Ultrathin aluminum oxide films: Al-sublattice structure and the effect of substrate on ad-metal adhesion

First principles density-functional slab calculations are used to study 5 A ngstrom (two O-layer) Al2O3 films on Ru(0001) and Al(111). Using larger uni t cells than in a recent study, it is found that the lowest energy stable f ilm has an even mix of tetrahedral (t) and octahedral to) site Al ions, and thus most closely resembles the Ic-phase of bulk alumina. Here, alternatin g zig-zag rows of t and o occur within the surface plane, resulting ina gre ater average lateral separation of the Al-ions than with pure t or o. A sec ond structure with an even mix of t and o has also been found, consisting o f alternating stripes. These patterns mix easily, can exist in three equiva lent directions on basal substrates, and can also be displaced laterally, s uggesting a mechanism for a loss of long-range order in the Al-sublattice. While the latter would cause the film to appear amorphous in diffraction ex periments, local coordination and film density are little affected. On a fi lm supported by rigid Ru(0001), overlayers of Cu, Pd, and Pt bind similarly as on bulk truncated alpha -Al2O3(0001). However, when the film is support ed by soft Al(111), the adhesion of Cu, Pd, and Pt metal overlayers is sign ificantly increased: oxide-surface Al atoms rise so only they contact the o verlayer, while substrate Al metal atoms migrate into the oxide film. Thus the binding energy of metal overlayers is strongly substrate dependent, and our numbers for the above Pd-overlayer systems bracket a recent experiment ally derived value for a film on NiAl(IIO). (C) 2000 Elsevier Science B.V. All rights reserved.