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

TL;DR

This study investigates how electrical currents induce resistance switching and barrier degradation in tunnel junctions with non-magnetic electrodes, revealing a transition from tunneling to metallic conduction and the formation of pinholes.

Contribution

It demonstrates current-driven ion displacement causing resistance switching and barrier degradation, including the transition from tunneling to metallic conduction in non-magnetic tunnel junctions.

Findings

Resistance switching is driven by current-induced ion displacement.

Barrier degradation leads to a transition from tunneling to metallic conduction.

Large currents cause irreversible barrier thinning and pinhole formation.

Abstract

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. The CIS effect is here studied in TJs with two thin (20 \AA) 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 resistance decrease 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 into 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 barrier thickness decreases (without the formation of pinholes). Only for later barrier degradation stages 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.