Experimental evidence for a negative-<i>U</i>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 state. From the thermally activated decay of the local mode lines and the observed threshold energies for the photoionization cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{p}}^{0}$(1) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}^{0}$(2), binding energies of 0.15 and 0.62 eV for the first and the second electron, respectively, are deduced.

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