Enhancement of quantum efficiency in nBn detectors with thin absorbers using plasmonic gratings

Abstract

For diffusion limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current. As the absorber thickness decreases, the lower dark current increases the signal-to-noise ratio to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. Here we discuss the use of a metallic grating to couple the incident light into laterally-propagating surface plasmon polariton (SPP) modes, so as to increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform grating profile without increasing the dark current. Using this process in conjunction with a 0.5 μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T = 78–240 K. An nBn structure with an InAs0.8Sb0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band, and can potentially reduce the dark current by a factor of 3–8 in addition to the 5× reduction due to the thinner absorber.