  The effect of outdiffusion of Mn interstitials from (Ga,Mn)As epitaxial
layers, caused by post-growth low-temperature annealing, on their electronic-
and band-structure properties has been investigated by modulation
photoreflectance (PR) spectroscopy. The annealing-induced changes in structural
and magnetic properties of the layers were examined with high-resolution X-ray
diffractometry and SQUID magnetometery, respectively. They confirmed an
outdiffusion of Mn interstitials from the layers and an enhancement in their
hole concentration, which were more efficient for the layer covered with a Sb
cap acting as a sink for diffusing Mn interstitials. The PR results revealing a
decrease in the band-gap-transition energy in the as-grown (Ga,Mn)As layers,
with respect to that in the reference GaAs one, are interpreted by assuming a
merging of the Mn-related impurity band with the host GaAs valence band. On the
other hand, an increase in the band-gap-transition energy in the annealed
(Ga,Mn)As layers is interpreted as a result of the Moss-Burstein shift of the
absorption edge due to the Fermi level location within the valence band,
determined by the enhanced free-hole concentration. The experimental results
are consistent with the valence-band origin of mobile holes mediating
ferromagnetic ordering in (Ga,Mn)As, in agreement with the Zener model for
ferromagnetic semiconductors.
