Study of the Energy Gaps in Bulk Gallium Single Crystals and Polycrystal Aluminum Films with High Impedance Tunnel Junctions

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Abstract
Results are presented suggesting that fabrication procedures for bulk tunnel junctions can be relaxed provided one uses sample junctions with large barrier thicknesses and, consequently, large junction impedances. The results of a study conducted on Al–Al2O3–Pb film junctions, ranging in impedance from 10 Ω to 4 MΩ, show that one can obtain good I‐V curves from high‐impedance junctions and that the normalized energy gaps obtained from these data are the same as those measured with low‐impedance samples. Bulk gallium tunnel junctions with impedances ranging from 1 kΩ to 20 MΩ exhibited energy gaps in the range of 3.4–3.8kTc for flat‐surfaced specimens and from 2.9 to 3.5kTc for round‐surfaced specimens. A best estimate for the anisotropy is 5% with an upper limit of 20%. Possible multiple gap structure, excess subharmonic currents, weak link behavior, and sample aging effects were observed.