Spatial correlation between Bi atoms in dilute GaAs1-xBix:: From random distribution to Bi pairing and clustering

We use x-ray absorption spectroscopy to investigate the local structure around Bi atoms in GaAs(1-x)Bi(x) layers grown on GaAs as a function of Bi concentration in order to detect short-range order. We find that static disorder in the Bi next-nearest-neighbor interatomic distances dramatically increases when the Bi concentration is increased. At 1.2% Bi concentration, the Bi atoms are randomly distributed whereas at 1.9%, they tend to form next-nearest-neighbor pairs. When the Bi concentration rises to 2.4%, our results suggest that some of the Bi atoms form small Bi clusters. Such strong deviations from a random distribution are likely to play an important role in the occurrence of the giant optical bowing recently measured in this alloy.