Aluminum oxide layers as possible components for layered tunnel barriers

We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma oxidation, before and after rapid thermal postannealing of the completed structures at temperatures up to 550 deg. C. Postannealing at temperatures above 300 deg. C results in a significant decrease of the tunneling conductance of thermally grown barriers, while plasma-grown barriers start to change only at annealing temperatures above 450 deg. C. Fitting the experimental I-V curves of the junctions using the results of the microscopic theory of direct tunneling shows that the annealing of thermally grown oxides at temperatures above 300 deg. C results in a substantial increase of their average tunnel barriers height, from ∼1.8 eV to ∼2.45 eV, versus the practically unchanged height of ∼2.0 eV for plasma-grown layers. This difference, together with high endurance of annealed barriers under electric stress (breakdown field above 10 MV/cm) may enable all-AlOx and SiO2/AlOx layered 'crested' barriers for advanced floating-gate memory applications