Time Resolved Optical Detection of Electron Loss and Migration in CdS

A beam of energetic electrons incident on a semiconductor produces a variety of effects depending on the primary beam energy and on the properties of the semiconductor. These effects include electron penetration, internai ionization and thermal deposition as well as external effects such as secondary and back scattered electrons, electron beam induced current (EBIC) and lattice strain. Modulated electron beams have been used for thermal wave imaging by the use of piezoelectric detectors in contact with the sample to monitor the modulated strain produced by the electron beam. This technique is termed Scanning Electron Acoustic Microscopy (SEAM). SEAM studies of integrated circuits have shown that subsurface features are imaged at depths controlled by the energy of the electron beam [1]. However, there is no adequate theory which describes this effect or the more generai question of image contrast in SEAM. This arises in part because SEAM images represent a convolution of thermal, acoustic and electron transport effects as well as the initial electron loss profile in the semiconductor. There is a need for improved understanding of electron injection, scattering, trapping and thermalization especially as they apply to the use of electron excitation beams for thermal wave imaging