Change in Co/Al-oxide/Co tunneling junction under constant voltage stress

The tunneling junctions Co(10nm)/Al-oxide/Co(50nm) were fabricated on a glass substrate using a metal mask by ion-beam sputtering. On the bottom Co electrode, a 2.5 nm thick Al film was deposited and was oxidized in pure oxygen gas for 12 - 216 h at 200 C. The electrical properties of the junction were measured by the four-probe method under a constant voltage and the current-voltage characteristics at low voltages were measured every half hour. The barrier parameters of Al-oxide layers are evaluated from a fit to the calculated current-voltage characteristics for the junction by using the Particle Swarm Optimization method with high accuracy. The oxide layer was assumed to comprise two different barriers. In case of the barrier with low potential height, oxidation proceeds by applying the voltage and the resistivity increases. In the case of hard breakdown, the barrier retains uniform at the breakdown with sudden change of the resistivity. In other case of the soft breakdown, the barrier shows segregation just before the breakdown.