Silicon Photonics: Fundamentals and Devices

4.9 Free-carrier Absorption

The semiconductors used in devices are doped with impurities, and as a result there exists a substantial amount of free electrons or holes. These carriers may absorb photons and make a transition from a state of wave vector k to another state of wave vector k′ in the same band. These transitions are shown in Figure 4.2 as process (D) and process (E), respectively, in the valence band and conduction band. These transitions are termed as free-hole (D) and free-electron (E) absorption or in general as free-carrier absorption.

A change in wave vector occurs in the absorption process and to conserve the wave vector, one must have . The photon wave vector is related to the frequency of radiation by . In order to satisfy both energy and momentum conservation, the energy of the photon should be prohibitively large, larger than the typical band gap. Therefore the radiative transitions just discussed cannot account for intraband free carrier absorption.

The intraband processes occur with the simultaneous action of a photon and another momentum-conserving agency, that is, scatterers like phonons, impurities, alloy disorder, or other imperfections. Again the process is a second-order transition and the energy of the photon is quite small compared to that needed for ...