We investigate and compare the performance of 30 layers strain-coupled quantum dot (SCQD) infrared photodetectors capped with one of two different layers: a quaternary (In 0.21 Al 0.21 Ga 0.58 As) or ternary (In 0.15 Ga 0.85 As) alloy of 30 Å and a GaAs layer with a thickness of 120-150 Å . Measurements of optical properties, spectral responsivity, and cross-sectional transmission electron microscopy were conducted. Results showed that quaternary capping yielded more superior multilayer QD infrared photodetectors than ternary capping. Quaternary capping resulted in enhanced dot size, order, and uniformity of the QD array. The presence of Al in the capped layer helped in the reduction in dark current density and spectral linewidth as well as led to higher electron confinement of the QDs and enhanced device detectivity. The vertically ordered SCQD system with quaternary capping exhibited higher peak detectivity (*10 10 cm Hz 1/2 /W) than that with ternary capping (*10 7 cm Hz 1/2 /W). In addition, a very low noise current density of *10 -16 A/cm 2 Hz 1/2 at 77 K was achieved with quaternary-capped QDs.
We report the effects of rapid thermal annealing on the optical, structural, and device properties of 30 layer straincoupled InAs/GaAs quantum dot infrared photodetectors. Stability in the photoluminescence peak is observed for annealing up to 800°C, which has not been previously reported. Cross-sectional transmission electron microscopy images show preservation of quantum dots is observed up to 800°C. Device with total capping thickness of 150 nm annealed at 750°C exhibit a fivefold enhancement in spectral intensity compared to as-grown devices and increase in the temperature of detector operation is observed from 100 to 140 K from the same device. The annealed devices exhibited a single-order enhancement in peak detectivity compared to as-grown quantum dot infrared photodetector.
Index Terms-Infrared detectors, photodetectors, photoluminescence, quantum dots. Subhananda Chakrabarti (M'06) received the M.Sc. and Ph.D. degrees from the from 2005 to 2006, and a Senior Researcher (RA2) with the University of Glasgow, Glasgow, U.K., from 2006 to 2007. He joined the Department of Electrical Engineering, IIT Bombay, Mumbai, India, in 2007.He has extensively researched molecular beam epitaxial growth, characterization, and fabrication of semiconductor optoelectronic materials and devices, such as quantum dot laser, intersubband detectors, and vertical-cavity surface-emitting lasers. He has authored or coauthored more than 160 papers in journals and conferences. He has also coauthored a couple of book chapters on intersubband quantum dot detectors.
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