Instabilities of simple models of C-As_icomplexes in gallium arsenide

Irradiation of GaAs:C produces C-related defects called C(1) centres. These are analysed using first-principles theory. The authors find that the stable defect has C3v symmetry, in agreement with FTIR results, but two obvious models, one with Asi lying at an anti-bonding site to CAs and the other with the interstitial anti-bonding to an on-site Ga neighbour, are unstable. They find that the interstitial displaces one of the Ga atoms bonded to CAs into a bond-centred configuration, leaving Asi as an anti-site defect. The calculated vibrational modes of this structure are in reasonable agreement with the observed ones.