Electronic aspects of the optical-absorption spectrum of theEL2 defect in GaAs

As far as its optical properties are concerned, the EL2 defect seems to be the same as the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ arsenic antisite defect in GaAs. Although it is widely accepted that lattice relaxation plays a role in determining the optical absorption of EL2, there are now reasons to question just how much lattice relaxation occurs. As an initial step towards resolving this issue, this paper presents a calculation of the optical absorption of the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ defect in which the calculational emphasis is on a realistic description of the electronic states. In this calculation, we ignore lattice relaxation altogether and, consequently, we find neither the experimentally observed zero-phonon line nor the multiphonon replicas. Aside from these and one other interesting discrepancy that can probably be explained by our neglect of lattice coupling, the spectra are reasonably similar to what is observed. One conclusion from the analysis presented here is that the hole cross section for the main donor level of EL2 has not been correctly measured in any of the experiments reported to date.