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Sample records for accelerator network gan

  1. Remote operations in a global accelerator network

    SciTech Connect

    Peggs, Steve; Satogata, Todd; Agarwal, Deborah; Rice, David

    2003-05-08

    The INTRODUCTION to this paper summarizes the history of the Global Accelerator Network (GAN) concept and the recent workshops that discussed the relationship between GAN and Remote Operations. The REMOTE OPERATIONS SCENARIOS section brings out the organizational philosophy embodied in GAN-like and to non-GAN-like scenarios. The set of major TOPICS RAISED AT THE WORKSHOPS are only partially resolved. COLLABORATION TOOLS are described and discussed, followed by examples of REMOTE ACCELERATOR CONTROL PROJECTS around the world.

  2. REMOTE OPERATIONS IN A GLOBAL ACCELERATOR NETWORK

    SciTech Connect

    PEGGS,S.SATOGATA,TAGARWAL,DRICE,D

    2003-05-12

    The INTRODUCTION to this paper summarizes the history of the Global Accelerator Network (GAN) concept and the recent workshops that discussed the relationship between GAN and Remote Operations. The REMOTE OPERATIONS SCENARIOS section brings out the organizational philosophy embodied in GAN-like and to non-GAN-like scenarios. The set of major TOPICS RAISED AT THE WORKSHOPS are only partially resolved. COLLABORATION TOOLS are described and discussed, followed by examples of REMOTE ACCELERATOR CONTROL PROJECTS around the world.

  3. Collaboration tools for the global accelerator network: Workshop Report

    SciTech Connect

    Agarwal, Deborah; Olson, Gary; Olson, Judy

    2002-09-15

    The concept of a ''Global Accelerator Network'' (GAN) has been put forward as a means for inter-regional collaboration in the operation of internationally constructed and operated frontier accelerator facilities. A workshop was held to allow representatives of the accelerator community and of the collaboratory development community to meet and discuss collaboration tools for the GAN environment. This workshop, called the Collaboration Tools for the Global Accelerator Network (GAN) Workshop, was held on August 26, 2002 at Lawrence Berkeley National Laboratory. The goal was to provide input about collaboration tools in general and to provide a strawman for the GAN collaborative tools environment. The participants at the workshop represented accelerator physicists, high-energy physicists, operations, technology tool developers, and social scientists that study scientific collaboration.

  4. Network acceleration techniques

    NASA Technical Reports Server (NTRS)

    Crowley, Patricia (Inventor); Awrach, James Michael (Inventor); Maccabe, Arthur Barney (Inventor)

    2012-01-01

    Splintered offloading techniques with receive batch processing are described for network acceleration. Such techniques offload specific functionality to a NIC while maintaining the bulk of the protocol processing in the host operating system ("OS"). The resulting protocol implementation allows the application to bypass the protocol processing of the received data. Such can be accomplished this by moving data from the NIC directly to the application through direct memory access ("DMA") and batch processing the receive headers in the host OS when the host OS is interrupted to perform other work. Batch processing receive headers allows the data path to be separated from the control path. Unlike operating system bypass, however, the operating system still fully manages the network resource and has relevant feedback about traffic and flows. Embodiments of the present disclosure can therefore address the challenges of networks with extreme bandwidth delay products (BWDP).

  5. Growth of GaN nanowall network on Si (111) substrate by molecular beam epitaxy.

    PubMed

    Zhong, Aihua; Hane, Kazuhiro

    2012-12-27

    GaN nanowall network was epitaxially grown on Si (111) substrate by molecular beam epitaxy. GaN nanowalls overlap and interlace with one another, together with large numbers of holes, forming a continuous porous GaN nanowall network. The width of the GaN nanowall can be controlled, ranging from 30 to 200 nm by adjusting the N/Ga ratio. Characterization results of a transmission electron microscope and X-ray diffraction show that the GaN nanowall is well oriented along the C axis. Strong band edge emission centered at 363 nm is observed in the spectrum of room temperature photoluminescence, indicating that the GaN nanowall network is of high quality. The sheet resistance of the Si-doped GaN nanowall network along the lateral direction was 58 Ω/. The conductive porous nanowall network can be useful for integrated gas sensors due to the large surface area-to-volume ratio and electrical conductivity along the lateral direction by combining with Si micromachining.

  6. GalaxyGAN: Generative Adversarial Networks for recovery of galaxy features

    NASA Astrophysics Data System (ADS)

    Schawinski, Kevin; Zhang, Ce; Zhang, Hantian; Fowler, Lucas; Krishnan Santhanam, Gokula

    2017-02-01

    GalaxyGAN uses Generative Adversarial Networks to reliably recover features in images of galaxies. The package uses machine learning to train on higher quality data and learns to recover detailed features such as galaxy morphology by effectively building priors. This method opens up the possibility of recovering more information from existing and future imaging data.

  7. Accelerating Learning By Neural Networks

    NASA Technical Reports Server (NTRS)

    Toomarian, Nikzad; Barhen, Jacob

    1992-01-01

    Electronic neural networks made to learn faster by use of terminal teacher forcing. Method of supervised learning involves addition of teacher forcing functions to excitations fed as inputs to output neurons. Initially, teacher forcing functions are strong enough to force outputs to desired values; subsequently, these functions decay with time. When learning successfully completed, terminal teacher forcing vanishes, and dynamics or neural network become equivalent to those of conventional neural network. Simulated neural network with terminal teacher forcing learned to produce close approximation of circular trajectory in 400 iterations.

  8. Neural network-based sensor signal accelerator.

    SciTech Connect

    Vogt, M. C.

    2000-10-16

    A strategy has been developed to computationally accelerate the response time of a generic electronic sensor. The strategy can be deployed as an algorithm in a control system or as a physical interface (on an embedded microcontroller) between a slower responding external sensor and a higher-speed control system. Optional code implementations are available to adjust algorithm performance when computational capability is limited. In one option, the actual sensor signal can be sampled at the slower rate with adaptive linear neural networks predicting the sensor's future output and interpolating intermediate synthetic output values. In another option, a synchronized collection of predictors sequentially controls the corresponding synthetic output voltage. Error is adaptively corrected in both options. The core strategy has been demonstrated with automotive oxygen sensor data. A prototype interface device is under construction. The response speed increase afforded by this strategy could greatly offset the cost of developing a replacement sensor with a faster physical response time.

  9. Neural Networks for Modeling and Control of Particle Accelerators

    DOE PAGES

    Edelen, A. L.; Biedron, S. G.; Chase, B. E.; ...

    2016-04-01

    Myriad nonlinear and complex physical phenomena are host to particle accelerators. They often involve a multitude of interacting systems, are subject to tight performance demands, and should be able to run for extended periods of time with minimal interruptions. Often times, traditional control techniques cannot fully meet these requirements. One promising avenue is to introduce machine learning and sophisticated control techniques inspired by artificial intelligence, particularly in light of recent theoretical and practical advances in these fields. Within machine learning and artificial intelligence, neural networks are particularly well-suited to modeling, control, and diagnostic analysis of complex, nonlinear, and time-varying systems,more » as well as systems with large parameter spaces. Consequently, the use of neural network-based modeling and control techniques could be of significant benefit to particle accelerators. For the same reasons, particle accelerators are also ideal test-beds for these techniques. Moreover, many early attempts to apply neural networks to particle accelerators yielded mixed results due to the relative immaturity of the technology for such tasks. For the purpose of this paper is to re-introduce neural networks to the particle accelerator community and report on some work in neural network control that is being conducted as part of a dedicated collaboration between Fermilab and Colorado State University (CSU). We also describe some of the challenges of particle accelerator control, highlight recent advances in neural network techniques, discuss some promising avenues for incorporating neural networks into particle accelerator control systems, and describe a neural network-based control system that is being developed for resonance control of an RF electron gun at the Fermilab Accelerator Science and Technology (FAST) facility, including initial experimental results from a benchmark controller.« less

  10. Neural Networks for Modeling and Control of Particle Accelerators

    SciTech Connect

    Edelen, A. L.; Biedron, S. G.; Chase, B. E.; Edstrom, D.; Milton, S. V.; Stabile, P.

    2016-04-01

    Myriad nonlinear and complex physical phenomena are host to particle accelerators. They often involve a multitude of interacting systems, are subject to tight performance demands, and should be able to run for extended periods of time with minimal interruptions. Often times, traditional control techniques cannot fully meet these requirements. One promising avenue is to introduce machine learning and sophisticated control techniques inspired by artificial intelligence, particularly in light of recent theoretical and practical advances in these fields. Within machine learning and artificial intelligence, neural networks are particularly well-suited to modeling, control, and diagnostic analysis of complex, nonlinear, and time-varying systems, as well as systems with large parameter spaces. Consequently, the use of neural network-based modeling and control techniques could be of significant benefit to particle accelerators. For the same reasons, particle accelerators are also ideal test-beds for these techniques. Moreover, many early attempts to apply neural networks to particle accelerators yielded mixed results due to the relative immaturity of the technology for such tasks. For the purpose of this paper is to re-introduce neural networks to the particle accelerator community and report on some work in neural network control that is being conducted as part of a dedicated collaboration between Fermilab and Colorado State University (CSU). We also describe some of the challenges of particle accelerator control, highlight recent advances in neural network techniques, discuss some promising avenues for incorporating neural networks into particle accelerator control systems, and describe a neural network-based control system that is being developed for resonance control of an RF electron gun at the Fermilab Accelerator Science and Technology (FAST) facility, including initial experimental results from a benchmark controller.

  11. Method Accelerates Training Of Some Neural Networks

    NASA Technical Reports Server (NTRS)

    Shelton, Robert O.

    1992-01-01

    Three-layer networks trained faster provided two conditions are satisfied: numbers of neurons in layers are such that majority of work done in synaptic connections between input and hidden layers, and number of neurons in input layer at least as great as number of training pairs of input and output vectors. Based on modified version of back-propagation method.

  12. Accelerating coordination in temporal networks by engineering the link order

    PubMed Central

    Masuda, Naoki

    2016-01-01

    Social dynamics on a network may be accelerated or decelerated depending on which pairs of individuals in the network communicate early and which pairs do later. The order with which the links in a given network are sequentially used, which we call the link order, may be a strong determinant of dynamical behaviour on networks, potentially adding a new dimension to effects of temporal networks relative to static networks. Here we study the effect of the link order on linear coordination (i.e., synchronisation) dynamics. We show that the coordination speed considerably depends on specific orders of links. In addition, applying each single link for a long time to ensure strong pairwise coordination before moving to a next pair of individuals does not often enhance coordination of the entire network. We also implement a simple greedy algorithm to optimise the link order in favour of fast coordination. PMID:26916093

  13. Modeling of UH-60A Hub Accelerations with Neural Networks

    NASA Technical Reports Server (NTRS)

    Kottapalli, Sesi

    2002-01-01

    Neural network relationships between the full-scale, flight test hub accelerations and the corresponding three N/rev pilot floor vibration components (vertical, lateral, and longitudinal) are studied. The present quantitative effort on the UH-60A Black Hawk hub accelerations considers the lateral and longitudinal vibrations. An earlier study had considered the vertical vibration. The NASA/Army UH-60A Airloads Program flight test database is used. A physics based "maneuver-effect-factor (MEF)", derived using the roll-angle and the pitch-rate, is used. Fundamentally, the lateral vibration data show high vibration levels (up to 0.3 g's) at low airspeeds (for example, during landing flares) and at high airspeeds (for example, during turns). The results show that the advance ratio and the gross weight together can predict the vertical and the longitudinal vibration. However, the advance ratio and the gross weight together cannot predict the lateral vibration. The hub accelerations and the advance ratio can be used to satisfactorily predict the vertical, lateral, and longitudinal vibration. The present study shows that neural network based representations of all three UH-60A pilot floor vibration components (vertical, lateral, and longitudinal) can be obtained using the hub accelerations along with the gross weight and the advance ratio. The hub accelerations are clearly a factor in determining the pilot vibration. The present conclusions potentially allow for the identification of neural network relationships between the experimental hub accelerations obtained from wind tunnel testing and the experimental pilot vibration data obtained from flight testing. A successful establishment of the above neural network based link between the wind tunnel hub accelerations and the flight test vibration data can increase the value of wind tunnel testing.

  14. Accelerated Training for Large Feedforward Neural Networks

    NASA Technical Reports Server (NTRS)

    Stepniewski, Slawomir W.; Jorgensen, Charles C.

    1998-01-01

    In this paper we introduce a new training algorithm, the scaled variable metric (SVM) method. Our approach attempts to increase the convergence rate of the modified variable metric method. It is also combined with the RBackprop algorithm, which computes the product of the matrix of second derivatives (Hessian) with an arbitrary vector. The RBackprop method allows us to avoid computationally expensive, direct line searches. In addition, it can be utilized in the new, 'predictive' updating technique of the inverse Hessian approximation. We have used directional slope testing to adjust the step size and found that this strategy works exceptionally well in conjunction with the Rbackprop algorithm. Some supplementary, but nevertheless important enhancements to the basic training scheme such as improved setting of a scaling factor for the variable metric update and computationally more efficient procedure for updating the inverse Hessian approximation are presented as well. We summarize by comparing the SVM method with four first- and second- order optimization algorithms including a very effective implementation of the Levenberg-Marquardt method. Our tests indicate promising computational speed gains of the new training technique, particularly for large feedforward networks, i.e., for problems where the training process may be the most laborious.

  15. Explicit integration with GPU acceleration for large kinetic networks

    DOE PAGES

    Brock, Benjamin; Belt, Andrew; Billings, Jay Jay; ...

    2015-09-15

    In this study, we demonstrate the first implementation of recently-developed fast explicit kinetic integration algorithms on modern graphics processing unit (GPU) accelerators. Taking as a generic test case a Type Ia supernova explosion with an extremely stiff thermonuclear network having 150 isotopic species and 1604 reactions coupled to hydrodynamics using operator splitting, we demonstrate the capability to solve of order 100 realistic kinetic networks in parallel in the same time that standard implicit methods can solve a single such network on a CPU. In addition, this orders-of-magnitude decrease in computation time for solving systems of realistic kinetic networks implies thatmore » important coupled, multiphysics problems in various scientific and technical fields that were intractable, or could be simulated only with highly schematic kinetic networks, are now computationally feasible.« less

  16. Explicit integration with GPU acceleration for large kinetic networks

    SciTech Connect

    Brock, Benjamin; Belt, Andrew; Billings, Jay Jay; Guidry, Mike W.

    2015-09-15

    In this study, we demonstrate the first implementation of recently-developed fast explicit kinetic integration algorithms on modern graphics processing unit (GPU) accelerators. Taking as a generic test case a Type Ia supernova explosion with an extremely stiff thermonuclear network having 150 isotopic species and 1604 reactions coupled to hydrodynamics using operator splitting, we demonstrate the capability to solve of order 100 realistic kinetic networks in parallel in the same time that standard implicit methods can solve a single such network on a CPU. In addition, this orders-of-magnitude decrease in computation time for solving systems of realistic kinetic networks implies that important coupled, multiphysics problems in various scientific and technical fields that were intractable, or could be simulated only with highly schematic kinetic networks, are now computationally feasible.

  17. Explicit integration with GPU acceleration for large kinetic networks

    SciTech Connect

    Brock, Benjamin; Belt, Andrew; Billings, Jay Jay; Guidry, Mike

    2015-12-01

    We demonstrate the first implementation of recently-developed fast explicit kinetic integration algorithms on modern graphics processing unit (GPU) accelerators. Taking as a generic test case a Type Ia supernova explosion with an extremely stiff thermonuclear network having 150 isotopic species and 1604 reactions coupled to hydrodynamics using operator splitting, we demonstrate the capability to solve of order 100 realistic kinetic networks in parallel in the same time that standard implicit methods can solve a single such network on a CPU. This orders-of-magnitude decrease in computation time for solving systems of realistic kinetic networks implies that important coupled, multiphysics problems in various scientific and technical fields that were intractable, or could be simulated only with highly schematic kinetic networks, are now computationally feasible.

  18. Linear induction accelerator and pulse forming networks therefor

    DOEpatents

    Buttram, Malcolm T.; Ginn, Jerry W.

    1989-01-01

    A linear induction accelerator includes a plurality of adder cavities arranged in a series and provided in a structure which is evacuated so that a vacuum inductance is provided between each adder cavity and the structure. An energy storage system for the adder cavities includes a pulsed current source and a respective plurality of bipolar converting networks connected thereto. The bipolar high-voltage, high-repetition-rate square pulse train sets and resets the cavities.

  19. The accelerated growth of the worldwide air transportation network

    NASA Astrophysics Data System (ADS)

    Azzam, Mark; Klingauf, Uwe; Zock, Alexander

    2013-01-01

    Mobility by means of air transportation has a critical impact on the global economy. Especially against the backdrop of further growth and an aggravation of the energy crisis, it is crucial to design a sustainable air transportation system. Current approaches focus on air traffic management. Nevertheless, also the historically evolved network offers great potential for an optimized redesign. But the understanding of its complex structure and development is limited, although modern network science supplies a great set of new methods and tools. So far studies analyzing air transportation as a complex network are based on divers and poor data, which are either merely regional or strongly bounded time-wise. As a result, the current state of research is rather inconsistent regarding topological coefficients and incomplete regarding network evolution. Therefore, we use the historical, worldwide OAG flight schedules data between 1979 and 2007 for our study. Through analyzing by far the most comprehensive data base so far, a better understanding of the network, its evolution and further implications is being provided. To our knowledge we present the first study to determine that the degree distribution of the worldwide air transportation network is non-stationary and is subject to densification and accelerated growth, respectively.

  20. Embedded Streaming Deep Neural Networks Accelerator With Applications.

    PubMed

    Dundar, Aysegul; Jin, Jonghoon; Martini, Berin; Culurciello, Eugenio

    2016-04-08

    Deep convolutional neural networks (DCNNs) have become a very powerful tool in visual perception. DCNNs have applications in autonomous robots, security systems, mobile phones, and automobiles, where high throughput of the feedforward evaluation phase and power efficiency are important. Because of this increased usage, many field-programmable gate array (FPGA)-based accelerators have been proposed. In this paper, we present an optimized streaming method for DCNNs' hardware accelerator on an embedded platform. The streaming method acts as a compiler, transforming a high-level representation of DCNNs into operation codes to execute applications in a hardware accelerator. The proposed method utilizes maximum computational resources available based on a novel-scheduled routing topology that combines data reuse and data concatenation. It is tested with a hardware accelerator implemented on the Xilinx Kintex-7 XC7K325T FPGA. The system fully explores weight-level and node-level parallelizations of DCNNs and achieves a peak performance of 247 G-ops while consuming less than 4 W of power. We test our system with applications on object classification and object detection in real-world scenarios. Our results indicate high-performance efficiency, outperforming all other presented platforms while running these applications.

  1. The ADVANCE network: accelerating data value across a national community health center network

    PubMed Central

    DeVoe, Jennifer E; Gold, Rachel; Cottrell, Erika; Bauer, Vance; Brickman, Andrew; Puro, Jon; Nelson, Christine; Mayer, Kenneth H; Sears, Abigail; Burdick, Tim; Merrell, Jonathan; Matthews, Paul; Fields, Scott

    2014-01-01

    The ADVANCE (Accelerating Data Value Across a National Community Health Center Network) clinical data research network (CDRN) is led by the OCHIN Community Health Information Network in partnership with Health Choice Network and Fenway Health. The ADVANCE CDRN will ‘horizontally’ integrate outpatient electronic health record data for over one million federally qualified health center patients, and ‘vertically’ integrate hospital, health plan, and community data for these patients, often under-represented in research studies. Patient investigators, community investigators, and academic investigators with diverse expertise will work together to meet project goals related to data integration, patient engagement and recruitment, and the development of streamlined regulatory policies. By enhancing the data and research infrastructure of participating organizations, the ADVANCE CDRN will serve as a ‘community laboratory’ for including disadvantaged and vulnerable patients in patient-centered outcomes research that is aligned with the priorities of patients, clinics, and communities in our network. PMID:24821740

  2. Accelerated Biofluid Filling in Complex Microfluidic Networks by Vacuum-Pressure Accelerated Movement (V-PAM).

    PubMed

    Yu, Zeta Tak For; Cheung, Mei Ki; Liu, Shirley Xiaosu; Fu, Jianping

    2016-09-01

    Rapid fluid transport and exchange are critical operations involved in many microfluidic applications. However, conventional mechanisms used for driving fluid transport in microfluidics, such as micropumping and high pressure, can be inaccurate and difficult for implementation for integrated microfluidics containing control components and closed compartments. Here, a technology has been developed termed Vacuum-Pressure Accelerated Movement (V-PAM) capable of significantly enhancing biofluid transport in complex microfluidic environments containing dead-end channels and closed chambers. Operation of the V-PAM entails a pressurized fluid loading into microfluidic channels where gas confined inside can rapidly be dissipated through permeation through a thin, gas-permeable membrane sandwiched between microfluidic channels and a network of vacuum channels. Effects of different structural and operational parameters of the V-PAM for promoting fluid filling in microfluidic environments have been studied systematically. This work further demonstrates the applicability of V-PAM for rapid filling of temperature-sensitive hydrogels and unprocessed whole blood into complex irregular microfluidic networks such as microfluidic leaf venation patterns and blood circulatory systems. Together, the V-PAM technology provides a promising generic microfluidic tool for advanced fluid control and transport in integrated microfluidics for different microfluidic diagnosis, organs-on-chips, and biomimetic studies.

  3. The Undiagnosed Diseases Network: Accelerating Discovery about Health and Disease.

    PubMed

    Ramoni, Rachel B; Mulvihill, John J; Adams, David R; Allard, Patrick; Ashley, Euan A; Bernstein, Jonathan A; Gahl, William A; Hamid, Rizwan; Loscalzo, Joseph; McCray, Alexa T; Shashi, Vandana; Tifft, Cynthia J; Wise, Anastasia L

    2017-02-02

    Diagnosis at the edges of our knowledge calls upon clinicians to be data driven, cross-disciplinary, and collaborative in unprecedented ways. Exact disease recognition, an element of the concept of precision in medicine, requires new infrastructure that spans geography, institutional boundaries, and the divide between clinical care and research. The National Institutes of Health (NIH) Common Fund supports the Undiagnosed Diseases Network (UDN) as an exemplar of this model of precise diagnosis. Its goals are to forge a strategy to accelerate the diagnosis of rare or previously unrecognized diseases, to improve recommendations for clinical management, and to advance research, especially into disease mechanisms. The network will achieve these objectives by evaluating patients with undiagnosed diseases, fostering a breadth of expert collaborations, determining best practices for translating the strategy into medical centers nationwide, and sharing findings, data, specimens, and approaches with the scientific and medical communities. Building the UDN has already brought insights to human and medical geneticists. The initial focus has been on data sharing, establishing common protocols for institutional review boards and data sharing, creating protocols for referring and evaluating patients, and providing DNA sequencing, metabolomic analysis, and functional studies in model organisms. By extending this precision diagnostic model nationally, we strive to meld clinical and research objectives, improve patient outcomes, and contribute to medical science.

  4. Accelerated Sensitivity Analysis in High-Dimensional Stochastic Reaction Networks.

    PubMed

    Arampatzis, Georgios; Katsoulakis, Markos A; Pantazis, Yannis

    2015-01-01

    Existing sensitivity analysis approaches are not able to handle efficiently stochastic reaction networks with a large number of parameters and species, which are typical in the modeling and simulation of complex biochemical phenomena. In this paper, a two-step strategy for parametric sensitivity analysis for such systems is proposed, exploiting advantages and synergies between two recently proposed sensitivity analysis methodologies for stochastic dynamics. The first method performs sensitivity analysis of the stochastic dynamics by means of the Fisher Information Matrix on the underlying distribution of the trajectories; the second method is a reduced-variance, finite-difference, gradient-type sensitivity approach relying on stochastic coupling techniques for variance reduction. Here we demonstrate that these two methods can be combined and deployed together by means of a new sensitivity bound which incorporates the variance of the quantity of interest as well as the Fisher Information Matrix estimated from the first method. The first step of the proposed strategy labels sensitivities using the bound and screens out the insensitive parameters in a controlled manner. In the second step of the proposed strategy, a finite-difference method is applied only for the sensitivity estimation of the (potentially) sensitive parameters that have not been screened out in the first step. Results on an epidermal growth factor network with fifty parameters and on a protein homeostasis with eighty parameters demonstrate that the proposed strategy is able to quickly discover and discard the insensitive parameters and in the remaining potentially sensitive parameters it accurately estimates the sensitivities. The new sensitivity strategy can be several times faster than current state-of-the-art approaches that test all parameters, especially in "sloppy" systems. In particular, the computational acceleration is quantified by the ratio between the total number of parameters over the

  5. Transport and optical properties of c-axis oriented wedge shaped GaN nanowall network grown by molecular beam epitaxy

    SciTech Connect

    Bhasker, H. P.; Dhar, S.; Thakur, Varun; Kesaria, Manoj; Shivaprasad, S. M.

    2014-02-21

    The transport and optical properties of wedge-shaped nanowall network of GaN grown spontaneously on cplane sapphire substrate by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) show interesting behavior. The electron mobility at room temperature in these samples is found to be orders of magnitude higher than that of a continuous film. Our study reveals a strong correlation between the mobility and the band gap in these nanowall network samples. However, it is seen that when the thickness of the tips of the walls increases to an extent such that more than 70% of the film area is covered, it behaves close to a flat sample. In the sample with lower surface coverage (≈40% and ≈60%), it was observed that the conductivity, mobility as well as the band gap increase with the decrease in the average tip width of the walls. Photoluminescence (PL) experiments show a strong and broad band edge emission with a large (as high as ≈ 90 meV) blue shift, compared to that of a continuous film, suggesting a confinement of carriers on the top edges of the nanowalls. The PL peak width remains wide at all temperatures suggesting the existence of a high density of tail states at the band edge, which is further supported by the photoconductivity result. The high conductivity and mobility observed in these samples is believed to be due to a “dissipation less” transport of carriers, which are localized at the top edges (edge states) of the nanowalls.

  6. Neural Network Based Representation of UH-60A Pilot and Hub Accelerations

    NASA Technical Reports Server (NTRS)

    Kottapalli, Sesi

    2000-01-01

    Neural network relationships between the full-scale, experimental hub accelerations and the corresponding pilot floor vertical vibration are studied. The present physics-based, quantitative effort represents an initial systematic study on the UH-60A Black Hawk hub accelerations. The NASA/Army UH-60A Airloads Program flight test database was used. A 'maneuver-effect-factor (MEF)', derived using the roll-angle and the pitch-rate, was used. Three neural network based representation-cases were considered. The pilot floor vertical vibration was considered in the first case and the hub accelerations were separately considered in the second case. The third case considered both the hub accelerations and the pilot floor vertical vibration. Neither the advance ratio nor the gross weight alone could be used to predict the pilot floor vertical vibration. However, the advance ratio and the gross weight together could be used to predict the pilot floor vertical vibration over the entire flight envelope. The hub accelerations data were modeled and found to be of very acceptable quality. The hub accelerations alone could not be used to predict the pilot floor vertical vibration. Thus, the hub accelerations alone do not drive the pilot floor vertical vibration. However, the hub accelerations, along with either the advance ratio or the gross weight or both, could be used to satisfactorily predict the pilot floor vertical vibration. The hub accelerations are clearly a factor in determining the pilot floor vertical vibration.

  7. GaN HEMTs

    NASA Astrophysics Data System (ADS)

    Anderson, Jonathan W.; Lee, Kyoung-Keun; Piner, Edwin L.

    2012-03-01

    Gallium nitride (GaN) has enormous potential for applications in high electron mobility transistors (HEMTs) used in RF and power devices. Intrinsic device properties such as high electron mobility, high breakdown voltage, very high current density, electron confinement in a narrow channel, and high electron velocity in the 2-dimensional electron gas of the HEMT structure are due in large part to the wide band gap of this novel semiconductor material system. This presentation discusses the properties of GaN that make it superior to other semiconductor materials, and outlines the research that will be undertaken in a new program at Texas State University to advance GaN HEMT technology. This program's aim is to further innovate the exceptional performance of GaN through improved material growth processes and epitaxial structure design.

  8. Real-time Optical Network for Accelerator Control

    SciTech Connect

    Lee, Keun

    2012-06-27

    The timing requirements of a modern accelerator complex call for several features. The first is a system for high precision relative timing among accelerator components. Stabilized fiber links have already been demonstrated to achieve sub-10 femtoseconds relative timing precision. The second is a system for timing distribution of absolute time with sufficient precision to identify a specific RF bucket. The White Rabbit technology is a promising candidate to deliver the absolute time that is linked to the GPS clock. In this study we demonstrated that these two technologies can be combined in a way that the absolute time information can be delivered to the stabilized fiber link system. This was accomplished by researching the design of the stabilized fiber and White Rabbit systems and devising adaptation modules that facilitate co-existence of both systems in the same FPGA environment. We built a prototype system using off-the-shelf products and implemented a proof-of-concept version of the FPGA firmware. The test verified that the White Rabbit features operate correctly under the stabilized fiber system environment. This work demonstrates that turn-key femtosecond timing systems with absolute time information can be built cost effectively and deployed in various accelerator environments. This will lead to many new applications in chemistry, biology and surface dynamics, to name a few.

  9. Accelerating Network Traffic Analytics Using Query-DrivenVisualization

    SciTech Connect

    Bethel, E. Wes; Campbell, Scott; Dart, Eli; Stockinger, Kurt; Wu,Kesheng

    2006-07-29

    Realizing operational analytics solutions where large and complex data must be analyzed in a time-critical fashion entails integrating many different types of technology. This paper focuses on an interdisciplinary combination of scientific data management and visualization/analysis technologies targeted at reducing the time required for data filtering, querying, hypothesis testing and knowledge discovery in the domain of network connection data analysis. We show that use of compressed bitmap indexing can quickly answer queries in an interactive visual data analysis application, and compare its performance with two alternatives for serial and parallel filtering/querying on 2.5 billion records worth of network connection data collected over a period of 42 weeks. Our approach to visual network connection data exploration centers on two primary factors: interactive ad-hoc and multiresolution query formulation and execution over n dimensions and visual display of then-dimensional histogram results. This combination is applied in a case study to detect a distributed network scan and to then identify the set of remote hosts participating in the attack. Our approach is sufficiently general to be applied to a diverse set of data understanding problems as well as used in conjunction with a diverse set of analysis and visualization tools.

  10. QuateXelero: An Accelerated Exact Network Motif Detection Algorithm

    PubMed Central

    Khakabimamaghani, Sahand; Sharafuddin, Iman; Dichter, Norbert; Koch, Ina; Masoudi-Nejad, Ali

    2013-01-01

    Finding motifs in biological, social, technological, and other types of networks has become a widespread method to gain more knowledge about these networks’ structure and function. However, this task is very computationally demanding, because it is highly associated with the graph isomorphism which is an NP problem (not known to belong to P or NP-complete subsets yet). Accordingly, this research is endeavoring to decrease the need to call NAUTY isomorphism detection method, which is the most time-consuming step in many existing algorithms. The work provides an extremely fast motif detection algorithm called QuateXelero, which has a Quaternary Tree data structure in the heart. The proposed algorithm is based on the well-known ESU (FANMOD) motif detection algorithm. The results of experiments on some standard model networks approve the overal superiority of the proposed algorithm, namely QuateXelero, compared with two of the fastest existing algorithms, G-Tries and Kavosh. QuateXelero is especially fastest in constructing the central data structure of the algorithm from scratch based on the input network. PMID:23874498

  11. Computational models reduce complexity and accelerate insight into cardiac signaling networks.

    PubMed

    Yang, Jason H; Saucerman, Jeffrey J

    2011-01-07

    Cardiac signaling networks exhibit considerable complexity in size and connectivity. The intrinsic complexity of these networks complicates the interpretation of experimental findings. This motivates new methods for investigating the mechanisms regulating cardiac signaling networks and the consequences these networks have on cardiac physiology and disease. Next-generation experimental techniques are also generating a wealth of genomic and proteomic data that can be difficult to analyze or interpret. Computational models are poised to play a key role in addressing these challenges. Computational models have a long history in contributing to the understanding of cardiac physiology and are useful for identifying biological mechanisms, inferring multiscale consequences to cell signaling activities and reducing the complexity of large data sets. Models also integrate well with experimental studies to explain experimental observations and generate new hypotheses. Here, we review the contributions computational modeling approaches have made to the analysis of cardiac signaling networks and forecast opportunities for computational models to accelerate cardiac signaling research.

  12. Accelerate!

    PubMed

    Kotter, John P

    2012-11-01

    The old ways of setting and implementing strategy are failing us, writes the author of Leading Change, in part because we can no longer keep up with the pace of change. Organizational leaders are torn between trying to stay ahead of increasingly fierce competition and needing to deliver this year's results. Although traditional hierarchies and managerial processes--the components of a company's "operating system"--can meet the daily demands of running an enterprise, they are rarely equipped to identify important hazards quickly, formulate creative strategic initiatives nimbly, and implement them speedily. The solution Kotter offers is a second system--an agile, networklike structure--that operates in concert with the first to create a dual operating system. In such a system the hierarchy can hand off the pursuit of big strategic initiatives to the strategy network, freeing itself to focus on incremental changes to improve efficiency. The network is populated by employees from all levels of the organization, giving it organizational knowledge, relationships, credibility, and influence. It can Liberate information from silos with ease. It has a dynamic structure free of bureaucratic layers, permitting a level of individualism, creativity, and innovation beyond the reach of any hierarchy. The network's core is a guiding coalition that represents each level and department in the hierarchy, with a broad range of skills. Its drivers are members of a "volunteer army" who are energized by and committed to the coalition's vividly formulated, high-stakes vision and strategy. Kotter has helped eight organizations, public and private, build dual operating systems over the past three years. He predicts that such systems will lead to long-term success in the 21st century--for shareholders, customers, employees, and companies themselves.

  13. Wafer-scale epitaxial lift-off of optoelectronic grade GaN from a GaN substrate using a sacrificial ZnO interlayer

    NASA Astrophysics Data System (ADS)

    Rajan, Akhil; Rogers, David J.; Ton-That, Cuong; Zhu, Liangchen; Phillips, Matthew R.; Sundaram, Suresh; Gautier, Simon; Moudakir, Tarik; El-Gmili, Youssef; Ougazzaden, Abdallah; Sandana, Vinod E.; Teherani, Ferechteh H.; Bove, Philippe; Prior, Kevin A.; Djebbour, Zakaria; McClintock, Ryan; Razeghi, Manijeh

    2016-08-01

    Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.

  14. To Break or to Brake Neuronal Network Accelerated by Ammonium Ions?

    PubMed Central

    Dynnik, Vladimir V.; Kononov, Alexey V.; Sergeev, Alexander I.; Teplov, Iliya Y.; Tankanag, Arina V.; Zinchenko, Valery P.

    2015-01-01

    Purpose The aim of present study was to investigate the effects of ammonium ions on in vitro neuronal network activity and to search alternative methods of acute ammonia neurotoxicity prevention. Methods Rat hippocampal neuronal and astrocytes co-cultures in vitro, fluorescent microscopy and perforated patch clamp were used to monitor the changes in intracellular Ca2+- and membrane potential produced by ammonium ions and various modulators in the cells implicated in neural networks. Results Low concentrations of NH4Cl (0.1–4 mM) produce short temporal effects on network activity. Application of 5–8 mM NH4Cl: invariably transforms diverse network firing regimen to identical burst patterns, characterized by substantial neuronal membrane depolarization at plateau phase of potential and high-amplitude Ca2+-oscillations; raises frequency and average for period of oscillations Ca2+-level in all cells implicated in network; results in the appearance of group of «run out» cells with high intracellular Ca2+ and steadily diminished amplitudes of oscillations; increases astrocyte Ca2+-signalling, characterized by the appearance of groups of cells with increased intracellular Ca2+-level and/or chaotic Ca2+-oscillations. Accelerated network activity may be suppressed by the blockade of NMDA or AMPA/kainate-receptors or by overactivation of AMPA/kainite-receptors. Ammonia still activate neuronal firing in the presence of GABA(A) receptors antagonist bicuculline, indicating that «disinhibition phenomenon» is not implicated in the mechanisms of networks acceleration. Network activity may also be slowed down by glycine, agonists of metabotropic inhibitory receptors, betaine, L-carnitine, L-arginine, etc. Conclusions Obtained results demonstrate that ammonium ions accelerate neuronal networks firing, implicating ionotropic glutamate receptors, having preserved the activities of group of inhibitory ionotropic and metabotropic receptors. This may mean, that ammonia

  15. DeepX: Deep Learning Accelerator for Restricted Boltzmann Machine Artificial Neural Networks.

    PubMed

    Kim, Lok-Won

    2017-03-08

    Although there have been many decades of research and commercial presence on high performance general purpose processors, there are still many applications that require fully customized hardware architectures for further computational acceleration. Recently, deep learning has been successfully used to learn in a wide variety of applications, but their heavy computation demand has considerably limited their practical applications. This paper proposes a fully pipelined acceleration architecture to alleviate high computational demand of an artificial neural network (ANN) which is restricted Boltzmann machine (RBM) ANNs. The implemented RBM ANN accelerator (integrating 1024 x 1024 network size, using 128 input cases per batch, and running at a 303-MHz clock frequency) integrated in a state-of-the art field-programmable gate array (FPGA) (Xilinx Virtex 7 XC7V-2000T) provides a computational performance of 301-billion connection-updates-per-second and about 193 times higher performance than a software solution running on general purpose processors. Most importantly, the architecture enables over 4 times (12 times in batch learning) higher performance compared with a previous work when both are implemented in an FPGA device (XC2VP70).

  16. First Steps Toward Incorporating Image Based Diagnostics Into Particle Accelerator Control Systems Using Convolutional Neural Networks

    SciTech Connect

    Edelen, A. L.; Biedron, S. G.; Milton, S. V.; Edelen, J. P.

    2016-12-16

    At present, a variety of image-based diagnostics are used in particle accelerator systems. Often times, these are viewed by a human operator who then makes appropriate adjustments to the machine. Given recent advances in using convolutional neural networks (CNNs) for image processing, it should be possible to use image diagnostics directly in control routines (NN-based or otherwise). This is especially appealing for non-intercepting diagnostics that could run continuously during beam operation. Here, we show results of a first step toward implementing such a controller: our trained CNN can predict multiple simulated downstream beam parameters at the Fermilab Accelerator Science and Technology (FAST) facility's low energy beamline using simulated virtual cathode laser images, gun phases, and solenoid strengths.

  17. Feasibility of Using Neural Network Models to Accelerate the Testing of Mechanical Systems

    NASA Technical Reports Server (NTRS)

    Fusaro, Robert L.

    1998-01-01

    Verification testing is an important aspect of the design process for mechanical mechanisms, and full-scale, full-length life testing is typically used to qualify any new component for use in space. However, as the required life specification is increased, full-length life tests become more costly and lengthen the development time. At the NASA Lewis Research Center, we theorized that neural network systems may be able to model the operation of a mechanical device. If so, the resulting neural network models could simulate long-term mechanical testing with data from a short-term test. This combination of computer modeling and short-term mechanical testing could then be used to verify the reliability of mechanical systems, thereby eliminating the costs associated with long-term testing. Neural network models could also enable designers to predict the performance of mechanisms at the conceptual design stage by entering the critical parameters as input and running the model to predict performance. The purpose of this study was to assess the potential of using neural networks to predict the performance and life of mechanical systems. To do this, we generated a neural network system to model wear obtained from three accelerated testing devices: 1) A pin-on-disk tribometer; 2) A line-contact rub-shoe tribometer; 3) A four-ball tribometer.

  18. Impact of the Cancer Prevention and Control Research Network: Accelerating the Translation of Research Into Practice.

    PubMed

    Ribisl, Kurt M; Fernandez, Maria E; Friedman, Daniela B; Hannon, Peggy A; Leeman, Jennifer; Moore, Alexis; Olson, Lindsay; Ory, Marcia; Risendal, Betsy; Sheble, Laura; Taylor, Vicky M; Williams, Rebecca S; Weiner, Bryan J

    2017-03-01

    The Cancer Prevention and Control Research Network (CPCRN) is a thematic network dedicated to accelerating the adoption of evidence-based cancer prevention and control practices in communities by advancing dissemination and implementation science. Funded by the Centers for Disease Control and Prevention and National Cancer Institute, CPCRN has operated at two levels: Each participating network center conducts research projects with primarily local partners as well as multicenter collaborative research projects with state and national partners. Through multicenter collaboration, thematic networks leverage the expertise, resources, and partnerships of participating centers to conduct research projects collectively that might not be feasible individually. Although multicenter collaboration is often advocated, it is challenging to promote and assess. Using bibliometric network analysis and other graphical methods, this paper describes CPCRN's multicenter publication progression from 2004 to 2014. Searching PubMed, Scopus, and Web of Science in 2014 identified 249 peer-reviewed CPCRN publications involving two or more centers out of 6,534 total. The research and public health impact of these multicenter collaborative projects initiated by CPCRN during that 10-year period were then examined. CPCRN established numerous workgroups around topics such as: 2-1-1, training and technical assistance, colorectal cancer control, federally qualified health centers, cancer survivorship, and human papillomavirus. This paper discusses the challenges that arise in promoting multicenter collaboration and the strategies that CPCRN uses to address those challenges. The lessons learned should broadly interest those seeking to promote multisite collaboration to address public health problems, such as cancer prevention and control.

  19. Acceleration of Deep Neural Network Training with Resistive Cross-Point Devices: Design Considerations

    PubMed Central

    Gokmen, Tayfun; Vlasov, Yurii

    2016-01-01

    In recent years, deep neural networks (DNN) have demonstrated significant business impact in large scale analysis and classification tasks such as speech recognition, visual object detection, pattern extraction, etc. Training of large DNNs, however, is universally considered as time consuming and computationally intensive task that demands datacenter-scale computational resources recruited for many days. Here we propose a concept of resistive processing unit (RPU) devices that can potentially accelerate DNN training by orders of magnitude while using much less power. The proposed RPU device can store and update the weight values locally thus minimizing data movement during training and allowing to fully exploit the locality and the parallelism of the training algorithm. We evaluate the effect of various RPU device features/non-idealities and system parameters on performance in order to derive the device and system level specifications for implementation of an accelerator chip for DNN training in a realistic CMOS-compatible technology. For large DNNs with about 1 billion weights this massively parallel RPU architecture can achieve acceleration factors of 30, 000 × compared to state-of-the-art microprocessors while providing power efficiency of 84, 000 GigaOps∕s∕W. Problems that currently require days of training on a datacenter-size cluster with thousands of machines can be addressed within hours on a single RPU accelerator. A system consisting of a cluster of RPU accelerators will be able to tackle Big Data problems with trillions of parameters that is impossible to address today like, for example, natural speech recognition and translation between all world languages, real-time analytics on large streams of business and scientific data, integration, and analysis of multimodal sensory data flows from a massive number of IoT (Internet of Things) sensors. PMID:27493624

  20. Acceleration of Deep Neural Network Training with Resistive Cross-Point Devices: Design Considerations.

    PubMed

    Gokmen, Tayfun; Vlasov, Yurii

    2016-01-01

    In recent years, deep neural networks (DNN) have demonstrated significant business impact in large scale analysis and classification tasks such as speech recognition, visual object detection, pattern extraction, etc. Training of large DNNs, however, is universally considered as time consuming and computationally intensive task that demands datacenter-scale computational resources recruited for many days. Here we propose a concept of resistive processing unit (RPU) devices that can potentially accelerate DNN training by orders of magnitude while using much less power. The proposed RPU device can store and update the weight values locally thus minimizing data movement during training and allowing to fully exploit the locality and the parallelism of the training algorithm. We evaluate the effect of various RPU device features/non-idealities and system parameters on performance in order to derive the device and system level specifications for implementation of an accelerator chip for DNN training in a realistic CMOS-compatible technology. For large DNNs with about 1 billion weights this massively parallel RPU architecture can achieve acceleration factors of 30, 000 × compared to state-of-the-art microprocessors while providing power efficiency of 84, 000 GigaOps∕s∕W. Problems that currently require days of training on a datacenter-size cluster with thousands of machines can be addressed within hours on a single RPU accelerator. A system consisting of a cluster of RPU accelerators will be able to tackle Big Data problems with trillions of parameters that is impossible to address today like, for example, natural speech recognition and translation between all world languages, real-time analytics on large streams of business and scientific data, integration, and analysis of multimodal sensory data flows from a massive number of IoT (Internet of Things) sensors.

  1. MS/MS similarity networking accelerated target profiling of triterpene saponins in Eleutherococcus senticosus leaves.

    PubMed

    Ge, Yue-Wei; Zhu, Shu; Yoshimatsu, Kayo; Komatsu, Katsuko

    2017-07-15

    The targeted mass information of compounds accelerated their discovery in a large volume of untargeted MS data. An MS/MS similarity networking is advanced in clustering the structural analogues, which benefits the collection of mass information of similar compounds. The triterpene saponins extracted from Eleutherococcus senticosus leaves (ESL), a kind of functional tea, have shown promise in the relief of Alzheimer's disease. In this work, a target-precursor list (TPL) generated using MS/MS similarity networking was employed to rapidly trace 106 triterpene saponins from the aqueous extracts of ESL, of which 49 were tentatively identified as potentially new triterpene saponins. Moreover, a compound database of triterpene saponins was established and successfully applied to uncover their distribution features in ESL samples collected from different areas.

  2. GPU technology as a platform for accelerating physiological systems modeling based on Laguerre-Volterra networks.

    PubMed

    Papadopoulos, Agathoklis; Kostoglou, Kyriaki; Mitsis, Georgios D; Theocharides, Theocharis

    2015-01-01

    The use of a GPGPU programming paradigm (running CUDA-enabled algorithms on GPU cards) in biomedical engineering and biology-related applications have shown promising results. GPU acceleration can be used to speedup computation-intensive models, such as the mathematical modeling of biological systems, which often requires the use of nonlinear modeling approaches with a large number of free parameters. In this context, we developed a CUDA-enabled version of a model which implements a nonlinear identification approach that combines basis expansions and polynomial-type networks, termed Laguerre-Volterra networks and can be used in diverse biological applications. The proposed software implementation uses the GPGPU programming paradigm to take advantage of the inherent parallel characteristics of the aforementioned modeling approach to execute the calculations on the GPU card of the host computer system. The initial results of the GPU-based model presented in this work, show performance improvements over the original MATLAB model.

  3. Accelerated intoxication of GABAergic synapses by botulinum neurotoxin A disinhibits stem cell-derived neuron networks prior to network silencing

    PubMed Central

    Beske, Phillip H.; Scheeler, Stephen M.; Adler, Michael; McNutt, Patrick M.

    2015-01-01

    Botulinum neurotoxins (BoNTs) are extremely potent toxins that specifically cleave SNARE proteins in peripheral synapses, preventing neurotransmitter release. Neuronal responses to BoNT intoxication are traditionally studied by quantifying SNARE protein cleavage in vitro or monitoring physiological paralysis in vivo. Consequently, the dynamic effects of intoxication on synaptic behaviors are not well-understood. We have reported that mouse embryonic stem cell-derived neurons (ESNs) are highly sensitive to BoNT based on molecular readouts of intoxication. Here we study the time-dependent changes in synapse- and network-level behaviors following addition of BoNT/A to spontaneously active networks of glutamatergic and GABAergic ESNs. Whole-cell patch-clamp recordings indicated that BoNT/A rapidly blocked synaptic neurotransmission, confirming that ESNs replicate the functional pathophysiology responsible for clinical botulism. Quantitation of spontaneous neurotransmission in pharmacologically isolated synapses revealed accelerated silencing of GABAergic synapses compared to glutamatergic synapses, which was consistent with the selective accumulation of cleaved SNAP-25 at GAD1+ pre-synaptic terminals at early timepoints. Different latencies of intoxication resulted in complex network responses to BoNT/A addition, involving rapid disinhibition of stochastic firing followed by network silencing. Synaptic activity was found to be highly sensitive to SNAP-25 cleavage, reflecting the functional consequences of the localized cleavage of the small subpopulation of SNAP-25 that is engaged in neurotransmitter release in the nerve terminal. Collectively these findings illustrate that use of synaptic function assays in networked neurons cultures offers a novel and highly sensitive approach for mechanistic studies of toxin:neuron interactions and synaptic responses to BoNT. PMID:25954159

  4. Evolution on neutral networks accelerates the ticking rate of the molecular clock

    PubMed Central

    Manrubia, Susanna; Cuesta, José A.

    2015-01-01

    Large sets of genotypes give rise to the same phenotype, because phenotypic expression is highly redundant. Accordingly, a population can accept mutations without altering its phenotype, as long as the genotype mutates into another one on the same set. By linking every pair of genotypes that are mutually accessible through mutation, genotypes organize themselves into neutral networks (NNs). These networks are known to be heterogeneous and assortative, and these properties affect the evolutionary dynamics of the population. By studying the dynamics of populations on NNs with arbitrary topology, we analyse the effect of assortativity, of NN (phenotype) fitness and of network size. We find that the probability that the population leaves the network is smaller the longer the time spent on it. This progressive ‘phenotypic entrapment’ entails a systematic increase in the overdispersion of the process with time and an acceleration in the fixation rate of neutral mutations. We also quantify the variation of these effects with the size of the phenotype and with its fitness relative to that of neighbouring alternatives. PMID:25392402

  5. Accelerated search for biomolecular network models to interpret high-throughput experimental data

    PubMed Central

    Datta, Suman; Sokhansanj, Bahrad A

    2007-01-01

    Background The functions of human cells are carried out by biomolecular networks, which include proteins, genes, and regulatory sites within DNA that encode and control protein expression. Models of biomolecular network structure and dynamics can be inferred from high-throughput measurements of gene and protein expression. We build on our previously developed fuzzy logic method for bridging quantitative and qualitative biological data to address the challenges of noisy, low resolution high-throughput measurements, i.e., from gene expression microarrays. We employ an evolutionary search algorithm to accelerate the search for hypothetical fuzzy biomolecular network models consistent with a biological data set. We also develop a method to estimate the probability of a potential network model fitting a set of data by chance. The resulting metric provides an estimate of both model quality and dataset quality, identifying data that are too noisy to identify meaningful correlations between the measured variables. Results Optimal parameters for the evolutionary search were identified based on artificial data, and the algorithm showed scalable and consistent performance for as many as 150 variables. The method was tested on previously published human cell cycle gene expression microarray data sets. The evolutionary search method was found to converge to the results of exhaustive search. The randomized evolutionary search was able to converge on a set of similar best-fitting network models on different training data sets after 30 generations running 30 models per generation. Consistent results were found regardless of which of the published data sets were used to train or verify the quantitative predictions of the best-fitting models for cell cycle gene dynamics. Conclusion Our results demonstrate the capability of scalable evolutionary search for fuzzy network models to address the problem of inferring models based on complex, noisy biomolecular data sets. This approach

  6. FastGCN: a GPU accelerated tool for fast gene co-expression networks.

    PubMed

    Liang, Meimei; Zhang, Futao; Jin, Gulei; Zhu, Jun

    2015-01-01

    Gene co-expression networks comprise one type of valuable biological networks. Many methods and tools have been published to construct gene co-expression networks; however, most of these tools and methods are inconvenient and time consuming for large datasets. We have developed a user-friendly, accelerated and optimized tool for constructing gene co-expression networks that can fully harness the parallel nature of GPU (Graphic Processing Unit) architectures. Genetic entropies were exploited to filter out genes with no or small expression changes in the raw data preprocessing step. Pearson correlation coefficients were then calculated. After that, we normalized these coefficients and employed the False Discovery Rate to control the multiple tests. At last, modules identification was conducted to construct the co-expression networks. All of these calculations were implemented on a GPU. We also compressed the coefficient matrix to save space. We compared the performance of the GPU implementation with those of multi-core CPU implementations with 16 CPU threads, single-thread C/C++ implementation and single-thread R implementation. Our results show that GPU implementation largely outperforms single-thread C/C++ implementation and single-thread R implementation, and GPU implementation outperforms multi-core CPU implementation when the number of genes increases. With the test dataset containing 16,000 genes and 590 individuals, we can achieve greater than 63 times the speed using a GPU implementation compared with a single-thread R implementation when 50 percent of genes were filtered out and about 80 times the speed when no genes were filtered out.

  7. Acceleration of spiking neural network based pattern recognition on NVIDIA graphics processors.

    PubMed

    Han, Bing; Taha, Tarek M

    2010-04-01

    There is currently a strong push in the research community to develop biological scale implementations of neuron based vision models. Systems at this scale are computationally demanding and generally utilize more accurate neuron models, such as the Izhikevich and the Hodgkin-Huxley models, in favor of the more popular integrate and fire model. We examine the feasibility of using graphics processing units (GPUs) to accelerate a spiking neural network based character recognition network to enable such large scale systems. Two versions of the network utilizing the Izhikevich and Hodgkin-Huxley models are implemented. Three NVIDIA general-purpose (GP) GPU platforms are examined, including the GeForce 9800 GX2, the Tesla C1060, and the Tesla S1070. Our results show that the GPGPUs can provide significant speedup over conventional processors. In particular, the fastest GPGPU utilized, the Tesla S1070, provided a speedup of 5.6 and 84.4 over highly optimized implementations on the fastest central processing unit (CPU) tested, a quadcore 2.67 GHz Xeon processor, for the Izhikevich and the Hodgkin-Huxley models, respectively. The CPU implementation utilized all four cores and the vector data parallelism offered by the processor. The results indicate that GPUs are well suited for this application domain.

  8. Effects of Al additives on growth of GaN polycrystals by the Na flux method

    NASA Astrophysics Data System (ADS)

    Imabayashi, Hiroki; Murakami, Kosuke; Matsuo, Daisuke; Honjo, Masatomo; Imanishi, Masayuki; Maruyama, Mihoko; Imade, Mamoru; Yoshimura, Masashi; Mori, Yusuke

    2017-03-01

    In this study, we investigated the growth of GaN polycrystals using the Al-added Na flux method. We studied the effects of Al on accelerating the nucleation and purity of GaN polycrystals. The yields of GaN crystals grown in Al-added Na flux were dramatically increased from those in Al-free Na flux, and the polycrystals grown by the Al-added Na flux method were highly transparent. As observed in secondary ion mass spectroscopy measurements, the Al content of the polycrystals was below the detection limit of 3 × 1016 atoms/cm3. From these results, the Al-added Na flux method is found to be appropriate for fabricating a large amount of GaN polycrystals without deteriorating the crystal quality.

  9. Accelerating the Mining of Influential Nodes in Complex Networks through Community Detection

    SciTech Connect

    Halappanavar, Mahantesh; Sathanur, Arun V.; Nandi, Apurba

    2016-05-31

    Computing the set of influential nodes with a given size to ensure maximal spread of influence on a complex network is a challenging problem impacting multiple applications. A rigorous approach to influence maximization involves utilization of optimization routines that comes with a high computational cost. In this work, we propose to exploit the existence of communities in complex networks to accelerate the mining of influential seeds. We provide intuitive reasoning to explain why our approach should be able to provide speedups without significantly degrading the extent of the spread of influence when compared to the case of influence maximization without using the community information. Additionally, we have parallelized the complete workflow by leveraging an existing parallel implementation of the Louvain community detection algorithm. We then conduct a series of experiments on a dataset with three representative graphs to first verify our implementation and then demonstrate the speedups. Our method achieves speedups ranging from 3x - 28x for graphs with small number of communities while nearly matching or even exceeding the activation performance on the entire graph. Complexity analysis reveals that dramatic speedups are possible for larger graphs that contain a correspondingly larger number of communities. In addition to the speedups obtained from the utilization of the community structure, scalability results show up to 6.3x speedup on 20 cores relative to the baseline run on 2 cores. Finally, current limitations of the approach are outlined along with the planned next steps.

