Persistent Photo-Hall Phenomenon and Near-Bandedge Absorption in Lightly n-Type LEC GaAs
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PERSISTENT PHOTO-HALL PHENOMENON AND NEAR-BANDEDGE ABSORPTION IN LIGHTLY n-TYPE LEC GaAs S. TUZEMEN AND M.R. BROZEL Department of Electrical Engineering & Electronics, UMIST, PO Box 88, Manchester, M60 1OD, UK ABSTRACT 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 ntype 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. INTRODUCTION Over the past several years, there has been considerable interest in the mapping of impurities and other crystal defects in bulk, Semi-Insulating GaAs. One of the most successful of these techniques, is the so-called EL2 imaging technique [ 11, where absorption from neutral EL2 centres is mapped over a substrate by using the broad infrared absorption band which occurs in the near infrared. This technique is non-destructive and is particularly valuable because EL2 centres control the compensation of residual acceptors in SI GaAs. In the preliminary stages of investigations into the wavelength dependence of these maps, Skolnick et al [21 demonstrated that the absorption image was rendered negative if the absorption at wavelengths from the bandedge to approximately 65 meV below the bandedge were mapped. This effect which takes place only at sample temperatures below - 140 K, was called Reverse Contrast (RC) by those authors. At that time, there was no definitive model for this absorption, and even after much further work, where it was established that this absorption was due to point defects, little advance was made [3,41. Measurements by Jimenez and co-workers [5] and by other groups [6,71, demonstrated that photoconductivity due to free holes could be excited in the same wavelength range as that exhibited by RC. These authors also demonstrated that such photoconductivity could be photoquenched, i.e. reduced by irradiating the sample with an intense beam of sub bandgap light. It was the demonstration of the photoquenchability of RC absorption under similar circumstances that allowed us to deduce that RC was a
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