Single-particle and collective excitations of a two-dimensional electron gas at the Cs/InAs(110) surface

The accumulation space-charge region at a semiconductor surface has been studied by a joint investigation of the plasmon excitation and the spectral density of the quasi-two-dimensional electron gas ~Q2DEG!. The analysis has been performed by means of high-resolution electron-energy-loss spectroscopy and highresolution ultraviolet photoemission, respectively. The accumulation layer was produced by depositing tiny amounts of Cs on the InAs~110! surface. By using a semiclassical dielectric model, the formation of the Q2DEG in the subsurface region was unambiguously proved by a satisfactory description of the coverage and primary energy dependence of the collective excitations. The characteristic parameters of the Q2DEG, i.e., charge density and width of the space-charge region, are determined. These results are in very good Agreement with the values deduced by self-consistently solving the Poisson and Schro¨dinger coupled equations, which also give the eigenvalue spectrum and spectral density as measured by photoemission.