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

Abstract

We present a computational study of the adhesive and structural properties of the Al/Al 2 O 3 interfaces as building blocks of the metal-insulator-metal (MIM) tunnel devices, where electron transport is accomplished via tunneling 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. Initially, we concentrate on the adhesive properties of the interfaces. To provide reliable results, we carefully assess the accuracy of the traditional methods used to examine Al/Al 2 O 3 systems. These are the most widely employed exchangecorrelation functionals-local-density approximation and two different generalized gradient approximations; the universal binding-energy relation for predicting equilibrium interfacial distances and adhesion energies; and the ideal work of separation as a measure of junction stability. In addition, we show that the established interpretation of the computed ideal work of separation might be misleading in predicting the optimal interface structures. Finally, we perform a detailed analysis of the atomic and interplanar relaxations in each junction, and identify their contributions to the tunnel barrier parameters. 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 atomic relaxations at the interface and interplanar relaxations in Al 2 O 3 may considerably change the width of the barrier and, thus, distort its uniformity. Both the effects may critically influence the performance of the MIM tunnel devices.

Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo

“…The starting structure was obtained upon relaxing O-terminated α-Al 2 O 3 (0001) (with 6 O layers, ∼11 Å) on Al (111) (with 7 Al layers, ∼14 Å) in FCC stacking with a periodic interface arrangement. This idealized interface matches the close-packed planes and directions of Al 2 O 3 and Al (i.e., (111) Al ||(0001)­Al 2 O 3 and , ), which has been reported in experiments of Al 2 O 3 grown on Al and Al deposited on sapphire , and is reported to be the most stable across a wide range of μ O . ,, To mimic epitaxial growth of Al 2 O 3 on Al, as observed during thermal oxidation of Al, , Al 2 O 3 (0001) is strained in-plane to match Al (111). To obtain the starting interface structure, the cell was allowed to relax along the c -axis, keeping the a- and b-lattice parameters fixed.…”

“…Note that apart from some structural and electronic similarities, none of the four thermodynamically stable structures correspond to the interfacial structures commonly reported for the Al 2 O 3 /Al interface in the ab initio literature. − This indicates that the idealized interfaces frequently used in DFT analyses of interfacial work of adhesion, electronic structure, bonding character, and so forth are likely metastable. It is worthwhile to add here that the structure labeled configuration 1 in Figure , with 12.5% V Al at the interface, corresponds well with the AIMD study of the Al 2 O 3 /liquid Al interface .…”

“…Computational studies have investigated the Al 2 O 3 /Al interface using density functional theory (DFT) and molecular dynamics (MD). First principles studies have primarily focused on identifying stable oxide terminations at the interface, , thickness-dependent phase stability of Al 2 O 3 on Al, and evaluating properties such as the work of adhesion , and tunneling efficiency across Al/Al 2 O 3 /Al tunnel junctions . DFT studies have also investigated the early stage growth of the oxide on Al, − proposing a barrierless mechanism for the initial oxidation of Al via Al extraction by adsorbed oxygen atoms and identifying the favorable surface and subsurface sites for oxygen adsorption. , Al 2 O 3 growth on Al has been studied using ab initio and classical MD , as well, by sequentially adding O 2 molecules over the Al surface.…”

First Principles, Explicit Interface Studies of Oxygen Vacancy and Chloride in Alumina Films for Corrosion Applications

“…46 In the appendix, we use a combinatorial approach to compare the structure with other interfacial models investigated in the literature. [24][25][26][27][28][29][30][31] As Ref. 24 points out, variations in the interfacial structure can change the work function by <0.5 eV.…”

“…[21][22][23] To model e − transfer effects on corrosion requires a quantum mechanical treatment of valence e − , with DFT being a standard compromise between accuracy and computational cost. Our first step is to apply DFT models with explicit Al|Al Previous DFT work that has studied the Al metal/Al 2 O 3 interface [24][25][26][27][28][29][30][31][32][33][34][35][36][37] has not simultaneously considered defects like oxygen vacancies. While oxygen vacancies in Al 2 O 3 have been examined using DFT, [38][39][40][41][42][43] most such calculations are conducted in the absence of metallic Al, which defines the Fermi level (E F ), in the simulation cell.…”