Crystalline oxide tunnel barriers formed by thermal oxidation of aluminum overlayers on superconductor surfaces

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Abstract
Crystalline oxide tunnel barriers were formed by thermal oxidation of thin Al overlayers on single-crystal and polycrystalline films of four refractory superconductors: Nb, Mo-Re, Nb3Sn, and NbN. Tunnel junctions were fabricated mostly with Pb-Bi and also with Nb, Mo-Re, and NbN counterelectrodes. Differences in the base electrode crystallinity, surface roughness, oxygen affinity, and native oxide properties influenced the average barrier height, the subgap and normal (above-gap) conductance and the occurrence of microshorts. An effect of the overlayer deposition method (evaporation versus sputtering) was also noticed. The comparison of properties of barriers obtained on different base electrodes and of those amorphized by ion milling lead to the observation that amorphous barriers are more uniform and representative of Al2O3 than the crystalline and epitaxial barriers. The properties of the latter are defined, in part, by the shunting native oxide channels which result from a nonuniform coverage of the base electrode.