Free carrier absorption and inter-subband transitions in imperfect heterostructures

We present the results of a quantum mechanical modelling of the free carrier absorption (FCA) in semiconductor heterolayers. Elastic and inelastic scatterers are considered with emphasis on the interface defects (optical phonons) contributions to the induced photon absorption for elastic (inelastic) scatterers. Various approaches to FCA are also presented (perturbation, Green's function technique). The connection between inter-subband absorption and FCA is thoroughly discussed. The absorption lineshape and its modification by suitable doping is presented. (invited review)