  10. Genomic and network patterns of schizophrenia genetic variation in human evolutionary accelerated regions.

    PubMed

    Xu, Ke; Schadt, Eric E; Pollard, Katherine S; Roussos, Panos; Dudley, Joel T

    2015-05-01

    The population persistence of schizophrenia despite associated reductions in fitness and fecundity suggests that the genetic basis of schizophrenia has a complex evolutionary history. A recent meta-analysis of schizophrenia genome-wide association studies offers novel opportunities for assessment of the evolutionary trajectories of schizophrenia-associated loci. In this study, we hypothesize that components of the genetic architecture of schizophrenia are attributable to human lineage-specific evolution. Our results suggest that schizophrenia-associated loci enrich in genes near previously identified human accelerated regions (HARs). Specifically, we find that genes near HARs conserved in nonhuman primates (pHARs) are enriched for schizophrenia-associated loci, and that pHAR-associated schizophrenia genes are under stronger selective pressure than other schizophrenia genes and other pHAR-associated genes. We further evaluate pHAR-associated schizophrenia genes in regulatory network contexts to investigate associated molecular functions and mechanisms. We find that pHAR-associated schizophrenia genes significantly enrich in a GABA-related coexpression module that was previously found to be differentially regulated in schizophrenia affected individuals versus healthy controls. In another two independent networks constructed from gene expression profiles from prefrontal cortex samples, we find that pHAR-associated schizophrenia genes are located in more central positions and their average path lengths to the other nodes are significantly shorter than those of other schizophrenia genes. Together, our results suggest that HARs are associated with potentially important functional roles in the genetic architecture of schizophrenia.

  11. Ghrelin accelerates synapse formation and activity development in cultured cortical networks

    PubMed Central

    2014-01-01

    Background While ghrelin was initially related to appetite stimulation and growth hormone secretion, it also has a neuroprotective effect in neurodegenerative diseases and regulates cognitive function. The cellular basis of those processes is related to synaptic efficacy and plasticity. Previous studies have shown that ghrelin not only stimulates synapse formation in cultured cortical neurons and hippocampal slices, but also alters some of the electrophysiological properties of neurons in the hypothalamus, amygdala and other subcortical areas. However, direct evidence for ghrelin’s ability to modulate the activity in cortical neurons is not available yet. In this study, we investigated the effect of acylated ghrelin on the development of the activity level and activity patterns in cortical neurons, in relation to its effect on synaptogenesis. Additionally, we quantitatively evaluated the expression of the receptor for acylated ghrelin – growth hormone secretagogue receptor-1a (GHSR-1a) during development. Results We performed electrophysiology and immunohistochemistry on dissociated cortical cultures from neonates, treated chronically with acylated ghrelin. On average 76 ± 4.6% of the cortical neurons expressed GHSR-1a. Synapse density was found to be much higher in ghrelin treated cultures than in controls across all age groups (1, 2 or 3 weeks). In all cultures (control and ghrelin treated), network activity gradually increased until it reached a maximum after approximately 3 weeks, followed by a slight decrease towards a plateau. During early developmental stages (1–2 weeks), the activity was much higher in ghrelin treated cultures and consequently, they reached the plateau value almost a week earlier than controls. Conclusions Acylated ghrelin leads to earlier network formation and activation in cultured cortical neuronal networks, the latter being a possibly consequence of accelerated synaptogenesis. PMID:24742241

  12. A FLUX ROPE NETWORK AND PARTICLE ACCELERATION IN THREE-DIMENSIONAL RELATIVISTIC MAGNETIC RECONNECTION

    SciTech Connect

    Kagan, Daniel; Milosavljevic, Milos; Spitkovsky, Anatoly

    2013-09-01

    We investigate magnetic reconnection and particle acceleration in relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet in a periodic geometry. We include a guide field that introduces an inclination between the reconnecting field lines and explore outside-of-the-current sheet magnetizations that are significantly below those considered by other authors carrying out similar calculations. Thus, our simulations probe the transitional regime in which the magnetic and plasma pressures are of the same order of magnitude. The tearing instability is the dominant mode in the current sheet for all guide field strengths, while the linear kink mode is less important even without the guide field, except in the lower magnetization case. Oblique modes seem to be suppressed entirely. In its nonlinear evolution, the reconnection layer develops a network of interconnected and interacting magnetic flux ropes. As smaller flux ropes merge into larger ones, the reconnection layer evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on plasma skin depth scales. Embedded in the flux ropes, we detect spatially and temporally intermittent sites of dissipation reflected in peaks in the parallel electric field. Magnetic dissipation and particle acceleration persist until the end of the simulations, with simulations with higher magnetization and lower guide field strength exhibiting greater and faster energy conversion and particle energization. At the end of our largest simulation, the particle energy spectrum attains a tail extending to high Lorentz factors that is best modeled with a combination of two additional thermal components. We confirm that the primary energization mechanism is acceleration by the electric field in the X-line region. The highest-energy positrons (electrons) are moderately beamed with median angles {approx}30 Degree-Sign -40 Degree

  13. Neural-network accelerated fusion simulation with self-consistent core-pedestal coupling

    NASA Astrophysics Data System (ADS)

    Meneghini, O.; Candy, J.; Snyder, P. B.; Staebler, G.; Belli, E.

    2016-10-01

    Practical fusion Whole Device Modeling (WDM) simulations require the ability to perform predictions that are fast, but yet account for the sensitivity of the fusion performance to the boundary constraint that is imposed by the pedestal structure of H-mode plasmas due to the stiff core transport models. This poster presents the development of a set of neural-network (NN) models for the pedestal structure (as predicted by the EPED model), and the neoclassical and turbulent transport fluxes (as predicted by the NEO and TGLF codes, respectively), and their self-consistent coupling within the TGYRO transport code. The results are benchmarked with the ones obtained via the coupling scheme described in [Meneghini PoP 2016]. By substituting the most demanding codes with their NN-accelerated versions, the solution can be found at a fraction of the computation cost of the original coupling scheme, thereby combining the accuracy of a high-fidelity model with the fast turnaround time of a reduced model. Work supported by U.S. DOE DE-FC02-04ER54698 and DE-FG02-95ER54309.

  14. Incipient fault detection and identification in process systems using accelerating neural network learning

    SciTech Connect

    Parlos, A.G.; Muthusami, J.; Atiya, A.F. . Dept. of Nuclear Engineering)

    1994-02-01

    The objective of this paper is to present the development and numerical testing of a robust fault detection and identification (FDI) system using artificial neural networks (ANNs), for incipient (slowly developing) faults occurring in process systems. The challenge in using ANNs in FDI systems arises because of one's desire to detect faults of varying severity, faults from noisy sensors, and multiple simultaneous faults. To address these issues, it becomes essential to have a learning algorithm that ensures quick convergence to a high level of accuracy. A recently developed accelerated learning algorithm, namely a form of an adaptive back propagation (ABP) algorithm, is used for this purpose. The ABP algorithm is used for the development of an FDI system for a process composed of a direct current motor, a centrifugal pump, and the associated piping system. Simulation studies indicate that the FDI system has significantly high sensitivity to incipient fault severity, while exhibiting insensitivity to sensor noise. For multiple simultaneous faults, the FDI system detects the fault with the predominant signature. The major limitation of the developed FDI system is encountered when it is subjected to simultaneous faults with similar signatures. During such faults, the inherent limitation of pattern-recognition-based FDI methods becomes apparent. Thus, alternate, more sophisticated FDI methods become necessary to address such problems. Even though the effectiveness of pattern-recognition-based FDI methods using ANNs has been demonstrated, further testing using real-world data is necessary.

  15. Kinetics of optically excited charge carriers at the GaN surface: Influence of catalytic Pt nanostructures

    SciTech Connect

    Winnerl, Andrea Pereira, Rui N.; Stutzmann, Martin

    2015-10-21

    In this work, we use GaN with different deposited Pt nanostructures as a controllable model system to investigate the kinetics of photo-generated charge carriers in hybrid photocatalysts. We combine conductance and contact potential difference measurements to investigate the influence of Pt on the processes involved in the capture and decay of photo-generated charge carriers at and close to the GaN surface. We found that in the presence of Pt nanostructures the photo-excitation processes are similar to those found in Pt free GaN. However, in GaN with Pt nanostructures, photo-generated holes are preferentially trapped in surface states of the GaN covered with Pt and/or in electronic states of the Pt and lead to an accumulation of positive charge there, whereas negative charge is accumulated in localized states in a shallow defect band of the GaN covered with Pt. This preferential accumulation of photo-generated electrons close to the surface is responsible for a dramatic acceleration of the turn-off charge transfer kinetics and a stronger dependence of the surface photovoltage on light intensity when compared to a Pt free GaN surface. Our study shows that in hybrid photocatalysts, the metal nanostructures induce a spatially inhomogeneous surface band bending of the semiconductor that promotes a lateral drift of photogenerated charges towards the catalytic nanostructures.

  16. Study of neutron irradiated structures of ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Gaubas, E.; Ceponis, T.; Deveikis, L.; Meskauskaite, D.; Miasojedovas, S.; Mickevicius, J.; Pavlov, J.; Pukas, K.; Vaitkus, J.; Velicka, M.; Zajac, M.; Kucharski, R.

    2017-04-01

    Study of the radiation damage in GaN-based materials becomes an important aspect for possible application of the GaN detectors in the harsh radiation environment at the Large Hadron Collider and at other particle acceleration facilities. Intentionally doped and semi-insulating bulk ammonothermal GaN materials were studied to reveal the dominant defects introduced by reactor neutron irradiations. These radiation defects have been identified by combining electron spin resonance and transmission spectroscopy techniques. Characteristics of carrier lifetime dependence on neutron irradiation fluence were examined. Variations of the response of the capacitor-type sensors with neutron irradiation fluence have been correlated with the carrier lifetime changes. The measurements of the photoconductivity and photoluminescence transients have been used to study the variation of the parameters of radiative and non-radiative recombination. The examined characteristics indicate that AT GaN as a particle sensing material is radiation hard up to high hadron fluences  ⩾1016 cm‑2.

  17. Extrinsic and Intrinsic Brain Network Connectivity Maintains Cognition across the Lifespan Despite Accelerated Decay of Regional Brain Activation

    PubMed Central

    Henson, Richard N.A.; Tyler, Lorraine K.; Razi, Adeel; Geerligs, Linda; Ham, Timothy E.; Rowe, James B.

    2016-01-01

    The maintenance of wellbeing across the lifespan depends on the preservation of cognitive function. We propose that successful cognitive aging is determined by interactions both within and between large-scale functional brain networks. Such connectivity can be estimated from task-free functional magnetic resonance imaging (fMRI), also known as resting-state fMRI (rs-fMRI). However, common correlational methods are confounded by age-related changes in the neurovascular signaling. To estimate network interactions at the neuronal rather than vascular level, we used generative models that specified both the neural interactions and a flexible neurovascular forward model. The networks' parameters were optimized to explain the spectral dynamics of rs-fMRI data in 602 healthy human adults from population-based cohorts who were approximately uniformly distributed between 18 and 88 years (www.cam-can.com). We assessed directed connectivity within and between three key large-scale networks: the salience network, dorsal attention network, and default mode network. We found that age influences connectivity both within and between these networks, over and above the effects on neurovascular coupling. Canonical correlation analysis revealed that the relationship between network connectivity and cognitive function was age-dependent: cognitive performance relied on neural dynamics more strongly in older adults. These effects were driven partly by reduced stability of neural activity within all networks, as expressed by an accelerated decay of neural information. Our findings suggest that the balance of excitatory connectivity between networks, and the stability of intrinsic neural representations within networks, changes with age. The cognitive function of older adults becomes increasingly dependent on these factors. SIGNIFICANCE STATEMENT Maintaining cognitive function is critical to successful aging. To study the neural basis of cognitive function across the lifespan, we studied a

  18. GaN High Power Devices

    SciTech Connect

    PEARTON,S.J.; REN,F.; ZHANG,A.P.; DANG,G.; CAO,X.A.; LEE,K.P.; CHO,H.; GILA,B.P.; JOHNSON,J.W.; MONIER,C.; ABERNATHY,C.R.; HAN,JUNG; BACA,ALBERT G.; CHYI,J.-I.; LEE,C.-M.; NEE,T.-E.; CHUO,C.-C.; CHI,G.C.; CHU,S.N.G.

    2000-07-17

    A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors, GaN heterostructure and metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

  19. A Method of Social Collaboration and Knowledge Sharing Acceleration for e-Learning System: The Distance Learning Network Scenario

    NASA Astrophysics Data System (ADS)

    Różewski, Przemysław

    Nowadays, e-learning systems take the form of the Distance Learning Network (DLN) due to widespread use and accessibility of the Internet and networked e-learning services. The focal point of the DLN performance is efficiency of knowledge processing in asynchronous learning mode and facilitating cooperation between students. In addition, the DLN articulates attention to social aspects of the learning process as well. In this paper, a method for the DLN development is proposed. The main research objectives for the proposed method are the processes of acceleration of social collaboration and knowledge sharing in the DLN. The method introduces knowledge-disposed agents (who represent students in educational scenarios) that form a network of individuals aimed to increase their competence. For every agent the competence expansion process is formulated. Based on that outcome the process of dynamic network formation performed on the social and knowledge levels. The method utilizes formal apparatuses of competence set and network game theories combined with an agent system-based approach.

  20. Bulk ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Dwiliński, R.; Doradziński, R.; Garczyński, J.; Sierzputowski, L. P.; Puchalski, A.; Kanbara, Y.; Yagi, K.; Minakuchi, H.; Hayashi, H.

    2009-05-01

    In this work, results of structural characterization of high-quality ammonothermal GaN are presented. Besides expected low dislocation density (being of the order of 10 3 cm -2) the most interesting feature seems perfect flatness of the crystal lattice of studied crystals. Regardless the size of crystals, lattice curvature radius exceeds 100 m, whereas better crystals reveal radius of several hundred meters and the best above 1000 m. Excellent crystallinity manifests in very narrow X-ray diffraction peaks of full-width at half-maximum (FWHM) values about 16 arcsec.

  1. A high efficiency C-band internally-matched harmonic tuning GaN power amplifier

    NASA Astrophysics Data System (ADS)

    Lu, Y.; Zhao, B. C.; Zheng, J. X.; Zhang, H. S.; Zheng, X. F.; Ma, X. H.; Hao, Y.; Ma, P. J.

    2016-09-01

    In this paper, a high efficiency C-band gallium nitride (GaN) internally-matched power amplifier (PA) is presented. This amplifier consists of 2-chips of self-developed GaN high-electron mobility transistors (HEMTs) with 16 mm total gate width on SiC substrate. New harmonic manipulation circuits are induced both in the input and output matching networks for high efficiency matching at fundamental and 2nd-harmonic frequency, respectively. The developed amplifier has achieved 72.1% power added efficiency (PAE) with 107.4 W output power at 5 GHz. To the best of our knowledge, this amplifier exhibits the highest PAE in C-band GaN HEMT amplifiers with over 100 W output power. Additionally, 1000 hours' aging test reveals high reliability for practical applications.

  2. Bulk GaN Ion Cleaving

    NASA Astrophysics Data System (ADS)

    Moutanabbir, O.; Gösele, U.

    2010-05-01

    Bulk or freestanding GaN is a key material in various devices other than the blue laser diodes. However, the high cost of bulk GaN wafers severely limits the large scale exploitation of these potential technologies. In this paper, we discuss some engineering issues involved in the application of the ion-cut process to split a thin layer from 2-inch freestanding GaN. This process combines the implantation of light ions and wafer bonding and can possibly be used to reduce the cost of the fabrication of GaN-based devices by allowing the transfer of several bulk quality thin layers from the same donor wafer. To achieve this multi-layer transfer several conditions must be fulfilled. Here issues related to bulk GaN surface irregularities and wafer bowing are discussed. We also describe a method to circumvent most of these problems and achieve high quality bonding.

  3. Optical properties of GaN pyramids

    SciTech Connect

    Zeng, K.C.; Lin, J.Y.; Jiang, H.X.; Yang, W.

    1999-03-01

    Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer grown under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; (iii) PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and (iv) the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO{sub 2} mask. {copyright} {ital 1999 American Institute of Physics.}

  4. Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Tripathy, S.; Chen, P.; Fonstad, C. G.

    2005-12-01

    We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

  5. The Formation and Characterization of GaN Hexagonal Pyramids

    NASA Astrophysics Data System (ADS)

    Zhang, Shi-Ying; Xiu, Xiang-Qian; Lin, Zeng-Qin; Hua, Xue-Mei; Xie, Zi-Li; Zhang, Rong; Zheng, You-Dou

    2013-05-01

    GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {101¯1¯} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

  6. Neural Network Models of Simple Mechanical Systems Illustrating the Feasibility of Accelerated Life Testing

    NASA Technical Reports Server (NTRS)

    Fusaro, Robert L.; Jones, Steven P.; Jansen, Ralph

    1996-01-01

    A complete evaluation of the tribological characteristics of a given material/mechanical system is a time-consuming operation since the friction and wear process is extremely systems sensitive. As a result, experimental designs (i.e., Latin Square, Taguchi) have been implemented in an attempt to not only reduce the total number of experimental combinations needed to fully characterize a material/mechanical system, but also to acquire life data for a system without having to perform an actual life test. Unfortunately, these experimental designs still require a great deal of experimental testing and the output does not always produce meaningful information. In order to further reduce the amount of experimental testing required, this study employs a computer neural network model to investigate different material/mechanical systems. The work focuses on the modeling of the wear behavior, while showing the feasibility of using neural networks to predict life data. The model is capable of defining which input variables will influence the tribological behavior of the particular material/mechanical system being studied based on the specifications of the overall system.

  7. Acceleration Studies

    NASA Technical Reports Server (NTRS)

    Rogers, Melissa J. B.

    1993-01-01

    Work to support the NASA MSFC Acceleration Characterization and Analysis Project (ACAP) was performed. Four tasks (analysis development, analysis research, analysis documentation, and acceleration analysis) were addressed by parallel projects. Work concentrated on preparation for and implementation of near real-time SAMS data analysis during the USMP-1 mission. User support documents and case specific software documentation and tutorials were developed. Information and results were presented to microgravity users. ACAP computer facilities need to be fully implemented and networked, data resources must be cataloged and accessible, future microgravity missions must be coordinated, and continued Orbiter characterization is necessary.

  8. Accelerated convergence of neural network system identification algorithms via principal component analysis

    NASA Astrophysics Data System (ADS)

    Hyland, David C.; Davis, Lawrence D.; Denoyer, Keith K.

    1998-12-01

    While significant theoretical and experimental progress has been made in the development of neural network-based systems for the autonomous identification and control of space platforms, there remain important unresolved issues associated with the reliable prediction of convergence speed and the avoidance of inordinately slow convergence. To speed convergence of neural identifiers, we introduce the preprocessing of identifier inputs using Principal Component Analysis (PCA) algorithms. Which automatically transform the neural identifier's external inputs so as to make the correlation matrix identity, resulting in enormous improvements in the convergence speed of the neural identifier. From a study of several such algorithms, we developed a new PCA approach which exhibits excellent convergence properties, insensitivity to noise and reliable accuracy.

  9. Mushroom structure of GaN template for epitaxial growth of GaN

    NASA Astrophysics Data System (ADS)

    Lee, Sung Bo; Kwon, Tae-Wan; Park, Jungwon; Jin Choi, Won; Sung Park, Hae

    2012-07-01

    In the present study, we show the formation of mushroom morphology produced by a ramp anneal of a low-temperature GaN buffer layer. Structural analysis by transmission electron microscopy indicates that the cap of the mushroom has the stable wurtzitic GaN structure, whereas the stem possesses the metastable zinc-blende structure. With the air gap introduced between the substrate and the cap of the mushroom structure, threading dislocations propagate along its stem. The formation of the mushroom morphology is suggested to result from the nucleation of wurtzitic GaN on the surface of the low-temperature buffer layer during the ramp anneal, followed by mass transport of GaN from the buffer layer, which remains zinc-blende during the anneal, to the surface, because wurtzitic GaN has the lower structure energy than zinc-blende GaN. This study extends limits of the conventional use of the buffer layer, laying the foundation for the development of low-cost recipes for achieving GaN templates with a low density of threading dislocations.

  10. Theoretical study of gallium nitride molecules, GaN2 and GaN4.

    PubMed

    Tzeli, Demeter; Theodorakopoulos, Giannoula; Petsalakis, Ioannis D

    2008-09-18

    The electronic and geometric structures of gallium dinitride GaN 2, and gallium tetranitride molecules, GaN 4, were systematically studied by employing density functional theory and perturbation theory (MP2, MP4) in conjunction with the aug-cc-pVTZ basis set. In addition, for the ground-state of GaN 4( (2)B 1) a density functional theory study was carried out combining different functionals with different basis sets. A total of 7 minima have been identified for GaN 2, while 37 structures were identified for GaN 4 corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces and bonding mechanisms for some low-lying electronic states of GaN 4. The dissociation energy of the ground-state GaN 2 ( X (2)Pi) is 1.1 kcal/mol with respect to Ga( (2)P) + N 2( X (1)Sigma g (+)). The ground-state and the first two excited minima of GaN 4 are of (2)B 1( C 2 v ), (2)A 1( C 2 v , five member ring), and (4)Sigma g (-)( D infinityh ) symmetry, respectively. The dissociation energy ( D e) of the ground-state of GaN 4, X (2)B 1, with respect to Ga( (2)P) + 2 N 2( X (1)Sigma g (+)), is 2.4 kcal/mol, whereas the D e of (4)Sigma g (-) with respect to Ga( (4)P) + 2 N 2( X (1)Sigma g (+)) is 17.6 kcal/mol.

  11. POF data link employing a display-type green LED fabricated from GaN material

    NASA Astrophysics Data System (ADS)

    Kato, Satoru; Kagami, Manabu; Ito, Hiroshi

    2003-07-01

    Data links incorporating a green light source at 520nm are required for use with polymethyl methacrylate (PMMA) plastic optical fiber (POF) systems because they have a lower attenuation coefficient compared with conventional red light sources at 650nm. Recently, green LEDs have been developed based on Gallium Nitride (GaN) materials, and high optical output power GaN green LED lamps are now commercially available for general use in display applications. In this paper, we describe in detail the fundamental characteristics of these GaN green LEDs that are due to be employed in POF data links. We evaluate the temperature coefficients of the optical output power and the center wavelength shift and also demonstrate a green LED POF data link that complies with IEEE 1394 S100 operation. GaN green LEDs seem to be promising candidates as light sources for the next generation of POF data links for automotive applications or for long distance In-house multimedia networks. This is because, as we will show, they can operate both at high temperatures and with reduced temperature sensitivity compared with red LEDs fabricated from AlGaInP materials.

  12. Resonantly enhanced selective photochemical etching of GaN

    NASA Astrophysics Data System (ADS)

    Trichas, E.; Kayambaki, M.; Iliopoulos, E.; Pelekanos, N. T.; Savvidis, P. G.

    2009-04-01

    Wavelength dependent photochemical etching of GaN films reveals a strong resonant enhancement of the photocurrent at the GaN gap, in close agreement with the excitonic absorption profile of GaN. The corresponding etching rate of GaN strongly correlates with the measured photocurrent. No photocurrent, nor etching is observed for AlGaN films under same excitation conditions. The method could pave the way to the development of truly selective etching of GaN on AlGaN for the fabrication of nitride based optoelectronic devices.

  13. Growth of self-standing GaN substrates

    NASA Astrophysics Data System (ADS)

    Lee, Hyun-Jae; Fujii, Katsushi; Goto, Takenari; Kim, Chinkyo; Chang, Jiho; Hong, Soon-Ku; Cho, Meoungwhan; Yao, Takafumi

    2010-03-01

    Large-sized and high-quality free standing GaN are required with the development of GaN-based devices. We have developed new techniques to reduce the price of GaN substrates. In this paper, we introduce a simple fabrication way of freestanding GaN substrate using hydride vapor phase epitaxy (HVPE). An evaporable buffer layer was applied for the fabrication of 2inch freestanding GaN to separate from a sapphire substrate, in other words, a freestanding GaN was fabricated only by HVPE (one-stop process) without any process.

  14. A 181 GOPS AKAZE Accelerator Employing Discrete-Time Cellular Neural Networks for Real-Time Feature Extraction.

    PubMed

    Jiang, Guangli; Liu, Leibo; Zhu, Wenping; Yin, Shouyi; Wei, Shaojun

    2015-09-04

    This paper proposes a real-time feature extraction VLSI architecture for high-resolution images based on the accelerated KAZE algorithm. Firstly, a new system architecture is proposed. It increases the system throughput, provides flexibility in image resolution, and offers trade-offs between speed and scaling robustness. The architecture consists of a two-dimensional pipeline array that fully utilizes computational similarities in octaves. Secondly, a substructure (block-serial discrete-time cellular neural network) that can realize a nonlinear filter is proposed. This structure decreases the memory demand through the removal of data dependency. Thirdly, a hardware-friendly descriptor is introduced in order to overcome the hardware design bottleneck through the polar sample pattern; a simplified method to realize rotation invariance is also presented. Finally, the proposed architecture is designed in TSMC 65 nm CMOS technology. The experimental results show a performance of 127 fps in full HD resolution at 200 MHz frequency. The peak performance reaches 181 GOPS and the throughput is double the speed of other state-of-the-art architectures.

  15. GaN membrane MSM ultraviolet photodetectors

    NASA Astrophysics Data System (ADS)

    Muller, A.; Konstantinidis, G.; Kostopoulos, A.; Dragoman, M.; Neculoiu, D.; Androulidaki, M.; Kayambaki, M.; Vasilache, D.; Buiculescu, C.; Petrini, I.

    2006-12-01

    GaN exhibits unique physical properties, which make this material very attractive for wide range of applications and among them ultraviolet detection. For the first time a MSM type UV photodetector structure was manufactured on a 2.2 μm. thick GaN membrane obtained using micromachining techniques. The low unintentionally doped GaN layer structure was grown by MOCVD on high resistivity (ρ>10kΩcm) <111> oriented silicon wafers, 500μm thick. The epitaxially grown layers include a thin AlN layer in order to reduce the stress in the GaN layer and avoid cracking. Conventional contact lithography, e-gun Ni/Au (10nm /200nm) evaporation and lift-off techniques were used to define the interdigitated Schottky metalization on the top of the wafer. Ten digits with a width of 1μm and a length of 100μm were defined for each electrode. The distance between the digits was also 1μm. After the backside lapping of the wafer to a thickness of approximately 150μm, a 400nm thick Al layer was patterned and deposited on the backside, to be used as mask for the selective reactive ion etching of silicon. The backside mask, for the membrane formation, was patterned using double side alignment techniques and silicon was etched down to the 2.2μm thin GaN layer using SF 6 plasma. A very low dark current (30ρA at 3V) was obtained. Optical responsivity measurements were performed at 1.5V. A maximum responsivity of 18mA/W was obtained at a wavelength of 370nm. This value is very good and can be further improved using transparent contacts for the interdigitated structure.

  16. Computational Approach for Securing Radiology-Diagnostic Data in Connected Health Network using High-Performance GPU-Accelerated AES.

    PubMed

    Adeshina, A M; Hashim, R

    2017-03-01

    Diagnostic radiology is a core and integral part of modern medicine, paving ways for the primary care physicians in the disease diagnoses, treatments and therapy managements. Obviously, all recent standard healthcare procedures have immensely benefitted from the contemporary information technology revolutions, apparently revolutionizing those approaches to acquiring, storing and sharing of diagnostic data for efficient and timely diagnosis of diseases. Connected health network was introduced as an alternative to the ageing traditional concept in healthcare system, improving hospital-physician connectivity and clinical collaborations. Undoubtedly, the modern medicinal approach has drastically improved healthcare but at the expense of high computational cost and possible breach of diagnosis privacy. Consequently, a number of cryptographical techniques are recently being applied to clinical applications, but the challenges of not being able to successfully encrypt both the image and the textual data persist. Furthermore, processing time of encryption-decryption of medical datasets, within a considerable lower computational cost without jeopardizing the required security strength of the encryption algorithm, still remains as an outstanding issue. This study proposes a secured radiology-diagnostic data framework for connected health network using high-performance GPU-accelerated Advanced Encryption Standard. The study was evaluated with radiology image datasets consisting of brain MR and CT datasets obtained from the department of Surgery, University of North Carolina, USA, and the Swedish National Infrastructure for Computing. Sample patients' notes from the University of North Carolina, School of medicine at Chapel Hill were also used to evaluate the framework for its strength in encrypting-decrypting textual data in the form of medical report. Significantly, the framework is not only able to accurately encrypt and decrypt medical image datasets, but it also

  17. MOVPE growth of GaN on 6-inch SOI-substrates: effect of substrate parameters on layer quality and strain

    NASA Astrophysics Data System (ADS)

    Lemettinen, J.; Kauppinen, C.; Rudzinski, M.; Haapalinna, A.; Tuomi, T. O.; Suihkonen, S.

    2017-04-01

    We demonstrate that higher crystalline quality, lower strain and improved electrical characteristics can be achieved in gallium nitride (GaN) epitaxy by using a silicon-on-insulator (SOI) substrate compared to a bulk silicon (Si) substrate. GaN layers were grown by metal–organic vapor phase epitaxy on 6-inch bulk Si and SOI wafers using the standard step graded AlGaN and AlN approach. The GaN layers grown on SOI exhibited lower strain according to x-ray diffraction analysis. Defect selective etching measurements suggested that the use of SOI substrate for GaN epitaxy reduces the dislocation density approximately by a factor of two. Furthermore, growth on SOI substrate allows one to use a significantly thinner AlGaN buffer compared to bulk Si. Synchrotron radiation x-ray topography analysis confirmed that the stress relief mechanism in GaN on SOI epitaxy is the formation of a dislocation network to the SOI device Si layer. In addition, the buried oxide layer significantly improves the vertical leakage characteristics as the onset of the breakdown is delayed by approximately 400 V. These results show that the GaN on the SOI platform is promising for power electronics applications.

  18. Germanium doping of GaN by metalorganic chemical vapor deposition for polarization screening applications

    NASA Astrophysics Data System (ADS)

    Young, N. G.; Farrell, R. M.; Iza, M.; Nakamura, S.; DenBaars, S. P.; Weisbuch, C.; Speck, J. S.

    2016-12-01

    We demonstrate n-type doping of GaN with Ge by MOCVD at high concentrations that are necessary to fully screen the polarization fields in c-plane InGaN/GaN quantum wells. Hall measurements show linear Ge incorporation with dopant flow rate and carrier concentrations exceeding 1×1020 cm-3. GaN:Ge layers exhibit excellent electron mobility, high conductivity, and contact resistivity comparable to the best unannealed contacts to Si-doped GaN. However, the surface morphology begins to degrade with Ge concentrations above 1×1019 cm-3, resulting in severe step bunching and a network of plateaus and trenches, even in layers as thin as 10 nm.

  19. Gallium Nitride (GaN) High Power Electronics (FY11)

    DTIC Science & Technology

    2012-01-01

    Gallium Nitride (GaN) High Power Electronics (FY11) by Kenneth A. Jones, Randy P. Tompkins, Michael A. Derenge, Kevin W. Kirchner, Iskander...Army Research Laboratory Adelphi, MD 20783-1197 ARL-TR-5903 January 2012 Gallium Nitride (GaN) High Power Electronics (FY11) Kenneth A...DSI 3. DATES COVERED (From - To) 4. TITLE AND SUBTITLE Gallium Nitride (GaN) High Power Electronics (FY11) 5a. CONTRACT NUMBER 5b. GRANT

  20. Dislocation annihilation in regrown GaN on nanoporous GaN template with optimization of buffer layer growth

    NASA Astrophysics Data System (ADS)

    Soh, C. B.; Hartono, H.; Chow, S. Y.; Chua, S. J.; Fitzgerald, E. A.

    2007-01-01

    Nanoporous GaN template has been fabricated by electrochemical etching to give hexagonal pits with nanoscale pores of size 20-50nm in the underlying grains. The effect of GaN buffer layer grown at various temperatures from 650to1015°C on these as-fabricated nanopores templates is investigated by transmission electron microscopy. The buffer layer grown at the optimized temperature of 850°C partially fill up the pores and voids with annihilation of threading dislocations, serving as an excellent template for high-quality GaN growth. This phenomenon is, however, not observed for the samples grown with other temperature buffer layers. Micro-Raman measurements show significant strain relaxation and improvement in the crystal quality of the overgrown GaN layer on nanoporous GaN template as compared to overgrown on conventional GaN template.

  1. Amphoteric arsenic in GaN

    SciTech Connect

    Wahl, U.; Correia, J. G.; Araujo, J. P.; Rita, E.; Soares, J. C.

    2007-04-30

    The authors have determined the lattice location of implanted arsenic in GaN by means of conversion electron emission channeling from radioactive {sup 73}As. They give direct evidence that As is an amphoteric impurity, thus settling the long-standing question as to whether it prefers cation or anion sites in GaN. The amphoteric character of As and the fact that As{sub Ga} 'antisites' are not minority defects provide additional aspects to be taken into account for an explanantion of the so-called miscibility gap in ternary GaAs{sub 1-x}N{sub x} compounds, which cannot be grown with a single phase for values of x in the range of 0.1

  2. TEM characterization of GaN nanowires

    SciTech Connect

    Liliental-Weber, Zuzanna; Gao, Y.H.; Bando, Y.

    2002-02-21

    Transmission electron microscopy was applied to study GaN nanowires grown on carbon nanotube surfaces by chemical reaction between Ga{sub 2}O and NH{sub 3} gas in a conventional furnace. These wires grew in two crystallographic directions, <2{und 11}0> and <01{und 1}0> (fast growth directions of GaN), in the form of whiskers covered by small elongated GaN platelets. The morphology of these platelets is similar to that observed during the growth of single crystals from a Ga melt at high temperatures under high nitrogen pressure. It is thought that growth of nanowires in two different crystallographic directions and the arrangement of the platelets to the central whisker may be influenced by the presence of Ga{sub 2}O{sub 3} (based on the observation of the energy dispersive x-ray spectra), the interplanar spacings in the wire, and the presence of defects on the interface between the central part of the nanowire and the platelets surrounding it.

  3. Ground Albedo Neutron Sensing (GANS) method for measurements of soil moisture in cropped fields

    NASA Astrophysics Data System (ADS)

    Andres Rivera Villarreyes, Carlos; Baroni, Gabriele; Oswald, Sascha E.

    2013-04-01

    Measurement of soil moisture at the plot or hill-slope scale is an important link between local vadose zone hydrology and catchment hydrology. However, so far only few methods are on the way to close this gap between point measurements and remote sensing. This study evaluates the applicability of the Ground Albedo Neutron Sensing (GANS) for integral quantification of seasonal soil moisture in the root zone at the scale of a field or small watershed, making use of the crucial role of hydrogen as neutron moderator relative to other landscape materials. GANS measurements were performed at two locations in Germany under different vegetative situations and seasonal conditions. Ground albedo neutrons were measured at (i) a lowland Bornim farmland (Brandenburg) cropped with sunflower in 2011 and winter rye in 2012, and (ii) a mountainous farmland catchment (Schaefertal, Harz Mountains) since middle 2011. At both sites depth profiles of soil moisture were measured at several locations in parallel by frequency domain reflectometry (FDR) for comparison and calibration. Initially, calibration parameters derived from a previous study with corn cover were tested under sunflower and winter rye periods at the same farmland. GANS soil moisture based on these parameters showed a large discrepancy compared to classical soil moisture measurements. Therefore, two new calibration approaches and four different ways of integration the soil moisture profile to an integral value for GANS were evaluated in this study. This included different sets of calibration parameters based on different growing periods of sunflower. New calibration parameters showed a good agreement with FDR network during sunflower period (RMSE = 0.023 m3 m-3), but they underestimated soil moisture in the winter rye period. The GANS approach resulted to be highly affected by temporal changes of biomass and crop types which suggest the need of neutron corrections for long-term observations with crop rotation. Finally

  4. Laser acceleration

    NASA Astrophysics Data System (ADS)

    Tajima, T.; Nakajima, K.; Mourou, G.

    2017-02-01

    The fundamental idea of Laser Wakefield Acceleration (LWFA) is reviewed. An ultrafast intense laser pulse drives coherent wakefield with a relativistic amplitude robustly supported by the plasma. While the large amplitude of wakefields involves collective resonant oscillations of the eigenmode of the entire plasma electrons, the wake phase velocity ˜ c and ultrafastness of the laser pulse introduce the wake stability and rigidity. A large number of worldwide experiments show a rapid progress of this concept realization toward both the high-energy accelerator prospect and broad applications. The strong interest in this has been spurring and stimulating novel laser technologies, including the Chirped Pulse Amplification, the Thin Film Compression, the Coherent Amplification Network, and the Relativistic Mirror Compression. These in turn have created a conglomerate of novel science and technology with LWFA to form a new genre of high field science with many parameters of merit in this field increasing exponentially lately. This science has triggered a number of worldwide research centers and initiatives. Associated physics of ion acceleration, X-ray generation, and astrophysical processes of ultrahigh energy cosmic rays are reviewed. Applications such as X-ray free electron laser, cancer therapy, and radioisotope production etc. are considered. A new avenue of LWFA using nanomaterials is also emerging.

  5. Linear Accelerators

    NASA Astrophysics Data System (ADS)

    Sidorin, Anatoly

    2010-01-01

    In linear accelerators the particles are accelerated by either electrostatic fields or oscillating Radio Frequency (RF) fields. Accordingly the linear accelerators are divided in three large groups: electrostatic, induction and RF accelerators. Overview of the different types of accelerators is given. Stability of longitudinal and transverse motion in the RF linear accelerators is briefly discussed. The methods of beam focusing in linacs are described.

  6. P-type doping of GaN

    SciTech Connect

    Wong, Raechelle Kimberly

    2000-04-01

    After implantation of As, As + Be, and As + Ga into GaN and annealing for short durations at temperatures as high as 1500 C, the GaN films remained highly resistive. It was apparent from c-RBS studies that although implantation damage did not create an amorphous layer in the GaN film, annealing at 1500 C did not provide enough energy to completely recover the radiation damage. Disorder recovered significantly after annealing at temperatures up to 1500 C, but not completely. From SIMS analysis, oxygen contamination in the AIN capping layer causes oxygen diffusion into the GaN film above 1400 C. The sapphire substrate (A1203) also decomposed and oxygen penetrated into the backside of the GaN layer above 1400 C. To prevent donor-like oxygen impurities from the capping layer and the substrate from contaminating the GaN film and compensating acceptors, post-implantation annealing should be done at temperatures below 1500 C. Oxygen in the cap could be reduced by growing the AIN cap on the GaN layer after the GaN growth run or by depositing the AIN layer in a ultra high vacuum (UHV) system post-growth to minimize residual oxygen and water contamination. With longer annealing times at 1400 C or at higher temperatures with a higher quality AIN, the implantation drainage may fully recover.

  7. GaN Electronics For High Power, High Temperature Applications

    SciTech Connect

    PEARTON,S.J.; REN,F.; ZHANG,A.P.; DANG,G.; CAO,X.A.; LEE,K.P.; CHO,H.; GILA,B.P.; JOHNSON,J.W.; MONIER,C.; ABERNATHY,C.R.; HAN,JUNG; BACA,ALBERT G.; CHYI,J.-I.; LEE,C.-M.; NEE,T.-E.; CHUO,C.-C.; CHU,S.N.G.

    2000-06-12

    A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers. GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

  8. Can Accelerators Accelerate Learning?

    NASA Astrophysics Data System (ADS)

    Santos, A. C. F.; Fonseca, P.; Coelho, L. F. S.

    2009-03-01

    The 'Young Talented' education program developed by the Brazilian State Funding Agency (FAPERJ) [1] makes it possible for high-schools students from public high schools to perform activities in scientific laboratories. In the Atomic and Molecular Physics Laboratory at Federal University of Rio de Janeiro (UFRJ), the students are confronted with modern research tools like the 1.7 MV ion accelerator. Being a user-friendly machine, the accelerator is easily manageable by the students, who can perform simple hands-on activities, stimulating interest in physics, and getting the students close to modern laboratory techniques.

  9. PARTICLE ACCELERATOR

    DOEpatents

    Teng, L.C.

    1960-01-19

    ABS>A combination of two accelerators, a cyclotron and a ring-shaped accelerator which has a portion disposed tangentially to the cyclotron, is described. Means are provided to transfer particles from the cyclotron to the ring accelerator including a magnetic deflector within the cyclotron, a magnetic shield between the ring accelerator and the cyclotron, and a magnetic inflector within the ring accelerator.

  10. GaN: Defect and Device Issues

    SciTech Connect

    Pearton, S.J.; Ren, F.; Shul, R.J.; Zolper, J.C.

    1998-11-09

    The role of extended and point defects, and key impurities such as C, O and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

  11. Yellow Luminescence Centers of GaN

    NASA Astrophysics Data System (ADS)

    Zhao, Guangyuan; Hubbard, Seth; Pavlidis, Dimitris

    2004-05-01

    The method for measuring Shockley-Read-Hall (SRH) lifetime of yellow centers of GaN was developed. The capture-section ratio (150) of hole to electron is extracted by comparing the experimental and theoretical results. A marked increase in the SRH lifetime (from 0.75 to 7.0 ns) with the increasing in Si doping density (from 1.5× 1017 to 8.8× 1018 cm-3) was observed, and it is attributed to some Si dopant substituting for the Ga vacancy. In addition, it is also found that the YL centers are an important factor limiting the performance of GaN-based devices.

  12. Cathodoluminescence characterization of suspended GaN nanomembranes

    NASA Astrophysics Data System (ADS)

    Stevens-Kalceff, M. A.; Tiginyanu, I. M.; Popa, V.; Braniste, T.; Brenner, P.

    2013-07-01

    Continuous suspended ˜15 nm thick gallium nitride (GaN) nano-membranes have been investigated using cathodoluminescence microanalysis. The GaN nanomembranes are fabricated by focused ion beam (FIB) pre-treatment of GaN epilayer surfaces followed by photoelectrochemical (PEC) etching. CL microanalysis enables high sensitivity, nanoscale spatial resolution detection of impurities, and defects, and is associated with key features of the suspended GaN nano-membranes. CL spectra and images of the suspended nano-membranes reveal the broad emission band at ˜2.2 eV which is associated with deep acceptor states and the near edge emission at ˜3.4 eV which is associated with free exciton transitions at 295 K. The near edge emission can be resolved into two components, one associated with emission from the nanomembrane and the other associated with CL from underlying GaN transmitted through the nanomembrane. CL spectroscopy gives insight into the physical properties and optical quality of the suspended GaN nano-membranes. Blue shift of the CL near band edge emission indicates that the suspended GaN nanomembranes exhibit the combined effects of quantum confinement and strain.

  13. GaN power devices for automotive applications

    NASA Astrophysics Data System (ADS)

    Uesugi, T.; Kachi, Tetsu

    2013-03-01

    GaN is an attractive material for high performance power devices. Vertical GaN power devices are suitable for high current operation, on the other hand, lateral GaN power devices, namely GaN lateral HEMTs have both low on-resistance and low parasitic capacitance. In addition, the GaN lateral HEMTs can be fabricated on Si substrate. We can get low conduction loss and low switching loss devices with low cost. So the GaN lateral HEMTs are suitable for subsystems like an air conditioner and an electric power steering. Serious technical issues about GaN power devices are a normally-off operation, a current collapse, and a high quality gate insulator. Several normally-off operation techniques have been proposed but there is no decisive method. An NH3 surface treatment and a SiO2 passivation are useful to suppress the current collapse. An Al2O3 deposited by ALD is excellent for gate insulator in breakdown and it has enough TDDB reliability under room temperature and 150°C.

  14. Coronal and interplanetary propagation, interplanetary acceleration, cosmic-ray observations by deep space network and anomalous component

    NASA Technical Reports Server (NTRS)

    Ng, C. K.

    1986-01-01

    The purpose is to provide an overview of the contributions presented in sessions SH3, SH1.5, SH4.6 and SH4.7 of the 19th International Cosmic Ray Conference. These contributed papers indicate that steady progress continues to be made in both the observational and the theoretical aspects of the transport and acceleration of energetic charged particles in the heliosphere. Studies of solar and interplanetary particles have placed emphasis on particle directional distributions in relation to pitch-angle scattering and magnetic focusing, on the rigidity and spatial dependence of the mean free path, and on new propagation regimes in the inner and outer heliosphere. Coronal propagation appears in need of correlative multi-spacecraft studies in association with detailed observation of the flare process and coronal magnetic structures. Interplanetary acceleration has now gone into a consolidation phase, with theories being worked out in detail and checked against observation.

  15. Comparative study on cost evaluation and network visualization of particle accelerator components for heavy ion inertial fusion

    NASA Astrophysics Data System (ADS)

    Inoue, A.; Takahashi, K.; Sasaki, T.; Kikuchi, T.; Harada, Nob; Barnard, J. J.

    2016-05-01

    By visualizing accelerator system components in heavy ion inertial fusion, the connection between the components becomes clear. We clarify an influential component on the entire cost by the relation of node connections due to the visualization result. Since a low cost component affects a high cost component, not only the cost estimation but also the relation between the components is considerable and important issue. A cost estimation result changing with an induction core cost indicates no influences in the rate of details.

  16. GaN for LED applications

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.

    1973-01-01

    In order to improve the synthesis of GaN the effect of various growth and doping parameters has been studied. Although Be, Li, Mg, and Dy can be used to overcompensate native donors, the most interesting acceptor element is Zn. The emission spectrum and the luminescence efficiency depend on the growth temperature (below 800 C), on the partial pressure of the doping impurity, and on the duration of growth. Blue-green electroluminescence with a power efficiency of 0.1 percent and a brightness of 850 fL (at 0.6 mA and 22.5 V) was obtained. Some diodes allow the color of the emitted light to change by reversing the polarity of the bias. Continuous operation of a diode over a period of 5 months showed no evidence of degradation. The luminescence properties of ion-implanted GaN were studied. Delay effects were found in the electroluminescence of diodes, although, with a dc bias, a 70-MHz modulation was possible.

  17. Development of GaN photocathodes for UV detectors

    NASA Astrophysics Data System (ADS)

    Siegmund, O.; Vallerga, J.; McPhate, J.; Malloy, J.; Tremsin, A.; Martin, A.; Ulmer, M.; Wessels, B.

    2006-11-01

    We have made substantial progress in the development of GaN photocathodes, including crystalline and polycrystalline GaN and InGaN coatings grown by chemical vapor deposition or molecular beam epitaxy on sapphire substrates. GaN and InGaN photocathodes have been developed with efficiencies up to 70% and cutoffs at ˜380 nm with low out of band response, and high stability and longevity. Samples have been processed and tested at ultra high vacuum to establish cathode process parameters, and some have been integrated into sealed tubes for long-term evaluation.

  18. Optical spectroscopy of cubic GaN in nanowires

    NASA Astrophysics Data System (ADS)

    Renard, J.; Tourbot, G.; Sam-Giao, D.; Bougerol, C.; Daudin, B.; Gayral, B.

    2010-08-01

    We show that highly homogeneous cubic GaN can be grown by plasma-assisted molecular beam epitaxy on wurtzite GaN nanowires. The line width of the donor bound exciton is below 3 meV and can reach 1.6 meV in the best parts of the studied sample. This allows to perform a detailed spectroscopy of cubic GaN, and, in particular, to determine the precise spectral positions of the donor bound exciton, the fundamental free exciton and the split-off exciton in a photoluminescence experiment.

  19. Strain Relief Analysis of InN Quantum Dots Grown on GaN

    PubMed Central

    2007-01-01

    We present a study by transmission electron microscopy (TEM) of the strain state of individual InN quantum dots (QDs) grown on GaN substrates. Moiré fringe and high resolution TEM analyses showed that the QDs are almost fully relaxed due to the generation of a 60° misfit dislocation network at the InN/GaN interface. By applying the Geometric Phase Algorithm to plan-view high-resolution micrographs, we show that this network consists of three essentially non-interacting sets of misfit dislocations lying along the directions. Close to the edge of the QD, the dislocations curve to meet the surface and form a network of threading dislocations surrounding the system. PMID:21794190

  20. Crystallography and elasticity of individual GaN nanotubes

    NASA Astrophysics Data System (ADS)

    Liu, Baodan; Bando, Yoshio; Wang, Mingsheng; Tang, Chengchun; Mitome, Masanori; Golberg, Dmitri

    2009-05-01

    High-purity, crystalline [001]-oriented GaN nanotubes with outer diameters of 200 nm or more, rough surfaces and irregular internal channels were synthesized under epitaxial growth on [001]-oriented sapphire substrates. Elastic property measurements on free-standing individual GaN nanotubes, using the in situ transmission electron microscopy (TEM) electromechanical resonance technique, pointed at an average Young's modulus E of 37 GPa and minimum quality factor of 320. These numbers are notably lower than those for previously reported GaN nanowires. The crystallography and chemistry of the GaN nanotubes were analyzed using TEM and energy dispersion x-ray spectroscopy (EDS). It is suggested that the lowered Young's modulus and quality factor of the nanotubes are mainly due to the surface roughness and defectiveness.

  1. Novel high frequency devices with graphene and GaN

    NASA Astrophysics Data System (ADS)

    Zhao, Pei

    This work focuses on exploring new materials and new device structures to develop novel devices that can operate at very high speed. In chapter 2, the high frequency performance limitations of graphene transistor with channel length less than 100 nm are explored. The simulated results predict that intrinsic cutoff frequency fT of graphene transistor can be close to 2 THz at 15 nm channel length. In chapter 3, we explored the possibility of developing a 2D materials based vertical tunneling device. An analytical model to calculate the channel potentials and current-voltage characteristics in a Symmetric tunneling Field-Effect-Transistor (SymFET) is presented. The symmetric resonant peak in SymFET is a good candidate for high-speed analog applications. Rest of the work focuses on Gallium Nitride (GaN), several novel device concepts based on GaN heterostructure have been proposed for high frequency and high power applications. In chapter 4, we compared the performance of GaN Schottky diodes on bulk GaN substrates and GaN-on-sapphire substrates. In addition, we also discussed the lateral GaN Schottky diode between metal/2DEGs. The advantage of lateral GaN Schottky diodes is the intrinsic cutoff frequency is in the THz range. In chapter 5, a GaN Heterostructure barrier diode (HBD) is designed using the polarization charge and band offset at the AlGaN/GaN heterojunction. The polarization charge at AlGaN/GaN interface behaves as a delta-doping which induces a barrier without any chemical doping. The IV characteristics can be explained by the barrier controlled thermionic emission current. GaN HBDs can be directly integrated with GaN HEMTs, and serve as frequency multipliers or mixers for RF applications. In chapter 6, a GaN based negative effective mass oscillator (NEMO) is proposed. The current in NEMO is estimated under the ballistic limits. Negative differential resistances (NDRs) can be observed with more than 50% of the injected electrons occupied the negative

  2. GaN nanorods coated with pure BN

    NASA Astrophysics Data System (ADS)

    Han, Wei-Qiang; Zettl, A.

    2002-12-01

    We report a method to efficiently synthesize gallium nitride (GaN) nanorods coated with insulating boron nitride (BN) layers. The GaN core is crystalline (with either a cubic zincblende or hexagonal wurtzite structure) and has diameters ranging from 10 to 85 nm and lengths up to 60 μm. The outer encapsulating BN shells with typical thicknesses less than 5 nm extend fully over, and adhere well to, the entire nanorod surface.

  3. Terahertz response of GaN thin films.

    PubMed

    Tsai, Tsong-Ru; Chen, Shi-Jie; Chang, Chih-Fu; Hsu, Sheng-Hsien; Lin, Tai-Yuan; Chi, Cheng-Chung

    2006-05-29

    The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the GaN film, but also estimates the relaxation time distribution function and average relaxation time.

  4. ARM MJO Investigation Experiment on Gan Island (AMIE-Gan) Science Plan

    SciTech Connect

    Long, CL; Del Genio, A; Deng, M; Fu, X; Gustafson, W; Houze, R; Jakob, C; Jensen, M; Johnson, R; Liu, X; Luke, E; May, P; McFarlane, S; Minnis, P; Schumacher, C; Vogelmann, A; Wang, Y; Webster, P; Xie, S; Zhang, C

    2011-04-11

    The overarching campaign, which includes the ARM Mobile Facility 2 (AMF2) deployment in conjunction with the Dynamics of the Madden-Julian Oscillation (DYNAMO) and the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) campaigns, is designed to test several current hypotheses regarding the mechanisms responsible for Madden-Julian Oscillation (MJO) initiation and propagation in the Indian Ocean area. The synergy between the proposed AMF2 deployment with DYNAMO/CINDY2011, and the corresponding funded experiment on Manus, combine for an overarching ARM MJO Investigation Experiment (AMIE) with two components: AMF2 on Gan Island in the Indian Ocean (AMIE-Gan), where the MJO initiates and starts its eastward propagation; and the ARM Manus site (AMIE-Manus), which is in the general area where the MJO usually starts to weaken in climate models. AMIE-Gan will provide measurements of particular interest to Atmospheric System Research (ASR) researchers relevant to improving the representation of MJO initiation in climate models. The framework of DYNAMO/CINDY2011 includes two proposed island-based sites and two ship-based locations forming a square pattern with sonde profiles and scanning precipitation and cloud radars at both island and ship sites. These data will be used to produce a Variational Analysis data set coinciding with the one produced for AMIE-Manus. The synergy between AMIE-Manus and AMIE-Gan will allow studies of the initiation, propagation, and evolution of the convective cloud population within the framework of the MJO. As with AMIE-Manus, AMIE-Gan/DYNAMO also includes a significant modeling component geared toward improving the representation of MJO initiation and propagation in climate and forecast models. This campaign involves the deployment of the second, marine-capable, AMF; all of the included measurement systems; and especially the scanning and vertically pointing radars. The campaign will include sonde

  5. Fabrication and properties of nanoporous GaN films

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Chua, S. J.; Sander, M. S.; Chen, P.; Tripathy, S.; Fonstad, C. G.

    2004-08-01

    Nanopore arrays with pore diameters of approximately 75nm were fabricated in GaN films by inductively coupled plasma etching using anodic aluminum oxide (AAO) films as etch masks. Nanoporous AAO films were formed on the GaN surface by evaporating an Al film onto a GaN epilayer and subsequently anodizing the aluminum. To minimize plasma-induced damage, the template was exposed to CF4-based plasma conditions. Scanning electron microscopy analysis shows that the diameter and the periodicity of the nanopores in the GaN were directly transferred from the original anodic alumina template. The pore diameter in the AAO film can be easily controlled by tuning the anodization conditions. Atomic force microscopy, photoluminescence, and micro-Raman techniques were employed to assess the etched GaN nanopore surface. This cost-effective, nonlithographic method to produce nano-patterned GaN templates is expected to be useful for growth and fabrication of nitride-based nanostructures and photonic band gap materials.

  6. High nitrogen pressure solution growth of GaN

    NASA Astrophysics Data System (ADS)

    Bockowski, Michal

    2014-10-01

    Results of GaN growth from gallium solution under high nitrogen pressure are presented. Basic of the high nitrogen pressure solution (HNPS) growth method is described. A new approach of seeded growth, multi-feed seed (MFS) configuration, is demonstrated. The use of two kinds of seeds: free-standing hydride vapor phase epitaxy GaN (HVPE-GaN) obtained from metal organic chemical vapor deposition (MOCVD)-GaN/sapphire templates and free-standing HVPE-GaN obtained from the ammonothermally grown GaN crystals, is shown. Depending on the seeds’ structural quality, the differences in the structural properties of pressure grown material are demonstrated and analyzed. The role and influence of impurities, like oxygen and magnesium, on GaN crystals grown from gallium solution in the MFS configuration is presented. The properties of differently doped GaN crystals are discussed. An application of the pressure grown GaN crystals as substrates for electronic and optoelectronic devices is reported.

  7. Role of the ganSPQAB Operon in Degradation of Galactan by Bacillus subtilis.

    PubMed

    Watzlawick, Hildegard; Morabbi Heravi, Kambiz; Altenbuchner, Josef

    2016-10-15

    Bacillus subtilis possesses different enzymes for the utilization of plant cell wall polysaccharides. This includes a gene cluster containing galactan degradation genes (ganA and ganB), two transporter component genes (ganQ and ganP), and the sugar-binding lipoprotein-encoding gene ganS (previously known as cycB). These genes form an operon that is regulated by GanR. The degradation of galactan by B. subtilis begins with the activity of extracellular GanB. GanB is an endo-β-1,4-galactanase and is a member of glycoside hydrolase (GH) family 53. This enzyme was active on high-molecular-weight arabinose-free galactan and mainly produced galactotetraose as well as galactotriose and galactobiose. These galacto-oligosaccharides may enter the cell via the GanQP transmembrane proteins of the galactan ABC transporter. The specificity of the galactan ABC transporter depends on the sugar-binding lipoprotein, GanS. Purified GanS was shown to bind galactotetraose and galactotriose using thermal shift assay. The energy for this transport is provided by MsmX, an ATP-binding protein. The transported galacto-oligosaccharides are further degraded by GanA. GanA is a β-galactosidase that belongs to GH family 42. The GanA enzyme was able to hydrolyze short-chain β-1,4-galacto-oligosaccharides as well as synthetic β-galactopyranosides into galactose. Thermal shift assay as well as electrophoretic mobility shift assay demonstrated that galactobiose is the inducer of the galactan operon regulated by GanR. DNase I footprinting revealed that the GanR protein binds to an operator overlapping the -35 box of the σ(A)-type promoter of Pgan, which is located upstream of ganS IMPORTANCE: Bacillus subtilis is a Gram-positive soil bacterium that utilizes different types of carbohydrates, such as pectin, as carbon sources. So far, most of the pectin degradation systems and enzymes have been thoroughly studied in B. subtilis Nevertheless, the B. subtilis utilization system of galactan, which is

  8. Growth of freestanding GaN using pillar-epitaxial lateral overgrowth from GaN nanocolumns

    NASA Astrophysics Data System (ADS)

    Bougrioua, Z.; Gibart, P.; Calleja, E.; Jahn, U.; Trampert, A.; Ristic, J.; Utrera, M.; Nataf, G.

