EXPERIMENTAL-EVIDENCE FOR A NEGATIVE-U CENTER IN GALLIUM-ARSENIDE RELATED TO OXYGEN

Fourier-transform infrared spectroscopy on the charge-state-dependent local modes of the off-center substitutional oxygen impurity in GaAs has revealed an inverted ordering of the two gap levels. The metastable one-electron state shows the characteristic disproportionation into the zero- and the two-electron states. From the thermally activated decay of the local mode lines and the observed threshold energies for the photoionization cross sections sigma-p0(1) and sigma-n0(2), binding energies of 0.15 and 0.62 eV for the first and second electron, respectively, are deduced.