ELECTRON COINCIDENCE SPECTROSCOPY STUDIES OF SECONDARY AND AUGER-ELECTRON GENERATION MECHANISMS

Electron coincidence spectroscopy in an ultrahigh vacuum scanning transmission electron microscope has been used to study the generation pathways for secondary (SE) and Auger electrons (AE) excited by high-energy incident electrons. Energy and momentum transfer of inelastically scattered 100 keV primary electrons have been correlated with energy selected SE and AE for both thin [111] oriented Si crystals and amorphous C films. Coincidence spectra from the valence excitation region indicate that bulk plasmon decay is not the primary production channel for SE in Si(111) and that SE result partially from the decay of ionizations from deep in the valence band. Energy deposition by the primary beam is responsible for SE production at excitation energies above the valence region. At most one SE is emitted from the entrance surface of a thin film for each inelastically scattered 100 keV primary electron. An enhancement in both the SE yield and generation probability is observed at the C K ionization edge. Correlations between energy loss electrons in the vicinity of the C K ionization edge and energy selected SE near the C KLL AE energy show a very sharp threshold in the generation probability. High-momentum transfer (spatially localized) inelastic scattering events are more efficient at creating SE than low-momentum transfer events. The high-spatial resolution obtained in SE images is explained using the Heisenberg uncertainty principle and the scattering angle dependence of the SE generation probability.