Metallization, Bonding, and Energetics of Ordered Phases of Al on Si(111)

Abstract

Many models have been proposed for understanding the pinning of the Fermi energy (Ef) at the metal-semiconductor interfaces1-3. Each of these models is based on the general assumption that Ef is pinned in response to the presence of surface states. Bardeen1 originally suggested that the surface states were intrinsic to the semiconductor surface, while Heine2 proposed that the pinning was due to the tails of the metal wavefunctions decaying into the gap of the semiconductor. Since Schottky barriers are known to be developed at submonolayer coverages and many times the first adsorbed metal atoms remove the intrinsic surface states, neither model provides a universal picture of the Schottky barrier formation. Phillips4 has pointed out that a complete picture of the Schottky barrier formation would require a description of the microscopic bonding at the metal-semiconductor interface. For each metalsemiconductor system, the surfaces states pinning Ef will depend on the bonding at the interface. Batra and Ciraci5 recentiy suggested that the nature of the pinning states can also change as a function of metal coverage. Their study5 of the pseudomorphic growth of Al -Ge(001) showed that at low coverage ( 1/2 monolayer (ML)) the Ge dangling bond is saturated by the formation of a strong Al-Ge chemisorption bond. At higher coverages (1 ML), the Al overlayer relaxed outward, resulting in a weakening of the chemisorption bond and an increased quasi 2D metallic character of the Al overlayer. In light of these interesting results, we have investigated the change in the bonding properties at the Si(l 11)-A1 interface as a function of Al coverage and adatom geometry. We find that the Al adatom geometry is critically dependent on the coverage, being four-fold coordinated at 1/3 ML and evolving to single-fold on-top site configuration at 1 ML. The resulting geometries reflect the strong covalent bonding between Si and Al at each coverage.