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Front-illuminated p-i-n GaN-based ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® n-GaN substrate having a low etch pit density (EPD) less than 5 × 104 [cm-2] and processed into 6×6 APD arrays. The devices employed N-ion implantation to achieve sidewall passivation. Evaluation of these 6×6 arrays will help to confirm the uniformity of the epitaxial materials and device processing. The maximum avalanche gain reached ~ 3×105 at the breakdown (current limited). The dark current density was 10-9 A/cm2 at reverse bias up to -20 V and the APDs exhibited a reverse breakdown voltage of 81 ± 1 V for all 36 devices without any leaky devices, confirming a high uniformity of the growth and fabrication processes.

Germanium offers many benefits over groups III-V materials when used for infrared detection. Most importantly, germanium is compatible with Complementary Metal Oxide Semiconductor (CMOS) manufacturing which allows for a low-cost, high-throughput device, and it does not require cooling, which many III-V devices do. With the deposition of germanium directly on silicon, there will be a thermally induced strain due to the difference in thermal expansion coefficients between the two materials. When a biaxial tensile strain is present, the direct bandgap of germanium is lowered to ~0.77 eV and is capable of absorbing longer wavelengths. We have used a two-step deposition process to create a strained germanium film in order to fabricate a photodetector device. Our device has a dark current of 1.35 nA and a photocurrent of 22.5 nA at 1570 nm wavelength. Next, we developed a model in order to compare theoretical results with experimental results. The results indicate that the model is in close agreement with our measured data, and we are therefore able to use it to model future devices.

Infrared (IR) technology plays a critical role in various military and civilian applications including target acquisition, surveillance, night vision, and target tracking. IR sensors and systems operating from the short-wave infrared (SWIR) to long-wave infrared (LWIR) spectra are being developed for defense and commercial system applications. Performance of these IR systems is substantially limited by signal loss due to reflection off the IR substrates and optical components. Optical coatings with high antireflection (AR) characteristics can overcome this limitation and thus enhance the performance of IR systems. We are developing and advancing high-performance antireflection (AR) coatings for a wide range of spectral bands on various substrates for a variety of defense and commercial applications. The AR coatings enhance the transmission of light through optical components and devices by significantly minimizing reflection losses, providing substantial improvements over conventional thin-film AR coating technologies. The optical properties of ARcoated optical components and sensor substrates have been measured and fine-tuned to achieve high levels of performance. In this paper, we review our latest work on robust nanostructure-based AR coatings, including recent efforts in the development of the nanostructured AR coatings on silicon and CdZnTe substrates as well as on ZnSe lenses.

GaN p-i-n ultraviolet avalanche photodiodes (UV-APDs) were fabricated from epitaxial structures grown on low-dislocation-density free-standing GaN substrates to form $4\times 4$ UV-APD arrays with a device size of $75\times 75~\mu \text{m}^{{ {2}}}$ . The devices in the UV-APD array showed a uniform and reliable distribution of breakdown voltage ( $V_{\text {BR}}$ ) and leakage current density. The average $V_{\text {BR}}$ of the 16 devices in one of the UV-APD arrays was 96±0.6 V, and the average dark current density ( $J_{R\_{}{\text {Dark}}}$ ) and photocurrent density ( $J_{R\_{}{\text {Photo}}}$ ) were measured to be (6.5±1.8) $\times 10^{-7}$ and (5.7±1.1) $\times 10^{{\text {-6}}}$ A/cm $^{{{2}}}$ at the reverse bias voltage of $V_{R}=48$ V (50% of the average onset point of $V_{\text {BR}}$ ), respectively. The reliable device performance was confirmed by performing multiple reverse bias $I$ – $V$ scans for the selected devices in the UV-APD array. We also observed the significantly enhanced spectral responsivity from the 142 to 5485 mA/W due to the strong carrier impact ionization at high reverse bias.

SiGe offers a low-cost alternative to conventional infrared sensor material systems such as InGaAs, InSb, and HgCdTe for developing near-infrared (NIR) photodetector devices that do not require cooling and can operate with relatively low dark current. As a result of the significant difference in thermal expansion coefficients between germanium (Ge) and silicon (Si), tensile strain incorporated into SiGe detector devices through specialized growth processes can extend their NIR wavelength range of operation. We have utilized high throughput, large-area complementary metal-oxide semiconductor (CMOS) technology to fabricate Ge based p-i-n (PIN) detector devices on 300 mm Si wafers. The two-step device fabrication process, designed to effectively reduce the density of defects and dislocations arising during deposition that form recombination centers which can result in higher dark current, involves low temperature epitaxial deposition of Ge to form a thin p+ seed layer, followed by higher temperature deposition of a thicker Ge intrinsic layer. Phosphorus was then ion-implanted to create devices with n+ regions of various doping concentrations. Secondary ion mass spectroscopy (SIMS) has been utilized to determine the doping profiles and material compositions of the layers. In addition, electrical characterization of the I-V photoresponse of different devices from the same wafer with various n+ region doping concentrations has demonstrated low dark current levels (down to below 1 nA at -1 V bias) and comparatively high photocurrent at reverse biases, with optimal response for doping concentration of 5 × 1019 cm-3.

