Far-infrared absorption of neutron-transmutation-doped germanium

An investigation of the far-infrared absorption of compensated p-type Ge at a temperature of 3 K is presented. The absorption mechanism which is of interest is due to photon-induced hopping transitions of charge carriers between impurity centers. The samples were prepared by neutron transmutation doping. Samples with carrier concentrations (Nₐ-Nd) ranging from 2.3×{}10¹⁵ to 2.6×{}10¹⁶ cm^-3 show a broad maximum peaked at frequency between 10 and 24 cm^-1. The absorption and the frequency of its maximum increase asymptotically with respect to time due to the evolution of the Ga-impurity concentration. It is found that at low frequencies the absorption coefficient is proportional to frequency. The overall behavior of the absorption spectra is found to be consistent with a theory based on the localized-pair model, but a theoretically predicted sharp peak at 20.4 cm^-1 on the absorption curve has not been observed in the experimental absorption spectra.