Nature of conduction-band surface resonances for Si(111) surfaces with and without chemisorbed overlayers

Conduction-band surface resonances up to 15 eV above the fundamental gap are calculated for clean Si(111), Si(111) with chemisorbed hydrogen, and Si(111) with chemisorbed chlorine. The studies are based on previously determined self-consistent pseudopotentials, which have been calculated in the repeated slab configuration. The influence of this artificial geometry on states above the vacuum level is discussed and the induced systematic errors are estimated. A number of strong surface resonances are found. Optical-absorption spectra between surface bound states and these resonances are calculated in the dipole approximation. The spectra show marked structure and anisotropy. They are compared with electron-energy-loss spectra and constant-initial-state photoelectron spectra.