Far-infrared and galvanomagnetic study of gallium doped PbTe

The far-infrared reflectivity spectra of PbTe(Ga) (NGa = 0.1, 0.2, and 0.4 at.%) alloys were measured in the 30-500 cm−1 spectral range at different temperatures and analyzed using a numerical fitting procedure based on the plasmon-phonon interaction model. The three local Ga modes at about 120, 150 and 220 cm−1 together with the strong plasmon-phonon coupling are observed. Using results of galvanomagnetic measurements carrier concentrations and the Fermi level temperature dependence are determined for illuminated and unilluminated samples. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with negative correlation energy. For both, p-type (NGa = 0.1 and 0.2 at.%) and n-type samples (NGa = 0.4 at.%) the Fermi level is in the forbiden band, but for n-type illuminated sample the Fermi level is in the conduction band at low temperature. Positions and concentrations of different Ga impurity states are obtained using the Gibbs distribution.