ENERGETICS AND LOCAL VIBRATIONS OF THE DX CENTER IN GAAS

The negatively charged broken-bond geometry is a strong candidate for the microscopic origin of the DX center in GaAs but the validity of the broken-bond model is still controversial. We thus examine the model for the Si-related DX center, performing large-scale supercell calculations within the local-density approximation. The highly efficient conjugate-gradient minimization technique is combined with the norm-conserving-pseudopotential method in order to include both short-range and medium-range lattice relaxations. First, we compare the total energy of the broken-bond geometry with that of the negatively charged on-site geometry, which is also a candidate for the DX center. It is found that the broken-bond geometry has a lower total energy, supporting the broken-bond model for the DX center. Second, the barrier height for the broken-bond geometry is found to be comparable to observed emission barriers for the DX center. Third, the broken-bond model is shown to be consistent with results of a Fourier-transform infrared-absorption measurement for the local vibrational frequency of Si in pressurized GaAs, although the neutral on-site model also reproduces the experimental results within the calculational accuracy. Most of the results in this work indicate that the broken-bond model is consistent with experiments. Finally, we find that the broken-bond model with the neutral charge state is unstable and thus expect that the Si atom moves to the substitutional site after the DX center is photoexcited.