Free-carrier absorption in quasi-two-dimensional semiconducting structures

The quantum theory of free-carrier absorption in semiconductors is extended to treat the case where the free carriers are confined in quasi-two-dimensional semiconducting structures such as layered heterojunctions, thin films, and inversion layers. As a result of the confinement of the carriers, the energy of motion of the carriers normal to the quasi-two-dimensional structure is size quantized. The free-carrier absorption coefficient is found to depend upon the polarization of the electromagnetic radiation relative to the direction normal to the quasi-two-dimensional structure and to be an oscillatory function of the thickness of the layer or film as a result of the size quantization. The free-carrier absorption coefficient is calculated for the case where the carriers are scattered by acoustic phonons via deformation-potential coupling.