Electronic Surface State and Plasmon-Phonon Coupled Excitations at the surface of Modulation-doped GaAs/AlGaAs Multiquantum Wells: A Study of High-Resolution Electron-Energy-Loss Spectroscopy(HREELS)

A selfconsistent framework for study of the surface electronic structure of finite superlattices has been proposed. Our emphasis is on the influence of accumulation or depletion layer on the surface states (Tamm states). We have used a modulated doping profile, including an accumulation or a depletion surface layer, in our selfconsistent calculations of potential and carrier density profile. We have found the existence of Tamm states above(depletion layer) or below(accumulation layer) the superlattice miniband. One of the Tamm states found crosses the Fermi energy in the energy gap when the depletion effect, resulting from dangling bonds, defects, impurities, etc, near the surface increases. In our case, near the top layer of the superlattice the depletion region was formed as a result of the pinning of the Fermi level below its position in intrinsic GaAs due to the midgap surface states.