Vacancy-group-V-impurity atom pairs in Ge crystals doped with P, As, Sb, and Bi

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 have been investigated. The vacancy-group-V-impurity atom pairs (E centers) have been induced by irradiation with 60Co γ rays and studied by means of capacitance transient techniques with the use of Au-Ge Schottky barriers. It is argued that the E centers in Ge have three charge states: double negative, single negative and neutral, and introduce two energy levels into the gap. There are 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 is accompanied by a large change in entropy (Δ5), 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(--/-)} is temperature dependent. In Ge crystals having shallow donor concentrations in the range 1013-1015 cm-3 at equilibrium conditions half-occupancy of the doubly negatively charged state of the vacancy-group-V-impurity atom pairs occurs when the Fermi level is at E c-(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 has been shown that the directly measured capture cross sections of electrons at the singly negatively charged E centers are temperature dependent and can be described by the multiphonon-assisted capture model. The first acceptor level of the E centers is in the lower part of the band gap. The formation of one vacancy-group-V-impurity atom complex results in the removal of at least two electrons from the conduction band in n-type Ge. It is thought that the E centers are responsible for the fast free carrier removal and n → p conversion of the conductivity type in oxygen-lean Ge crystals upon electron- or γ-irradiation at room temperature. The thermal stability of the E centers in Ge has been found to increase with an increase in the size of donor atoms.