PHOTOIONIZATION OF THE DX(TE) CENTERS IN ALXGA1-XAS - EVIDENCE FOR A NEGATIVE-U CHARACTER OF THE DEFECT

A detailed analysis of the photoionization of the DX(Te) centers in AlxGa1-xAs (0.25 < x < 0.55) has provided experimental evidence for the negative-U character of the defect. A variety of phenomenological models were considered but only the assumption that in ground state the DX center binds two electrons and forms a negative-U system allowed us to quantitatively describe the observed ionization kinetics at different temperatures and light intensities. The intermediate state of the process is not the effective-mass X- or GAMMA-like excited state of the DX center, but rather the neutral (DX)0 state. This is strongly coupled to the lattice in the same way as the ground (DX)- state.