Flexible copper indium gallium diselenide (CIGS) solar cells on lightweight substrates can deliver high specific powers. Flexible lightweight CIGS solar cells are also primary candidates for building-integrated panels. In all applications, CIGS cells can greatly benefit from the application of broadband and wide-angle AR coating technology. The AR coatings can significantly improve the transmittance of light over the entire CIGS absorption band spectrum. Increased short-circuit current has been observed after integrating AR coated films onto baseline solar panels. NREL’s System Advisor Model (SAM) has predicted up to 14% higher annual power output on AR integrated vertical or building-integrated panels. The combination of lightweight flexible substrates and advanced device designs employing nanostructured optical coatings together have the potential to achieve flexible CIGS modules with enhanced efficiencies and specific power.

Infrared (IR) sensing technologies and systems operating from the near-infrared (NIR) to long-wave infrared (LWIR) spectra are being developed for a variety of defense and commercial systems applications. Reflection losses affecting a significant portion of the incident signal limits the performance of IR sensing systems. One of the critical technologies that will overcome this limitation and enhance the performance of IR sensing systems is the development of advanced antireflection (AR) coatings. Magnolia is actively involved in the development and advancement of ultrahigh performance AR coatings for a wide variety of defense and commercial applications. Ultrahigh performance nanostructured AR coatings have been demonstrated for UV to LWIR spectral bands using various substrates. The AR coatings enhance the optical transmission through optical components and devices by significantly minimizing reflection losses, a substantial improvement over conventional thin-film AR coating technologies. Nanostructured AR coatings are fabricated using a tunable self-assembly process on substrates that are transparent for a given spectrum of interest ranging from UV to LWIR. The nanostructured multilayer structures have been designed, developed and optimized for various optoelectronic applications. The optical properties of the AR-coated optical components and sensor substrates have been measured and fine-tuned to achieve a predicted high level of performance of the coatings. In this paper, we review our latest work on high quality nanostructure-based AR coatings, including recent efforts towards the development of nanostructured AR coatings on IR-transparent substrates.

SiGe p-i-n photodetectors have been fabricated on 300 mm (12”) diameter silicon (Si) wafers utilizing high throughput, large-area complementary metal-oxide semiconductor (CMOS) technologies. These Ge photodetectors are designed to operate in room temperature environments without cooling, and thus have potential size and cost advantages over conventional cooled infrared detectors. The two-step fabrication process for the p-i-n photodetector devices, designed to minimize the formation of defects and threading dislocations, involves low temperature epitaxial growth of a thin p+ (boron) Ge seed/buffer layer, followed by higher temperature deposition of a thicker Ge intrinsic layer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated uniform layer compositions with well defined layer interfaces and reduced dislocation density. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) was likewise employed to analyze the doping levels of the p+ and n+ layers. Current-voltage (I-V) measurements demonstrated that these SiGe photodetectors, when exposed to incident visible-NIR radiation, exhibited dark currents down below 1 μA and significant enhancement in photocurrent at -1 V. The zero-bias photocurrent was also relatively high, showing a minimal drop compared to that at -1 V bias.

Photodetectors based on germanium which do not require cooling and can provide good near-infrared (NIR) detection performance offer a low-cost alternative to conventional infrared sensors based on material systems such as InGaAs, InSb, and HgCdTe. As a result of the significant difference in thermal expansion coefficients between germanium and silicon, tensile strain incorporated into Ge epitaxial layers deposited on Si utilizing specialized growth processes can extend the operational range of detection to 1600 nm and longer wavelengths. We have fabricated Ge based PIN photodetectors on 300 mm diameter Si wafers to take advantage of high throughput, large-area complementary metal-oxide semiconductor (CMOS) technology. This device fabrication process involves low temperature epitaxial deposition of Ge to form a thin p+ (boron) Ge seed/buffer layer, and subsequent higher temperature deposition of a thicker Ge intrinsic layer. This is followed by selective ion implantation of phosphorus of various concentrations to form n+ Ge regions, deposition of a passivating oxide cap, and then top copper contacts to complete the PIN detector devices. Various techniques including transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) have been employed to characterize the material and structural properties of the epitaxially grown layers and fabricated detector devices, and these results are presented. The I-V response of the photodetector devices with and without illumination was also measured, for which the Ge based photodetectors consistently exhibited low dark currents of around ~1 nA at -1 V bias.

EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers from 0.4 to 1.6 micron and beyond to 1.8 microns. One of the critical technologies that will enhance the EO/IR sensor performance is the development of high quality nanostructure based antireflection coating. In this paper, we will discuss our modeling approach and experimental results for using oblique angle nanowires growth technique for extending the application for UV, Visible and NIR sensors and their utility for longer wavelength application. The AR coating is designed by using a genetic algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the wavelength range of 250 nm to 2500 nm and 0° to 40° angle of incidence. These nanostructure AR coatings have shown to enhance the optical transmission in the band of interest and minimize the reflection loss to less than 3 percent substantial improvement from the thin film AR coatings technology.

