Photoemission from alpha and beta phases of the GaAs(001)-c(4 x 4) surface

We prepared alpha- and beta surface phases of GaAs(0 0 1)-c(4 x 4) reconstruction by molecular beam epitaxy (MBE) using As-4 and As-2 molecular beams, respectively, and examined them by angle-resolved ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation as an excitation source. The UPS valence band spectra and the XPS 3d core level data show pronounced differences corresponding to the surface composition and the atomic structure of the two phases, as proposed in the literature. In UPS, the beta phase is characterized by an intensive surface state 0.5 eV below the top of the valence band at low photon energy, while an analogous peak in the alpha phase spectra is missing. The surface state is interpreted in terms of dangling bonds on As dimers. The As3d and Ga3d core level photoelectron lines exhibit phase-specific shapes as well as differences in the number, position and intensity of their deconvoluted components. The location of various atoms in the surface and subsurface layers is discussed; our photoemission results support models of the beta phase and the alpha phase with As-As dimers and Ga-As heterodimers, respectively. (C) 2009 Elsevier B.V. All rights reserved.