Evolution of interface excitations under phase transition in two-dimensional layer of Cs on GaAs(100) and (111)

The evolution of electronic excitation spectra under deposition of Cs on th e polar GaAs faces (100) and (111) with various surface reconstructions is experimentally studied by means of high-resolution electron energy loss spe ctroscopy (EELS). Similar to the Cs/GaAs(110) surface studied earlier, for all crystal faces and surface reconstructions wt: found two energy loss pea ks in the band gap of GaAs at Cs coverages exceeding 0.5 monolayer (ML). Th e analysis of the loss spectra proves that the rise of the loss peaks is du e to a phase transition at which dispersed isolated Cs adatoms condense int o densely packed clusters, eventually merging in a two-dimensional overlaye r with metallic-type excitations. A simple model of plasmon excitations in a metallic layer placed on a dielectric substrate explains qualitatively th e essential features of EELS experiments. (C) 2001 Elsevier Science B.V. Al l rights reserved.