Complexes, clustering, and native-defect-assisted diffusion of aluminum in silicon

We study from first principles the energetics, complex formation, and diffusion of Al in crystalline Si. We first consider the interaction of Al with self-interstitials Si-i: at variance with B and In, the complex formed by substitutional Al-Si and a self-interstitial Si-i is metastable and trasforms into interstitial Al-i in the tetrahedral site. Therefore in the presence of excess self-interstitials Si-i, Al will go off-site and cease to be an acceptor. Al-i can then bind to Al-Si, forming a trigonal complex (also not an acceptor) compatible with the properties of the Si-G19 center. A further sizable energy gain is obtained forming the deep-donor Al-Al-self-interstitial complex. We thus predict that a sufficiently high externally generated Si-i population will cause the deactivation, and presumably the clustering and disordering, of the Al acceptor in Si. As to Al-vacancy interaction, Al-Si and a first-neighbor Si vacancy spontaneously form a divacancy-interstitial complex, similarly to In. We evaluate the activation energy for diffusion for the various mechanisms investigated, and find it to be predominantly interstitial-assisted; calculated activation energies are in good general agreement with experiment.