BINDING OF ELECTRONS BY NITROGEN PAIRS IN GAP

Calculations concerning the electron energy levels associated with nitrogen pairs in gallium phosphide are reported. A model Hamiltonian, containing impurity potentials representing the substitution of two phosphorus atoms of variable separation by nitrogen atoms is assumed. The resulting electron binding energies are calculated within the pseudopotential formulation using a Green function method which has previously met with some success in dealing with similar problems. Using a nitrogen potential which gives a bound electron state at the conduction-band edge for the single nitrogen impurity, it is found that the electron levels of the pairs range from approximately=0-100 meV in broad agreement with experiment. However, the ordering of the calculated levels does not agree with the currently accepted assignment. A brief discussion is given of the possible causes for this discrepancy and its relation to the nature of binding of N in GaP.