Raman spectroscopy of impurity states in gallium-doped PbTe

We present Raman spectra and results of galvanomagnetic measurements of PbTe single crystals, doped with gallium, between 10 and 300 K. The effect of persistent photoconductivity depends on the gallium concentration. In all samples well-resolved peaks were observed at about 104 cm(-1) (impurity-induced PbTe LO mode) and 166 cm(-1) at all temperatures. Another mode appears at about 117 cm(-1) at temperature below 250 K. One additional mode, at about 188 cm(-1), is observed in PbTe + 0.4 at% Ga. These modes are discussed in terms of local vibrations of impurities corresponding to different Ga charge states.