Cascade Capture of Electrons in Solids

Enormous capture cross sections in the range 10^-15 cm^2 to 10^-12 cm^2 have been observed for a wide variety of Coulomb attractive centers in Si and Ge, some involving binding energies many times the Debye energy. Whereas multiphonon transitions to the ground state yield cross sections five to ten orders of magnitude too small, capture into excited states of large radius followed by a cascade of one-phonon transitions leads to cross sections of the right order of magnitude. The initial capturing event is likely to involve an optical phonon or an intervalley collision in the room temperature range, but the acoustic phonon contribution will predominate at low temperatures. Subsequent collisions may eject the electron or cause it to increase its binding energy. The "sticking probability, " or probability of eventual capture into the ground state, becomes significant for binding energies of order kT. As the temperature is reduced capture into orbits of larger radius becomes effective, and, at least for the acoustic phonon case the cross section increases rapidly with decreasing temperature, and with decreasing electron energy. The large cross sections 10^-17 cm^2 to 10^-15 cm^2 found for neutral centers can be explained on a similar basis, the attractive potential in this case being provided by the large polarizability of the neutral center.