Extrinsic effects on the DC output characteristics of AlGaN/GaN HFETs with 1 m gate lengths are examined. The devices investigated were fabricated on MOCVD-grown AlGaN/ GaN heterostructures on sapphire substrates. An analytical model that takes into account parasitic resistances and thermal effects is constructed, and its results are compared with experimental data. With parameters determined from characterization experiments on the same wafer and from independent theoretical results, the agreement between the data and the model predictions is found to be very good. The model is then applied to performance predictions for devices with improved series resistances and heat sinking.

The structural and electronic properties of wide-bandgap semiconductors consisting of aluminum, gallium, and indium nitrides and their alloys make them excellent candidates for use in constructing solidstate optical devices operating from the visible to the ultraviolet. Fabrication of these materials is being approached from two perspectives: reactive magnetron sputtering and metal-organic chemical vapor deposition. Successful synthesis of these metal nitrides will ultimately lead to simple (light-emitting diode) and complex (laser) sources, nonlinear optical elements, and semiconductor detectors, all matched to the wavelength of the spectral region of interest.

: In this report, we compare the performance of the Lucas-Kanade algorithm with feature tracking with the bioplausible optical flow algorithm to achieve pose stabilization in the context of one-dimensional (1-D) attitude stabilization. These results have been benchmarked against an ideal controller in both simulation and robotic experimentation. Within both environments, the accuracy, stability, and settling time were evaluated as a function of contrast and system noise. Both algorithms achieved reasonable performance when compared to the ideal controller, but in most cases the Lucas-Kanade algorithm outperformed the bioplausible algorithm. Within the evaluated serial implementation, the Lucas-Kanade algorithm was also faster. It is anticipated that both performance and processing speed will improve for the bioplausible algorithm when it is implemented in a parallel instantiation.

: Due to increasing complexity, multi-material incompatibilities, and size constraints of electronics, no single monolithic technology can adequately realize the full spectrum of Army-relevant applications. The integration of a wide range of devices, each fabricated in different technologies and performing a well-defined task, would enable the construction of high performance systems. We use a custom microfabrication process flow to align dies within a silicon template wafer to obtain a common planar surface between them. The resulting silicon wafer with embedded chips would then be compatible with the U.S. Army Research Laboratory s (ARL) high quality factor (Q), state-of-the-art multilevel copper electroplating process. This approach permits high density interconnects and through silicon vias, reduces pad parasitics and allows researchers to construct highly miniaturized, modular heterogeneous platforms for Army systems. Future work will include testing and characterization of prototype heterogeneously integrated designs.

The study of brain-computer interfaces (BCIs) has undergone 30 years of intense development and has grown into a rich and diverse field. BCIs are technologies that enable direct communication between the human brain and external devices. Conventionally, wet electrodes have been employed to obtain unprecedented sensitivity to high-temporal-resolution brain activity; recently, the growing availability of various sensors that can be used to detect high-quality brain signals in a wide range of clinical and everyday environments is being exploited. This development of biosensing neurotechnologies and the desire to implement them in real-world applications have led to the opportunity to develop augmented BCIs (ABCIs) in the upcoming decades. An ABCI is similar to a BCI in that it relies on biosensors that record signals from the brain in everyday environments; the signals are then processed in real time to monitor the behavior of the human. To use an ABCI as a mobile brain imaging technique for everyday, real-life applications, the sensors and the corresponding device must be lightweight and the equipment response time must be short. This study presents an overview of the wide range of biosensor approaches currently being applied to ABCIs, from their use in the laboratory to their application in clinical and everyday use. The basic principles of each technique are described along with examples of current applications of cutting-edge neuroscience research. In summary, we show that ABCI techniques continue to grow and evolve, incorporating new technologies and advances to address ever more complex and important neuroscience issues, with advancements that are envisioned to lead to a wide range of real-life applications.

: Passively powered devices are becoming more essential when designing centimeter-scale communication capable devices. Power components are often the majority of the weight in small robots, so eliminating the onboard power source will give engineers and scientists the ability to scale down devices. An X-band wafer level antenna on a 16- by 16-mm wafer that will harvest power from a base station with the intent of providing drive current to a low-power device such as a spin torque nano-oscillator (STNO) has been designed. A circuit that will rectify the propagating radio frequency (RF) waves and provide the STNO with a constant DC drive current was also designed. The STNO output radiation consists of an amplitude- and frequency-modulated signal that allows data to be transmitted from the STNO. Remotely powering the STNO will enable it to transmit data on the condition of a battlefield sensor back to the base station. This research was performed in the summer of 2009 as part of the Science Outreach for Army Research (SOAR) program.

