Infra-red Absorption in Semiconductors

Infra-red absorption in semiconductors is classified into four different types according to the mechanism: (i) intrinsic absorption associated with electron excitation across the energy gap; (ii) absorption due to the presence of free carriers; (iii) absorption associated with impurities or lattice defects; and (iv) absorption associated with lattice vibration. A general introduction is followed by some theoretical discussions. Electron excitation between different energy bands is discussed with emphasis on the intrinsic absorption edge. For the absorption by free carriers, the effects of electron scattering by lattice vibration and by impurity centres are considered. Absorption associated with localized electronic states is briefly discussed. Experimental results are discussed for four different semiconductors: germanium, silicon, indium antimonide, and tellurium. All four types of absorption have been investigated to some extent for germanium and silicon. The work done on these materials provides a pattern for infra-red studies on semiconductors. The absorption edge in indium antimonide is affected by the carrier concentration. Long wave-length absorption shows interesting behaviour, and the observed effects attributed to lattice vibration have provided information regarding the type of binding in the crystal. Tellurium, having an optical axis, is doubly refracting. Both the absorption edge and the absorption associated with free carriers depend on the direction of polarization of the radiation.