Energy Structure in Photoelectric Emission from Cs-Covered Silicon and Germanium

Photoelectric yield and energy distributions are given for clean-cleaved and cesium-covered (111) surfaces of silicon and germanium. A monolayer of cesium lowers the work function of each to ∼1.6 eV and reveals detailed structure in both yield and distributions. This structure is directly related to that seen in reflectivity and the major features in silicon agree well with those predicted by theoretical calculations of Brust, Cohen, and Phillips, assuming direct transitions in an energy-band model derived by the pseudopotential method. Varying bulk doping from extreme $n$ to $p$ type produces large changes in energy distributions and yield caused by the changes in band bending induced just beneath the surface. Such band-bending effects are much stronger for silicon than for germanium. It is concluded that excitation is predominantly a direct volume process and that emitted electrons originate from a mean depth ranging from 20 Å to several hundred Å as the electron energy varies from 6 to 3 eV above the valence-band maximum.