Structural details of Al/Al2O3 junctions and their role in formation of electron tunnel barriers

TL;DR

This computational study investigates how atomic-scale geometrical details of Al/Al2O3 interfaces influence the formation and properties of electron tunnel barriers in MIM devices, emphasizing surface irregularities and interlayer relaxations.

Contribution

It provides a detailed analysis of the atomic and interplanar relaxations at Al/Al2O3 interfaces and assesses the accuracy of common computational methods for modeling these junctions.

Findings

Surface irregularities lower the tunnel barrier height.

Interplanar relaxations in Al2O3 layers can contract the barrier by up to 13%.

Relaxations away from the interface have minimal impact on tunnelling.

Abstract

We present a computational study of the adhesive and structural properties of the Al/Al2O3 interfaces as building blocks of the Metal-Insulator-Metal (MIM) tunnel devices, where electron transport is accomplished via tunnelling mechanism through the sandwiched insulating barrier. The main goal of this paper is to understand, on the atomic scale, the role of the geometrical details in the formation of the tunnel barrier profiles. To provide reliable results, we carefully assess the accuracy of the traditional methods used to examine Al/Al2O3 interfaces. These are the most widely employed exchange-correlation functionals, LDA, PBE and PW91, the Universal Binding Energy Relation (UBER) for predicting equilibrium interfacial distances and adhesion energies, and the ideal work of separation as a measure of junction stability. Finally, we perform a detailed analysis of the atomic and interplanar relaxations in each junction. Our results imply that the structural irregularities on the surface of the Al film have a significant contribution to lowering the tunnel barrier height, while interplanar relaxations in the Al film, away from the immediate interface do not have a notable impact on the tunnelling properties. On the other hand, up to 5-7 layers of Al2O3 may be involved in shaping the tunnel barriers. Interplanar relaxations of these layers depend on the geometry of the interface and may result in the net contraction by 13% relative to the corresponding thickness in the bulk oxide. This is a significant amount as the tunnelling probability depends exponentially on the barrier width.