We investigated the electrical contact characteristics of indium tin oxide (ITO)/doped hydrogenated amorphous silicon (a-Si:H) junctions. For efficient collection of photo-generated carriers, photovoltaic and photodetector devices require good ohmic contacts with transparent electrodes. The amorphous-Si thin films were sputter deposited on ITO coated glass substrates. As-deposited p-type a-Si:H on ITO formed nearly ohmic type contacts and further annealing did not improve the contact characteristics. On the other hand, as-deposited n-type a-Si:H on ITO formed an ohmic contact, while further annealing resulted in a Schottky type contact. The ITO contact with p-type silicon semiconductor is a robust ohmic contact for Si based optoelectronic devices.

High-resolution imaging in ultraviolet (UV) bands has many a pplications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. The next frontier is to develop UV avalanche photodiode (UV-APD) arrays with high gain to demonstrate high-resolution imaging. We compare performance characteristics of front-illuminated Al 0.05 Ga 0.95 N UV-APDs grown on a free-standing (FS) GaN substrate and a GaN/sapphire template. UV-APDs grown on a FS-GaN substrate show lower dark current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapph ire template. In addition, stable avalanche gain higher than 5×10 5 and a significant increase in the responsivity of UV-APDs grown on a FS-GaN substrate are observed as a result of avalanche multiplication at high reverse bias. We believe that the high crystalline quality of Al 0.05 Ga 0.95 N UV-APDs grown on a FS-GaN substrate with low dislocation density is responsible for the observed improvement of low leakage currents, high performance photodetector characteristics, and reliability of the devices. Keywords: GaN, AlGaN, avalanche photodiodes, ultraviolet, high gain

SiGe offers a low-cost alternative to conventional infrared sensor material systems such as InGaAs, InSb, and HgCdTe for developing near-infrared (NIR) photodetector devices that do not require cooling and can offer high bandwidths and responsivities. As a result of the significant difference in thermal expansion coefficients between germanium and silicon, tensile strain incorporated into Ge epitaxial layers deposited on Si utilizing specialized growth processes can extend the operational range of detection to 1600 nm and longer wavelengths. We have fabricated SiGe based PIN detector devices on 300 mm diameter Si wafers in order to take advantage of high throughput, large-area complementary metal-oxide semiconductor (CMOS) technology. This device fabrication process involves low temperature epitaxial deposition of Ge to form a thin p+ seed/buffer layer, followed by higher temperature deposition of a thicker Ge intrinsic layer. An n+-Ge layer formed by ion implantation of phosphorus, passivating oxide cap, and then top copper contacts complete the PIN photodetector design. Various techniques including transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) have been employed to characterize the material and structural properties of the epitaxial growth and fabricated detector devices. In addition, electrical characterization was performed to compare the I-V dark current vs. photocurrent response as well as the time and wavelength varying photoresponse properties of the fabricated devices, results of which are likewise presented.

High-voltage InGaAs quantum well solar cells have been demonstrated in a thin-film format, utilizing structures that employ advanced band gap engineering to suppress non-radiative recombination and expose the limiting radiative component of the diode current. In particular, multiple InGaAs quantum well structures fabricated via epitaxial lift-off exhibit one-sun open circuit voltages as high as 1.05 V. The dark diode characteristics of these high-voltage III-V photovoltaic devices are compared to the radiative current calculated from the measured external quantum efficiency using a generalized detailed balance model specifically adapted for optically-thin absorber structures. The fitted n=1 component of the diode current is found to match the calculated radiative dark current when assuming negligible photon recycling, suggesting this thin-film multiple quantum well structure is operating close to the radiative limit.

ZnO has distinct advantages over competing technologies such as GaN. Two advantages are the inherent improvement in ultraviolet (UV) brightness, necessary for the biological sensor application where the signal-to-noise ratio (SNR) is enhanced by a bright UV source, and the second is the availability of ZnO lattice-matched substrates, which will result in lower defect densities than GaN, higher manufacturing yield, and then lower cost. The ZnO material system’s advantage in exciton binding energy of 60 MeV, a three-time improvement over GaN, will result in UV emitters with superior performance.1

Near-infrared (NIR) sensors operating at room temperatures are critical for a variety of commercial and military applications including detecting mortar fire and muzzle flashes. SiGe technology offers a low-cost alternative to conventional IR sensor technologies such as InGaAs, InSb, and HgCdTe for developing NIR micro-sensors that will not require any cooling and can operate with high bandwidths and comparatively low dark currents. Since Ge has a larger thermal expansion coefficient than Si, tensile strain may be incorporated into detector devices during the growth process, enabling an extended operating wavelength range above 1600 nm. SiGe based pin photodetectors have advantages of high stability, low noise, and high responsivity compared to metal-semiconductor-metal (MSM) devices. We have developed a process flow and are fabricating SiGe detector devices on 12” (300 mm) silicon wafers in order to take advantage of high throughput, large-area leading-edge silicon based CMOS technology that provides small feature sizes with associated device cost/density scaling advantages. The fabrication of the detector devices is facilitated by a two-step growth process incorporating initial low temperature growth of Ge/SiGe to form a thin strain-relaxed layer, followed by high temperature growth to deposit a thicker absorbing film, and subsequent high temperature anneal. This growth process is designed to effectively reduce dark current and enhance detector performance by reducing the number of defects and threading dislocations which form recombination centers during the growth process. Various characterization techniques have been employed to determine the properties of the epitaxially deposited Ge/SiGe layers, and the corresponding results are discussed.

