Binding energies of electrons by nitrogen pairs in GaP

Theoretical calculations of binding energies of electrons by nitrogen pairs in GaP are reported. The calculations are based on the Koster-Slater Green's-function equation and the central-cell defect-potential approximation of Hjalmarson et al. [Phys. Rev. Lett. 44, 810 (1980)]. The defect-potential parameters Vₛ and Vₚ are adjusted to fit the experimental binding energies of electrons by single N impurity and seven (NN)₁ (x=1,2,...,7) pairs. The results are in general agreement for the first time with experiments for either range or ordering of binding energies, and thus strongly support the Hopfield-Thomas-Lynch model for isoelectronic traps. Besides, excited electronic states of (NN)₁, (NN)₃, (NN)₅, and (NN)₆ are reported. The energy value of the (NN)₁ excited state supports the speculation of Cohen et al.