Negative-U, off-centerOAsin GaAs and its relation to theEL3 level

Using comparative local-vibrational-mode and deep-level transient spectroscopy, we have studied the off-center ${\mathrm{O}}_{\mathrm{As}}$ defect in high-resistivity, undoped GaAs and neutron-transmutation-doped, n-type samples from the same starting material. The results fully support that this defect has a negative-U character. They also reveal that its deeper, two-electron level corresponds to the chemically unidentified EL3-defect level in GaAs at ${\mathit{E}}_{\mathit{c}}$-0.58 eV.