    2007-12-01

    Dislocation-free and strain-free GaN nanopillars, grown on Si by molecular beam epitaxy, were used as nanoseeds for a new form of epitaxial lateral overgrowth (ELO) by metalorganic vapour phase epitaxy (MOVPE) until full coalescence. Such overgrown GaN films are almost relaxed and were used as templates for producing thick GaN layers by halide vapour phase epitaxy (HVPE). The final GaN film is easily separated from the starting Si substrate. This is henceforth a new technology to produce freestanding GaN. The GaN crystal quality was assessed by transmission electron microscopy (TEM), photo- and cathodoluminescence (PL, CL). It was seen that the pillar-ELO is produced from a limited number of nanopillars. Some dislocations and basal stacking faults are formed during the coalescence. However, those that propagate parallel to the substrate do not replicate in the top layer and it is expected that the thickened material present a reduced defect density.

  9. DLTS study of n-type GaN grown by MOCVD on GaN substrates

    NASA Astrophysics Data System (ADS)

    Tokuda, Y.; Matsuoka, Y.; Ueda, H.; Ishiguro, O.; Soejima, N.; Kachi, T.

    2006-10-01

    Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm 2 of substrate A and on an area of 1×1 cm 2 of substrate B. Two traps labeled B1 (Ec-0.23 eV) and B2 (Ec-0.58 eV) are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.

  10. Harnessing person-generated health data to accelerate patient-centered outcomes research: the Crohn's and Colitis Foundation of America PCORnet Patient Powered Research Network (CCFA Partners).

    PubMed

    Chung, Arlene E; Sandler, Robert S; Long, Millie D; Ahrens, Sean; Burris, Jessica L; Martin, Christopher F; Anton, Kristen; Robb, Amber; Caruso, Thomas P; Jaeger, Elizabeth L; Chen, Wenli; Clark, Marshall; Myers, Kelly; Dobes, Angela; Kappelman, Michael D

    2016-05-01

    The Crohn's and Colitis Foundation of America Partners Patient-Powered Research Network (PPRN) seeks to advance and accelerate comparative effectiveness and translational research in inflammatory bowel diseases (IBDs). Our IBD-focused PCORnet PPRN has been designed to overcome the major obstacles that have limited patient-centered outcomes research in IBD by providing the technical infrastructure, patient governance, and patient-driven functionality needed to: 1) identify, prioritize, and undertake a patient-centered research agenda through sharing person-generated health data; 2) develop and test patient and provider-focused tools that utilize individual patient data to improve health behaviors and inform health care decisions and, ultimately, outcomes; and 3) rapidly disseminate new knowledge to patients, enabling them to improve their health. The Crohn's and Colitis Foundation of America Partners PPRN has fostered the development of a community of citizen scientists in IBD; created a portal that will recruit, retain, and engage members and encourage partnerships with external scientists; and produced an efficient infrastructure for identifying, screening, and contacting network members for participation in research.

  11. Spectroscopic measurements of the surface stoichiometry of chemical vapor deposited GaN

    NASA Astrophysics Data System (ADS)

    Craft, H. S.; Rice, A. L.; Collazo, R.; Sitar, Z.; Maria, J.-P.

    2011-02-01

    We report on the surface stoichiometry of Ga-polar GaN films grown by metalorganic chemical vapor deposition as studied by x-ray photoelectron spectroscopy. GaN film surfaces are found to be Ga-rich, with Ga:N ratios ranging from 1.3:1 to 3.2:1. In vacuo ion-beam sputter/annealing studies show that these treatments drive the apparent Ga:N surface composition farther from unity, either through a decrease in surface contamination, oxidation of the surface, or both. Simple annealing experiments decrease the Ga:N ratio. The measured GaN ratio is correlated with the GaN growth time, suggesting that residual Ga precursor after growth interacts with the GaN surface as it cools.

  12. Future accelerators (?)

    SciTech Connect

    John Womersley

    2003-08-21

    I describe the future accelerator facilities that are currently foreseen for electroweak scale physics, neutrino physics, and nuclear structure. I will explore the physics justification for these machines, and suggest how the case for future accelerators can be made.

  13. Tailoring GaN semiconductor surfaces with biomolecules.

    PubMed

    Estephan, Elias; Larroque, Christian; Cuisinier, Frédéric J G; Bálint, Zoltán; Gergely, Csilla

    2008-07-24

    Functionalization of semiconductors constitutes a crucial step in using these materials for various electronic, photonic, biomedical, and sensing applications. Within the various possible approaches, selection of material-binding biomolecules from a random biological library, based on the natural recognition of proteins or peptides toward specific material, offers many advantages, most notably biocompatibility. Here we report on the selective functionalization of GaN, an important semiconductor that has found broad uses in the past decade due to its efficient electroluminescence and pronounced chemical stability. A 12-mer peptide ("GaN_probe") with specific recognition for GaN has evolved. The subtle interplay of mostly nonpolar hydrophobic and some polar amino acidic residues defines the high affinity adhesion properties of the peptide. The interaction forces between the peptide and GaN are quantified, and the hydrophobic domain of the GaN_probe is identified as primordial for the binding specificity. These nanosized binding blocks are further used for controlled placement of biotin-streptavidin complexes on the GaN surface. Thus, the controlled grow of a new, patterned inorganic-organic hybrid material is achieved. Tailoring of GaN by biological molecules can lead to a new class of nanostructured semiconductor-based devices.

  14. Native defects in GaN: a hybrid functional study

    NASA Astrophysics Data System (ADS)

    Diallo, Ibrahima Castillo; Demchenko, Denis

    Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

  15. Pulsed laser annealing of Be-implanted GaN

    NASA Astrophysics Data System (ADS)

    Wang, H. T.; Tan, L. S.; Chor, E. F.

    2005-11-01

    Postimplantation thermal processing of Be in molecular-beam-epitaxy-grown GaN by rapid thermal annealing (RTA) and pulsed laser annealing (PLA) was investigated. It has been found that the activation of Be dopants and the repair of implantation-induced defects in GaN films cannot be achieved efficiently by conventional RTA alone. On the other hand, good dopant activation and surface morphology and quality were obtained when the Be-implanted GaN film was annealed by PLA with a 248 nm KrF excimer laser. However, observations of off-resonant micro-Raman and high-resolution x-ray-diffraction spectra indicated that crystal defects and strain resulting from Be implantation were still existent after PLA, which probably degraded the carrier mobility and limited the activation efficiency to some extent. This can be attributed to the shallow penetration depth of the 248 nm laser in GaN, which only repaired the crystal defects in a thin near-surface layer, while the deeper defects were not annealed out well. This situation was significantly improved when the Be-implanted GaN was subjected to a combined process of PLA followed by RTA, which produced good activation of the dopants, good surface morphology, and repaired bulk and surface defects well.

  16. Optimization-driven identification of genetic perturbations accelerates the convergence of model parameters in ensemble modeling of metabolic networks.

    PubMed

    Zomorrodi, Ali R; Lafontaine Rivera, Jimmy G; Liao, James C; Maranas, Costas D

    2013-09-01

    The ensemble modeling (EM) approach has shown promise in capturing kinetic and regulatory effects in the modeling of metabolic networks. Efficacy of the EM procedure relies on the identification of model parameterizations that adequately describe all observed metabolic phenotypes upon perturbation. In this study, we propose an optimization-based algorithm for the systematic identification of genetic/enzyme perturbations to maximally reduce the number of models retained in the ensemble after each round of model screening. The key premise here is to design perturbations that will maximally scatter the predicted steady-state fluxes over the ensemble parameterizations. We demonstrate the applicability of this procedure for an Escherichia coli metabolic model of central metabolism by successively identifying single, double, and triple enzyme perturbations that cause the maximum degree of flux separation between models in the ensemble. Results revealed that optimal perturbations are not always located close to reaction(s) whose fluxes are measured, especially when multiple perturbations are considered. In addition, there appears to be a maximum number of simultaneous perturbations beyond which no appreciable increase in the divergence of flux predictions is achieved. Overall, this study provides a systematic way of optimally designing genetic perturbations for populating the ensemble of models with relevant model parameterizations.

  17. Structural defects in bulk GaN

    NASA Astrophysics Data System (ADS)

    Liliental-Weber, Z.; dos Reis, R.; Mancuso, M.; Song, C. Y.; Grzegory, I.; Porowski, S.; Bockowski, M.

    2014-10-01

    Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

  18. Characterization of electrospun GaN nanofibers

    NASA Astrophysics Data System (ADS)

    Ramos, Idalia; Melendez, Anamaris; Morales, Kristle; Campo, Eva M.; Santiago-Aviles, Jorge J.

    2010-03-01

    Gallium Nitride shows characteristics pertinent to optoelectronics and gas sensing applications. Nanofibers have been produced using electrospinning and a precursor composed of Gallium (III) Nitrate Hydrate dissolved in Dimethyl-Acetamide and Cellulose Acetate in Acetone and DMA. The resulting nanofibers were sintered at 400C in nitrogen for one hour to decompose the polymer, the furnace atmosphere switched to ammonia and the fibers sintered for periods of 3, 5 and 7 hrs at 900C. They showed morphologies with unclear dependence on processing parameters. X-ray Diffraction revealed the evolution towards wurtzite phase through annealing. From line broadening we estimate a crystalline domain size of about 12 nm. Transmission Electron Microscopy suggests nucleation and growth of X-tallites while Fourier-Transform Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy confirm the material evolution towards crystallinity and the production of wurtzite GaN nanofibers. I-V characteristics of single nanofibers show linearity with increments in conductivity for those fibers ammoniated during longer periods of time. Ongoing efforts aim at improving fabrication, sensing and photoluminescence characterization.

  19. Desorption Induced Formation of Negative Nanowires in GaN

    SciTech Connect

    Stach, E.A.; Kim, B.-J.

    2011-06-01

    We report in-situ transmission electron microscopy studies of the formation of negative nanowires created by thermal decomposition of single crystal GaN. During annealing, vertical negative nanowires are formed in [0 0 0 1] by preferential dissociation of GaN along the 1 0 {bar 1} 0 prism planes, while lateral negative nanowires grow in close-packed 1 0 {bar 1} 0 by the self-catalytic solid-liquid-vapor (SLV) mechanism. Our quantitative measurements show that the growth rates of the laterally grown negative nanowires are independent of the wire diameter, indicating that the rate-limiting step is the decomposition of GaN on the surface of the Ga droplets that catalyze their creation. These nanoscale features offer controllable templates for the creation and integration of a broad range of nanoscale materials systems, with potential applications in nanoscale fluidics.

  20. Application of GaN for photoelectrolysis of water

    NASA Astrophysics Data System (ADS)

    Puzyk, M. V.; Usikov, A. S.; Kurin, S. Yu; Puzyk, A. M.; Fomichev, A. D.; Ermakov, I. A.; Kovalev, D. S.; Papchenko, B. P.; Helava, H.; Makarov, Yu N.

    2015-11-01

    GaN layers of n-type and p-type conductivity grown by HVPE on sapphire substrates were used as working electrodes for water electrolysis, photoelectrolysis and hydrogen gas generation. Specifically the water splitting process is discussed. Corrosion of the GaN materials is also considered. The hydrogen production rate under 365-nm UV LED irradiation of the GaN and external bias was 0.3 ml/(cm2*h) for an n-GaN photoanode (n∼8×1016 cm-3) in 1M Na2SO4 electrolyte and 1.2 ml/(cm2*h) for an n-GaN photoanode (n∼1×1017 cm-3) in 1M KOH electrolyte.

  1. Gate stack engineering for GaN lateral power transistors

    NASA Astrophysics Data System (ADS)

    Yang, Shu; Liu, Shenghou; Liu, Cheng; Hua, Mengyuan; Chen, Kevin J.

    2016-02-01

    Developing optimal gate-stack technology is a key to enhancing the reliability and performance of GaN insulated-gate devices for high-voltage power switching applications. In this paper, we discuss current challenges and review our recent progresses in gate-stack technology development toward high-performance and high-reliability GaN power devices, including (1) interface engineering that creates a high-quality dielectric/III-nitride interface with low trap density; (2) barrier-layer engineering that enables optimal trade-off between performance and stability; (3) bulk quality and reliability enhancement of the gate dielectric. These gate-stack techniques in terms of new process development and device structure design are valuable to realize highly reliable and competitive GaN power devices.

  2. Stability of Carbon Incorpoated Semipolar GaN(1101) Surface

    NASA Astrophysics Data System (ADS)

    Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori

    2011-08-01

    The structural stability of carbon incorporated GaN(1101) surfaces is theoretically investigated by performing first-principles pseudopotential calculations. The calculated surface formation energies taking account of the metal organic vapor phase epitaxy conditions demonstrate that several carbon incorporated surfaces are stabilized depending on the growth conditions. Using surface phase diagrams, which are obtained by comparing the calculated adsorption energy with vapor-phase chemical potentials, we find that the semipolar surface forms NH2 and CH2 below ˜1660 K while the polar GaN(0001) surface with CH3 is stabilized below ˜1550 K. This difference could be one of possible explanations for p-type doping on the semipolar GaN(1101) surface.

  3. Biosensors based on GaN nanoring optical cavities

    NASA Astrophysics Data System (ADS)

    Kouno, Tetsuya; Takeshima, Hoshi; Kishino, Katsumi; Sakai, Masaru; Hara, Kazuhiko

    2016-05-01

    Biosensors based on GaN nanoring optical cavities were demonstrated using room-temperature photoluminescence measurements. The outer diameter, height, and thickness of the GaN nanorings were approximately 750-800, 900, and 130-180 nm, respectively. The nanorings functioned as whispering-gallery-mode (WGM)-type optical cavities and exhibited sharp resonant peaks like lasing actions. The evanescent component of the WGM was strongly affected by the refractive index of the ambient environment, the type of liquid, and the sucrose concentration of the analyzed solution, resulting in shifts of the resonant wavelengths. The results indicate that the GaN nanorings can potentially be used in sugar sensors of the biosensors.

  4. Desorption induced formation of negative nanowires in GaN

    NASA Astrophysics Data System (ADS)

    Kim, Bong-Joong; Stach, Eric A.

    2011-06-01

    We report in-situ transmission electron microscopy studies of the formation of negative nanowires created by thermal decomposition of single crystal GaN. During annealing, vertical negative nanowires are formed in [0 0 0 1] by preferential dissociation of GaN along the {1 0 1¯ 0} prism planes, while lateral negative nanowires grow in close-packed <1 0 1¯ 0> by the self-catalytic solid-liquid-vapor (SLV) mechanism. Our quantitative measurements show that the growth rates of the laterally grown negative nanowires are independent of the wire diameter, indicating that the rate-limiting step is the decomposition of GaN on the surface of the Ga droplets that catalyze their creation. These nanoscale features offer controllable templates for the creation and integration of a broad range of nanoscale materials systems, with potential applications in nanoscale fluidics.

  5. GaN nanowire tips for nanoscale atomic force microscopy.

    PubMed

    Behzadirad, Mahmoud; Nami, Mohsen; Rishinaramagalam, Ashwin; Feezell, Daniel; Busani, Tito

    2017-04-07

    Imaging of high-aspect-ratio nanostructures with sharp edges and straight walls in nanoscale metrology by Atomic Force Microscopy (AFM) has been challenging due to the mechanical properties and conical geometry of the majority of available commercial tips. Here we report on the fabrication of GaN probes for nanoscale metrology of high-aspect-ratio structures to enhance the resolution of AFM imaging and improve the durability of AFM tips. GaN nanowires (NWs) were fabricated using bottom-up and top-down techniques and bonded to Si cantilevers to scan vertical trenches on Si substrates. Over several scans, the GaN probes demonstrated excellent durability while scanning uneven structures and showed resolution enhancements in topography images, independent of scan direction, compared to commercial Si tips.

  6. Mg doping and its effect on the semipolar GaN(1122) growth kinetics

    SciTech Connect

    Lahourcade, L.; Wirthmueller, A.; Monroy, E.; Chauvat, M. P.; Ruterana, P.; Laufer, A.; Eickhoff, M.

    2009-10-26

    We report the effect of Mg doping on the growth kinetics of semipolar GaN(1122) synthesized by plasma-assisted molecular-beam epitaxy. Mg tends to segregate on the surface, inhibiting the formation of the self-regulated Ga film which is used as a surfactant for the growth of undoped and Si-doped GaN(1122). We observe an enhancement of Mg incorporation in GaN(1122) compared to GaN(0001). Typical structural defects or polarity inversion domains found in Mg-doped GaN(0001) were not observed for the semipolar films investigated in the present study.

  7. Conductivity based on selective etch for GaN devices and applications thereof

    DOEpatents

    Zhang, Yu; Sun, Qian; Han, Jung

    2015-12-08

    This invention relates to methods of generating NP gallium nitride (GaN) across large areas (>1 cm.sup.2) with controlled pore diameters, pore density, and porosity. Also disclosed are methods of generating novel optoelectronic devices based on porous GaN. Additionally a layer transfer scheme to separate and create free-standing crystalline GaN thin layers is disclosed that enables a new device manufacturing paradigm involving substrate recycling. Other disclosed embodiments of this invention relate to fabrication of GaN based nanocrystals and the use of NP GaN electrodes for electrolysis, water splitting, or photosynthetic process applications.

  8. A DFT study on NEA GaN photocathode with an ultrathin n-type Si-doped GaN cap layer

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Kong, Yike; Diao, Yu

    2016-10-01

    Due to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.

  9. Critical issues for homoepitaxial GaN growth by molecular beam epitaxy on hydride vapor-phase epitaxy-grown GaN substrates

    NASA Astrophysics Data System (ADS)

    Storm, D. F.; Hardy, M. T.; Katzer, D. S.; Nepal, N.; Downey, B. P.; Meyer, D. J.; McConkie, Thomas O.; Zhou, Lin; Smith, David J.

    2016-12-01

    While the heteroepitaxial growth of gallium nitride-based materials and devices on substrates such as SiC, sapphire, and Si has been well-documented, the lack of a cost-effective source of bulk GaN crystals has hindered similar progress on homoepitaxy. Nevertheless, freestanding GaN wafers are becoming more widely available, and there is great interest in growing GaN films and devices on bulk GaN substrates, in order to take advantage of the greatly reduced density of threading dislocations, particularly for vertical devices. However, homoepitaxial GaN growth is far from a trivial task due to the reactivity and different chemical sensitivities of N-polar (000_1) and Ga-polar (0001) GaN surfaces, which can affect the microstructure and concentrations of impurities in homoepitaxial GaN layers. In order to achieve high quality, high purity homoepitaxial GaN, it is necessary to investigate the effect of the ex situ wet chemical clean, the use of in situ cleaning procedures, the sensitivity of the GaN surface to thermal decomposition, and the effect of growth temperature. We review the current understanding of these issues with a focus on homoepitaxial growth of GaN by molecular beam epitaxy (MBE) on c-plane surfaces of freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE), as HVPE-grown substrates are most widely available. We demonstrate methods for obtaining homoepitaxial GaN layers by plasma-assisted MBE in which no additional threading dislocations are generated from the regrowth interface and impurity concentrations are greatly reduced.

  10. Development of Partial-Charge Potential for GaN

    SciTech Connect

    Gao, Fei; Devanathan, Ram; Oda, Takuji; Weber, William J.

    2006-09-01

    Partial-charged potentials for GaN are systematically developed that describes a wide range of structural properties, where the reference data for fitting the potential parameters are taken from ab initial calculations or experiments. The present potential model provides a good fit to different structural geometries and high pressure phases of GaN. The high-pressure transition from wurtzite to rock-salt structure is correctly described yielding the phase transition pressure of about 55 GPa, and the calculated volume change at the transition is in good agreement with experimental data. The results are compared with those obtained by ab initio simulations.

  11. Transmission electron microscopy of electrospun GaN nanofibers

    NASA Astrophysics Data System (ADS)

    Robles-García, Joshua L.; Meléndez, Anamaris; Yates, Douglas; Santiago-Avilés, Jorge J.; Ramos, Idalia; Campo, Eva M.

    2011-06-01

    We have reported earlier progress in producing polycrystalline wurtzite-polymorph and photo-conductive GaN nanofibers by electrospinning. This paper shows grain stacking during heat treatment and suggests the need to understand nucleation and grain growth following electrospinning. Transmission Electron Microscopy (TEM) analysis of GaN shows brittle fibers, grain stacking, and unfinished grain nucleation. X-Ray Diffraction analysis confirmed dominant hexagonal 101-wurtzite preferential overall orientation and the incipient grains are of high crystalline quality as seen by high resolution TEM.

  12. Dislocation luminescence in GaN single crystals under nanoindentation

    PubMed Central

    2014-01-01

    This work presents an experimental study on the dislocation luminescence in GaN by nanoindentation, cathodoluminescence, and Raman. The dislocation luminescence peaking at 3.12 eV exhibits a series of special properties in the cathodoluminescence measurements, and it completely disappears after annealing at 500°C. Raman spectroscopy shows evidence for existence of vacancies in the indented region. A comprehensive investigation encompassing cathodoluminescence, Raman, and annealing experiments allow the assignment of dislocation luminescence to conduction-band-acceptor transition involving Ga vacancies. The nanoscale plasticity of GaN can be better understood by considering the dislocation luminescence mechanism. PMID:25593548

  13. Redistribution of Implanted Dopants in GaN

    SciTech Connect

    Fu, M.; Gao, X.A.; Han, J.; Pearton, S.J.; Rieger, D.J.; Scarvepalli, V. Sekhar, J.A.; Shul, R.J.; Singh, R.K.; Wilson, R.G.; Zavada, J.M.; Zolper, J.C.

    1998-11-20

    Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of 3-5x1014 cm-2 and annealed at temperatures up to 1450 *C. No redistribution of any of the elements is detectable by Secondary Ion Mass Spectrometry, except for Be, which displays an apparent damage-assisted diffusion at 900 "C. At higher temperatures there is no further movement of the Be, suggesting that the point defect flux that assists motion at lower temperatures has been annealed. Effective diffusivities are <2X 1013 cm2.sec-1 at 1450 `C for each of the dopants in GaN.

  14. Ferromagnetism in undoped One-dimensional GaN Nanowires

    SciTech Connect

    Jeganathan, K. E-mail: jagan@physics.bdu.ac.in; Purushothaman, V.; Debnath, R.; Arumugam, S.

    2014-05-15

    We report an intrinsic ferromagnetism in vertical aligned GaN nanowires (NW) fabricated by molecular beam epitaxy without any external catalyst. The magnetization saturates at ∼0.75 × emu/gm with the applied field of 3000 Oe for the NWs grown under the low-Gallium flux of 2.4 × 10{sup −8} mbar. Despite a drop in saturation magnetization, narrow hysteresis loop remains intact regardless of Gallium flux. Magnetization in vertical standing GaN NWs is consistent with the spectral analysis of low-temperature photoluminescence pertaining to Ga-vacancies associated structural defects at the nanoscale.

  15. GaN surface states investigated by electrochemical studies

    NASA Astrophysics Data System (ADS)

    Winnerl, Andrea; Garrido, Jose A.; Stutzmann, Martin

    2017-03-01

    We present a systematic study of electrochemically active surface states on MOCVD-grown n-type GaN in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy over a wide range of potentials and frequencies. In order to alter the surface states, the GaN samples are either etched or oxidized, and the influence of the surface treatment on the defect-mediated charge transfer to the electrolyte is investigated. Etching in HCl removes substoichiometric GaO x , and leads to a pronounced density of electrochemically active surface states. Oxidation effectively removes these surface states.

  16. Chemical mechanical polishing of freestanding GaN substrates

    NASA Astrophysics Data System (ADS)

    Huaiyue, Yan; Xiangqian, Xiu; Zhanhui, Liu; Rong, Zhang; Xuemei, Hua; Zili, Xie; Ping, Han; Yi, Shi; Youdou, Zheng

    2009-02-01

    Chemical mechanical polishing (CMP) has been used to produce smooth and scratch-free surfaces for GaN. In the aqueous solution of KOH, GaN is subjected to etching. At the same time, all surface irregularities, including etch pyramids, roughness after mechanical polishing and so on will be removed by a polishing pad. The experiments had been performed under the condition of different abrasive particle sizes of the polishing pad. Also the polishing results for different polishing times are analyzed, and chemical mechanical polishing resulted in an average root mean square (RMS) surface roughness of 0.565 nm, as measured by atomic force microscopy.

  17. Self-assembled GaN hexagonal micropyramid and microdisk

    SciTech Connect

    Lo Ikai; Hsieh, C.-H.; Hsu, Y.-C.; Pang, W.-Y.; Chou, M.-C.

    2009-02-09

    The self-assembled GaN hexagonal micropyramid and microdisk were grown on LiAlO{sub 2} by plasma-assisted molecular-beam epitaxy. It was found that the (0001) disk was established with the capture of N atoms by most-outside Ga atoms as the (1x1) surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from M-plane GaN.

  18. Highly transparent ammonothermal bulk GaN substrates

    SciTech Connect

    Jiang, WK; Ehrentraut, D; Downey, BC; Kamber, DS; Pakalapati, RT; Do Yoo, H; D'Evelyn, MP

    2014-10-01

    A novel apparatus has been employed to grow ammonothermal (0001) gallium nitride (GaN) with diameters up to 2 in The crystals have been characterized by x-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), and optical spectroscopy. High crystallinity GaN with FWHM values about 20-50 arcsec and dislocation densities below 1 x 10(5) cm(-2) have been obtained. High optical transmission was achieved with an optical absorption coefficient below 1 cm(-1) at a wavelength of 450 nm. (C) 2014 Elsevier B.V. All rights reserved.

  19. GaN nanowire arrays by a patterned metal-assisted chemical etching

    NASA Astrophysics Data System (ADS)

    Wang, K. C.; Yuan, G. D.; Wu, R. W.; Lu, H. X.; Liu, Z. Q.; Wei, T. B.; Wang, J. X.; Li, J. M.; Zhang, W. J.

    2016-04-01

    We developed an one-step and two-step metal-assisted chemical etching method to produce self-organized GaN nanowire arrays. In one-step approach, GaN nanowire arrays are synthesized uniformly on GaN thin film surface. However, in a two-step etching processes, GaN nanowires are formed only in metal uncovered regions, and GaN regions with metal-covering show nano-porous sidewalls. We propose that nanowires and porous nanostructures are tuned by sufficient and limited etch rate, respectively. PL spectra shows a red-shift of band edge emission in GaN nanostructures. The formation mechanism of nanowires was illustrated by two separated electrochemical reactions occur simultaneously. The function of metals and UV light was illustrated by the scheme of potential relationship between energy bands in Si, GaN and standard hydrogen electrode potential of solution and metals.

  20. Properties of amorphous GaN from first-principles simulations

    NASA Astrophysics Data System (ADS)

    Cai, B.; Drabold, D. A.

    2011-08-01

    Amorphous GaN (a-GaN) models are obtained from first-principles simulations. We compare four a-GaN models generated by “melt-and-quench” and the computer alchemy method. We find that most atoms tend to be fourfold, and a chemically ordered continuous random network is the ideal structure for a-GaN albeit with some coordination defects. Where the electronic structure is concerned, the gap is predicted to be less than 1.0 eV, underestimated as usual by a density functional calculation. We observe a highly localized valence tail and a remarkably delocalized exponential conduction tail in all models generated. Based upon these results, we speculate on potential differences in n- and p-type doping. The structural origin of tail and defect states is discussed. The vibrational density of states and dielectric function are computed and seem consistent with experiment.

  1. Basic ammonothermal GaN growth in molybdenum capsules

    NASA Astrophysics Data System (ADS)

    Pimputkar, S.; Speck, J. S.; Nakamura, S.

    2016-12-01

    Single crystal, bulk gallium nitride (GaN) crystals were grown using the basic ammonothermal method in a high purity growth environment created using a non-hermetically sealed molybdenum (Mo) capsule and compared to growths performed in a similarly designed silver (Ag) capsule and capsule-free René 41 autoclave. Secondary ion mass spectrometry (SIMS) analysis revealed transition metal free (<1×1017 cm-3) GaN crystals. Anomalously low oxygen concentrations ((2-6)×1018 cm-3) were measured in a {0001} seeded crystal boule grown using a Mo capsule, despite higher source material oxygen concentrations ((1-5)×1019 cm-3) suggesting that molybdenum (or molybdenum nitrides) may act to getter oxygen under certain conditions. Total system pressure profiles from growth runs in a Mo capsule system were comparable to those without a capsule, with pressures peaking within 2 days and slowly decaying due to hydrogen diffusional losses. Measured Mo capsule GaN growth rates were comparable to un-optimized growth rates in capsule-free systems and appreciably slower than in Ag-capsule systems. Crystal quality replicated that of the GaN seed crystals for all capsule conditions, with high quality growth occurring on the (0001) Ga-face. Optical absorption and impurity concentration characterization suggests reduced concentrations of hydrogenated gallium vacancies (VGa-Hx).

  2. Refractive index of erbium doped GaN thin films

    SciTech Connect

    Alajlouni, S.; Sun, Z. Y.; Li, J.; Lin, J. Y.; Jiang, H. X.; Zavada, J. M.

    2014-08-25

    GaN is an excellent host for erbium (Er) to provide optical emission in the technologically important as well as eye-safe 1540 nm wavelength window. Er doped GaN (GaN:Er) epilayers were synthesized on c-plane sapphire substrates using metal organic chemical vapor deposition. By employing a pulsed growth scheme, the crystalline quality of GaN:Er epilayers was significantly improved over those obtained by conventional growth method of continuous flow of reaction precursors. X-ray diffraction rocking curve linewidths of less than 300 arc sec were achieved for the GaN (0002) diffraction peak, which is comparable to the typical results of undoped high quality GaN epilayers and represents a major improvement over previously reported results for GaN:Er. Spectroscopic ellipsometry was used to determine the refractive index of the GaN:Er epilayers in the 1540 nm wavelength window and a linear dependence on Er concentration was found. The observed refractive index increase with Er incorporation and the improved crystalline quality of the GaN:Er epilayers indicate that low loss GaN:Er optical waveguiding structures are feasible.

  3. Photoluminescence of Zn-implanted GaN

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.; Hutchby, J. A.

    1974-01-01

    The photoluminescence spectrum of Zn-implanted GaN peaks at 2.87 eV at room temperature. The emission efficiency decreases linearly with the logarithm of the Zn concentration in the range from 1 x 10 to the 18th to 20 x 10 to the 18th Zn/cu cm.

  4. Networks.

    ERIC Educational Resources Information Center

    Maughan, George R.; Petitto, Karen R.; McLaughlin, Don

    2001-01-01

    Describes the connectivity features and options of modern campus communication and information system networks, including signal transmission (wire-based and wireless), signal switching, convergence of networks, and network assessment variables, to enable campus leaders to make sound future-oriented decisions. (EV)

  5. Luminescence properties of defects in GaN

    NASA Astrophysics Data System (ADS)

    Reshchikov, Michael A.; Morkoç, Hadis

    2005-03-01

    Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects (vacancies, interstitial, and antisites), intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of

  6. LINEAR ACCELERATOR

    DOEpatents

    Colgate, S.A.

    1958-05-27

    An improvement is presented in linear accelerators for charged particles with respect to the stable focusing of the particle beam. The improvement consists of providing a radial electric field transverse to the accelerating electric fields and angularly introducing the beam of particles in the field. The results of the foregoing is to achieve a beam which spirals about the axis of the acceleration path. The combination of the electric fields and angular motion of the particles cooperate to provide a stable and focused particle beam.

  7. Mechanism of stress control for GaN growth on Si using AlN interlayers

    NASA Astrophysics Data System (ADS)

    Suzuki, Michihiro; Nakamura, Akihiro; Nakano, Yoshiaki; Sugiyama, Masakazu

    2017-04-01

    For the purpose of controlling the wafer bow of GaN-on-Si structure, in situ curvature transient during the growth of a GaN layer on an AlN interlayer was investigated systematically by estimating the compressive strain applied to the GaN layer with the progress of the layer growth. The compressive strain was dependent on the morphology of the GaN surface prior to the growth of the AlN interlayer. It was found that the transition sequence from GaN growth to AlN growth induces roughening of the GaN surface and both high NH3 partial pressure and the short transition time were effective for reducing the roughness of the GaN surface beneath the AlN interlayer. The improved transition sequence increased the compressive strain in GaN by a factor of 2.5. The AlN grown at the same temperature as that of GaN was beneficial in both better surface morphology and the reduction of the transition time between GaN growth and AlN growth. With this high-temperature AlN interlayer, its thickness is another important factor governing the compressive strain in GaN. To get AlN relaxed for applying the compressive strain to GaN, the AlN layer should be thicker but too thick layer after relaxation results in surface roughening, which in turn introduces defects to the overlying GaN layer and reduces the compressive strain by partial lattice relaxation of GaN.

  8. Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

    PubMed

    Liu, Qingyun; Liu, Baodan; Yang, Wenjin; Yang, Bing; Zhang, Xinglai; Labbé, Christophe; Portier, Xavier; An, Vladimir; Jiang, Xin

    2017-04-11

    Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001)GaN∥(0001)sapphire and (101[combining macron]0)GaN∥(112[combining macron]0)sapphire. Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

  9. Substitutional and interstitial oxygen in wurtzite GaN

    NASA Astrophysics Data System (ADS)

    Wright, A. F.

    2005-11-01

    Density-functional theory was used to compute energy-minimum configurations and formation energies of substitutional and interstitial oxygen (O) in wurtzite GaN. The results indicate that O substituted at a N site (ON) acts as a single donor with the ionized state (ON+1) being the most stable O state in p-type GaN. In n-type GaN, interstitial O (OI) is predicted to be a double acceptor and O substituted at a Ga site (OGa) is predicted to be a triple acceptor. The formation energies of these two species are comparable to that of ON in n-type GaN and, as such, they should form and compensate the ON donors. The extent of compensation was estimated for both Ga-rich and N-rich conditions with a total O concentration of 1017cm-3. Ga-rich conditions yielded negligible compensation and an ON concentration in excess of 9.9×1016cm-3. N-rich conditions yielded a 25% lower ON concentration, due to the increased stability of OI and OGa relative to ON, and moderate compensation. These findings are consistent with experimental results indicating that O acts as a donor in GaN(O). Complexes of ON with the Mg acceptor and OI with the Si donor were examined. Binding energies for charge-conserving reactions were ⩾0.5eV, indicating that these complexes can exist in equilibrium at room temperature. Complexes of ON with the Ga vacancy in n-type GaN were also examined and their binding energies were 1.2 and 1.4eV, indicating that appreciable concentrations can exist in equilibrium even at elevated temperatures.

  10. Specific peptide for functionalization of GaN

    NASA Astrophysics Data System (ADS)

    Estephan, E.; Larroque, C.; Cloitre, T.; Cuisinier, F. J. G.; Gergely, C.

    2008-04-01

    Nanobiotechnology aims to exploit biomolecular recognition and self-assembly capabilities for integrating advanced materials into medicine and biology. However frequent problems are encountered at the interface of substrate-biological molecule, as the direct physical adsorption of biological molecules is dependent of unpredictable non-specific interactions with the surface, often causing their denaturation. Therefore, a proper functionalization of the substrate should avoid a loss of biological activity. In this work we address the functionalization of the semiconductor GaN (0001) for biosensing applications. The basic interest of using III-V class semiconductors is their good light emitting properties and a fair chemical stability that allows various applications of these materials. The technology chosen to elaborate GaN-specific peptides is the combinatorial phage-display method, a biological screening procedure based on affinity selection. An M13 bacteriophage library has been used to screen 10 10 different peptides against the GaN (0001) surface to finally isolate one specific peptide. The preferential attachment of the biotinylated selected peptide onto the GaN (0001), in close proximity to a surface of different chemical and structural composition has been demonstrated by fluorescence microscopy. Further physicochemical studies have been initiated to evaluate the semiconductor-peptide interface and understand the details in the specific recognition of peptides for semiconductor substrates. Fourier Transform Infrared spectroscopy in Attenuated Total Reflection mode (FTIR-ATR) has been employed to prove the presence of peptides on the surface. Our Atomic Force Microscopy (AFM) studies on the morphology of the GaN surface after functionalization revealed a total surface coverage by a very thin, homogeneous peptide layer. Due to its good biocompatibility, functionalized GaN devices might evolve in a new class of implantable biosensors for medical applications.

  11. Acceleration switch

    DOEpatents

    Abbin, Jr., Joseph P.; Devaney, Howard F.; Hake, Lewis W.

    1982-08-17

    The disclosure relates to an improved integrating acceleration switch of the type having a mass suspended within a fluid filled chamber, with the motion of the mass initially opposed by a spring and subsequently not so opposed.

  12. Acceleration switch

    DOEpatents

    Abbin, J.P. Jr.; Devaney, H.F.; Hake, L.W.

    1979-08-29

    The disclosure relates to an improved integrating acceleration switch of the type having a mass suspended within a fluid filled chamber, with the motion of the mass initially opposed by a spring and subsequently not so opposed.

  13. ION ACCELERATOR

    DOEpatents

    Bell, J.S.

    1959-09-15

    An arrangement for the drift tubes in a linear accelerator is described whereby each drift tube acts to shield the particles from the influence of the accelerating field and focuses the particles passing through the tube. In one embodiment the drift tube is splii longitudinally into quadrants supported along the axis of the accelerator by webs from a yoke, the quadrants. webs, and yoke being of magnetic material. A magnetic focusing action is produced by energizing a winding on each web to set up a magnetic field between adjacent quadrants. In the other embodiment the quadrants are electrically insulated from each other and have opposite polarity voltages on adjacent quadrants to provide an electric focusing fleld for the particles, with the quadrants spaced sufficienily close enough to shield the particles within the tube from the accelerating electric field.

  14. LINEAR ACCELERATOR

    DOEpatents

    Christofilos, N.C.; Polk, I.J.

    1959-02-17

    Improvements in linear particle accelerators are described. A drift tube system for a linear ion accelerator reduces gap capacity between adjacent drift tube ends. This is accomplished by reducing the ratio of the diameter of the drift tube to the diameter of the resonant cavity. Concentration of magnetic field intensity at the longitudinal midpoint of the external sunface of each drift tube is reduced by increasing the external drift tube diameter at the longitudinal center region.

  15. Linear induction accelerator

    DOEpatents

    Buttram, M.T.; Ginn, J.W.

    1988-06-21

    A linear induction accelerator includes a plurality of adder cavities arranged in a series and provided in a structure which is evacuated so that a vacuum inductance is provided between each adder cavity and the structure. An energy storage system for the adder cavities includes a pulsed current source and a respective plurality of bipolar converting networks connected thereto. The bipolar high-voltage, high-repetition-rate square pulse train sets and resets the cavities. 4 figs.

  16. Atomic-Level Study of Melting Behavior of GaN Nanotubes

    SciTech Connect

    Wang, Zhiguo; Zu, Xiaotao; Gao, Fei; Weber, William J.

    2006-09-20

    Molecular dynamics simulations with a Stillinger-Weber potential have been used to investigate the melting behavior of wurtzite-type single crystalline GaN nanotubes. The simulations show that the melting temperature of GaN nanotubes is much lower than that of bulk GaN, which may be associated with the large surface-to-volume ratio of the nanotubes. The melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a GaN slab. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. The melting temperature of a single-crystalline GaN nanotube with [100]-oriented lateral facets is higher than that with [110]-oriented lateral facets for the same thickness.

  17. Photo-induced Doping in GaN Epilayers with Graphene Quantum Dots

    PubMed Central

    Lin, T. N.; Inciong, M. R.; Santiago, S. R. M. S.; Yeh, T. W.; Yang, W. Y.; Yuan, C. T.; Shen, J. L.; Kuo, H. C.; Chiu, C. H.

    2016-01-01

    We demonstrate a new doping scheme where photo-induced carriers from graphene quantum dots (GQDs) can be injected into GaN and greatly enhance photoluminescence (PL) in GaN epilayers. An 8.3-fold enhancement of PL in GaN is observed after the doping. On the basis of time-resolved PL studies, the PL enhancement is attributed to the carrier transfer from GQDs to GaN. Such a carrier transfer process is caused by the work function difference between GQDs and GaN, which is verified by Kelvin probe measurements. We have also observed that photocurrent in GaN can be enhanced by 23-fold due to photo-induced doping with GQDs. The improved optical and transport properties from photo-induced doping are promising for applications in GaN-based optoelectronic devices. PMID:26987403

  18. Fabrication and characterization of GaN nanowire doubly clamped resonators

    SciTech Connect

    Maliakkal, Carina B. Mathew, John P.; Hatui, Nirupam; Rahman, A. Azizur; Deshmukh, Mandar M.; Bhattacharya, Arnab

    2015-09-21

    Gallium nitride (GaN) nanowires (NWs) have been intensely researched as building blocks for nanoscale electronic and photonic device applications; however, the mechanical properties of GaN nanostructures have not been explored in detail. The rigidity, thermal stability, and piezoelectric properties of GaN make it an interesting candidate for nano-electromechanical systems. We have fabricated doubly clamped GaN NW electromechanical resonators on sapphire using electron beam lithography and estimated the Young's modulus of GaN from resonance frequency measurements. For wires of triangular cross section with side ∼90 nm, we obtained values for the Young's modulus to be about 218 and 691 GPa, which are of the same order of magnitude as the values reported for bulk GaN. We also discuss the role of residual strain in the nanowire on the resonant frequency and the orientation dependence of the Young's modulus in wurtzite crystals.

  19. Graphene oxide assisted synthesis of GaN nanostructures for reducing cell adhesion.

    PubMed

    Yang, Rong; Zhang, Ying; Li, Jingying; Han, Qiusen; Zhang, Wei; Lu, Chao; Yang, Yanlian; Dong, Hongwei; Wang, Chen

    2013-11-21

    We report a general approach for the synthesis of large-scale gallium nitride (GaN) nanostructures by the graphene oxide (GO) assisted chemical vapor deposition (CVD) method. A modulation effect of GaN nanostructures on cell adhesion has been observed. The morphology of the GaN surface can be controlled by GO concentrations. This approach, which is based on the predictable choice of the ratio of GO to catalysts, can be readily extended to the synthesis of other materials with controllable nanostructures. Cell studies show that GaN nanostructures reduced cell adhesion significantly compared to GaN flat surfaces. The cell-repelling property is related to the nanostructure and surface wettability. These observations of the modulation effect on cell behaviors suggest new opportunities for novel GaN nanomaterial-based biomedical devices. We believe that potential applications will emerge in the biomedical and biotechnological fields.

  20. Structural properties of free-standing 50 mm diameter GaN waferswith (101_0) orientation grown on LiAlO2

    SciTech Connect

    Jasinski, Jacek; Liliental-Weber, Zuzanna; Maruska, Herbert-Paul; Chai, Bruce H.; Hill, David W.; Chou, Mitch M.C.; Gallagher, John J.; Brown, Stephen

    2005-09-27

    (10{und 1}0) GaN wafers grown on (100) face of {gamma}-LiAlO{sub 2} were studied using transmission electron microscopy. Despite good lattice matching in this heteroepitaxial system, high densities of planar structural defects in the form of stacking faults on the basal plane and networks of boundaries located on prism planes inclined to the layer/substrate interface were present in these GaN layers. In addition, significant numbers of threading dislocations were observed. High-resolution electron microscopy indicates that stacking faults present on the basal plane in these layers are of low-energy intrinsic I1type. This is consistent with diffraction contrast experiments.

  1. Radiation effects in GaN devices and materials (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Sun, Ke-Xun; Nelson, Ron; Yeamans, Charles

    2016-10-01

    Gallium Nitride (GaN) is a wide-bandgap semiconductor having excellent radiation properties. GaN crystal is ionic-covalent with significant iconicity resulting in stronger molecular bond strength, which in in turn leads to excellent radiation hardness. Further, GaN has ultrafast carrier relaxation time. GaN transistors are promising for high-frequency applications due to their large bandgap (3.9eV) and higher breakdown field (<5MV/cm). These exceptional characteristics make GaN suitable to operate in high radiation flux environment such as fusion plasma facilities, for ultrafast detection. The expected detector temporal response is faster than 0.01-1 ns. We have been systematically testing neutron radiation effects in GaN devices and materials at Los Alamos Neutron Science Center (LANSCE) with ever increased neutron fluence levels, and at National Ignition Facility (NIF) high foot, high yield shots. In 2013 LANSCE run cycle, we tested GaN UV LED devices at 3.1E11 neutrons/cm^2. In 2015-2016 LANSCE run cycles, we have been operating three neutron beam lines with fluence level 1.2E11, 1.5E13, and 1E15 neutrons/cm^2. The irradiated samples include GaN UV LEDs, GaN HEMTs, and GaN substrates. In the experiments up to 2015 run cycle, we have characterized electrical and optical performances of GaN device before and after neutron irradiation, including the device IV curve measurements monitored at over the three months neutron irradiation time, and device IV curve measurements before and after NIF high yield shot irradiation. We observed no substantial degradation. These experiments firmly established GaN devices as the radiation hard platform of the next generation fusion plasma diagnostic instruments.

  2. Networking.

    ERIC Educational Resources Information Center

    Duvall, Betty

    Networking is an information giving and receiving system, a support system, and a means whereby women can get ahead in careers--either in new jobs or in current positions. Networking information can create many opportunities: women can talk about how other women handle situations and tasks, and previously established contacts can be used in…

  3. On the Helmert-blocking technique: its acceleration by block Choleski decomposition and formulae to insert observations into an adjusted network

    PubMed Central

    Del Rio, Eduardo; Oliveira, Leonardo

    2015-01-01

    The Helmert-blocking technique is a common approach to adjust large geodetic networks like Europeans and Brazilians. The technique is based upon a division of the network into partial networks called blocks. This way, the global network adjustment can be done by manipulating these blocks. Here we show alternatives to solve the block system that arises from the application of the technique. We show an alternative that optimizes its implementation as the elapsed processing time is decreased by about 33%. We also show that to insert observations into an adjusted network it is not necessary to readjust the whole network. We show the formulae to insert new observations into an adjusted network that are more efficient than simply readjusting the whole new network. PMID:26064634

  4. Understanding of surface pit formation mechanism of GaN grown in MOCVD based on local thermodynamic equilibrium assumption

    NASA Astrophysics Data System (ADS)

    Zhi-Yuan, Gao; Xiao-Wei, Xue; Jiang-Jiang, Li; Xun, Wang; Yan-Hui, Xing; Bi-Feng, Cui; De-Shu, Zou

    2016-06-01

    Frank’s theory describes that a screw dislocation will produce a pit on the surface, and has been evidenced in many material systems including GaN. However, the size of the pit calculated from the theory deviates significantly from experimental result. Through a careful observation of the variations of surface pits and local surface morphology with growing temperature and V/III ratio for c-plane GaN, we believe that Frank’s model is valid only in a small local surface area where thermodynamic equilibrium state can be assumed to stay the same. If the kinetic process is too vigorous or too slow to reach a balance, the local equilibrium range will be too small for the center and edge of the screw dislocation spiral to be kept in the same equilibrium state. When the curvature at the center of the dislocation core reaches the critical value 1/r 0, at the edge of the spiral, the accelerating rate of the curvature may not fall to zero, so the pit cannot reach a stationary shape and will keep enlarging under the control of minimization of surface energy to result in a large-sized surface pit. Project supported by the National Natural Science Foundation of China (Grant Nos. 11204009 and 61204011) and the Beijing Municipal Natural Science Foundation, China (Grant No. 4142005).

  5. Dependence of adhesion strength between GaN LEDs and sapphire substrate on power density of UV laser irradiation

    NASA Astrophysics Data System (ADS)

    Park, Junsu; Sin, Young-Gwan; Kim, Jae-Hyun; Kim, Jaegu

    2016-10-01

    Selective laser lift-off (SLLO) is an innovative technology used to manufacture and repair micro-light-emitting diode (LED) displays. In SLLO, laser is irradiated to selectively separate micro-LED devices from a transparent sapphire substrate. The light source used is an ultraviolet (UV) laser with a wavelength of 266 nm, pulse duration of 20 ns, and repetition rate of 30 kHz. Controlled adhesion between a LED and the substrate is key for a SLLO process with high yield and reliability. This study examined the fundamental relationship between adhesion and laser irradiation. Two competing mechanisms affect adhesion at the irradiated interface between the GaN LED and sapphire substrate: Ga precipitation caused by the thermal decomposition of GaN and roughened interface caused by thermal damage on the sapphire. The competition between these two mechanisms leads to a non-trivial SLLO condition that needs optimization. This study helps understand the SLLO process, and accelerate the development of a process for manufacturing micro-LED displays via SLLO for future applications.

  6. Advanced technology of GaN based tunable violet laser with external cavity for holographic data storage

    NASA Astrophysics Data System (ADS)

    Mori, Naoki; Dejima, Norihiro; Higashiura, Atsushi; Omori, Masaki; Higuchi, Yu

    2016-09-01

    We successfully completed the development of a GaN based Tunable laser for Tapestry holographic data storage through collaborative research with InPhase Technologies in 2010. After the collaborative research, with the aim to achieve further advance development for commercial storage use, we have continued to improve the laser characteristics, especially coherence property and high optical output power are significant issues for holographic data storage. Longitudinal single mode lasing is one of most important property in hologram recording; therefore we addressed to optimize laser diode structure for external cavity laser. In parallel to that, we have reviewed not only a Laser diode structure but also laser drive processing, and have successfully developed a function. In the case of performing high visibility hologram recording, prior to exposing to medium, both the laser driving current and the wavelength are slightly adjusted to achieve single mode lasing. We call this function "Mode Stabilizer". Mode Stabilizer can automatically execute the adjustments in combination with an internal mode sensor for visibility sensing. This function is advantageous in that erroneous page-recording can be avoided. Moreover, we achieved high optical output power of 100 mW increased from conventional 45 mW, by optimizing the device structure of GaN Laser diode. With this high optical output power, acceleration of recording bit rate becomes possible.

  7. Spontaneous nucleation and growth of GaN nanowires: the fundamental role of crystal polarity.

    PubMed

    Fernández-Garrido, Sergio; Kong, Xiang; Gotschke, Tobias; Calarco, Raffaella; Geelhaar, Lutz; Trampert, Achim; Brandt, Oliver

    2012-12-12

    We experimentally investigate whether crystal polarity affects the growth of GaN nanowires in plasma-assisted molecular beam epitaxy and whether their formation has to be induced by defects. For this purpose, we prepare smooth and coherently strained AlN layers on 6H-SiC(0001) and SiC(0001̅) substrates to ensure a well-defined polarity and an absence of structural and morphological defects. On N-polar AlN, a homogeneous and dense N-polar GaN nanowire array forms, evidencing that GaN nanowires form spontaneously in the absence of defects. On Al-polar AlN, we do not observe the formation of Ga-polar GaN NWs. Instead, sparse N-polar GaN nanowires grow embedded in a Ga-polar GaN layer. These N-polar GaN nanowires are shown to be accidental in that the necessary polarity inversion is induced by the formation of Si(x)N. The present findings thus demonstrate that spontaneously formed GaN nanowires are irrevocably N-polar. Due to the strong impact of the polarity on the properties of GaN-based devices, these results are not only essential to understand the spontaneous formation of GaN nanowires but also of high technological relevance.

  8. Si in GaN -- On the nature of the background donor

    SciTech Connect

    Wetzel, C.; Chen, A.L.; Suski, T.; Ager, J.W. III; Walukiewicz, W.

    1996-08-01

    A characterization of the Si impurity in GaN is performed by Raman spectroscopy. Applying hydrostatic pressure up to 25 GPa the authors study the behavior of the LO phonon-plasmon mode in a series of high mobility Si doped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observed in Si doped samples. The authors find that Si is a shallow hydrogenic donor throughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.

  9. Growth and properties of bulk single crystals of GaN

    SciTech Connect

    Suski, T.; Perlin, P.; Leszczynski, M.

    1996-11-01

    In this paper the authors review recent developments in the growth of bulk GaN crystals by a high-pressure, high-temperature method. They also provide information on various physical properties of bulk GaN material. Then, some preliminary results on the homoepitaxial growth of GaN are given. In the second part of this paper the authors discuss the following problems: the possible origin of the large free electron concentration in undoped GaN material, the parasitic effect of yellow luminescence and the nature of Zn- and Mg-acceptors.

  10. Fabrication of a Lateral Polarity GaN MESFET: An Exploratory Study

    DTIC Science & Technology

    2007-06-27

    the sheet resistance between Ga- polar and mixed-polar GaN films. Highly resistive Ga-polar GaN is advantageous in optoelectronic and electronic device...re si st an ce [ M Ω /s q. ] 108642 2nd nitridation time [min] (b) Figure 5. Sheet resistance of GaN grown on AlN region for the first set of LT...AlN layers deposited for 4, 6, and 8 min. 2nd nitridation time after ex-situ process was kept at 950°C for 1 min. (b) Sheet resistance of GaN grown on

  11. Homoepitaxial growth of catalyst-free GaN wires on N-polar substrates

    NASA Astrophysics Data System (ADS)

    Chen, X. J.; Perillat-Merceroz, G.; Sam-Giao, D.; Durand, C.; Eymery, J.

    2010-10-01

    The shape of c-oriented GaN nanostructures is found to be directly related to the crystal polarity. As evidenced by convergent beam electron diffraction applied to GaN nanostructures grown by metal-organic vapor phase epitaxy on c-sapphire substrates: wires grown on nitridated sapphire have the N-polarity ([0001¯]) whereas pyramidal crystals have Ga-polarity ([0001]). In the case of homoepitaxy, the GaN wires can be directly selected using N-polar GaN freestanding substrates and exhibit good optical properties. A schematic representation of the kinetic Wulff's plot points out the effect of surface polarity.

  12. Structural defects in GaN revealed by Transmission Electron Microscopy

    DOE PAGES

    Liliental-Weber, Zuzanna

    2014-09-08

    This paper reviews the various types of structural defects observed by Transmission Electron Microscopy in GaN heteroepitaxial layers grown on foreign substrates and homoepitaxial layers grown on bulk GaN substrates. The structural perfection of these layers is compared to the platelet self-standing crystals grown by High Nitrogen Pressure Solution. Defects in undoped and Mg doped GaN are discussed. Lastly, some models explaining the formation of inversion domains in heavily Mg doped layers that are possible defects responsible for the difficulties of p-doping in GaN are also reviewed.

  13. Characterizations of GaN film growth by ECR plasma chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Fu, Silie; Chen, Junfang; Zhang, Hongbin; Guo, Chaofen; Li, Wei; Zhao, Wenfen

    2009-06-01

    The electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR-MOPECVD) is adopted to grow GaN films on (0 0 0 1) α-Al2O3 substrate. The gas sources are pure N2 and trimethylgallium (TMG). Optical emission spectroscopy (OES) and thermodynamic analysis of GaN growth are applied to understand the GaN growth process. The OES of ECR plasma shows that TMG is significantly dissociated in ECR plasma. Reactants N and Ga in the plasma, obtained easily under the self-heating condition, are essential for the GaN growth. They contribute to the realization of GaN film growth at a relatively low temperature. The thermodynamic study shows that the driving force for the GaN growth is high when N2:TMG>1. Furthermore, higher N2:TMG flow ratio makes the GaN growth easier. Finally, X-ray diffraction, photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN films. The results demonstrate that the ECR-MOPECVD technology is favorable for depositing GaN films at low temperatures.

