Optical Properties

Abstract

The interaction of photons with the electrons of a semiconductor provides a large amount of information: the position of an emission or absorption line indicates the energy separating two levels; the lowest photon energy at which a spectral structure begins reveals the threshold for a class of transitions; the shape of a spectrum is a measure of the transition probability or of the distribution of states, etc. In the absorption process, a photon of known energy excites an electron from a lower into a higher energy state. Thus, absorption in a semiconductor occurs due to band-to-band transitions, exciton formation, transitions between localized levels and bands, and between the ground states of localized levels and their excited states. Free-carrier absorption within a band also occurs (this carrier absorption must be removed by compensation of the free carriers, in order to allow observation of other absorption processes). Finally, resonances due to the vibrational states of the lattice and defects also induce absorption as shown in [Ref. 1.1, Chap. 5].