Influence of structural defects on carrier recombination and current gain in an InGaAs/AIGaAs/GaAs heterojunction phototransistor

We have studied the influence of structural defects on the spatial variation of radiative and nonradiative recombination in an InGaAs/AlGaAs/GaAs resonant cavity enhanced npn heterojunction phototransistor (HPT) structure using cathodoluminescence (CL) and electron beam-induced current (EBIC) imaging. Absorber layers of InGaAs/GaAs multiple quantum wells (MQWs) are used to provide the photosensitivity for light with wavelengths that are transparent to the GaAs substrate. The current gain of the HPT under an applied bias voltage depends on the hole accumulation efficiency in the base and therefore on the hole lifetime. Strain relaxation-induced misfit dislocations in the MQWs are found to create regions of enhanced nonradiative recombination thereby reducing locally the hole accumulation efficiency and current gain. The reduction in the local EBIC signal caused by the dark line defects is less than ∼20%, suggesting that misfit dislocations in this sample have a relatively small impact on overall device performance. EBIC and CL are found to be excellent complementary probes of the hole accumulation efficiency and relative recombination rates, owing to the high spatial resolution (∼1 μm) of excitation of the electron beam used in these techniques. The temperature dependence of the EBIC and spectrally integrated CL images is examined for 85≤T≤300 K, and reveals that nonradiative recombination in the vicinity of misfit dislocations is predominantly thermally activated with observed spatial variations in lifetime and activation energy.