  14. Optical and field emission properties of layer-structure GaN nanowires

    SciTech Connect

    Cui, Zhen; Li, Enling; Shi, Wei; Ma, Deming

    2014-08-15

    Highlights: • The layer-structure GaN nanowires with hexagonal-shaped cross-sections are produced via a process based on the CVD method. • The diameter of the layer-structure GaN nanowire gradually decreases from ∼500 nm to ∼200 nm along the wire axis. • The layer-structure GaN nanowire film possesses good field emission property. - Abstract: A layer-structure gallium nitride (GaN) nanowires, grown on Pt-coated n-type Si (1 1 1) substrate, have been synthesized using chemical vapor deposition (CVD). The results show: (1) SEM indicates that the geometry structure is layer-structure. HRTEM indicates that GaN nanowire’s preferential growth direction is along [0 0 1] direction. (2) The room temperature PL emission spectrum of the layer-structure GaN nanowires has a peak at 375 nm, which proves that GaN nanowires have potential application in light-emitting nano-devices. (3) Field-emission measurements show that the layer-structure GaN nanowires film has a low turn-on field of 4.39 V/μm (at room temperature), which is sufficient for electron emission devices, field emission displays and vacuum nano-electronic devices. The growth mechanism for GaN nanowires has also been discussed briefly.

  15. Demonstration of flexible thin film transistors with GaN channels

    NASA Astrophysics Data System (ADS)

    Bolat, S.; Sisman, Z.; Okyay, A. K.

    2016-12-01

    We report on the thin film transistors (TFTs) with Gallium Nitride (GaN) channels directly fabricated on flexible substrates. GaN thin films are grown by hollow cathode plasma assisted atomic layer deposition (HCPA-ALD) at 200 °C. TFTs exhibit 103 on-to-off current ratios and are shown to exhibit proper transistor saturation behavior in their output characteristics. Gate bias stress tests reveal that flexible GaN TFTs have extremely stable electrical characteristics. Overall fabrication thermal budget is below 200 °C, the lowest reported for the GaN based transistors so far.

  16. Electron beam accelerator with magnetic pulse compression and accelerator switching

    DOEpatents

    Birx, D.L.; Reginato, L.L.

    1984-03-22

    An electron beam accelerator is described comprising an electron beam generator-injector to produce a focused beam of greater than or equal to .1 MeV energy electrons; a plurality of substantially identical, aligned accelerator modules to sequentially receive and increase the kinetic energies of the beam electron by about .1-1 MeV per module. Each accelerator module includes a pulse-forming network that delivers a voltage pulse to the module of substantially .1-1 MeV maximum energy over a time duration of less than or equal to 1 ..mu..sec.

  17. Electron beam accelerator with magnetic pulse compression and accelerator switching

    DOEpatents

    Birx, Daniel L.; Reginato, Louis L.

    1987-01-01

    An electron beam accelerator comprising an electron beam generator-injector to produce a focused beam of .gtoreq.0.1 MeV energy electrons; a plurality of substantially identical, aligned accelerator modules to sequentially receive and increase the kinetic energies of the beam electrons by about 0.1-1 MeV per module. Each accelerator module includes a pulse-forming network that delivers a voltage pulse to the module of substantially 0.1-1 MeV maximum energy over a time duration of .ltoreq.1 .mu.sec.

  18. Electron beam accelerator with magnetic pulse compression and accelerator switching

    DOEpatents

    Birx, Daniel L.; Reginato, Louis L.

    1988-01-01

    An electron beam accelerator comprising an electron beam generator-injector to produce a focused beam of .gtoreq.0.1 MeV energy electrons; a plurality of substantially identical, aligned accelerator modules to sequentially receive and increase the kinetic energies of the beam electrons by about 0.1-1 MeV per module. Each accelerator module includes a pulse-forming network that delivers a voltage pulse to the module of substantially .gtoreq.0.1-1 MeV maximum energy over a time duration of .ltoreq.1 .mu.sec.

  19. Improved photoelectrochemical performance of GaN nanopillar photoanodes.

    PubMed

    Narangari, Parvathala Reddy; Karuturi, Siva Krishna; Lysevych, Mykhaylo; Hoe Tan, Hark; Jagadish, Chennupati

    2017-04-18

    In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.

  20. Size dictated thermal conductivity of GaN

    SciTech Connect

    Thomas Edwin Beechem; McDonald, Anthony E.; Fuller, Elliot James; Talin, Albert Alec; Rost, Christina M.; Maria, Jon -Paul; Gaskins, John T.; Hopkins, Patrick E.; Allerman, Andrew A.

    2016-04-01

    The thermal conductivity on n- and p-type doped gallium nitride (GaN) epilayers having thickness of 3-4 μm was investigated using time domain thermoreflectance (TDTR). Despite possessing carrier concentrations ranging across 3 decades (1015 – 1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends–and their overall reduction relative to bulk–are explained leveraging established scattering models where it is shown that size effects play a primary role in limiting thermal conductivity for layers even tens of microns thick. GaN device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.

  1. Selective excitation of the yellow luminescence of GaN

    SciTech Connect

    Colton, J.S.; Yu, P.Y.; Teo, K.L.; Weber, E.R.; Grzegory, I.; Uchida, K.

    1999-07-01

    The yellow luminescence of n-type GaN has been studied with selective excitation using a combination of Ar ion and dye lasers. Narrower structures whose peak energies follow the excitation photon energy over the width of the yellow luminescence have been observed. Unlike the yellow luminescence excited by above band gap excitations, these fine structures exhibits thermal activated quenching behavior. We propose that these fine structures are due to emission occurring at complexes of shallow donors and deep acceptors which can be resonantly excited by photons with energies below the band gap. The activation energy deduced from their intensity is that for delocalization of electrons out of the complexes. Our results therefore suggest that there is more than one recombination channel (usually assumed to be due to distant donor-acceptor pairs) to the yellow luminescence in GaN.

  2. Improved photoelectrochemical performance of GaN nanopillar photoanodes

    NASA Astrophysics Data System (ADS)

    Reddy Narangari, Parvathala; Krishna Karuturi, Siva; Lysevych, Mykhaylo; Tan, Hark Hoe; Jagadish, Chennupati

    2017-04-01

    In this work, we report on the photoelectrochemical (PEC) investigation of n-GaN nanopillar (NP) photoanodes fabricated using metal organic chemical vapour deposition and the top-down approach. Substantial improvement in photocurrents is observed for GaN NP photoanodes compared to their planar counterparts. The role of carrier concentration and NP dimensions on the PEC performance of NP photoanodes is further elucidated. Photocurrent density is almost doubled for doped NP photoanodes whereas no improvement is noticed for undoped NP photoanodes. While the diameter of GaN NP is found to influence the onset potential, carrier concentration is found to affect both the onset and overpotential of the electrodes. Optical and electrochemical impedance spectroscopy characterisations are utilised to further explain the PEC results of NP photoanodes. Finally, improvement in the photostability of NP photoanodes with the addition of NiO as a co-catalyst is investigated.

  3. Ground Albedo Neutron Sensing (GANS) for Measurement of Integral Soil Water Content at the Small Catchment Scale

    NASA Astrophysics Data System (ADS)

    Rivera Villarreyes, C.; Baroni, G.; Oswald, S. E.

    2012-12-01

    Soil water content at the plot or hill-slope scale is an important link between local vadose zone hydrology and catchment hydrology. One largest initiative to cover the measuring gap of soil moisture between point scale and remote sensing observations is the COSMOS network (Zreda et al., 2012). Here, cosmic-ray neutron sensing, which may be more precisely named ground albedo neutron sensing (GANS), is applied. The measuring principle is based on the crucial role of hydrogen as neutron moderator compared to others landscape materials. Soil water content contained in a footprint of ca. 600 m diameter and a depth ranging down to a few decimeters is inversely correlated to the neutron flux at the air-ground interface. This approach is now implemented, e.g. in USA (Zreda et al., 2012) and Germany (Rivera Villarreyes et al., 2011), based on its simple installation and integral measurement of soil moisture at the small catchment scale. The present study performed Ground Albedo Neutron Sensing on farmland at two locations in Germany under different vegetative situations (cropped and bare field) and different seasonal conditions (summer, autumn and winter). Ground albedo neutrons were measured at (i) a farmland close to Potsdam and Berlin cropped with corn in 2010, sunflower in 2011 and winter rye in 2012, and (ii) a mountainous farmland catchment (Schaefertal, Harz Mountains) since middle 2011. In order to test this methodology, classical soil moisture devices and meteorological data were used for comparison. Moreover, several calibration approaches, role of vegetation cover and transferability of calibration parameters to different times and locations were also evaluated. Observations suggest that GANS can overcome the lack of data for hydrological processes at the intermediate scale. Soil moisture from GANS compared quantitatively with mean values derived from a network of classical devices under vegetated and non- vegetated conditions. The GANS approach responded well

  4. Erbium Doped GaN Lasers by Optical Pumping

    DTIC Science & Technology

    2016-07-13

    P.O. Box 12211 Research Triangle Park, NC 27709-2211 Er doped GaN, gain medium, high energy laser, optical pump REPORT DOCUMENTATION PAGE 11. SPONSOR...Nanophotonics Center, Texas Tech University Lubbock, TX 79409-3102 jingyu.lin@ttu.edu; hx.jiang@ttu.edu I. Summary of Progress High energy and...emerging technologies. The optical gain medium is the heart of a high energy laser (HEL) system. Comparing with the presently dominant gain material

  5. Particle acceleration

    NASA Technical Reports Server (NTRS)

    Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Alissandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.

    1986-01-01

    Data is compiled from Solar Maximum Mission and Hinothori satellites, particle detectors in several satellites, ground based instruments, and balloon flights in order to answer fundamental questions relating to: (1) the requirements for the coronal magnetic field structure in the vicinity of the energization source; (2) the height (above the photosphere) of the energization source; (3) the time of energization; (4) transistion between coronal heating and flares; (5) evidence for purely thermal, purely nonthermal and hybrid type flares; (6) the time characteristics of the energization source; (7) whether every flare accelerates protons; (8) the location of the interaction site of the ions and relativistic electrons; (9) the energy spectra for ions and relativistic electrons; (10) the relationship between particles at the Sun and interplanetary space; (11) evidence for more than one acceleration mechanism; (12) whether there is single mechanism that will accelerate particles to all energies and also heat the plasma; and (13) how fast the existing mechanisms accelerate electrons up to several MeV and ions to 1 GeV.

  6. Plasma accelerator

    DOEpatents

    Wang, Zhehui; Barnes, Cris W.

    2002-01-01

    There has been invented an apparatus for acceleration of a plasma having coaxially positioned, constant diameter, cylindrical electrodes which are modified to converge (for a positive polarity inner electrode and a negatively charged outer electrode) at the plasma output end of the annulus between the electrodes to achieve improved particle flux per unit of power.

  7. Accelerated Achievement

    ERIC Educational Resources Information Center

    Ford, William J.

    2010-01-01

    This article focuses on the accelerated associate degree program at Ivy Tech Community College (Indiana) in which low-income students will receive an associate degree in one year. The three-year pilot program is funded by a $2.3 million grant from the Lumina Foundation for Education in Indianapolis and a $270,000 grant from the Indiana Commission…

  8. ACCELERATION INTEGRATOR

    DOEpatents

    Pope, K.E.

    1958-01-01

    This patent relates to an improved acceleration integrator and more particularly to apparatus of this nature which is gyrostabilized. The device may be used to sense the attainment by an airborne vehicle of a predetermined velocitv or distance along a given vector path. In its broad aspects, the acceleration integrator utilizes a magnetized element rotatable driven by a synchronous motor and having a cylin drical flux gap and a restrained eddy- current drag cap deposed to move into the gap. The angular velocity imparted to the rotatable cap shaft is transmitted in a positive manner to the magnetized element through a servo feedback loop. The resultant angular velocity of tae cap is proportional to the acceleration of the housing in this manner and means may be used to measure the velocity and operate switches at a pre-set magnitude. To make the above-described dcvice sensitive to acceleration in only one direction the magnetized element forms the spinning inertia element of a free gyroscope, and the outer housing functions as a gimbal of a gyroscope.

  9. MgCaO Dry Etching on GaN

    NASA Astrophysics Data System (ADS)

    Hlad, M.; Ren, F.

    2005-11-01

    MgCaO films grown by rf plasma-assisted Molecular Beam Epitaxy and capped with Sc2O3 are promising candidates as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors (HEMTs) and metal-oxide semiconductor HEMTs (MOS-HEMTs) respectively. Two different plasma chemistries were examined for etching these thin films on GaN. Inductively Coupled Plasmas of CH4/H2/Ar produced etch rates only in the range 20-70 å/min, comparable to the Ar sputter rates under the same conditions. Similarly slow MgCaO etch rates (˜100 å/min) were obtained with Cl2/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher. The MgCaO removal rates are limited by the low volatilities of the respective etch products. The CH4/H2/Ar plasma chemistry produced a selectivity of around 2 or etching the MgCaO with respect to GaN.

  10. Epitaxially-Grown GaN Junction Field Effect Transistors

    SciTech Connect

    Baca, A.G.; Chang, P.C.; Denbaars, S.P.; Lester, L.F.; Mishra, U.K.; Shul, R.J.; Willison, C.G.; Zhang, L.; Zolper, J.C.

    1999-05-19

    Junction field effect transistors (JFET) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The DC and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (gm) of 48 mS/mm and a maximum source-drain current of 270 mA/mm are achieved on a 0.8 µ m gate JFET device at VGS= 1 V and VDS=15 V. The intrinsic transconductance, calculated from the measured gm and the source series resistance, is 81 mS/mm. The fT and fmax for these devices are 6 GHz and 12 GHz, respectively. These JFETs exhibit a significant current reduction after a high drain bias is applied, which is attributed to a partially depleted channel caused by trapped hot-electrons in the semi-insulating GaN buffer layer. A theoretical model describing the current collapse is described, and an estimate for the length of the trapped electron region is given.

  11. Metal contacts on ZnSe and GaN

    SciTech Connect

    Duxstad, Kristin Joy

    1997-05-01

    Recently, considerable interest has been focused on the development of blue light emitting materials and devices. The focus has been on GaN and ZnSe, direct band gap semiconductors with bands gaps of 3.4 and 2.6 eV, respectively. To have efficient, reliable devices it is necessary to have thermally and electrically stable Ohmic contacts. This requires knowledge of the metal-semiconductor reaction behavior. To date few studies have investigated this behavior. Much information has accumulated over the years on the behavior of metals on Si and GaAs. This thesis provides new knowledge for the more ionic wide band gap semiconductors. The initial reaction temperatures, first phases formed, and phase stability of Pt, Pd, and Ni on both semiconductors were investigated. The reactions of these metals on ZnSe and GaN are discussed in detail and correlated with predicted behavior. In addition, comparisons are made between these highly ionic semiconductors and Si and GaAs. The trends observed here should also be applicable to other II-VI and III-Nitride semiconductor systems, while the information on phase formation and stability should be useful in the development of contacts for ZnSe and GaN devices.

  12. Dislocation core structures in Si-doped GaN

    SciTech Connect

    Rhode, S. L. Fu, W. Y.; Sahonta, S.-L.; Kappers, M. J.; Humphreys, C. J.; Horton, M. K.; Pennycook, T. J.; Dusane, R. O.; Moram, M. A.

    2015-12-14

    Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in plan-view geometry of GaN films with a range of Si-doping levels and dislocation densities ranging between (5 ± 1) × 10{sup 8} and (10 ± 1) × 10{sup 9} cm{sup −2}. All a-type (edge) dislocation core structures in all samples formed 5/7-atom ring core structures, whereas all (a + c)-type (mixed) dislocations formed either double 5/6-atom, dissociated 7/4/8/4/9-atom, or dissociated 7/4/8/4/8/4/9-atom core structures. This shows that Si-doping does not affect threading dislocation core structures in GaN. However, electron beam damage at 300 keV produces 4-atom ring structures for (a + c)-type cores in Si-doped GaN.

  13. Theoretical investigation of GaN carbon doped

    NASA Astrophysics Data System (ADS)

    Espitia Rico, M. J.; Moreno Armenta, M. G.; Rodríguez, J. A.; Takeuchi, N.

    2016-02-01

    In this work we used first principles calculations in the frame of density functional theory (DFT) in order to study the structural and electronic properties of GaN doped with carbon. The computational calculations were carried out by a method based on plane waves pseudopotentials, as implemented in the Quantum Espresso code. In the wurtzite type GaN supercell the nitrogen atoms were replaced by carbon atoms (C by N) and then also the gallium atoms by carbon atoms (C by Ga). The carbon concentrations in the GaN volume was set as x=25, 50 y 75%. For each concentration x of carbon the formation energy was calculated for the substitutions C by N and CxGa. We found that it is more energetically favourable that the carbon atoms occupy the positions of the nitrogen atoms (C by N), because in all the x concentrations of carbon the formation energies were lower than that in the substitutions (C by Ga). It was found that the new compounds CxGaN1-x have higher bulk moduli. So they are very rigid. This property makes them good candidates for applications in hard coatings or devices for high power and temperatures. Analysis of the density of states show that the new CxGaN1-x ternary compound have metallic behaviour that comes essentially from the hybridization states N-p and C-p cross the Fermi level.

  14. Theoretical and Experiment Study of Cathodoluminescence of GaN

    NASA Astrophysics Data System (ADS)

    Ben Nasr, F.; Matoussi, A.; Salh, R.; Boufaden, T.; Guermazi, S.; Fitting, H.-J.; Eljani, B.; Fakhfakh, Z.

    2007-09-01

    In this work, we report the theoretical and experimental results of cathodoluminescence (CL) from GaN layers with thickness (1-3) micron grown at 800 °C by MOVPE on silicon substrate. The CL measurements were performed in a digital scanning electron microscope DSM 960 at room temperature. The CL spectra recorded at room temperature (RT) show the main UV peak at 3.42 eV of the fundamental transition and a broad yellow band at 2.2 eV attributed the intrinsic defects and extrinsic dopants and impurities. The simulation of the CL excitation and intensity is developed using consistent 2-D model based on the electron beam energy dissipation and taking into account the effects of carrier diffusion, internal absorption and the recombination processes in GaN. Then, we have investigated the evolution of the CL intensity from GaN as a function the electron beam energy in the range Eo = (5-20) keV. A comparative study between experimental and simulated CL spectra at room temperature is presented.

  15. Nucleation conditions for catalyst-free GaN nanowires

    NASA Astrophysics Data System (ADS)

    Bertness, K. A.; Roshko, A.; Mansfield, L. M.; Harvey, T. E.; Sanford, N. A.

    2007-03-01

    We have examined the initial steps for catalyst-free growth of GaN nanowires by molecular beam epitaxy (MBE) on Si (1 1 1) substrates using AlN buffer layers. These wires form spontaneously under high N-to-Ga ratios for a growth temperature range of about 810-830 °C. Field emission scanning electron microscopy (FESEM) shows that part of the GaN forms a "matrix layer" that also grows with the [0 0 0 1] direction perpendicular to the substrate surface. This layer contains small, dense hexagonal pits in which the nanowires nucleate. Using both FESEM and atomic force microscopy (AFM), we identify the pit facets as {1 0 1¯ 2} planes. The nucleation studies show that the use of an AlN buffer layer is essential to the regular formation of the nanowires and matrix layers under our growth conditions. Our typical AlN buffer layer is 40-50 nm thick. We conclude that the nucleation mechanism for nanowires includes formation of nanocolumns in the AlN buffer layer. The propagation of the nanowires in GaN growth appears to be driven by differences in growth rates among crystallographic planes under N-rich conditions.

  16. Understanding the pyramidal growth of GaN

    SciTech Connect

    Rouviere, J.L.; Arlery, M.; Bourret, A.

    1996-11-01

    By a combination of conventional, HREM and CBED TEM experiments the authors have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on (0001)Al{sub 2}O{sub 3}. They experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimized buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75 nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighboring material: the GaN layers grown on (0001)Al{sub 2}O{sub 3} are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.

  17. Magnesium diffusion profile in GaN grown by MOVPE

    NASA Astrophysics Data System (ADS)

    Benzarti, Z.; Halidou, I.; Bougrioua, Z.; Boufaden, T.; El Jani, B.

    2008-07-01

    The diffusion of magnesium has been studied in GaN layers grown on sapphire substrate by atmospheric pressure metalorganic vapor-phase-epitaxy (MOVPE) in a "home-made" reactor. Secondary Ion Mass Spectroscopy (SIMS) was used to visualise the Mg profiles in two kinds of multi-sublayer GaN structures. One structure was grown with a variable flow of Ga precursor (TMG) and the second one with a variable growth temperature. In both cases, the Mg dopant precursor (Cp 2Mg) flow was kept constant. Using the second Fick's law to fit the experimental SIMS data, we have deduced an increasing then a saturating Mg diffusion coefficient versus the Mg concentration. Mg incorporation was found to get higher for lower growth rate, i.e. when TMG flow is reduced. Furthermore, based on the temperature-related behaviour we have found that the activation energy for Mg diffusion coefficient in GaN was 1.9 eV. It is suggested that Mg diffuses via substitutional sites.

  18. Abnormal selective area growth of irregularly-shaped GaN structures on the apex of GaN pyramids and its application for wide spectral emission

    NASA Astrophysics Data System (ADS)

    Yu, Yeon Su; Lee, Jun Hyeong; Ahn, Hyung Soo; Yang, Min

    2014-12-01

    We report on the growth and the characterization of three-dimensional randomly-shaped InGaN/GaN structures selectively grown on the apex of GaN pyramids for the purpose of enlarging the emission spectral range. We found that the variations in the shape and the size of the three-dimensional GaN structures depend on the growth temperature and the surface area for selective growth under intentional turbulence in the gas stream. The selectively grown GaN structures grown at 1020 °C have irregular shape, while the samples grown at 1100 °C have rather uniform hexagonal pyramidal shapes. Irregularly shaped GaN structures were also obtained on the apex of GaN pyramids when the SiO2 mask was removed to 1/10 of the total height of the underlying GaN pyramid. When only 1/5 of the SiO2 mask was removed, however, the selectively grown GaN structures had similar hexagonal pyramidal shapes resembling those of the underlying GaN pyramids. The CL (Cathodoluminescence) spectra of the InGaN layers grown on the randomly shaped GaN structures showed a wide emission spectral range from 388 to 433 nm due to the non-uniform thickness and spatially inhomogeneous indium composition of the InGaN layers. This new selective growth method might have great potential for applications of non-phosphor white light emitting diodes (LEDs) with optimized growth conditions for InGaN active layers of high indium composition and with optimum process for fabrication of electrodes for electrical injection.

  19. Network Cosmology

    PubMed Central

    Krioukov, Dmitri; Kitsak, Maksim; Sinkovits, Robert S.; Rideout, David; Meyer, David; Boguñá, Marián

    2012-01-01

    Prediction and control of the dynamics of complex networks is a central problem in network science. Structural and dynamical similarities of different real networks suggest that some universal laws might accurately describe the dynamics of these networks, albeit the nature and common origin of such laws remain elusive. Here we show that the causal network representing the large-scale structure of spacetime in our accelerating universe is a power-law graph with strong clustering, similar to many complex networks such as the Internet, social, or biological networks. We prove that this structural similarity is a consequence of the asymptotic equivalence between the large-scale growth dynamics of complex networks and causal networks. This equivalence suggests that unexpectedly similar laws govern the dynamics of complex networks and spacetime in the universe, with implications to network science and cosmology. PMID:23162688

  20. Network cosmology.

    PubMed

    Krioukov, Dmitri; Kitsak, Maksim; Sinkovits, Robert S; Rideout, David; Meyer, David; Boguñá, Marián

    2012-01-01

    Prediction and control of the dynamics of complex networks is a central problem in network science. Structural and dynamical similarities of different real networks suggest that some universal laws might accurately describe the dynamics of these networks, albeit the nature and common origin of such laws remain elusive. Here we show that the causal network representing the large-scale structure of spacetime in our accelerating universe is a power-law graph with strong clustering, similar to many complex networks such as the Internet, social, or biological networks. We prove that this structural similarity is a consequence of the asymptotic equivalence between the large-scale growth dynamics of complex networks and causal networks. This equivalence suggests that unexpectedly similar laws govern the dynamics of complex networks and spacetime in the universe, with implications to network science and cosmology.

  1. Multi-wavelength emitting InGan/GaN quantum well grown on V-shaped gan(1101) microfacet.

    PubMed

    Kang, Eun-Sil; Ju, Jin-Woo; Kim, Jin Soo; Ahn, Haeng-Keun; Lee, June Key; Kim, Jin Hyeok; Shin, Dong-Chan; Lee, In-Hwan

    2007-11-01

    InGaN/GaN multiple quantum wells (MQWs) were successfully grown on the inclined GaN(1101) microfacets. Conventional photolithography and subsequent growth of GaN were employed to generate the V-shaped microfacets along (1120) direction. The well-developed microfacets observed by scanning electron microscopy and the clear transmission electron microscope interfacial images indicated that the MQW was successfully grown on the GaN microfacets. Interestingly, cathodoluminescence (CL) spectra measured on the microfacets showed a continuous change in the luminescence peak positions. The CL peaks were shifted to a longer wavelength from 420 nm to 440 nm as the probing points were changed along upward direction. This could be attributed to the nonuniform distribution of the In composition and/or the wavefunction overlapping between adjacent wells. Present works thus propose a novel route to fabricate a monolithic white light emitting diode without phosphors by growing the InGaN/GaN MQWs on (1101) facet.

  2. Particle Accelerators in China

    NASA Astrophysics Data System (ADS)

    Zhang, Chuang; Fang, Shouxian

    As the special machines that can accelerate charged particle beams to high energy by using electromagnetic fields, particle accelerators have been widely applied in scientific research and various areas of society. The development of particle accelerators in China started in the early 1950s. After a brief review of the history of accelerators, this article describes in the following sections: particle colliders, heavy-ion accelerators, high-intensity proton accelerators, accelerator-based light sources, pulsed power accelerators, small scale accelerators, accelerators for applications, accelerator technology development and advanced accelerator concepts. The prospects of particle accelerators in China are also presented.

  3. Compact accelerator

    DOEpatents

    Caporaso, George J.; Sampayan, Stephen E.; Kirbie, Hugh C.

    2007-02-06

    A compact linear accelerator having at least one strip-shaped Blumlein module which guides a propagating wavefront between first and second ends and controls the output pulse at the second end. Each Blumlein module has first, second, and third planar conductor strips, with a first dielectric strip between the first and second conductor strips, and a second dielectric strip between the second and third conductor strips. Additionally, the compact linear accelerator includes a high voltage power supply connected to charge the second conductor strip to a high potential, and a switch for switching the high potential in the second conductor strip to at least one of the first and third conductor strips so as to initiate a propagating reverse polarity wavefront(s) in the corresponding dielectric strip(s).

  4. BICEP's acceleration

    SciTech Connect

    Contaldi, Carlo R.

    2014-10-01

    The recent Bicep2 [1] detection of, what is claimed to be primordial B-modes, opens up the possibility of constraining not only the energy scale of inflation but also the detailed acceleration history that occurred during inflation. In turn this can be used to determine the shape of the inflaton potential V(φ) for the first time — if a single, scalar inflaton is assumed to be driving the acceleration. We carry out a Monte Carlo exploration of inflationary trajectories given the current data. Using this method we obtain a posterior distribution of possible acceleration profiles ε(N) as a function of e-fold N and derived posterior distributions of the primordial power spectrum P(k) and potential V(φ). We find that the Bicep2 result, in combination with Planck measurements of total intensity Cosmic Microwave Background (CMB) anisotropies, induces a significant feature in the scalar primordial spectrum at scales k∼ 10{sup -3} Mpc {sup -1}. This is in agreement with a previous detection of a suppression in the scalar power [2].

  5. Nanoheteroepitaxy of GaN on AlN/Si(111) nanorods fabricated by nanosphere lithography

    NASA Astrophysics Data System (ADS)

    Lee, Donghyun; Shin, In-Su; Jin, Lu; Kim, Donghyun; Park, Yongjo; Yoon, Euijoon

    2016-06-01

    Nanoheteroepitaxy (NHE) of GaN on an AlN/Si(111) nanorod structure was investigated by metal-organic chemical vapor deposition. Silica nanosphere lithography was employed to fabricate a periodic hexagonal nanorod array with a narrow gap of 30 nm between the nanorods. We were successful in obtaining a fully coalesced GaN film on the AlN/Si(111) nanorod structure. Transmission electron microscopy revealed that threading dislocation (TD) bending and termination by stacking faults occurred near the interface between GaN and the AlN/Si(111) nanorods, resulting in the reduction of TD density for the NHE GaN layer. The full width at half-maximum of the X-ray rocking curve for (102) plane of the NHE GaN was found to decrease down to 728 arcsec from 1005 arcsec for the GaN layer on a planar AlN/Si(111) substrate, indicating that the crystalline quality of the NHE GaN was improved. Also, micro-Raman measurement showed that tensile stress in the NHE GaN layer was reduced significantly as much as 70% by introducing air voids between the nanorods.

  6. Opportunities and challenges in GaN metal organic chemical vapor deposition for electron devices

    NASA Astrophysics Data System (ADS)

    Matsumoto, Koh; Yamaoka, Yuya; Ubukata, Akinori; Arimura, Tadanobu; Piao, Guanxi; Yano, Yoshiki; Tokunaga, Hiroki; Tabuchi, Toshiya

    2016-05-01

    The current situation and next challenge in GaN metal organic chemical vapor deposition (MOCVD) for electron devices of both GaN on Si and GaN on GaN are presented. We have examined the possibility of increasing the growth rate of GaN on 200-mm-diameter Si by using a multiwafer production MOCVD machine, in which the vapor phase parasitic reaction is well controlled. The impact of a high-growth-rate strained-layer-superlattice (SLS) buffer layer is presented in terms of material properties. An SLS growth rate of as high as 3.46 µm/h, which was 73% higher than the current optimum, was demonstrated. As a result, comparable material properties were obtained. Next, a typical result of GaN doped with Si of 1 × 1016 cm-3 grown at the growth rate of 3.7 µm/h is shown. For high-voltage application, we need a thick high-purity GaN drift layer with a low carbon concentration, of less than 1016 cm-3. It is shown that achieving a high growth rate by precise control of the vapor phase reaction is still challenge in GaN MOCVD.

  7. Terahertz study of m-plane GaN thin fims

    NASA Astrophysics Data System (ADS)

    Quadir, Shaham; Jang, Der-Jun; Lin, Ching-Liang; Lo, Ikai

    2014-03-01

    We investigate the optical properties of m-plane GaN thin films using the terahertz time domain spectroscopy. The m-plane GaN thin films were grown on γ-LiAlO2 substrates with buffer layers of low temperature grown GaN. The thin films were illuminated with terahertz radiation generated by a LT-GaAs antenna and the transmitted signal was detected by a ZnTe crystal. The polarization of the terahertz wave was chosen to be either parallel or perpendicular to the GaN [0001] direction. We compared the transmitted signals of the m-plane GaN thin films to that of the LAO substrate. The samples as well as the LAO substrate exhibited polarization dependence of absorption in terahertz spectrum. The carrier densities and the mobilities were derived from the transmittance of the THz wave using extended Drude model. We found, in all samples, both the carrier densities and mobilities along the GaN [0001] direction were smaller than those along the GaN [1120] direction due to the stripe formation along the GaN [1120].

  8. Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles

    PubMed Central

    Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel

    2016-01-01

    Summary Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle–cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN. PMID:27826507

  9. Integral Quantification of Soil Water Content at the Intermediate Catchment Scale by Ground Albedo Neutron Sensing (GANS)

    NASA Astrophysics Data System (ADS)

    Rivera Villarreyes, C. A.; Baroni, G.; Oswald, S. E.

    2012-04-01

    Soil water content at the plot or hill-slope scale is an important link between local vadose zone hydrology and catchment hydrology. However, so far only few methods are on the way to close this gap between point measurements and remote sensing. One new measurement methodology for integral quantifications of mean areal soil water content at the intermediate catchment scale is the aboveground sensing of cosmic-ray neutrons, more precisely ground albedo neutron sensing (GANS). Ground albedo natural neutrons, are generated by collisions of secondary cosmic rays with land surface materials (soil, water, biomass, snow, etc). Neutrons measured at the air/ground interface correlate with soil moisture contained in a footprint of ca. 600 m diameter and a depth ranging down to a few decimeters. This correlation is based on the crucial role of hydrogen as neutron moderator compared to others landscape materials. The present study performed ground albedo neutron sensing in different locations in Germany under different vegetative situations (cropped and bare field) and different seasonal conditions (summer, autumn and winter). Ground albedo neutrons were measured at (i) a farmland close to Potsdam (Brandenburg, Germany) cropped with corn in 2010 and sunflowers in 2011, and (ii) a mountainous farmland catchment (Schaefertal, Harz Mountains, Germany) in 2011. In order to test this method, classical soil moisture devices and meteorological data were used for comparison. Moreover, calibration approach, and transferability of calibration parameters to different times and locations are also evaluated. Our observations suggest that GANS can overcome the lack of data for hydrological processes at the intermediate scale. Soil water content from GANS compared quantitatively with mean water content values derived from a network of classical devices (RMSE = 0.02 m3/m3 and r2 = 0.98) in three calibration periods with cropped-field conditions. Then, same calibration parameters corresponded

  10. Semipolar single component GaN on planar high index Si(11h) substrates

    SciTech Connect

    Ravash, Roghaiyeh; Blaesing, Juergen; Dadgar, Armin; Krost, Alois

    2010-10-04

    We present metal organic vapor phase epitaxy growth of polarization reduced, single component GaN on nonpatterned Si(112), Si(113), Si(114), Si(115), and Si(116) substrates. We find that the inclination angle of GaN c-axis with respect to the surface normal depends on the angle between Si(111) and above mentioned Si(11h)-surfaces. The growth of the GaN layer is essentially performed as c-axis oriented growth on the naturally occurring Si(111) facets of these Si(11h)-surfaces. The c-axis tilt-angle of GaN crystallites depends on the Si-surface direction and increases from Si(112) to Si(116) planes. GaN layers are investigated by x-ray analysis and scanning electron microscopy.

  11. Hole-induced d0 ferromagnetism enhanced by Na-doping in GaN

    NASA Astrophysics Data System (ADS)

    Zhang, Yong; Li, Feng

    2017-02-01

    The d0 ferromagnetism in wurtzite GaN is investigated by the first-principle calculations. It is found that spontaneous magnetization occurs if sufficient holes are injected in GaN. Both Ga vacancy and Na doping can introduce holes into GaN. However, Ga vacancy has a high formation energy, and is thus unlikely to occur in a significant concentration. In contrast, Na doping has relatively low formation energy. Under N-rich growth condition, Na doping with a sufficient concentration can be achieved, which can induce half-metallic ferromagnetism in GaN. Moreover, the estimated Curie temperature of Na-doped GaN is well above the room temperature.

  12. Step-induced misorientation of GaN grown on r-plane sapphire

    SciTech Connect

    Smalc-Koziorowska, J.; Dimitrakopulos, G. P.; Sahonta, S.-L.; Komninou, Ph.; Tsiakatouras, G.; Georgakilas, A.

    2008-07-14

    In the growth of nonpolar (1120) a-plane GaN on r-plane (1102) sapphire by plasma-assisted molecular beam epitaxy, misoriented crystallites are observed close to the substrate. They have average diameter {approx}10 nm and are oriented with the (0001){sub GaN} plane approximately parallel to the (2113){sub sapph.} plane and [0110]{sub GaN} parallel [1101]{sub sapph.}. This semipolar orientation is promoted by a low misfit (2.4%) between (1011){sub GaN} and (1210){sub sapph.} planes. Its introduction, after nitridation treatment, is due to GaN nucleation on (2113){sub sapph.} step facets inclined at 26 deg. relative to the r-plane. Two variants are observed, leading to twinning when they abut inside the epilayer.

  13. Vertical nonpolar growth templates for light emitting diodes formed with GaN nanosheets

    NASA Astrophysics Data System (ADS)

    Yeh, Ting-Wei; Lin, Yen-Ting; Ahn, Byungmin; Stewart, Lawrence S.; Daniel Dapkus, P.; Nutt, Steven R.

    2012-01-01

    We demonstrate that nonpolar m-plane surfaces can be generated on uniform GaN nanosheet arrays grown vertically from the (0001)-GaN bulk material. InGaN/GaN multiple quantum wells (MQWs) grown on the facets of these nanosheets are demonstrated by cross-sectional transmission electron microscopy. Owing to the high aspect ratio of the GaN nanosheet structure, the MQWs predominantly grow on nonpolar GaN planes. The results suggest that GaN nanosheets provide a conduction path for device fabrication and also a growth template to reduce the piezoelectric field inside the active region of InGaN-based light emitting diodes.

  14. Prospects for the application of GaN power devices in hybrid electric vehicle drive systems

    NASA Astrophysics Data System (ADS)

    Su, Ming; Chen, Chingchi; Rajan, Siddharth

    2013-07-01

    GaN, a wide bandgap semiconductor successfully implemented in optical and high-speed electronic devices, has gained momentum in recent years for power electronics applications. Along with rapid progress in material and device processing technologies, high-voltage transistors over 600 V have been reported by a number of teams worldwide. These advances make GaN highly attractive for the growing market of electrified vehicles, which currently employ bipolar silicon devices in the 600-1200 V class for the traction inverter. However, to capture this billion-dollar power market, GaN has to compete with existing IGBT products and deliver higher performance at comparable or lower cost. This paper reviews key achievements made by the GaN semiconductor industry, requirements of the automotive electric drive system and remaining challenges for GaN power devices to fit in the inverter application of hybrid vehicles.

  15. Hydrogen sensing characteristics of semipolar (112{sup ¯}2) GaN Schottky diodes

    SciTech Connect

    Hyeon Baik, Kwang; Kim, Hyonwoong; Jang, Soohwan; Lee, Sung-Nam; Lim, Eunju; Pearton, S. J.; Ren, F.

    2014-02-17

    The hydrogen detection characteristics of semipolar (112{sup ¯}2) plane GaN Schottky diodes were investigated and compared to c-plane Ga- and N-polar and nonpolar a-plane (112{sup ¯}0) GaN diodes. The semipolar GaN diodes showed large current response to 4% hydrogen in nitrogen gas with an accompanying Schottky barrier reduction of 0.53 eV at 25 °C, and the devices exhibited full recovery to the initial current level upon switching to a nitrogen ambient. The current-voltage characteristics of the semipolar devices remained rectifying after hydrogen exposure, in sharp contrast to the case of c-plane N-polar GaN. These results show that the surface atom configuration and polarity play a strong role in hydrogen sensing with GaN.

  16. Improvement of Crystalline Quality in GaN Films by Air-Bridged Lateral Epitaxial Growth

    NASA Astrophysics Data System (ADS)

    Kidoguchi, Isao; Ishibashi, Akihiko; Sugahara, Gaku; Tsujimura, Ayumu; Ban, Yuzaburoh

    2000-05-01

    Air-bridged lateral epitaxial growth (ABLEG), a new technique of lateral growth of GaN films, has been developed using low-pressure metalorganic vapor phase epitaxy. A previously grown 1-μm-thick GaN film is grooved along the < 1{\\bar 1}00 >\\textrm{GaN} direction, and the bottoms of the trenches and the sidewalls are covered with a silicon nitride mask. A free-standing GaN material is regrown from the exposed (0001) surface of the ridged GaN seed structure. Cross-sectional transmission electron microscopy analysis reveals that the dislocations originating from the underlying seed GaN extend straight in the < 0001 > direction and dislocations do not propagate into the wing region. The wing region also exhibits a smooth surface and the root mean square roughness is found to be 0.088 nm by atomic force microscopy measurement of the (0001) face of the wing region.

  17. Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature

    NASA Astrophysics Data System (ADS)

    Koblmüller, G.; Reurings, F.; Tuomisto, F.; Speck, J. S.

    2010-11-01

    The effect of Ga/N flux ratio on surface morphology, incorporation of point defects and electrical transport properties of GaN films grown by plasma-assisted molecular beam epitaxy in a recently developed high-temperature growth regime was investigated. The homoepitaxial (0001) GaN films grown at ˜780-790 °C showed smoothest morphologies near the cross-over between N-rich and Ga-rich growth (0.75<Ga/N<1.1) contrasting previous observations for low-temperature growth. The higher-quality growth near Ga/N˜1 resulted from lower thermal decomposition rates and was corroborated by slightly lower Ga vacancy concentrations [VGa], lower unintentional oxygen incorporation, and improved electron mobilities. The consistently low [VGa], i.e., ˜1016 cm-3 for all films attribute further to the significant benefits of the high-temperature growth regime.

  18. Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN /GaN multi-quantum wells

    NASA Astrophysics Data System (ADS)

    Keller, S.; Schaake, C.; Fichtenbaum, N. A.; Neufeld, C. J.; Wu, Y.; McGroddy, K.; David, A.; DenBaars, S. P.; Weisbuch, C.; Speck, J. S.; Mishra, U. K.

    2006-09-01

    GaN nanopillar and nanostripe arrays with embedded InGaN /GaN multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in NH3/N2 mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric (101¯5) reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on c-plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.

  19. Polarity of semipolar wurtzite crystals: X-ray photoelectron diffraction from GaN(101⁻1) and GaN(202⁻1) surfaces

    SciTech Connect

    Romanyuk, O. Jiříček, P.; Bartoš, I.; Paskova, T.

    2014-09-14

    Polarity of semipolar GaN(101⁻1) (101⁻1⁻) and GaN(202⁻1) (202⁻1⁻) surfaces was determined with X-ray photoelectron diffraction (XPD) using a standard MgKα source. The photoelectron emission from N 1s core level measured in the a-plane of the crystals shows significant differences for the two crystal orientations within the polar angle range of 80–100° from the (0001) normal. It was demonstrated that XPD polar plots recorded in the a-plane are similar for each polarity of the GaN(101⁻1) and GaN(202⁻1) crystals if referred to (0001) crystal axes. For polarity determinations of all important GaN(h0h⁻l) semipolar surfaces, the above given polar angle range is suitable.

  20. 1.2 kV GaN Schottky barrier diodes on free-standing GaN wafer using a CMOS-compatible contact material

    NASA Astrophysics Data System (ADS)

    Liu, Xinke; Liu, Qiang; Li, Chao; Wang, Jianfeng; Yu, Wenjie; Xu, Ke; Ao, Jin-Ping

    2017-02-01

    In this paper, we report the formation of vertical GaN Schottky barrier diodes (SBDs) on a 2-in. free-standing (FS) GaN wafer, using CMOS-compatible contact material. By realizing an off-state breakdown voltage V BR of 1200 V and an on-state resistance R on of 7 mΩ·cm2, the FS-GaN SBDs fabricated in this work achieve a power device figure-of-merit V\\text{BR}2/R\\text{on} of 2.1 × 108 V2·Ω-1·cm-2 on a high quality GaN wafer. In addition, the fabricated FS-GaN SBDs show the highest I on/I off current ratio of ˜2.3 × 1010 among the GaN SBDs reported in the literature.

  1. Influence of GaN column diameter on structural properties for InGaN nanocolumns grown on top of GaN nanocolumns

    NASA Astrophysics Data System (ADS)

    Oto, Takao; Mizuno, Yutaro; Yanagihara, Ai; Miyagawa, Rin; Kano, Tatsuya; Yoshida, Jun; Sakakibara, Naoki; Kishino, Katsumi

    2016-11-01

    The influence of GaN column diameter DGaN on structural properties was systematically investigated for InGaN nanocolumns (NCs) grown on top of GaN NCs. We demonstrated a large critical layer thickness of above 400 nm for In0.3Ga0.7N/GaN NCs. The structural properties were changed at the boundary of DGaN=D0 (˜120 nm). Homogeneous InGaN NCs grew axially on the GaN NCs with DGaN≤D0, while InGaN-InGaN core-shell structures were spontaneously formed on the GaN NCs with DGaN>D0. These results can be explained by a growth system that minimizes the total strain energy of the NCs.

  2. Improved emission efficiency of a-plane GaN light emitting diodes with silica nano-spheres integrated into a-plane GaN buffer layer

    NASA Astrophysics Data System (ADS)

    Park, S. H.; Park, J.; You, D.-J.; Joo, K.; Moon, D.; Jang, J.; Kim, D.-U.; Chang, H.; Moon, S.; Song, Y.-K.; Lee, G.-D.; Jeon, H.; Xu, J.; Nanishi, Y.; Yoon, E.

    2012-05-01

    A simple and inexpensive technique to improve the emission efficiency of nonpolar a-plane light emitting diodes (LEDs) is proposed. The 3-dimensional growth nature of a-plane GaN was utilized to form the regrowth template of a-plane GaN. Subsequently, the controlled integration of silica nano-spheres (CIS) into the regrowth template is performed to improve the crystal quality of a-plane GaN by epitaxial lateral overgrowth method. In addition, the CIS improves light extraction by the scattering process. The light output power from the CIS a-plane GaN LEDs showed 130%-150% increase compared to that of LED without silica nano-spheres.

  3. Advanced concepts for acceleration

    SciTech Connect

    Keefe, D.

    1986-07-01

    Selected examples of advanced accelerator concepts are reviewed. Such plasma accelerators as plasma beat wave accelerator, plasma wake field accelerator, and plasma grating accelerator are discussed particularly as examples of concepts for accelerating relativistic electrons or positrons. Also covered are the pulsed electron-beam, pulsed laser accelerator, inverse Cherenkov accelerator, inverse free-electron laser, switched radial-line accelerators, and two-beam accelerator. Advanced concepts for ion acceleration discussed include the electron ring accelerator, excitation of waves on intense electron beams, and two-wave combinations. (LEW)

  4. Accelerators and the Accelerator Community

    SciTech Connect

    Malamud, Ernest; Sessler, Andrew

    2008-06-01

    In this paper, standing back--looking from afar--and adopting a historical perspective, the field of accelerator science is examined. How it grew, what are the forces that made it what it is, where it is now, and what it is likely to be in the future are the subjects explored. Clearly, a great deal of personal opinion is invoked in this process.

  5. Rokumi-jio-gan-Containing Prescriptions Attenuate Oxidative Stress, Inflammation, and Apoptosis in the Remnant Kidney

    PubMed Central

    Park, Chan Hum; Lee, Sul Lim; Okamoto, Takuya; Tanaka, Takashi; Yokozawa, Takako

    2012-01-01

    Two Rokumi-jio-gan-containing prescriptions (Hachimi-jio-gan and Bakumi-jio-gan) were selected to examine their actions in nephrectomized rats. Each prescription was given orally to rats for 10 weeks after the excision of five-sixths of their kidney volumes, and its effect was compared with non-nephrectomized and normal rats. Rats given Hachimi-jio-gan and Bakumi-jio-gan showed an improvement of renal functional parameters such as serum urea nitrogen, creatinine, creatinine clearance, and urinary protein. The nephrectomized rats exhibited the up-regulation of nicotinamide adenine dinucleotide phosphate oxidase subunits, c-Jun N-terminal kinase (JNK), phosphor-JNK, c-Jun, transforming growth factor-β1, nuclear factor-kappa B, cyclooxygenase-2, inducible nitric oxide synthase, monocyte chemotactic protein-1, intracellular adhesion molecule-1, Bax, cytochrome c, and caspase-3, and down-regulation of NF-E2-related factor 2, heme oxygenase-1, and survivin; however, Bakumi-jio-gan administration acts as a regulator in inflammatory reactions caused by oxidative stress in renal failure. Moreover, the JNK pathway and apoptosis-related protein expressions, Bax, caspase-3, and survivin, were ameliorated to the normal levels by Hachimi-jio-gan administration. The development of renal lesions, glomerular sclerosis, tubulointerstitial damage, and arteriolar sclerotic lesions, estimated by histopathological evaluation and scoring, was strong in the groups administered Hachimi-jio-gan rather than Bakumi-jio-gan. This study suggests that Rokumi-jio-gan-containing prescriptions play a protective role in the progression of renal failure. PMID:23243456

  6. Enhancement in wafer bow of free-standing GaN substrates due to high-dose hydrogen implantation: implications for GaN layer transfer applications

    NASA Astrophysics Data System (ADS)

    Singh, R.; Radu, I.; Bruederl, G.; Eichler, C.; Haerle, V.; Gösele, U.; Christiansen, S. H.

    2007-04-01

    Two-inch free-standing GaN wafers were implanted by 100 keV H+2 ions with a dose of 1.3 × 1017 cm-2 at room temperature. The hydrogen implantation induced damage in GaN extends between 230 to 500 nm from the surface as measured by cross-sectional transmission electron microscopy (XTEM). The wafer bow of the free-standing GaN wafers was measured using a Tencor long range profilometer on a scan length of 48 mm before and after the hydrogen implantation. Before implantation the bow of two different free-standing GaN wafers (named A and B) with different thicknesses was 1.5 µm and 6 µm, respectively. Initially, both wafers were concave in shape. After implantation the bow changed to convex with a value of 36 µm for wafer A and a value of 32 µm for wafer B. High dose hydrogen implantation leads to an in-plane compressive stress in the top damaged layer of the GaN, which is responsible for the enhancement of wafer bow and change of bow direction. The high value of bow after implantation hinders the direct wafer bonding of the free-standing GaN wafers to sapphire or any other handle wafers. Tight bonding between hydrogen implanted GaN wafers and the handle wafers is a necessary requirement for the successful layer transfer of thin GaN layers onto other substrates based on wafer bonding and layer splitting (Smart-cut).

  7. Impact accelerations

    NASA Technical Reports Server (NTRS)

    Vongierke, H. E.; Brinkley, J. W.

    1975-01-01

    The degree to which impact acceleration is an important factor in space flight environments depends primarily upon the technology of capsule landing deceleration and the weight permissible for the associated hardware: parachutes or deceleration rockets, inflatable air bags, or other impact attenuation systems. The problem most specific to space medicine is the potential change of impact tolerance due to reduced bone mass and muscle strength caused by prolonged weightlessness and physical inactivity. Impact hazards, tolerance limits, and human impact tolerance related to space missions are described.

  8. ACCELERATION PHYSICS CODE WEB REPOSITORY.

    SciTech Connect

    WEI, J.

    2006-06-26

    In the framework of the CARE HHH European Network, we have developed a web-based dynamic accelerator-physics code repository. We describe the design, structure and contents of this repository, illustrate its usage, and discuss our future plans, with emphasis on code benchmarking.

  9. Accelerator Physics Code Web Repository

    SciTech Connect

    Zimmermann, F.; Basset, R.; Bellodi, G.; Benedetto, E.; Dorda, U.; Giovannozzi, M.; Papaphilippou, Y.; Pieloni, T.; Ruggiero, F.; Rumolo, G.; Schmidt, F.; Todesco, E.; Zotter, B.W.; Payet, J.; Bartolini, R.; Farvacque, L.; Sen, T.; Chin, Y.H.; Ohmi, K.; Oide, K.; Furman, M.; /LBL, Berkeley /Oak Ridge /Pohang Accelerator Lab. /SLAC /TRIUMF /Tech-X, Boulder /UC, San Diego /Darmstadt, GSI /Rutherford /Brookhaven

    2006-10-24

    In the framework of the CARE HHH European Network, we have developed a web-based dynamic accelerator-physics code repository. We describe the design, structure and contents of this repository, illustrate its usage, and discuss our future plans, with emphasis on code benchmarking.

  10. Visions for the future of particle accelerators

    NASA Astrophysics Data System (ADS)

    Romaniuk, Ryszard S.

    2013-10-01

    The ambitions of accelerator based science, technology and applications far exceed the present accelerator possibilities. Accelerator science and technology is one of a key enablers of the developments in the particle physic, photon physics and also applications in medicine and industry. The paper presents a digest of the research results and visions for the future in the domain of accelerator science and technology in Europe, shown during the final fourth annual meeting of the EuCARD - European Coordination of Accelerator Research and Development. The conference concerns building of the research infrastructure, including advanced photonic and electronic systems for servicing large high energy physics experiments. There are debated a few basic groups of such systems like: measurement - control networks of large geometrical extent, multichannel systems for large amounts of metrological data acquisition, precision photonic networks of reference time, frequency and phase distribution. The main subject is however the vision for the future of particle accelerators and next generation light sources.

  11. Fabrication of GaN Microporous Structure at a GaN/Sapphire Interface as the Template for Thick-Film GaN Separation Grown by HVPE

    NASA Astrophysics Data System (ADS)

    Chen, Jianli; Cheng, Hongjuan; Zhang, Song; Lan, Feifei; Qi, Chengjun; Xu, Yongkuan; Wang, Zaien; Li, Jing; Lai, Zhanping

    2016-10-01

    In this paper, a microporous structure at the GaN/sapphire interface has been obtained by an electrochemical etching method via a selective etching progress using an as-grown GaN/sapphire wafer grown by metal organic chemical vapor deposition. The as-prepared GaN interfacial microporous structure has been used as a template for the following growth of thick-film GaN crystal by hydride vapor phase epitaxy (HVPE), facilitating the fabrication of a free-standing GaN substrate detached from a sapphire substrate. The evolution of the interfacial microporous structure has been investigated by varying the etching voltages and time, and the formation mechanism of interfacial microporous structure has been discussed in detail as well. Appropriate interfacial microporous structure is beneficial for separating the thick GaN crystal grown by HVPE from sapphire during the cooling down process. The separation that occurred at the place of interfacial microporous can be attributed to the large thermal strain between GaN and sapphire. This work realized the fabrication of a free-standing GaN substrate with high crystal quality and nearly no residual strain.

  12. Optical properties of GaN wurtzite quantum wires

    NASA Astrophysics Data System (ADS)

    Zhang, X. W.; Xia, J. B.

    2006-03-01

    The electronic structure and optical properties of freestanding GaN wurtzite quantum wires are studied in the framework of six-band effective-mass envelope function theory. It is found that the electron states are either twofold or fourfold degenerate. There is a dark exciton effect when the radius R of GaN wurtzite quantum wires is in the range of [0.7, 10.9] nm. The linear polarization factors are calculated in three cases, the quantum confinement effect (finite long wire), the dielectric effect and both effects (infinitely long wire). It is found that the linear polarization factor of a finite long wire whose length is much less than the electromagnetic wavelength decreases as R increases, is very close to unity (0.979) at R = 1 nm, and changes from a positive value to a negative value around R = 4.1 nm. The linear polarization factor of the dielectric effect is 0.934, independent of radius, as long as the radius remains much less than the electromagnetic wavelength. The result for the two effects shows that the quantum confinement effect gives a correction to the dielectric effect result. It is found that the linear polarization factor of very long (treated approximately as infinitely long) quantum wires is in the range of [0.8, 1]. The linear polarization factors of the quantum confinement effect of CdSe wurtzite quantum wires are calculated for comparison. In the CdSe case, the linear polarization factor of R = 1 nm is 0.857, in agreement with the experimental results (Hu et al 2001 Science 292 2060). This value is much smaller than unity, unlike 0.979 in the GaN case, mainly due to the big spin-orbit splitting energy Δso of CdSe material with wurtzite structure.

  13. Growth of thick GaN layers on laser-processed sapphire substrate by hydride vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Koyama, Koji; Aida, Hideo; Kim, Seong-Woo; Ikejiri, Kenjiro; Doi, Toshiro; Yamazaki, Tsutomu

    2014-10-01

    A 600 μm thick GaN layer was successfully grown by hydride vapor phase epitaxy by replacing the standard sapphire substrate with that processed by a focused laser beam within the substrate. The effects of the laser processing on the curvature and cracking of the GaN layer were investigated. Microscopic observations of the interior of the thick GaN layer revealed that the laser-processed substrate suppressed the generation of microcracks in the GaN layer. In addition, the laser processing was also found to reduce the change in the curvature during the GaN layer growth in comparison to that on the standard substrate. It is shown that the overlapping microcracks observed in the GaN layer on the standard sapphire substrate lead to serious cracking after thick GaN layer growth.

  14. MOCVD growth of GaN on Si through novel substrate modification techniques

    NASA Astrophysics Data System (ADS)

    Gagnon, Jarod C.

