Properties of molecular-beam epitaxy-grown GaNAs from optical spectroscopy

${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}$ layers with different nitrogen concentrations x grown on (001)GaAs substrates by molecular-beam epitaxy have been studied by photoluminescence, optical absorption, and Raman spectroscopy. The content of nitrogen in the layers was determined by x-ray diffraction and secondary-ion-mass spectrometry. The samples can be classified in three categories with respect to the concentration of N: with doping nitrogen concentration, with average content of N less than 0.3, and with x close to 1. From optical measurements and from analysis of x-ray diffraction spectra, different phases are observed in the ${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}$ layers: GaAs, GaN, and the solid ternary solution ${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}.$ In Raman spectra both GaAs-like and GaN-like optical phonons are observed. We have estimated the fundamental band-gap energy in the ${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}$ alloy with low nitrogen concentration up to $\text{x=0.04}$ from absorption measurements, and in ${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}$ with high nitrogen concentration $\text{x>0.96}$ from photoluminescence spectra. Fitting of the experimental data for low x values gives a constant bowing parameter as big as $\text{b=−18\hspace{0.17em}}\mathrm{eV}.$ This value predicts the band-gap energy for the high nitrogen concentration in agreement with experimental data. Consequently, ${\mathrm{GaN}}_x{\mathrm{As}}_{\text{1−x}}$ is predicted to be semimetallic in the range $\text{0.12<x<0.75.}$