The electronic structure of SWNTs was investigated using the complementary techniques of single molecule photoluminescence spectroscopy and ultrafast optical spectroscopy. We found that photoexcited electrons in SWNTs isolated in surfactant micelles decay through many channels, exhibiting a range of decay times (~200 fs to ~ 120 ps). The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime corresponds to the band-edge relaxation time. Fluorescence spectra from single SWNTs are well described by a single, Lorentzian lineshape. However, nanotubes with identical structure fluoresce over a distribution of peak positions and line widths not observed in ensemble studies, caused by localized defects and electrostatic perturbations. Unlike for most other single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at 300K. This lack of photoluminescence intensity blinking or bleaching demonstrates that SWNTs have the potential to provide a stable, single molecule infrared photon source, allowing for the exciting possibility of single nanotube integrated photonic devices and biophotonic sensors.

Much progress has been made in recent years in the fabrication and understanding of the operations of Single-walled carbon nanotube (SWNT) field effect transistors (FETs). Nevertheless, it remains a challenge to develop highly scaled device structures for nanotube transistors and push to the performance limit of these molecular materials. The purpose of this work is to fabricate highly scaled SWNT field effect transistors (FETs) and approach the performance limit of carbon nanotube FETs.

Quantum dots have been fabricated in an individual single-wall carbon nanotube, and their single electron transport has been measured at low temperatures with magnetic field up to 5T. The Coulomb diamonds have shown two and four electron periodicities in different gate voltage ranges. The results can be understood by the two- or four-electron shell filling model. In magnetic fields, the Zeeman splitting of single particle levels has been observed. This means that the single spin polarization is realized when an odd number of electrons are in the dot. The experimental observation suggests an important step for realization of the spin qubit.

We have fabricated carbon nanotube (CNT) FETs using position-controlled growth technique. The CNT-FETs showed p-type conduction. The chirality of the CNT FET channel was determined by using microphotocurrent spectroscopy. Novel peapods FETs with fullerenes inserted in the CNTs were also fabricated. They showed ambipolar I-V characteristics with both n- and p-type conductions depending on the gate voltage. The ambipolar behavior was explained based on the Schottky-barrier-controlled transistor model, where the transistor action occurs primarily by changing the Schottky contact resistance by the gate voltage. It has been shown that the bandgap of the peapod FETs was dependent on what kind of fullerene was inserted in the CNT channel. We have also demonstrated that it was possible to control the conduction type of the FET channel by choosing contact metal with different work function without any doping in the CNT channel.

We demonstrate the advantages of carbon nanotubes (CNTs) for ULSI interconnections, which we believe that it must be one of the most promising applications for metallic carbon nanotubes. We report present status of CNT material technologies and the potential of metallic CNT vias for future ULSI interconnections.

Carbon nano-tubes are alternatives to conventional metal/silicon tips for field emission sources. They exhibit extraordinary field emission properties because of their high electrical conductivity, their high aspect ratio "whisker-like" shape for optimum geometrical field enhancement, and remarkable thermal stability. This paper will review the PECVD growth process, and the micro-fabrication techniques needed to produce well defined carbon nano-tube based micro-electron sources for use in a variety of applications.

We present here, a novel approach for the membrane-based synthesis, also called template synthesis of arrays of nanomaterials with monodispersed geometrical features. The basic principle is to grow or generate the desired material inside the pores of a nanoporous alumina membrane. The pores of are synthesised parallel to the surface of the substrate by performing the anodic oxidation of an aluminium thin film laterally, i.e. parallel to the surface of the substrate, instead of perpendicular as usually done. We obtain highly regular and ordered pore arrays, with a minimum pore size in the range of ~3 to 4 nm, which to the best of our knowledge is the smallest reported to date for anodic alumina membranes. After anodic oxidation, the pores of the lateral alumina membranes have been electrochemically “filled” with Te nanowires. Such porous alumina structures may allow to control the in-plane organisation of arrays of template-grown nanowires and carbon nanotubes for reproducible device fabrication.

In this paper, we will introduce a non-lithographic technique that utilizes self-organized, highly ordered anodized aluminum oxide (AAO) porous membrane as a template to form a variety of periodic nanometric superstructures. Materials as different as metals, semiconductors, and carbon nanotubes (CNT) can be adapted into this platform with equal ease. The flexibility with materials, the accurate control over the fabrication process, and the command over the alumina template attributes offers us a new degree of freedom of engineering various physical properties by templating the nanostructures with the desired shape, size, composition, and doping. The versatile nanometric superstructure platform realized by this unique approach can enable a broad range of applications including infrared detector and quantum dot lasers and so on.

Aligned carbon nanotubes (CNT’s) have been found to form on both the Si and C faces of silicon carbide (SiC) wafers at high temperature. The CNT’s form when the SiC wafer is exposed to temperatures in the range 1400-1700°C under moderate vacuum. The CNT’s are aligned roughly parallel to the surface. After a half hour at 1700°C under vacuum of 10^-4torr, a near continuous CNT layer about 250nm thick is formed. The entire surface of the SiC is covered with CNT’s including both single and multiwalled tubes, and some graphitic carbon. SEM, TEM, AFM, XPS and Raman scattering measurements have been used to analyse the CNT/SiC structures. The metal catalyst free CNT’s on SiC exhibit low density of structural defects and are very straight. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. It is speculated that CNT's growth is catalysed by low concentrations of residual oxygen in the chamber during growth. The vacuum conditions can significantly affect CNT's growth. Single wall carbon nanotubes are evident in Raman spectra on the samples grown at 10^-3 Torr, not on these grown at 10^-5Torr.