    GaN is a semiconductor material with great potential for use in high power electronics and optoelectronics due to the high electron mobility, high breakdown voltage, high thermal stability, and large direct bandgap of GaN. Si is a desirable substrate material for GaN heteroepitaxy due to the low cost of production, large wafer sizes available, and current widespread use in the electronics industry. The growth of GaN/Si devices suffers from the lattice and CTE mismatches between GaN and Si and therefore multiple methods of strain reduction have been employed to counter these effects. In this work we presented two novel methods of substrate modification to promote the growth of device quality GaN on Si. Initial work focused on the implantation of AlN/Si(111) substrates with N+ ions below the AlN/Si(111) interface. A reduction in the initial compressive stress in GaN films as well as the degree of tensile stress generation during growth was observed on implanted samples. Optical microscopy of the GaN surfaces showed reduced channeling crack density on implanted substrates. Transmission electron microscopy (TEM) studies showed a disordered layer in the Si substrate at the implantation depth which consisted of a mixture of polycrystalline and amorphous Si. Evidence was provided to suggest that the disordered layer at the implantation depth was acting as a compliant layer which decoupled the GaN film from the bulk Si substrate and partially accommodated the tensile stress formed during growth and cooling. A reduction in threading dislocation (TD) density on ion implanted substrates was also observed. Additional studies showed that by increasing the lateral size of AlN islands, the tensile growth stress and TD density in GaN films on ion implanted substrates could be further reduced. XRD studies showed an expansion of the AlN lattice on implanted substrates with larger lateral island sizes. The final tensile growth stress of films on implanted substrates was further

  15. H enhancement of N vacancy migration in GaN.

    SciTech Connect

    Wixom, Ryan R.; Wright, Alan Francis

    2005-06-01

    We have used density functional theory to investigate diffusion of V{sub N}{sup +} in the presence of H{sup +}. Optimal migration pathways were determined using the climbing image nudged elastic band and directed dimer methods. Our calculations indicate that the rate-limiting barrier for VN{sub N}{sup +} migration will be reduced by 0.58 eV by interplay with H{sup +}, which will enhance migration by more than an order of magnitude at typical GaN growth temperatures.

  16. Transport mechanisms in Schottky diodes realized on GaN

    NASA Astrophysics Data System (ADS)

    Amor, Sarrah; Ahaitouf, Ali; Ahaitouf, Abdelaziz; Salvestrini, Jean Paul; Ougazzaden, Abdellah

    2017-03-01

    This work is focused on the conducted transport mechanisms involved on devices based in gallium nitride GaN and its alloys. With considering all conduction mechanisms of current, its possible to understanded these transport phenomena. Thanks to this methodology the current-voltage characteristics of structures with unusual behaviour are further understood and explain. Actually, the barrier height (SBH) is a complex problem since it depends on several parameters like the quality of the metal-semiconductor interface. This study is particularly interesting as solar cells are made on this material and their qualification is closely linked to their transport properties.

  17. High field effects of GaN HEMTs.

    SciTech Connect

    Barker, Joy; Shul, Randy John

    2004-09-01

    This report represents the completion of a Laboratory-Directed Research and Development (LDRD) program to develop and fabricate geometric test structures for the measurement of transport properties in bulk GaN and AlGaN/GaN heterostructures. A large part of this study was spent examining fabrication issues related to the test structures used in these measurements, due to the fact that GaN processing is still in its infancy. One such issue had to do with surface passivation. Test samples without a surface passivation, often failed at electric fields below 50 kV/cm, due to surface breakdown. A silicon nitride passivation layer of approximately 200 nm was used to reduce the effects of surface states and premature surface breakdown. Another issue was finding quality contacts for the material, especially in the case of the AlGaN/GaN heterostructure samples. Poor contact performance in the heterostructures plagued the test structures with lower than expected velocities due to carrier injection from the contacts themselves. Using a titanium-rich ohmic contact reduced the contact resistance and stopped the carrier injection. The final test structures had an etch constriction with varying lengths and widths (8x2, 10x3, 12x3, 12x4, 15x5, and 16x4 {micro}m) and massive contacts. A pulsed voltage input and a four-point measurement in a 50 {Omega} environment was used to determine the current through and the voltage dropped across the constriction. From these measurements, the drift velocity as a function of the applied electric field was calculated and thus, the velocity-field characteristics in n-type bulk GaN and AlGaN/GaN test structures were determined. These measurements show an apparent saturation velocity near to 2.5x10{sup 7} cm/s at 180 kV/cm and 3.1x10{sup 7} cm/s, at a field of 140 kV/cm, for the bulk GaN and AlGaN heterostructure samples, respectively. These experimental drift velocities mark the highest velocities measured in these materials to date and confirm

  18. Radiation enhanced basal plane dislocation glide in GaN

    NASA Astrophysics Data System (ADS)

    Yakimov, Eugene B.; Vergeles, Pavel S.; Polyakov, Alexander Y.; Lee, In-Hwan; Pearton, Stephen J.

    2016-05-01

    A movement of basal plane segments of dislocations in GaN films grown by epitaxial lateral overgrowth under low energy electron beam irradiation (LEEBI) was studied by the electron beam induced current (EBIC) method. Only a small fraction of the basal plane dislocation segments were susceptible to irradiation and the movement was limited to relatively short distances. The effect is explained by the radiation enhanced dislocation glide (REDG) in the structure with strong pinning. A dislocation velocity under LEEBI with a beam current lower than 1 nA was estimated as about 10 nm/s. The results assuming the REDG for prismatic plane dislocations were presented.

  19. First-Principles Study of Defects in GaN

    DTIC Science & Technology

    2009-07-29

    energies and other properties of the Gai – VGa and Ni – VN Frenkel pairs in GaN. o Results on the binding energies and stabilities of Frenkel pairs as...Frenkel pair are interstitial Ga ( Gai ) and Ga vacancy (VGa). Gallium interstitial can occur in 3+, 2+, and 1+ charge states, depending on the Fermi...distance to the six Ga nearest neighbors are roughly the same (to within 0.15 Å). The defect level of Gai is found to lie in the upper part of the

  20. Phonon-plasmon coupled modes in GaN

    NASA Astrophysics Data System (ADS)

    Dyson, A.

    2009-04-01

    The phonon lifetime in GaN is known to exhibit a dependence on electron density. Recent noise measurements have also shown the lifetime to be temperature dependent. The source of these dependences is the coupling of the phonon and plasmon populations through the dielectric function. The effect of this anharmonicity is illustrated by comparing the frequency and wavevector dependent coupled-mode momentum relaxation rate with the phonon momentum relaxation rate obtained by Callen. A simple model that includes the anharmonic interaction and phonon migration yields phonon lifetimes depending on both electron density and temperature.

  1. Field emission from quantum size GaN structures

    NASA Astrophysics Data System (ADS)

    Yilmazoglu, O.; Pavlidis, D.; Litvin, Yu. M.; Hubbard, S.; Tiginyanu, I. M.; Mutamba, K.; Hartnagel, H. L.; Litovchenko, V. G.; Evtukh, A.

    2003-12-01

    Whisker structures and quantum dots fabricated by photoelectrochemical (PEC) etching of undoped and doped metalorganic chemical vapor deposition (MOCVD)-grown GaN (2×10 17 or 3×10 18 cm -3) are investigated in relation with their field-emission characteristics. Different surface morphologies, corresponding to different etching time and photocurrent, results in different field-emission characteristics with low turn-on voltage down to 4 V/μm and the appearance of quantum-size effect in the I- V curves.

  2. Modelling of GaN quantum dot terahertz cascade laser

    NASA Astrophysics Data System (ADS)

    Asgari, A.; Khorrami, A. A.

    2013-03-01

    In this paper GaN based spherical quantum dot cascade lasers has been modelled, where the generation of the terahertz waves are obtained. The Schrödinger, Poisson, and the laser rate equations have been solved self-consistently including all dominant physical effects such as piezoelectric and spontaneous polarization in nitride-based QDs and the effects of the temperature. The exact value of the energy levels, the wavefunctions, the lifetimes of electron levels, and the lasing frequency are calculated. Also the laser parameters such as the optical gain, the output power and the threshold current density have been calculated at different temperatures and applied electric fields.

  3. Photoluminescence enhancement from GaN by beryllium doping

    NASA Astrophysics Data System (ADS)

    García-Gutiérrez, R.; Ramos-Carrazco, A.; Berman-Mendoza, D.; Hirata, G. A.; Contreras, O. E.; Barboza-Flores, M.

    2016-10-01

    High quality Be-doped (Be = 0.19 at.%) GaN powder has been grown by reacting high purity Ga diluted alloys (Be-Ga) with ultra high purity ammonia in a horizontal quartz tube reactor at 1200 °C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produced GaN powders with 100% reaction efficiency. Doping was achieved by dissolving beryllium into the gallium metal. The powders synthesized by this method regularly consist of two particle size distributions: large hollow columns with lengths between 5 and 10 μm and small platelets in a range of diameters among 1 and 3 μm. The GaN:Be powders present a high quality polycrystalline profile with preferential growth on the [10 1 bar 1] plane, observed by means of X-ray diffraction. The three characteristics growth planes of the GaN crystalline phase were found by using high resolution TEM microscopy. The optical enhancing of the emission in the GaN powder is attributed to defects created with the beryllium doping. The room temperature photoluminescence emission spectra of GaN:Be powders, revealed the presence of beryllium on a shoulder peak at 3.39 eV and an unusual Y6 emission at 3.32eV related to surface donor-acceptor pairs. Also, a donor-acceptor-pair transition at 3.17 eV and a phonon replica transition at 3.1 eV were observed at low temperature (10 K). The well-known yellow luminescence band coming from defects was observed in both spectra at room and low temperature. Cathodoluminescence emission from GaN:Be powders presents two main peaks associated with an ultraviolet band emission and the yellow emission known from defects. To study the trapping levels related with the defects formed in the GaN:Be, thermoluminescence glow curves were obtained using UV and β radiation in the range of 50 and 150 °C.

  4. A modeling study of GaN growth by MOVPE

    SciTech Connect

    Safvi, S.A.; Kuech, T.F.; Redwing, J.M.; Tischler, M.A.

    1996-11-01

    A model for the growth of gallium nitride in a vertical metalorganic vapor phase epitaxy reactor is presented. For a mixture of non-dilute gases, the flow temperature and concentration profiles are predicted. The results show that the growth of GaN epilayers is through an intermediate adduct of TMG and ammonia. Growth rates are predicted based on simple reaction mechanisms and compared with those obtained experimentally. Loss of adduct species due to polymerization leads to lowering in growth rate. An attempt to quantify loss of reacting species is made based on experimentally observed growth rates.

  5. Accelerator system and method of accelerating particles

    NASA Technical Reports Server (NTRS)

    Wirz, Richard E. (Inventor)

    2010-01-01

    An accelerator system and method that utilize dust as the primary mass flux for generating thrust are provided. The accelerator system can include an accelerator capable of operating in a self-neutralizing mode and having a discharge chamber and at least one ionizer capable of charging dust particles. The system can also include a dust particle feeder that is capable of introducing the dust particles into the accelerator. By applying a pulsed positive and negative charge voltage to the accelerator, the charged dust particles can be accelerated thereby generating thrust and neutralizing the accelerator system.

  6. GaN quantum dot polarity determination by X-ray photoelectron diffraction

    NASA Astrophysics Data System (ADS)

    Romanyuk, O.; Bartoš, I.; Brault, J.; Mierry, P. De; Paskova, T.; Jiříček, P.

    2016-12-01

    Growth of GaN quantum dots (QDs) on polar and semipolar GaN substrates is a promising technology for efficient nitride-based light emitting diodes (LED). The QDs crystal orientation typically repeats the polarity of the substrate. In case of non-polar or semipolar substrates, the polarity of QDs is not obvious. In this article, the polarity of GaN QDs and of underlying layers was investigated nondestructively by X-ray photoelectron diffraction (XPD). Polar and semipolar GaN/Al0.5Ga0.5N heterostructures were grown on the sapphire substrates with (0001) and (1 1 bar 00) orientations by molecular beam epitaxy (MBE). Polar angle dependence of N 1s core-level photoelectron intensities were measured from GaN QDs and compared with the corresponding experimental curves from free-standing GaN crystals. It is confirmed experimentally, that the crystalline orientation of polar (0001) GaN QDs follows the orientation of the (0001) sapphire substrate. In case of semipolar GaN QDs grown on (1 1 bar 00) sapphire substrate, the (11 2 bar 2) polarity of QDs was determined.

  7. Photoelectrochemical water splitting on nanoporous GaN thin films for energy conversion under visible light

    NASA Astrophysics Data System (ADS)

    Cao, Dezhong; Xiao, Hongdi; Fang, Jiacheng; Liu, Jianqiang; Gao, Qingxue; Liu, Xiangdong; Ma, Jin

    2017-01-01

    Nanoporous (NP) GaN thin films, which were fabricated by an electrochemical etching method at different voltages, were used as photoelectrodes during photoelectrochemical (PEC) water splitting in 1 M oxalic acid solution. Upon illumination at a power density of 100 mW cm‑2 (AM 1.5), water splitting is observed in NP GaN thin films, presumably resulting from the valence band edge which is more positive than the redox potential of the oxidizing species. In comparison with NP GaN film fabricated at 8 V, NP GaN obtained at 18 V shows nearly twofold enhancement in photocurrent with the maximum photo-to-hydrogen conversion efficiency of 1.05% at ~0 V (versus Ag/AgCl). This enhancement could be explained with (i) the increase of surface area and surface states, and (ii) the decrease of resistances and carrier concentration in the NP GaN thin films. High stability of the NP GaN thin films during the PEC water splitting further confirms that the NP GaN thin film could be applied to the design of efficient solar cells and solar fuel devices.

  8. Fabrication of photonic crystal circuits based on GaN ultrathin membranes by maskless lithography

    NASA Astrophysics Data System (ADS)

    Volciuc, Olesea; Braniste, Tudor; Sergentu, Vladimir; Ursaki, Veaceslav; Tiginyanu, Ion M.; Gutowski, Jürgen

    2015-06-01

    We report on maskless fabrication of photonic crystal (PhC) circuits based on ultrathin (d ~ 15 nm) nanoperforated GaN membranes exhibiting a triangular lattice arrangement of holes with diameters of 150 nm. In recent years, we have proposed and developed a cost-effective technology for GaN micro- and nanostructuring, the so-called surface charge lithography (SCL), which opened wide possibilities for a controlled fabrication of GaN ultrathin membranes. SCL is a maskless approach based on direct writing of negative charges on the surface of a semiconductor by a focused ion beam (FIB). These charges shield the material against photo-electrochemical (PEC) etching. Ultrathin GaN membranes suspended on specially designed GaN microstructures have been fabricated using a technological route based on SCL with two selected doses of ion beam treatment. Calculation of the dispersion law in nanoperforated membranes in the approximation of scalar waves is indicative of the occurrence of surface and bulk modes, and there is a range of frequencies where only surface modes can exist. Advantages of the occurrence of two types of modes in ultrathin nanoperforated GaN membranes from the point of view of their incorporation in photonic and optoelectronic integrated circuits are discussed. Along with this, we present the results of a comparative analysis of persistent photoconductivity (PPC) and optical quenching (OQ) effects occurring in continuous and nanoperforated ultrathin GaN suspended membranes, and assess the mechanisms behind these phenomena.

  9. Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Wang, Honggang; Wang, Meishan; Kong, Yike

    2016-09-01

    For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin n-type GaN cap layer on p-type GaN emission layer, a p-n heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an n-type doped cap layer which contributes to the decreasing of work function. After the growth of n-type GaN cap layer, the atom structure near the p-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by n-type GaN cap layer.

  10. Fabrication of GaN nanotubular material using MOCVD with aluminum oxide membrane

    NASA Astrophysics Data System (ADS)

    Jung, Woo-Gwang; Jung, Se-Hyuck; Kung, Patrick; Razeghi, Manijeh

    2006-02-01

    GaN nanotubular material is fabricated with aluminum oxide membrane in MOCVD. SEM, XRD, TEM and PL are employed to characterize the fabricated GaN nanotubular material. An aluminum oxide membrane with ordered nano holes is used as template. Gallium nitride is deposited at the inner wall of the nano holes in aluminum oxide template, and the nanotubular material with high aspect ratio is synthesized using the precursors of TMG and ammonia gas. Optimal synthesis condition in MOCVD is obtained successfully for the gallium nitride nanotubular material in this research. The diameter of GaN nanotube fabricated is approximately 200 ~ 250 nm and the wall thickness is about 40 ~ 50 nm. GaN nanotubular material consists of numerous fine GaN particulates with sizes ranging 15 to 30 nm. The composition of gallium nitride is confirmed to be stoichiometrically 1:1 for Ga and N by EDS. XRD and TEM analyses indicate that grains in GaN nanotubular material have nano-crystalline structure. No blue shift is found in the PL spectrum on the GaN nanotubular material fabricated in aluminum oxide template.

  11. Fabrication of GaN nanotubular material using MOCVD with an aluminium oxide membrane

    NASA Astrophysics Data System (ADS)

    Jung, Woo-Gwang; Jung, Se-Hyuck; Kung, Patrick; Razeghi, Manijeh

    2006-01-01

    GaN nanotubular material is fabricated with an aluminium oxide membrane in MOCVD. SEM, XRD, TEM and PL are employed to characterize the fabricated GaN nanotubular material. An aluminium oxide membrane with ordered nanoholes is used as a template. Gallium nitride is deposited at the inner wall of the nanoholes in the aluminium oxide template, and the nanotubular material with high aspect ratio is synthesized using the precursors of TMG and ammonia gas. Optimal synthesis conditions in MOCVD are obtained successfully for the gallium nitride nanotubular material in this research. The diameter of the GaN nanotube fabricated is approximately 200-250 nm and the wall thickness is about 40-50 nm. GaN nanotubular material consists of numerous fine GaN particulates with size range 15-30 nm. The composition of gallium nitride is confirmed to be stoichiometrically 1:1 for Ga and N by EDS. XRD and TEM analyses indicate that the grains in GaN nanotubular material have a nano-crystalline structure. No blue shift is found in the PL spectrum on the GaN nanotubular material fabricated in an aluminium oxide template.

  12. Air-bridged lateral epitaxial overgrowth of GaN thin films

    NASA Astrophysics Data System (ADS)

    Kidoguchi, Isao; Ishibashi, Akihiko; Sugahara, Gaku; Ban, Yuzaburoh

    2000-06-01

    A promising technique of selective lateral epitaxy, namely air-bridged lateral epitaxial overgrowth, is demonstrated in order to reduce the wing tilt as well as the threading dislocation density in GaN thin films. A seed GaN layer was etched to make ridge-stripe along <11¯00>GaN direction and a GaN material was regrown from the exposed (0001) top facet of the ridged GaN seed structures, whose sidewalls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor phase epitaxy. The density of dislocations in the wing region was reduced to be <107cm-2, which was at least two orders of magnitude lower than that of underlying GaN. The magnitude of the wing tilt was determined to be 0.08° by x-ray diffraction (XRD) measurements, which was smaller than other lateral epitaxial overgrown GaN thin films. The full width at half maximum of XRD for the wing region was 138 arc sec, indicating high uniformity of c-axis orientation.

  13. Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

    NASA Astrophysics Data System (ADS)

    Zhong, Guohua; Zhang, Kang; He, Fan; Ma, Xuhang; Lu, Lanlan; Liu, Zhuang; Yang, Chunlei

    2012-09-01

    Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce-Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin-orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.

  14. Annealing effects on polycrystalline GaN using nitrogen and ammonia ambients

    NASA Astrophysics Data System (ADS)

    Ariff, A.; Zainal, N.; Hassan, Z.

    2016-09-01

    This paper describes effects of using post-annealing treatment in different conditions on the properties of polycrystalline GaN layer grown on m-plane sapphire substrate by electron beam (e-beam) evaporator. Without annealing, GaN surface was found to have a low RMS roughness with agglomeration of GaN grains in a specific direction and the sample consisted of gallium oxide (Ga2O3) material. When the post-annealing treatment was carried out in N2 ambient at 650 °C, initial re-crystallization of the GaN grains was observed while the evidence of Ga2O3 almost disappeared. As the NH3 annealing was conducted at 950 °C, more effect of re-crystallization occurred but with less grains coalescence. Three dominant XRD peaks of GaN in (10 1 bar 0) , (0002) and (10 1 bar 1) orientations were evident. Near band edge (NBE) related emission in GaN was also observed. The significant improvement was attributed to simultaneous recrystallization and effective reduction of N deficiency density. The post-annealing in a mixture of N2 and NH3 ambient at 950 °C was also conducted, but has limited the effectiveness of the N atoms to incorporate on the GaN layer due to 'clouding' effect by the inert N2 gas. Further increase in the annealing temperature at 980 °C and 1100 °C, respectively caused severe deteriorations of the structural and optical properties of the GaN layer. Overall, this work demonstrated initial potential in improving polycrystalline GaN material in simple and inexpensive manner.

  15. Growth of GaN nanotubes and nanowires

    NASA Astrophysics Data System (ADS)

    He, Maoqi; Zhou, Piezhen; Mohammad, S. Noor; Halpern, Joshua; Jacobs, Randy; Sarney, Wendy; Salamanca-Riba, Lourdes

    2001-03-01

    We have grown GaN nanowires and nanotubes by direct reaction of Ga metal vapor with NH3 in a simple tube furnace. By varying the ammonia flow rate (20-120 sccm) and temperature (800-1000 ^oC) we have been able to control onset of growth, and the diameters of these cylindrically symmetric nanostructures. The length of the nanowires and nanotubes is determined by the duration of the growth cycle. The nanowires and nanotubes that we have studied are single crystal and have a wurtzite structure. In the first 90 minutes, a 1 or 2 μ thick amorphous GaN matrix is deposited. Soon after pebble like polycrystalline hillocks form. Single crystal nanowires sprout from the sides of the platelets. The diameter of the nanowire is the same as the thickness of the plate, that is the smallest dimension of the face from which it grows. Since the platelet thickness depends on temperature and the ammonia, the thickness of the nanowires is proportional to the temperature and ammonia flow rate. At higher temperatures and flow rates nanotubes and rods start to grow from the flat surface of the plates

  16. Magneto-ballistic transport in GaN nanowires

    NASA Astrophysics Data System (ADS)

    Santoruvo, Giovanni; Allain, Adrien; Ovchinnikov, Dmitry; Matioli, Elison

    2016-09-01

    The ballistic filtering property of nanoscale crosses was used to investigate the effect of perpendicular magnetic fields on the ballistic transport of electrons on wide band-gap GaN heterostructures. The straight scattering-less trajectory of electrons was modified by a perpendicular magnetic field which produced a strong non-linear behavior in the measured output voltage of the ballistic filters and allowed the observation of semi-classical and quantum effects, such as quenching of the Hall resistance and manifestation of the last plateau, in excellent agreement with the theoretical predictions. A large measured phase coherence length of 190 nm allowed the observation of universal quantum fluctuations and weak localization of electrons due to quantum interference up to ˜25 K. This work also reveals the prospect of wide band-gap GaN semiconductors as a platform for basic transport and quantum studies, whose properties allow the investigation of ballistic transport and quantum phenomena at much larger voltages and temperatures than in other semiconductors.

  17. Orthodox etching of HVPE-grown GaN

    SciTech Connect

    Weyher, J.L.; Lazar, S.; Macht, L.; Liliental-Weber, Z.; Molnar,R.J.; Muller, S.; Nowak, G.; Grzegory, I.

    2006-08-10

    Orthodox etching of HVPE-grown GaN in molten eutectic of KOH + NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.

  18. High Quality, Low Cost Ammonothermal Bulk GaN Substrates

    SciTech Connect

    Ehrentraut, D; Pakalapati, RT; Kamber, DS; Jiang, WK; Pocius, DW; Downey, BC; McLaurin, M; D'Evelyn, MP

    2013-12-18

    Ammonothermal GaN growth using a novel apparatus has been performed on c-plane, m-plane, and semipolar seed crystals with diameters between 5 mm and 2 in. to thicknesses of 0.5-3 mm. The highest growth rates are greater than 40 mu m/h and rates in the 10-30 mu m/h range are routinely observed for all orientations. These values are 5-100x larger than those achieved by conventional ammonothermal GaN growth. The crystals have been characterized by X-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), optical spectroscopy, and capacitance-voltage measurements. The crystallinity of the grown crystals is similar to or better than that of the seed crystals, with FWHM values of about 20-100 arcsec and dislocation densities of 1 x 10(5)-5 x 10(6) cm(-2). Dislocation densities below 10(4) cm(-2) are observed in laterally-grown crystals. Epitaxial InGaN quantum well structures have been successfully grown on ammonothermal wafers. (C) 2013 The Japan Society of Applied Physics

  19. Europium doping of zincblende GaN by ion implantation

    SciTech Connect

    Lorenz, K.; Franco, N.; Darakchieva, V.; Alves, E.; Roqan, I. S.; O'Donnell, K. P.; Trager-Cowan, C.; Martin, R. W.; As, D. J.; Panfilova, M.

    2009-06-01

    Eu was implanted into high quality cubic (zincblende) GaN (ZB-GaN) layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction (XRD) and Rutherford backscattering/channeling spectrometry. A low concentration (<10%) of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their (0001) planes aligned with the (111) planes of the cubic lattice. Implantation of Eu causes an expansion of the lattice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN (W-GaN). For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the <110> direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence (PL), and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.

  20. ODENDOR of Defects in GaN Epitaxial Layers

    NASA Astrophysics Data System (ADS)

    Glaser, E. R.; Kennedy, T. A.; Carlos, W. E.; Freitas, J. A., Jr.; Wickenden, A. E.; Koleske, D. D.

    1998-03-01

    Optically-detected electron-nuclear double resonance (ODENDOR) at 24 GHz has been performed on a set of GaN epitaxial layers (undoped and Si-doped) grown on Al_2O3 to identify and locate residual defects and dopants.(E.R. Glaser et al., submitted to Phys. Rev. B) The first observation was made by the Paderborn group.(F.K. Koschnick et al., Phys. Rev. B 54, R11042 (1996).) Strong ODENDOR with resolved quadrupole structure was observed between 6 and 14 MHz on the g = 1.951 effective-mass (EM) donor resonance found on the 2.2 eV emission bands from two high-resistivity films (n <= 1x10^16 cm-3). The lines are assigned to ^69,71Ga lattice nuclei coupled to the residual donors through a weak hyperfine interaction (<= 1 MHz). The quadrupole splitting of ~ 2.3 MHz found for ^69Ga varies between samples and in comparison to published values for strain-free GaN by 15-25 %. The symmetry of the electric field gradient provides evidence that the donors are located in the crystallites rather than near grain boundaries. ODENDOR was not found between 1.5 and 140 MHz on the g = 1.991 deep defect resonances. Possible reasons for the absence of signals will be discussed.

  1. Atomic layer deposition of GaN at low temperatures

    SciTech Connect

    Ozgit, Cagla; Donmez, Inci; Alevli, Mustafa; Biyikli, Necmi

    2012-01-15

    The authors report on the self-limiting growth of GaN thin films at low temperatures. Films were deposited on Si substrates by plasma-enhanced atomic layer deposition using trimethylgallium (TMG) and ammonia (NH{sub 3}) as the group-III and -V precursors, respectively. GaN deposition rate saturated at 185 deg. C for NH{sub 3} doses starting from 90 s. Atomic layer deposition temperature window was observed from 185 to {approx}385 deg. C. Deposition rate, which is constant at {approx}0.51 A/cycle within the temperature range of 250 - 350 deg. C, increased slightly as the temperature decreased to 185 deg. C. In the bulk film, concentrations of Ga, N, and O were constant at {approx}36.6, {approx}43.9, and {approx}19.5 at. %, respectively. C was detected only at the surface and no C impurities were found in the bulk film. High oxygen concentration in films was attributed to the oxygen impurities present in group-V precursor. High-resolution transmission electron microscopy studies revealed a microstructure consisting of small crystallites dispersed in an amorphous matrix.

  2. A GaN photonic crystal membrane laser.

    PubMed

    Lin, Cheng-Hung; Wang, Jyh-Yang; Chen, Cheng-Yen; Shen, Kun-Ching; Yeh, Dong-Ming; Kiang, Yean-Woei; Yang, C C

    2011-01-14

    The implementation of a series of optically pumped GaN photonic crystal (PhC) membrane lasers is demonstrated at room temperature. The photonic crystal is composed of a scalene-triangular arrangement of circular holes in GaN. Three defect structures are fabricated for comparing their lasing characteristics with those of perfect PhC. It is observed that all the lasing defect modes have lasing wavelengths very close to the band-edge modes in the perfect PhC structure. Although those lasing modes, including band-edge and defect modes, have different optical pump thresholds, different lasing spectral widths, different quality factors (Q factors), and different polarization ratios, all their polarization distributions show maxima in the directions around one of the hole arrangement axes. The similar lasing characteristics between the band-edge and defect modes are attributed to the existence of extremely narrow partial band gaps for forming the defect modes. Also, the oriented polarization properties are due to the scalene-triangle PhC structure. In one of the defect lasing modes, the lasing threshold is as low as 0.82 mJ cm(-2), the cavity Q factor is as large as 1743, and the polarization ratio is as large as 25.4. Such output parameters represent generally superior lasing behaviors when compared with previously reported implementations of similar laser structures.

  3. Nearly stress-free substrates for GaN homoepitaxy

    NASA Astrophysics Data System (ADS)

    Hermann, M.; Gogova, D.; Siche, D.; Schmidbauer, M.; Monemar, B.; Stutzmann, M.; Eickhoff, M.

    2006-08-01

    High-quality 300 μm thick GaN crack-free layers grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire without buffer layers and separated from the substrate by laser lift-off were investigated by high resolution X-ray diffraction (XRD), low-temperature photoluminescence and cathodoluminescence. All these characterization techniques confirm the high structural quality of the resulting material. Lateral X-ray mapping of the free-standing bulk-like GaN shows a homogeneous compressive stress of less than 40 MPa and a heterogeneous stress of about 80 MPa. The formation of twin grains (domains) were observed both in the reciprocal space mapping of the (2 0 .5) reflection and in rocking curve measurements. The latter ones revealed an estimated lateral coherence length of about 1.2 μm. The crystallite size along the c-axis is estimated to be larger than 20 μm. An upper limit of the density of dislocations with a component of the Burgers vector along the c-axis (screw and mixed type) of 1.3×10 7 cm -2 was extracted from the XRD data, while transmission electron microscopy measurements revealed a dislocation density of 1.7×10 7 cm -2. Thus, these layers are suitable as lattice-parameter and thermal-expansion matched substrates for strain-free homoepitaxy of GaN-based device heterostructures.

  4. Excitation mechanisms of Er optical centers GaN epilayers

    NASA Astrophysics Data System (ADS)

    Hawkins, Matthew; Jiang, Hongxing; Lin, Jingyu; Zavada, John; Vinh, Nguyen

    We report direct evidence of two mechanisms responsible for the excitation of optically active Er3 + ions in GaN epilayers grown by metal-organic chemical vapor deposition. These mechanisms, resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, lead to narrow emission lines from isolated and the defect-related Er optical centers. However, these centers have different photoluminescence spectra, local defect environments, decay dynamics, and excitation cross sections. The photoluminescence at 1.54 micrometer from the isolated Er optical center which can be excited by either mechanism has the same decay dynamics, but possesses a much higher excitation cross-section under band-to-band excitation. In contrast, the photoluminescence at 1.54 micrometer from the defect-related Er optical center can only be observed through band-to-band excitation but has the largest excitation cross-section. These results explain the difficulty in achieving gain in Er doped GaN and indicate approaches for realization of optical amplification, and possibly lasing, at room temperature.

  5. Research on quantum efficiency of GaN wire photocathode

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Diao, Yu; Kong, Yike

    2017-02-01

    On the basis of three-dimensional continuity equation in semiconductors and finite difference method, the carrier concentration and the quantum efficiency of GaN wire photocathode as a function of incident photon energy are achieved. Results show that the quantum efficiency of the wire photocathode is largely enhanced compared with the conventional planar photocathode. The superiority of the wire photocathode is reflected in its structure with surrounding surfaces. The quantum efficiency of the wire photocathode largely depends on the wire width, surface reflectivity, surface escape probability and incident angle of light. The back interface recombination rate, however, has little influences on the quantum efficiency of the wire photocathode. The simulation results suggest that the optimal width for photoemission is 150-200 nm. Besides, the quantum efficiency increases and decreases linearly with increasing surface escape probability and surface reflectivity, respectively. With increasing ratio of wire spacing to wire height, the optimal incident angle of light is reduced. These simulations are expected to guide the preparation of a better performing GaN wire photocathode.

  6. Highly c-axis oriented growth of GaN film on sapphire (0001) by laser molecular beam epitaxy using HVPE grown GaN bulk target

    SciTech Connect

    Kushvaha, S. S.; Kumar, M. Senthil; Maurya, K. K.; Dalai, M. K.; Sharma, Nita D.

    2013-09-15

    Growth temperature dependant surface morphology and crystalline properties of the epitaxial GaN layers grown on pre-nitridated sapphire (0001) substrates by laser molecular beam epitaxy (LMBE) were investigated in the range of 500–750 °C. The grown GaN films were characterized using high resolution x-ray diffraction, atomic force microscopy (AFM), micro-Raman spectroscopy, and secondary ion mass spectroscopy (SIMS). The x-ray rocking curve full width at a half maximum (FWHM) value for (0002) reflection dramatically decreased from 1582 arc sec to 153 arc sec when the growth temperature was increased from 500 °C to 600 °C and the value further decreased with increase of growth temperature up to 720 °C. A highly c-axis oriented GaN epitaxial film was obtained at 720 °C with a (0002) plane rocking curve FWHM value as low as 102 arc sec. From AFM studies, it is observed that the GaN grain size also increased with increasing growth temperature and flat, large lateral grains of size 200-300 nm was obtained for the film grown at 720 °C. The micro-Raman spectroscopy studies also exhibited the high-quality wurtzite nature of GaN film grown on sapphire at 720 °C. The SIMS measurements revealed a non-traceable amount of background oxygen impurity in the grown GaN films. The results show that the growth temperature strongly influences the surface morphology and crystalline quality of the epitaxial GaN films on sapphire grown by LMBE.

  7. Highly c-axis oriented growth of GaN film on sapphire (0001) by laser molecular beam epitaxy using HVPE grown GaN bulk target

    NASA Astrophysics Data System (ADS)

    Kushvaha, S. S.; Kumar, M. Senthil; Maurya, K. K.; Dalai, M. K.; Sharma, Nita D.

    2013-09-01

    Growth temperature dependant surface morphology and crystalline properties of the epitaxial GaN layers grown on pre-nitridated sapphire (0001) substrates by laser molecular beam epitaxy (LMBE) were investigated in the range of 500-750 °C. The grown GaN films were characterized using high resolution x-ray diffraction, atomic force microscopy (AFM), micro-Raman spectroscopy, and secondary ion mass spectroscopy (SIMS). The x-ray rocking curve full width at a half maximum (FWHM) value for (0002) reflection dramatically decreased from 1582 arc sec to 153 arc sec when the growth temperature was increased from 500 °C to 600 °C and the value further decreased with increase of growth temperature up to 720 °C. A highly c-axis oriented GaN epitaxial film was obtained at 720 °C with a (0002) plane rocking curve FWHM value as low as 102 arc sec. From AFM studies, it is observed that the GaN grain size also increased with increasing growth temperature and flat, large lateral grains of size 200-300 nm was obtained for the film grown at 720 °C. The micro-Raman spectroscopy studies also exhibited the high-quality wurtzite nature of GaN film grown on sapphire at 720 °C. The SIMS measurements revealed a non-traceable amount of background oxygen impurity in the grown GaN films. The results show that the growth temperature strongly influences the surface morphology and crystalline quality of the epitaxial GaN films on sapphire grown by LMBE.

  8. Size dependence of melting of GaN nanowires with triangular cross sections

    SciTech Connect

    Wang, Zhiguo; Zu, Xiaotao; Gao, Fei; Weber, William J.

    2007-02-15

    Molecular dynamics simulations have been used to study the melting of GaN nanowires with triangular cross-sections. The curve of the potential energy, along with the atomic configuration is used to monitor the phase transition. The thermal stability of GaN nanowires is dependent on the size of the nanowires. The melting temperature of the GaN nanowires increases with the increasing of area cross-section of the nanowires to a saturation value. An interesting result is that of the nanowires start to melt from the edges, then the surface, and extends to the inner regions of nanowires as temperature increases.

  9. Interfacial Structure and Chemistry of GaN on Ge(111)

    NASA Astrophysics Data System (ADS)

    Zhang, Siyuan; Zhang, Yucheng; Cui, Ying; Freysoldt, Christoph; Neugebauer, Jörg; Lieten, Ruben R.; Barnard, Jonathan S.; Humphreys, Colin J.

    2013-12-01

    The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5∶4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5∶4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5∶4 lattice-matching growth of GaN.

  10. Stress and Defect Control in GaN Using Low Temperature Interlayers

    SciTech Connect

    Akasaki, I.; Amano, H.; Chason, E.; Figiel, J.; Floro, J.A.; Han, J.; Hearne, S.; Iwaya, M.; Kashima, T.; Katsuragcawa, M.

    1998-12-04

    In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.

  11. Etching of GaN layers at electrolysis under UV-irradiation

    NASA Astrophysics Data System (ADS)

    Zubenko, T. K.; Puzyk, M. V.; Stozharov, V. M.; Ermakov, I. A.; Kovalev, D. S.; Ivanova, S. A.; Usikov, A. S.; Medvedev, O. S.; Papchenko, B. P.; Kurin, S. Yu; Antipov, A. A.; Chernyakov, A. E.

    2016-08-01

    Etching of the GaN layers in 1M KOH aqua solution under irradiation was studied by the electro-stimulated photolysis using N2-laser (337 nm, 60 W/m2) as a light source. It was observed that the size and the depth of the failure monotonically depend on the optical power and the irradiation time of the N2 laser and the GaN layer type of conductivity. The GaN layers etching rate was evaluated. A mechanism of the failure in the n-GaN layers is discussed.

  12. Acceleration modules in linear induction accelerators

    NASA Astrophysics Data System (ADS)

    Wang, Shao-Heng; Deng, Jian-Jun

    2014-05-01

    The Linear Induction Accelerator (LIA) is a unique type of accelerator that is capable of accelerating kilo-Ampere charged particle current to tens of MeV energy. The present development of LIA in MHz bursting mode and the successful application into a synchrotron have broadened LIA's usage scope. Although the transformer model is widely used to explain the acceleration mechanism of LIAs, it is not appropriate to consider the induction electric field as the field which accelerates charged particles for many modern LIAs. We have examined the transition of the magnetic cores' functions during the LIA acceleration modules' evolution, distinguished transformer type and transmission line type LIA acceleration modules, and re-considered several related issues based on transmission line type LIA acceleration module. This clarified understanding should help in the further development and design of LIA acceleration modules.

  13. Centralized digital control of accelerators

    SciTech Connect

    Melen, R.E.

    1983-09-01

    In contrasting the title of this paper with a second paper to be presented at this conference entitled Distributed Digital Control of Accelerators, a potential reader might be led to believe that this paper will focus on systems whose computing intelligence is centered in one or more computers in a centralized location. Instead, this paper will describe the architectural evolution of SLAC's computer based accelerator control systems with respect to the distribution of their intelligence. However, the use of the word centralized in the title is appropriate because these systems are based on the use of centralized large and computationally powerful processors that are typically supported by networks of smaller distributed processors.

  14. Genetic diversity of Histoplasma capsulatum isolated from infected bats randomly captured in Mexico, Brazil, and Argentina, using the polymorphism of (GA)(n) microsatellite and its flanking regions.

    PubMed

    Taylor, Maria Lucia; Hernández-García, Lorena; Estrada-Bárcenas, Daniel; Salas-Lizana, Rodolfo; Zancopé-Oliveira, Rosely M; García de la Cruz, Saúl; Galvão-Dias, Maria A; Curiel-Quesada, Everardo; Canteros, Cristina E; Bojórquez-Torres, Georgina; Bogard-Fuentes, Carlos A; Zamora-Tehozol, Erick

    2012-02-01

    The genetic diversity of 47 Histoplasma capsulatum isolates from infected bats captured in Mexico, Brazil, and Argentina was studied, using sequence polymorphism of a 240-nucleotides (nt) fragment, which includes the (GA)(n) length microsatellite and its flanking regions within the HSP60 gene. Three human clinical strains were used as geographic references. Based on phylogenetic analyses of 240-nt fragments achieved, the relationships among H. capsulatum isolates were resolved using neighbour-joining and maximum parsimony methods. The tree topologies obtained by both methods were identical and highlighted two major clusters of isolates. Cluster I had three sub-clusters (Ia, Ib, and Ic), all of which contained Mexican H. capsulatum samples, while cluster II consisted of samples from Brazil and Argentina. Sub-cluster Ia included only fungal isolates from the migratory bat Tadarida brasiliensis. An average DNA mutation rate of 2.39 × 10(-9) substitutions per site per year was estimated for the 240-nt fragment for all H. capsulatum isolates. Nucleotide diversity analysis of the (GA)(n) and flanking regions from fungal isolates of each cluster and sub-cluster underscored the high similarity of cluster II (Brazil and Argentina), sub-clusters Ib, and Ic (Mexico). According to the genetic distances among isolates, a network of the 240-nt fragment was graphically represented by (GA)(n) length haplotype. This network showed an association between genetic variation and both the geographic distribution and the ecotype dispersion of H. capsulatum, which are related to the migratory behaviour of the infected bats studied.

  15. Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs

    PubMed Central

    Shih, Huan-Yu; Shiojiri, Makoto; Chen, Ching-Hsiang; Yu, Sheng-Fu; Ko, Chung-Ting; Yang, Jer-Ren; Lin, Ray-Ming; Chen, Miin-Jang

    2015-01-01

    High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors. PMID:26329829

  16. Progress on plasma accelerators

    SciTech Connect

    Chen, P.

    1986-05-01

    Several plasma accelerator concepts are reviewed, with emphasis on the Plasma Beat Wave Accelerator (PBWA) and the Plasma Wake Field Accelerator (PWFA). Various accelerator physics issues regarding these schemes are discussed, and numerical examples on laboratory scale experiments are given. The efficiency of plasma accelerators is then revealed with suggestions on improvements. Sources that cause emittance growth are discussed briefly.

  17. High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

    SciTech Connect

    Qi, Meng; Zhao, Yuning; Yan, Xiaodong; Li, Guowang; Verma, Jai; Fay, Patrick; Nomoto, Kazuki; Zhu, Mingda; Hu, Zongyang; Protasenko, Vladimir; Song, Bo; Xing, Huili Grace; Jena, Debdeep; Bader, Samuel

    2015-12-07

    Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm{sup 2} is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm{sup 2} is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

  18. Controlled morphology of regular GaN microrod arrays by selective area growth with HVPE

    NASA Astrophysics Data System (ADS)

    Lekhal, Kaddour; Bae, Si-Young; Lee, Ho-Jun; Mitsunari, Tadashi; Tamura, Akira; Deki, Manato; Honda, Yoshio; Amano, Hiroshi

    2016-08-01

    The selective area growth (SAG) of GaN was implemented on patterned GaN/sapphire templates by hydride vapor phase epitaxy (HVPE) to fabricate regular arrays of Ga-polar GaN microrods. The control of growth parameters such as H2/N2 carrier gas ratio, growth temperature, and absolute NH3/HCl gas flow resulted in changes in the growth morphology. In particular, for an optimized mixed-carrier gas ratio of H2 to N2, we achieved vertically well-aligned microrods. The topmost regions of the GaN microrods were terminated with pyramidal facets, indicating typical Ga polarity. The optical properties of the grown microrods were characterized by cathodoluminescence (CL) at a low temperature. This revealed that the GaN microrods had high crystal quality since they exhibited suppressed yellow luminescence as well as strong band edge emission.

  19. Thermal Conductivity and Large Isotope Effect in GaN from First Principles

    SciTech Connect

    Lindsay, L.; Broido, D. A.; Reinecke, T. L.

    2012-08-28

    We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb. In GaN we find a large increase to the thermal conductivity with isotopic enrichment, ~65% at room temperature. We show that both the high thermal conductivity and its enhancement with isotopic enrichment in GaN arise from the weak coupling of heat-carrying acoustic phonons with optic phonons. This weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap between acoustic and optic phonons compared to other materials. Rigorous understanding of these features in GaN gives important insights into the interplay between intrinsic phonon-phonon scattering and isotopic scattering in a range of materials.

  20. Depth dependence of defect density and stress in GaN grown on SiC

    SciTech Connect

    Faleev, N.; Temkin, H.; Ahmad, I.; Holtz, M.; Melnik, Yu.

    2005-12-15

    We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29 to 30 {mu}m. High level of residual elastic strain was found in thin (0.29 to 0.73 {mu}m thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2x10{sup 7} cm{sup -2}, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

  1. Radiative defects in GaN nanocolumns: Correlation with growth conditions and sample morphology

    SciTech Connect

    Lefebvre, P.; Fernandez-Garrido, S.; Grandal, J.; Ristic, J.; Sanchez-Garcia, M.-A.; Calleja, E.

    2011-02-21

    Low-temperature photoluminescence is studied in detail in GaN nanocolumns (NCs) grown by plasma-assisted molecular beam epitaxy under various conditions (substrate temperature and impinging Ga/N flux ratio). The relative intensities of the different emission lines, in particular those related to structural defects, appear to be correlated with the growth conditions, and clearly linked to the NC sample morphology. We demonstrate, in particular, that all lines comprised between 3.10 and 3.42 eV rapidly lose intensity when the growth conditions are such that the NC coalescence is reduced. The well-known line around 3.45 eV, characteristic of GaN NC samples, shows, however, a behavior that is exactly the opposite of the other lines, namely, for growth conditions leading to reduced NC coalescence, this line tends to become more prominent, thus proving to be intrinsic to individual GaN NCs.

  2. Formation of helical dislocations in ammonothermal GaN substrate by heat treatment

    NASA Astrophysics Data System (ADS)

    Horibuchi, Kayo; Yamaguchi, Satoshi; Kimoto, Yasuji; Nishikawa, Koichi; Kachi, Tetsu

    2016-03-01

    GaN substrate produced by the basic ammonothermal method and an epitaxial layer on the substrate was evaluated using synchrotron radiation x-ray topography and transmission electron microscopy. We revealed that the threading dislocations present in the GaN substrate are deformed into helical dislocations and the generation of the voids by heat treatment in the substrate for the first observation in the GaN crystal. These phenomena are formed by the interactions between the dislocations and vacancies. The helical dislocation was formed in the substrate region, and not in the epitaxial layer region. Furthermore, the evaluation of the influence of the dislocations on the leakage current of Schottky barrier diodes fabricated on the epitaxial layer is discussed. The dislocations did not affect the leakage current characteristics of the epitaxial layer. Our results suggest that the deformation of dislocations in the GaN substrate does not adversely affect the epitaxial layer.

  3. Surface potential barrier in m-plane GaN studied by contactless electroreflectance

    NASA Astrophysics Data System (ADS)

    Janicki, Lukasz; Misiewicz, Jan; Cywiński, Grzegorz; Sawicka, Marta; Skierbiszewski, Czeslaw; Kudrawiec, Robert

    2016-02-01

    Contactless electroreflectance (CER) is used to study the surface potential barrier in m-plane GaN UN+ [GaN (d = 20,30,50,70 nm)/GaN:Si] structures grown by using molecular beam epitaxy. Clear bandgap-related transitions followed by Franz-Keldysh oscillations (FKO) have been observed in the CER spectra of all samples at room temperature. The built-in electric fields in the undoped cap layers have been determined from the FKO period. From the built-in electric field and the undoped GaN layer thickness, the Fermi level location at the air-exposed m-plane GaN surface has been estimated as 0.42 ± 0.05 eV below the conduction band.

  4. Thermal conductivity and large isotope effect in GaN from first principles.

    PubMed

    Lindsay, L; Broido, D A; Reinecke, T L

    2012-08-31

    We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb. In GaN we find a large increase to the thermal conductivity with isotopic enrichment, ~65% at room temperature. We show that both the high thermal conductivity and its enhancement with isotopic enrichment in GaN arise from the weak coupling of heat-carrying acoustic phonons with optic phonons. This weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap between acoustic and optic phonons compared to other materials. Rigorous understanding of these features in GaN gives important insights into the interplay between intrinsic phonon-phonon scattering and isotopic scattering in a range of materials.

  5. Boron doped GaN and InN: Potential candidates for spintronics

    NASA Astrophysics Data System (ADS)

    Fan, S. W.; Huang, X. N.; Yao, K. L.

    2017-02-01

    The full potential linearized augmented plane wave method together with the Tran-Blaha modified Becke-Johnson potential is utilized to investigate the electronic structures and magnetism for boron doped GaN and InN. Calculations show the boron substituting nitrogen (BN defects) could induce the GaN and InN to be half-metallic ferromagnets. The magnetic moments mainly come from the BN defects, and each BN defect would produce the 2.00 μB total magnetic moment. The electronic structures indicate the carriers-mediated double exchange interaction plays a crucial role in forming the ferromagnetism. Positive chemical pair interactions imply the BN defects would form the homogeneous distribution in GaN and InN matrix. Moderate formation energies suggest that GaN and InN with BN defects could be fabricated experimentally.

  6. Above room-temperature ferromagnetism of Mn delta-doped GaN nanorods

    SciTech Connect

    Lin, Y. T.; Wadekar, P. V.; Kao, H. S.; Chen, T. H.; Chen, Q. Y.; Tu, L. W.; Huang, H. C.; Ho, N. J.

    2014-02-10

    One-dimensional nitride based diluted magnetic semiconductors were grown by plasma-assisted molecular beam epitaxy. Delta-doping technique was adopted to dope GaN nanorods with Mn. The structural and magnetic properties were investigated. The GaMnN nanorods with a single crystalline structure and with Ga sites substituted by Mn atoms were verified by high-resolution x-ray diffraction and Raman scattering, respectively. Secondary phases were not observed by high-resolution x-ray diffraction and high-resolution transmission electron microscopy. In addition, the magnetic hysteresis curves show that the Mn delta-doped GaN nanorods are ferromagnetic above room temperature. The magnetization with magnetic field perpendicular to GaN c-axis saturates easier than the one with field parallel to GaN c-axis.

  7. High-resistance GaN epilayers with low dislocation density via growth mode modification

    NASA Astrophysics Data System (ADS)

    Xu, Z. Y.; Xu, F. J.; Wang, J. M.; Lu, L.; Yang, Z. J.; Wang, X. Q.; Shen, B.

    2016-09-01

    High-resistance GaN with low dislocation density adopting growth mode modification has been investigated by metalorganic chemical vapor deposition. The sheet resistance of the order of 1016 Ω/sq has been achieved at room temperature by diminishing the oxygen impurity level close to the substrate with an AlN blocking layer. Attributed to this method which offers more freedom to tailor the growth mode, a three-dimensional (3D) growth process is introduced by adjusting the growth pressure and temperature at the initial stage of the GaN epitaxy to improve the crystalline quality. The large 3D GaN grains formed during this period roughen the surface, and the following coalescence of the GaN grains causes threading dislocations bending, which finally remarkably reduces the dislocation density.

  8. Modeling and Simulation of a Gallium Nitride (GaN) Betavoltaic Energy Converter

    DTIC Science & Technology

    2016-06-01

    ARL-TR-7675 ● JUNE 2016 US Army Research Laboratory Modeling and Simulation of a Gallium Nitride (GaN) Betavoltaic Energy ...Laboratory Modeling and Simulation of a Gallium Nitride (GaN) Betavoltaic Energy Converter by Marc S Litz and Johnny A Russo Sensors and Electron...GaN) Betavoltaic Energy Converter 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) William B Ray II, Marc S

  9. Linearity and Efficiency Performance of GaN HEMTs with Digital Pre-Distortion Correction (Preprint)

    DTIC Science & Technology

    2006-07-01

    AFRL-ML-WP-TP-2007-414 LINEARITY AND EFFICIENCY PERFORMANCE OF GaN HEMTs WITH DIGITAL PRE-DISTORTION CORRECTION (PREPRINT) M.J. Poulton, W.K...EFFICIENCY PERFORMANCE OF GaN HEMTs WITH DIGITAL PRE-DISTORTION CORRECTION (PREPRINT) 5c. PROGRAM ELEMENT NUMBER 62102F 5d. PROJECT NUMBER 4348...device performance using Digital Pre-Distortion (DPD) correction. Additionally, both drain voltage and current were optimized to provide high

  10. 10kW TWT Transition to GaN IRE

    DTIC Science & Technology

    2015-03-31

    replacement for the TWT. Phase I covered by this IRE, was to do market research on commercially available GaN transistors as a substitute for traveling wave...Phase I covered by this IRE, was to do market research on commercially available GaN transistors as a substitute for traveling wave tubes in high... Market Research Findings ........................................................................................ 8 3.1.2. Design Approach

  11. Strain dependent electron spin dynamics in bulk cubic GaN

    SciTech Connect

    Schaefer, A.; Buß, J. H.; Hägele, D.; Rudolph, J.; Schupp, T.; Zado, A.; As, D. J.

    2015-03-07

    The electron spin dynamics under variable uniaxial strain is investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. Spin relaxation is found to be approximately independent of the applied strain, in complete agreement with estimates for Dyakonov-Perel spin relaxation. Our findings clearly exclude strain-induced relaxation as an effective mechanism for spin relaxation in cubic GaN.

  12. GaN Stress Evolution During Metal-Organic Chemical Vapor Deposition

    SciTech Connect

    Amano, H.; Chason, E.; Figiel, J.; Floro, J.A.; Han, J.; Hearne, S.; Hunter, J.; Tsong, I.

    1998-10-14

    The evolution of stress in gallium nitride films on sapphire has been measured in real- time during metal organic chemical vapor deposition. In spite of the 161%0 compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050"C. Furthermore, in-situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling.

  13. Mass transport, faceting and behavior of dislocations in GaN

    SciTech Connect

    Nitta, S.; Kashima, T.; Kariya, M.; Yukawa, Y.; Yamaguchi, S.; Amano, H.; Akasaki, I.

    2000-07-01

    The behavior of threading dislocations during mass transport of GaN was investigated in detail by transmission electron microscopy. Mass transport occurred at the surface. Therefore, growing species are supplied from the in-plane direction. The behavior of threading dislocations was found to be strongly affected by the mass transport process as well as the high crystallographic anisotropy of the surface energy of the facets particular to GaN.

  14. Gadolinium-Based GaN for Neutron Detection with Gamma Discrimination

    DTIC Science & Technology

    2016-06-01

    diodes since the proposed neutron convertor is Gd, which emits fast electrons upon neutron capture. Diode N7 appeared to show a good response to a 14C...research. Workable GaN Schottky diode radiation detectors were successfully fabricated and tested. The first year has, however, seen a significant effort...Epitaxy (HVPE) grown GaN substrate (Kyma technology [3]) was employed and the sandwich structure Schottky diode was fabricated, on which both alpha and

  15. Evolution of deep centers in GaN grown by hydride vapor phaseepitaxy

    SciTech Connect

    Fang, Z.-Q.; Look, D.C.; Jasinski, J.; Benamara, M.; Liliental-Weber, Z.; Molnar, R.J.

    2001-04-18

    Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.

  16. Determination of the nitrogen vacancy as a shallow compensating center in GaN doped with divalent metals.

    PubMed

    Buckeridge, J; Catlow, C R A; Scanlon, D O; Keal, T W; Sherwood, P; Miskufova, M; Walsh, A; Woodley, S M; Sokol, A A

    2015-01-09

    We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p-type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.

  17. Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au /GaN Schottky contacts

    NASA Astrophysics Data System (ADS)

    Wang, R. X.; Xu, S. J.; Shi, S. L.; Beling, C. D.; Fung, S.; Zhao, D. G.; Yang, H.; Tao, X. M.

    2006-10-01

    Under identical preparation conditions, Au /GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers.

  18. Carbon nanotube assisted Lift off of GaN layers on sapphire

    NASA Astrophysics Data System (ADS)

    Long, Hao; Feng, Xiaohui; Wei, Yang; Yu, Tongjun; Fan, Shoushan; Ying, Leiying; Zhang, Baoping

    2017-02-01

    Laser lift off (LLO) was one of the most essential processes in fabrication of vertical GaN-based LEDs. However, traditional laser lift off of GaN on sapphire substrates needed high laser energy threshold, which deteriorated the GaN crystal. In this paper, it was found that inserting carbon nanotube between GaN and sapphire could effectively reduce the laser energy threshold in GaN LLO, from 1.5 J /cm2 of conventional GaN/sapphire to 1.3 J /cm2 of CNT inserted GaN/sapphire. The temperature distributions at the GaN/sapphire interfaces with and without CNTs were simulated by the finite elements calculation under laser irradiation. It was found that, due to the higher laser absorption coefficient of CNT, the CNT played as a powerful heating wire, sending out the thermal outside to elevate the GaN's temperature, and thus reduce the laser threshold for LLO. Raman and photoluminescence measurements indicated that residual stress of GaN membranes was as small as 0.3 GPa by the carbon nanotube assisted LLO. This work not only opens new application of CNTs, but also demonstrates the potential of high performance blue and green LEDs.

