Definition of IR Spectral Band [1]. 

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... model the sensor and system performance, we have assumed the pixel size for a high sensitivity, detector size of 5-20 microns for the UV detector array. The fill factor of 70% is assumed typical for these small pixels. Typical quantum efficiencies have been assumed to be in the 70% range for the PIN diode and APD [5-6]. The model uses as default, an amp noise of 15 electrons per frame time, a dark current of 1e-15 amps for a 5 micron pixel or 4 nA/cm 2 or 200 electrons or about 14 noise electrons, and scene noise is effectively zero in the solar-blind region. The model from the MODTRAN runs shown in figure 3, the daytime irradiance in the UV is insignificant in the solar-blind region. The drop-off from 0.30 microns to 0.26 microns illus‐ trates the requirement for a UV detector with spectral response is in the solar-blind region. Figure 4 shows the UV spectral radiance at midday and the comparative laser illumination of the target at 1 km for a 6 milliradian beam divergence for powers of 1 mW and 10 mW. The left plot in the figure shows that the transmittance improves with longer UV wavelengths for all three levels of aerosols and is sufficient for 1 km lengths in our solar-blind region. To achieve high-resolution day-night imaging and identification of targets, the following conditions and requirements must be met. While linear detection (no APD and no laser illumination) is fine for muzzle flashes and images of nearby combatants illuminated by live fire (a millisecond event), laser illumination is required for cold targets (facial recognition, profile recognition). A continuous laser and 33 msec integrations are adequate if enough laser power is available. If not, a pulsed laser with nanosecond integrations and APD detectors are required to reduce atmosphere scatter and improve detector sensitivity. 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 [7]. Highly ordered vertical arrays of ZnO nanowires (NWs) have been grown on substrates including silicon, SiO , GaN, and sapphire using a metal organic chemical vapor deposition (MOCVD) growth process [7]. The structural and optical properties of the grown vertically aligned ZnO NW arrays have been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) measurements [7-10]. 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 [11-12]. 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 defense and commercial applications [13]. Zinc oxide is a versatile functional material that provides a biocompatible material system with a unique wide direct energy band gap and exhibits both semiconducting and piezoelectric properties. ZnO is transparent to visible light and can be made highly conductive by doping. Bulk ZnO has a bandgap of 3.37 eV that includes emissions in the solar blind ultraviolet (UV) spectral band (~240-280 nm), making it suitable for UV detector applications [7]. Over this wavelength range, solar radiation is completely absorbed by the ozone layer of the earth’s atmosphere, so the background solar radiation at the earth’s surface is essentially zero. This enhances the capability of UV sensors in missile warning systems to detect targets such as missile plumes and flames emitting in this region. ZnO is the basis for the one of the richest families of nanostructures among all materials taking into accounts both structure and properties. ZnO growth morphologies have been demon‐ strated for nanowires, nanobelts, nanocages, nanocombs, nanosprings, nanorings, and nanohelixes [7]. The development of ZnO nanowire (NW) based UV detectors offers high UV sensitivity and low visible sensitivity for missile warning related applications. Demonstration of devices using single ZnO NW strands has been widely reported in literature [7-16]. However, the development of reliable 2D arrays of aligned ZnO NWs has proven more challenging. The demonstration of reliable 2D arrays requires (1) correlation of growth process and growth parameters with the material quality of ZnO NWs, (2) correlation of the electrical and optical performance with growth parameters and fabrication processes, and (3) addressing system design challenges [17-18]. With conventional NW growth methods including electrochemical deposition, hydrothermal synthesis, and molecular beam epitaxy (MBE), it is generally difficult to scale up and control NW growth. Electrochemical deposition is well suited for large scale production but does not allow control over the NW orientation. Hydrothermal synthesis is a low temperature and low- cost process that allows growth of NWs on flexible substrates without metal catalysts, but the direction and morphology of the NWs cannot be well-controlled with this method [8-10]. The MBE method allows monitoring of the structural quality during NW growth; however, this type of synthesis often requires use of metal catalysts as a seed layer [10], which introduces undesired defects to the structure, decreasing the crystal quality [12-16]. Chemical vapor deposition (CVD) also requires catalysts at the NW tips, and using this method the tips of the grown NWs were observed to be flat, with vertical