In-Situ Growth of Superconducting Bi-Sr-Ca-Cu-O Thin Films by Activated Reactive Co-evaporation
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In-Situ Growth of Superconducting Activated Reactive Co-evaporation K. Yoshikawa, T. Fujitsu Limited, Japan
Bi-Sr-Ca-Cu-O Thin Films by
Satoh, N. Sasaki, and M. Nakano 1015 Kamikadanaka Nakahara-ku, Kawasaki,
211
ABSTRACT cooling conditions on surface The effect of in-situ roughness and superconducting properties have been studied. on (100) MgO Bi-Sr-Ca-Cu-O thin films were grown in-situ The substrates at 700°C by activated reactive co-evaporation. cool-down in 760 Torr oxygen showed a transition films of 100 K and zero resistance temperature (Tc(onset)) Smooth surface was obtained temperature (Tc (zero)) of 65 K. for the sample cooled-down in oxygen plasma. INTRODUCTION surface as well as Electronic applications require flat surface requires in-situ Obtaining a flat high Tc and Jc. processes, because higher temperature post-annealing causes rough surfaces. In-situ fabrication of Y-Ba-Cu-O [1],[2] and has been intensively studied. We report the Bi-Sr-Ca-Cu-O[3] effect of in-situ cooling conditions on surface roughness and superconducting transition temperature (Tc). EXPERIMENTAL Films were grown on (100) MgO substrate by the activated reactive co-evaporation
technique[4].
The
Table 1. Growth conditions. Substrate
(100) MgO
Substrate temperature
700C
0.4A/s two Growth rate machine has 1 X 10- 4 Torr Oxygen pressure electron-beam guns, and two resistively heated 125W RF power boats; Bi 2O and SrF were evaporaEed from guns, and Ca and Cu metals from resistively boats. Oxygen was introduced directly onto the heated The distance between substrate and the introducing substrate. tube was 5 mm. RF plasma was generated between the sources and the substrate to enhance oxidation and crystallization. Growth conditions are summarized in Table 1. cooling of in-situ the effect We have investigated conditions on surface roughness and Tc for different cooling The cooling conditions were as follows; rate and atmosphere. Torr oxygen. (a) Slow cooling (2*C / minute) in 1x10-4 4 Torr oxygen plasma. (b) Slow cooling (2'C / minute) in 1x10(c) Rapid cooling (30'C / minute) in 760 Torr oxygen after oxygen plasma discharge for 20 minutes. (d) Rapid cooling (30*C / minute) in 760 Torr nitrogen. (e) Slow cooling (2*C / minute) in 760 Torr oxygen after oxygen plasma discharge for 20 minutes. The 20-minutes plasma-discharge in (c) and (e) was carried out to prevent the filament of EB guns and heated boats from Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society
708
oxidizing. That is, this duration is for cooling filaments and boats. Chemical composition was determined by Rutherford backscattering spectrometry (RBS). The crystal structure of the film was studied using X-ray diffraction (XRD). Resistivity was measured with the conventional four-probe method. RESULTS Film thicknesses are from 120 to 150 nm. compositions
Table 2. Film compositions.
Film are
summarized in Table 2. Figure 1 shows the film resistivity for different in-situ cooling conditions. Samples (a) show (d) and semiconductor-like
(a) 2
2.0
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