Photoionization of the DX(Te) centers in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As: Evidence for a negative-U character of the defect

A detailed analysis of the photoionization of the DX(Te) centers in ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As (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 Γ-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${)}^{\mathrm{-}}$ state.