Band structure and optical properties of ${\mathrm{In}}_y{\mathrm{Ga}}_{1-y}{\mathrm{As}}_{1-x}{\mathrm{N}}_x$ alloys

We have carried out comprehensive studies of the nitrogen-induced modifications of the electronic structure of ${\mathrm{In}}_y{\mathrm{Ga}}_{1-y}{\mathrm{As}}_{1-x}{\mathrm{N}}_x$ alloys. Temperature- and composition-dependent optical absorption spectra have been measured on free-standing layers of ${\mathrm{In}}_y{\mathrm{Ga}}_{1-y}{\mathrm{As}}_{1-x}{\mathrm{N}}_x$ thin films lattice matched to GaAs with $0<~x=3y<~0.025$ in the photon energy range 0.8–2.5 eV. The measurements provided information on the optical transitions at the Γ point of the Brillouin zone. Spectroscopic ellipsometry measurements performed in a wide photon energy range 1.5–5.5 eV have been used to determine the energy dependence of the dielectric function as well as energies of $E_1,$ $E_0^{\prime },$ and $E_2$ critical point transitions. Measurements of the plasma edge frequency for samples with different electron concentrations have been used to determine the dispersion relation for the lowest conduction band. The results show a large effect of nitrogen on the optical spectra and on the dispersion relations for the conduction band states close to the Γ point. They can be consistently explained in terms of the recently proposed band anticrossing model. On the other hand, the observed small effect of N on the transition energies at the high-energy critical points can be well understood within a virtual crystal approximation. The insensitivity of the high-energy critical point transitions to the N content is in disagreement with the theoretical calculations predicting a large effect of N incorporation on the energies of X and L conduction band minima.