Observation of fluctuation-induced tunneling conduction in micrometer-sized tunnel junctions

Micrometer-sized Al/AlO_x/Y tunnel junctions were fabricated by the electron-beam lithography technique. The thin (≈ 1.5-2 nm thickness) insulating AlO_x layer was grown on top of the Al base electrode by O₂ glow discharge. The zero-bias conductances G(T) and the current-voltage characteristics of the junctions were measured in a wide temperature range 1.5-300 K. In addition to the direct tunneling conduction mechanism observed in low-G junctions, high-G junctions reveal a distinct charge transport process which manifests the thermally fluctuation-induced tunneling conduction (FITC) through short nanoconstrictions.We ascribe the experimental realization of the FITC mechanism to originating from the formations of "hot spots" (incomplete pinholes) in the AlO_x layer owing to large junction-barrier interfacial roughness.