O-IN GAP - A NEGATIVE-U CENTER

We have determined the equilibrium configurations of O+, O0 and O- states for O in GaP using a self-consistent semi-empirical method. A centred configuration is found for both the O+ and the O0 states, consistent with experimental results, while the O- state has the O atom heavily lattice relaxed from its original O0 state. The energy barrier between the metastable centred O- and the stable heavily lattice-relaxed off-centred O- is only 0.12 eV, small enough to be compatible with experimental findings, with the defect levels (+/0) and (0/-) exhibiting an inversion in the ordering. Thus, a negative-U behaviour is demonstrated here and a charge-transfer reaction is postulated for O- in the GaP system to account for the large lattice relaxation upon capture of the second electron.