Electronic structure of heavily and randomly nitrogen doped GaAs near the fundamental band gap

On increasing the nitrogen doping concentration in GaAs, states associated with isolated. paired and clustered (i.e.. more complex configurations) nit rogen atoms sequentially appear, with their energy levels being resonant fo r the isolated center and most of the pairs and becoming bound for a couple of pairs and clusters. At a nitrogen mole concentration of x similar to 0. 1%, the shallow nitrogen bound states have merged with the GaAs band edge, which effectively gives rise to a band gap reduction, but the deeper nitrog en bound states persist as discrete levels. We study the behavior of nitrog en at this "transition" concentration, using various techniques (photolumin escence under selective excitation. electroreflectance. and Raman scatterin g), in order to gain insight into the large band gap reduction observed at all nitrogen concentrations. The validity of a few existing models proposed for explaining the large band gap reduction will be briefly discussed.