Role of strain and properties of N clusters at the onset of the alloy limit in GaAs1-xNx

In GaAs1-xNx, the band gap energy decreases very rapidly with x and the electron effective mass shows a quite unusual compositional dependence characterized by a sudden doubling for x congruent to 0.1%. In this work, we investigate the origin of this behavior by photoluminescence measurements under hydrostatic pressure in as-grown and hydrogenated GaAs0.9989N0.0011 samples. First, we show that two nitrogen pair states emitting at 1.488 and 1.508 eV contribute mainly, but to a different extent, in determining the steep increase in the electron mass observed for x congruent to 0.1%. Tight-binding supercell calculations assign the 1.488 eV levels to isolated N pairs and the 1.508 eV states to N pairs perturbed by a nearby N atom, in disagreement with previous attributions but consistent with the electron mass data. Second, photoluminescence at high hydrostatic pressure discloses that these N pair states show quite different rates of passivation by hydrogen. By combining these findings with the calculated lattice energies associated with each N complex, we conclude that strain relaxation is a key mechanism driving the interaction of hydrogen with N atoms in GaAs1-xNx.