We have previously developed a simple pyrolytic method for large-scale production of aligned multi-wall carbon nanotube arrays perpendicular to the substrate. These aligned carbon nanotube arrays can be transferred onto various substrates of particular interest (e.g. polymer films for organic optoelectronic devices) in either a patterned or non-patterned fashion. The well-aligned structure provides additional advantages for not only an efficient device construction but also for surface functionalization. The surface functionalization of aligned carbon nanotubes is particularly attractive, as it allows surface characteristics of the aligned carbon nanotubes to be tuned to meet specific requirements for particular applications while their alignment structure can be largely retained. These aligned carbon nanotubes with tunable surface characteristics are of great significance to various practical applications ranging from sensors to electronics. Single-strand DNA chains could be covalently immobilized onto plasma-activated aligned carbon nanotubes for sensing complementary DNA and/or target DNA chains of specific sequences with a high sensitivity and selectivity. Furthermore, glucose oxidase (GOX) could also be immobilized onto the aligned carbon nanoutbe arrays by electropolymerization of pyrrole in the presence of GOX. The resultant GOX-containing polypyrrole-carbon nanotube coaxial nanowires were shown to be promising new sensing active materials for making advanced glucose sensors with a high sensitivity.

We provide a general technique for evaluating the performance of an optical sensor for the detection of chemical warfare agents (CWAs) in realistic environments and present data from a simulation model based on a field deployed discretely tunable ¹³CO₂ laser photoacoustic spectrometer (L-PAS). Results of our calculations show the sensor performance in terms of usable sensor sensitivity as a function of probability of false positives (PFP). The false positives arise from the presence of many other gases in the ambient air that could be interferents. Using the L-PAS as it exists today, we can achieve a detection threshold of about 4 ppb for the CWAs while maintaining a PFP of less than 1:10⁶. Our simulation permits us to vary a number of parameters in the model to provide guidance for performance improvement. We find that by using a larger density of laser lines (such as those obtained through the use of tunable semiconductor lasers), improving the detector noise and maintaining the accuracy of laser frequency determination, optical detection schemes can make possible CWA sensors having sub-ppb detection capability with <1:10⁸ PFP. We also describe the results of a preliminary experiment that verifies the results of the simulation model. Finally, we discuss the use of continuously tunable quantum cascade lasers in L-PAS for CWA and TIC detection.

The design of a miniature capacitive microphone is discussed with specific application to photoacoustic instrumentation. It is demonstrated that a microphone design optimized for use in PAS can provide significant improvements in performance, with positive implications on the overall system performance. Furthermore, it is shown that MEMS-based microphone technology allows for important improvements in microphone vibration sensitivity, size, cost and ruggedness with direct application to portable PAS systems.

We report the cw operation of quantum cascade lasers that do not require cryogenic cooling and emit at λ = 4.7-6.2 μm. At 200 K, more than 1 W of output power is obtained from 12-μm-wide stripes, with a wall-plug efficiency (η_wall) near 10%. Room-temperature cw operation has also been demonstrated, with a maximum output power of 640 mW (η_wall = 4.5%) at 6 μm and 260 mW (η_wall = 2.3%) at 4.8 μm. Far-field characterization indicates that whereas the beam quality remains close to the diffraction limit in all of the tested lasers, in the devices emitting at 6.2 μm the beam tends to steer by as much as 5-10° degrees in either direction with varying temperature and pump current.

Laser sources operating in spectral region 2 - 4 μm are in demand for ultra-sensitive laser spectroscopy, medical diagnostics, home security, industrial process monitoring, infrared countermeasures, optical wireless communications, etc. Currently, solid state lasers and optical parametric oscillators and amplifiers are used as coherent light sources in this spectral region. Solid state and parametric sources are being optically pumped by near infrared diode lasers. This intermediate energy transfer step from near infrared pumping diode to mid infrared emitting device reduces power-conversion system efficiency. Development of the highly efficient semiconductor diode lasers operating in 2 - 4 μm spectral region will significantly improve the performance of the many existing systems and enable new applications. In this work we will describe major breakthrough in the development of the high power room temperature operated mid-IR semiconductor lasers. The performance limitations of the devices based on type-I and type-II quantum well (QW) active region design will be analyzed. Future directions in device performance optimization and enhancement of the wavelength for high power room temperature operation will be discussed.

Recent advances in current-pumped, bandgap-engineered semiconductor lasers have dramatically impacted laser-based sensor concepts for in-situ trace species measurement and standoff sensing applications. These devices allow a common technology platform to access strong fundamental vibrational absorption transitions of many gases, liquids, and solids in the mid-wave and long-wave IR, as well as far-IR, or THz. The THz wavelength region is particularly interesting for applications related to structure penetrating detection of hidden materials and biomolecular spectroscopy. This presentation will briefly review the important properties of these lasers as they apply to sensor design and present highlights of recent sensor development activity for trace gas analysis in environmental and biomedical applications, remote sensing LIDAR systems, and detection of hidden explosives.

During the last several years, Pacific Northwest National Lab has developed a remote sensing system designed to detect trace chemicals present in the atmosphere. Using Frequency Modulated Differential Absorption LIDAR (FM DIAL) techniques chemical signatures have been observed over pathlengths ranging from several hundred meters to several kilometers. Throughout the development process, we have encountered many challenges. Some of these have been overcome but others will require more novel solutions.

