A means of enhancing transmission to enable higher efficiency detector and imaging array performance from ultraviolet (UV) to infrared (IR) bands involves deposition of novel nanostructured optical layers with tunable refractive index properties. This nanostructured antireflection (AR) coating technology provides broadband and omnidirectional suppression of light reflection/scattering to substantially increase transmission. The multilayer AR coatings can be furthermore custom-designed for operation over specific wavebands for a wide range of potential optical applications, particularly for maximizing electro-optic and IR radiation transmission onto the surfaces of detector arrays to greatly enhance sensitivity and efficiency over mid-wave infrared (MWIR) and long-wave infrared (LWIR) bands of detection. Advanced AR nanostructured coatings have been fabricated using a novel growth process and developed for high AR performance, on substrates including GaSb and Si, as well as on GaSb-based focal plane array (FPA) devices, for IR band sensing applications. These nanostructured coatings further minimize reflection losses to provide substantial improvements in increased transmission over thin film AR coating technologies such as quarter wavelength stacks, further enabling higher quantum efficiency and broadband detection of MWIR and LWIR band radiation from the AR-coated optical sensing and FPA imaging devices. Here we review recent developments of this high-performance nanostructured AR coating technology for advancing NASA Earth Science sensing and imaging applications.
The development of a scalable, low cost, low power, and room temperature operating detector technology capable of high spatial resolution imaging over IR bands of interest can advance and extend space and satellite sensing capabilities such as remote sensing and earth observation. Conventional infrared (IR) band photodetectors based on HgCdTe material operating over short-wave infrared (SWIR), mid-wave infrared (MWIR), and/or long-wave infrared (LWIR) bands often require external cooling to achieve high performance IR sensing and imaging. By integrating bilayers of doped graphene to function as a high mobility channel to aid the recombination of photogenerated carriers, a graphene-enhanced IR photodetector with HgCdTe absorbing layer can be developed providing high performance uncooled detection over SWIR, MWIR, and LWIR (~1.3-14 μm) bands. Development of the high-performance IR photodetector technology involves ptype doping of graphene bilayers deposited on Si/SiO2 with boron using a spin-on dopant (SOD) process, and then transfer of the doped bilayer graphene onto HgCdTe substrates. The graphene-enhanced HgCdTe SWIR/MWIR/LWIR band detectors were analyzed and characterized material and optoelectronically to demonstrate high performance IR detection at room temperature for advanced NASA Earth Science, defense, and commercial applications.
The growth and development of antireflection (AR) coatings comprising alternating optical layers has enabled reduced losses due to reflection of radiation off substrates and optical components, with associated substantial improvements in overall optical and imaging performance for a wide range of devices and applications. A novel means of enhancing transmission for improved detector and system performance has involved the growth of nanostructured optical layers offering tunable refractive index properties that provide broadband and omnidirectional suppression of light reflection/scattering while increasing transmission. These nanostructured AR coatings can be custom designed for specific wavebands from the ultraviolet (UV) to long-wave infrared (LWIR) for various photonic applications, such as when the need for increased sensitivity over a given wavelength range requires maximizing the transmission of light, e.g., onto the surfaces of detectors and imaging devices. We have developed advanced, optimized nanostructured AR coatings fabricated using a proprietary e-beam deposition process on GaSb, Si and various other types of substrates and sensors to provide broadband high AR performance, particularly for IR band sensing applications. These nanostructured coatings provide substantial improvements over conventional thin film AR coating technologies including quarter wavelength stacks by further minimizing reflection losses and increasing transmission over a wide range of light incidence angles on optical detectors and focal plane array (FPA) imaging devices. Here we review the latest developments of this high-performance nanoengineered AR coating technology in view of advancing NASA Earth Science sensing and imaging infrared (IR) band applications.
The development of a scalable, low cost, low power, and room temperature operating MWIR detector technology capable of high spatial resolution infrared (IR) imaging is of substantial interest and utility towards the advancement of space and satellite technologies, including remote sensing and earth observation capabilities. However, conventional mid-wave infrared (MWIR) band photodetectors based on HgCdTe material typically require external cooling to achieve sufficient sensing performance, adding significant size, weight, and power restrictions and requirements. By incorporating bilayers of p+-doped graphene to function as a high mobility channel enhancing recombination of photogenerated carriers within HgCdTe absorbing material, a graphene-enhanced HgCdTe photodetector capable of providing uncooled detection over the 2-5 μm MWIR band has been developed. This high performance MWIR band detector technology comprises graphene bilayers initially deposited on Si/SiO2 doped with boron and annealed using a spin-on dopant (SOD) process, that are subsequently transferred onto HgCdTe. Raman spectroscopy, secondary-ion mass spectroscopy (SIMS), and I-V photocurrent testing were used to analyze dopant levels, structural properties of the bilayer graphene prior to and following doping and transfer, and detector IR photoresponse, respectively, of the graphene-enhanced detector devices through various stages of the development process. These room-temperature operating graphene-HgCdTe MWIR detectors have demonstrated enhanced MWIR detection performance to benefit certain NASA Earth Science, defense, and commercial applications.
