Overview

The fundamental energy gaps of most semiconductors span the energy range from zero to about 6 eV. Photons of sufficient energy can excite electrons from the filled valence bands to the empty conduction bands. As a result, the optical spectra of semiconductors provide a rich source of information on their electronic properties. In many semiconductors, photons can also interact with lattice vibrations and with electrons localized on defects, thus making optical techniques also useful for studying these excitations. Their optical properties are the basis of many important applications of semiconductors, such as lasers, light emitting diodes, and photodetectors.

Figure 6.1 shows schematically some of the optical processes which can occur when a medium is illuminated by light. At the surface of the medium, a fraction of the incident light is reflected and the rest transmitted. Inside the medium some of the radiation may be absorbed or scattered while the remainder passes through the sample. Some of the absorbed electromagnetic waves may be dissipated as heat or reemitted at a different frequency. The latter process is known as photoluminescence. Electromagnetic waves are scattered by inhomogeneities inside the medium. These inhomogeneities may be static or dynamic. An example of a dynamic fluctuation is the density fluctuation associated with an acoustic wave. Scattering of light by acoustic waves is usually referred to as Brillouin scattering [6.1a]. (This phenomenon was independently discovered by Mandelstam [6.1b].) Scattering of light by other elementary excitations, such as optical phonons or plasmons, is known...