Spintronics detection technology can add significant functionality to bioassay detection systems and improve their precision. Photonic detection is presently the dominant technology in bioassay systems. Optical detectors on mechanical scanners are used to read the vast majority of commercially produced DNA chips. Introduction of these optical systems into non-laboratory applications is greatly impeded by the cost, size, and complexity of optical systems. Magnetics can help by providing detectors and actuators that are inherently rugged, small, and mass-manufacturable. Magnetic nanoparticles are used in many biochemical assays as labels for concentration, manipulation and, more recently, detection. Typically one attaches the magnetic particles to the biochemical species of interest (target) using a chemically specific binding interaction. Once bound, the labels enable the manipulation of the target species through the application of magnetic forces. Spintronic sensors, specifically Giant Magnetoresistive (GMR) and Spin Dependent Tunneling (SDT), sensors have been developed to detect and quantify labels in two main formats: flowing in a microfluidic channel, and immobilized labels on a chip surface.

Optical characterization is a powerful tool for assessment of spin-injection properties in spin-LEDs. We show a comparison of spin-dependent transport properties in LEDs with different spin-injector structures including ferromagnet/tunnel-barrier or Schottky-barrier injectors and magnetic semiconductors. Using these devices, room temperature spin-injection and voltage-controlled injection has been achieved. The optical technique used is the Oblique Hanle Effect. Features such as spin-nuclear interactions can be clearly demonstrated in such spin-LED structures. Finally we discuss the design of a spin-dependent contact that can supply both electrical spin injection as electrical spin detection.

We present S and P polarized measurements of artificial bianisotropic magnetic metamaterials with resonant behavior at infrared frequencies. These metamaterials consist of an array of micron sized (~40μm) copper rings fabricated upon a quartz substrate. Simulation of the reflectance is obtained through a combination of electromagnetic eigenmode simulation and Jones matrix analysis, and we find excellent agreement with the experimental data. It is shown that although the artificial magnetic materials do indeed exhibit a magnetic response, care must be taken to avoid an undesirable electric dipole resonance, due to lack of reflection symmetry in one orientation. The effects of bianisotropy on negative index are detailed and shown to be beneficial for certain configurations of the material parameters.

The high frequency permeability of two types of artificial materials are investigated. Composites made of ferromagnetic wires and conductive wires have been manufactured, and their microwave properties have been investigated. Experimental results are reported, and a model is proposed that accounts for the observations. It is underlined that textile technology is very attractive and cost-effective to manufacture metamaterials. The permeability of systems composed of magneto-mechanical oscillators has been investigated theoretically. It is shown that these systems are described by equations similar to that of gyromagnetism. The figure of merits of conventional soft permeable materials and of these magneto mechanical systems are discussed.

The MDA/AS is developing advanced capabilities in the IR sensor area for ballistic missile defense. Infrared sensors play an important role to every layer of ballistic missile defense systems. Current missile defense systems use infrared sensors with material systems such as InSb and HgCdTe. They are sufficient to counter current threat level. However, with the Missile Defense Agency’s (MDA) new capability based acquisition, the Advanced Systems (AS) of MDA is pushing the state-of-the-art of infrared technology to a higher level. The new infrared sensors will have the capability to cover a broad range of spectrum, have higher sensitivity, uniformity and operabilities, large format, multicolor, with a much better manufacturability and affordability. Examples are strained layer type-II superlattices, quantum well infrared photodetectors, HgCdTe on Si, PbSnTe, and other novel concepts. This presentation will discuss what IR sensors are used in current missile defense systems, what is new in technology development, and its system applications.

Researchers engaged in the development of infrared detector technologies may well benefit from a broader understanding of their products from the perspective of the end-user. An appreciation of how this technology is to be used by system designers, many of whom possess only a rudimentary understanding of quantum physics, is highly germane. Answers to questions like: "What device technology will be employed," "How will the device be used?" and "What are the impacts on signal-to-noise?" are of critical importance. In this paper, some of the fundamentals of hit-to-kill missile defense technology are examined in a largely non-mathematical context. From its "Star Wars" inception during the Reagan administration, to today’s Missile Defense Agency, the core requirement of missile defense has not changed - find the threat and destroy it before it reaches its destination. This fundamental requirement, while conceptually straightforward, is extraordinarily difficult to satisfy, and is almost exclusively dependent on our ability to detect and designate a relatively small, very fast-moving, room-temperature object, at great distances, and usually in a severe environment of shock and vibration further clouded by error and uncertainty. With an obvious bias toward passive IR detection and associated focal plane array characteristics, the flight of a fictitious interceptor is followed from launch to impact. At various points along the interceptor’s trajectory, a "peel the onion" approach is utilized to expose increasingly detailed layers of behavior, including the eventual release of the kinetic kill vehicle, and its autonomous flight to a body-to-body impact with its target. Various sources of error and their impact on the success of the mission are examined, and an overall understanding of the key features of the infrared seeker and its critical role in missile defense are ultimately developed.

HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm² in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048 x 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6 in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 x 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations.

W-structured type-II superlattices (W-SLs) were initially developed to increase the gain of mid-wave infrared (MWIR) lasers. The design addressed the reduced optical transition matrix elements due to the spatial displacement between valence and conduction band wavefunctions in the type-II superlattice (T2SL), and further improved the differential optical gain by providing a mostly two-dimensional density of states. As a result, W-SL and W interband cascade lasers have lower thresholds and higher pulsed and cw operating temperatures than any other III-V interband MWIR lasers. These same features give W-SLs desirable properties for IR detectors, and here we report for the first time on characteristics of W-SLs used for long-wave and very long-wave IR photodiodes. IR transmission measurements of W and conventional T2SL photodiodes revealed absorption characteristics that are well described by theory, including line shape and peak absorption coefficient values which are about a factor of 2 greater in the W-SLs. Similarly, the low temperature photoluminescence shows much higher and sharper emission intensity in the W-SLs. While the W-SLs have demonstrated superior optical properties, as predicted, additional work is needed to achieve higher detector quantum efficiency. Results suggest that the excess carrier collection in the W-structures is reduced with respect to similar T2SL structures, especially for the lowest energy state. Possible mechanisms of excess carrier loss, as well as new designs to improve charge collection, in the W-SL, will be discussed.

Recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg_1-xCd_xTe (HgCdTe) multilayer heterostructures on GaAs/CdTe substrates for uncooled infrared photodetectors is presented. The optimum conditions for the growth of single layers and complex multilayer heterostructures have been established. One of the crucial stages of HgCdTe epitaxy is CdTe nucleation on GaAs substrate. Successful composite substrates have been obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition and doping profiles. This requires for IMP pairs to be very thin and grown in a short time. Arsenic and iodine have been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is essential for stable and reproducible doping. In situ anneal seems to be sufficient for iodine doping at any required level. In contrast, efficient As doping with near 100% activation requires ex situ anneal at near saturated mercury vapors. As a result we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including photoconductors, photoelectromagnetic and photovoltaic detectors. The present generation of uncooled long wavelength infrared devices is based on multijunction photovoltaic devices. Near-BLIP performance is possible at ≈ 230 K with optical immersion. These devices are especially promising as 7.8-9.5-μm detectors, indicating the potential for achieving detectivities above 10⁹ cmHz^1/2/W.

Type-II superlattices composed of alternating thin layers of InAs and GaSb, have been shown to be a highly flexible infrared materials system in which the energy band gap can be adjusted anywhere between 360 meV and 40 meV. These superlattices (SLs) are the III-V equivalent to the well established Hg_xCd_1-xTe alloys used for infrared detection in the short, mid and long wavelength bands of the infrared spectrum. There are many possible designs for these superlattices that will produce the same narrow band gap by adjusting individual layer thicknesses and interface composition. Systematic growth and characterization studies were performed to determine optimum superlattice designs suitable for infrared detection in the 3 to 5 μm wavelength band. For these studies the individual layer thicknesses were less than 35Å. The effects of adding different thickness InSb-like interfaces were also studied. Through precision molecular beam epitaxy, design changes as small as 3Å to the SL layers could be studied. Significant changes were observed in the infrared photoresponse spectra of the various SL samples. The infrared properties of the various designs of these type-II superlattices were modeled using an 8-band Envelope Function Approximation. The infrared photoresponse spectra, combined with quantum mechanical modeling of predicted absorption spectra, were a key factor in the design optimization of the InAs/GaSb superlattices with band gaps in the range of 200 to 360 meV.

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024x1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17 mK at a 95K operating temperature with f/2.5 optics at 300K background and the LWIR detector array has demonstrated a NEΔT of 13 mK at a 70K operating temperature with the same optical and background conditions as the MWIR detector array. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90K and 70K operating temperatures respectively, with similar optical and background conditions. In this paper, we will discuss the performance in terms of quantum efficiency, NEΔT, uniformity, operability, and modulation transfer functions.

Leakage currents limit the operation of high performance type II InAs/GaSb superlattice photodiode technology. Surface leakage current becomes a dominant limiting factor, especially at the scale of a focal plane array pixel (< 25 μm) and must be addressed. A reduction of the surface state density, unpinning the Fermi level at the surface, and appropriate termination of the semiconductor crystal are all aims of effective passivation. Recent work in the passivation of type II InAs\GaSb superlattice photodetectors with aqueous sulfur-based solutions has resulted in increased R₀A products and reduced dark current densities by reducing the surface trap density. Additionally, photoluminescence of similarly passivated type II InAs/GaSb superlattice and InAs GaSb bulk material will be discussed.

We report an InGaAs/InGaP/GaAs quantum dot infrared photodetector grown by metalorganic chemical vapor deposition with detectivity of 1.3x10¹¹ cmHz1/2/W at 77K and 1.2x10¹⁰ cmHz^1/2/W at 120K. Modeling of the Quantum dot energy levels showed us that increased photoresponse could be obtained by doping the quantum dots to 4 electrons per dot instead of the usual 2 electrons per dot. This happens because the primary photocurrent transition is from the first excited state to a higher excited state. Increasing the quantum doping in our device yielded significant responsivity improvement and much higher detectivity as a result. This paper discusses the performance of this higher doping device and compares it to our previously reported device with lower doping.

Dark current has become a significant limiting factor for the development of the Type II InAs/GaSb superlattices technology. Experimental results showed that at liquid nitrogen temperature the dominating dark current under reverse bias is the generation-recombination current before the tunneling current turns on. Recent research on the source of the dark current indicated that the Auger recombinations might play a very important role in the superlattice diode dark current. With proper design of the superlattice structure, we have been able to reduce the dark current several orders of magnitude in the LWIR range. The superlattice diode performance was also improved dramatically. Infrared focal plane arrays based on these superlattices will also be discussed.