  19. Hybrid device based on GaN nanoneedles and MEH-PPV/PEDOT:PSS polymer

    SciTech Connect

    Shin, Min Jeong; Gwon, Dong-Oh; Lee, Chan-Mi; Lee, Gang Seok; Jeon, In-Jun; Ahn, Hyung Soo; Yi, Sam Nyung; Ha, Dong Han

    2015-08-15

    Highlights: • A hybrid device was demonstrated by using MEH-PPV, PEDOT:PSS, and GaN nanoneedles. • I–V curve of the hybrid device showed its rectification behaviour, similar to a diode. • EL peak originated by the different potential barriers at MEH-PPV and GaN interface. - Abstract: A hybrid device that combines the properties of organic and inorganic semiconductors was fabricated and studied. It incorporated poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as organic polymers and GaN nanoneedles as an inorganic semiconductor. Layers of the two polymers were spin coated on to the GaN nanoneedles. The one peak in the electroluminescence spectrum originated from the MEH-PPV layer owing to the different potential barriers of electrons and holes at its interface with the GaN nanoneedles. However, the photoluminescence spectrum showed peaks due to both GaN nanoneedles and MEH-PPV. Such hybrid structures, suitably developed, might be able to improve the efficiency of optoelectronic devices.

  20. Correlation between macroscopic transport parameters and microscopic electrical properties in GaN

    NASA Astrophysics Data System (ADS)

    Witte, H.; Krtschil, A.; Schrenk, E.; Fluegge, K.; Dadgar, A.; Krost, A.

    2005-02-01

    In GaN layers grown by metal-organic vapor phase epitaxy on sapphire substrates the temperature-dependent Hall (TDH) and photo-Hall-effect (PHE) measurements show essential differences between undoped and Si-doped GaN. In undoped GaN the maximum of the Hall mobility occurs at temperatures near 300K with a low value. In PHE, an illumination introduces an enhancement of the mobility and a decrease of the electron density. In contrast, in Si-doped GaN the maximum Hall mobility is higher by a factor of 10 and is observed at temperatures between 100 and 180K. The photoinduced changes in the mobility and electron density are only marginal. Intensity dependent PHE measurements suggest the existence of internal potential barriers caused by inhomogeneities in the undoped samples. These results are combined with the surface-potential roughness on a microscale, as determined by scanning surface-potential microscopy (SSPM). In SSPM the undoped layers show strong potential fluctuations while they are lower for the Si-doped GaN samples. A correlation among the rms roughness of the surface potential, the maximum Hall mobility in TDH, and the maximum changes of the photo-Hall mobility is observed. In undoped GaN the mobility seems to be determined by the scattering at inner potential barriers stemming from structural inhomogeneities.

  1. Strain-induced step bunching in orientation-controlled GaN on Si

    NASA Astrophysics Data System (ADS)

    Narita, Tetsuo; Iguchi, Hiroko; Horibuchi, Kayo; Otake, Nobuyuki; Hoshi, Shinichi; Tomita, Kazuyoshi

    2016-05-01

    We report a technique for the fabrication of high-quality GaN-on-silicon (Si) substrates for use in various power applications. GaN epitaxial layers were generated on Si(111) vicinal faces that had been previously covered with a thin coating of Al2O3 to control the orientation of the AlN seed layers. We obtained orientation-controlled GaN layers and found a linear relationship between the GaN c-axis and Si[111] tilt angles. As a result, the threading dislocation density in the AlN seed layer was reduced and high-quality GaN layers were generated. The X-ray rocking curves for these layers exhibited full width at half maximum values of 390‧‧ and 550‧‧ for the (004) and (114) reflections, respectively. Significant step bunching was observed on a GaN(0001) vicinal face produced using this technique, attributed to strain-induced attractive interactions between steps. Thus, by controlling the strain near the surface layer, we achieved the step flow growth of GaN on Si.

  2. Size dictated thermal conductivity of GaN

    DOE PAGES

    Thomas Edwin Beechem; McDonald, Anthony E.; Fuller, Elliot James; ...

    2016-04-01

    The thermal conductivity on n- and p-type doped gallium nitride (GaN) epilayers having thickness of 3-4 μm was investigated using time domain thermoreflectance (TDTR). Despite possessing carrier concentrations ranging across 3 decades (1015 – 1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends–and their overall reduction relative to bulk–are explained leveraging established scattering models where it is shown that size effects play a primary role in limiting thermal conductivity for layers even tens of microns thick. GaNmore » device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.« less

  3. Ge doping of GaN beyond the Mott transition

    NASA Astrophysics Data System (ADS)

    Ajay, A.; Schörmann, J.; Jiménez-Rodriguez, M.; Lim, C. B.; Walther, F.; Rohnke, M.; Mouton, I.; Amichi, L.; Bougerol, C.; Den Hertog, M. I.; Eickhoff, M.; Monroy, E.

    2016-11-01

    We present a study of germanium as n-type dopant in wurtzite GaN films grown by plasma-assisted molecular-beam epitaxy, reaching carrier concentrations of up to 6.7  ×  1020 cm-3 at 300 K, well beyond the Mott density. The Ge concentration and free carrier density were found to scale linearly with the Ge flux in the studied range. All the GaN:Ge layers present smooth surface morphology with atomic terraces, without trace of pits or cracks, and the mosaicity of the samples has no noticeable dependence on the Ge concentration. The variation of the GaN:Ge band gap with the carrier concentration is consistent with theoretical calculations of the band gap renormalization due to electron-electron and electron-ion interaction, and Burstein-Moss effect.

  4. Influence of dopants on defect formation in GaN

    SciTech Connect

    Liliental-Weber, Z.; Jasinski, J.; Benamara, M.; Grzegory, I.; Porowski, S.; Lampert, D.J.H.; Eiting, C.J.; Dupuis R.D.

    2001-10-15

    Influence of p-dopants (Mg and Be) on the structure of GaN has been studied using Transmission Electron Microscopy (TEM). Bulk GaN:Mg and GaN:Be crystals grown by a high pressure and high temperature process and GaN:Mg grown by metal-organic chemical-vapor deposition (MOCVD) have been studied. Structural dependence on growth polarity was observed in the bulk crystals. Spontaneous ordering in bulk GaN:Mg on c-plane (formation of Mg-rich planar defects with characteristics of inversion domains) was observed for growth in the N to Ga polar direction (N polarity). On the opposite site of the crystal (growth in the Ga to N polar direction) Mg-rich pyramidal defects empty inside (pinholes) were observed. Both these defects were also observed in MOCVD grown crystals. Pyramidal defects were also observed in the bulk GaN:Be crystals.

  5. Electron spin dynamics in cubic GaN

    NASA Astrophysics Data System (ADS)

    Buß, J. H.; Schupp, T.; As, D. J.; Brandt, O.; Hägele, D.; Rudolph, J.

    2016-12-01

    The electron spin dynamics in cubic GaN is comprehensively investigated by time-resolved magneto-optical Kerr-rotation spectroscopy over a wide range of temperatures, magnetic fields, and doping densities. The spin dynamics is found to be governed by the interplay of spin relaxation of localized electrons and Dyakonov-Perel relaxation of delocalized electrons. Localized electrons significantly contribute to spin relaxation up to room temperature at moderate doping levels, while Dyakonov-Perel relaxation dominates for high temperatures or degenerate doping levels. Quantitative agreement to Dyakonov-Perel theory requires a larger value of the spin-splitting constant than theoretically predicted. Possible reasons for this discrepancy are discussed, including the role of charged dislocations.

  6. Dislocation generation in GaN by dicing process

    NASA Astrophysics Data System (ADS)

    Taguchi, Hideyuki; Kitahara, Amane; Miyake, Syugo; Nakaue, Akimitu; Nishikawa, Atsushi; Fujiwara, Yasufumi

    2013-03-01

    In order to analyze effect of the dicing process on the GaN epitaxial layer, the GaN-wafer is cut in sizes of the 0.7 mm square and the 1.7 mm square. The crystal characteristics of the GaN-chips have been measured using X-ray measurements and Raman spectra measurements. The full-width half maximum (FWHM) values of the X-ray rocking curves of (0002), (10-13) and (10-12) of the 0.7 mm square GaN-chip become wider than that of before the dicing process. The E2 (high) peak of Raman spectra at the edge in the 0.7mm square GaN-chip is shifted to lower wave number. In consideration of crystallography, we infer from these results that both the crystal strains and the screw dislocations have been generated during the dicing process.

  7. Intrinsic polarization control in rectangular GaN nanowire lasers

    DOE PAGES

    Li, Changyi; Liu, Sheng; Luk, Ting S.; ...

    2016-02-01

    In this study, we demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444kW/cm2 and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control overmore » the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.« less

  8. Intrinsic polarization control in rectangular GaN nanowire lasers

    SciTech Connect

    Li, Changyi; Liu, Sheng; Luk, Ting S.; Figiel, Jeffrey J.; Brener, Igal; Brueck, S. R. J.; Wang, George T.

    2016-02-01

    In this study, we demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444kW/cm2 and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.

  9. Spontaneous emission enhancement in micropatterned GaN

    NASA Astrophysics Data System (ADS)

    Niehus, M.; Sanguino, P.; Monteiro, T.; Soares, M. J.; Schwarz, R.

    2004-10-01

    With two interfering pulses from the fourth harmonic of a Nd-YAG laser we burnt a periodic lattice structure into the surface of GaN thin films. The lattice period of this permanent grating could be controlled between less than one and several tens of microns. Above the decomposition threshold, nitrogen evades from the sample surface, and the residual metallic gallium accumulates in the form of tiny droplets at the surfaces. The patterned structure shows structural similarities with microcavities. The question arises if the residual metallic gallium may act as a partially reflecting mirror. To test this hypothesis, we studied the steady-state and transient photoluminescence through the modulation of light emerging from the ubiquitous broad "yellow" photoluminescence band. The microlattice is evidenced by energy-equidistant spontaneous emission enhancement peaks in the steady-state photoluminescence spectra. We suggest that the partial reflection due to the residual metallic gallium leads to the observed enhancement effect.

  10. Microstructure of laterally overgrown GaN layers

    SciTech Connect

    Liliental-Weber, Zuzanna; Cherns, David

    2001-04-03

    Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.

  11. GaN Based Electronics And Their Applications

    NASA Astrophysics Data System (ADS)

    Ren, Fan

    2002-03-01

    The Group III-nitrides were initially researched for their promise to fill the void for a blue solid state light emitter. Electronic devices from III-nitrides have been a more recent phenomenon. The thermal conductivity of GaN is three times that of GaAs. For high power or high temperature applications, good thermal conductivity is imperative for heat removal or sustained operation at elevated temperatures. The development of III-N and other wide bandgap technologies for high temperature applications will likely take place at the expense of competing technologies, such as silicon-on-insulator (SOI), at moderate temperatures. At higher temperatures (>300°C), novel devices and components will become possible. The automotive industry will likely be one of the largest markets for such high temperature electronics. One of the most noteworthy advantages for III-N materials over other wide bandgap semiconductors is the availability of AlGaN/GaN and InGaN/GaN heterostructures. A 2-dimensional electron gas (2DEG) has been shown to exist at the AlGaN/GaN interface, and heterostructure field effect transistors (HFETs) from these materials can exhibit 2DEG mobilities approaching 2000 cm2 / V?s at 300K. Power handling capabilities of 12 W/mm appear feasible, and extraordinary large signal performance has already been demonstrated, with a current state-of-the-art of >10W/mm at X-band. In this talk, high speed and high temperature AlGaN/GaN HEMTs as well as MOSHEMTs, high breakdown voltage GaN (>6KV) and AlGaN (9.7 KV) Schottky diodes, and their applications will be presented.

  12. Zn-dopant dependent defect evolution in GaN nanowires

    NASA Astrophysics Data System (ADS)

    Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

    2015-10-01

    Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101&cmb.macr;3), (101&cmb.macr;1) and (202&cmb.macr;1), as well as Type I stacking faults (...ABABC&cmb.b.line;BCB...), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABAC&cmb.b.line;BA...) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a

  13. High electron mobility GaN grown under N-rich conditions by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Koblmueller, G.; Wu, F.; Mates, T.; Speck, J. S.; Fernandez-Garrido, S.; Calleja, E.

    2007-11-26

    An alternative approach is presented for the plasma-assisted molecular beam epitaxy of high-quality GaN. Under N-rich growth conditions, an unexpected layer-by-layer growth mode was found for a wide range of growth temperatures in the GaN thermal decomposition regime (>750 deg. C). Consequently, superior surface morphologies with roughness of less than 1 nm (rms) have been achieved. For lightly Si-doped GaN films, room-temperature electron mobilities exceeding 1100 cm{sup 2}/V s were measured, surpassing the commonly insulating nature of GaN grown under N-rich conditions at low temperature.

  14. A growth diagram for plasma-assisted molecular beam epitaxy of GaN nanocolumns on Si(111)

    SciTech Connect

    Fernandez-Garrido, S.; Grandal, J.; Calleja, E.; Sanchez-Garcia, M. A.; Lopez-Romero, D.

    2009-12-15

    The morphology of GaN samples grown by plasma-assisted molecular beam epitaxy on Si(111) was systematically studied as a function of impinging Ga/N flux ratio and growth temperature (730-850 deg. C). Two different growth regimes were identified: compact and nanocolumnar. A growth diagram was established as a function of growth parameters, exhibiting the transition between growth regimes, and showing under which growth conditions GaN cannot be grown due to thermal decomposition and Ga desorption. Present results indicate that adatoms diffusion length and the actual Ga/N ratio on the growing surface are key factors to achieve nanocolumnar growth.

  15. High-resistivity GaN buffer templates and their optimization for GaN-based HFETs

    NASA Astrophysics Data System (ADS)

    Hubbard, S. M.; Zhao, G.; Pavlidis, D.; Sutton, W.; Cho, E.

    2005-11-01

    High-resistance (HR) GaN templates for AlGaN/GaN heterojunction field effect transistor (HFET) applications were grown using organometallic vapor phase epitaxy. The GaN sheet resistance was tuned using final nucleation layer (NL) annealing temperature and NL thickness. Using an annealing temperature of 1033 °C and NL thickness of 26 nm, GaN with sheet resistance of 10 10 Ω/sq was achieved, comparable to that of Fe-doped GaN. Material characterization results show that the high-resistance GaN is achieved due to compensating acceptor levels that may be introduced through edge-type threading dislocations. Optimization of annealing temperature and NL thickness provided a means to maximize GaN sheet resistance without significantly degrading material quality. In situ laser reflectance was used to correlate the NL properties to sheet resistance and material quality, providing a figure of merit for expected sheet resistance. AlGaN/GaN HFET layers grown using HR GaN templates with R of 10 10 Ω/sq gave surface and interface roughness of 14 and 7 Å, respectively. The 2DEG Hall mobility and sheet charge of HFETs grown using HR GaN templates was comparable to similar layers grown using unintentionally doped (UID) GaN templates.

  16. Crystallographic Wet Chemical Etching of Semipolar GaN (11-22) Grown on m-Plane Sapphire Substrates.

    PubMed

    Kim, Jae-Kwan; Lee, Sung Nam; Song, Keun-Man; Yoon, Jae-Sik; Lee, Ji-Myon

    2015-07-01

    This paper reports the etch rates and etched surface morphology of semipolar GaN using a potassium hydroxide (KOH) solution. Semipolar (11-22) GaN could be etched easily using a KOH solution and the etch rate was higher than that of Ga-polar c-plane GaN (0001). The etch rate was anisotropic and the highest etch rate was measured to be approximately 116 nm/min for the (1011) plane and 62 nm/min for the (11-20) plane GaN using a 4 M KOH solution at 100 °C, resulting in specific surface features, such as inclined trigonal cells.

  17. Growth and characterization of GaN thin films on Si(111) substrates using SiC intermediate layer

    SciTech Connect

    Lim, K.Y.; Lee, K.J.; Park, C.I.; Kim, K.C.; Choi, S.C.; Lee, W.H.; Suh, E.K.; Yang, G.M.; Nahm, K.S.

    2000-07-01

    GaN films have been grown atop Si-terminated 3C-SiC intermediate layer on Si(111) substrates using low pressure metalorganic chemical vapor deposition (LP-MOCVD). The SiC intermediate layer was grown by chemical vapor deposition (CVD) using tetramethylsilane (TMS) as the single source precursor. The Si terminated SiC surface was obtained by immediately flow of SiH{sub 4} gas after growth of SiC film. LP-MOCVD growth of GaN on 3C-SiC/Si(111) was carried out with trimethylgallium (TMG) and NH{sub 3}. Single crystalline hexagonal GaN layers can be grown on Si terminated SiC intermediate layer using an AlN or GaN buffer layer. Compared with GaN layers grown using a GaN buffer layer, the crystal qualities of GaN films with AlN buffer layers are extremely improved. The GaN films were characterized by x-ray diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). Full width at half maximum (FWHM) of double crystal x-ray diffraction (DCXD) rocking curve for GaN (0002) on 3C-SiC/Si(111) was 890 arcsec. PL near band edge emission peak position and FWHM at room temperature are 3.38 eV and 79.35 meV, respectively.

  18. High active nitrogen flux growth of GaN by plasma assisted molecular beam epitaxy

    SciTech Connect

    McSkimming, Brian M. Speck, James S.; Chaix, Catherine

    2015-09-15

    In the present study, the authors report on a modified Riber radio frequency (RF) nitrogen plasma source that provides active nitrogen fluxes more than 30 times higher than those commonly used for plasma assisted molecular beam epitaxy (PAMBE) growth of gallium nitride (GaN) and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N{sub 2} gas flow rates between 5 and 25 sccm while varying the plasma source's RF forward power from 200 to 600 W. The highest growth rate, and therefore the highest active nitrogen flux, achieved was ∼7.6 μm/h. For optimized growth conditions, the surfaces displayed a clear step-terrace structure with an average RMS roughness (3 × 3 μm) on the order of 1 nm. Secondary ion mass spectroscopy impurity analysis demonstrates oxygen and hydrogen incorporation of 1 × 10{sup 16} and ∼5 × 10{sup 17}, respectively. In addition, the authors have achieved PAMBE growth of GaN at a substrate temperature more than 150 °C greater than our standard Ga rich GaN growth regime and ∼100 °C greater than any previously reported PAMBE growth of GaN. This growth temperature corresponds to GaN decomposition in vacuum of more than 20 nm/min; a regime previously unattainable with conventional nitrogen plasma sources. Arrhenius analysis of the decomposition rate shows that samples with a flux ratio below stoichiometry have an activation energy greater than decomposition of GaN in vacuum while samples grown at or above stoichiometry have decreased activation energy. The activation energy of decomposition for GaN in vacuum was previously determined to be ∼3.1 eV. For a Ga/N flux ratio of ∼1.5, this activation energy was found to be ∼2.8 eV, while for a Ga/N flux ratio of ∼0.5, it was found to be ∼7.9 eV.

  19. Future accelerator technology

    SciTech Connect

    Sessler, A.M.

    1986-05-01

    A general discussion is presented of the acceleration of particles. Upon this foundation is built a categorization scheme into which all accelerators can be placed. Special attention is devoted to accelerators which employ a wake-field mechanism and a restricting theorem is examined. It is shown how the theorem may be circumvented. Comments are made on various acceleration schemes.

  20. ACCELERATION AND THE GIFTED.

    ERIC Educational Resources Information Center

    GIBSON, ARTHUR R.; STEPHANS, THOMAS M.

    ACCELERATION OF PUPILS AND SUBJECTS IS CONSIDERED A MEANS OF EDUCATING THE ACADEMICALLY GIFTED STUDENT. FIVE INTRODUCTORY ARTICLES PROVIDE A FRAMEWORK FOR THINKING ABOUT ACCELERATION. FIVE PROJECT REPORTS OF ACCELERATED PROGRAMS IN OHIO ARE INCLUDED. ACCELERATION IS NOW BEING REGARDED MORE FAVORABLY THAN FORMERLY, BECAUSE METHODS HAVE BEEN…

  1. Laser driven ion accelerator

    DOEpatents

    Tajima, Toshiki

    2005-06-14

    A system and method of accelerating ions in an accelerator to optimize the energy produced by a light source. Several parameters may be controlled in constructing a target used in the accelerator system to adjust performance of the accelerator system. These parameters include the material, thickness, geometry and surface of the target.

  2. Laser driven ion accelerator

    DOEpatents

    Tajima, Toshiki

    2006-04-18

    A system and method of accelerating ions in an accelerator to optimize the energy produced by a light source. Several parameters may be controlled in constructing a target used in the accelerator system to adjust performance of the accelerator system. These parameters include the material, thickness, geometry and surface of the target.

  3. Piezo-generator integrating a vertical array of GaN nanowires

    NASA Astrophysics Data System (ADS)

    Jamond, N.; Chrétien, P.; Houzé, F.; Lu, L.; Largeau, L.; Maugain, O.; Travers, L.; Harmand, J. C.; Glas, F.; Lefeuvre, E.; Tchernycheva, M.; Gogneau, N.

    2016-08-01

    We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ˜12.7 mW cm-3. This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

  4. Piezo-generator integrating a vertical array of GaN nanowires.

    PubMed

    Jamond, N; Chrétien, P; Houzé, F; Lu, L; Largeau, L; Maugain, O; Travers, L; Harmand, J C; Glas, F; Lefeuvre, E; Tchernycheva, M; Gogneau, N

    2016-08-12

    We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

  5. Crystallographically tilted and partially strain relaxed GaN grown on inclined {111} facets etched on Si(100) substrate

    NASA Astrophysics Data System (ADS)

    Ansah Antwi, K. K.; Soh, C. B.; Wee, Q.; Tan, Rayson J. N.; Yang, P.; Tan, H. R.; Sun, L. F.; Shen, Z. X.; Chua, S. J.

    2013-12-01

    High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the {111} facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si{111} growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (IYL/INBE) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E2(high) optical phonon mode at 565.224 ± 0.001 cm-1 with a narrow full width at half maximum of 1.526 ± 0.002 cm-1 was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si{111} surface etched on Si(100).

  6. Crystallographically tilted and partially strain relaxed GaN grown on inclined (111) facets etched on Si(100) substrate

    SciTech Connect

    Ansah Antwi, K. K.; Soh, C. B.; Wee, Q.; Tan, Rayson J. N.; Tan, H. R.; Yang, P.; Sun, L. F.; Shen, Z. X.; Chua, S. J.

    2013-12-28

    High resolution X-ray diffractometry (HR-XRD), Photoluminescence, Raman spectroscopy, and Transmission electron microscope measurements are reported for GaN deposited on a conventional Si(111) substrate and on the (111) facets etched on a Si(100) substrate. HR-XRD reciprocal space mappings showed that the GaN(0002) plane is tilted by about 0.63° ± 0.02° away from the exposed Si(111) growth surface for GaN deposited on the patterned Si(100) substrate, while no observable tilt existed between the GaN(0002) and Si(111) planes for GaN deposited on the conventional Si(111) substrate. The ratio of integrated intensities of the yellow to near band edge (NBE) luminescence (I{sub YL}/I{sub NBE}) was determined to be about one order of magnitude lower in the case of GaN deposited on the patterned Si(100) substrate compared with GaN deposited on the conventional Si(111) substrate. The Raman E{sub 2}(high) optical phonon mode at 565.224 ± 0.001 cm{sup −1} with a narrow full width at half maximum of 1.526 ± 0.002 cm{sup −1} was measured, for GaN deposited on the patterned Si(100) indicating high material quality. GaN deposition within the trench etched on the Si(100) substrate occurred via diffusion and mass-transport limited mechanism. This resulted in a differential GaN layer thickness from the top (i.e., 1.8 μm) of the trench to the bottom (i.e., 0.3 μm) of the trench. Mixed-type dislocation constituted about 80% of the total dislocations in the GaN grown on the inclined Si(111) surface etched on Si(100)

  7. The acceleration sensitivity of quartz crystal oscillators: a review.

    PubMed

    Filler, R L

    1988-01-01

    A tutorial on navigation, radar, and identification systems is presented. The topics discussed are the consequences of acceleration sensitivity in crystal oscillators on the Allan variance, including the effects of sinusoidal and random vibration, phase noise and integrated phase jitter; the vector nature of quartz resonator sensitivity; the theoretical description of the cause of the acceleration sensitivity of quartz resonators; techniques for the measurement of acceleration sensitivity; and the effect of frequency multiplication on acceleration effect. Various techniques currently being used or developed for reducing the effective acceleration sensitivity are considered. The techniques fall into three general categories: reduction of the acceleration sensitivity of the resonator; passive techniques that use compensating elements in the oscillator feedback loop, e.g. a second resonator or an acceleration sensitivity capacitor; and active acceleration compensation schemes that sense the acceleration and feedback a compensating signal to a tuning network.

  8. GaN Initiative for Grid Applications (GIGA)

    SciTech Connect

    Turner, George

    2015-07-03

    For nearly 4 ½ years, MIT Lincoln Laboratory (MIT/LL) led a very successful, DoE-funded team effort to develop GaN-on-Si materials and devices, targeting high-voltage (>1 kV), high-power, cost-effective electronics for grid applications. This effort, called the GaN Initiative for Grid Applications (GIGA) program, was initially made up of MIT/LL, the MIT campus group of Prof. Tomas Palacios (MIT), and the industrial partner M/A Com Technology Solutions (MTS). Later in the program a 4th team member was added (IQE MA) to provide commercial-scale GaN-on-Si epitaxial materials. A basic premise of the GIGA program was that power electronics, for ubiquitous utilization -even for grid applications - should be closer in cost structure to more conventional Si-based power electronics. For a number of reasons, more established GaN-on-SiC or even SiC-based power electronics are not likely to reach theses cost structures, even in higher manufacturing volumes. An additional premise of the GIGA program was that the technical focus would be on materials and devices suitable for operating at voltages > 1 kV, even though there is also significant commercial interest in developing lower voltage (< 1 kV), cost effective GaN-on-Si devices for higher volume applications, like consumer products. Remarkable technical progress was made during the course of this program. Advances in materials included the growth of high-quality, crack-free epitaxial GaN layers on large-diameter Si substrates with thicknesses up to ~5 μm, overcoming significant challenges in lattice mismatch and thermal expansion differences between Si and GaN in the actual epitaxial growth process. Such thick epilayers are crucial for high voltage operation of lateral geometry devices such as Schottky barrier (SB) diodes and high electron mobility transistors (HEMTs). New “Normally-Off” device architectures were demonstrated – for safe operation of power electronics circuits. The trade-offs between lateral and

  9. Nearly 4-Inch-Diameter Free-Standing GaN Wafer Fabricated by Hydride Vapor Phase Epitaxy with Pit-Inducing Buffer Layer

    NASA Astrophysics Data System (ADS)

    Sato, Tadashige; Okano, Shinya; Goto, Takenari; Yao, Takafumi; Seto, Ritsu; Sato, Akira; Goto, Hideki

    2013-08-01

    A free-standing GaN wafer was fabricated by depositing a GaN buffer that induced the formation of pits (hereafter, pit-inducing GaN buffer) on a low-temperature-grown GaN buffer on the sapphire substrate. A high-temperature-grown GaN layer was grown on the pit-inducing GaN buffer that induced the formation of pits on the high-temperature-grown GaN layer. The pit-inducing buffer suppresses crack formation in the thick GaN film thereby releasing growth stress. Thermal stress in GaN on a sapphire system is also discussed on the basis of calculations utilizing a bilayer model. We have succeeded in the fabrication of a nearly 4-in.-diameter free-standing GaN thick wafer with a pit-inducing GaN buffer by one-stop hydride vapor phase epitaxy, which will lead to a low-cost fabrication of free-standing GaN wafers.

  10. Influence of stress on optical transitions in GaN nanorods containing a single InGaN/GaN quantum disk

    SciTech Connect

    Zhuang, Y. D.; Shields, P. A.; Allsopp, D. W. E.; Bruckbauer, J.; Edwards, P. R.; Martin, R. W.

    2014-11-07

    Cathodoluminescence (CL) hyperspectral imaging has been performed on GaN nanorods containing a single InGaN quantum disk (SQD) with controlled variations in excitation conditions. Two different nanorod diameters (200 and 280 nm) have been considered. Systematic changes in the CL spectra from the SQD were observed as the accelerating voltage of the electron beam and its position of incidence are varied. It is shown that the dominant optical transition in the SQD varies across the nanorod as a result of interplay between the contributions of the deformation potential and the quantum-confined Stark effect to the transition energy as consequence of radial variation in the pseudomorphic strain.

  11. GaN Microwave DC-DC Converters

    NASA Astrophysics Data System (ADS)

    Ramos Franco, Ignacio

    Increasing the operating frequency of switching converters can have a direct impact in the miniaturization and integration of power converters. The size of energy-storage passive components and the difficulty to integrate them with the rest of the circuitry is a major challenge in the development of a fully integrated power supply on a chip. The work presented in this thesis attempts to address some of the difficulties encountered in the design of high-frequency converters by applying concepts and techniques usually used in the design of high-efficiency power amplifiers and high-efficiency rectifiers at microwave frequencies. The main focus is in the analysis, design, and characterization of dc-dc converters operating at microwave frequencies in the low gigahertz range. The concept of PA-rectifier duality, where a high-efficiency power amplifier operates as a high-efficiency rectifier is investigated through non-linear simulations and experimentally validated. Additionally, the concept of a self-synchronous rectifier, where a transistor rectifier operates synchronously without the need of a RF source or driver is demonstrated. A theoretical analysis of a class-E self-synchronous rectifier is presented and validated through non-linear simulations and experiments. Two GaN class-E2 dc-dc converters operating at a switching frequency of 1 and 1.2 GHz are demonstrated. The converters achieve 80 % and 75 % dc-dc efficiency respectively and are among the highest-frequency and highest-efficiency reported in the literature. The application of the concepts established in the analysis of a self-synchronous rectifier to a power amplifier culminated in the development of an oscillating, self-synchronous class-E 2 dc-dc converter. Finally, a proof-of-concept fully integrated GaN MMIC class-E 2 dc-dc converter switching at 4.6 GHz is demonstrated for the first time to the best of our knowledge. The 3.8 mm x 2.6 mm chip contains distributed inductors and does not require any

  12. Nanostructural engineering of nitride nucleation layers for GaN substrate dislocation reduction.

    SciTech Connect

    Koleske, Daniel David; Lee, Stephen Roger; Lemp, Thomas Kerr; Coltrin, Michael Elliott; Cross, Karen Charlene; Thaler, Gerald

    2009-07-01

    With no lattice matched substrate available, sapphire continues as the substrate of choice for GaN growth, because of its reasonable cost and the extensive prior experience using it as a substrate for GaN. Surprisingly, the high dislocation density does not appear to limit UV and blue LED light intensity. However, dislocations may limit green LED light intensity and LED lifetime, especially as LEDs are pushed to higher current density for high end solid state lighting sources. To improve the performance for these higher current density LEDs, simple growth-enabled reductions in dislocation density would be highly prized. GaN nucleation layers (NLs) are not commonly thought of as an application of nano-structural engineering; yet, these layers evolve during the growth process to produce self-assembled, nanometer-scale structures. Continued growth on these nuclei ultimately leads to a fully coalesced film, and we show in this research program that their initial density is correlated to the GaN dislocation density. In this 18 month program, we developed MOCVD growth methods to reduce GaN dislocation densities on sapphire from 5 x 10{sup 8} cm{sup -2} using our standard delay recovery growth technique to 1 x 10{sup 8} cm{sup -2} using an ultra-low nucleation density technique. For this research, we firmly established a correlation between the GaN nucleation thickness, the resulting nucleation density after annealing, and dislocation density of full GaN films grown on these nucleation layers. We developed methods to reduce the nuclei density while still maintaining the ability to fully coalesce the GaN films. Ways were sought to improve the GaN nuclei orientation by improving the sapphire surface smoothness by annealing prior to the NL growth. Methods to eliminate the formation of additional nuclei once the majority of GaN nuclei were developed using a silicon nitride treatment prior to the deposition of the nucleation layer. Nucleation layer thickness was determined

  13. Interface Electronic State Characterization of Plasma Enhanced Atomic Layer Deposited Dielectrics on GaN

    NASA Astrophysics Data System (ADS)

    Yang, Jialing

    In this dissertation, the interface chemistry and electronic structure of plasma-enhanced atomic layer deposited (PEALD) dielectrics on GaN are investigated with x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). Three interrelated issues are discussed in this study: (1) PEALD dielectric growth process optimization, (2) interface electronic structure of comparative PEALD dielectrics on GaN, and (3) interface electronic structure of PEALD dielectrics on Ga- and N-face GaN. The first study involved an in-depth case study of PEALD Al2O3 growth using dimethylaluminum isopropoxide, with a special focus on oxygen plasma effects. Saturated and self-limiting growth of Al2O3 films were obtained with an enhanced growth rate within the PEALD temperature window (25--220 °C). The properties of Al2O3 deposited at various temperatures were characterized to better understand the relation between the growth parameters and film properties. In the second study, the interface electronic structures of PEALD dielectrics on Ga-face GaN films were measured. Five promising dielectrics (Al2O3, HfO2, SiO2, La2O 3, and ZnO) with a range of band gap energies were chosen. Prior to dielectric growth, a combined wet chemical and in-situ H 2/N2 plasma clean process was employed to remove the carbon contamination and prepare the surface for dielectric deposition. The surface band bending and band offsets were measured by XPS and UPS for dielectrics on GaN. The trends of the experimental band offsets on GaN were related to the dielectric band gap energies. In addition, the experimental band offsets were near the calculated values based on the charge neutrality level model. The third study focused on the effect of the polarization bound charge of the Ga- and N-face GaN on interface electronic structures. A surface pretreatment process consisting of a NH4OH wet chemical and an in-situ NH3 plasma treatment was applied to remove carbon contamination, retain monolayer oxygen coverage, and

  14. On the origin of threading dislocations in GaN films

    NASA Astrophysics Data System (ADS)

    Moram, M. A.; Ghedia, C. S.; Rao, D. V. S.; Barnard, J. S.; Zhang, Y.; Kappers, M. J.; Humphreys, C. J.

    2009-10-01

    A series of GaN films were grown by metalorganic vapor phase epitaxy on nitrided sapphire using an initial annealed low-temperature nucleation layer (LT-NL), without employing any conventional threading dislocation (TD) reduction methods. Film thicknesses ranging from the LT-NL to 500 nm were used. The island network morphology was investigated at each growth stage using atomic force microscopy. Data from cathodoluminescence studies showed initially uniform luminescence, followed by the gradual development of bright (low TD) regions which had lateral sizes different from the island sizes at all times and which continued to increase in size after coalescence. The formation of low-energy arrays of a-type TDs also continued after island coalescence. X-ray diffraction, transmission electron microscopy (TEM) and AFM data indicated that the highest (a +c)-type TD densities were found in the LT-NL, but subsequently decreased due to TD loop formation (promoted by island facets) and reaction to produce a-type TDs. a-type TD densities were also high in the LT-NL but subsequently increased slightly, due to the reaction of (a +c)-type TDs. A very sharp dynamical `correlation' peak was also observed in XRD of the LT-NL, related to TDs with an a-component. Furthermore, defect formation was observed within the LT-NL using high-resolution TEM. These data are consistent with TD formation predominantly in the LT-NL, followed by TD movement at elevated growth temperatures. Initially, coalesced films had a high TD density with a spatially random TD arrangement, but progressively altered into a lower TD density, spatially clustered arrangement during growth. This type of microstructure may mistakenly be interpreted as arising from island coalescence.

  15. Thermal evolution of microstructure in ion-irradiated GaN

    SciTech Connect

    Bae, In-Tae; Jiang, Weilin; Wang, Chong M.; Weber, William J.; Zhang, Yanwen

    2009-04-20

    The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. Upon thermal annealing, the thickness of the amorphous layer decreases by ~13.1 %, and nano-beam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to debond and react with the Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an atomic volume change of 18.9 % for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7 % after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.

  16. Ultrathin GaN quantum disk nanowire LEDs with sub-250 nm electroluminescence

    NASA Astrophysics Data System (ADS)

    Sarwar, A. T. M. Golam; May, Brelon J.; Chisholm, Matthew F.; Duscher, Gerd J.; Myers, Roberto C.

    2016-04-01

    By quantum confining GaN at monolayer thickness with AlN barriers inside of a nanowire, deep ultraviolet LEDs are demonstrated. Full three-dimensional strain dependent energy band simulations are carried out within multiple quantum disk (MQD) GaN/AlN nanowire superlattice heterostructures. It is found that, even within the same nanowire MQD, the emission energy of the ultrathin GaN QDs varies from disk to disk due to the changing strain distribution and polarization charge induced energy band bending along the axial nanowire direction. MQD heterostructures are grown by plasma-assisted molecular beam epitaxy to form self-assembled catalyst-free nanowires with 1 to 2 monolayer thick GaN insertions within an AlN matrix. Photoluminescence peaks are observed at 295 nm and 283 nm from the 2 ML and 1 ML thick MQD samples, respectively. Polarization-doped nanowire LEDs are grown incorporating 1 ML thick GaN MQD active regions from which we observe deep ultraviolet electroluminescence. The shortest LED wavelength peak observed is 240 nm and attributed to electron hole recombination within 1 ML thick GaN QDs.

  17. Transport properties, specific heat and thermal conductivity of GaN nanocrystalline ceramic

    SciTech Connect

    Sulkowski, Czeslaw; ChuchmaLa, Andrzej; Zaleski, Andrzej J.; Matusiak, Marcin; Mucha, Jan; GLuchowski, PaweL; Strek, WiesLaw

    2010-10-15

    The structural and transport properties (resistivity, thermopower and Hall effect), specific heat and thermal conductivity have been measured for GaN nanocrystalline ceramic prepared by hot pressing. It was found that the temperature dependence of resistivity in temperature range 10-300 K shows the very low activation energy, which is ascribed to the shallow donor doping originating in amorphous phase of sample. The major charge carriers are electrons, what is indicated by negative sign of Hall constant and Seebeck coefficient. The thermopower attains large values (-58 {mu}V/K at 300 K) and was characterized by linear temperature dependence which suggests the diffusion as a major contribution to Seebeck effect. The high electron concentration of 1.3x10{sup 19} cm{sup -3} and high electronic specific heat coefficient determined to be 2.4 mJ/molK{sup 2} allow to conclude that GaN ceramic demonstrates the semimetallic-like behavior accompanied by very small mobility of electrons ({approx}0.1 cm{sup 2}/V s) which is responsible for its high resistivity. A low heat conductivity of GaN ceramics is associated with partial amorphous phase of GaN grains due to high pressure sintering. - Graphical Abstract: Thermal resistivity and thermopower measurements indicates the high phonon scattering and lack of phonon-drag contribution to thermopower in GaN nanoceramics pressed under 4 GPa at 800 {sup o}C.

  18. Electron spin relaxation in two polymorphic structures of GaN

    NASA Astrophysics Data System (ADS)

    Kang, Nam Lyong

    2015-03-01

    The relaxation process of electron spin in systems of electrons interacting with piezoelectric deformation phonons that are mediated through spin-orbit interactions was interpreted from a microscopic point of view using the formula for the electron spin relaxation times derived by a projection-reduction method. The electron spin relaxation times in two polymorphic structures of GaN were calculated. The piezoelectric material constant for the wurtzite structure obtained by a comparison with a previously reported experimental result was {{P}pe}=1.5 × {{10}29} eV {{m}-1}. The temperature and magnetic field dependence of the relaxation times for both wurtzite and zinc-blende structures were similar, but the relaxation times in zinc-blende GaN were smaller and decreased more rapidly with increasing temperature and magnetic field than that in wurtzite GaN. This study also showed that the electron spin relaxation for wurtzite GaN at low density could be explained by the Elliot-Yafet process but not for zinc-blende GaN in the metallic regime.

  19. Spontaneously grown GaN and AlGaN nanowires

    NASA Astrophysics Data System (ADS)

    Bertness, K. A.; Roshko, A.; Sanford, N. A.; Barker, J. M.; Davydov, A. V.

    2006-01-01

    We have identified crystal growth conditions in gas-source molecular beam epitaxy (MBE) that lead to spontaneous formation of GaN nanowires with high aspect ratio on Si (1 1 1) substrates. The nanowires were oriented along the GaN c-axis and normal to the substrate surface. Unlike in many other reports of GaN nanowire growth, no metal catalysts were used. Low growth rates at substrate temperatures near 820 °C were combined with high nitrogen flux (partially dissociated with RF plasma excitation) to form well-separated GaN wires with diameters from 50 to 250 nm in diameter and lengths ranging from 2 to 7 μm. The nanowires grew out of an irregular matrix layer containing deep faceted holes. X-ray diffraction indicated that the wires were fully relaxed and aligned to the silicon substrate. The growth morphology was strongly affected by the presence of Al and Be. The changes suggest that surface diffusion is a primary driving force in the growth of GaN nanowires with MBE.

  20. Ultrathin GaN quantum disk nanowire LEDs with sub-250 nm electroluminescence

    SciTech Connect

    Chisholm, Matthew F.; Golam Sarwar, A. T. M.; Myers, Roberto C.; Mays, Brelon J.; Duscher, Gerd J.

    2016-03-18

    By quantum confining GaN at monolayer thickness with AlN barriers inside of a nanowire, deep ultraviolet LEDs are demonstrated. Full three-dimensional strain dependent energy band simulations are carried out within multiple quantum disk (MQD) GaN/AlN nanowire superlattice heterostructures. It is found that, even within the same nanowire MQD, the emission energy of the ultrathin GaN QDs varies from disk to disk due to the changing strain distribution and polarization charge induced energy band bending along the axial nanowire direction. MQD heterostructures are grown by plasma-assisted molecular beam epitaxy to form self-assembled catalyst-free nanowires with 1 to 2 monolayer thick GaN insertions within an AlN matrix. Photoluminescence peaks are observed at 295 nm and 283 nm from the 2 ML and 1 ML thick MQD samples, respectively. Polarization-doped nanowire LEDs are grown incorporating 1 ML thick GaN MQD active regions from which we observe deep ultraviolet electroluminescence. As a result, the shortest LED wavelength peak observed is 240 nm and attributed to electron hole recombination within 1 ML thick GaN QDs.

  1. Improved crystalline quality of N-polar GaN epitaxial layers grown with reformed flow-rate-modulation technology

    NASA Astrophysics Data System (ADS)

    Zhang, Heng; Zhang, Xiong; Wang, Shuchang; Wang, Xiaolei; Zhao, Jianguo; Wu, Zili; Dai, Qian; Yang, Hongquan; Cui, Yiping

    2017-01-01

    A reformed flow-rate-modulation technology was developed for the metalorganic vapor phase epitaxy (MOVPE) growth of the N-polar GaN epitaxial layers. To improve the crystalline quality of the N-polar GaN epitaxial layers, a GaN nucleation layer was grown at relatively low temperature with carefully-controlled pulsed supply of Ga source and showed diverse morphology with atomic force microscope (AFM). Furthermore, the electrical and optical properties of the grown N-polar GaN epitaxial layers were investigated extensively by means of Hall effect, photoluminescence (PL), and X-ray rocking curve (XRC) measurements. The characterization results revealed that as compared with the N-polar GaN epitaxial layer grown over the conventional GaN nucleation layer which was deposited with continuous supply of both N and Ga sources, the electrical and optical properties of the N-polar GaN epitaxial layer grown with optimized supply of Ga source for the GaN nucleation layer were significantly improved.

  2. One-step fabrication of porous GaN crystal membrane and its application in energy storage

    NASA Astrophysics Data System (ADS)

    Zhang, Lei; Wang, Shouzhi; Shao, Yongliang; Wu, Yongzhong; Sun, Changlong; Huo, Qin; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

    2017-03-01

    Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.

  3. One-step fabrication of porous GaN crystal membrane and its application in energy storage

    PubMed Central

    Zhang, Lei; Wang, Shouzhi; Shao, Yongliang; Wu, Yongzhong; Sun, Changlong; Huo, Qin; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

    2017-01-01

    Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application. PMID:28281562

  4. One-step fabrication of porous GaN crystal membrane and its application in energy storage.

    PubMed

    Zhang, Lei; Wang, Shouzhi; Shao, Yongliang; Wu, Yongzhong; Sun, Changlong; Huo, Qin; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

    2017-03-10

    Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.

  5. A New Method to Modify Two-Dimensional Electron Gas Density by GaN Cap Etching

    NASA Astrophysics Data System (ADS)

    Li, Zhongda; Chow, T. Paul

    2013-08-01

    We have experimentally demonstrated a new method for modifying the two-dimensional electron density (2DEG) at the AlGaN/GaN interface by etching of the GaN cap layer on top of the AlGaN. GaN MOS capacitors have been fabricated on samples with partially or fully etched GaN cap, and the 2DEG density has been extracted. The results show a linear relation between the 2DEG density and the thickness of the GaN cap being etched. We have also fabricated van der Pauw structures and obtained the 2DEG density using Hall measurements, and the results are consistent with that from the GaN MOS capacitors.

  6. Synthesis of GaN nanowires on gold-coated SiC substrates by novel pulsed electron deposition technique

    NASA Astrophysics Data System (ADS)

    Lei, M.; Yang, H.; Li, P. G.; Tang, W. H.

    2008-01-01

    A two-step approach for macro-synthesis of GaN nanowires was developed in this study. GaN nanoparticles were firstly synthesized through a facile solid-state reaction using an organic reagent dicyandiamide (C 2N 4H 4) and Ga 2O 3 as precursors. Subsequently, single-crystalline wurtzite GaN nanowires were grown on gold-coated 6H-SiC substrates via novel pulsed electron deposition (PED) technique using the as-prepared GaN nanoparticles as target, which provides a new route employing nanoparticles as precursors to fabricate GaN nanowires. It is found that pulsed electron ablation induced Ga and N plasma directly towards the gold-coated substrate to initialize the vapor-liquid-solid (VLS) growth processes. The morphological and structural properties were investigated in detail and Raman scattering spectrum of these nanowires presented some new features.

  7. The depth-profiled carrier concentration and scattering mechanism in undoped GaN film grown on sapphire

    NASA Astrophysics Data System (ADS)

    Huang, Y.; Chen, X. D.; Fung, S.; Beling, C. D.; Ling, C. C.; Wei, Z. F.; Xu, S. J.; Zhi, C. Y.

    2004-07-01

    Temperature-dependent Hall (TDH) measurements and confocal micro-Raman spectroscopy have been used to study the free carrier spatial distribution and scattering mechanism in unintentionally doped GaN film grown on the sapphire substrate with the method of metalorganic chemical vapor deposition. Both the TDH data and the depth-profiled Raman spectra agreed with the existence of a nonuniform spatial distribution of free carriers in the GaN film with a highly conductive layer of ˜1 μm thickness near the GaN sapphire boundary. With the consideration of this parallel conduction channel adjacent to GaN sapphire boundary, detailed analysis of the TDH mobility data suggests that a relatively high concentration of nitrogen vacancies exists and nitrogen vacancy scattering has an important influence on limiting the electron mobility in the bulk film of the present GaN sample.

  8. Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation

    NASA Astrophysics Data System (ADS)

    Oshima, Yuichi; Eri, Takeshi; Shibata, Masatomo; Sunakawa, Haruo; Kobayashi, Kenji; Ichihashi, Toshinari; Usui, Akira

    2003-01-01

    We have developed a novel technique for preparing large-scale freestanding GaN wafers. Hydride vapor phase epitaxy (HVPE) growth of thick GaN layer was performed on a GaN template with a thin TiN film on the top. After the cooling process of the HVPE growth, the thick GaN layer was easily separated from the template by the assistance of many voids generated around the TiN film. As a result, a freestanding GaN wafer was obtained. The wafer obtained had a diameter of 45 mm, and a mirror-like surface. The-full-width-at-half-maximum (FWHM) of (0002) and (10\\bar{1}0) peaks in the X-ray rocking curve profile were 60 and 92 arcsec, respectively. The dislocation density was evaluated at 5× 106 cm-3 by etch pit density measurement.

  9. AMIE Gan Island Ancillary Disdrometer Field Campaign Report

    SciTech Connect

    Oue, Mariko

    2016-04-01

    As part of the U.S. Department of Energy (DOE)’s Atmospheric Radiation Measurement Climate Research Facility (ARM) Madden-Julian Oscillation (MJO) Investigation Experiment (AMIE), in January 2012 a disdrometer observation took place with the second ARM Mobile Facility (AMF2), the Scanning ARM Cloud Radar (SACR), the Texas A&M SMART-R C-band radar, and the National Center for Atmospheric Research (NCAR) dual wavelength S- and Ka-bands polarimetric (SPolKa) radar on Gan Island, Maldives. In order to measure raindrop size distributions, a disdrometer of Nagoya University, Japan, was set up close to the ARM Two-Dimensional (2D) Video Disdrometer (2DVD). The SMART-R and SPolKa radars performed range-height-indicator scanning in the direction of the disdrometer site. Comparing the disdrometer data with 2DVD data, the raindrop size distribution data will be calibrated. Furthermore, the analysis of the raindrop size distribution and radar data will be expected to clarify the microphysics in tropical convective clouds.

  10. Vertical Graphene-base transistor on GaN substrate

    NASA Astrophysics Data System (ADS)

    Zubair, Ahmad; Saadat, Omair; Song, Yi; Kong, Jing; Dresselhaus, Mildred; Palacios, Tomas

    2014-03-01

    The high carrier mobility, saturation velocity and thermal conductivity make graphene an attractive candidate for RF electronics. In addition to conventional lateral transistors, several alternative vertical device structures like hot electron transistors have been demonstrated to be promising for RF applications. The unique combination of sub-nanometer thickness and high conductivity makes graphene an excellent base material for hot electron transistors by lowering the base transit time in these vertical devices. The demonstrated graphene-base hot electron transistor performance is limited by low current density and low common-base current gain. In this work, we fabricated a graphene-base transistor on GaN/AlGaN heterostructure. We studied the tunneling from GaN/AlGaN heterojunction to graphene and compared with other demonstrated vertical graphene-base devices. We also investigated the effect of AlGaN thickness and different filtering barriers on both room temperature and low temperature transport characteristics of the fabricated devices. With careful design and optimization of the structure, graphene-base transistors on GaN substrate can be a potential candidate for future graphene RF electronics.

  11. Radiation Characterization of Commercial GaN Devices

    NASA Technical Reports Server (NTRS)

    Harris, Richard D.; Scheick, Leif Z.; Hoffman, James P.; Thrivikraman, Tushar; Jenabi, Masud; Gim, Yonggyu; Miyahira, Tetsuo

    2011-01-01

    Radiative feedback from primordial protostars and final mass of the first star Commercially available devices fabricated from GaN are beginning to appear from a number of different suppliers. Based on previous materials and prototype device studies, it is expected that these commercial devices will be quite tolerant to the types of radiation encountered in space. This expectation needs to be verified and the study described herein was undertaken for that purpose. All of the parts discussed in this report are readily available commercially. The parts chosen for study are all targeted for RF applications. Three different studies were performed: 1) a preliminary DDD/TID test of a variety of part types was performed by irradiating with 50 MeV protons, 2) a detailed DDD/TID study of one particular part type was performed by irradiating with 50 MeV protons, and 3) a SEB/SEGR test was performed on a variety of part types by irradiating with heavy ions. No significant degradation was observed in the tests performed in this study.

  12. Optical Properties of GaN and ZnO

    NASA Astrophysics Data System (ADS)

    Song, J.-H.

    A brief review on the optical properties of wurtzite ZnO and GaN is presented in this chapter with an emphasis on comparison between the materials. The properties of free excitons and impurity-bound excitons, such as their energetic positions and binding energies, are summarized. The localization energy and the ionization energy of the dominant impurities obtained by emission spectroscopy are also presented. Typical aspects of emissions from donor—acceptor pairs, free-to-bound transition, and deep level recombination are discussed. Several experimental characteristics of the relevant heterostructures, InGaN/GaN and MgZnO/ZnO, are also given below. Basic optical methods characterizing the effects of internal electric fields and carrier-localization are summarized. The unique properties of polarization sensitive emissions from nonpolar films are presented. Based on the valence band structures, the polarization selection rules can be obtained in simpler forms. Some recent reports will also be introduced stating that the anisotropic strain in nonpolar films plays an important role in deciding the polarization selectivity. The results of Raman spectroscopy are summarized in the end, with the emphasis on deciding the residual strain and the carrier concentration.

  13. Low energy electron beam induced vacancy activation in GaN

    SciTech Connect

    Nykaenen, H.; Suihkonen, S.; Sopanen, M.; Kilanski, L.

    2012-03-19

    Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy (MOVPE) is presented. The GaN samples are irradiated with a 5-20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiated GaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive V{sub Ga}-H{sub n} complexes that can be activated by H removal during low energy electron irradiation.

  14. Low energy electron beam induced vacancy activation in GaN

    NASA Astrophysics Data System (ADS)

    Nykänen, H.; Suihkonen, S.; Kilanski, L.; Sopanen, M.; Tuomisto, F.

    2012-03-01

    Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy (MOVPE) is presented. The GaN samples are irradiated with a 5-20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiated GaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive VGa-Hn complexes that can be activated by H removal during low energy electron irradiation.

  15. Study of electrical properties of single GaN nanowires grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Mozharov, A. M.; Komissarenko, F. E.; Vasiliev, A. A.; Bolshakov, A. D.; Moiseev, E. I.; Mukhin, M. S.; Cirlin, G. E.; Mukhin, I. S.

    2016-08-01

    Electrical properties of single GaN nanowires grown by means of molecular beam epitaxy with N-plasma source were studied. Ohmic contacts connected to single n-type GaN wires were produced by the combination of electron beam lithography, metal vacuum evaporation and rapid thermal annealing technique. The optimal annealing temperature to produce ohmic contacts implemented in the form of Ti/Al/Ti/Au stack has been determined. By means of 2-terminal measurement wiring diagram the conductivity of single NW has been obtained for NWs with different growth parameters. The method of MESFET measurement circuit layout of single GaN nanowires (NWs) has been developed. In accordance with performed numerical calculation, free carriers' concentration and mobility of single NWs could be independently estimated using MESFET structure.

  16. Semipolar AlN and GaN on Si(100): HVPE technology and layer properties

    NASA Astrophysics Data System (ADS)

    Bessolov, V.; Kalmykov, A.; Konenkova, E.; Kukushkin, S.; Myasoedov, A.; Poletaev, N.; Rodin, S.

    2017-01-01

    Hydride vapor phase epitaxy (HVPE) growth of semipolar AlN and GaN layers on planar Si(100) substrates with SiC nanolayer is investigated. It is shown experimentally that the solid-phase epitaxial formation of a specially oriented SiC nucleation layer followed by epitaxy of AlN layer by HVPE at low rates enables growth of aluminum and gallium nitrides in the semipolar direction. For the best GaN(20-23) layers obtained, the full width at half maximum (FWHM) value for the x-ray diffraction rocking curve is 24 arcmin. The photoluminescence spectrum of the semipolar GaN measured at 4 K exhibits bands related to basal-plane and prismatic stacking faults (BSF and PSF).

  17. Pyramidal defects in Mg-doped GaN in light of strain-energy minimization

    NASA Astrophysics Data System (ADS)

    Lee, Dong Nyung

    2011-12-01

    The planar segregation gives rise to stress and strain fields which are approximated by a uniaxial character in a displacement controlled system. In this condition, the elastic strain energy is proportional to Young's modulus. Young's modulus of GaN is minimized when the directions normal to a conical segregation surface make about 48° with the c-axis of hexagonal GaN, which is close to the angle 47.3° between the c-axis and the directions normal to the {112¯3} planes. This implies that the formation of pyramidal defects in magnesium-doped GaN can be a compromise between minimization of the elastic strain energy due to segregation of magnesium and the planar segregation.