Hydrogen removal from Mg-doped GaN is necessary to activate p-type conductivity, but the exact chemical process is not yet clear. We have investigated this issue by monitoring the intensity of an electron paramagnetic resonance (EPR) signal attributed to Mg through a series of isochronal and isothermal anneals between 600 and 1000 °C. Measurements made on GaN:Mg epitaxial layers deposited on SiC and annealed between 700 and 850 °C indicate that the Mg-related EPR signal increases with temperature as expected for depassivation of a Mg complex by removal of hydrogen. However, data obtained outside this temperature range suggest that additional processes may occur. For example, as-deposited films contain a signal resembling the Mg acceptor that is quenched by a 650 °C N 2 anneal. Also, for all samples, N 2 annealing at T>850 °C irreversibly decreases the signal thought to be due to Mg. Although the presence of the signal in the as-grown films is not fully understood, the effects observed at T>850 °C may be attributed to preferential N-desorption from Mg-N-H complexes.

High dose Si has been implanted into MOCVD grown high resistivity and n-type GaN in the 26–500°C temperature range. The implant activation varies widely (30 -> 100%) depending on, what energy level is assigned to the Si, the implantation and annealing temperatures, and the quality of the substrate. The usable maximum temperature for activation is limited by the severe decomposition of the GaN. After l050°C 15s RTA Ga liquid droplet formation has been observed by SEM. This decomposition changes the surface morphology but did not introduce measurable change in the electrical properties up to 1150°C /120s RTA.

The lack of a suitable, lattice matched substrate for the growth of the group III nitrides typically restricts GaN film growth to substrates such as sapphire or SiC, despite the large lattice and thermal mismatch. With the use of AlN or GaN nucleation layers (NL), GaN films of sufficient quality have been produced for blue LEDs. However, for laser and large-area microwave device applications, the large number of dislocations (> 10 8 cm −2 ) limit device performance, and techniques are desired to reduce dislocation density during the growth process. Here, we demonstrate how low temperature AlN interlayers (IL) sandwiched between high temperature (HT) GaN layers can be used to improve the electrical, optical, and structural properties of Si doped GaN films. A nearly two-fold increase in mobility is observed in Si doped GaN grown using 5 AlN IL compared to GaN grown on a single AlN NL. For GaN films grown on multiple AlN IL, cross-sectional transmission electron microscopy images reveal a significant reduction in the screw dislocation density and photoluminescence spectra reveal a reduction in yellow band intensity. An analysis of the electrical data based on a single donor/single acceptor model suggests that the improved electron mobility is the result of a reduced acceptor concentration in the top GaN film. The reduction in the calculated acceptor concentration may be associated with the reduction of the screw dislocation density.

We have shown the ability to grow thin, high mobility, GaN channel layers on high quality, highly resistive GaN. The growth, characteristics, and device results of two types of MESFET structures were discussed. The first device structure consists of a 2000Ǻ, 2 × 1017cm−3 Si-doped channel layer, grown on 3µm of highly resistive GaN, while die second structure (recessed-gate MESFET) had a 1000Ǻ, 1 × 1018cm−3 Si-doped, n+ capping layer deposited on a 2000Ǻ, 2 × 1017cm−3 Si-doped channel layer. The first MESFET structure was operational at 500°C which is the highest reported operating temperature for a GaN device while the recessed-gate MESFET had a gmas high as 41mS/mm, which is the highest reported value for a GaN MESFET.

We report the dielectric functions of various GaN samples as measured by spectroscopic ellipsometry. Structure related to the A and B excitons is resolved at room temperature, in principle allowing strain to be assessed. However, the data indicate that dead-layer and dispersion effects are present, preventing a simple interpretation. We discuss various complications including the Edn/dE contribution to dispersion, which is important for laser action. Our data appear to indicate that the spin-orbit splitting of GaN is about 15 meV, somewhat larger than the currently accepted value of about 11 meV.

Excitonic recombination processes in GaN films grown by low pressure metalorganic chemical vapor deposition technique have been studied in the temperature range between 6K to 320K by photoluminescence spectroscopy. Low temperature photoluminescence spectra of high resistivity films show well-resolved spectral features associated with the excitonic interband transitions. A detailed spectral analysis allowed us to estimate the exciton binding energy and the energy gap. Spectral studies of Si-doped GaN films demonstrate that the high energy recombination processes in these films are dominated by excitons bound to neutral Si donors. Comparison between the recombination channels in high resistivity and in Si-doped films indicated that Si has a larger exciton binding energy than the unknown pervasive donor in undoped materials. These results confirm the excellent electronic properties of the undoped and doped films.

: The field of biomimetics has grown in recent years as interest in using biology as an inspiration for technology has grown. Biology constantly optimizes mechanisms, materials, and integrated systems through natural selection. These systems and materials can be incorporated into a variety of applications, using the technology that nature has already developed as a launch point for novel solutions to engineering problems. Nature's mechanisms accomplish a variety of sensory, communications, and processing functions in low signal-to-noise ratio environments on the millimeter- to centimeter-scale, using very limited amounts of power. Many of these biological analogs function more reliably and are more sophisticated than the engineered systems that current technology can provide. This survey of selected biological analogs for low-power communication suggests mechanisms that biology has used to communicate and that could be realized in millimeter- to centimeter-scale engineered autonomous systems, with the objective of providing biomimetic inspiration for future technologies.