In this work, we investigate the DC and high-frequency performance of heterostructure InGaAs/InAlAs metal-semiconductor-metal (MSM) photodetectors fabricated using e-beam nanolithography. The device finger electrodes are approximately 250 nm in width with gaps of 100 nm, and are arranged interdigitally in a circular geometry with ~4.5 μm outer diameter. The InGaAs absorption layer is 200 nm thick. High-bandgap material layers are utilized to form carrier transport barriers and to enhance Schottky barrier heights at the contacts. Also, a buried Bragg reflector (resonant cavity enhanced structure) serves to increase the photodetector responsivity given the very thin absorption layer thickness. Current-voltage measurements of seven devices were taken at room temperature under dark and illuminated conditions. Additionally, the frequency performance was evaluated using a 1550-nm diode laser with an integrated 50-GHz electroabsorption modulator and a microwave network analyzer. A fairly large range of dark currents from 100 pA-1 μA is observed. External responsivities for these small devices also vary widely between 0.05-0.24 A/W. Under high-frequency intensity modulation, the devices exhibit an RC-limited and/or diffusion-limited performance, with usable bandwidths to 15 GHz. Physical origins of the device and parasitic capacitances are discussed as well as optical diffraction effects, both of which are thought to contribute to the limited high-frequency performance.

Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared detection. Quantum dot infrared photodetectors (QDIPs) have been predicted to have better performance than QWIPs including higher operation temperature and normal incidence detection. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition (MOCVD). The device structures consist of multiple stacks of InAs quantum dots with InP barriers. High detectivities in the range of 10¹⁰cmHz^1/2/W were obtained at 77K. The measurements at higher temperatures show better temperature dependent performance than QWIP. However, the performances of QDIPs are still far from the expected. One of the reasons is the low quantum efficiency due to the low fill factor of quantum dots layer. Resonant cavity enhanced QDIP has been studied to increase the quantum efficiency. Different schemes of mirrors using free carrier plasma and distributed Bragg reflector are discussed.

Nanopillar devices have been fabricated from GaInAs/InP QWIP material grown by MOCVD. Using electron beam lithography and reactive ion etching techniques, large, regular arrays of nanopillars with controllable diameters ranging from 150 nm to less than 40 nm have been reproducibly formed. Photoluminescence experiments demonstrate a strong peak wavelength blue shift for nanopillar structures compared to the as-grown quantum well material. Top and bottom metal contacts have been realized using a polyimide planarization and etchback procedure. I-V and noise measurements have been performed. Optical measurements indicate photoconductive response in selected nanopillar arrays. Device peak wavelength response occurs at about 8 μm with peak device responsivity of 420 mA/W. Peak detectivity of 3×10⁸ cmHz^1/2/W has been achieved at -1V bias and 30 K.

Design, fabrication, and characterization of high-performance Al_xGa_1-xN-based photodetectors for solar-blind applications are reported. Al_xGa_1-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA leakage currents at 6 V and 12 V reverse bias were measured on p-i-n and Schottky photodiode samples respectively. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9x10¹⁴ cmHz^1/2/W^-1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.15 A/W and 0.11 A/W peak responsivity values at 267 nm and 261 nm respectively. All samples displayed true solar-blind response with cut-off wavelengths smaller than 280 nm. A visible rejection of 4x10⁴ was achieved with Schottky detector samples. High speed measurements at 267 nm resulted in fast pulse responses with >GHz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.

Back-illuminated solar-blind ultraviolet p-i-n photodetectors and focal plane arrays are investigated. We initially study single-pixel devices and then discuss the hybridization to a read-out integrated circuit to form focal plane arrays for solar-blind UV imaging. The photodetectors consist of an AlGaN p-i-n active region grown atop a high quality AlN template layer with a ~1 μm thick Al_0.5Ga_0.5N:Si-In co-doped low-resistance UV-transparent lateral conduction layer. The material is processed into a 320 x 256 array of 25 μm x 25 μm pixels using standard lithographic techniques. Typical pixels demonstrate a peak responsivity of 93 mA/W at 278 nm; this corresponds to an external quantum efficiency of 42%. The uniformity of the array is discussed, and a selection of sample images from the solar-blind focal plane array is included. In addition, recent attempts to achieve shorter wavelength deep UV back-illuminated p-i-n photodetector and focal plane arrays are also discussed.

In this paper we propose a new type of nitride LED structure based on “wrinkled” rather than flat Multiple Quantum Wells (MQWs) grown along the polar [0001] direction. We show that this novel approach of employing “wrinkled” MQWs leads to the improvement of both the internal quantum efficiency as well the extraction efficiency. Identical GaN/AlGaN MQWs with well and barrier widths of 7 nm, were grown on both the textured and the smooth GaN templates by plasma-assisted MBE and their optical properties were evaluated by photoluminescence (PL) and cathodoluminescence (CL) measurements. The PL spectra of the smooth and “wrinkled” MQWs have significant differences. The PL from the smooth quantum wells has a single peak at 396 nm, consistent with the expected red-shift from the PL spectra of the bulk GaN films due to the quantum confined Stark effect (QCSE). The PL peak from the wrinkled QWs occurs at 358 nm, which is blue-shifted with respect to the PL spectra of the bulk GaN films, a result consistent with QWs having a square configuration. Furthermore, we found that the integrated photoluminescence intensity from the “wrinkled” multiple quantum wells (WMQWs) was about 700 times higher than that of the smooth MQWs. We attribute this significant enhancement of the photoluminescence from the WMQWs partly to enhancement in light extraction through the textured surface and partly to enhanced spontaneous emission rate. We believe that the increase in the internal quantum efficiency is due to the reduction of the QCSE, since the quantum wells are not perpendicular to polar [0001] direction. Further enhancement in internal quantum efficiency is expected due to quantum carrier confinement from "wedge" electronic eigen-modes. The latter has its origin to the transition in the carrier behavior from 2D to 1D due to the V-shaped intersecting planes of the quantum wells, and thus the "wedges" behave as quantum wires, which cause localization and trapping of excitons.

