Energy- and angular-dependent secondary-electron emission from a silicon (111) 7 × 7 surface. Emission from bulk states

Abstract

Energy- and angular-dependent secondary-electron emission (EADSEE) spectra from a silicon (111) 7 \ifmmode\times\else\texttimes\fi{} 7 reconstructed surface are reported. Only spectra which consist of electrons that have emerged from bulk states are considered here. The spectra are interpreted in terms of the conventional three-step model, in which electrons are excited into high-lying conduction states, move to the surface, and emerge into the vacuum. Microscopic models are considered for the processes that contribute to each of the three steps. The processes which are important in determining the shape of these EADSEE spectra have been identified by comparing the predictions of the models with the observed spectra. The shape of the EADSEE spectra between 6 and 40 eV is determined mostly by the incoherent scattering that occurs as the electrons cross the reconstructed layer. This scattering obliterates the structure in the internal hot-electron distribution, structure which is observed in the spectra of electrons which emerge from a (111) 1 \ifmmode\times\else\texttimes\fi{} 1 unreconstructed surface. For emergence energies below 6 eV there is evidence for a number of processes. It is found that the spectral shape of the internal flux is dominated by structure in the density of conduction states, as first described by Kane. This shape is modified somewhat by energy-dependent transport losses. Most of the observed low-energy ( 6 eV) electrons have emerged coherently through the surface. The energy and angular structure in the low-energy secondary flux is affected in a number of ways in this emergence process. Evidence is seen for diffraction, refraction, and reflection of the emerging electrons. Diffraction at the surface causes peaks in the internal electron distribution to appear at angles of emergence not otherwise expected. Refraction of the outgoing waves leads to a cosine dependence of the angular plot of the secondary current for fixed energies. Reflection of the outgoing electrons at the surface barrier causes a decrease in the number of secondaries. With certain assumptions the reflection (transmission) coefficient of electrons at the barrier, and the wave-vector dependence of this coefficient, can be deduced from the data.