Evidence for large configuration-induced band-gap fluctuations in GaAs1-xNx alloys

We have measured the near-band-gap absorption of ${\text{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ thin films with $xl0.012$. The spectra were analyzed with a model which allows a precise determination of the band gap and of the width of the optical transitions; the latter is found to increase, for $xg0.002$, well beyond what is expected in a III-V alloy. Ab initio calculations were performed within the generalized-gradient approximation with the exchange-correlation functional of Engel and Vosko [Phys. Rev. B 47, 13164 (1993)]. They reveal that the band gap depends markedly on nitrogen atomic configuration. The anomalous broadening of the intrinsic optical transitions thus gives strong evidence for configuration-induced band-gap fluctuations.