The effect of epitaxy on the upper critical fields of evaporated niobium films
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NTRODUCTION
II. EXPERIMENTAL
Highly-reliable Josephson tunnel junctions composed of refractory hard materials including superconducting niobium or its nitride have recently been studied for a number of electronics applications including use as ultrafast switching elements for a future computer system.1"3 Niobium is an attractive superconductor to be used as a material for Josephson junctions and their integrated circuits. The reason for it is because Nb usually has a coherence length £ (—400 A at 4.2 K) much longer and a penetration depth X (—800 A at 4.2 K) shorter than those of any other superconducting Nb compound, the highest transition temperature Tc (9.3 K) of elemental superconductors, mechanical ruggedness, and chemical stability. Moreover, single-crystal Nb can be expected to be improved in the following as compared with a polycrystalline one; namely, the purity, the homogeneity, the electronic scattering and flux trapping at lattice defects, the coherence length, the penetration depth, the mechanical properties, and the surface flatness, which are key problems for fabricating high-performance Josephson junctions and integrated circuits. Therefore, for it the use of highquality single-crystal Nb films as junction electrodes, ground planes, and strip lines may be desirable. However, there are only a few reports concerning properties of highquality single-crystal Nb films.4"6
Nb films with the thickness of —2000 A were deposited at the rates of 3-30 A/s on single-crystal a-Al 2 O 3 (1T02) and (0001), MgO (100), and a-SiO 2 (0001) substrates between 450 and 750 °C by using the same electron-beam evaporation method that was described in our previous paper.5 The film deposition was carried out in a high vacuum at pressure of the 10~6 Pa scale. The structure and crystallinity of deposited films were examined by x-ray and electron diffraction methods. The compositional variations in depth of the films were probed by a standard Augersputter depth profiling technique. The growth morphology of the films was examined by a scanning electron microscope (SEM). And the thicknesses of the films were measured by a Tolansky multiple-beam interference method. Superconducting transition temperatures Tc, normal resistivities pn at 10 K, and upper critical fields Bc2 at 4.2 K in transverse magnetic fields for the obtained Nb films were measured by a standard four-terminal method. The films were cooled with liquid helium, and then temperatures were measured with a calibrated germaniumresistance thermometer. Bc2 measurements of the films were for two orientations: normal to the film surface (5 c21 ) and parallel to the film surface (Bc2||)- Stationary magnetic fields applied to the films were generated by a superconducting magnet.
From this point of view, we have recently reported the successfully grown high-quality single-crystal Nb films with high Tc of —9.3 K and high residual resistance ratio ^300/^10 UP t 0 ~200. 5 In this paper, we describe basic upper critical fields of such single-crystal Nb films in relation to the mor
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