Optical sensing technology is critical for optical communication, defense and security applications. Advances in optoelectronics materials in the UV, Visible and Infrared, using nanostructures, and use of novel materials such as CNT and Graphene have opened doors for new approaches to apply device design methodology that are expected to offer enhanced performance and low cost optical sensors in a wide range of applications. This paper is intended to review recent advancements and present different device architectures and analysis. The chapter will briefly introduce the basics of UV and Infrared detection physics and various wave bands of interest and their characteristics [1, 2] We will cover the UV band (200-400 nm) and address some of the recent advances in nanostructures growth and characterization using ZnO/MgZnO based technologies and their applications. Recent advancements in design and development of CNT and Graphene based detection technologies have shown promise for optical sensor applications. We will present theoretical and experimental results on these device and their potential applications in various bands of interest.

Fossil fuels are the most highly used sources for energy generation. But as energy needs increase day by day, and fossil fuels are consumed at ever faster rates, there is a great need for alternative energy sources. Renewable sources such as wind and solar can be exploited in a wide range of geographical areas and could effectively replace fossil fuels. For example, the Earth receives over 8 million quads of BTU (British thermal units) annually, meaning that there is enough solar energy available to fulfill all the energy requirements of the human race. However, due to the low efficiencies with which current solar cell technologies convert light into electricity, only a small fraction of the available solar energy can be harnessed. Deployment of solar cells will increase if their efficiency can be improved without increasing their cost. A novel concept known as the intermediate band solar cell (IBSC) paves the way for increasing solar cell efficiency.1 In an IBSC, sub-bandgap photons that would be wasted in a conventional solar cell can be harvested effectively to create a higher photocurrent. Semiconductor quantum dots (QDs) are perhaps the best choice to create an intermediate band in a single-junction solar cell due to the inherent tunability of their shape, size, and quantum confinement properties. For an IBSC to work, the QD system being used must satisfy certain conditions in terms of bandgaps and band alignments. For maximum efficiency, the QD and host material bandgaps should be 0.7 and 1.93eV, respectively. There have been numerous attempts to use established QD systems for IBSCs, including indium gallium arsenide/gallium arsenide—In(Ga)As:GaAs)—gallium antimonide/gallium arsenide (GaSb:GaAs), and indium arsenide/gallium arsenide Figure 1. Schematic of our aluminum arsenide/antimonide (AlAsSb, with the composition AlAs0:56Sb0:44) p-i-n intermediate band solar cell (IBSC). This cell contains 10 layers of indium arsenide (InAs) quantum dots (QDs). Gallium arsenide (GaAs) and gallium arsenide/ antimonide (GaAs0:95Sb0:05) cladding layers are used below and above the QDs, respectively, for better morphology and to tune the photoluminescence spectra.

InAs/GaSb type-II superlattices (T2-SLs) are of great interest as they provide a lot of band engineering flexibility. A wide variety of unipolar barrier structures have been investigated with this material system. In this report, we will present our recent work on the development of low noise long-wave infrared (LWIR) InAs/GaSb T2-SLs photodetectors. By adopting a so-called pBiBn design, the dark current of LWIR photodetectors is greatly suppressed. The LWIR pBiBn device has demonstrated a dark current density as low as 1.42×10 -5 A/cm 2 at -60 mV, and R 0 A of 5365 Ωcm 2 at 76 K. A peak detectivity at 7.8 μm of 7.7×10 11 cmHz 1/2 W -1 is obtained at 76 K. Further effort to reduce the operating bias is also reported. By refining the energy-band alignment, a 2-μm-thick LWIR pBiBn device has demonstrated a single pass (no AR coating) quantum efficiency of 20% at 10 μm under zero-bias at 77 K. We have recently extended our efforts to further reduce the dark current by using an interband cascade (IC) photodetector structure. Some further details about the device operation and results will be discussed.

We present a new material platform for uncooled bolometric infrared detection, consisting of a composite membrane of carbon nanotubes and a non-conductive, volume phase-change polymer. Devices using this platform have achieved temperature coefficients of resistance (TCR) in excess of - 40%/K at 300 K, an order of magnitude larger than commercial materials.

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO2, GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process. The structural and optical properties of the grown vertically aligned ZnO NW arrays were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements. The photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Envisioned applications for such sensors/FPAs potentially include threat detection and threat warning.

This paper describes our efforts on the development of low dark current long-wave infrared (LWIR) photodetectors based on type-II InAs/GaSb strained superlattices. By adopting a so-called pBiBn structure, a hybrid between the conventional PIN structure and unipolar barrier concepts, suppressed dark current and near-zero-bias operation are obtained, respectively. The LWIR photodetector has a dark current density as low as 1.42×10-5 A/cm2 at -60 mV, and R0A of 5365 Ωcm2 at 76 K. The measured peak detectivity at 10.2 µm of 8.7×1010 cmHz1/2W-1 is obtained at -60 mV at 76 K. To further improve the device performances, a newer design with longer cut-off wavelength targeted for near zero-bias was also realized. This 2-µm-thick device exhibits a quantum efficiency of 20% at 10 µm under zero-bias.