Losses due to reflection of radiation off substrates and optical components often substantially inhibit the performance of detector and imaging systems. A novel means of enhancing transmission for improved detector and system performance involves the growth of nanostructured optical layers offering tunable refractive index properties, enabling broadband and omnidirectional suppression of light reflection/scattering while increasing transmission. These nanostructured antireflection (AR) coatings can be custom designed for specific wavebands from the ultraviolet (UV) to infrared (IR) for many potential optical applications, particularly when maximizing electro-optic and IR radiation transmission onto the surface of detectors is required to increase their sensitivity over various bands.
The optimized AR nanostructured coatings were fabricated using a proprietary deposition process for high broadband AR performance. We have developed and advanced the AR coatings on GaSb, Si and various other substrate types particularly for IR band sensing applications. These nanostructured coatings provide substantial improvements over more conventional thin film AR coating technologies such as quarter wavelength stacks by further minimizing reflection losses and increasing transmission over a wide range of light incidence angles on optical detector and imaging devices. In this paper we review the latest developments in the high-performance nanoengineered AR coating technology for advancing NASA Earth Science sensing and imaging for various IR bands.
Conventional mid-wave infrared (MWIR) band photodetectors based on HgCdTe material typically require external cooling to achieve sufficient sensing performance. The development of a scalable, low cost, low power, and room temperature operating MWIR detector technology capable of high spatial resolution IR imaging can greatly augment space and satellite sensing capabilities such as remote sensing and earth observation. By integrating bilayers of p + -doped graphene to function as a high mobility channel enhancing recombination of photogenerated carriers, a graphene-enhanced photodetector comprising HgCdTe absorbing material can provide higher performance uncooled detection over the 2-5 μm MWIR band. This high performance MWIR band detector technology consists of graphene bilayers on Si/SiO2 substrates doped with boron using a spin-on dopant (SOD) process, and subsequently transferred onto HgCdTe substrates. Boron doping levels and structural properties of the graphene bilayers were analyzed using Raman spectroscopy, Xray photoelectron spectroscopy (XPS), and secondary-ion mass spectroscopy (SIMS) throughout various stages of the development process including undoped, boron-doped, and following transfer onto HgCdTe/CdTe substrates. The developed room-temperature operating graphene-enhanced HgCdTe MWIR detectors have demonstrated through device modeling and optical and electrical characterization enhanced MWIR detection performance for NASA Earth Science, defense, and commercial applications.
Nanostructured antireflection (AR) coatings reducing optical reflections and maximizing radiation transmitted onto the surfaces of substrates, optics and optical devices such as detectors have many potential optical applications over ultraviolet (UV) to infrared (IR) wavebands including for NASA sensor applications. Through nanoengineering optical layers and tuning their refractive indexes, broadband and omnidirectional suppression of light reflection and scattering is achievable with increased optical transmission for enhanced IR detector and system performance over a wide range of light incidence angles. AR nanostructures have been developed that enable the realization of optimal AR coatings with high laser damage thresholds and high reliability in extreme low temperature environments and under launch conditions. These advanced nanostructured AR coatings we have developed and tested on GaSb and IR detector arrays devices primarily for 3-5 and 8-14 µm MWIR/LWIR applications provide substantial improvements over more conventional thin film AR coating technologies such as quarter-wavelength coatings. The growth of step-graded nanostructured layers using a process involving deposition at different tilt angles has produced single-layer AR coatings utilizing ZnS demonstrating below 4% reflectance, compared to ~34% reflectance for uncoated GaSb, across LWIR bands of interest with substantial improvement in quantum efficiency. In this paper we review and present latest developments and testing results for these high-performance nanostructure-based AR coatings for advanced LWIR band NASA Earth Science sensing and imaging applications.
