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Negative-U properties of off-centre substitutional oxygen in gallium arsenide

Recent infrared absorption studies of semi-insulating GaAs have revealed an electrically active, oxygen-related defect with remarkable spectroscopic and microscopic properties. This defect, the structural analogue of the oxygen vacancy centre in Si, occurs in three charge states, the zero-, one- and two-electron states. The experimental fingerprint for each charge state is the local mode frequency which shows a characteristic charge-state-induced shift. Dependent on the Fermi potential, at thermal equilibrium only the local modes corresponding to the zero- or the two-electron state are experimentally observable. The metastable one-electron state disproportionates spontaneously into the zero- and the two-electron states. The energy positions of the associated gap levels are at Ec-0.14 eV and at Ec-0.58 eV for the first and the second electron, respectively. These assignments are derived from the thermally activated decay of the local mode lines and, independently, from the threshold energies of the optical transitions sigma p0(1) and sigma n0(2). Through a comparative deep-level transient spectroscopy and infrared absorption study on neutron-transmutation-doped, n-type samples the second electron level is identified as the well known EL3 level.