the oxygen pressure was kept the most uniform and also where it was measured most accurately (to within a factor of 2, as described above). Since uniformity of pressure is closely linked to uniformity of composition, this means that Region IV corresponds to where we had the most compositionally uniform films. For this reason it is in this region that we looked carefully at the compositional dependence of the film properties. By cooldown we refer to the process by which the film is brought from deposition temperature and pressure to room temperature and pressure. On the p-T diagram of Fig. 2 this corresponds to going to the upper right corner. Many different paths are possible, but they are not equivalent. In fact, there are some indications that they are not even always reversible.20 For molecular oxygen we have tried to maximize oxygenation during cooldown by going first to higher pressure and then to lower temperature, thereby keeping the diffusion kinetics fast for the uptake of oxygen. This corresponds to going straight up on the diagram and then after reaching 10 Torr going to the right. For atomic oxygen and ozone the films were kept under the growth oxygen conditions during the cooling to temperatures less than 400 °C, after which the chamber was vented to oxygen. III. FILM CHARACTERIZATION
Films were routinely characterized by x-ray diffraction and electron microprobe analysis. Selected samples were characterized by SEM, plan view TEM, and crosssectional TEM. Electrical characterization consisted of dc resistive transitions, conductance measurements with an ac two-coil mutual inductance probe, surface resistance measurements at 10 GHz, penetration depth measurements using a microwave stripline resonator, critical currents from magnetization measurements with a vibrating sample magnetometer, and transport critical currents. A Picker 4-circle diffractometer with Cu Ka radiation was used for x-ray diffraction study.
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