Nitrogen-Activated Transitions, Level Repulsion, and Band Gap Reduction inGaAs1−xNxwithx<0.03

We report electroreflectance spectra for a series of ${\mathrm{GaAs}}_{1-x}{\mathrm{N}}_x$ samples with $x<\mathrm{}0.03\mathrm{}$. For all samples, the fundamental band gap transition $\left(E_0\right)$ and the transition from the spin-orbit split-off valence band $(E_0+{\Delta }_0)$ are observed. For samples with $x\ge 0.008$, an additional transition $\left(E_{+}\right)$ is observed. With increasing nitrogen content, the increase in $E_{+}$ is linear in, and nearly equal to, the band gap reduction indicative of a nitrogen-induced level repulsion. The directly observed $E_{+}$ transition may arise from either a nitrogen-related resonant level or a disorder-activated indirect transition.