Electrical current induced pinhole formation and insulator-metal transition in tunnel junctions

Current induced resistance switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and was attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. Here the CIS effect is studied in TJs with two thin (20 Å) non-magnetic (NM) Ta electrodes inserted above and below the insulating barrier. We observe resistance (R) switching for positive applied electrical current (flowing from the bottom to the top lead), characterized by a continuous decrease in resistance and associated with current-driven displacement of metallic ions from the bottom electrode into the barrier (thin barrier state). For negative currents, displaced ions return to their initial positions in the electrode and the electrical resistance gradually increases (thick barrier state). We measured the temperature (T) dependence of the electrical resistance of both thin- and thick-barrier states (R(b) and R(B), respectively). Experiments showed a weaker R(T) variation when the tunnel junction is in the R(b) state, associated with a smaller tunnel contribution. By applying large enough electrical currents we induced large irreversible R decreases in the studied TJs, associated with barrier degradation. We then monitored the evolution of the R(T) dependence for different stages of barrier degradation. In particular, we observed a smooth transition from tunnel- to metallic-dominated transport. The initial degradation stages are related to irreversible decreases in barrier thickness (without the formation of pinholes). Only for later stages of barrier degradation do we have the appearance of metallic paths between the two electrodes that, however, do not lead to metallic-dominated transport for small enough pinhole radius.