The novel infrared imaging technique for assessment of undoped semi-insulating (SI) GaAs substrates known as Reverse Contrast is known to result from absorption from point defects whose concentrations approximately anti-correlate with those of EL2, deep donor defects. The absorption that occurs within - 65 meV of Eg in cooled samples is sufficiently strong that commercial wafers can be mapped with simple infrared CCTV imaging equipment. RC defects are thought to be very deep acceptors with an ionization energy close to the Conduction Band. Concentrations of RC defects are not measurable in SI GaAs as they are un-ionized in the dark. Like EL2 defects they can be photo-quenched by irradiation with mid-gap light.
In this paper, we present novel Hall Effect measurements on very lightly Te-doped n-type GaAs where at least a fraction of RC defects are ionized. A permanent increase in the carrier concentration is observed after photo-quenching corresponding to the bleaching of RC defects. The carrier concentration returns to its initial value at the same temperature at which the absorption of RC defects is recovered. This result allows a calibration for the absorption coefficient to be found.