We report on the growth and fabrication of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with peak emission of below 255 nm. In order to achieve such short wavelength UV LEDs, the Al mole fractions in the device layers should be greater than ~60%. This introdues serious challenges on the growth and doping of Al_xGa_1-xN epilayers. However, with the aid of a high-quality AlN template layer and refinement of the growth conditions we have been able to demonstrate UV LEDs emitting below 255 nm. The ratio of the intensity of the electroluminescence primary peak to that of the secondary peak (related to Mg deep levels) is ~ 18:1 at moderate injection current levels. Milliwatt level output powers have been achieved for these deep UV LEDs.

This work focuses on the development of materials and growth techniques suitable for future spintronic device applications. Metal-organic chemical vapor deposition (MOCVD) was used to grow high-quality epitaxial films of varying thickness and manganese doping levels by introducing bis-cyclopentadienyl as the manganese source. High-resolution X-ray diffraction indicates that no macroscopic second phases are formed during growth, and Mn containing films are similar in crystalline quality to undoped films Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. The mean surface roughnesses of optimally grown Ga_1-xMn_xN films were almost identical to that from the as-grown template layers, with no change in growth mechanism or morphology. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the structural, magnetic and opto-electronic properties. SQUID measurements showed an apparent ferromagnetic hysteresis behavior which persisted to room temperature. An optical absorption band around 1.5 eV was observed via transmission studies. This band is assigned to the internal Mn³⁺ transition between the ⁵E and the partially filled ⁵T₂ levels of the ⁵D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn³⁺ to Mn²⁺ was found to effectively suppress these transitions resulting in a reduction of the magnetization. The structural quality, and the presence of Mn²⁺ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions indicative of clustering were observed. The absence of doping-induced strain in Ga_1-xMn_xN was observed by Raman spectroscopy.

Femtosecond time-resolved and continuous wave optical techniques have been used to study fundamental nanoscale materials issues in III-nitride semiconductors relevant to the realization of high quality ultraviolet light emitters and photodetectors. It is demonstrated that compositional fluctuations in AlGaN active regions grown by plasma-assisted MBE can be employed to create nanoscale spatial localization that enhances the luminescence efficiency and PL lifetime (300-400 ps) despite high defect density (>10¹⁰cm^-2) by inhibiting movement of carriers to nonradiative sites. Significant enhancement of this phenomenon has been obtained in a DH LED structure grown on a lower defect density (mid-10⁹cm^-2) AlGaN template, with PL lifetime increased by nearly a factor of two, corresponding to a defect density in the mid-10⁷ cm^-2 range, and only a 3.3 times drop in PL intensity when the temperature is raised from 12 K to room temperature, suggesting up to ~ 30% internal quantum efficiency. Femtosecond, time-resolved electroabsorption measurements of nanoscale high field transport in an AlGaN/GaN heterojunction p-i-n diode show an onset of velocity overshoot at an electric field of ~105 kV/cm for transport in the c-direction of wurzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ~300 kV/cm this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass. In addition, these calculations show that similar behavior is not expected for transport in the basal plane until much higher fields are attained, with important implications for the design of high power, high frequency electronics and avalanche photodetectors.

The development of ZnO nanoparticles in poly(styrene-acrylic acid), [PS]_m/[PAA]_n, diblock copolymer on large area Si wafers is reviewed and the study of ZnO nanoparticle properties in PS-PAA copolymer system with different parameters, m/n of 159/63 and 106/17, is reported. These diblock copolymers are expected to exhibit spherical nanosized domains of the acrylic acid blocks due to microphase separation in solid phase but with domain size and density dependent on their respective molecular block lengths. Self-assembly of ZnO nanoparticles within these domains enabled control of nanoparticle size and density. The self-assembly process involved the doping of copolymer with ZnCl₂ precursor followed by delivery of nanocomposite films onto wafers by a spincasting process and subsequent conversion of ZnCl₂ to ZnO. The chemical analysis of these nanocomposite films and nanoparticles were performed using X-ray Photoelectron spectroscopy (XPS) which verified the conversion to ZnO. Imaging of the nanocomposite films with atomic force microscopy (AFM) showed that particles in [PS]₁₅₉/[PAA]₆₃ and [PS]₁₀₆/[PAA]₁₇ have a size distribution of 250-350 nm and 10-60 nm, respectively. Effect of precursor overdoping on size distribution of the nanoparticles is also discussed.

GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. This problem has its origin in (1) intrinsic factors such as GaN's relatively low exciton binding energy (~24meV) and (2) extrinsic factors including the poor availability of native substrates good enough to significantly suppress the defect density. Indeed, the quality and availability of large-area bulk GaN substrates is currently considered a key problem for the continuing development of improved GaN-based devices. Since development of bulk GaN substrates of suitable quality has proven very difficult, a considerable amount of effort is also being directed towards the development of alternative substrates which offer advantages compared to those in widespread use (c-sapphire and 6H SiC). ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED.