Zinc oxide (ZnO) is a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties that has a diverse group of growth morphologies. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the ultraviolet (UV) spectral band. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO 2 , GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process. The structural and optical properties of the grown vertically aligned ZnO NW arrays were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements. Compared to conventional UV sensors, detectors based on ZnO NWs offer high UV sensitivity and low visible sensitivity, and are expected to exhibit low noise, high quantum efficiency, extended lifetimes, and have low power requirements. The photoresponse switching properties of NW array based sensing devices have been measured with intermittent exposure to UV radiation, where the devices were found to switch between low and high conductivity states at time intervals on the order of a few seconds. Furthermore, NW based UV sensors and focal plane arrays (FPAs) show promise for imaging in the near marine boundary layer, an area extending up to about six meters above the ocean surface characterized by a relatively high degree of aerosols and turbulence. Envisioned applications for such sensors/FPAs potentially integrated into submarine photonic masts (which would maintain their effectiveness even in bright daylight conditions) include threat detection and threat warning.

The scalability of nanostructured, self-assembled antireflection (AR) coatings has been demonstrated on 6-inch glass and silicon wafers. Ultra-high transmittance through these large-area coatings has been confirmed by measuring the short circuit current of a CIGS-based thin film photovoltaic (PV) device placed below the large-area AR-coated glass wafer. At normal light incidence, the light transmitted through the AR coated glass wafer yields 5% more short-circuit current compared to the uncoated glass wafer. At off-angle incidence, the light transmitted through the AR-coated wafer yields nearly 20% higher short-circuit current compared to light transmitted through an uncoated glass wafer. The large-area AR coating preserves ultra-high transmittance over a wide range of incident angles and has the potential to enhance PV device performance from dawn to dusk.

Flexible photovoltaic cells with a specific power of more than 275 W/kg have been demonstrated by depositing copper indium gallium diselenide (CIGS) absorber layers on ultra lightweight and highly durable titanium foil. Advanced device designs employing nanostructured optical coatings and exploiting optical cavity effects provide a pathway to further increase power-generating capability. Single-junction CIGS devices can potentially outperform multi-junction III-V structures in some environments, including under high air mass terrestrial spectrums.

The temperature coeffi cient of resistance of a carbon nanotube nanocom- posite with the non-conductive phase-change hydrogel Poly(N-isopropy- lacrylamide) is studied. This nanocomposite is found to achieve the largest reported temperature coeffi cient of resistance, ≈ − 10%/ ° C, observed in carbon nanotube-polymer nanocomposites to date. The giant temperature coeffi - cients of resistance results from a volume-phase-transition that is induced by the humidity present in the surrounding atmosphere and that enhances the temperature dependence of the resistivity via direct changes in the tunneling resistance that electrons experience in moving between nearby carbon nano- tubes. The bolometric photoresponses of this new material are also studied. The nanocomposite's enhanced responses to temperature and humidity give it great potential for sensor applications and uncooled infrared detection.

We present the performance of a unipolar barrier long-wave type-II InAs/GaSb superlattice (SLS) photodetector with a 50% cut-off wavelength of approximately 8.7 microns. In this study, the ability to lower dark current densities over traditional PIN diodes is presented by way of hetero-structure engineering of a pBiBn structure utilizing superlattice Ptype (P) and N-type (N) contacts, Intrinsic (I) superlattice active (absorber) region, and unipolar superlattice electron and hole blocking (b) layers. The spectral response of this pBiBn detector structure was determined using a Fourier Transform Infrared (FTIR) Spectrometer and the quantum efficiency (QE) was determined using a narrow 6250 nm narrow band filter and a 500K blackbody source. A diode structure designed, grown, and fabricated in this study yielded a dark current density of less than one (1) mA/cm 2 at a reverse bias of 150 mV and a specific detectivity value of greater than 10 11 Jones at 77K. In addition to single point temperature measurements, a variable temperature study (80K-300K) of the dark current is presented for a diode demonstrating diffusion limited dark current from 160K down to 80K.

Ultra-high, omnidirectional transmittance through a coated glass window is demonstrated over the entire accessible portion of the solar spectrum. The average broadband transmittance has been increased to greater than 98.5% at normal incidence, and exceeds 97.8% at all wavelengths between 440 nm and 1800 nm, significantly outperforming conventional MgF 2 coated glass. The measured improvement in transmittance results from coating the window with a new class of materials consisting of porous SiO 2 nanorods. The step-graded antireflection structure also exhibits excellent omnidirectional performance, enabling average broadband transmittance in excess of 96% at incident angles as high as 70°.

High resolution imaging in the UV band has a lot of applications in Defense and Commercial Applications. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We also disuses our recent efforts on development of APD's using MOCVD of GaN/ AlGaN. We present an analytical model that can predict sensor performance in the UV band using p-i-n or APD detectors with and without gain and other detector and sensor parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard MODTRAN atmospheres in the UV band using detector arrays with unity gain and with high gain APD along with continuous or pulsed UV lasers.

