An inverted ordering of the acceptor level (-/0) and the donor level (0/+) of the Si-related DX center was demonstrated by means of photo-emission deep-level transient spectroscopy. A new photo-induced deep-level transient spectroscopic peak was observed at low temperatures (60K) in Ga0.74Al0.26As and at high temperatures (140K) in Ga0.65Al0.35As. The activation energies, for emission and capture in the case of acceptor levels and metastable donor levels, were determined in both samples. This observation of a metastable level of the DX center confirmed the inverted ordering of the energy levels. It revealed an effective negative correlation energy (negative-U), and the existence of a thermodynamically unstable DX0 state.

S.Ghosh, V.Kumar: Physical Review B, 1997, 55[7], 4042-5