The evolution of ZnO nanowires has been studied under supersaturation of Zn metal species with and without a ZnO thin-film buffer layer on α-Al2O3 deposited by the pulsed laser ablation technique. The nanowires had diameters in the range of 30 nm to 50 nm and lengths in the range of 5 μm to 10 μm with clear hexagonal shape and \( [000\bar{1}] \), \( [10\bar{1}1] \), and \( [10\bar{1}0] \) facets. X-ray diffraction (XRD) measurements indicated crystalline properties for the ZnO nanostructures grown on pulsed laser deposition (PLD) ZnO nucleation layers. The optical properties were analyzed by photoluminescence (PL) and cathodoluminescence (CL) measurements. The ZnO nanowires were found to emit strong ultraviolet (UV) light at 386 nm and weak green emission as observed by PL measurements. The stoichiometry of Zn and O was found to be close to 1 by x-ray photoelectron spectroscopy (XPS) measurements. The process-dependent growth properties of ZnO nanostructures can be harnessed for future development of nanoelectronic components including optically pumped lasers, optical modulators, detectors, electron emitters, and gas sensors.

This paper investigates the generation of power at terahertz frequencies by a single-walled semiconducting carbon nanotube (s-SWCNT). The prediction of negative differential resistance (NDR) in s-SWCNTs allows for their consideration as a Gunn-type oscillator. Here we consider the regime of limited-space-charge accumulation within nanotubes biased with a potential along the tube axis. This regime minimizes the growth of high-field domain regions, which may be destructive to the nanotube, and allows for efficient high-power operation. Results show that a high-power, efficient, miniaturized, room temperature source of terahertz radiation is possible by appropriate biasing of the s-SWCNT element in the NDR region of operation. Nanotubes of diameter (d) 0.8–4.5 nm are considered. The generated ac power (Pac) is found to range in the μW/μm range, reaching values as high as 13 μW/μm at high bias and small diameter. Very large generation efficiencies (η) were found with a maximum value of 20% at high bias and small d. For a fixed dc bias field to NDR threshold field ratio, performance parameters are found to decrease with increasing s-SWCNT tube diameter as Pac∼d−2 and η∼d−1/3. Frequencies of operation where found to span the terahertz regime, indicating that a s-SWCNT may serve as the active element in terahertz oscillator diodes.

: In this report, we describe the complex impedance of spin torque nano oscillator (STNO) devices. We determined that the STNO is a nonreactive, real-resistance device with single-valued resistance in the broadband frequency range of 500 MHz to 10 GHz (the limit of our test conditions), and that STNOs do not require an external circuit or conjugate matching to operate in a transmitter configuration. We report the first demonstration of the low-power (250 pW), high-frequency (9 GHz) microwave output from an antenna-coupled discrete 50 nm diameter magnetic STNO radiating through air over a distance of 1 m. Amplitude and frequency modulation of the output radiation was used to transmit information from the STNO through microwave antennas. In addition to our identification of the STNO as a highly unusual broadband component that is frequency agile over at least four octaves of frequency without conjugate matching, the STNO is inherently radiation hard with an extremely low operating voltage (<0.25 V) compared to solid-state electronic devices including field-effect transistors (FETs), impact ionization avalanche transit time (IMPATT) diodes, and Gunn diodes. The present results establish the viability of using this class of nanoelectronic devices for frequency-agile communications applications at high frequencies.

We simulate hot optical phonon decay in small diameter (<0.7 nm) carbon nanotubes, solving the phonon Boltzmann transport equation using Monte Carlo methods incorporating the full phonon spectrum and phonon-phonon scattering. Results indicate decay times inversely proportional to the lattice temperature with negligible dependence on hot phonon polarization/wave vector or nanotube diameter and chirality. Nonequilibrium optical phonons with energies of ∼0.2 eV decay by emitting two ∼0.1 eV optical phonons, corresponding to out-of-plane polarization modes in graphitic materials. Modes polarized perpendicular to the nanotube axis may allow manipulation of hot phonon effects by the near environment of the nanotube.

This paper investigates the electronic properties of single-walled carbon nanotube field-effect transistors (SWCNT-FETs) in which the SWCNT element is coated with a charged dielectric. The presence of remote charge on the surface of the dielectric is considered to effect carrier transport in the nanotube as a result of both carrier-scattering and gate screening. Nanotube device characteristics are simulated using the multi- subband Boltzmann transport method incorporating scattering from both phonons and remote charges. This allows assessment of the sensitivity of a nanotube FET to the presence of a charged dielectric coating during room temperature operation. Results show remote charge scattering affects the diameter (d) dependence of the peak conductance and peak field-effect mobility of carbon nanotube devices. Under phonon-limited transport conditions, these peak values increase as ~ d and ~ d 2 , respectively. When remote charge scattering is significant, peak values cease to vary with diameter once a critical diameter reached. Charge scattering is found to particularly degrade device current at gate voltages that allow carriers scattering into or out of a subband minimum. Furthermore, simulations show that intersubband scattering resulting from asymmetry in the circumferential remote charge density becomes increasingly important as the nanotube length decreases. The authors propose that remote charge scattering effects may be applicable in sensing devices allowing for the identification of the charge on a functionalized CNT coating.