Unintentional doping and native defects in the AlGaAsSb barrier of <i>nBn</i> photodetectors and their effect on detector dark current

The unintentional p-type doping concentration in the AlGaAsSb barrier of typical mid-wave infrared nBn detectors is predicted by TCAD simulations to generate substantial depletion volume, and consequently, depletion current magnitudes exceeding the diffusion current component. However, diffusion-limited performance of conventional, barrier-on-absorber nBn detectors is routinely achieved. In contrast, investigation into an inverted, absorber-on-barrier nBn structure reveals depletion currents in line with TCAD expectations. To further investigate this phenomenon, devices from an ostensibly fully symmetric nBn structure with equal absorber thicknesses on either side of the barrier are grown, fabricated, and characterized with dark current and capacitance measurements to probe both volumes from the same device. The symmetric nBn, when biased such that carrier collection occurs across the barrier-on-absorber interface (as is the case in the conventional device structure), exhibits diffusion-limited dark current. Similarly, when biased such that carrier collection occurs across the absorber-on-barrier interface (as is the case in the inverted device structure), the device exhibits depletion-limited dark current. Capacitance voltage profiles provide experimental validation of the equivalent doping of each absorber volume; however, a peak capacitance occurring at negative bias similarly reflects the asymmetry in the structure. Secondary ion mass spectrometry and scanning transmission electron microscopy are used to investigate the physical origins of this barrier asymmetry, in which a high O concentration is found at the upper barrier interface while a layer of AlAs-like material is seen at the bottom interface. The implementation of these interface defects into TCAD simulations enables accurate modeling of the observed electrical behavior.