Abstract
A new mechanism of absorption of infrared radiation at low temperatures in n-type GaAs, InP and CdTe is investigated. Photon energies are about two thirds of donor ionization energy. Donor concentrations up to 2*1014 cm-3 in GaAs, 5*1014 cm-3 in InP and 2*1015 cm-3 in CdTe, and compensations up to 0.2 are considered. A model which is based on photon-induced transitions of electrons from 2p sigma u- to 3d sigma g-like states of ionized donor pairs is used. The formula for the absorption coefficient is derived and numerical results are presented. The transitions can be also detected as changes in DC conductivity.