Quantum structured solar cells seek to harness a wide spectrum of photons at high voltages by embedding low energy-gap wells or dots within a high energy-gap matrix. Quantum well and quantum dot solar cells have the potential to deliver ultra-high power conversion efficiencies in single junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide range of environmental conditions. In this paper, we will briefly review the theoretical underpinnings of quantum well and quantum dot photovoltaic devices, and summarize recent experimental efforts developing quantum-structured solar cell devices. In a specific example, test devices utilizing radiation-hard, III-V nitride materials have been built using both bulk and multiple quantum well (MQW) structures. Photovoltaic devices with an InGaN MQW structure are shown to outperform devices employing a thicker, bulk InGaN layer. These results, along with the underlying theoretical foundations, suggest that quantum well and quantum dot structures can enhance the performance of photovoltaic devices for a variety of defense applications.

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPA's. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS Bolometers.

Next Generation EO/IR focal plane arrays using nanostructure materials are being developed for a variety of Defense and Homeland Security Sensor Applications. Several different nanomaterials are being evaluated for these applications. These include ZnO nanowires, GaN Nanowires and II-VI nanowires, which have demonstrated large signal to noise ratio as a wide band gap nanostructure material in the UV band. Similarly, the work is under way using Carbon Nanotubes (CNT) for a high speed detector and focal plane array as two-dimensional array as bolometer for IR bands of interest, which can be implemented for the sensors for homeland security applications. In this paper, we will discuss the sensor design and model predicting performance of an EO/IR focal plane array and Sensor that can cover the UV to IR bands of interest. The model can provide a robust means for comparing performance of the EO/IR FPA's and Sensors that can operate in the UV, Visible-NIR (0.4- 1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR bands (8-14μ). This model can be used as a tool for predicting performance of nanostructure arrays under development. We will also discuss our results on growth and characterization of ZnO nanowires and CNT's for the next generation sensor applications. We also present several approaches for integrated energy harvesting using nanostructure based solar cells and Nanogenerators that can be used to supplement the energy required for nanostructure based sensors.

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper will discuss performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPA's. We will present results on the approach and device design for reducing the dark current in SiGe detector arrays. We will discuss electrical and optical properties of SiGe arrays at room temperature and as a function of temperature. We will also discuss future integration path for SiGe devices with other Silicon-based technology for defense and Commercial Applications.

High resolution imaging in the UV band has a lot of applications in Defense and Commercial Applications. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. UVAPD's have been demonstrated as discrete devices demonstrating gain. The next frontier is to develop UV APD arrays with high gain to demonstrate high resolution imaging. We also disuses our recent efforts on development of APD's using MOCVD of GaN/ AlGaN. We present an analytical model that can predict sensor performance in the UV band using p-i-n or APD detectors with and without gain and other detector and sensor parameters for a desired UV band of interest. SNR's can be modeled from illuminated targets at various distances with high resolution under standard MODTRAN atmospheres in the UV band using detector arrays with unity gain and with high gain APD along with continuous or pulsed UV lasers.

Nitride semiconductors possess a number of unique material properties applicable to energy harvesting photovoltaic devices, including a large range of energy gaps, superior radiation resistance, and tolerance to high temperatures. We present here our experimental results related to the self-assembled InN quantum dots formed on Si substrates. We have been successful at synthesizing InN quantum dots using the metal-organic chemical vapor deposition (MOCVD) process. We demonstrate the synthesis of a high density of InN dots exhibiting excellent structural and optical properties. An unprecedented range of absorption energies, ranging from the infrared to the ultraviolet, can be obtained by embedding InN-based quantum dots in a wide band gap GaN barrier. The combination of energy-gaps accessible to III-V nitride materials may be used to reap the benefits of advance quantum dot device concepts involving hot carrier effects or multiple carrier generation processes.

EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers from 0.4 to 1.6 micron. One of the critical technologies that will enhance the EO/IR sensor performance is the development of high quality nanostructure based antireflection coating. Prof. Fred Schubert and his group have used the TiO2 and SiO2 graded-index nanowires / nanorods deposited by obliqueangle deposition, and, for the first time, demonstrated their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN-air interface over the UV band. This was achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05, the lowest value so far reported In this paper, we will discuss our modeling approach and experimental results for using oblique angle nanowires growth technique for extending the application for UV, Visible and NIR sensors and their utility for longer wavelength application. The AR coating is designed by using a genetic algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the wavelength range of 400 nm to 2500 nm and 0° to 40° angle of incidence. The measured average optical transmittance of an uncoated glass substrate between 1000 nm and 2000 nm is improved from 92.6% to 99.3% at normal incidence by using a two-layer nanostructured AR coating deposited on both surfaces of the glass substrate.

EO/IR nanosensors are being developed for a variety of Defense and Commercial Systems Applications. These include UV, Visible, NIR, MWIR and LWIR nanotechnology based Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost SiGe visible and near infrared band that covers performance up to 1.6 micron. The use of nanowires for developing high quality antireflection coatings that allows minimizing the reflection loss is discussed. We have explored the possibility of using nanostructures grown by oblique angle deposition technique. A graded-index coating with different index profiles has been investigated for broadband antireflection properties, particularly with air as the ambient medium. In this paper, we present, modeling and experimental results for nanostructure AR coatings for UV, Visible and calculations for NIR sensors and also their utility for longer wavelength application.