  18. A low cost, green method to synthesize GaN nanowires

    PubMed Central

    Zhao, Jun-Wei; Zhang, Yue-Fei; Li, Yong-He; Su, Chao-hua; Song, Xue-Mei; Yan, Hui; Wang, Ru-Zhi

    2015-01-01

    The synthesis of gallium nitride nanowires (GaN NWs) by plasma enhanced chemical vapor deposition (PECVD) are successfully demonstrated in this work. The simple and green synthesis route is to introduce gallium oxide (Ga2O3) and nitrogen (N2) for the growth of nanowires. The prepared GaN nanowires have a single crystalline wurtzite structure, which the length of some nanowires is up to 20 μm, with a maximum diameter about 140 nm. The morphology and quantity of the nanowires can be modulated by the growth substrate and process parameters. In addition, the photoluminescence and field emission properties of the prepared GaN nanowires have been investigated, which were found to be largely affected by their structures. This work renders an environmentally benign strategy and a facile approach for controllable structures on nanodevice. PMID:26643613

  19. Lattice-matched HfN buffer layers for epitaxy of GaN on Si

    SciTech Connect

    Armitage, Robert; Yang, Qing; Feick, Henning; Gebauer, Joerg; Weber, Eicke R.; Shinkai, Satoko; Sasaki, Katsutaka

    2002-05-08

    Gallium nitride is grown by plasma-assisted molecular-beam epitaxy on (111) and (001) silicon substrates using sputter-deposited hafnium nitride buffer layers. Wurtzite GaN epitaxial layers are obtained on both the (111) and (001) HfN/Si surfaces, with crack-free thickness up to 1.2 (mu)m. Initial results for GaN grown on the (111) surface show a photoluminescence peak width of 17 meV at 11 K, and an asymmetric x-ray rocking curve width of 20 arcmin. Wurtzite GaN on HfN/Si(001) shows reduced structural quality and peculiar low-temperature luminescence features. However, growth on the (001) surface results in nearly stress-free films, suggesting that much thicker crack-free layers could be obtained.

  20. P-type doping of GaN(000\\bar{1}) by magnesium ion implantation

    NASA Astrophysics Data System (ADS)

    Narita, Tetsuo; Kachi, Tetsu; Kataoka, Keita; Uesugi, Tsutomu

    2017-01-01

    Magnesium ion implantation has been performed on a GaN(000\\bar{1}) substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current-voltage characteristics of p-n diodes fabricated on GaN(000\\bar{1}) showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN(000\\bar{1}).

  1. Synthesis, morphology and optical properties of GaN and AlGaN semiconductor nanostructures

    SciTech Connect

    Kuppulingam, B. Singh, Shubra Baskar, K.

    2014-04-24

    Hexagonal Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) nanoparticles were synthesized by sol-gel method using Ethylene Diamine Tetra Acetic acid (EDTA) complex route. Powder X-ray diffraction (PXRD) analysis confirms the hexagonal wurtzite structure of GaN and Al{sub 0.25}Ga{sub 0.75}N nanoparticles. Surface morphology and elemental analysis were carried out by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscopy (EDX). The room temperature Photoluminescence (PL) study shows the near band edge emission for GaN at 3.35 eV and at 3.59 eV for AlGaN nanoparticles. The Aluminum (Al) composition of 20% has been obtained from PL emission around 345 nm.

  2. Nonresonant tunneling phonon depopulated GaN based terahertz quantum cascade structures

    NASA Astrophysics Data System (ADS)

    Freeman, Will; Karunasiri, Gamani

    2013-04-01

    GaN based terahertz quantum cascade structures are theoretically studied. Since the Fröhlich interaction is ˜15 times higher in GaN than in GaAs, level broadening makes obtaining appreciable optical gain difficult even with a large population inversion. A density matrix Monte Carlo method is used to calculate the broadening of the optical gain spectra as a function of lattice temperature. We find by using a proposed method of nonresonant tunneling and electron-longitudinal-optical phonon scattering for depopulation of the lower lasing state, that it is possible to sufficiently isolate the upper lasing state and control the lower lasing state lifetime to obtain high optical gain in GaN. The results predict lasing out to 300 K which is significantly higher than for GaAs based structures.

  3. Visualization of GaN surface potential using terahertz emission enhanced by local defects

    NASA Astrophysics Data System (ADS)

    Sakai, Yuji; Kawayama, Iwao; Nakanishi, Hidetoshi; Tonouchi, Masayoshi

    2015-09-01

    Wide-gap semiconductors have received significant attention for their advantages over existing semiconductors in energy-efficient power devices. To realize stable and reliable wide-gap semiconductor devices, the basic physical properties, such as the electric properties on the surface and at the interface, should be revealed. Here, we report visualization of terahertz (THz) emission from the surface of GaN, which is excited by ultraviolet femtosecond laser pulses. We found that the THz emission is enhanced by defects related to yellow luminescence, and this phenomenon is explained through the modification of band structures in the surface depletion layer owing to trapped electrons at defect sites. Our results demonstrate that the surface potential in a GaN surface could be detected by laser-induced THz emission. Moreover, this method enables feasible evaluation of the distribution of non-radiative defects, which are undetectable with photoluminescence, and it contributes to the realization normally-off GaN devices.

  4. Visualization of GaN surface potential using terahertz emission enhanced by local defects.

    PubMed

    Sakai, Yuji; Kawayama, Iwao; Nakanishi, Hidetoshi; Tonouchi, Masayoshi

    2015-09-09

    Wide-gap semiconductors have received significant attention for their advantages over existing semiconductors in energy-efficient power devices. To realize stable and reliable wide-gap semiconductor devices, the basic physical properties, such as the electric properties on the surface and at the interface, should be revealed. Here, we report visualization of terahertz (THz) emission from the surface of GaN, which is excited by ultraviolet femtosecond laser pulses. We found that the THz emission is enhanced by defects related to yellow luminescence, and this phenomenon is explained through the modification of band structures in the surface depletion layer owing to trapped electrons at defect sites. Our results demonstrate that the surface potential in a GaN surface could be detected by laser-induced THz emission. Moreover, this method enables feasible evaluation of the distribution of non-radiative defects, which are undetectable with photoluminescence, and it contributes to the realization normally-off GaN devices.

  5. Long-Lived, Coherent Acoustic Phonon Oscillations in GaN Single Crystals

    SciTech Connect

    Wu, S.; Geiser, P.; Jun, J.; Karpinski, J.; Park, J.-R.; Sobolewski, R.

    2006-01-31

    We report on coherent acoustic phonon (CAP) oscillations studied in high-quality bulk GaN single crystals with a two-color femtosecond optical pump-probe technique. Using a far-above-the-band gap ultraviolet excitation (~270 nm wavelength) and a near-infrared probe beam (~810 nm wavelength), the long-lived, CAP transients were observed within a 10 ns time-delay window between the pump and probe pulses, with a dispersionless (proportional to the probe-beam wave vector) frequency of ~45 GHz. The measured CAP attenuation corresponded directly to the absorption of the probe light in bulk GaN, indicating that the actual (intrinsic) phonon-wave attenuation in our crystals was significantly smaller than the measured 65.8 cm^-1 value. The velocity of the phonon propagation was equal to the velocity of sound in GaN.

  6. Measurement of the electrostatic edge effect in wurtzite GaN nanowires

    SciTech Connect

    Henning, Alex; Rosenwaks, Yossi; Klein, Benjamin; Bertness, Kris A.; Blanchard, Paul T.; Sanford, Norman A.

    2014-11-24

    The electrostatic effect of the hexagonal corner on the electronic structure in wurtzite GaN nanowires (NWs) was directly measured using Kelvin probe force microscopy (KPFM). By correlating electrostatic simulations with the measured potential difference between the nanowire face and the hexagonal vertices, the surface state concentration and band bending of GaN NWs were estimated. The surface band bending is important for an efficient design of high electron mobility transistors and for opto-electronic devices based on GaN NWs. This methodology provides a way to extract NW parameters without making assumptions concerning the electron affinity. We are taking advantage of electrostatic modeling and the high precision that KPFM offers to circumvent a major source of uncertainty in determining the surface band bending.

  7. Migration mechanisms and diffusion barriers of carbon and native point defects in GaN

    NASA Astrophysics Data System (ADS)

    Kyrtsos, Alexandros; Matsubara, Masahiko; Bellotti, Enrico

    2016-06-01

    Carbon related defects are readily incorporated in GaN due to its abundance during growth both with MBE and MOCVD techniques. Employing first-principles calculations, we compute the migration barriers of carbon interstitials and we discuss possible relevant mechanisms of diffusion in the wurtzite GaN crystal. In addition, we calculate the migration barriers for the diffusion of the native defects of the crystal, i.e., gallium and nitrogen interstitials and vacancies. The minimum energy path and the migration barriers of these defects are obtained using the nudged elastic band method with the climbing image modification. In addition, the dimer method is used to independently determine the results. The results yield a quantitative description of carbon diffusion in GaN allowing for the determination of the most preferable migration paths.

  8. Investigation of HCl-based surface treatment for GaN devices

    SciTech Connect

    Okada, Hiroshi; Shinohara, Masatohi; Kondo, Yutaka; Sekiguchi, Hiroto; Yamane, Keisuke; Wakahara, Akihiro

    2016-02-01

    Surface treatments of GaN in HCl-based solutions are studied by X-ray photoelectron spectroscopy (XPS) and electrical characterization of fabricated GaN surfaces. A dilute-HCl treatment (HCl:H{sub 2}O=1:1) at room temperature and a boiled-HCl treatment (undiluted HCl) at 108°C are made on high-temperature annealed n-GaN. From the XPS study, removal of surface oxide by the dilute-HCl treatment was found, and more thoroughly oxide-removal was confirmed in the boiled-HCl treatment. Effect of the surface treatment on electrical characteristics on AlGaN/GaN transistor is also studied by applying treatment processes prior to the surface SiN deposition. Increase of drain current is found in boiled-HCl treated samples. The results suggest that the boiled-HCl treatment is effective for GaN device fabrication.

  9. Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil

    NASA Astrophysics Data System (ADS)

    Calabrese, Gabriele; Corfdir, Pierre; Gao, Guanhui; Pfüller, Carsten; Trampert, Achim; Brandt, Oliver; Geelhaar, Lutz; Fernández-Garrido, Sergio

    2016-05-01

    We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit an equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photoluminescence spectrum of the nanowire ensemble is not influenced or degraded by the bending of the substrate. The present results pave the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils.

  10. Electrical characterization of ensemble of GaN nanowires grown by the molecular beam epitaxy technique

    NASA Astrophysics Data System (ADS)

    Kolkovsky, Vl.; Zytkiewicz, Z. R.; Sobanska, M.; Klosek, K.

    2013-08-01

    High quality Schottky contacts are formed on GaN nanowires (NWs) structures grown by the molecular beam epitaxy technique on Si(111) substrate. The current-voltage characteristics show the rectification ratio of about 103 and the leakage current of about 10-4 A/cm2 at room temperature. From the capacitance-voltage measurements the free carrier concentration in GaN NWs is determined as about 1016 cm-3. Two deep levels (H200 and E280) are found in the structures containing GaN NWs. H200 is attributed to an extended defect located at the interface between the substrate and SiNx or near the sidewalls at the bottom of the NWs whereas E280 is tentatively assigned to a gallium-vacancy- or nitrogen interstitials-related defect.

  11. Development of semipolar (11-22) LEDs on GaN templates

    NASA Astrophysics Data System (ADS)

    Corbett, B.; Quan, Z.; Dinh, D. V.; Kozlowski, G.; O'Mahony, D.; Akhter, M.; Schulz, S.; Parbrook, P.; Maaskant, P.; Caliebe, M.; Hocker, M.; Thonke, K.; Scholz, F.; Pristovsek, M.; Han, Y.; Humphreys, C. J.; Brunner, F.; Weyers, M.; Meyer, T. M.; Lymperakis, L.

    2016-03-01

    We report on blue and green light-emitting-diodes (LEDs) grown on (11-22)-GaN templates. The templates were created by overgrowth on structured r-plane sapphire substrates. Low defect density, 100 mm diameter GaN templates were obtained by metal organic vapour phase epitaxy (VPE) and hydride VPE techniques. Chemical-mechanical polishing was used to obtain smooth surfaces for the subsequent growth of LED structures. Ohmic contacts to the p-type GaN were obtained despite the lower activated acceptor levels. The LEDs show excellent output power and fast carrier dynamics. Freestanding LEDs have been obtained by use of laser-lift-off. The work is the result of collaboration under the European Union funded ALIGHT project.

  12. Strong photoluminescence emission from GaN grown on amorphous silica substrates by gas source MBE

    NASA Astrophysics Data System (ADS)

    Iwata, K.; Asahi, H.; Asami, K.; Kuroiwa, R.; Gonda, S.

    1998-06-01

    GaN layers are grown on amorphous fused silica glass substrates by gas source MBE using an ion removed electron cyclotron resonance (ECR) radical cell. Reflection high-energy electron diffraction (RHEED) and X-ray diffraction measurements reveal that they are polycrystalline. However, they show a strong photoluminescence emission peak without deep level emission. The emission peak is red-shifted by about 150 meV from that of the excitonic emission peak of GaN grown on a sapphire substrate and has wide spectral half-width (˜250 meV at 77 K). The peak is not corresponding to the donor-acceptor pair (DAP) emission but is excitonic from the excitation power and temperature dependence of PL spectrum. These optical properties indicate that GaN layers grown on a glass substrate are promising for fabrication of large area and low cost light emitting devices and solar cells.

  13. Ammonothermal synthesis of GaN using Ba(NH2)2 as mineralizer

    NASA Astrophysics Data System (ADS)

    Hertrampf, J.; Alt, N. S. A.; Schlücker, E.; Knetzger, M.; Meissner, E.; Niewa, R.

    2016-12-01

    It is demonstrated that hexagonal GaN can be obtained under ammonothermal conditions (125 MPa and 723 K) using Ba(NH2)2 as mineralizer. The hexagonal wurtzite-type GaN crystallites are several μm in diameter, as examined by scanning electron microscopy. This is to our knowledge the first successful ammonothermal GaN synthesis using an alkaline-earth metal as mineralizer. Ba[Ga(NH2)4]2 was identified as intermediate species in the ammonothermal synthesis process. The formation of h-GaN using Sr(NH2)2 as mineralizer was indicated only at higher temperatures above 1000 K.

  14. On the phenomenon of large photoluminescence red shift in GaN nanoparticles.

    PubMed

    Slimane, Ahmed Ben; Najar, Adel; Elafandy, Rami; San-Román-Alerigi, Damián P; Anjum, Dalaver; Ng, Tien Khee; Ooi, Boon S

    2013-07-31

    We report on the observation of broad photoluminescence wavelength tunability from n-type gallium nitride nanoparticles (GaN NPs) fabricated using the ultraviolet metal-assisted electroless etching method. Transmission and scanning electron microscopy measurements performed on the nanoparticles revealed large size dispersion ranging from 10 to 100 nm. Nanoparticles with broad tunable emission wavelength from 362 to 440 nm have been achieved by exciting the samples using the excitation power-dependent method. We attribute this large wavelength tunability to the localized potential fluctuations present within the GaN matrix and to vacancy-related surface states. Our results show that GaN NPs fabricated using this technique are promising for tunable-color-temperature white light-emitting diode applications.

  15. Control of GaN crystal habit by solution stirring in the Na-flux method

    NASA Astrophysics Data System (ADS)

    Murakami, Kosuke; Imade, Mamoru; Imanishi, Masayuki; Honjo, Masatomo; Imabayashi, Hiroki; Matsuo, Daisuke; Nakamura, Kosuke; Maruyama, Mihoko; Yoshimura, Masashi; Mori, Yusuke

    2017-01-01

    In our previous study, we succeeded in fabricating low-curvature GaN wafers with low dislocation density by the Na-flux coalescence growth technique. However, the crystals consisted of many pyramidal grains with (10\\bar{1}1) facets, leading to an increase in the oxygen concentration in the crystal, an increase in the lattice constant, and blackening. In this study, we attempted to improve the crystal habit of the GaN crystals by employing a solution-stirring technique in the coalescence growth on multipoint seeds. Scanning electron microscope images indicated that the c-face area became larger by increasing the stirring rate and growth period. We concluded that solution stirring in the Na-flux coalescence growth technique is an effective approach to improve the crystal habit and uniformize the lattice constant of GaN crystals.

  16. Botulinum toxin detection using AlGaN /GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Wang, Yu-Lin; Chu, B. H.; Chen, K. H.; Chang, C. Y.; Lele, T. P.; Tseng, Y.; Pearton, S. J.; Ramage, J.; Hooten, D.; Dabiran, A.; Chow, P. P.; Ren, F.

    2008-12-01

    Antibody-functionalized, Au-gated AlGaN /GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN /GaN HEMT drain-source current showed a rapid response of less than 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng/ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN /GaN HEMTs for botulinum toxin detection.

  17. The pyroelectric coefficient of free standing GaN grown by HVPE

    NASA Astrophysics Data System (ADS)

    Jachalke, Sven; Hofmann, Patrick; Leibiger, Gunnar; Habel, Frank S.; Mehner, Erik; Leisegang, Tilmann; Meyer, Dirk C.; Mikolajick, Thomas

    2016-10-01

    The present study reports on the temperature dependent pyroelectric coefficient of free-standing and strain-free gallium nitride (GaN) grown by hydride vapor phase epitaxy (HVPE). The Sharp-Garn method is applied to extract the pyroelectric coefficient from the electrical current response of the crystals subjected to a sinusoidal temperature excitation in a range of 0 °C to 160 °C. To avoid compensation of the pyroelectric response by an internal conductivity, insulating GaN crystals were used by applying C, Mn, and Fe doping during HVPE growth. The different pyroelectric coefficients observed at room temperature due to the doping correlate well with the change of the lattice parameter c. The obtained data are compared to previously published theoretical and experimental values of thin film GaN and discussed in terms of a strained lattice.

  18. A survey on GaN- based devices for terahertz photonics

    NASA Astrophysics Data System (ADS)

    Ahi, Kiarash; Anwar, Mehdi

    2016-09-01

    With fast growing of the photonics and power electronic systems, the need for high power- high frequency semiconductor devices is sensed tremendously. GaN provides the highest electron saturation velocity, breakdown voltage and operation temperature, and thus combined frequency-power performance among commonly used semiconductors. With achieving the first THz image in just two decades ago, generation and detection of terahertz (THz) radiation is one of the most emerging photonic areas. The industrial needs for compact, economical, high resolution and high power THz imaging and spectroscopy systems are fueling the utilization of GaN for the realizing of the next generation of THz systems. As it is reviewed in this paper, the mentioned characteristics of GaN together with its capabilities of providing high 2-dimentional election densities and large longitudinal-optical phonon of 90 meV, make it one of the most promising semiconductor materials for the future of the THz generation, detection, mixing, and frequency multiplication. GaN- based devices have shown capabilities of operating in the upper THz frequency band of 5- 12 THz with relatively high photon densities and in room temperature. As a result, THz imaging and spectroscopy systems with high resolutions and depths of penetrations can be realized via utilizing GaN- based devices. In this paper, a comprehensive review on the history and state of the art of the GaN- based electronic devices, including plasma HFETs, NDRs, HDSDs, IMPATTs, QCLs, HEMTs, Gunn diodes and TeraFETs together with their impact on the future of THz imaging and spectroscopy systems is provided.

  19. Hybrid density functional theory studies of AlN and GaN under uniaxial strain.

    PubMed

    Qin, Lixia; Duan, Yifeng; Shi, Hongliang; Shi, Liwei; Tang, Gang

    2013-01-30

    The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).

  20. Hybrid density functional theory studies of AlN and GaN under uniaxial strain

    NASA Astrophysics Data System (ADS)

    Qin, Lixia; Duan, Yifeng; Shi, Hongliang; Shi, Liwei; Tang, Gang

    2013-01-01

    The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).

  1. Accelerating Particles with Plasma

    ScienceCinema

    Litos, Michael; Hogan, Mark

    2016-07-12

    Researchers at SLAC explain how they use plasma wakefields to accelerate bunches of electrons to very high energies over only a short distance. Their experiments offer a possible path for the future of particle accelerators.

  2. Peak acceleration limiter

    NASA Technical Reports Server (NTRS)

    Chapman, C. P.

    1972-01-01

    Device is described that limits accelerations by shutting off shaker table power very rapidly in acceleration tests. Absolute value of accelerometer signal is used to trigger electronic switch which terminates test and sounds alarm.

  3. Linear Accelerator (LINAC)

    MedlinePlus

    ... equipment? How is safety ensured? What is this equipment used for? A linear accelerator (LINAC) is the ... Therapy (SBRT) . top of page How does the equipment work? The linear accelerator uses microwave technology (similar ...

  4. Accelerating Particles with Plasma

    SciTech Connect

    Litos, Michael; Hogan, Mark

    2014-11-05

    Researchers at SLAC explain how they use plasma wakefields to accelerate bunches of electrons to very high energies over only a short distance. Their experiments offer a possible path for the future of particle accelerators.

  5. Improved plasma accelerator

    NASA Technical Reports Server (NTRS)

    Cheng, D. Y.

    1971-01-01

    Converging, coaxial accelerator electrode configuration operates in vacuum as plasma gun. Plasma forms by periodic injections of high pressure gas that is ionized by electrical discharges. Deflagration mode of discharge provides acceleration, and converging contours of plasma gun provide focusing.

  6. Accelerator Technology Division

    NASA Astrophysics Data System (ADS)

    1992-04-01

    In fiscal year (FY) 1991, the Accelerator Technology (AT) division continued fulfilling its mission to pursue accelerator science and technology and to develop new accelerator concepts for application to research, defense, energy, industry, and other areas of national interest. This report discusses the following programs: The Ground Test Accelerator Program; APLE Free-Electron Laser Program; Accelerator Transmutation of Waste; JAERI, OMEGA Project, and Intense Neutron Source for Materials Testing; Advanced Free-Electron Laser Initiative; Superconducting Super Collider; The High-Power Microwave Program; (Phi) Factory Collaboration; Neutral Particle Beam Power System Highlights; Accelerator Physics and Special Projects; Magnetic Optics and Beam Diagnostics; Accelerator Design and Engineering; Radio-Frequency Technology; Free-Electron Laser Technology; Accelerator Controls and Automation; Very High-Power Microwave Sources and Effects; and GTA Installation, Commissioning, and Operations.

  7. Accelerators, Colliders, and Snakes

    NASA Astrophysics Data System (ADS)

    Courant, Ernest D.

    2003-12-01

    The author traces his involvement in the evolution of particle accelerators over the past 50 years. He participated in building the first billion-volt accelerator, the Brookhaven Cosmotron, which led to the introduction of the "strong-focusing" method that has in turn led to the very large accelerators and colliders of the present day. The problems of acceleration of spin-polarized protons are also addressed, with discussions of depolarizing resonances and "Siberian snakes" as a technique for mitigating these resonances.

  8. Computing Equilibrium Shapes of Wurtzite Crystals: The Example of GaN.

    PubMed

    Li, Hong; Geelhaar, Lutz; Riechert, Henning; Draxl, Claudia

    2015-08-21

    Crystal morphologies are important for the design and functionality of devices based on low-dimensional nanomaterials. The equilibrium crystal shape (ECS) is a key quantity in this context. It is determined by surface energies, which are hard to access experimentally but can generally be well predicted by first-principles methods. Unfortunately, this is not necessarily so for polar and semipolar surfaces of wurtzite crystals. By extending the concept of Wulff construction, we demonstrate that ECSs can nevertheless be obtained for this class of materials. For the example of GaN, we identify different crystal shapes depending on the chemical potential, shedding light on experimentally observed GaN nanostructures.

  9. Injection Laser Using Rare Earth Doped GaN Thin Films for Visible and Infrared Applications

    DTIC Science & Technology

    2010-05-01

    GaN and AlN thin films on sapphire and silicon carbide ( SiC ) substrates. The methyl derivatives of the elements, namely trimethylgallium (TMGa) and...molecular beam epitaxy and electrical characterization of Si-doped zinc blende GaN films deposited on ? SiC coated (001) Si substrates," Appl. Phys. Lett...34InGaN Multi-Quantum-Well-Structure Laser Diodes with Cleaved Mirror Cavity Facets," Jpn. J. Appl. Phy., vol. 35, p. 15, 1996. [32] M. A. Khan, J. N

  10. Eliminating stacking faults in semi-polar GaN by AlN interlayers

    SciTech Connect

    Dadgar, A.; Ravash, R.; Veit, P.; Schmidt, G.; Mueller, M.; Dempewolf, A.; Bertram, F.; Wieneke, M.; Christen, J.; Krost, A.

    2011-07-11

    We report on the elimination of stacking faults by the insertion of low-temperature AlN interlayers in nearly (1016) and (1104) oriented semi-polar GaN grown by metalorganic vapor phase epitaxy on Si(112) and Si(113), respectively. The elimination of these defects is visualized by cathodoluminescence (CL) as well as scanning transmission electron microscopy (STEM) and STEM-CL. A possible annihilation mechanism is discussed which leads to the conclusion that the elimination mechanism is most likely valid for all layers with (1101) surfaces, enabling heteroepitaxial semi- and non-polar GaN free from stacking faults.

  11. GaN ultraviolet detector based demonstrator board for UV-index monitoring

    NASA Astrophysics Data System (ADS)

    Song, Man; Xie, Feng; Wang, Jun; Wang, Tanglin; Guo, Jin

    2015-04-01

    Currently, various types of III nitride-based materials have been successfully used for short-wavelength optoelectronic devices. The GaN ultraviolet detector has been wildly used for UV-Index(UVI) monitoring, UV curing and water disinfection. The global solar UVI describes the levels of solar UV radiation at the Earth's surface. The higher the UVI value, the greater the potential damage to the skin and eyes. The UVI monitoring demonstrator board with GaN detector is briefly introduced in this paper.

  12. Thermal Conductivity of GaN Nanotubes Simulated by Nonequilibrium Molecular Dynamics

    SciTech Connect

    Wang, Zhiguo; Gao, Fei; Crocombette, J.-P.; Zu, Xiaotao; Yang, Li; Weber, William J.

    2007-04-15

    Thermal conductivity of GaN nanotubes along the tube axis is investigated over the temperature range of 600K-2300K using homogeneous nonequilibrium molecular dynamics. In general, the thermal conductivity of nanotubes is smaller than that for the bulk GaN single crystal. The thermal conductivity is also found to decrease with temperature and increase with increasing wall thickness of the nanotubes. The change of phonon spectrum and surface inelastic scattering may account for the reduction of thermal conductivity in the nanotubes, while thermal softening and high frequency phonon interactions at high temperatures may provide an explanation for its decrease with increasing temperature.

  13. Characterization of GaN microstructures grown by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Lo, Ikai; Pang, Wen-Yuan; Hsu, Yu-Chi; Hsieh, Chia-Ho; Shih, Cheng-Hung; Chou, Mitch M. C.; Chen, Wen-Yen; Hsu, Tzu-Min; Hsu, Gary Z. L.

    2013-06-15

    The characterization of GaN microstructures grown by plasma-assisted molecular beam epitaxy on LiAlO{sub 2} substrate was studied by cathodoluminescence and photoluminescence measurements. We demonstrated that the cathodoluminescence from oblique semi-polar surfaces of mushroom-shaped GaN was much brighter than that from top polar surface due to the reduction of polarization field on the oblique semi-polar surfaces. It implies that the oblique semi-polar surface is superior for the light-emitting surface of wurtzite nano-devices.

  14. Modeling of radiation damage recovery in particle detectors based on GaN

    NASA Astrophysics Data System (ADS)

    Gaubas, E.; Ceponis, T.; Pavlov, J.

    2015-12-01

    The pulsed characteristics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift-diffusion models. The drift-diffusion current simulations have been implemented by employing the commercial software package Synopsys TCAD Sentaurus. The bipolar drift regime has been analyzed. The possible internal gain in charge collection through carrier multiplication processes determined by impact ionization has been considered in order to compensate carrier lifetime reduction due to radiation defects introduced into GaN material of detector.

  15. An Optimal Density Functional Theory Method for GaN and ZnO

    SciTech Connect

    Yu, H.G.

    2011-08-25

    We report an optimal DFT method (bBLYP) for studying the GaN and ZnO systems. It is developed by modifying the exchange functional in the hybrid BLYP method in order to overcome the flaw of traditional DFT that often predict a rather small band gap for those semiconductors. Results show that the bBLYP method can describe not only correct band gaps of both GaN and ZnO wurtzite crystals, but also accurate properties of relevant small molecules. The application study of crystal-cut nanoparticles and nanowires reveals a new mechanism for band gap narrowing in GaN/ZnO.

  16. Simulation of optimum parameters for GaN MSM UV photodetector

    NASA Astrophysics Data System (ADS)

    Alhelfi, Mohanad A.; Ahmed, Naser M.; Hashim, M. R.; Al-Rawi, Ali Amer; Hassan, Z.

    2016-07-01

    In this study the optimum parameters of GaN M-S-M photodetector are discussed. The evaluation of the photodetector depends on many parameters, the most of the important parameters the quality of the GaN film and others depend on the geometry of the interdigited electrode. In this simulation work using MATLAB software with consideration of the reflection and absorption on the metal contacts, a detailed study involving various electrode spacings (S) and widths (W) reveals conclusive results in device design. The optimum interelectrode design for interdigitated MSM-PD has been specified and evaluated by effect on quantum efficiency and responsivity.

  17. Deep Centers and Their Capture Barriers in MOCVD-Grown GaN

    DTIC Science & Technology

    2002-01-01

    Kim, A. Botchkarev, H. Morkoc, "Deep level defects in n-type GaN grown by molecular beam epitaxy," Applied Physics Letters 72, pp. 1211-1213, 1998. 34...photoionization cross section are due to a lattice relaxation associated with a capture barrier for the DX center. Polyakov , et al. has also pointed out that...ohmic contacts to n-type GaN," Materials Science and Engineering B59 , pp. 358-361, 1999. 10. J. Rennie, M. Onomura, S. Nunoue, G. Hatakoshi, H

  18. First-Principles Study of Defects in GaN, AlN and Their Alloys

    DTIC Science & Technology

    2010-08-31

    studied the similar type of pairs in GaN, i.e. Gai -VGa. These types of defect complexes are called Frenkel pairs which are believed to form when the...defects have been carried out.[17-21] In our previous program, we studied the GaGa i V− pairs in GaN. We found that Gai can occur in 3+, 2+, and 1...charge states, depending on the Fermi energy of the sample. VGa can occur in 3−, 2−, 1−, and neutral charge states. Because Gai and VGa have

  19. Simulations of Operation Dynamics of Different Type GaN Particle Sensors

    PubMed Central

    Gaubas, Eugenijus; Ceponis, Tomas; Kalesinskas, Vidas; Pavlov, Jevgenij; Vysniauskas, Juozas

    2015-01-01

    The operation dynamics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift-diffusion models. The drift-diffusion current simulations have been implemented by employing the software package Synopsys TCAD Sentaurus. The monopolar and bipolar drift regimes have been analyzed by using dynamic models based on the Shockley-Ramo theorem. The carrier multiplication processes determined by impact ionization have been considered in order to compensate carrier lifetime reduction due to introduction of radiation defects into GaN detector material. PMID:25751080

  20. Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching

    NASA Astrophysics Data System (ADS)

    Tiginyanu, I. M.; Ursaki, V. V.; Zalamai, V. V.; Langa, S.; Hubbard, S.; Pavlidis, D.; Föll, H.

    2003-08-01

    GaN nanocolumns with transverse dimensions of about 50 nm were obtained by illumination-assisted anodic etching of epilayers grown by metalorganic chemical vapor deposition on sapphire substrates. The photoluminescence spectroscopy characterization shows that the as-grown bulk GaN layers suffer from compressive biaxial strain of 0.5 GPa. The majority of nanocolumns are fully relaxed from strain, and the room-temperature luminescence is free excitonic. The high quality of the columnar nanostructures evidenced by the enhanced intensity of the exciton luminescence and by the decrease of the yellow luminescence is explained by the peculiarities of the anodic etching processing.

  1. Determination of satellite valley position in GaN emitter from photoexcited field emission investigations

    NASA Astrophysics Data System (ADS)

    Semenenko, M.; Yilmazoglu, O.; Hartnagel, H. L.; Pavlidis, D.

    2011-01-01

    Argon plasma etched GaN field-emitter rods with nanometer-scale diameter were fabricated on GaN grown on an n+-GaN substrate. Their electron field emission properties were investigated both without and under illumination by using light sources with various wavelengths. The Fowler-Nordheim current-voltage characteristics of the cathodes show a change in slope for illuminated cathodes. The electron affinity difference ΔE between the different valleys in the conduction band has been ascertained and is in the range from 1.18 up to 1.21 eV.

  2. Doping induced structural stability and electronic properties of GaN nanotubes.

    PubMed

    Srivastava, Anurag; Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

    2014-01-01

    The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1-4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend.

  3. Doping Induced Structural Stability and Electronic Properties of GaN Nanotubes

    PubMed Central

    Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

    2014-01-01

    The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1–4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend. PMID:24707225

  4. Nucleation and Growth of GaN on GaAs (001) Substrates

    SciTech Connect

    Drummond, Timothy J.; Hafich, Michael J.; Heller, Edwin J.; Lee, Stephen R.; Liliental-Weber, Zuzanna; Ruvimov, Sergei; Sullivan, John P.

    1999-05-03

    The nucleation of GaN thin films on GaAs is investigated for growth at 620 "C. An rf plasma cell is used to generate chemically active nitrogen from N2. An arsenic flux is used in the first eight monolayer of nitride growth to enhance nucleation of the cubic phase. Subsequent growth does not require an As flux to preserve the cubic phase. The nucleation of smooth interfaces and GaN films with low stacking fault densities is dependent upon relative concentrations of active nitrogen species in the plasma and on the nitrogen to gallium flux ratio.

  5. Optimization of GaN Nanorod Growth Conditions for Coalescence Overgrowth

    DTIC Science & Technology

    2016-02-04

    serves as the catalyst for precipitating GaN below the droplet when its absorption of N atoms reaches the super-saturation condition. In this vapor-liquid...C. Chèze, L. Geelhaar, B. Jenichen, H. Riechert, Different growth rates for catalyst -induced and self-induced GaN nanowires, Appl. Phys. Lett. 97...15] W. Guo, M. Zhang, A. Banerjee, P. Bhattacharya, Catalyst -free InGaN/GaN DISTRIBUTION A: Distribution approved for public release. 9

  6. X-ray detectors based on GaN Schottky diodes

    SciTech Connect

    Duboz, Jean-Yves; Frayssinet, Eric; Chenot, Sebastien; Reverchon, Jean-Luc; Idir, Mourad

    2010-10-18

    GaN Schottky diodes have been fabricated and tested as x-ray detectors in the range from 6 to 21 keV. The spectral response has been measured and is compared to its theoretical value. The study of the response and its temporal dynamics as a function of the bias allows to identify a photovoltaic behavior at low bias and a photoconductive one at larger reverse biases. The GaN diode turned out to be linear as a function of the incident power. The noise and detectivity are given and discussed.

  7. Functionalizing the GaN(0001)-(1 x 1) Surface. I. The Chemisorption of Aniline

    DTIC Science & Technology

    2001-11-01

    tion. Extending this work to wide-bandgap materials such as SiC and GaN offers advantages over the use of Si as a substrate . In optoelectronic devices...the wide bandgap of 6H–SiC (3.0 eV) or GaN (3.4 eV) permits optical access to the organic layer through the substrate as well as through space...occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the organic adsorbate with the substrate band edges. This opens the

  8. Habit control during growth on GaN point seed crystals by Na-flux method

    NASA Astrophysics Data System (ADS)

    Honjo, Masatomo; Imanishi, Masayuki; Imabayashi, Hiroki; Nakamura, Kosuke; Murakami, Kosuke; Matsuo, Daisuke; Maruyama, Mihoko; Imade, Mamoru; Yoshimura, Masashi; Mori, Yusuke

    2017-01-01

    The formation of the pyramidal habit is one of the requirements for the dramatic reduction of dislocations during growth on a tiny GaN seed called a “point seed”. In this study, we focus on controlling the growth habit to form a pyramidal shape in order to reduce the number of dislocations in the c-growth sector during growth on GaN point seeds. High temperature growth was found to change the growth habit from the truncated pyramidal shape to the pyramidal shape. As a result, the number of dislocations in the c-growth sector tended to decrease with increasing growth temperature.

  9. Investigation of structural and optical properties of GaN on flat and porous silicon

    NASA Astrophysics Data System (ADS)

    Abud, Saleh H.; Selman, Abbas M.; Hassan, Z.

    2016-09-01

    In this work, gallium nitride (GaN) layers were successfully grown on Flat-Si and porous silicon (PSi) using a radio frequency-magnetron sputtering system. Field emission scanning electron microscopy and atomic force microscopy images showed that the grown film on Flat-Si had smoother surface, even though there were some cracks on it. Furthermore, the X-ray diffraction measurements showed that the peak intensity of all the grown layers on PSi was higher than that of the grown layer on Flat-Si. Our detailed observation showed that PSi is a promising substrate to obtain GaN films.

  10. Ge doped GaN with controllable high carrier concentration for plasmonic applications

    SciTech Connect

    Kirste, Ronny; Hoffmann, Marc P.; Sachet, Edward; Bobea, Milena; Bryan, Zachary; Bryan, Isaac; Maria, Jon-Paul; Collazo, Ramón; Sitar, Zlatko; Nenstiel, Christian; Hoffmann, Axel

    2013-12-09

    Controllable Ge doping in GaN is demonstrated for carrier concentrations of up to 2.4 × 10{sup 20} cm{sup −3}. Low temperature luminescence spectra from the highly doped samples reveal band gap renormalization and band filling (Burstein-Moss shift) in addition to a sharp transition. Infrared ellipsometry spectra demonstrate the existence of electron plasma with an energy around 3500 cm{sup −1} and a surface plasma with an energy around 2000 cm{sup −1}. These findings open possibilities for the application of highly doped GaN for plasmonic devices.

  11. Acceleration: It's Elementary

    ERIC Educational Resources Information Center

    Willis, Mariam

    2012-01-01

    Acceleration is one tool for providing high-ability students the opportunity to learn something new every day. Some people talk about acceleration as taking a student out of step. In actuality, what one is doing is putting a student in step with the right curriculum. Whole-grade acceleration, also called grade-skipping, usually happens between…

  12. Angular Acceleration without Torque?

    ERIC Educational Resources Information Center

    Kaufman, Richard D.

    2012-01-01

    Hardly. Just as Robert Johns qualitatively describes angular acceleration by an internal force in his article "Acceleration Without Force?" here we will extend the discussion to consider angular acceleration by an internal torque. As we will see, this internal torque is due to an internal force acting at a distance from an instantaneous center.

  13. Accelerated test design

    NASA Technical Reports Server (NTRS)

    Mcdermott, P. P.

    1980-01-01

    The design of an accelerated life test program for electric batteries is discussed. A number of observations and suggestions on the procedures and objectives for conducting an accelerated life test program are presented. Equations based on nonlinear regression analysis for predicting the accelerated life test parameters are discussed.

  14. Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study.

    PubMed

    Qin, Zhenzhen; Qin, Guangzhao; Zuo, Xu; Xiong, Zhihua; Hu, Ming

    2017-03-23

    Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK(-1)) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon-phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon-phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga-N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for

  15. Fiber Accelerating Structures

    SciTech Connect

    Hammond, Andrew P.; /Reed Coll. /SLAC

    2010-08-25

    One of the options for future particle accelerators are photonic band gap (PBG) fiber accelerators. PBG fibers are specially designed optical fibers that use lasers to excite an electric field that is used to accelerate electrons. To improve PBG accelerators, the basic parameters of the fiber were tested to maximize defect size and acceleration. Using the program CUDOS, several accelerating modes were found that maximized these parameters for several wavelengths. The design of multiple defects, similar to having closely bound fibers, was studied to find possible coupling or the change of modes. The amount of coupling was found to be dependent on distance separated. For certain distances accelerating coupled modes were found and examined. In addition, several non-periodic fiber structures were examined using CUDOS. The non-periodic fibers produced several interesting results and promised more modes given time to study them in more detail.

  16. High brightness electron accelerator

    DOEpatents

    Sheffield, Richard L.; Carlsten, Bruce E.; Young, Lloyd M.

    1994-01-01

    A compact high brightness linear accelerator is provided for use, e.g., in a free electron laser. The accelerator has a first plurality of acclerating cavities having end walls with four coupling slots for accelerating electrons to high velocities in the absence of quadrupole fields. A second plurality of cavities receives the high velocity electrons for further acceleration, where each of the second cavities has end walls with two coupling slots for acceleration in the absence of dipole fields. The accelerator also includes a first cavity with an extended length to provide for phase matching the electron beam along the accelerating cavities. A solenoid is provided about the photocathode that emits the electons, where the solenoid is configured to provide a substantially uniform magnetic field over the photocathode surface to minimize emittance of the electons as the electrons enter the first cavity.

  17. Distributed Representations Accelerate Evolution of Adaptive Behaviours

    PubMed Central

    Stone, James V

    2007-01-01

    Animals with rudimentary innate abilities require substantial learning to transform those abilities into useful skills, where a skill can be considered as a set of sensory–motor associations. Using linear neural network models, it is proved that if skills are stored as distributed representations, then within-lifetime learning of part of a skill can induce automatic learning of the remaining parts of that skill. More importantly, it is shown that this “free-lunch” learning (FLL) is responsible for accelerated evolution of skills, when compared with networks which either 1) cannot benefit from FLL or 2) cannot learn. Specifically, it is shown that FLL accelerates the appearance of adaptive behaviour, both in its innate form and as FLL-induced behaviour, and that FLL can accelerate the rate at which learned behaviours become innate. PMID:17676948

  18. Gradual tilting of crystallographic orientation and configuration of dislocations in GaN selectively grown by vapour phase epitaxy methods

    PubMed

    Kuwan; Tsukamoto; Taki; Horibuchi; Oki; Kawaguchi; Shibata; Sawaki; Hiramatsu

    2000-01-01

    Cross-sectional transmission electron microscope (TEM) observation was performed for selectively grown gallium nitride (GaN) in order to examine the dependence of GaN microstructure on the growth conditions. The GaN films were grown by hydride vapour phase epitaxy (HVPE) or metalorganic vapour phase epitaxy (MOVPE) on GaN covered with a patterned mask. Thin foil specimens for TEM observation were prepared with focused ion beam (FIB) machining apparatus. It was demonstrated that the c-axis of GaN grown over the terrace of the mask tilts towards the centre of the terrace when the GaN is grown in a carrier gas of N2. The wider terrace results in a larger tilting angle if other growth conditions are identical. The tilting is attributed to 'horizontal dislocations' (HDs) generated during the overgrowth of GaN on the mask terrace. The HDs in HVPE-GaN have a semi-loop shape and are tangled with one another, while those in MOVPE-GaN are straight and lined up to form low-angle grain boundaries.

  19. Growth optimization and characterization of GaN epilayers on multifaceted (111) surfaces etched on Si(100) substrates

    SciTech Connect

    Ansah-Antwi, KwaDwo Konadu Chua, Soo Jin; Soh, Chew Beng; Liu, Hongfei

    2015-11-15

    The four nearest Si(111) multifaceted sidewalls were exposed inside an array of 3 μm-wide square holes patterned on an Si(100) substrate, and this patterned Si(100) substrate was used as a substrate for the deposition of a gallium nitride (GaN) epilayer. Subsequently the effect that the growth pressure, the etched-hole profiles, and the etched-hole arrangement had upon the quality of the as-grown GaN was investigated. The coalescence of the as-grown GaN epilayer on the exposed Si(111) facets was observed to be enhanced with reduced growth pressure from 120 to 90 Torr. A larger Si(001) plane area at the bottom of the etched holes resulted in bidirectional GaN domains, which resulted in poor material quality. The bidirectional GaN domains were observed as two sets of six peaks via a high-resolution x-ray diffraction phi scan of the GaN(10-11) reflection. It was also shown that a triangular array of etched holes was more desirable than square arrays of etched holes for the growth high-quality and continuous GaN films.

  20. Yi-gan san restores behavioral alterations and a decrease of brain glutathione level in a mouse model of schizophrenia.

    PubMed

    Makinodan, Manabu; Yamauchi, Takahira; Tatsumi, Kouko; Okuda, Hiroaki; Noriyama, Yoshinobu; Sadamatsu, Miyuki; Kishimoto, Toshifumi; Wanaka, Akio

    2009-01-01

    The traditional Chinese herbal medicine yi-gan san has been used to cure neuropsychological disorders. Schizophrenia can be one of the target diseases of yi-gan san. We aimed at evaluating the possible use of yi-gan san in improving the schizophrenic symptoms of an animal model. Yi-gan san or distilled water was administered to mice born from pregnant mice injected with polyinosinic-polycytidilic acid or phosphate buffered saline. The former is a model of schizophrenia based on the epidemiological data that maternal infection leads to psychotic disorders including schizophrenia in the offspring. Prepulse inhibition and sensitivity to methamphetamine in open field tests were analyzed and the total glutathione content of whole brains was measured. Yi-gan san reversed the decrease in prepulse inhibition, hypersensitivity to methamphetamine and cognitive deficits found in the model mice to the level of control mice. Total glutathione content in whole brains was reduced in the model mice but was restored to normal levels by yi-gan san treatment. These results suggest that yi-gan san may have ameliorating effects on the pathological symptoms of schizophrenia.

  1. Nobel Lecture: Growth of GaN on sapphire via low-temperature deposited buffer layer and realization of p -type GaN by Mg doping followed by low-energy electron beam irradiation*

    NASA Astrophysics Data System (ADS)

    Amano, Hiroshi

    2015-10-01

    This is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smart television and display systems using blue LEDs. The most important work was done in the mid to late 1980s. The background to the author's work and the process by which the technology enabling the growth of GaN and the realization of p -type GaN was established are reviewed.

  2. Growth of GaN on Sapphire via Low-Temperature Deposited Buffer Layer and Realization of p-Type GaN by Mg Doping Followed by Low-Energy Electron Beam Irradiation

    NASA Astrophysics Data System (ADS)

    Amano, Hiroshi

    2015-12-01

    This is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smart television and display systems using blue LEDs. The most important work was done in the mid- to late 80s. The background to the author's work and the process by which the technology enabling the growth of GaN and the realization of p-type GaN was established are reviewed.

  3. Acceleration in astrophysics

    SciTech Connect

    Colgate, S.A.

    1993-12-31

    The origin of cosmic rays and applicable laboratory experiments are discussed. Some of the problems of shock acceleration for the production of cosmic rays are discussed in the context of astrophysical conditions. These are: The presumed unique explanation of the power law spectrum is shown instead to be a universal property of all lossy accelerators; the extraordinary isotropy of cosmic rays and the limited diffusion distances implied by supernova induced shock acceleration requires a more frequent and space-filling source than supernovae; the near perfect adiabaticity of strong hydromagnetic turbulence necessary for reflecting the accelerated particles each doubling in energy roughly 10{sup 5} to {sup 6} scatterings with negligible energy loss seems most unlikely; the evidence for acceleration due to quasi-parallel heliosphere shocks is weak. There is small evidence for the expected strong hydromagnetic turbulence, and instead, only a small number of particles accelerate after only a few shock traversals; the acceleration of electrons in the same collisionless shock that accelerates ions is difficult to reconcile with the theoretical picture of strong hydromagnetic turbulence that reflects the ions. The hydromagnetic turbulence will appear adiabatic to the electrons at their much higher Larmor frequency and so the electrons should not be scattered incoherently as they must be for acceleration. Therefore the electrons must be accelerated by a different mechanism. This is unsatisfactory, because wherever electrons are accelerated these sites, observed in radio emission, may accelerate ions more favorably. The acceleration is coherent provided the reconnection is coherent, in which case the total flux, as for example of collimated radio sources, predicts single charge accelerated energies much greater than observed.

  4. Effect of residual stress on the microstructure of GaN epitaxial films grown by pulsed laser deposition

    NASA Astrophysics Data System (ADS)

    Wang, Haiyan; Wang, Wenliang; Yang, Weijia; Zhu, Yunnong; Lin, Zhiting; Li, Guoqiang

    2016-04-01

    The stress-free GaN epitaxial films have been directly grown by pulsed laser deposition (PLD) at 850 °C, and the effect of different stress on the microstructure of as-grown GaN epitaxial films has been explored in detail. The as-grown stress-free GaN epitaxial films exhibit very smooth surface without any particles and grains, which is confirmed by the smallest surface root-mean-square roughness of 2.3 nm measured by atomic force microscopy. In addition, they also have relatively high crystalline quality, which is proved by the small full-width at half maximum values of GaN(0002) and GaN (10 1 bar 2) X-ray rocking curves as 0.27° and 0.68°, respectively. However, when the growth temperature is lower or higher than 850 °C, internal or thermal stress would be increased in as-grown GaN epitaxial films. To release the larger stress, a great number of dislocations are generated. Many irregular particulates, hexagonal GaN gains and pits are therefore produced on the films surface, and the crystalline quality is greatly reduced consequently. This work has demonstrated the direct growth of stress-free GaN epitaxial films with excellent surface morphology and high crystalline quality by PLD, and presented a comprehensive study on the origins and the effect of stress in GaN layer. It is instructional to achieve high-quality nitride films by PLD, and shows great potential and broad prospect for the further development of high-performance GaN-based devices.

  5. The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning

    NASA Astrophysics Data System (ADS)

    Wang, Ding; Zhang, Minglu; Zhuang, Huaijuan; Chen, Xu; Wang, Xianying; Zheng, Xuejun; Yang, Junhe

    2017-02-01

    (GaN)1-x(ZnO)x composite nanofibers with hollow structure were prepared by initial electrospinning, and the subsequent calcination and nitridation. The structure and morphology characteristics of samples were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The characterization results showed the phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x solid-solution under ammonia atmosphere. The preparation conditions were explored and the optimum nitridation temperature and holding time are 750 °C and 2 h, respectively. The photocatalytic properties of (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios were investigated by degrading Rhodamine B under the visible light irradiation. The photocatalytic activity sequence is (GaN)1-x(ZnO)x (Ga:Zn = 1:2) > (GaN)1-x(ZnO)x (Ga:Zn = 1:3) > ZnO nanofibers > (GaN)1-x(ZnO)x (Ga:Zn = 1:4) > (GaN)1-x(ZnO)x (Ga:Zn = 1:1). The photocatalytic mechanism of the (GaN)1-x(ZnO)x hollow nanofibers was further studied by UV-vis diffuse reflectance spectra. The excellent photocatalytic performance of (GaN)1-x(ZnO)x hollow nanofibers was attributed to the narrow band gap and high surface area of porous nanofibers with hollow structure.

  6. MOCVD of BN and GaN thin films on silicon: new attempt of GaN growth with BN buffer layer

    NASA Astrophysics Data System (ADS)

    Boo, Jin-Hyo; Rohr, Carsten; Ho, Wilson

    1998-06-01

    Highly oriented polycrystalline h-BN thin films were deposited on silicon substrates in the temperature range of 600-900°C from the single molecular precursor of borane-triethylamine complex, (C 2H 5) 3N : BH 3, by supersonic jet assisted chemical vapor deposition. Hydrogen was used as carrier gas, and additional nitrogen was supplied by either ammonia through a nozzle or nitrogen via a remote microwave plasma. Hexagonal GaN films were also grown on Si(1 0 0) with h-BN buffer layers at temperatures between 550 and 750°C with dual supersonic molecular beam sources. Triethylgallium, (C 2H 5) 3Ga, and ammonia, NH 3, were used as precursors. Hydrogen was used as seeding gas for the precursors, providing a wide range of possible kinetic energies for the supersonic beams. The h-BN buffer layers and the GaN films were characterized in situ by Auger electron spectroscopy (AES), and ex situ by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and optical transmission. This is the first report of growing the h-BN films on silicon substrates from the single source precursor of borane-triethylamine complex and new attempts of GaN film growth on silicon with BN buffer layer.

  7. Ultraviolet InGaN and GaN Single-Quantum-Well-Structure Light-Emitting Diodes Grown on Epitaxially Laterally Overgrown GaN Substrates

    NASA Astrophysics Data System (ADS)

    Mukai, Takashi; Nakamura, Shuji

    1999-10-01

    Ultraviolet (UV) InGaN and GaN single-quantum-well-structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only under high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG even under low-and high-current operations, due to the lack of localized energy states formed by alloy composition fluctuations. When the emission wavelengths were in the blue and green regions, EQE was almost the same between LEDs on both ELOG and sapphire due to a large number of deep localized energy states formed by large alloy composition fluctuations. The localized energy states are responsible for the high efficiency of InGaN-based LEDs in spite of a large number of dislocations.

  8. Accelerating Climate Simulations Through Hybrid Computing

    NASA Technical Reports Server (NTRS)

    Zhou, Shujia; Sinno, Scott; Cruz, Carlos; Purcell, Mark

    2009-01-01

    Unconventional multi-core processors (e.g., IBM Cell B/E and NYIDIDA GPU) have emerged as accelerators in climate simulation. However, climate models typically run on parallel computers with conventional processors (e.g., Intel and AMD) using MPI. Connecting accelerators to this architecture efficiently and easily becomes a critical issue. When using MPI for connection, we identified two challenges: (1) identical MPI implementation is required in both systems, and; (2) existing MPI code must be modified to accommodate the accelerators. In response, we have extended and deployed IBM Dynamic Application Virtualization (DAV) in a hybrid computing prototype system (one blade with two Intel quad-core processors, two IBM QS22 Cell blades, connected with Infiniband), allowing for seamlessly offloading compute-intensive functions to remote, heterogeneous accelerators in a scalable, load-balanced manner. Currently, a climate solar radiation model running with multiple MPI processes has been offloaded to multiple Cell blades with approx.10% network overhead.

  9. An exact accelerated stochastic simulation algorithm.

    PubMed

    Mjolsness, Eric; Orendorff, David; Chatelain, Philippe; Koumoutsakos, Petros

    2009-04-14

    An exact method for stochastic simulation of chemical reaction networks, which accelerates the stochastic simulation algorithm (SSA), is proposed. The present "ER-leap" algorithm is derived from analytic upper and lower bounds on the multireaction probabilities sampled by SSA, together with rejection sampling and an adaptive multiplicity for reactions. The algorithm is tested on a number of well-quantified reaction networks and is found experimentally to be very accurate on test problems including a chaotic reaction network. At the same time ER-leap offers a substantial speedup over SSA with a simulation time proportional to the 23 power of the number of reaction events in a Galton-Watson process.

  10. Utilisation of GaN and InGaN/GaN with nanoporous structures for water splitting

    SciTech Connect

    Benton, J.; Bai, J.; Wang, T.

    2014-12-01

    We report a cost-effective approach to the fabrication of GaN based nanoporous structure for applications in renewable hydrogen production. Photoelectrochemical etching in a KOH solution has been employed to fabricate both GaN and InGaN/GaN nanoporous structures with pore sizes ranging from 25 to 60 nm, obtained by controlling both etchant concentration and applied voltage. Compared to as-grown planar devices the nanoporous structures have exhibited a significant increase of photocurrent with a factor of up to four times. An incident photon conversion efficiency of up to 46% around the band edge of GaN has been achieved.

  11. Combined atomic force microscopy and scanning tunneling microscopy imaging of cross-sectioned GaN light-emitting diodes.

    PubMed

    Bender, J W; Salmon, M E; Russell, P E

    2003-01-01

    Cross-sectional scanning tunneling microscopy (STM) was combined with atomic force microscopy (AFM) over the same area to characterize a cross-sectioned GaN light emitting diode. Because GaN is typically grown on a non-native substrate and also forms a wurtzite crystal structure, a cryogenic cleaving technique was developed to generate smooth surfaces. The depletion region surrounding the p-n junction was clearly identified using STM. Furthermore, by imaging under multiple sample biases, distinctions between the n-doped and p-doped GaN could be made.

  12. Growth of low-threading-dislocation-density GaN on graphene by hydride vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    He, Shunyu; Xu, Yu; Qi, Lin; Li, Zongyao; Cao, Bing; Wang, Chinhua; Zhang, Jicai; Wang, Jianfeng; Xu, Ke

    2017-03-01

    Recently, gallium nitride (GaN) films grown on graphene have been widely studied. Here, we have grown low-threading-dislocation-density GaN films on graphene by hydride vapor phase epitaxy (HVPE). The full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) of the GaN films were 276 and 350 arcsec at the 0002 and 10\\bar{1}2 reflections, respectively. This shows that the threading dislocation densities are on the order of magnitude of 108 cm‑2, which is consistent with the results of cathodoluminescence (CL).

  13. Simulation of electrical characteristics of GaN vertical Schottky diodes

    NASA Astrophysics Data System (ADS)

    Łukasiak, Lidia; Jasiński, Jakub; Jakubowski, Andrzej

    2016-12-01

    Reverse current of GaN vertical Schottky diodes is simulated using Silvaco ATLAS to optimize the geometry for the best performance. Several physical quantities and phenomena, such as carrier mobility and tunneling mechanism are studied to select the most realistic models. Breakdown voltage is qualitatively estimated based on the maximum electric field in the structure.

