Inelastic scattering of electrons from ionic crystals with a highly conducting overlayer

The purpose of the present study has been to examine the manner in which inelastic electron scattering from ionic crystal surfaces is modified by the presence of a conducting overlayer as its thickness passes from the limit of zero coverage through two-dimensional film formation and then onto bulk growth. Specifically, we present both experiments and theory on the effects of thin (17 {}) silver overlayers on the vibrational spectrum of gallium arsenide single-crystal surfaces. Clean (100) Te-doped GaAs samples are characterized by an intense surface optical phonon at 36.2±{}0.5 meV and a weaker plasmon-phonon coupled mode at 12-13 meV, depending on the doping level. Both modes are completely screened by a (3.0±{}0.3)-{}-thick uniform overlayer of silver in good agreement with theoretical predictions (assuming a silver overlayer with bulk dielectric properties). In addition, modes characteristic of the coupled Ag-GaAs system are observed. The effects of silver film thickness and metal island formation are also discussed.