ANALYSIS OF SEMICONDUCTORS AND INSULATORS BY HIGH-RESOLUTION ELECTRON-ENERGY LOSS SPECTROSCOPY - PROSPECTS FOR QUANTIFICATION

High‐resolution electron energy loss spectroscopy (HREELS) is now widely known and recognized for its very significant contributions to the characterization of molecules adsorbed on well‐defined surfaces: many chemical surface reactions were understood successfully (adsorption sites, reaction scheme, time‐resolved experiments) through the analysis of electron‐induced vibrational spectra. Semiconductors and insulators form other classes of materials that contribute to vibrational spectra by intrinsic vibrations: compound semiconductors, and insulators present acoustic and optical phonons, i.e. collective lattice vibrations, whereas organic insulators (like polymers) offer numerous molecular groups to the vibrational excitation by the electron beam (extrinsic vibrations). Quantitative analysis of the surface of those materials rests on the understanding of the interaction mechanisms between monochromatized electrons and intrinsic or extrinsic vibrations. Three scattering processes have been recognized to contribute to HREEL spectra: dipole interaction, impact scattering and resonance scattering. They have been characterized theoretically and experimentally through their dependence versus variation of the electron beam energy and/or of the scattering angular geometry. Recent experiments have shown that the analysis of intrinsic phonons is very well understood for insulators. As a consequence, quantitative analysis of compound semiconductors is possible and brings additional information on the surface stoichiometry (of AlxGa1−xAs, for example) and on the doping level. For organic polymers, however, the excitation of molecular vibrations does not—up to now—appear to follow a simple scheme: all the scattering mechanisms (dipolar, impact and resonance) appear to play a significant role; a true quantification will be possible only through a systematic study of model compounds, and will require further theoretical treatments. Copyright © 1990 John Wiley & Sons Ltd.