UV response of ZnO nanowire nanosensor has been studied under ambient condition. By utilizing Schottky contact instead of Ohmic contact in device fabrication, the UV sensitivity of the nanosensor has been improved by four orders of magnitude, and the reset time has been drastically reduced from 417 to 0.8 s. By further surface functionalization with function polymers, the reset time has been reduced to 20 ms even without correcting the electronic response of the measurement system. These results demonstrate an effective approach for building high response and fast reset UV detectors. © 2009 American Institute of Physics. DOI: 10.1063/1.3133358 Ultraviolet UV photon detectors have a wide range of applications from environmental monitoring, missile launching detection, space research, high temperature flame detection to optical communications. 1 For these applications, fast response time, fast reset time, high selectivity, high responsivity, and good signal-to-noise ratio are commonly desired characteristics. 2 For UV photon detector based on polycrystalline ZnO thin film, a slow response time ranging from a few minutes to several hours is commonly observed. 3,4 Due

EO/IR Sensors have been developed for a variety of Military Systems Applications. These include UV, Visible, SWIR, MWIR and LWIR Sensors. The conventional SWIR Sensors using InGaAs Focal Plane Array (FPA) can operate in 0.4 - 1.8 micron region. Similarly, MWIR Sensors use InSb and HgCdTe based FPA's that are sensitive in 3-5 and 8-14 micron region. DOD investments in the last 10 years have provided the necessary building blocks for the IR Sensors that are being deployed in the field. In this paper, we discuss recent developments and work under way to develop Next Generation nanostructure based EO/IR detectors that can potentially cover UV, Visible and IR regions of interest. The critical technologies being developed include ZnO nanostructures with wide band gap for UV detection and Carbon Nanostructures that have shown the feasibility for IR detection. Experimental results on ZnO based nanostructures demonstrate enhanced UV sensitivity and path forward for larger arrays. Similarly, recent works on carbon nanostructures have shown the feasibility of IR detection. Combining the two technologies in a sensor can provide multispectral capability.

Next Generation EO/IR Sensors using Nanostructures are being developed for a variety of Defense Applications. In addition, large area IRFPA's are being developed on low cost substrates. In this paper, we will discuss the capabilities of a EO/IR Sensor Model to provide a robust means for comparing performance of infrared FPA's and Sensors that can operate in the visible and infrared spectral bands that coincide with the atmospheric windows - UV, Visible-NIR (0.4-1.8μ), SWIR (2.0-2.5μ), MWIR (3-5μ), and LWIR (8-14μ). The model will be able to predict sensor performance and also functions as an assessment tool for single-color and for multi-color imaging. The detector model can also characterize ZnO, Si, SiGe, InGaAs, InSb, HgCdTe and Nanostructure based Sensors. The model can predict performance by also placing the specific FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR). This model has been used as a tool for predicting performance of state-of-the-art detector arrays and nanostructure arrays under development. Results of the analysis can be presented for various targets for each of the focal plane technologies for a variety of missions.

In this Paper we present growth and characterization of ZnO nanowires on wideband gap substrates, such as SiC and GaN. Experimental results on the ZnO nanowires grown on p-SiC and p-GaN are presented with growth morphology, structure analysis, and dimensionality control. We also present experimental results on individual nanowires such as I-V measurements and UV sensitivity measurements with use of polymer coating on ZnO nanowires. The ZnO nanowires can be used for a variety of nanoscale optical and electronics applications.

High sensitivity photodetectors are of great significance in developing a new generation of optoelectronic devices in sensing, imaging, and other applications. In this Communication, highly sensitive UV detectors based on ZnO nanobelts (NBs) are achieved by polymer surface-functionalization. The UV-induced photoconductance in ZnO NBs increases by 5 orders of magnitude after functionalizing, using a polymer that exhibits large UV absorption. The huge increase in photoconductance is attributed to an electron−hole generation process as assisted by the energy states in the polymer. This study suggests that, by selecting polymers with different wavelengths of UV absorption, highly sensitive UV detectors with a large range of detection wavelengths can be fabricated using ZnO NBs.

This paper discusses the capabilities of a EO/IR sensor model developed to provide a robust means for comparative assessments of infrared FPA's and sensors operating in the infrared spectral bands that coincide with the atmospheric windows - SW1 (1.0-1.8mm), sMW (2-2.5mm), MW (3-5mm), and LW (8-12mm). The applications of interest include thermal imaging, threat warning, missile interception, UAV surveillance, forest fire and agricultural crop health assessments, and mine detection. As a true imaging model it also functions as an assessment tool for single-band and multi-color imagery. The detector model characterizes InGaAs, InSb, HgCdTe, QWIP and microbolometer sensors for spectral response, dark currents and noise. The model places the specified FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR) and creates two-point corrected imagery complete with 3-D noise image effects. Analyses are possible for both passive and active laser illuminated scenes for simulated state-of-the-art IR FPA's and Avalanche Photodiode Detector (APD) arrays. Simulated multispectral image comparisons expose various scene components of interest which are illustrated using the imaging model. This model has been exercised here as a predictive tool for the performance of state-of-the-art detector arrays in optical systems in the five spectral bands (atmospheric windows) from the SW to the LW and as a potential testbed for prototype sensors. Results of the analysis will be presented for various targets for each of the focal plane technologies for a variety of missions.

