Soft-x-ray-induced secondary-electron emission from semiconductors and insulators: Models and measurements

Secondary-electron energy distribution curves (EDC's) and the total secondary-electron yields relative to such for gold have been measured for seven semiconductors for which electron-electron scattering losses within the emitter were considered dominant and for nine insulators (alkali halides) for which electron-phonon scattering losses were expected to be dominant in the transport process. The secondary-electron spectra were excited by Al-$K\ensuremath{\alpha}$ (1487 eV) photons and were measured from evaporated dielectric films (of about 0.3 \ensuremath{\mu} thickness) on conducting substrates with an electrostatic hemispherical analyzer of about 0.03-eV resolution. Some of the dielectric photoemitters have appreciably narrower energy distributions and higher yields than has gold; CuI and CsI have EDC widths at half-maximum of about one-third of that for gold, and yield values of 11 and 30 times greater. The FWHM and secondary-electron yield for gold were measured to be about 4 eV and 0.50 electrons per normally incident photon, respectively. The shapes of the EDC's were found to be essentially unchanged for photon excitation in the 0.1-10-keV region. Strong structural features appear only in the alkali halide EDC's, and it is proposed that these are mainly the result of single-electron promotion of secondaries from the valence band by plasmon deexcitation. A relatively simple model for x-ray photoemission has been developed which assumes that direct excitation of secondaries by photoelectron and Auger-electron "primaries" is the dominant excitation mechanism, and accounts for both electron-electron and electron-phonon scattering in the transport process. Free-electron conduction-band descriptions are assumed. The theoretical and experimental curves are in satisfactory agreement.