Present microbolometer technology for infrared (IR) sensing and imaging has featured microbridges comprising Si3N4 as well as VOx materials and shown decent performance for IR band detection applications. Nevertheless, further integration of carbon nanotubes (CNTs) and graphene can improve the temperature coefficient of resistance (TCR) to provide even higher dynamic range. For the development of high performance and low noise IR microbolometer detectors with improved TCR, vanadium oxide (VOx) layers were grown on 4-inch SiO2/Si wafers as well as on Si substrates using a DC sputtering process with flow of oxygen and argon gases. From energy-dispersive X-ray spectroscopy (EDS) measurements of the sputter-assisted VOx layer growth it was determined that reduced Ar:O flow resulted in lower measured O/V ratios, and therefore more optimal stochiometric properties in the VOx layers. Likewise, analysis of scanning electron microscopy (SEM) images demonstrated that DC sputtering power had a substantial impact on the deposition rates and corresponding VOx layer thickness. Using a gas flow ratio of 18.7:1.3, with DC sputtering powers of approximately 300 W, V/O ratios in the 1.8-1.9 target range and 200 nm target thicknesses, respectively, were achievable in VOx layer growth on SiO2/Si substrates. The electrical and performance properties of these optimized VOx layer test structures were then measured and characterized in view of integration with graphene and single wall and multiwall carbon nanotubes (CNTs) for advanced long-wave infrared (LWIR) detection. These demonstrated significant noise reductions and as well as enhancements in the TCR, indicating the potential for improved noise equivalent temperature difference (NETD) for high imaging cameras and microbolometer focal plane array (FPA) performance for defense and commercial LWIR sensing applications.
Conventional photodetectors based on HgCdTe material and designed to absorb mid-wave infrared (MWIR) band wavelengths typically require cryogenic or at minimum thermoelectric cooling to maintain adequate levels of infrared (IR) sensing performance. This cooling requirement invariably entails augmentations in size, power, and cost, which for space and satellite applications such as remote sensing and earth observation generally are limiting in scope and potentially prohibitive. Here we report a scalable, low cost, low power, and small footprint room temperature operating MWIR sensing device involving the integration of bilayer graphene functioning as a high mobility channel with HgCdTe material, to limit the recombination of photogenerated carriers and achieve higher performance detection over the 2-5 μm MWIR without the need of an additional cooling mechanism. For the development of these graphene-enhanced HgCdTe MWIR photodetectors, graphene bilayers on Si/SiO2 substrates were doped with boron using a spin-on dopant (SOD) process, and then transferred onto HgCdTe substrates for enhanced higher-mobility photodetection. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and secondary-ion mass spectroscopy (SIMS) were employed to analyze dopant levels and structural properties of the graphene through various stages of the development process and characterize the p-doped graphene following doping and transfer. The features and enhanced performance of the room-temperature operating graphene-based HgCdTe MWIR detectors were demonstrated through modeling, material characterization, and measurements of detector IR sensitivity and response performance.
A scalable, low cost, low power, and small footprint uncooled mid-wave infrared (MWIR) sensing technology capable of measuring thermal dynamics with high spatial resolution can be of great benefit to space and satellite applications such as remote sensing and earth observation. Conventional photodetectors designed to absorb MWIR band wavelengths have often been based on HgCdTe material and typically require cooling. However, through integration of bilayer graphene functioning as a high mobility channel with HgCdTe material in photodetectors, higher performance detection over the 2-5 μm MWIR band may be enabled and facilitated primarily by thus limiting recombination of photogenerated carriers in these detectors. This high performance MWIR band detector technology is being developed and tested for NASA Earth Science, defense, and commercial applications. Graphene bilayers on Si/SiO2 substrates are doped with boron using a spin-on dopant (SOD) process and then transferred onto HgCdTe substrates for enhanced mobility photodetection applications. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and secondary-ion mass spectroscopy (SIMS) were utilized for analysis of dopant levels and structural properties of the graphene throughout various stages of the development process to characterize the p-doped graphene following doping and transfer. The enhanced performance and functional capabilities of the room-temperature operating graphene-based HgCdTe MWIR detectors and arrays are thereby demonstrated through modeling, material development and characterization, and device optimization.