alignment. The samples were characterized by scanning electron microscopy (SEM) utilizing a Quanta FEG 250 system, and X-ray diffraction (XRD) using Bruker D-8 Advance X-ray diffractometer with a wavelength of 1.5406 Å corresponding to the Cu Kα line. In addition, photolumines‐ cence (PL) measurements were performed at room temperature using a Linconix HeCd UV laser emitting at a wavelength of 325 nm. A Si detector in conjunction with at lock-in amplifier and chopper were used to measure the PL from the beam reflected off the sample at the output over the desired wavelength range [18-20]. SEM was performed to explore the NWs morphology. Figure 5 show the synthesized ZnO NWs on the various substrates, which can be generally seen to have uniform distribution density. The ZnO NWs grown on sapphire [Figure 5(a)] had approximate diameters of 50-70 nm and lengths in the range of 1-2 μm. NWs grown on SiO 2 [Figure 5(b)] had diameters of 150-200 nm and lengths of 1-2 μm, and were the least vertically oriented and associated with a relatively high lattice mismatch. NWs grown on the Si (111) substrate [Figure 5(c)] had a slightly random orientation, also having diameters in the range of 150-200 nm and lengths from 1-2 μm. Finally, the NWs grown on GaN [Figure 5(d)] showed strong vertical orientation, with diameters of 20-40 nm and lengths of 0.7-1.0 μm [20]. Figure 6 shows the XRD pattern for the ZnO NWs grown on p-Si, GaN, and SiO 2 substrates [10]. The inset of Figure 2 shows dominant peaks related to ZnO (002). The peak at 34o (2θ) for ZnO grown on p-Si and SiO 2 substrates incorporated the overlapping of ZnO NWs (002) and ZnO thin film (002). An additional diffraction peak associated with GaN was present for the GaN/sapphire substrate. ZnO NWs oriented along the (002) direction had full-widths at half maxima (FWHM) and c-lattice constants of 0.0498 (θ) and 5.1982 Å at 34.48° (2θ) for p-Si, 0.0497(θ) and 5.1838 Å at 34.58° (2θ) for GaN, 0.0865(θ) and 5.1624° at 34.38o (2θ) for SiO 2 , and 0.0830 ̊(θ) and 5.2011 Å at 34.46o (2θ) for sapphire. The quality of the ZnO epilayers utilized as seed layers to grow ZnO NWs was also charac‐ terized. The ZnO thin films were oriented along (002) and had a maximum at 34.58o with FWHM of 0.0697 (θ) for p-Si, maximum of 34.58o with FWHM of 0.0684 (θ) for GaN, and maximum of 34.43o with FWHM of 0.0557 (θ) for SiO 2 . Additional shallow diffraction peaks were observed for NWs grown on p-Si and SiO 2 , which are attributed to ZnO (100, 101, 102 and 110) as can be seen from Figure 6. As shown in Figure 7, for ZnO NW growth on sapphire major peaks were observed for ZnO (002) at 34.46° (2θ) and Al 2 O at 37.91° (2θ), with a minor peak for ZnO (101) at 36.34° (2θ). Figure 8 shows the PL spectra for ZnO NWs grown on p-Si, GaN, and SiO 2 substrates [10]. The room temperature PL measurements were performed using a ~280 nm light source. Single peaks located at 380 nm having a FWHM of 14.69 nm and at 378 nm having a FWHM of 15 nm were observed for p-Si and SiO 2 substrates, respectively, corresponding to the recombination of excitons through an exciton-exciton collision process ...

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... defects related to Zn or O vacancies were observed, which can be attributed to the confinement of defects at the ZnO thin film/substrate interface. For the ZnO NWs grown on GaN, a predominant peak with a FWHM of 18.18 nm was observed at 378 nm. High stress was evident for ZnO NWs grown on GaN, which can be observed in Figure 2; this can contribute to the broadening of the peak in comparison to p-Si and SiO 2 . Shallow peaks identified at 474 nm and 490 nm through Lorentzian decomposition are attributed to oxygen interstitial and oxygen vacancies, respectively [20]. A UV LED lamp acquired from Sensor Electronic Technology Inc. was used to characterize the UV Photoresponse of the ZnO NW arrays [20]. The lamp comprises eight separate AlGaN based UV LEDs in a TO-3 package spanning the 240-370 nm wavelength range, with a customized power supply capable of independently monitoring and controlling the current of all or any of the LEDs. The Photoresponse was determined by first applying voltage between indium contacts on the front and back sides of a Si NW sample and measuring the resulting current in the dark, and then repeating this procedure while the sample was exposed to radiation from a UV LED at a specific wavelength. Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to radiation at 370 nm. This device was found to switch between low and high conductivity states in approximately 3 s, a faster response than most reported thus far for ZnO Figure 9 shows the on-off switching characteristics of a ZnO vertical array NW device when exposed to NW based UV detectors. Figure 9(a) shows a mounted and wire bonded NW UV 3x9 pixel array detector device. Incorporation of Mg allows the detector response to be shifted to shorter wavelengths to provide detection in the solar blind region. This device was tested by applying a bias between the top contacts on the pixels, which are apparent in Figure 9(b), and the back contact. ZnO nanowires based arrays offer high sensitivity and have potential application in UV imaging systems. ZnO nanowire array based UV detectors have no moving parts, high quantum efficiency, extended lifetimes, low noise, low power requirements, and offer high sensitivity. ZnO nanowires have also been evaluated for providing remote power for the stand alone sensors. This type of application has been extensively studied by Professor Z.L. Wang and his team at Georgia Tech [21, 22]. They have shown that ZnO nanowires can be used as nano- generators for providing remote power using the Piezo-electric effect. Photovoltaic cells or solar cells are a popular renewable energy technology, relying on approaches such as inorganic p-n junctions, organic thin films, and organic-inorganic heterojunction. However, a solar cell works only under sufficient light illumination, which depends on the location the devices will be deployed, as well as the time of the day and the weather. Considering that mechanical energy is widely available in our living environment, They have demonstrated [21] the first hybrid cell for concurrently harvesting solar and mechanical energy through simply integrating a dye-sensitized solar cell (DSSC) and a piezoelectric nanogener‐ ator on the two sides of a common substrate. After this, in order to solve the encapsulation problem from liquid electrolyte leakage in the first back-to-back integrated HC, early in 2011, Xu and Wang improved the prototype design of the HC and developed a compact solid state solar cell. This innovative design convoluted the roles played by the NW array to simultane‐ ously perform their functionality in a nanogenerator and a DSSC. The design and the per‐ formance are shown in figure 11. Based on these demonstrations of HCs for concurrently harvesting solar and mechanical energies, they have. reported an optical fi ber-based three-dimensional (3D) hybrid cell, consisting of a dye-sensitized solar cell for harvesting solar energy and a nanogenerator for harvesting mechanical energy; these are fabricated coaxially around a single fi ber as a core– shell structure (Figure 11). The optical fiber, which is fl exible and allows remote transmission of light, serves as the substrate for the 3D DSSC for enhancing the electron transport property and the surface area, and making it suitable for solar power generation at remote/concealed locations. The inner layer of the HC is the DSSC portion, which is based on a radically grown ZnO NW array on an optical fiber with ITO as the bottom electrode. The dye-sensitized ZnO NW array was encapsulated by a stainless steel capillary tube with a Pt-coated inner wall as the photo- anode for the DSSC. The stainless steel tube also serves as the bottom electrode for the outer layer of the nanogenerator, with densely packed ZnO NWs grown on its outer wall. Another exciting application of ZnO nanowires is designing, fabricating, and integrating arrays of nanodevices into a functional system are key to transferring nanoscale science into applicable nanotechnology as shown in Figure 12. Recent work [22] on three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as an active taxel-addressable pressure/force sensor matrix for tactile imaging. Using the piezoelectric polarization charges created at a metal- semiconductor interface under strain to gate/modulate the transport process of local charge carriers, we designed independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied to the devices into local electronic controlling signals. The device matrix can achieve shape-adaptive high-resolution tactile imaging and self- powered, multidimensional active sensing. The 3D piezotronic transistor array may have applications in human-electronics interfacing, smart skin, and micro- and nano-electrome‐ chanical systems. High resolution imaging in UV bans 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 will discuss model that can predict sensor performance in the UV band using APD’s with various gain and other parameters for a desired UV band of interest. SNR’s can be modeled from illuminated targets at various distances with high resolution under standard atmospheres in the UV band and the solar blind region using detector arrays with unity gain and with high gain APD’s [23-26]. Figure 13 presents the relationship between the alloy composition of Gallium and Aluminum in Al x Ga 1-x N that determines the cut-off wavelength of the UV detector for p-i-n [23-24] and also for UV APD’s. Deep Ultra Violet (DUV) will require addition of larger composition of Aluminum in Al Ga N. [25]. Figure 14 presents the High-Temperature MOCVD system by Aixtron. This new reactor design and capability has the ability to grow high quality GaN and AlGaN material with doping for GaN/AlGaN UVAPD applications [26]. Figure 15 presents the device structure of a back-side illuminated AlGaN UV APD. The substrate in this device structure is double side polished AlN substrate. The use of AlN substrate allows the UV APD device structure to be back-side illuminated and can be inte‐ grated with silicon CMOS electronics. Figure 16 presents the Reciprocal Space mapping of AlGaN on AlN substrate and Sapphire substrate. The data for sapphire substrate ...

