Experimental and theoretical studies of theE+optical transition inGaAsNalloys

We present a joint experimental and theoretical study of the optical properties of $\mathrm{GaAsN}$ alloys. The ${E}_{+}$ optical transition is identified by variable-angle ellipsometry measurements. It induces an edge characteristic of a ${M}_{0}$ critical point in the imaginary part of the dielectric function of the alloys. The experimental results are modeled by ab initio calculations of the dielectric function of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ in the nitrogen concentration range $x=0.009--0.037$. The origin of the ${E}_{+}$ transition is analyzed by calculating contributions from different conduction states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ to the dielectric function. Our study confirms that optical transitions from the valence band to electronic states of ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ derived from $L$ states of the conduction band of $\mathrm{GaAs}$ are at the origin of the ${E}_{+}$ edge.