Tunnel spectroscopy of electron sub-bands on Si surfaces

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Abstract
Reports tunnelling studies of quantised electron space-charge layers on (111) and (100) Si surfaces. Planar metal-SiO2-Si junctions were prepared on non-degenerate n- and p-type substrates. Metals of different work functions, lower than the Si electron affinity, were used as counter electrodes, yielding surface electron densities in the range (1-20)*1012 cm2. In samples on p-type substrates the surface density of the ionised acceptors was varied by the application of a bias voltage between the substrate and the electron channel. From the bias position of the structures induced by the sub-band edges the minimum energies of occupied and empty sub-bands relative to the semiconductor Fermi level are determined as functions of the depletion charge density. The surface density of electrons in the nth sub-band is obtained from the difference EF-En. The density of states of the lowest sub-band is obtained from a measurement of the Fermi energy in this sub-band as a function of its occupancy, which was varied by changing the depletion charge density. The results correspond to a valley degeneracy factor gv=6 and a density-of-states mass mD/m0=0.36+or-0.04 for (111) Si and to gv=2 and mD/m0=0.20+or-0.02 for (100) Si, which is in accordance with predictions of the effective mass theory.