Tight-binding molecular dynamics simulations at 0K were performed in order to study the effect of vacancies and antisites, in various charge states, upon electronic and structural properties. Relaxation was incorporated into the model and, for each defect, calculations were made of the local atomic structure, the volume change upon relaxation, the formation energy (including chemical potential contributions), and the ionization levels. It was found that Ga vacancies tended to relax by an amount which was independent of the charge state. This result was consistent with positron lifetime measurements. The calculations also predicted that Ga vacancies would exhibit a negative-U effect, and would have a triply negative charge state for most values of the electron chemical potential. On the other hand, the relaxation of As vacancies depended sensitively upon the charge state. The model confirmed the 2 experimentally observed ionization levels for this defect, which were just below the conduction-band minimum. In the same way, Ga antisites exhibited large relaxations. In the neutral state, the relaxation was so great that it led to a broken-bond configuration; in close agreement with first-principles calculations. This system also exhibited a negative-U effect for values of the electron chemical potential which were near to the mid-gap. In the case of As antisites, only a weak relaxation was found, which was independent of the charge. The model predicted that the neutral state of the defect would be the ground state for values of the electron chemical potential which were near to, and above, the mid-gap. This supported the view that the EL2 defect was a neutral As antisite. Upon comparing the formation energies of the various defects, it was concluded that, for all values of the atomic chemical potential, antisites were more likely to occur than vacancies.

H.Seong, L.J.Lewis: Physical Review B, 1995, 52[8], 5675-84