The atomic configuration of the O negative-U center, which had a Ga-O-Ga structure, was investigated using first-principles calculations. The stable atomic configurations and formation energies were calculated for two Ga-O-Ga structures. One of these was Ga-{O-VAs}-Ga, which had been proposed as being the atomic structure of the negative-U center. The other was Ga-Oi-Ga, as proposed here. Five charge states, ranging from 1 to -3, were considered. For both Ga-{O-VAs}-Ga and Ga-Oi-Ga, the stable atomic configuration depended upon the charge state. In the case of Ga-{O-VAs}-Ga, the calculated atomic configurations could not explain the experimentally observed trends in the local vibrational-mode absorption frequencies for various charge states. The calculated formation energy, as a function of the Fermi level, indicated that the Ga-{O-VAs}-Ga structure did not have a negative-U nature. On the other hand, the calculated results for the Ga-{O-VAs}-Ga structure explained experimental results such as the local vibrational-mode absorption frequencies for various charge states, and the negative-U nature. It was concluded that the atomic configuration of the negative-U center was Ga-Oi-Ga, rather than Ga-{O-VAs}-Ga.

A.Taguchi, H.Kageshima: Physical Review B, 1998, 57[12], R6779-82