Reflection high-energy electron-diffraction and photoemission spectroscopy study of GaAs(001) surface modified by Se adsorption

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Abstract
Modification of the GaAs surface induced by the adsorption of Se atoms in a vacuum is studied by reflection high-energy electron diffraction (RHEED) and surface-sensitive photoemission spectroscopy. Se-induced variation of the surface structure is examined by RHEED for GaAs(001) surfaces with different surface compositions prepared by molecular-beam epitaxy, and a diagram of the reconstructions for the Se-GaAs(001) system is presented. Se-stabilized surfaces show a (2×1) reconstruction irrespective of the initial As and Ga surface composition. In addition, two intermediate surfaces are also observed for partially selenized surfaces. These intermediate surfaces are characterized in the RHEED patterns by a 1/3-order streaking in the [11¯0] direction. Measurements of core-level photoemission peak intensities show considerable As loss at the (2×1) surface. An analysis of observed Ga, As, and Se 3d core-level peak intensities for the (2×1) surface is made, assuming the layer attenuation model. It suggests that the surface is terminated with a full atomic layer of Se atoms, which are bonded with Ga atoms of the next layer. It also suggests the occurrence of Se-As exchange near the surface leading to the formation of a selenide layer a few monolayers thick. The Se 3d spectrum of the (2×1) surface shows the presence of two chemically shifted components, which are tentatively attributed to the Se atoms terminating the surface and those occupying the As sites below the surface in the proposed layer model. Binding-energy shifts of the As 3d core-level peak and valence-band edge suggest that the band is nearly flat for the (2×1) surface, indicative of the Fermi-level unpinning occurring at the surface. Its mechanism is discussed in the proposed structural model.