It was shown that the charge state of the 73Ge antisite defect that arose in anionic sub-lattices of PbS, PbSe and PbTe after radioactive transformation of 73As does not depend on the position of the Fermi level, whereas the 73Ge centre in cationic sub-lattices of PbS and PbSe represents a 2-electron donor with the negative correlation energy: the Mössbauer spectrum for the n-type samples corresponds to the neutral state of the donor centre (Ge2+), while this spectrum corresponds to the doubly ionized state (Ge4+) of the centre in the p-type samples. In partially compensated PbSe samples, a fast electron exchange between the neutral and ionized donor centres was realized. It was shown by the method of Mössbauer spectroscopy for the 119Sn isotope that the Ge-related energy levels were located higher than the levels formed in the band gap of these semiconductors by the impurity Sn atoms.

Two-Electron Germanium Centers with a Negative Correlation Energy in Lead Chalcogenides. E.I.Terukov, A.V.Marchenko, A.V.Zaĭtseva, P.P.Seregin: Semiconductors, 2007, 41[12], 1413-8