Direct and Indirect Excitation Processes in Photoelectric Emission from Silicon

The spectral dependence of saturation photoelectric emission has been studied for atomically-clean (111) silicon surfaces which were prepared by cleavage in high vacuum. The observed spectra, and their dependence on sample doping, are interpreted as being due to a volume excitation process which is modified by space charge band bending effects. Both direct and indirect optical excitation thresholds are observed, at 5.45 eV and 5.15 eV, respectively, with the latter being equal to the electron affinity, $\chi$, plus the energy gap, $E_G$. The spectral dependence of the direct excitation process is in agreement with the theoretical model developed by Kane, in which there is a complete absence of scattering either in the bulk or at the surface for those excited electrons which are emitted. The indirect process is also in agreement with Kane's theory. The dependence of the yield on sample doping, in conjunction with the theoretical model, may be used to determine a direct-flight escape depth for excited electrons of 25 ű5 Šfor electron energies about 5.5 eV above the valence-band maximum.