Angle-resolved photoemission studies of GaAs(100) surfaces grown by molecular-beam epitaxy

Angle-resolved photoemission techniques were used to study GaAs(100) surfaces grown by molecular-beam epitaxy. Three well-ordered surfaces with the $c(4\ifmmode\times\else\texttimes\fi{}4)$, $c(2\ifmmode\times\else\texttimes\fi{}8)$, and $c(6\ifmmode\times\else\texttimes\fi{}4)$ reconstructed surface structures and a disordered As-covered (1\ifmmode\times\else\texttimes\fi{}1) surface were investigated in the photon-energy range of 12-70 eV. All major sharp peaks in the normal-emission spectra were found to be transitions from bulk valence bands; therefore, transitions due to surface states were either weak or broad. The variations in bulk-peak intensities for different surface structures were related to surface diffraction effects. For photon energies greater than about 20 eV, primary-cone peaks for transitions from the upper bulk valence bands to a broadened and shifted free-electron-like final band were identified, and energy dispersion relations for the upper valence bands were determined along the [100] direction. Surface umklapp transitions were quite strong due to the presence of many short surface reciprocal-lattice vectors. To characterize these surfaces, angle-integrated photo-emission measurements of valence bands and core levels were made, and the positions of the Fermi level for these surfaces were determined.