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... L, d are the length and diameter of the CNT Using average values for CNTs gives the following numerical value for the thermal resistance of a CNT R T ~ 5 x 10 8 (L/d 2 ) Where R T is in units of degrees K/Watt, L is in units of microns and d is in units of nanometers. So a tube that is one micron long and one nanometer in diameter will have a thermal resistance R T of approximately 5 x 10 8 K/W. Net IR Radiation Power Absorbed: Now that we have the thermal diffusion and capacitance we are almost ready to begin solving the above heat flow equation to determine the temperature of the bolometer. However, before doing so, we need to determine H , the net IR power absorbed by the bolometer. We determine this power using the Stefan-Boltzmann Law of blackbody radiation, which relates the net power absorbed to the temperatures of the subject and the bolometer using the following expression: H net = σAε ( T obj 4 − T b 4 ) Where H net , σ, A, ε, T obj and T b are heat absorbed by the bolometer, the Stefan-Boltzmann constant, cross-sectional area, emissivity, object of interest temperature and bolometer absorber temperature, respectively. Figure 1 below shows the net IR power absorbed by the absorber as a function of bolometer temperature for radiating objects at 20 o C and 36.5 o C. Cooling the bolometer by 30 o C below room temperature allows for significantly more power to be absorbed, which can give rise to a much stronger signal. Calculating the Bolometer Temperature Distribution: Using the aforementioned expressions for C , K and H , we expand on previous work and convert the heat flow equation above into a thermal network, illustrated in Figure 2 [6]. In actuality, there are thousands of nodes in the network 30, for each which resistor we calculate represents the the temperature thermal resistance for each. of a CNT in series with the In Figure 30, each resistor represents the thermal resistance of a CNT in series with the thermal resistance between adjacent CNTs. In addition, the capacitors represent the thermal capacity of a CNT, while the current sources represent the net IR radiation absorbed by each CNT. This thermal network contains thousands of nodes, and there is an equation relating the thermal resistance, capacitance and net power for each node. This system of equations is then solved for the temperature as a function of position and time throughout the bolometer absorber [54]. Results of these calculations for are shown in Figure 31 for different types of CNT networks. Here, we assumed that the net absorbed power is 1 nW, and the pixel is tightly packed with the CNTs. The entire pixel’s temperature map is obtained with a 100×100 tempera‐ ture resolution. For the tubes, we used two different thermal resistance values: 5×10 8 and 1×10 9 K/W. As expected, higher the thermal resistance, higher the temperature difference from the ambient. We note that in general thermal resistance also rises with increasing temperature, resulting in further heating of hot spots compared to the case that this dependency is ignored. The temperature gradient of the contacts legs connecting the film to the readout IC (ROIC) is clearly shown. To read the temperature that the bolometer pixel reaches after an exposure to infrared radiation, one needs to measure the electrical resistance of the pixel. By comparing this resistance to a look-up table or using the a-priori knowledge of temperature coefficient of resistance, the pixel temperature can be determined. Therefore, in addition to having a large thermal resistance which translates into higher temperature rises, a large temperature coefficient of electrical resistance (TCR) is desirable to achieve a higher temperature resolution. Here TCR is defined as the change in electrical resistance per degree Kelvin divided by the absolute electrical resistance measured at the quiescent point, as follows: TCR = 1 d R e Thus, the pixel electrical resistance after it reaches a temperature that is ∆T above its ambient becomes R e (T) = R e (To)(1+TCR) . Using this relationship, the pixel temperature is calculated. To obtain a high temperature resolution, a large change in electrical resistance is needed upon heating. To achieve this, a substantial increase either in electron concentration or velocity (for a given electric field) is necessary. And to this end, materials with junctions where thermionic emission or tunneling are the electrical current bottlenecks offer a good solution. As the tunneling current exponentially rises with temperature, the effective change in their electrical resistance due to temperature becomes large compared to those observed in bulk materials where the change is proportional T γ and γ is generally < 2. Here, a film of CNTs is proposed as the bolometer pixel material, since it is likely to have large thermal resistance and TCR values simultaneously. Both of these favorable properties are partially owed to the junctions between the tubes. As the electrical current flows along the mat, it needs to jump from one tube to the next where they intersect. At this intersection, the carriers see a potential barrier that they need to tunnel through which gives rise to exponential increase in current upon heating. Assuming that the electron transport across this barrier is governed by a Fowler-Nordheim-type tunneling or thermionic emission, the expected TCR values can be calculated using the following ...

