General comparisons: Switching between states and motion

For wide-bandgap ionic solids, electron–hole recombination energies can exceed defect formation energies. In semiconductors, the gap is smaller, and usually significantly less than the energy needed to create a defect. Defect production is predicted to occur only in special cases, as in porous silicon (Caldas et al. 1997) and perhaps in other nanocrystals. Most wide-gap ionic solids have strong electron–phonon coupling, so that self-trapping of excitons and certain carriers is common. In semiconductors, the lower electron–phonon coupling makes self-trapping unlikely. Even excitonic complexes (e.g. Kuvalovskii et al. 1986) are relatively ineffective, since the electrons and holes tend to recombine one at a time. The role of electronic excitation takes a different form in semiconductors from that observed in wide-gap ionic solids. In semiconductors, it is processes involving impurities, or pre-existing defects such as dislocations, which are affected. The major consequences of excitation are enhanced processes like diffusion, or the production of metastable, non-equilibrium states of defects. The ways in which defect processes are influenced by excitations are often identified only after frustrations. A common example might be that devices survive for long times in storage, but degrade rapidly when used because of dislocation climb.

Discussions of materials modification imply modification controlled in space or time, or with selected defect states. Control in space is common, e.g. lithography and photography.