Advancements Toward High Operating Temperature Small Pixel Infrared Focal Plane Arrays: Superlattice Heterostructure Engineering, Passivation, and Open-Circuit Voltage Architecture

Autor: Specht, Teressa Rose
Jazyk: angličtina
Rok vydání: 2020
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Druh dokumentu: Text
Popis: Infrared detector technology has proven useful in a wide range of imaging applications including, but not limited to, astronomy, military surveillance, industrial manufacturing processes, medical diagnostics, and automotive applications. Infrared detectors and sensors have developed into sophisticated structures since their inception in the 1950's when infrared detectors and scanning systems were first introduced. Present-day imagers consist of staring large format focal plane arrays (FPAs) based on HgCdTe, InSb, and III-V strained layer superlattice (SLS) material systems. Type II superlattice (T2SL) detector structures based on the III-V SLS, are emerging as a versatile material system due to their low Auger recombination, high effective electron mass, and detection tunability across the mid-wave infrared (MWIR, 3-5 μm) and long-wave infrared (LWIR, 8-12 μm) wavelength regions.However, several physical factors currently limit the T2SL detectors in the context of high-density and high operating temperature (HOT) conditions in the MWIR and LWIR. Currently, the low operating temperature of these detectors is costly as imaging degrades and becomes unstable with higher detector operating temperature. Imager systems are also trending toward larger format FPAs with smaller pixel pitch, therefore scaling to high-density arrays comes with adverse radiometric effects. In this dissertation, scientific research challenges limiting current SLS detector technology from achieving high-density, HOT conditions are identified and investigated to understand the underlying device physics and mitigate poor performance with three main contributions: (1) superlattice heterostructure engineering, (2) detector surface leakage current suppression with passivation, and (3) demonstration of an open-circuit voltage photodetector (VocP) architecture. Through superlattice heterostructure engineering, we designed, simulated, fabricated, and tested unipolar barrier nBp detectors with InAs/GaSb, InAs/InGaSb, and InAsSb/GaAsSb SLS material systems. The measured results were compared to simulations retrieved from the NRL Bands™ k·p modeling tool to understand the differences between ideal detector behavior and the behavior of the as-grown devices. The nBp detector dark current results of the as-grown sample led to an important conclusion that fully delineating the barrier detector unintentionally inverts p-type absorber designs, confirming previous p-type absorber research, and also confirmed in this work to occur in InAs/GaSb LWIR SLS absorbers. We concluded this study with several paths forward to further optimize these designs for future LWIR detector structures, including the use of passivation.We investigated the use of passivation on a dual-band LWIR InAs/GaSb pBp barrier detector and a MWIR InAs/InAsSb pn detector to reduce the surface leakage current seen in these detectors. By using aluminum oxide and zinc oxide via atomic layer deposition (ALD), we were able to employ these passivation techniques through fabrication and measure the dark current to understand the surface effects on the fully delineated detector pixels. Results between passivated and unpassivated detectors were compared using variable area diode arrays, and show that both the aluminum oxide ALD film and the zinc oxide ALD treatment reduced the sidewall surface leakage current on these single pixel detectors by at least two orders of magnitude, where the passivated samples showed bulk-limited dark current characteristics over a range of diode sizes from 50 μm2 to 500 μm2 under reverse-bias voltage at 77 K. Further optimization to improve the bulk-limited performance at other biases and temperatures is needed, but the techniques used for this research have attained bulk-limited performance of small pixel detectors. Lastly, a more device-centered approach was developed where we re-examined the relative advantages of using the reverse-bias photocurrent of a photodetector versus using the open-circuit voltage under the same conditions. We investigated the detector physics through analytical modeling, fabrication, integration and test of a VocP detector and explored the potential of using this architecture for small pixels in FPAs under HOT conditions. The comparison of the developed models to the measured data support the premise that the open-circuit voltage operation can be modeled using standard diode physics. Further analysis found favorable operating conditions for the open-circuit voltage detector through noise equivalent temperature difference (NEDT) models using standard radiometric optics for high density FPAs. The resultant radiometric demonstration of the VocP architecture provided good agreement between the developed model and the measured data over three orders of magnitude in irradiance ranging from 1015 – 1018 photons/s-cm2. We believe the VocP detector provides a solution to support large format FPAs with small pixel pitch and small capacitor ROICs for HOT MWIR and LWIR operation. Applications that can trade off longer integration times for increased sensitivity and dynamic range will especially benefit from the VocP architecture, ultimately filling a technological void in infrared imaging under HOT MWIR conditions.Overall, this research has expanded the fundamental boundaries in infrared detector technology through material system designs and processes, and through the critical role of the detector to readout integrated circuit (ROIC) interface for future advancements toward high-density, HOT FPAs.
Databáze: Networked Digital Library of Theses & Dissertations
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