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... Chapter covers recent advances in nanostructured based detector technology, materials and devices for optical sensing applications. The authors have many years of experience working nanotechnologies that include a variety of semiconductors and other advanced materials such as GaN, ZnO, Si/SiGe, CNT and Graphene for optical sensing applications. Optical sensing technology is critical for defense and commercial applications including optical communication. 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. We will cover the UV band (200-400 nm) and address some of the recent advances in nano‐ structures growth and characterization using GaN/AlGaN, ZnO/MgZnO based technologies and their applications. We will also discuss nanostructure based Si/SiGe technologies (400-1700 nm) that will cover various bands of interest in visible-near infrared for detection and optical communication applications. The chapter will also discuss some of the theoretical and experimental results in these detector technologies. Recent advancements in design and development of CNT 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. The Ultraviolet spectrum has been of interest for a variety of sensors for defense and com‐ mercial applications. The UV band is from 250-400 nanometers as shown in the figure 1. This band can be further divided into UVA and UVB bands. Each of these bands has applications for sensors, detectors and LED applications. The word “infrared” refers to a broad portion of the electromagnetic spectrum that spans a wavelength range from 1.0 um to beyond 30 um everything between visible light and micro‐ wave radiation. Much of the infrared spectrum is not useful for ground- or sea-based imaging because the radiation is blocked by the atmosphere. The remaining portions of the spectrum are often called “atmospheric transmission windows,” and define the infrared bands that are usable on Earth. The infrared spectrum is loosely segmented into near infrared (NIR, 0.8-1.1um), short wave infrared (SWIR, 0.9-2.5um), mid wave infrared (MWIR, 3-5um), long wave infrared (LWIR, 8-14um), very long wave infrared (VLWIR, 12- 25um) and far infrared (FIR, > 25um), as shown in Figure 2. The MWIR- LWIR wavebands are important for the imaging of objects that emit thermal radiation, while the NIR-SWIR bands are good for imaging scenes that reflect light, similar to visible light. Since NIR and SWIR are so near to the visible bands, their behavior is similar to the more familiar visible light. Energy in these bands must be reflected from the scene in order to produce good imagery, which means that there must be some external illumination source. Both NIR and SWIR imaging systems can take advantage of sunlight, moonlight, starlight, and an atmospheric phenomenon called “nightglow," but typically require some type of artificial illumination at night. In lieu of photon starved scenes, arrays of infrared Light Emitting Diodes (LEDs) can provide a very cost effective solution for short-range illumination. However, achieving good performance at distances of over hundreds of meters requires more directed illumination, such as a focused beam from a laser or specialized spotlight, although special consideration of eye-safety issues is required. Imagery for identification of targets at various distances uses visible cameras, image intensi‐ fiers, shortwave IR cameras and long wave uncooled cameras. Each have distinct advantages and disadvantages and are each useful under specific sets of conditions such as light level, thermal conditions, and level of atmospheric obscuration. The shortest wavelength is desired for spatial resolution which allows for small pixels and large formats. [2- 6] Visible cameras, if adequate light level is present, can provide high resolution, but for long range identification even under moonlit and starlit illuminations, long integration times and large optics are required and dust, smoke and fog easily defeat a single visible camera. Image intensifiers and SWIR cameras are useful in many conditions as the SWIR penetrates fog easily but requires fairly clear night skies for the upper atmospheric airglow light source, and image intensifiers require a certain level of celestial (starlight, moonlight) or light pollution irradiance. Both the SWIR and image intensifiers are limited by the diffraction resolution of the NIR to SWIR wavelengths [5-6]. For optimal resolution, the visible or ultraviolet spectrum is preferable; however, active (laser) illumination is required for long-range night imaging. Covert UV illumination is preferred over the visible and the atmosphere transmits fairly well at the longer UV wavelengths. The covert active system for high-resolution identification modeled in this paper consists of a UV laser source and a silicon CCD, AlGaN or AlGaN APD focal plane array with pixels as small as 4 microns that are spectrally tuned for the solar-blind region of the UV spectrum. The solar- blind region is optimal as virtually all of the solar radiation is absorbed at the higher altitudes leaving a pitch dark terrain even in bright day, yet for sea-level path lengths of 1 km and shorter; the UV atmospheric transmittance is still acceptable. This combination is ideal for exploitation by a UV illuminator and UV FPA sensor. Current UV lasers can provide either continuous or pulsed energy at levels detectable by solar-blind UV detectors under relatively small optics and at 30 Hz frame rates, providing real-time high- resolution (on the order of 1 cm at 1 km) imagery. At these illumination levels and target ranges, both standard PN, PIN and APD UV detectors and silicon CCD’s can be used for target identification. The model has been developed and used to include the combined effects of detector and electronics, atmospheric transmittance and UV background radiance, target size, range and reflectance, and UV laser attributes to simulate and predict both CW and pulsed laser imaging performance and to assist in the design of this prototype system ...