Origin of the magnetic circular dichroism of absorption of the arsenic antisite in GaAs and Al_xGa_1-xAs

We have investigated the transitions associated with magnetic circular dichroism of absorption (MCDA) in GaAs by studying the MCDA spectrum as a function of stoichiometry in Al_xGa_1-xAs grown at low temperatures via molecular-beam epitaxy. The main result of the stoichiometry-dependence study is that the spectrum of the singly ionized arsenic antisite can be well explained within the valence-band model, that is, the optical transitions are from the valence band to the A₁ ground state. The competing internal transition model can be refuted. The stoichiometry dependence also suggests that the observed diamagnetic spectrum of the neutral arsenic antisite could be linked to the zero-phonon line of the optical-absorption spectrum.