Multiple-charge impurity states of the transition and noble metal ions in semiconductors

Two possible ways of creating multiple-charge deep levels of d-metal ions in semiconductors are considered. The case of the different charge states corresponding to the different d-shell configurations is discussed with respect to transition-metal ions in A_IIB_VI and A _IIIB_V compounds. A model in which the double-charge states are created without d-valence change is proposed for noble metal impurities. In this case the wavefunction of the first electron in the impurity level is the superposition of the d shell and valence band states, and the wavefunction of the second localised electron is made only of the band states. The authors discuss also the occupation rules for the impurity levels and show that one obtains a wrong distribution function for the localised electron when one uses the Hartree approximation for the d-d correlation energy U. The true 'atomic' distribution function can be obtained including U in zero approximation.