Metallization and Fermi-level movement at the Cs/GaAs(110) interfaces

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Abstract
The room-temperature (RT) and 110-K low-temperature (LT) Cs/GaAs(110) interfaces have been studied with use of photoelectron spectroscopy. The overlayer metallization is investigated by following the density of states near the Fermi cutoff, the free-electron-plasma loss features, and the Fermi-level movement of n- and p-type GaAs. The criteria of metallicity are proposed. One monolayer (ML) Cs (which is the saturation coverage at RT) is not metallic at either RT or LT, and the full metallicity is established at 2 ML coverage. Development of the overlayer metallicity after the first monolayer is accompanied by a striking increase in electron scattering. The band bending (large for p-type GaAs but little for n-type GaAs) at low Cs coverages (up to 0.1 ML) is predominantly due to the surface donor states originating from the chemisorbed Cs adatoms. A one-to-one correlation is established between the number of surface donor states and that of the deposited Cs atoms before p-type GaAs band bending reaches a maximum value [1.25 eV at LT and 1.1 eV at RT above the valence-band maximum (VBM)]. The Fermi-level stabilization (0.75 and 0.5 eV above the VBM for n- and p-type GaAs, respectively) is due to the defects at the RT interfaces. At LT it takes place only when the overlayer becomes metallic. In this case, the defects become inefficient because of metal screening, and the metal-induced gap states are responsible for the Fermi-level pinning (0.78 eV above the VBM).