Local-environment effect on the nitrogen bound state inGaPxAs1−xalloys: Experiments and coherent-potential approximation theory

In $\mathrm{Ga}{\mathrm{P}}_x{\mathrm{As}}_{1-x}:\mathrm{N}$ alloys ($0.6\le x\le 1$) the luminescence due to the radiative recombination of nitrogen bound excitons (${\mathrm{N}}_x$) is shifted toward lower energies with respect to excitation spectra ($A$). This shift is attributed to the band broadening generated by local disorder around nitrogen atoms. The broadening and the shift-composition dependences are characterized by their appearance even for very low As concentration and their saturation for $x<\mathrm{}0.8\mathrm{}$. The temperature dependence of the luminescence and excitation lines is consistent with a model which we suggest and according to which the luminescence band is generated by the radiative recombination of nitrogen bound excitons which have thermalized toward the bottom of the excitation band broadened by alloy disorder, i.e., by the occurrence of different local configurations As-P around nitrogen atoms. Theoretically, we stress the importance of taking into account the perturbation of the electronic structure brought about by the alloy disorder; to this end we use the coherent-potential approximation (CPA). We find a good agreement with the experimental bowing parameter of the band edges. By using an extension of the CPA incorporating the local environment effect, we account for the evolution of the bandwidth, energy positions, and cross sections of the excitation band ($A$) versus the alloy composition.