A comparison of photoluminescence imaging and confocal photoluminescence microscopy in the study of diffusion near isolated extended defects in GaAs

Extended defects like dislocations augment recombination and reduce the local density of photogenerated carriers. We use photoluminescence imaging and confocal photoluminescence microscopy to study the diffusion of free carriers toward these defect-related depletion regions in GaAs. Both techniques reveal important changes in the size of the darkened region as the photoexcitation is reduced. Under lower illumination, the affected region generally expands, reflecting the impact of longer carrier lifetimes. But the physics of the confocal measurement, with highly localized excitation and detection, is very different from the physics involved in far-field imaging of photoluminescence under spatially uniform illumination. In particular, the radiative efficiency falls precipitously as the confocal excitation is reduced below a threshold of approximately 1 KW/cm², and we observe a simultaneous transformation in the confocal contrast profile. We attribute this dramatic change to trapping by bulk point defects, which are saturated when the effective photogenerated carrier density is sufficiently high.