Cathodoluminescence (CL) is the emission of low energy photons in the range from approximately 1 eV to 5 eV (infrared, visible, and ultraviolet light) as a result of inelastic scattering of the high energy beam electrons (Fig. 28.1). Materials that can emit such photons are insulators or semiconductors which have an electronic structure with a filled valence band of allowed energy states that is separated by a gap of disallowed energy states from the empty conduction band, as shown schematically in Fig. 28.2a. Inelastic scattering of the beam electron can transfer energy to a weakly bound valence electron promoting it to the empty conduction band, leaving a positively charged “hole” in the conduction band. When a free electron and a positive hole are attracted and recombine, the energy difference is expressed as a photon, as illustrated in Fig. 28.2b. Because the possible energy transitions and the resulting photon emission are defined by the intrinsic properties of a high purity material, such as the band-gap energy but also including energy levels that result from physical defects such as lattice vacancies, rather than by the influence of impurity atoms, this type of CL is referred to as “intrinsic CL emission.” Since the valence electron promoted to the conduction band can receive a range of possible kinetic energies depending on the details of the initial scattering, the photons emitted during free electron–hole recombination can have a range of energies, resulting in broad band CL photon emission. Because of the great mismatch in the velocity of the high energy (keV) beam electron and the low energy (eV) valence electron, this is not an efficient process and in general CL emission is very weak. The ionization cross section is maximized for electrons with three to five times the binding energy of the valence electrons, so that most efficient energy transfer to initiate CL emission occurs from the more energetic slow SE (>10 eV) and the fast SE (hundreds of eV) also created by inelastic scattering of the primary electron.
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