Ab initio calculation of optical properties with excitonic effects in wurtzite In_xGa_1-xN and In_xAl_1-xN alloys

By combining modern many-body approaches with a cluster expansion scheme, frequency-dependent dielectric functions including excitonic and local-field effects are computed for wurtzitic group-III nitride alloys with varying composition x. The quasiparticle electronic structure required to construct the quasielectron-quasihole pair Hamiltonian for each cluster is approximated using a LDA+U+Δ approach. Two different cluster statistics are employed to perform configurational averages for the frequency-dependent complex dielectric functions. Comparing the resulting composition dependence of peak positions and intensities to experimental data allows conclusions regarding the distribution of the group-III cations in the alloys.