In this Paper we present growth and characterization of ZnO nanowires on a variety of substrates, such as Silicon and SiC. Experimental results on the ZnO nanowires grown on Si and SiC are presented with growth morphology, structure analysis, and dimensionality control. The ZnO nanowires can be used for a variety of nanoscale optical and electronics sensors.

GaN / Al 1-x Ga x N-based hetero-structures have demonstrated a versatility in RF electronic applications which is practically unmatched by any other material system. There are many device structures under consideration for use in RF and Power amplifiers, suitable for both commercial and military applications. In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating SiC substrates. Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance mapping, optical reflectance, and high-resolution X-ray diffraction. Several of the wafers were fabricated into HEMT devices, and the results on these devices are also presented.

In this paper we will present innovative design approach for UV Focal Plane Array for Photon Counting Applications. This Focal Plane includes the building a large area silicon micro-channel plate (MCP) using GaN photocathode. In this paper, we will present the design and simulation of silicon micro-channel plate with GaN photocathode with large area array with 2 micron pores and 3 micron pitch. We will also present results for the ICP-RIE process to fabricate 2 micron pores, and Growth of high conductivity GaN photocathodes using MOCVD to produce 40% QE. We will also discuss approaches for development of readout architecture and circuit for Silicon based MCP 4096x4096 UV FPA. The readout architecture scheme uses a series of charge sensitive amplifiers (CSA) to boost the charge received on each anode. The signal from each CSA is then passed into a shaping amplifier (SA) that produces a smooth waveform. The peak of the smoothed waveform is captured by a sample and hold (S/H) circuit that holds the signal until an analog to digital (A/D) converter can sample it. Details of the ROIC circuit will be presented.

This model was developed to provide a means for comparative assessments of HgCdTe FPA's and sensors operating in the infrared spectral bands that coincide with the atmospheric windows - (SW1(1.5-1.8μ), SW2(2-2.5μ), MW(3-5μ), and LW(8-12μ). As a true imaging model it also functions as an assessment tool for single-band imagery and for multi-color imagery. The HgCdTe model characterizes both n-on-p and p-on-n homojunctions and heterostructures. Diffusion and depletion dark currents and RoA's are calculated for the three common configurations (mesa heterojunction, planar ionimplanted or diffused junction, and the vertically integrated photodiode). The model places the specified FPA into an optical system, evaluates system performance (NEI, NETD, MRTD, and SNR) and creates two-point corrected imagery complete with 3D noise image effects. This model was exercised here as a predictive tool for performance of state-of-the- art detector arrays in optical systems in the four spectral bands (atmospheric windows) from the SW to the MW (1.5- 1.8, 2.0-2.5, 3.4-4.2 and 4.5-5.0 um) which are the bands commonly considered for hot target and plume exhaust detection. Results from the literature and model runs for various target and scene sets show promise for HgCdTe FPA's and sensors developed for the 2-2.5 μm band for a variety of missions such as threat detection from UAV or satellite platforms, perimeter defense, and high-altitude intercepts.

LWIR and Multi-color LWIR Focal Planes are being developed for a variety of Space applications. A variety of Focal Plane array technologies, that includes HgCdTe, PbSnTe and other novel technologies are being developed. These detectors FPAs will require High quality two-dimensional, small unit cell, Silicon CMOS based ROICs for efficient read-out circuits. This Paper will discuss some of the salient features of various approaches being developed for the IR focal plane applications. We will also present Trade-off analysis for design of a trans-impedance input amplifier with reset and compact signal average for low noise performance. We will also discuss the various approaches for design of an A/D converter with high linearity and speed. We will discuss approaches to achieve ROIC with low noise floor, high dynamic range and high frame rate.

High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for manyspaceapplications. Reliable focal plane arrays are needed for these applications that can operate in space environment withoutanydegradation.In this paper, we will present various LWIR and Multicolor Focal Plane architectures currentlybeing evaluated forLWIR and Multicolor applications that include focal planematerials such as HgCdTe, PbSnTe, QWIP and otherSuperlattice device structures.We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includesCdTe, Si 3 N 4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hardmaterials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflectioncoatings with high optical qualitywith controlled physical properties. INTRODUCTON Reliable LWIR focal planearrays are needed for space applications that can operate in space environment without anydegradation. Some of the technologies being developed for the Next Generation include HgCdTe (1, 2), PbSnTe (3) andQWIP (4) IR focal plane arrays. While HgCdTe and PbSnTe are being grown on silicon substrates, the QWIP focalplane arrays use GaAs substrates. The advantages of Silicon substrate for HgCdTe and PbSnTe, include the ability to

GaN /AlGaN transistors are being developed for a variety of RF electronic devices that will eventually replace GaAs- and silicon-based devices for commercial and military applications. In this paper, we present results from the optimization of the growth conditions for GaN/AlGaN HEMT structures. The HEMT epitaxial layers are grown via MOCVD. We demonstrate that the key to high quality HEMT structures is the ability to grow uniform AlGaN layers. Details of the structural, electrical and optical characteristics of the HEMT epitaxial layers are presented. In addition, we present results on an innovative ICP etching used for HEMT fabrication. This technique allows for low damage device processing and improved reliability.