Adhesion, atomic structure, and bonding at theAl(111)/α−Al2O3(0001)interface: A first principles study

Abstract

We have performed a series of ab initio calculations to determine the atomic structure, ideal work of adhesion $({\mathcal{W}}_{\mathrm{ad}\mathrm{}}),$ and bonding character of the $\mathrm{Al}(111)/\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ interface. Six candidate interface geometries were considered, including Al and O terminations of the oxide. Minimization of the Hellman-Feynman forces resulted in substantial changes to the atomic structure of the metal near the interface, wherein some atoms adopted positions consistent with a continuation of the oxide's Al-sublattice crystal structure across the interface. Consequently, the lowest-energy structures (i.e., having the largest ${\mathcal{W}}_{\mathrm{ad}\mathrm{}})$ are those that facilitate this ``oxide extension'' mechanism. By applying several methods of analysis we have thoroughly characterized the electronic structure and have determined that Al-O bonds constitute the primary interfacial bonding interaction. These bonds are very similar to the cation-anion bonds found in the oxide bulk and are mainly ionic, yet maintain a small amount of covalent character. In addition, there is evidence of metal-cation bonding at the optimal Al-terminated interface. Taking into account recent theoretical and experimental evidence suggesting an Al termination of the clean oxide surface, our calculations predict ${\mathcal{W}}_{\mathrm{ad}\mathrm{}}=1.36{\mathrm{J}/\mathrm{m}}^{2}$ [local density approximation (LDA)] and 1.06 ${\mathrm{J}/\mathrm{m}}^{2}$ [generalized gradient approximation (GGA)] for the optimal Al-terminated structure, which are in good agreement with the experimental value of 1.13 ${\mathrm{J}/\mathrm{m}}^{2}$ as scaled to 0 K. These values are approximately an order of magnitude smaller than what is found for the optimal O-terminated interface: 10.70 ${\mathrm{J}/\mathrm{m}}^{2}$ (LDA) and 9.73 ${\mathrm{J}/\mathrm{m}}^{2}$ (GGA). Although cleavage preferentially occurs at the interface for the Al termination, strong bonding at the O-terminated interface favors cleavage within the metal.