The growth and overlap of research in the fields of photonic crystals (PhCs) and nonlinear optics should lead to new exciting, active functions and high-efficiency nonlinear optical applications because nonlinear optical effects in PhCs may allow for significant advances in various optical processes by using resonantly stored light and anomalous band dispersions, in combination with nonlinear optical host materials. To show the experimental and theoretical evidences of great potentials of the nonlinear PhCs for the realization of the high-efficient and very compact nonlinear optical devices and applications, we have examined the typical second- and third-order nonlinear optical responses and the photonic band structure of the nonlinear two-dimensional (2D) PhC waveguides. We have demonstrated for the first time the second harmonic generation (SHG), the sum-frequency mixing (SFM) in the ultraviolet (UV) region, and the strong enhancement of the SHG intensity originating from the photonic band resonance in this waveguide. Moreover, by probing the nonlinear optical changes of the band resonances in angle-resolved reflectivity,we have shown direct evidence that the nonlinear optical changes arising from modifications of the photonic bands by purely optical means are dominated by the dispersion nature and the group velocity of the photonic bands, which are essential for the realization of desirable nonlinear applications, such as practical all-optical switching devices with very low operational power and ultra-small dimensions. These results agree well with the bhavior predicted from band structures, indicating that the design of nonlinear optical properties of material systems is realistically possible by band dispersion and group velocity engineering.

One of the most important concerns in photonic integrated circuits is the reduction of the propagation path for optical interconnections leading so to a size decrease of active and passive components such as filters or multichannel wavelength (de)multiplexers. In this aim, it is necessary to use monomode optical waveguides with a high optical confinement via a strong index contrast. So the structures reported here consist in 300 nm-wide GaAs nano-wires and nano-gratings inserted in a polymer matrix. The technological process we present hereby allows fabricating symmetric waveguides with square cross-section.

Electric currents flowing perpendicularly through a spin valve produce a spin transfer torque on magnetization of the free layer. The spin torque displays many unique features that are different from the magnetic field driven torques. This article reviews current understanding on the current-driven magnetization dynamics, with the emphasis on chaotic dynamics.

Experiments of spin injection, from a normal metal to a ferromagnet, show that the magnetization of the ferromagnet can be switched with high current pulses, without the need of a magnetic field. In order to describe the experimental observations, two different kinds of electronic populations are defined for the transport: the spin polarized current inside the normal metal (paramagnetic spin-polarized current) and the spin-polarized current inside the ferromagnet (ferromagnetic spin polarization). The redistribution of the electronic populations is then investigated in the framework of a four-channel approach (up and down spin polarization for paramagnetic and ferromagnetic currents). This mechanism couples the electronic transport (Boltzmann equations to the dynamics of the magnetization (Landau-Lifshitz-Gilbert equation), and leads to define a magnetic temperature which depends on the current injection.

Photonic crystals have a great deal of potential in the area of integrated optics. Waveguides with small bends may be formed allowing compact integrated photonic circuits to be formed. Two-dimensional planar photonic crystals are examined for use as integrated photonic circuits. They, however, possess an inherent loss associated with coupling to radiation modes in the surrounding media. Thus, three-dimensional photonic circuits are required in order to realize very low loss integrated photonic crystal circuits. A three-dimensional photonic crystal that is only a few periods thick, quasi-3-D, and can support low loss waveguides embedded in it needs to be identified. We examine several candidate 3-D photonic crystal structures to determine a suitable structure for the fabrication of quasi-3-D photonic crystals.

We present our studies on quantum efficiency (QE), dark counts, and noise equivalent power (NEP) of the latest generation of nanostructured NbN superconducting single-photon detectors (SSPDs) operated at 2.0 K. Our SSPDs are based on 4 nm-thick NbN films, patterned by electron beam lithography as highly-uniform 100÷120-nm-wide meander-shaped stripes, covering the total area of 10x10 μm² with the meander filling factor of 0.7. Advances in the fabrication process and low-temperature operation lead to QE as high as ~30-40% for visible-light photons (0.56 μm wavelength)-the saturation value, limited by optical absorption of the NbN film. For 1.55 μm photons, QE was ~20% and decreased exponentially with the wavelength reaching ~0.02% at the 5-μm wavelength. Being operated at 2.0-K temperature the SSPDs revealed an exponential decrease of the dark count rate, what along with the high QE, resulted in the NEP as low as 5x10^-21 W/Hz^-1/2, the lowest value ever reported for near-infrared optical detectors. The SSPD counting rate was measured to be above 1 GHz with the pulse-to-pulse jitter below 20 ps. Our nanostructured NbN SSPDs operated at 2.0 K significantly outperform their semiconducting counterparts and find practical applications ranging from noninvasive testing of CMOS VLSI integrated circuits to ultrafast quantum communications and quantum cryptography.

We will discuss the advantages of the realization of SiC/SiO₂ quantum structures and their optical absorption properties. Our calculations suggest that it is possible to extend the range of the optical absorption of such structures to build applications that operates from infrared to UV spectrum using a single material and band-structure engineering.

In this work, we will discuss Si-/SiGe and high-κ oxides nanostructures. The exciton properties of strained Si/Si_1-χGe_χ quantum well (QW) are calculated taking into account interface effects and both possibilities of the band lineup of the conduction-band offset, type-I and type-II. Our numerical results show that interface fluctuations of only 10 Å in a Si/Si_1-χGe_χ 50 Å type-I QW (type-II QW) leads to a 25 meV (10 meV) blueshift of the exciton (transition) energy. Concerning high- κ nanostructures, our simulation was performed in order to analyse how the charge image effects can modify the electronics properties in Si/HfO₂ and Si/SrTiO₃ based quantum wells. The results of Si/SrTiO₃ (Si/HfO₂) quantum wells indicate a recombination emission difference, as compared with the case when no charge image effects are included, of the order of 1.5 eV (0.7 eV).

The consequences of the use of high-k dielectrics in nanocrystal based non-volatile flash memories focusing on the electrical and electronic properties are investigated through computational simulations. In the light of these results, we discuss several aspects which must be addressed for the design of such devices. We focus on nanocrystals flash memories with HfO₂ and SiO₂ for analysis. Due to significant reductions of the single-electron tunneling time and improvements on the data retention, high-k dielectrics offers important improvements for the non-volatile flash memories technology.

