Ideal Al-Ge(001) interface: From chemisorption to metallization of the Al overlayer

Abstract

The electronic structure of the ideal Al-Ge(001) interface is calculated from the submonolayer chemisorption regime to multilayer Al deposition. In the chemisorption regime we have performed total-energy calculations to determine configurational parameters and cohesive energies. Here we used the slab model in the framework of the local-density-functional theory and a self-consistent- field pseudopotential approach. We found that aluminum in the first-layer bridging positions is more stable (by about 1 eV) than in the on-top configuration. This implies that the chemisorbed Al atoms prefer to continue the bulk Ge lattice. The equilibrium bond lengths in the on-top and bridging sites are calculated to be 2.4 and 2.3, respectively, which are close to the sum of the atomic radii (2.5). At higher coverages the bond length is shown to be elongated with important consequences. Using these geometrical parameters, we have calculated electronic states responsible for the Fermi-level pinning at a semiempirical level in order to include a large number of Ge(13) and Al (up to 8) layers. At submonolayer coverage it is found that the chemisorption bond and metal states dominate the energy spectrum near the band-gap region, and pin the Fermi level. As metal coverage increases the interface Al becomes metallic and the chemisorption bond states recede, leading to less directional interactions. At high coverages the metal-induced gap states are primarily responsible for the Fermi-level pinning. © 1984 The American Physical Society.