Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers

TL;DR

This study compares the effects of thermal and plasma oxidation on aluminum oxide tunnel barriers in Nb/Al/AlOx/Nb junctions, revealing how post-annealing modifies their electrical properties and potential for memory device applications.

Contribution

It provides a detailed analysis of how different oxidation methods and annealing temperatures affect the tunnel barrier height and stability of aluminum oxide layers in superconducting junctions.

Findings

Thermally-grown oxides show increased barrier height after annealing above 300°C.

Plasma-grown oxides are less affected by annealing below 450°C.

Annealed barriers exhibit high endurance under electric stress.

Abstract

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 post-annealing of the completed structures at temperatures up to 550 deg C. Post-annealing 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.