  14. Cathodoluminescence of GaN nanorods and nanowires grown by thermal evaporation

    NASA Astrophysics Data System (ADS)

    Guzmán, G.; Herrera, M.

    2014-02-01

    GaN nanorods and nanowires have been grown by thermal evaporation of GaN on Au/Si (1 0 0) substrates. The nanorods recorded a surface decorated with numerous grains with an average size of about 100 nm. The nanowires grew onto the surface of the nanorods exhibiting multiple bends along them. TEM measurements revealed the formation of irregular porous and a polycrystalline structure in the nanowires with diameter higher than 100 nm, while the nanowires with lower diameter showed a tubular structure with wall thickness of 10 nm. The luminescence of the samples recorded three bands centered at about 2.1, 2.74, and 3.2 eV, attributed to the GaN yellow emission and to the blue and UV emissions of the β-Ga2O3, respectively. Ga-ion irradiation in samples revealed a decrease in the intensity of the β-Ga2O3 blue emission attributed to the elimination of gallium vacancies. A thermal annealing treatment at 800 °C in N2 atmosphere generated a quenching of the GaN yellow emission, due to the elimination of nitrogen vacancies.

  15. Electrical properties of Si-doped GaN prepared using pulsed sputtering

    NASA Astrophysics Data System (ADS)

    Arakawa, Yasuaki; Ueno, Kohei; Imabeppu, Hideyuki; Kobayashi, Atsushi; Ohta, Jitsuo; Fujioka, Hiroshi

    2017-01-01

    In this study, we investigated the basic electrical properties of Si-doped wurtzite GaN films prepared using a low-temperature pulsed sputtering deposition (PSD) process. We found that the electron concentration can be controlled in the range between 1.5 × 1016 and 2.0 × 1020 cm-3. For lightly Si-doped GaN ([Si] = 2.1 × 1016 cm-3), the room temperature (RT) electron mobility was as high as 1008 cm2 V-1 s-1, which was dominantly limited by polar optical phonon scattering. Moreover, we found that heavily Si-doped GaN prepared using PSD exhibited an RT mobility as high as 110 cm2 V-1 s-1 at an electron concentration of 2 × 1020 cm-3, which indicated that the resistivity of this film was almost as small as those of typical transparent conductive oxides such as indium tin oxide. At lower temperatures, the electron mobility increased to 1920 cm2 V-1 s-1 at 136 K, and the temperature dependence was well explained by conventional scattering models. These results indicate that Si-doped GaN prepared using PSD is promising not only for the fabrication of GaN-based power devices but also for use as epitaxial transparent electrode materials for nitride based optical devices.

  16. Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

    SciTech Connect

    Verma, Jai; Kandaswamy, Prem Kumar; Protasenko, Vladimir; Verma, Amit; Grace Xing, Huili; Jena, Debdeep

    2013-01-28

    We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.

  17. GaN nano-pyramid arrays as an efficient photoelectrode for solar water splitting

    NASA Astrophysics Data System (ADS)

    Hou, Y.; Yu, X.; Syed, Z. Ahmed; Shen, S.; Bai, J.; Wang, T.

    2016-11-01

    A prototype photoelectrode has been fabricated using a GaN nano-pyramid array structure grown on a cost-effective Si (111) substrate, demonstrating a significant improvement in performance of solar-powered water splitting compared with any planar GaN photoelectrode. Such a nano-pyramid structure leads to enhanced optical absorption as a result of a multi-scattering process which can effectively produce a reduction in reflectance. A simulation based on a finite-difference time-domain approach indicates that the nano-pyramid architecture enables incident light to be concentrated within the nano-pyramids as a result of micro-cavity effects, further enhancing optical absorption. Furthermore, the shape of the nano-pyramid further facilitates the photo-generated carrier transportation by enhancing a hole-transfer efficiency. All these features as a result of the nano-pyramid configuration lead to a large photocurrent of 1 mA cm-2 under an illumination density of 200 mW cm-2, with a peak incident photon-to-current conversion efficiency of 46.5% at ˜365 nm, around the band edge emission wavelength of GaN. The results presented are expected to pave the way for the fabrication of GaN based photoelectrodes with a high energy conversion efficiency of solar powered water splitting.

  18. GaN nanostructure-based light emitting diodes and semiconductor lasers.

    PubMed

    Viswanath, Annamraju Kasi

    2014-02-01

    GaN and related materials have received a lot of attention because of their applications in a number of semiconductor devices such as LEDs, laser diodes, field effect transistors, photodetectors etc. An introduction to optical phenomena in semiconductors, light emission in p-n junctions, evolution of LED technology, bandgaps of various semiconductors that are suitable for the development of LEDs are discussed first. The detailed discussion on photoluminescence of GaN nanostructures is made, since this is crucial to develop optical devices. Fabrication technology of many nanostructures of GaN such as nanowires, nanorods, nanodots, nanoparticles, nanofilms and their luminescence properties are given. Then the optical processes including ultrafast phenomena, radiative, non-radiative recombination, quantum efficiency, lifetimes of excitons in InGaN quantum well are described. The LED structures based on InGaN that give various important colors of red, blue, green, and their design considerations to optimize the output were highlighted. The recent efforts in GaN technology are updated. Finally the present challenges and future directions in this field are also pointed out.

  19. Mechanism and crucial parameters on GaN nanocluster formation in a silica matrix

    NASA Astrophysics Data System (ADS)

    Kioseoglou, J.; Katsikini, M.; Termentzidis, K.; Karakostas, I.; Paloura, E. C.

    2017-02-01

    The formation of wurtzite GaN nanoclusters in an amorphous silica matrix, via gallium and nitrogen ion implantation and rapid thermal annealing, is identified using Extended X Ray Absorption Fine Structure analysis. The mechanism and the crucial parameters that rule the formation of the nanoclusters are established by the use of molecular dynamics simulations. The dominant structural parameters are found to be the concentration of the silicon and oxygen vacancies that are formed during the implantation and the annealing temperature. It is concluded that annealing at 1400 K and 8% Ga/Si and 12% N/O ratios are needed for the formation of GaN nanoclusters. In addition to that, the GaN nanocluster formation is accomplished only when the vacancy concentrations of silicon and oxygen atoms are equal to 10% and 20%, respectively. Finally, the observation of various snapshots upon an increase of the annealing duration indicates the coalescence of smaller GaN nuclei towards larger ones, designating that the Ostwald ripening is a dominant mechanism.

  20. Uniaxial strain effects on the optoelectronic properties of GaN nanowires

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Kong, Yike; Wang, Meishan

    2016-09-01

    Considering the importance of strain engineering on semiconductors, GaN nanowires under uniaxial compression deformation and stretch deformation are researched using first principle calculations with density functional theory. It is found that the deformation will destroy the stability of the nanowires except a weak stretch. The compression deformation is more difficult than the stretch deformation. Besides, the work function of the nanowires is reduced under increasing compression while that under increasing stretch is reversed. With increasing diameter, the band gaps of the nanowires gradually exhibit a linear decreasing relation as the elongation of uniaxial length of GaN nanowires. With increasing compression, the band gaps change from direct to indirect. The optical calculations exhibit a redshift for the imaginary part of dielectric function. This study demonstrates strain engineering can effectively adjust the optoelectronic characteristics of GaN nanowire. Moderate compression, which induces a lower work function with a direct band gap, can improve the photoemission performance of GaN nanowires.

  1. GaN Haeckelite Single-Layered Nanostructures: Monolayer and Nanotubes

    PubMed Central

    Camacho-Mojica, Dulce C.; López-Urías, Florentino

    2015-01-01

    Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube´s chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures. PMID:26658148

  2. Evaluation of the influence mode on the CVC GaN HEMT using numerical modeling

    NASA Astrophysics Data System (ADS)

    Parnes, Ya M.; Tikhomirov, V. G.; Petrov, V. A.; Gudkov, A. G.; Marzhanovskiy, I. N.; Kukhareva, E. S.; Vyuginov, V. N.; Volkov, V. V.; Zybin, A. A.

    2016-08-01

    Done numerically simulated the effects of certain modes of operation on the CVC of field microwave transistors on the basis of heterostructures AlGaN / GaN (HEMT). The results of these studies suggest the possibility of quite efficient use of numerical simulation for the development of HEMT microwave transistors allowing for the real instrument designs.

  3. Fine structure of the red luminescence band in undoped GaN

    SciTech Connect

    Reshchikov, M. A.; Usikov, A.; Helava, H.; Makarov, Yu.

    2014-01-20

    Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RL band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.

  4. GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium

    SciTech Connect

    Bartram, Michael E.; Creighton, J. Randall

    1999-05-26

    Isotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N15H3 did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N2 of mixed isotopes below the onset of GaN sublimation, This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence G-aN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia.

  5. GaN IMPATT diode: a photo-sensitive high power terahertz source

    NASA Astrophysics Data System (ADS)

    Mukherjee, Moumita; Mazumder, Nilraton; Roy, Sitesh Kumar; Goswami, Kushalendu

    2007-12-01

    The prospects of wurtzite phase single-drift-region (SDR), flat and single-low-high-low (SLHL) type GaN IMPATT devices as terahertz sources are studied through a simulation experiment. The study indicates that GaN IMPATT diodes are capable of generating high RF power (at least 2.5 W) at around 1.45 THz with high efficiency (17-20%). The superior electronic properties of GaN make this a promising candidate for IMPATT operation in the THz regime, unapproachable by conventional Si, GaAs and InP based IMPATT diodes. The effect of parasitic series resistance on the THz performance of the device is further simulated. It is interesting to note that the presence of a charge bump in a flatly doped SDR structure reduces the value of parasitic series resistance by 22%. The effects of photo- illumination on the devices are also investigated using a modified double iterative simulation technique. Under photo-illumination (i) the negative conductance and (ii) the negative resistance of the devices (both flat and SLHL) decrease, while the frequency of operation and the device quality factor shift upwards. However, the upward shift in operating frequency is found to be more (~16 GHz) in the case of the SLHL SDR IMPATT device. The study indicates that GaN IMPATT is a promising opto-sensitive high power THz source.

  6. Fabrications and application of single crystalline GaN for high-performance deep UV photodetectors

    NASA Astrophysics Data System (ADS)

    Velazquez, R.; Aldalbahi, A.; Rivera, M.; Feng, P.

    2016-08-01

    High-quality single crystalline Gallium Nitride (GaN) semiconductor has been synthesized using molecule beam epitaxy (MBE) technique for development of high-performance deep ultraviolet (UV) photodetectors. Thickness of the films was estimated by using surface profile meter and scanning electron microscope. Electronic states and elemental composition of the films were obtained using Raman scattering spectroscopy. The orientation, crystal structure and phase purity of the films were examined using a Siemens x-ray diffractometer radiation. The surface microstructure was studied using high resolution scanning electron microscopy (SEM). Two types of metal pairs: Al-Al, Al-Cu or Cu-Cu were used for interdigital electrodes on GaN film in order to examine the Schottky properties of the GaN based photodetector. The characterizations of the fabricated prototype include the stability, responsivity, response and recovery times. Typical time dependent photoresponsivity by switching different UV light source on and off five times for each 240 seconds at a bias of 2V, respectively, have been obtained. The detector appears to be highly sensitive to various UV wavelengths of light with very stable baseline and repeatability. The obtained photoresponsivity was up to 354 mA/W at the bias 2V. Higher photoresponsivity could be obtained if higher bias was applied but it would unavoidably result in a higher dark current. Thermal effect on the fabricated GaN based prototype was discussed.

  7. Diffusion mechanism and the thermal stability of fluorine ions in GaN after ion implantation

    SciTech Connect

    Wang, M. J.; Yuan, L.; Chen, K. J.; Xu, F. J.; Shen, B.

    2009-04-15

    The diffusion mechanisms of fluorine ions in GaN are investigated by means of time-of-flight secondary ion mass spectrometry. Instead of incorporating fluorine ions close to the sample surface by fluorine plasma treatment, fluorine ion implantation with an energy of 180 keV is utilized to implant fluorine ions deep into the GaN bulk, preventing the surface effects from affecting the data analysis. It is found that the diffusion of fluorine ions in GaN is a dynamic process featuring an initial out-diffusion followed by in- diffusion and the final stabilization. A vacancy-assisted diffusion model is proposed to account for the experimental observations, which is also consistent with results on molecular dynamic simulation. Fluorine ions tend to occupy Ga vacancies induced by ion implantation and diffuse to vacancy rich regions. The number of continuous vacancy chains can be significantly reduced by a dynamic thermal annealing process. As a result, strong local confinement and stabilization of fluorine ions can be obtained in GaN crystal, suggesting excellent thermal stability of fluorine ions for device applications.

  8. Incorporation of Mg in Free-Standing HVPE GaN Substrates

    NASA Astrophysics Data System (ADS)

    Zvanut, M. E.; Dashdorj, J.; Freitas, J. A.; Glaser, E. R.; Willoughby, W. R.; Leach, J. H.; Udwary, K.

    2016-06-01

    Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm-3 and 1019 cm-3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor-shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm-3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

  9. GaN nano-pyramid arrays as an efficient photoelectrode for solar water splitting.

    PubMed

    Hou, Y; Yu, X; Syed, Z Ahmed; Shen, S; Bai, J; Wang, T

    2016-11-11

    A prototype photoelectrode has been fabricated using a GaN nano-pyramid array structure grown on a cost-effective Si (111) substrate, demonstrating a significant improvement in performance of solar-powered water splitting compared with any planar GaN photoelectrode. Such a nano-pyramid structure leads to enhanced optical absorption as a result of a multi-scattering process which can effectively produce a reduction in reflectance. A simulation based on a finite-difference time-domain approach indicates that the nano-pyramid architecture enables incident light to be concentrated within the nano-pyramids as a result of micro-cavity effects, further enhancing optical absorption. Furthermore, the shape of the nano-pyramid further facilitates the photo-generated carrier transportation by enhancing a hole-transfer efficiency. All these features as a result of the nano-pyramid configuration lead to a large photocurrent of 1 mA cm(-2) under an illumination density of 200 mW cm(-2), with a peak incident photon-to-current conversion efficiency of 46.5% at ∼365 nm, around the band edge emission wavelength of GaN. The results presented are expected to pave the way for the fabrication of GaN based photoelectrodes with a high energy conversion efficiency of solar powered water splitting.

  10. Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

    SciTech Connect

    Bajaj, Sanyam Shoron, Omor F.; Park, Pil Sung; Krishnamoorthy, Sriram; Akyol, Fatih; Hung, Ting-Hsiang; Reza, Shahed; Chumbes, Eduardo M.; Khurgin, Jacob; Rajan, Siddharth

    2015-10-12

    We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors (HEMTs). Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 × 10{sup 7 }cm/s at a low sheet charge density of 7.8 × 10{sup 11 }cm{sup −2}. An optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated longitudinal optical (LO) phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

  11. Photoluminescence Observation of GaN Thin Films Treated by Inductively-Coupled Plasmas

    NASA Astrophysics Data System (ADS)

    Nakamura, Keiji; Itoh, Noriyoshi; Nakano, Yoshitaka; Sugai, Hideo

    2011-10-01

    This paper reports observations of photoluminescence from plasma-treated GaN thin films. A 10 mTorr Ar ICP was used, and irradiation of 313 nm ultraviolet (UV) light from Hg-Xe light source induced the photoluminescence of the GaN film. In both in-situ and ex-situ observations, significant yellow luminescence was observed visually, and the ex-situ observed luminescence ranges in a wavelength of 500-800 nm corresponding to defect-states-related transition. The measurements also revealed that the luminescence also contains UV emission at a wavelength of ~365 nm attributed to transition related to near band edges. In order to examine effects of the plasma on the luminescence, the ex-situ observation was made as a function of the plasma treatment time. As the treatment time increased, both the UV and the luminescence intensity decreased, and the decrease in the emission became significant when the 313 nm UV light was irradiated onto the plasma-exposed GaN surface. These results suggested that plasma-induced defect formation leads to the luminescence degradation, and that the photoluminescence observation will be useful for damage monitoring of the GaN surface. This work is partly supported by the 2nd stage Knowledge Cluster Initiative and Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

  12. Fabrication of non-polar GaN based highly responsive and fast UV photodetector

    NASA Astrophysics Data System (ADS)

    Gundimeda, Abhiram; Krishna, Shibin; Aggarwal, Neha; Sharma, Alka; Sharma, Nita Dilawar; Maurya, K. K.; Husale, Sudhir; Gupta, Govind

    2017-03-01

    We report the fabrication of ultraviolet photodetector on non-polar (11-20), nearly stress free, Gallium Nitride (GaN) film epitaxially grown on r-plane (1-102) sapphire substrate. High crystalline film leads to the formation of two faceted triangular islands like structures on the surface. The fabricated GaN ultraviolet photodetector exhibited a high responsivity of 340 mA/W at 5 V bias at room temperature which is the best performance reported for a-GaN/r-sapphire films. A detectivity of 1.24 × 109 Jones and noise equivalent power of 2.4 × 10-11 WHz-1/2 were also attained. The rise time and decay time of 280 ms and 450 ms have been calculated, respectively, which were the fastest response times reported for non-polar GaN ultraviolet photodetector. Such high performance devices substantiate that non-polar GaN can serve as an excellent photoconductive material for ultraviolet photodetector based applications.

  13. Design and analysis of vertical-channel gallium nitride (GaN) junctionless nanowire transistors (JNT).

    PubMed

    Seo, Jae Hwa; Yoon, Young Jun; Lee, Hwan Gi; Yoo, Gwan Min; Jo, Young-Woo; Son, Dong-Hyeok; Lee, Jung-Hee; Cho, Eou-Sik; Cho, Seongjae; Kang, In Man

    2014-11-01

    Vertical-channel gallium nitride (GaN) junctionless nanowire transistor (JNT) has been designed and characterized by technology computer-aided design (TCAD) simulations. Various characteristics such as wide bandgap, strong polariztion field, and high electron velocity make GaN one of the attractive materials in advanced electronics in recent times. Nanowire-structured GaN can be applicable to various transistors for enhanced electrical performances by its geometrical feature. In this paper, we analyze the direct-current (DC) characteristics depending on various channel doping concentrations (N(ch)) and nanowire radii (R(NW)). Furthermore, the radio-frequency (RF) characteristics under optimized conditions are extracted by small-signal equivalent circuit modeling. For the optimally designed vertical GaN JNT demonstrated on-state current (I(on)) of 345 μA/μm and off-state current (I(off)) of 3.7 x 10(-18) A/μm with a threshold voltage (V(t)) of 0.22 V, and subthreshold swing (S) of 68 mV/dec. Besides, f(T) and f(max) under different operating conditions (gate voltage, V(GS)) have been obtained.

  14. The controlled growth of GaN microrods on Si(111) substrates by MOCVD

    NASA Astrophysics Data System (ADS)

    Foltynski, Bartosz; Garro, Nuria; Vallo, Martin; Finken, Matthias; Giesen, Christoph; Kalisch, Holger; Vescan, Andrei; Cantarero, Andrés; Heuken, Michael

    2015-03-01

    In this paper, a selective area growth (SAG) approach for growing GaN microrods on patterned SiNx/Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) is studied. The surface morphology, optical and structural properties of vertical GaN microrods terminated by pyramidal shaped facets (six { 10 1 bar 1} planes) were characterized using scanning electron microscopy (SEM), room temperature photoluminescence (PL) and Raman spectroscopy, respectively. Measurements revealed high-quality GaN microcolumns grown with silane support. Characterized structures were grown nearly strain-free (central frequency of Raman peak of 567±1 cm-1) with crystal quality comparable to bulk crystals (FWHM=4.2±1 cm-1). Such GaN microrods might be used as a next-generation device concept for solid-state lighting (SSL) applications by realizing core-shell InGaN/GaN multi-quantum wells (MQWs) on the n-GaN rod base.

  15. Ultrathin GaN quantum disk nanowire LEDs with sub-250 nm electroluminescence

    DOE PAGES

    Chisholm, Matthew F.; Golam Sarwar, A. T. M.; Myers, Roberto C.; ...

    2016-03-18

    By quantum confining GaN at monolayer thickness with AlN barriers inside of a nanowire, deep ultraviolet LEDs are demonstrated. Full three-dimensional strain dependent energy band simulations are carried out within multiple quantum disk (MQD) GaN/AlN nanowire superlattice heterostructures. It is found that, even within the same nanowire MQD, the emission energy of the ultrathin GaN QDs varies from disk to disk due to the changing strain distribution and polarization charge induced energy band bending along the axial nanowire direction. MQD heterostructures are grown by plasma-assisted molecular beam epitaxy to form self-assembled catalyst-free nanowires with 1 to 2 monolayer thick GaNmore » insertions within an AlN matrix. Photoluminescence peaks are observed at 295 nm and 283 nm from the 2 ML and 1 ML thick MQD samples, respectively. Polarization-doped nanowire LEDs are grown incorporating 1 ML thick GaN MQD active regions from which we observe deep ultraviolet electroluminescence. As a result, the shortest LED wavelength peak observed is 240 nm and attributed to electron hole recombination within 1 ML thick GaN QDs.« less

  16. An introduction to acceleration mechanisms

    SciTech Connect

    Palmer, R.B.

    1987-05-01

    This paper discusses the acceleration of charged particles by electromagnetic fields, i.e., by fields that are produced by the motion of other charged particles driven by some power source. The mechanisms that are discussed include: Ponderamotive Forces, Acceleration, Plasma Beat Wave Acceleration, Inverse Free Electron Laser Acceleration, Inverse Cerenkov Acceleration, Gravity Acceleration, 2D Linac Acceleration and Conventional Iris Loaded Linac Structure Acceleration. (LSP)

  17. High Efficiency m-plane LEDs on Low Defect Density Bulk GaN Substrates

    SciTech Connect

    David, Aurelien

    2012-10-15

    Solid-state lighting is a key technology for reduction of energy consumption in the US and worldwide. In principle, by replacing standard incandescent bulbs and other light sources with sources based on light-emitting diodes (LEDs), ultimate energy efficiency can be achieved. The efficiency of LEDs has improved tremendously over the past two decades, however further progress is required for solid- state lighting to reach its full potential. The ability of an LED at converting electricity to light is quantified by its internal quantum efficiency (IQE). The material of choice for visible LEDs is Gallium Nitride (GaN), which is at the basis of blue-emitting LEDs. A key factor limiting the performance of GaN LEDs is the so-called efficiency droop, whereby the IQE of the LED decreases significantly at high current density. Despite decades of research, efficiency droop remains a major issue. Since high-current operation is necessary for practical lighting applications, reducing droop is a major challenge for the scientific community and the LED industry. Our approach to solving the droop issue is the use of newly available low-defect-density bulk GaN non-polar substrates. In contrast to the standard foreign substrates (sapphire, silicon carbide, silicon) used in the industry, we have employed native bulk GaN substrates with very low defect density, thus ensuring exquisite material quality and high IQE. Whereas all commercial LEDs are grown along the c-plane crystal direction of GaN, we have used m-plane non-polar substrates; these drastically modify the physical properties of the LED and enable a reduction of droop. With this approach, we have demonstrated very high IQE performance and low droop. Our results focused on violet and blue LEDs. For these, we have demonstrated very high peak IQEs and current droops of 6% and 10% respectively (up to a high current density of 200A.cm-2). All these results were obtained under electrical operation. These high IQE and low droop

  18. Schooling in Times of Acceleration

    ERIC Educational Resources Information Center

    Buddeberg, Magdalena; Hornberg, Sabine

    2017-01-01

    Modern societies are characterised by forms of acceleration, which influence social processes. Sociologist Hartmut Rosa has systematised temporal structures by focusing on three categories of social acceleration: technical acceleration, acceleration of social change, and acceleration of the pace of life. All three processes of acceleration are…

  19. Uniformly accelerated black holes

    NASA Astrophysics Data System (ADS)

    Letelier, Patricio S.; Oliveira, Samuel R.

    2001-09-01

    The static and stationary C metric are examined in a generic framework and their interpretations studied in some detail, especially those with two event horizons, one for the black hole and another for the acceleration. We find that (i) the spacetime of an accelerated static black hole is plagued by either conical singularities or a lack of smoothness and compactness of the black hole horizon, (ii) by using standard black hole thermodynamics we show that accelerated black holes have a higher Hawking temperature than Unruh temperature of the accelerated frame, and (iii) the usual upper bound on the product of the mass and acceleration parameters (<1/27) is just a coordinate artifact. The main results are extended to accelerated rotating black holes with no significant changes.

  20. The Dielectric Wall Accelerator

    SciTech Connect

    Caporaso, George J.; Chen, Yu-Jiuan; Sampayan, Stephen E.

    2009-01-01

    The Dielectric Wall Accelerator (DWA), a class of induction accelerators, employs a novel insulating beam tube to impress a longitudinal electric field on a bunch of charged particles. The surface flashover characteristics of this tube may permit the attainment of accelerating gradients on the order of 100 MV/m for accelerating pulses on the order of a nanosecond in duration. A virtual traveling wave of excitation along the tube is produced at any desired speed by controlling the timing of pulse generating modules that supply a tangential electric field to the tube wall. Because of the ability to control the speed of this virtual wave, the accelerator is capable of handling any charge to mass ratio particle; hence it can be used for electrons, protons and any ion. The accelerator architectures, key technologies and development challenges will be described.

  1. Enhanced functionality in GaN and SiC devices by using novel processing

    NASA Astrophysics Data System (ADS)

    Pearton, S. J.; Abernathy, C. R.; Gila, B. P.; Ren, F.; Zavada, J. M.; Park, Y. D.

    2004-11-01

    Some examples of recent advances in enhancing or adding functionality to GaN and SiC devices through the use of novel processing techniques are discussed. The first example is the use of ion implantation to incorporate transition metals such as Mn, Cr and Co at atomic percent levels in the wide bandgap semiconductors to produce room temperature ferromagnetism. A discussion is given of the phase space within which single-phase material can be obtained and the requirements for demonstrating the presence of a true dilute magnetic semiconductor. The ability to make GaN and SiC ferromagnetic leads to the possibility of magnetic devices with gain, spin FETs operating at low voltages and spin polarized light emitters. The second example is the use of novel oxides such as Sc 2O 3 and MgO as gate dielectrics or surface passivants on GaN. True inversion behavior has been demonstrated in gated MOS-GaN diodes with implanted n-regions supplying the minority carriers need for inversion. These oxide layers also effectively mitigate current collapse in AlGaN/GaN HEMTs through their passivation of surface states in the gate-drain region. The third example is the use of laser drilling to make through-wafer via holes in SiC, sapphire and GaN. The ablation rate is sufficiently high that this maskless, serial process appears capable of achieving similar throughput to the more conventional approach of plasma etching of vias. The fourth example is the use of either ungated AlGaN/GaN HEMTs or simple GaN and SiC Schottky diodes as sensors for chemicals, biogens, radiation, combustion gases or strain. The sensitivity of either the channel carrier density or the barrier height to changes in surface condition make these materials systems ideal for compact robust sensors capable of operating at elevated temperatures.

  2. A new system for sodium flux growth of bulk GaN. Part I: System development

    NASA Astrophysics Data System (ADS)

    Von Dollen, Paul; Pimputkar, Siddha; Alreesh, Mohammed Abo; Albrithen, Hamad; Suihkonen, Sami; Nakamura, Shuji; Speck, James S.

    2016-12-01

    Though several methods exist to produce bulk crystals of gallium nitride (GaN), none have been commercialized on a large scale. The sodium flux method, which involves precipitation of GaN from a sodium-gallium melt supersaturated with nitrogen, offers potentially lower cost production due to relatively mild process conditions while maintaining high crystal quality. We successfully developed a novel apparatus for conducting crystal growth of bulk GaN using the sodium flux method which has advantages with respect to prior reports. A key task was to prevent sodium loss or migration from the growth environment while permitting N2 to access the growing crystal. We accomplished this by implementing a reflux condensing stem along with a reusable capsule containing a hermetic seal. The reflux condensing stem also enabled direct monitoring of the melt temperature, which has not been previously reported for the sodium flux method. Furthermore, we identified and utilized molybdenum and the molybdenum alloy TZM as a material capable of directly containing the corrosive sodium-gallium melt. This allowed implementation of a crucible-free system, which may improve process control and potentially lower crystal impurity levels. Nucleation and growth of parasitic GaN (;PolyGaN;) on non-seed surfaces occurred in early designs. However, the addition of carbon in later designs suppressed PolyGaN formation and allowed growth of single crystal GaN. Growth rates for the (0001) Ga face (+c-plane) were up to 14 μm/h while X-ray omega rocking (ω-XRC) curve full width half-max values were 731″ for crystals grown using a later system design. Oxygen levels were high, >1019 atoms/cm3, possibly due to reactor cleaning and handling procedures.

  3. Spatial distribution of carrier concentration in un-doped GaN film grown on sapphire

    NASA Astrophysics Data System (ADS)

    Huang, Y.; Chen, X. D.; Beling, C. D.; Fung, S.; Ling, C. C.

    2004-03-01

    The depth and lateral dependent carrier concentration of un-intentionally doped GaN film grown on sapphire substrate have been studied by temperature-dependent Hall effect measurement, confocal micro-Raman spectroscopy and capacitance-voltage (C-V) measurements. The depth-dependent free carrier concentration extracted from the depth-profiled Raman spectra confirms a non-uniform spatial distribution of free carriers in the GaN film with a highly conductive layer of 1 m thickness near the GaN/sapphire boundary. The temperature dependent Hall data have been analyzed using two-layer model to extract the carrier concentration in the GaN bulk film and in the parallel conduction channel adjacent to the GaN/sapphire boundary. The carrier concentrations of the two layers derived from the Raman technique and the Hall measurements agree with each other. The lateral-dependent carrier concentration of the 2-inch GaN epitaxial wafer has also been studied by micro-Raman spectroscopy and C-V measurements. The line-shape fitting of the Raman A1(LO) coupled modes taken from horizontal lateral-different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. These data are compared well with a lateral-dependent carrier concentration map of the wafer revealed by C-V measurements. The study in the article indicates that Raman spectroscopy of the LO phonon-plasmon mode can be used as a nondestructive and reliable, in situ diagnostic for GaN wafer production.

  4. On the piezoelectric coupling constant of epitaxial Mg-doped GaN

    NASA Astrophysics Data System (ADS)

    Xu, X.; Woods, R. C.

    2010-07-01

    The electromechanical coupling coefficient ( k2) of an Mg-doped GaN epilayer has been evaluated by Lee et al. [IEEE Trans Electron Dev 2001;48:524-9] as (4.3 ± 0.3)%, which is much higher than those of other comparable III-V materials. As this is a surprising result, the present paper reports an independent determination of k2 from a similar Mg-doped GaN epilayer for comparison. A 2 μm thick Mg-doped GaN epilayer ((0 0 0 1) orientation) was grown on a c-plane sapphire substrate. The atomic concentration of Mg was 1.2 × 10 18/cm 3. Surface-acoustic wave (SAW) delay-line filters were fabricated using evaporated aluminium and "lift-off" techniques, and consisted of pairs of interdigital transducers (IDTs). Each IDT had 150 pairs of double-fingers. The SAW propagation direction in the sapphire substrate was [ 1¯ 1 0 0]. Acoustic wavelengths of 32 μm, 40 μm, 48 μm and 56 μm were used in the present work. The insertion loss of these filters was found to be more than 80 dB. This allows an upper bound of the electromechanical coupling coefficient ( k2) of the Mg-doped GaN epilayer to be calculated as 1 × 10 -4%. Our value, significantly lower than previously reported, shows that an Mg-doped GaN epilayer may not be such a promising material for SAW devices as appeared at first sight.

  5. Optically pulsed electron accelerator

    DOEpatents

    Fraser, J.S.; Sheffield, R.L.

    1985-05-20

    An optically pulsed electron accelerator can be used as an injector for a free electron laser and comprises a pulsed light source, such as a laser, for providing discrete incident light pulses. A photoemissive electron source emits electron bursts having the same duration as the incident light pulses when impinged upon by same. The photoemissive electron source is located on an inside wall of a radiofrequency-powered accelerator cell which accelerates the electron burst emitted by the photoemissive electron source.

  6. Optically pulsed electron accelerator

    DOEpatents

    Fraser, John S.; Sheffield, Richard L.

    1987-01-01

    An optically pulsed electron accelerator can be used as an injector for a free electron laser and comprises a pulsed light source, such as a laser, for providing discrete incident light pulses. A photoemissive electron source emits electron bursts having the same duration as the incident light pulses when impinged upon by same. The photoemissive electron source is located on an inside wall of a radio frequency powered accelerator cell which accelerates the electron burst emitted by the photoemissive electron source.

  7. ACCELERATION RESPONSIVE SWITCH

    DOEpatents

    Chabrek, A.F.; Maxwell, R.L.

    1963-07-01

    An acceleration-responsive device with dual channel capabilities whereby a first circuit is actuated upon attainment of a predetermined maximum acceleration level and when the acceleration drops to a predetermined minimum acceleriltion level another circuit is actuated is described. A fluid-damped sensing mass slidably mounted in a relatively frictionless manner on a shaft through the intermediation of a ball bushing and biased by an adjustable compression spring provides inertially operated means for actuating the circuits. (AEC)

  8. The foxhole accelerating structure

    SciTech Connect

    Fernow, R.C.; Claus, J.

    1992-07-17

    This report examines some properties of a new type of open accelerating structure. It consists of a series of rectangular cavities, which we call foxholes, joined by a beam channel. The power for accelerating the particles comes from an external radiation source and enters the cavities through their open upper surfaces. Analytic and computer calculations are presented showing that the foxhole is a suitable structure for accelerating relativistic electrons.

  9. Cerebral circulation during acceleration stress

    NASA Astrophysics Data System (ADS)

    Cirovic, Srdjan

    A mathematical model of the cerebrovascular system has been developed to examine the influence of acceleration on cerebral circulation. The objective is to distinguish the main factors that limit cerebral blood flow in pilots subjected to accelerations which exceed the gravitational acceleration of the earth (Gz > 1). The cerebrovascular system was approximated by an open-loop network of elastic tubes and the flow in blood vessels was modeled according to a one-dimensional theory of flow in collapsible tubes. Since linear analysis showed that the speed of pulse propagation in the intracranial vessels should not be modified by the skull constraint, the same governing equations were used for the intracranial vessels as for the rest of the network. The steady and pulsatile components of the cerebrospinal fluid pressure were determined from the condition that the cranial volume must be conserved. After the qualitative aspects of the model results were verified experimentally, the open-loop geometry was incorporated into a global mathematical model of the cardiovascular system. Both the mathematical models and the experiment show that cerebral blood flow diminishes for Gz > 1 due to an increase in the resistance of the large veins in the neck, which collapse as soon as the venous pressure becomes negative. In contrast, the conservation of the cranial volume requires that the cerebrospinal and venous pressure always be approximately the same, and the vessels contained in the cranial cavity do not collapse. Positive pressure breathing provides protection by elevating blood arterial and venous pressures at the heart, thus preventing the venous collapse and maintaining the normal cerebral vascular resistance.

  10. Particle acceleration in flares

    NASA Technical Reports Server (NTRS)

    Benz, Arnold O.; Kosugi, Takeo; Aschwanden, Markus J.; Benka, Steve G.; Chupp, Edward L.; Enome, Shinzo; Garcia, Howard; Holman, Gordon D.; Kurt, Victoria G.; Sakao, Taro

    1994-01-01

    Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, gamma-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.

  11. Charged particle accelerator grating

    DOEpatents

    Palmer, Robert B.

    1986-01-01

    A readily disposable and replaceable accelerator grating for a relativistic particle accelerator. The grating is formed for a plurality of liquid droplets that are directed in precisely positioned jet streams to periodically dispose rows of droplets along the borders of a predetermined particle beam path. A plurality of lasers are used to direct laser beams into the droplets, at predetermined angles, thereby to excite the droplets to support electromagnetic accelerating resonances on their surfaces. Those resonances operate to accelerate and focus particles moving along the beam path. As the droplets are distorted or destroyed by the incoming radiation, they are replaced at a predetermined frequency by other droplets supplied through the jet streams.

  12. Charged particle accelerator grating

    DOEpatents

    Palmer, Robert B.

    1986-09-02

    A readily disposable and replaceable accelerator grating for a relativistic particle accelerator. The grating is formed for a plurality of liquid droplets that are directed in precisely positioned jet streams to periodically dispose rows of droplets along the borders of a predetermined particle beam path. A plurality of lasers are used to direct laser beams into the droplets, at predetermined angles, thereby to excite the droplets to support electromagnetic accelerating resonances on their surfaces. Those resonances operate to accelerate and focus particles moving along the beam path. As the droplets are distorted or destroyed by the incoming radiation, they are replaced at a predetermined frequency by other droplets supplied through the jet streams.

  13. Accelerator-based BNCT.

    PubMed

    Kreiner, A J; Baldo, M; Bergueiro, J R; Cartelli, D; Castell, W; Thatar Vento, V; Gomez Asoia, J; Mercuri, D; Padulo, J; Suarez Sandin, J C; Erhardt, J; Kesque, J M; Valda, A A; Debray, M E; Somacal, H R; Igarzabal, M; Minsky, D M; Herrera, M S; Capoulat, M E; Gonzalez, S J; del Grosso, M F; Gagetti, L; Suarez Anzorena, M; Gun, M; Carranza, O

    2014-06-01

    The activity in accelerator development for accelerator-based BNCT (AB-BNCT) both worldwide and in Argentina is described. Projects in Russia, UK, Italy, Japan, Israel, and Argentina to develop AB-BNCT around different types of accelerators are briefly presented. In particular, the present status and recent progress of the Argentine project will be reviewed. The topics will cover: intense ion sources, accelerator tubes, transport of intense beams, beam diagnostics, the (9)Be(d,n) reaction as a possible neutron source, Beam Shaping Assemblies (BSA), a treatment room, and treatment planning in realistic cases.

  14. High Gradient Accelerator Research

    SciTech Connect

    Temkin, Richard

    2016-07-12

    The goal of the MIT program of research on high gradient acceleration is the development of advanced acceleration concepts that lead to a practical and affordable next generation linear collider at the TeV energy level. Other applications, which are more near-term, include accelerators for materials processing; medicine; defense; mining; security; and inspection. The specific goals of the MIT program are: • Pioneering theoretical research on advanced structures for high gradient acceleration, including photonic structures and metamaterial structures; evaluation of the wakefields in these advanced structures • Experimental research to demonstrate the properties of advanced structures both in low-power microwave cold test and high-power, high-gradient test at megawatt power levels • Experimental research on microwave breakdown at high gradient including studies of breakdown phenomena induced by RF electric fields and RF magnetic fields; development of new diagnostics of the breakdown process • Theoretical research on the physics and engineering features of RF vacuum breakdown • Maintaining and improving the Haimson / MIT 17 GHz accelerator, the highest frequency operational accelerator in the world, a unique facility for accelerator research • Providing the Haimson / MIT 17 GHz accelerator facility as a facility for outside users • Active participation in the US DOE program of High Gradient Collaboration, including joint work with SLAC and with Los Alamos National Laboratory; participation of MIT students in research at the national laboratories • Training the next generation of Ph. D. students in the field of accelerator physics.

  15. FFAGS for rapid acceleration

    SciTech Connect

    Carol J. Johnstone and Shane Koscielniak

    2002-09-30

    When large transverse and longitudinal emittances are to be transported through a circular machine, extremely rapid acceleration holds the advantage that the beam becomes immune to nonlinear resonances because there is insufficient time for amplitudes to build up. Uncooled muon beams exhibit large emittances and require fast acceleration to avoid decay losses and would benefit from this style of acceleration. The approach here employs a fixed-field alternating gradient or FFAG magnet structure and a fixed frequency acceleration system. Acceptance is enhanced by the use only of linear lattice elements, and fixed-frequency rf enables the use of cavities with large shunt resistance and quality factor.

  16. Acceleration of polarized protons in circular accelerators

    SciTech Connect

    Courant, E.D.; Ruth, R.D.

    1980-09-12

    The theory of depolarization in circular accelerators is presented. The spin equation is first expressed in terms of the particle orbit and then converted to the equivalent spinor equation. The spinor equation is then solved for three different situations: (1) a beam on a flat top near a resonance, (2) uniform acceleration through an isolated resonance, and (3) a model of a fast resonance jump. Finally, the depolarization coefficient, epsilon, is calculated in terms of properties of the particle orbit and the results are applied to a calculation of depolarization in the AGS.

  17. Improved control over spontaneously formed GaN nanowires in molecular beam epitaxy using a two-step growth process.

    PubMed

    Zettler, J K; Corfdir, P; Geelhaar, L; Riechert, H; Brandt, O; Fernández-Garrido, S

    2015-11-06

    We investigate the influence of modified growth conditions during the spontaneous formation of GaN nanowires (NWs) on Si(111) in plasma-assisted molecular beam epitaxy. We find that a two-step growth approach, where the substrate temperature is increased during the nucleation stage, is an efficient method to gain control over the area coverage, average diameter, and coalescence degree of GaN NW ensembles. Furthermore, we also demonstrate that the growth conditions employed during the incubation time that precedes nanowire nucleation do not influence the properties of the final nanowire ensemble. Therefore, when growing GaN NWs at elevated temperatures or with low Ga/N ratios, the total growth time can be reduced significantly by using more favorable growth conditions for nanowire nucleation during the incubation time.

  18. In situ GaN decomposition analysis by quadrupole mass spectrometry and reflection high-energy electron diffraction

    SciTech Connect

    Fernandez-Garrido, S.; Calleja, E.; Koblmueller, G.; Speck, J. S.

    2008-08-01

    Thermal decomposition of wurtzite (0001)-oriented GaN was analyzed: in vacuum, under active N exposure, and during growth by rf plasma-assisted molecular beam epitaxy. The GaN decomposition rate was determined by measurements of the Ga desorption using in situ quadrupole mass spectrometry, which showed Arrhenius behavior with an apparent activation energy of 3.1 eV. Clear signatures of intensity oscillations during reflection high-energy electron diffraction measurements facilitated complementary evaluation of the decomposition rate and highlighted a layer-by-layer decomposition mode in vacuum. Exposure to active nitrogen, either under vacuum or during growth under N-rich growth conditions, strongly reduced the GaN losses due to GaN decomposition.

  19. Tolerance of GaAs as an original substrate for HVPE growth of free standing GaN

    NASA Astrophysics Data System (ADS)

    Suzuki, Mio; Sato, T.; Suemasu, T.; Hasegawa, F.

    2004-09-01

    In order to investigate possibility of thick GaN growth on a GaAs substrate by halide vapar phase epitaxy (HVPE), GaN was grown on GaAs(111)/Ti wafer with Ti deposited by E-gun. It was found that surface treatment of the GaAs substrate by HF solution deteriorated greatly the tolerence of GaAs and that Ti can protected GaAs from erosion by NH3. By depositing Ti on GaAs(111)A surface, a millor-like GaN layer could be grown at 1000 °C for 1 hour without serious deterioration of the original GaAs substrate. By increasing the growth rate, a thick free standing GaN will be obtained with GaAs as an original substrate in near future.

  20. Synchrotron radiation x-ray topography and defect selective etching analysis of threading dislocations in GaN

    SciTech Connect

    Sintonen, Sakari Suihkonen, Sami; Jussila, Henri; Tuomi, Turkka O.; Lipsanen, Harri; Rudziński, Mariusz; Knetzger, Michael; Meissner, Elke; Danilewsky, Andreas

    2014-08-28

    The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

  1. The origin of yellow band emission and cathodoluminescence of Au-catalyzed wurtzite GaN nanowires

    NASA Astrophysics Data System (ADS)

    Zhang, R. S.; Jiao, J. F.; Wu, X.

    2016-06-01

    GaN nanowires with large yield are directly synthesized by simply ammoniating the gallium oxide powders in the presence of ammonia gas at 1000 °C, under the assistance of Au nanocatalysts. The microstructure and crystallinity of as-synthesized GaN nanowires are well studied by using high-resolution transmission electron microscope (HRTEM) and some structural defects such as stacking faults are found in the GaN nano-crystal. Cathodoluminescence measurement shows that a strong near-band-edge (NBE) emission band centered at 384 nm and a broad yellow band in the range of 500-800 nm are observed. Finally, the growth mechanism and possible optical emission process of GaN nanowires are discussed.

  2. Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy

    SciTech Connect

    Afroz Faria, Faiza; Guo Jia; Zhao Pei; Li Guowang; Kumar Kandaswamy, Prem; Wistey, Mark; Xing Huili; Jena, Debdeep

    2012-07-16

    Ti/Al/Ni/Au ohmic contacts were formed on heavily doped n{sup +} metal-polar GaN samples with various Si doping concentrations grown by molecular beam epitaxy. The contact resistivity (R{sub C}) and sheet resistance (R{sub sh}) as a function of corresponding GaN free carrier concentration (n) were measured. Very low R{sub C} values (<0.09 {Omega} mm) were obtained, with a minimum R{sub C} of 0.035 {Omega} mm on a sample with a room temperature carrier concentration of {approx}5 Multiplication-Sign 10{sup 19} cm{sup -3}. Based on the systematic study, the role of R{sub C} and R{sub sh} is discussed in the context of regrown n{sup +} GaN ohmic contacts for GaN based high electron mobility transistors.

  3. Very strong photoluminescence emission from GaN grown on amorphous silica substrate by gas source MBE

    NASA Astrophysics Data System (ADS)

    Asahi, H.; Iwata, K.; Tampo, H.; Kuroiwa, R.; Hiroki, M.; Asami, K.; Nakamura, S.; Gonda, S.

    1999-05-01

    Polycrystalline GaN layers showing very strong photoluminescence (PL) intensities are successfully grown on amorphous fused silica (SiO 2) substrates by gas source molecular beam epitaxy (MBE) using an ion removed electron cyclotron resonance radical cell. The PL intensity is larger than that of undoped single crystalline GaN grown on sapphire by gas source MBE and is comparable to that of Si-doped single crystalline GaN grown on sapphire by metalorganic vapor-phase epitaxy at Nichia Chemical. The PL peak emission is considered to be excitonic. Undoped GaN layers grown on silica substrates exhibit n-type conduction and both n- and p-type conductions are achieved by impurity doping. These results open up the area of "Polycrystalline Semiconductor Photonics".

  4. Step-flow growth mode instability of N-polar GaN under N-excess

    SciTech Connect

    Chèze, C.

    2013-08-12

    GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards <1010>, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

  5. InGaN light emitting diodes with a nanopipe layer formed from the GaN epitaxial layer.

    PubMed

    Hsu, Wei-Ju; Chen, Kuei-Ting; Huang, Wan-Chun; Wu, Chia-Jung; Dai, Jing-Jie; Chen, Sy-Hann; Lin, Chia-Feng

    2016-05-30

    A Si-heavy doped GaN:Si epitaxial layer is transformed into a directional nanopipe GaN layer through a laser-scribing process and a selectively electrochemical (EC) etching process. InGaN light-emitting diodes (LEDs) with an EC-treated nanopipe GaN layer have a high light extraction efficiency. The direction of the nanopipe structure was directed perpendicular to the laser scribing line and was guided by an external bias electric field. An InGaN LED structure with an embedded nanopipe GaN layer can enhance external quantum efficiency through a one-step epitaxial growth process and a selective EC etching process. A birefringence optical property and a low effective refractive index were observed in the directional-nanopipe GaN layer.

  6. Microstructures of GaN and In{sub x}Ga{sub 1-x}N films grown by MOCVD on free-standing GaN templates

    SciTech Connect

    Jasinski, J.; Liliental-Weber, Z.; Huang, D.; Reshchikov, M.A.; Yun, F.; Morkoc, H.; Sone, C.; Park, S.S.; Lee, K.Y.

    2002-04-30

    We summarize structural properties of thick HVPE GaN templates from the point of view of their application as substrates for growth of nitride layers. This is followed by the results of optical and structural studies, mostly transmission electron microscopy, of nitride layers grown by MOCVD on top of the HVPE substrates. The results indicate high structural quality of these layers with a low density of threading dislocations (in the range of 10{sup 6} cm{sup -2}). Convergent beam electron diffraction studies showed that the MOCVD GaN films have Ga-polarity, the same polarity as the HVPE GaN substrates. Structural studies of an InGaN layer grown on top of the MOCVD GaN film showed the presence of two layers, which differed in lattice parameter and composition. The upper layer, on the top of the structure had a c-lattice parameter about 2% larger than that of GaN and contained 10.3 {+-} 0.8% of In. Values measured for the thinner, intermediate layer adjacent to the GaN layer were about 2 .5 times lower.

  7. Scaling FFAG accelerator for muon acceleration

    SciTech Connect

    Lagrange, JB.; Planche, T.; Mori, Y.

    2011-10-06

    Recent developments in scaling fixed field alternating gradient (FFAG) accelerators have opened new ways for lattice design, with straight sections, and insertions like dispersion suppressors. Such principles and matching issues are detailed in this paper. An application of these new concepts is presented to overcome problems in the PRISM project.

  8. Scaling FFAG accelerator for muon acceleration

    NASA Astrophysics Data System (ADS)

    Lagrange, JB.; Planche, T.; Mori, Y.

    2011-10-01

    Recent developments in scaling fixed field alternating gradient (FFAG) accelerators have opened new ways for lattice design, with straight sections, and insertions like dispersion suppressors. Such principles and matching issues are detailed in this paper. An application of these new concepts is presented to overcome problems in the PRISM project.

  9. Angular velocities, angular accelerations, and coriolis accelerations

    NASA Technical Reports Server (NTRS)

    Graybiel, A.

    1975-01-01

    Weightlessness, rotating environment, and mathematical analysis of Coriolis acceleration is described for man's biological effective force environments. Effects on the vestibular system are summarized, including the end organs, functional neurology, and input-output relations. Ground-based studies in preparation for space missions are examined, including functional tests, provocative tests, adaptive capacity tests, simulation studies, and antimotion sickness.

  10. Dislocation confinement in the growth of Na flux GaN on metalorganic chemical vapor deposition-GaN

    SciTech Connect

    Takeuchi, S. Asazu, H.; Nakamura, Y.; Sakai, A.; Imanishi, M.; Imade, M.; Mori, Y.

    2015-12-28

    We have demonstrated a GaN growth technique in the Na flux method to confine c-, (a+c)-, and a-type dislocations around the interface between a Na flux GaN crystal and a GaN layer grown by metalorganic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate. Transmission electron microscopy (TEM) clearly revealed detailed interface structures and dislocation behaviors that reduced the density of vertically aligned dislocations threading to the Na flux GaN surface. Submicron-scale voids were formed at the interface above the dislocations with a c component in MOCVD-GaN, while no such voids were formed above the a-type dislocations. The penetration of the dislocations with a c component into Na flux GaN was, in most cases, effectively blocked by the presence of the voids. Although some dislocations with a c component in the MOCVD-GaN penetrated into the Na flux GaN, their propagation direction changed laterally through the voids. On the other hand, the a-type dislocations propagated laterally and collectively near the interface, when these dislocations in the MOCVD-GaN penetrated into the Na flux GaN. These results indicated that the dislocation propagation behavior was highly sensitive to the type of dislocation, but all types of dislocations were confined to within several micrometers region of the Na flux GaN from the interface. The cause of void formation, the role of voids in controlling the dislocation behavior, and the mechanism of lateral and collective dislocation propagation are discussed on the basis of TEM results.

  11. Induction linear accelerators

    NASA Astrophysics Data System (ADS)

    Birx, Daniel

    1992-03-01

    Among the family of particle accelerators, the Induction Linear Accelerator is the best suited for the acceleration of high current electron beams. Because the electromagnetic radiation used to accelerate the electron beam is not stored in the cavities but is supplied by transmission lines during the beam pulse it is possible to utilize very low Q (typically<10) structures and very large beam pipes. This combination increases the beam breakup limited maximum currents to of order kiloamperes. The micropulse lengths of these machines are measured in 10's of nanoseconds and duty factors as high as 10-4 have been achieved. Until recently the major problem with these machines has been associated with the pulse power drive. Beam currents of kiloamperes and accelerating potentials of megavolts require peak power drives of gigawatts since no energy is stored in the structure. The marriage of liner accelerator technology and nonlinear magnetic compressors has produced some unique capabilities. It now appears possible to produce electron beams with average currents measured in amperes, peak currents in kiloamperes and gradients exceeding 1 MeV/meter, with power efficiencies approaching 50%. The nonlinear magnetic compression technology has replaced the spark gap drivers used on earlier accelerators with state-of-the-art all-solid-state SCR commutated compression chains. The reliability of these machines is now approaching 1010 shot MTBF. In the following paper we will briefly review the historical development of induction linear accelerators and then discuss the design considerations.

  12. Accelerator Science: Why RF?

    SciTech Connect

    Lincoln, Don

    2016-12-21

    Particle accelerators can fire beams of subatomic particles at near the speed of light. The accelerating force is generated using radio frequency technology and a whole lot of interesting features. In this video, Fermilab’s Dr. Don Lincoln explains how it all works.

  13. Particle Acceleration in Jets

    NASA Technical Reports Server (NTRS)

    Nishikawa, Ken-Ichi

    2005-01-01

    Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma ray burst (GRBs), and Galactic microquasar systems usually have power-law emission spectra. Fermi acceleration is the mechanism usually assumed for the acceleration of particles in astrophysical environments.

  14. Accelerators Beyond The Tevatron?

    SciTech Connect

    Lach, Joseph; /Fermilab

    2010-07-01

    Following the successful operation of the Fermilab superconducting accelerator three new higher energy accelerators were planned. They were the UNK in the Soviet Union, the LHC in Europe, and the SSC in the United States. All were expected to start producing physics about 1995. They did not. Why?

  15. Accelerators (3/5)

    ScienceCinema

    None

    2016-07-12

    1a) Introduction and motivation 1b) History and accelerator types 2) Transverse beam dynamics 3a) Longitudinal beam dynamics 3b) Figure of merit of a synchrotron/collider 3c) Beam control 4) Main limiting factors 5) Technical challenges Prerequisite knowledge: Previous knowledge of accelerators is not required.

  16. Diagnostics for induction accelerators

    SciTech Connect

    Fessenden, T.J.

    1996-04-01

    The induction accelerator was conceived by N. C. Christofilos and first realized as the Astron accelerator that operated at LLNL from the early 1960`s to the end of 1975. This accelerator generated electron beams at energies near 6 MeV with typical currents of 600 Amperes in 400 ns pulses. The Advanced Test Accelerator (ATA) built at Livermore`s Site 300 produced 10,000 Ampere beams with pulse widths of 70 ns at energies approaching 50 MeV. Several other electron and ion induction accelerators have been fabricated at LLNL and LBNL. This paper reviews the principal diagnostics developed through efforts by scientists at both laboratories for measuring the current, position, energy, and emittance of beams generated by these high current, short pulse accelerators. Many of these diagnostics are closely related to those developed for other accelerators. However, the very fast and intense current pulses often require special diagnostic techniques and considerations. The physics and design of the more unique diagnostics developed for electron induction accelerators are presented and discussed in detail.

  17. Accelerators (4/5)

    ScienceCinema

    None

    2016-07-12

    1a) Introduction and motivation 1b) History and accelerator types 2) Transverse beam dynamics 3a) Longitudinal beam dynamics 3b) Figure of merit of a synchrotron/collider 3c) Beam control 4) Main limiting factors 5) Technical challenges Prerequisite knowledge: Previous knowledge of accelerators is not required.

  18. Measuring Model Rocket Acceleration.

    ERIC Educational Resources Information Center

    Jenkins, Randy A.

    1993-01-01

    Presents an experiment that measures the acceleration and velocity of a model rocket. Lift-off information is transmitted to a computer that creates a graph of the velocity. Discusses the analysis of the computer-generated data and differences between calculated and experimental velocity and acceleration of several rocket types. (MDH)

  19. Microscale acceleration history discriminators

    DOEpatents

    Polosky, Marc A.; Plummer, David W.

    2002-01-01

    A new class of micromechanical acceleration history discriminators is claimed. These discriminators allow the precise differentiation of a wide range of acceleration-time histories, thereby allowing adaptive events to be triggered in response to the severity (or lack thereof) of an external environment. Such devices have applications in airbag activation, and other safety and surety applications.

  20. Accelerators (5/5)

    ScienceCinema

    None

    2016-07-12

    1a) Introduction and motivation 1b) History and accelerator types 2) Transverse beam dynamics 3a) Longitudinal beam dynamics 3b) Figure of merit of a synchrotron/collider 3c) Beam control 4) Main limiting factors 5) Technical challenges Prerequisite knowledge: Previous knowledge of accelerators is not required.