Mechanism for Limiting Thickness of Thin Oxide Films on Aluminum

Abstract

A first-principles account of the observed limiting thickness of oxide films formed on aluminum during oxidizing conditions is presented. The results uncover enhanced bonding of oxygen to thin alumina films in contact with metallic aluminum that stems from charge transfer between a reconstructed oxide-metal interface and the adsorbed molecules. The first-principles results are compared with the traditional CabreraMott (CM) model, which is a classical continuum model. Within the CM model, charged surface oxygen species and metal ions generate a (Mott) potential that drives oxidation. An apparent limiting thickness is observed as the oxidation rate decreases rapidly with film growth. The present results support experimental estimates of the Mott potential and film thicknesses. In contrast to the CM model, however, the calculations reveal a real limiting thickness that originates from a diminishing oxygen adsorption energy beyond a certain oxide film thickness. Oxidation of metals has tremendous impact on society, and corrosion alone costs the U.S. trillions of dollars each year [1]. Under controlled conditions, however, oxidation plays an important role; thin oxides are used as catalysts, sensors, dielectrics, and corrosion inhibitors. For the latter purpose, metals such as aluminum and rhodium are preferred, as they develop protective oxide layers that inhibit bulk oxidation. Owing to its importance, there have been numerous experimental and theoretical efforts concerning the fundamental understanding of oxidation processes and, especially, the initial growth.The seminal work of Cabrera and Mott (CM) of the mid20th century still remains the key theoretical model for growth of thin oxides on metals, see Fig. 1 [2]. According to CM, electrons from the Fermi level of the metal substrate (ϵ F ) traverse the developing oxide film (with a band gap of E g ) by either tunneling or thermionic emission to acceptor levels (φ a ) of oxygen species, thereby, forming different types of anions (O 2− 2 , O − 2 , O 2− , O − ) on the oxide surface. The negative anions and the positive counterpart at the metal-oxide interface generate an electric potential, called the Mott potential (V M ), which effectively lowers the energy barriers for migration of cations and/or anions through the oxide. This leads to a high oxidation rate even at low temperatures. As the oxide thickness increase, the additional effect of the Mott potential diminishes, and the oxidation process effectively stops at an apparent limiting thickness. It could be noted that although the original CM model discusses charge transfer through tunneling or thermionic emission, the actual transfer is probably defect or polaron mediated. However, this issue is not addressed in the present Letter.Oxidation of aluminum is the prototypical process owing to the simple but versatile electronic structure of aluminum when discussing thin film growth, and this system has played a central role in the establishment of the CM model [3,4]. Recently, Zhou et al. [5,6] showed that the th...

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