Charge accumulation layer induced by potassium adsorption on hydrogen terminated Si(111)(1x1)

By ultraviolet photoemission spectroscopy and high-resolution electron energy loss spectroscopy we have detected the formation of a charge accumulation layer at the surface of a hydrogenated Si(111)(1 x 1) single crystal at very low potassium coverages. By UPS we have followed the evolution of the band edges relative to the Fermi level, leading to a downwards band-bending (BB) around 340 meV, at about 0.05 of the K saturation coverage. This means that the conduction band bottom is below the Fermi level, producing the accumulation of charge carriers just below the surface. At increasing coverages the BB retains its downward trend, reducing its value to 100 meV at K saturation. The presence of an accumulation layer, consisting of a quasi two-dimensional electron gas, is unambiguously confirmed by the anomalous behaviour of the quasi-elastic peak in the HREEL spectra. Its width strongly increases at the lowest coverage and remains quite large up to the coverage at which UPS shows that the Fermi level leaves the CB minimum. (C) 1998 Elsevier Science B.V. All rights reserved.