The performance of optical and imaging systems may be limited considerably by losses due to reflection of signals off substrates and optical components. Nanoengineered optical layers offering tunable refractive index properties provide broadband and omnidirectional suppression of light reflection/scattering with increased optical transmission for enhanced detector and system performance. These nanostructured antireflection (AR) coatings custom designed for specific wavebands from the ultraviolet (UV) to infrared (IR) have many potential optical applications, particularly in maximizing light and IR radiation transmitted onto the surfaces of detectors to increase their sensitivity, for various NASA systems. Through fabrication of these AR nanostructures at various tilt angles of deposition, optimized AR coatings having high laser damage thresholds and reliable in extreme low temperature environments and under launch conditions may be realized. We have developed and advanced such AR nanostructures on GaSb substrates as well as GaSb-based detector devices for 8-14 µm LWIR applications. In this paper we review the latest findings and measurements in the development of these high-performance nanostructure-based AR coatings primarily for advanced LWIR band to NASA Earth Science sensing and imaging applications.
Broadband antireflection (AR) optical coatings covering the ultraviolet (UV) to infrared (IR) spectral bands have many potential applications for various NASA systems. The performance of these systems is significantly limited by signal loss due to reflection off substrates and optical components. Tunable nanoengineered optical layers offer omnidirectional suppression of light reflection/scattering with increased optical transmission to enhance detector and system performance particularly over IR band wavelengths. Nanostructured AR coatings enable the realization of optimal AR coatings with high laser damage thresholds and reliability in extreme low temperature environments and under launch conditions for various NASA applications. We are developing and advancing high-performance AR coatings on GaSb and various other substrate types for spectral bands ranging from UV to LWIR. The nanostructured AR coatings enhance transmission of light through optical components and detector devices by greatly minimizing reflection losses over range of incidence angles, providing substantial improvements over more conventional thin film AR coating technologies. In this paper, we review our latest developments in high performance nanostructurebased AR coatings, focusing primarily on recent efforts in designing and fabricating AR coatings for the LWIR spectral band for performance improvements in airborne and space detector applications.
Broadband antireflection (AR) optical coatings covering the ultraviolet (UV) to infrared (IR) spectral bands have many potential applications for various NASA systems. The performance of these systems is substantially limited by signal loss due to reflection off substrates and optical components. Tunable nanoengineered optical layers offer omnidirectional suppression of light reflection/scattering with increased optical transmission to enhance detector and system performance. Nanostructured AR coatings enable realization of optimal AR coatings with high laser damage thresholds and reliability in extreme low temperature environments and under launch conditions for various NASA applications. We are developing and advancing high-performance AR coatings on various substrates for spectral bands ranging from the UV to IR. The nanostructured AR coatings enhance transmission of light through optical components and devices by significantly minimizing reflection losses over a wide range of incidence angles, providing substantial improvements over conventional thin film AR coating technologies. In this paper, we review our latest work on high performance nanostructure-based AR coatings, including recent efforts in the development of the nanostructured AR coatings for various sensor applications over the 2-5 μm MWIR and 8-12 μm LWIR spectral bands.
High performance detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. The graphene-based HgCdTe detector technology involves integration of graphene with HgCdTe photodetectors allowing higher performance detection over 2-5 μm compared with photodetectors using only HgCdTe material. The graphene layer functioning as a high mobility channel reduces recombination of photogenerated carriers in the detector to further enhance performance. Graphene bilayers on Si/SiO2 substrates have been doped with boron using a spin-on dopant (SOD) process. The p-doped graphene is then transferred onto HgCdTe substrates for high mobility layers in MWIR photodetectors. Various characterization techniques including Raman spectroscopy and secondary-ion mass spectroscopy (SIMS) have analyzed dopant levels and properties of the graphene throughout various stages of development to qualify and quantify the graphene doping and transfer. The objective of this work is demonstration of graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is enablement of a scalable, low cost, low power, and small footprint uncooled MWIR sensing technology capable of measuring thermal dynamics with better spatial resolution for applications such as remote sensing and earth observation.
High performance detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. The graphene-based HgCdTe detector technology involves the integration of graphene with HgCdTe photodetectors that combines the best of both materials, and allows for higher MWIR (2-5 μm) detection performance compared with photodetectors using only HgCdTe material. The interfacial barriers between the HgCdTe-based absorber and the graphene act as a tunable rectifier that reduces the recombination of photogenerated carriers in the detector. The graphene layer also acts as high mobility channel that whisks away carriers before they recombine, further enhancing detection performance. This makes them much more practical and useful for MWIR sensing applications such as remote sensing and earth observation, e.g., in smaller satellite platforms (CubeSat) for measurement of thermal dynamics with better spatial resolution. The objective of this work is to demonstrate graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is the enablement of a scalable, low cost, low power, and small footprint infrared technology component that offers high performance, while opening doors for new earth observation measurement capabilities.
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