Stoichiometry and adhesion of ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$

We examine the relative stability of both stoichiometric and nonstoichiometric ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$ interfaces and ${\mathrm{Al}}_2{\mathrm{O}}_3$ 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 ${\mathrm{Al}}_2{\mathrm{O}}_3\mathrm{}\left(0001\right)\mathrm{}$ 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 ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$ interface. Our results are consistent with Al migration into the Nb at temperatures of the order of ${10}^3\mathrm{K},$ as reported experimentally.