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 $V_s$ and $V_p$ 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.