We propose a new model for the DX center in AlxGa1-xAs, for which symmetry-conserving lattice relaxation (breathing distortion) is followed by successive symmetry-lowering lattice relaxation (Jahn-Teller distortion) induced by electron capture at a triplet resonant state in the conduction band. The degeneracy of the triplet state is lifted upon electron capture, and consequently a singlet state within the forbridden gap (the DX center) as well as a resonant doublet state appears. We argue that the photoionization of the DX center is an intracenter optical transition between the singlet and doublet states, and that thermal emission and capture of electrons occur through the resonant triplet state. The sequential lattice relaxation model proposed here provides a natural explantation for the puzzling experimental data available up to now. © 1990 IOP Publishing Ltd.
