Stoichiometry and adhesion of Nb/Al₂O₃

We examine the relative stability of both stoichiometric and nonstoichiometric Nb/Al₂O₃ interfaces and Al₂O₃ surfaces. Results of first-principles computations of surface and interfacial atomic relaxations, electron-density distributions, and total energies are presented. We found that while the Al-terminated Al₂O₃(0001) surface is stable relative to the O-terminated surface, interface formation with Nb can reverse the stability, depending on the oxygen partial pressure. This interfacial structure is consistent with recent experimental results. Finally, we computed some of the energetics associated with the diffusion of Al into Nb across the Nb/Al₂O₃ interface. Our results are consistent with Al migration into the Nb at temperatures of the order of 10³K, as reported experimentally.