First-principles calculational study of the exoelectron emission of oxygen-adsorbed aluminum surfaces

Abstract

The first-principles calculation, based upon the ultrasoft pseudopotential, was carried out on clean and O-adsorbed Al surfaces in order to investigate the emission mechanism of exoelectrons from Al surfaces. The Al (001) surface was chosen to simulate the previous experiments using Al tips [Tagawa et al., Appl. Surf. Sci. 72, 259 (1993)]. The ultrasoft pseudopotential-based first-principles calculation provided a lattice constant within 1% of the difference from the experimental value and also showed good agreement with the surface states measured by an angle-resolved photoelectron spectroscopy. The computational results indicated a high local density of states (LDOS) near the Fermi level, exceeded by the O adsorption of the Al (001) surface. It was also calculated that the LDOS peak was located 0.2 eV below the Fermi level of the O-covered Al (001). Considering the work function of the clean Al (001) surface (4.4 eV) and its decrease due to the O adsorption (-0.6 eV), this electron-filled LDOS peak was located 4.0 eV below the vacuum level. Calculated results revealed good agreement with the experimental data showing the exoelectron peak at 4.0 eV below the vacuum level [Tagawa et al., Appl. Phys. Lett. 61, 1471 (1992)]. From the first-principles calculation results reported here, the low work-function patch (LWP) model was introduced to the Al surface. This LWP model explained the origin of the exoelectron emission phenomenon in the early stage of oxidation on the Al surface.