Si_1-xGe_x is a good candidate as a substitute material for Si in a low-power and high-speed semiconductor device technologies. Optical devices, such as heterojunction bipolar transistors, are already in industrial production. The samples are grown on Si(001) with both n- and p-type impurities and with different Ge concentrations. The linear optical response of Si_1-xGe_x is investigated theoretically using a full-potential linearized augmented plane wave method with respect to composition x. The calculated real and imaginary parts of the dielectric function ε(ω) = ε₁(ω) + iε₂(ω) were found to be in good agreement with recent spectroscopic ellipsometry measurements performed by Bahng et al., J. Phys.: Condens. Matter 13, 777 (2001). We also perform absorption measurements for different type of samples showing the variation of energy gaps as a function of Ge concentrations.

The effects of period, geometry, and thickness on optically thick metal films perforated with subwavelength apertures have been an area of recent investigation, both experimentally and theoretically. It has been shown that the spectra of these films can be scaled proportionally to the period. Different geometries can change peak positions, and the thickness of a metal film can determine the degree of transmission for the entire spectrum. This tunability allows the positioning of a single peak to a wavelength of interest, but the position of all other peaks are dependent on the peak of interest and cannot be positioned independently. Further, the peak intensities are all related. This lack of individual control over specific peaks limits the future applicability of plasmonic films in optoelectronic and photonic devices. Transmission spectra that can be tuned by the polarization of the incident light can be fabricated with novel array structures. Further, control over the transmission spectra can be gained by using highly anisotropic apertures to modifying specific surface plasmon polariton modes and their associated peaks. Surface plasmon polariton modes that are not in the direct path of these anisotropic apertures are not significantly affected. This allows for controlled engineering of the intensity of peaks within the visible, greatly increasing the tailorability of the spectral characteristics of plasmonic films.

We review recent progress in the growth and characterization of Si_1-xGe_x islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si_1-xGe_x coverage, and the effect of growth parameters or post-growth annealing on the shape of the islands and dots. We outline some of the structural, and optical properties of Si_1-xGe_x islands and review recent progress in the determination of their composition and strain distribution. We describe the use of undulated Si_1-xGe_x islands superlattices for infrared detection at telecommunication wavelengths. We discuss various approaches currently being investigated to engineer Si_1-xGe_x quantum dots and in particular control their size, density and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence nucleation and growth of Ge islands. We also show how low temperature Si homoepitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

The ability to control and transform the morphology and optical properties of porous silicon (PS) interface arrays can have important implications for displays and sensors. The optoelectronic properties and interaction sensing capabilities of PS can be varied as a function of pore size and pore morphology as dictated by the manipulation of a surface structure which can be controlled with current density, the solution composition of the electrochemical etches used to prepare the pores, and careful post etch surface treatments. These treatments influence the time dependent photoluminescence (PL) emission from PS. The variation of surface structure as it effects the PL from these PS pore arrays is outlined and discussed within the framework of detailed molecular electronic structure calculations which model the excited state structure giving rise to the UV-visible PL emission from PS. Applications varying from displays to sensors to micro-reactors will be considered. Porous silicon interfaces have also been transformed within the framework of nanotechnology to create highly efficient sensors displaying a rapid, reversible, sensitive, and selective response to HCl, NH₃, CO, and NO down to the ppb level. Photoluminescence induced metallization is used to obtain a highly efficient, <20Ω, electrical contact to the sensor, allowing operation at a bias voltage of 1-10mV. The introduction of gold and tin-based nanostructures to a micro/nanoporous PS array selectively modifies its impedance response to considerably improve the detection of the NH₃, CO, and NO. Through FFT analysis, a gas response can be acquired and filtered on a drifting baseline, further improving sensitivity. New nanoscale exclusive techniques have been developed for the rapid formation of highly efficient TiO_2-xN_x nanophotocatalysts, operative and tunable throughout the visible wavelength region, to be used in conjunction with porous silicon hybrid nanopore coated micropores to form novel and efficient solar pumped sensor arrays.

Since the invention of the Quantum Cascade Laser (QCL) a decade ago an impressive progress has been achieved from first low temperature pulsed laser emission to continuous wave operation at room temperature. Distributed feedback (DFB) lasers working in pulsed mode at ambient temperatures and covering a broad spectral range in the mid infrared (MIR) are commercially available now. For many industrial applications e.g. automotive exhaust control and process monitoring, laser spectroscopy is an established technique, generally using near infrared (NIR) diode lasers. However, the mid infrared (MIR) spectral region is of special interest because of much stronger absorption lines compared to NIR. The status of QCL devices, system development and applications is reviewed. Special emphasis is given to the situation in Europe where a remarkable growth of QCL related R&D can be observed.

Type-I diode lasers based on the (AlGaIn)(AsSb) material family are ideally suited to cover the 2-3 μm wavelength range. In this paper recent progress in terms of output power, beam quality and wavelength tunability is reported, achieved for broad-area and tapered single emitters as well as for linear broad-area laser arrays. Special attention has been paid to the reduction of the fast axis far-field beam divergence, employing improved vertical waveguide laser designs. Furthermore, tapered diode lasers have been developed in order to increase the slow axis beam quality at high output powers. An improved beam quality is of particular importance as many applications, including coupling the laser output into an optical fiber or into an external resonator, require diode lasers with a low beam divergence and a high brightness rather than sheer output power.