Design, fabrication, and characterization of indium(gallium,aluminum)arsenic/(gallium,aluminum)arsenic quantum dot infrared photodetectors for high-temperature operation.

Abstract

Infrared (IR) detectors are used in a range of imaging applications, including environmental monitoring, medical diagnosis, and space science. The typical components of an IR camera system include: light-collecting optics, a focal plane array (FPA) for detection and signal processing, a cooling system for photodetector FPAs, and electronics for displaying images. The cost of IR cameras can be reduced significantly if traditional FPA cooling systems (liquid-nitrogen dewars) are replaced by thermo-electric coolers. Such a design change requires the development of IR photodetectors that operate at elevated temperatures (&ge;120K). Quantum dot infrared photodetectors (QDIPs) have great potential to provide the requisite detector array performance at high temperatures due to three-dimensional quantum confinement of the detector active region. Low dark current (&sim;10<super>-5</super>A/cm<super> 2</super> at 78K), high responsivity (&sim;1A/W), and high detectivity (&sim;10<super> 11</super>cmHz<super>1/2</super>/W) are design benchmarks for IR photodetector arrays. Through the device fabrication and characterization of several QDIP heterostructure designs, as well as preliminary QDIP array studies, this work demonstrates that QDIPs meet many FPA requirements. Theoretical modeling and experimental verification of dark current in QDIPs determined that a 70-layer InAs/GaAs QDIP with 50nm GaAs barriers yields extremely low dark currents (J_dark = 2.8 x 10<super>-8 </super>A/cm<super>2</super>, T = 78K, V_bias = -1.0V), contributing to high-temperature operation (175K). These 70-layer QDIPs also yield estimated D*_Peak and NEDeltaT values of 10<super>11</super>cmHz<super> 1/2</super>/W and 15mK, respectively, at operating bias and T = 100K, representing state-of-the-art QDIP performance. An InAs dot-in-a-well QDIP featuring AlAs/GaAs superlattice barriers demonstrated high responsivity and conversion efficiency (R_Peak = 2.5A/W, &eegr;_conv = 70%, T = 78K, V_bias = -1.5V) due to increased dot density and carrier confinement. Dark current non-uniformity was measured at several temperatures across a (4 x 4) InAs/GaAs QDIP array. At 100K, the dark current non-uniformity in the QDIP array was 12%, comparable to other IR photodetector technologies. The imaging capability of QDIPs was demonstrated through a raster-scan imaging experiment in which a small (13 x 13) interconnected InAs/GaAs QDIP array imaged a hot plate heating element at 500&deg;C. To conclude, this dissertation research has advanced the state-of-the-art in QDIP performance, demonstrating that it is reasonable to incorporate these devices into a FPA operating at 150K for detection from 3--5mum.