The electronic properties and thermal stability of centers incorporating a vacancy and a group-V-impurity atom (P, As, Sb, or Bi) in Ge crystals were investigated. The vacancy-group-V-impurity atom pairs (E centers) were induced by irradiation with 60Co γ-rays and studied by means of capacitance transient techniques with the use of Au-Ge Schottky barriers. It was argued that the E centers in Ge had 3 charge states: double negative, single negative and neutral, and introduce 2 energy levels into the gap. There were pronounced changes in the activation energies of charge carrier emission for the particular states with the changes in the type of impurity atoms. The emission of an electron from the doubly negatively charged state of the centers was accompanied by a large change in entropy (ΔS), so, the free energy of the electron ionization, ΔG(––/–) = ΔH(––/–)–TΔS(––/–), changes significantly with temperature. Consequently, the position of the second acceptor level of the E centers {E(––/–) = Ec– ΔG(––/–)} was temperature dependent. In Ge crystals having shallow donor concentrations in the range 1013–1015/cm3 at equilibrium conditions half-occupancy of the doubly negatively charged state of the vacancy-group-V-impurity atom pairs occurred when the Fermi level was at Ec–(0.18–0.22)eV. Changes in the entropy of ionization and the energy of electron emission for the double negative state of the E centers follow the Meyer-Neldel rule. It was shown that the directly measured capture cross sections of electrons at the singly negatively charged E centers were temperature dependent and could be described by the multiphonon-assisted capture model. The first acceptor level of the E centers was in the lower part of the band gap. The formation of one vacancy–group-V-impurity atom complex resulted in the removal of at least 2 electrons from the conduction band in n-type Ge. It was thought that the E centers were responsible for the fast free carrier removal and n → p conversion of the conductivity type in O-lean Ge crystals upon electron- or γ-irradiation at room temperature. The thermal stability of the E centers in Ge was found to increase with an increase in the size of donor atoms.

Vacancy–Group-V-Impurity Atom Pairs in Ge Crystals Doped with P, As, Sb and Bi. V.P.Markevich, I.D.Hawkins, A.R.Peaker, K.V.Emtsev, V.V.Emtsev, V.V.Litvinov, L.I.Murin, L.Dobaczewski: Physical Review B, 2004, 70[23], 235213 (7pp)