Inverted order of acceptor and donor levels of the Si-related DX center in AlxGa1-xAs

We have demonstrated the inverted ordering of the acceptor level (-/0) and the donor level (0/+) of the Si-related DX center in AlxGa1-xAs by photoemission deep-level transient spectroscopy (DLTS). The new photoinduced DLTS peak has been observed at low temperature (60 K) in Al0.26Ga0.74As and at high temperature (140 K) in Al0.35Ga0.65As. Activation energies for emission and capture for acceptor level and metastable donor level have been determined in both samples with different AlAs mole fractions. The observation of this metastable level of the DX center proves the inverted ordering of the energy levels that reveals an effective negative correlation energy (negative U) with the existence of a thermodynamically unstable DX(0) state.