The dependence of the fundamental band gap and higher-lying critical-point energies of dilute-nitrogen Ga_(1-y)In_(y)As_(1-x)N_(x) epilayers on nitrogen mole fraction (x), for x≤0.0125, and temperature, from 20 to 300 K, was investigated by photoreflectance spectroscopy. The band gap, E_(G), was found to decrease with increasing x in a highly nonlinear manner. The bowing parameter (the second-order parameter b in a quadratic expression for the dependence of E_(G) on x) was found to become less negative with increasing x; the value of b changed from -50 eV, at very low nitrogen fraction, to -20 eV, at x>0.01. These results strongly suggest that nitrogen-related impurity levels arise within the band gap of dilute-nitrogen Ga_(1-y)In_(y)As_(1-x)N_(x) alloys.
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