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 a ccumulation layer at the surface of a hydrogenated Si(111)(1 x 1) sing le crystal at very low potassium coverages. By UPS we have followed th e 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 s aturation coverage. This means that the conduction band bottom is belo w 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 qua si-elastic peak in the HREEL spectra. Its width strongly increases at the lowest coverage and remains quite large up to the coverage at whic h UPS shows that the Fermi level leaves the CB minimum. (C) 1998 Elsev ier Science B.V. All rights reserved.