Theory of carbon complexes in aluminum arsenide

The structural and dynamic properties of carbon defects in aluminum arsenide are investigated using first-principles local-density-functional cluster theory. The method accounts satisfactorily for the structure and phonon modes of AlAs. The carbon acceptor and donor possess triplet modes in the band gap between the acoustic and optic branches as well as localized triplet modes. The local mode of the acceptor lies within 40 ${\mathrm{cm}}^{\mathrm{-}1}$ of the observed mode. Four modes of a close-by acceptor pair lie within 40 ${\mathrm{cm}}^{\mathrm{-}1}$ of the local mode of the isolated acceptor and this supports a previous assignment of four satellite lines seen in heavily doped material to this defect. The modes of a donor-acceptor pair are investigated but there is no evidence of their existence.