Oxygen-doped gallium arsenide crystals have been investigated by Fourier transform infrared spectroscopy. Samples with the Fermi level at about below the conduction band exhibit under thermal equilibrium conditions both stable states of electrically active oxygen , identified optically by the associated local vibrational modes at 730 and . Based on the analysis of compensating centers in the material and comparative Hall measurements, the charge states and , respectively, are assigned to these bands. This result favors the (two arsenic antisites and one off-center substitutional oxygen atom) microscopic model for the defect, presented recently, and is in disagreement with the earlier isolated models. Systematic investigations related to the quantitative optical spectroscopy of this center using optically induced conversion experiments confirm that the absorption strengths of the two bands differ by 15%. A calibration factor of is suggested for the band at , derived indirectly from the variation of the and charge states with the carbon acceptor concentration.
The calibration factor is defined as , where is the defect concentration per and is the integrated absorption.