Electrical and Computer Engineering Commons^™

Ingan Staircase Electron Injector For Reduction Of Electron Overflow In Ingan Light Emitting Diodes, X. Ni, X. Li, J. Lee, S. Liu, Vitaliy Avrutin, Ü. Özgür, Hadis Morkoç, A. Matulionis, T. Paskova, G. Mulholland, K. R. Evans

Electrical and Computer Engineering Publications

Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial …

Carrier Velocity In Inaln/Aln/Gan Heterostructure Field Effect Transistors On Fe-Doped Bulk Gan Substrates, J. H. Leach, M. Wu., X. Ni, X. Li, J. Xie, Ü. Özgür, Hadis Morkoç, T. Paskova, E. Preble, K. R. Evans, Chang-Zhi Lu

Electrical and Computer Engineering Publications

We report microwave characteristics of field effect transistors employing InAlN/AlN/GaN heterostructures grown on low-defect-density bulk Fe-doped GaN substrates. We achieved unity current gain cutoff frequencies of 14.3 and 23.7 GHz for devices with gate lengths of 1 and 0.65 μm, respectively. Measurements as a function of applied bias allow us to estimate the average carrier velocity in the channel to be ∼1.0×107 cm/sec for a 1 μm device. Additionally, we found nearly no gate lag in the devices, which is considered a precondition for good performance under large signal operation.

Effect Of Hot Phonon Lifetime On Electron Velocity In Inaln/Aln/Gan Heterostructure Field Effect Transistors On Bulk Gan Substrates, J. H. Leach, C. Y. Zhu, M. Wu, X. Ni, X. Li, J. Xie, Ü. Özgür, Hadis Morkoç, J. Liberis, E. Šermukšnis, A. Matulionis, T. Paskova, E. Preble, K. R. Evans

Electrical and Computer Engineering Publications

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructurefield effect transistors(HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields …

Large Dielectric Tuning And Microwave Phase Shift At Low Electric Field In Epitaxial Ba0.5sr0.5tio3 On Srtio3, J. H. Leach, H. Liu, V. Avrutin, B. Xiao, Ü. Özgür, H. Morkoç, J. Das, Y. Y. Song, C. E. Patton

Electrical and Computer Engineering Publications

Dielectric properties of annealed and as-grown ferroelectricBa0.5Sr0.5TiO3 (BST) grown by pulsed laser deposition on sputtered BST seed layers on strontium titanate (STO) substrates were investigated at microwave frequencies in the realm of tunability of its dielectric constant as well as phase shifters based on this material. The as-grown layers were nearly fully relaxed with measured lattice parameters nearly identical to those of bulk BST. The tuning of the relative dielectric constant (∼1750 at zero bias at 10 GHz) of the annealed BST was found to be as high as 59% and 56% at 10 and 19 GHz, respectively. The analysis …

Bias Dependent Two-Channel Conduction In Inaln/Aln/Gan Structures, J. H. Leach, X. Ni, X. Li, M. Wu, Ü. Özgür, H. Morkoç, L. Zhou, D. A. Cullen, D. J. Smith, H. Cheng, Ç. Kurdak, J. R. Meyer, I. Vurgaftman

Electrical and Computer Engineering Publications

Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that …

Hot Electron Effects On Efficiency Degradation In Ingan Light Emitting Diodes And Designs To Mitigate Them, X. Ni, X. Li, J. Lee, S. Liu, V. Avrutin, Ü. Özgür, H. Morkoç, A. Matulionis

Electrical and Computer Engineering Publications

Hot electrons and the associated ballistic and quasiballistic transport, heretofore neglected endemically, across the active regions of InGaN light emitting diodes (LEDs) have been incorporated into a first order simple model which explains the experimental observations of electron spillover and the efficiency degradation at high injection levels. The model is in good agreement with experiments wherein an adjustable barrier hot electron stopper, commonly called the electron blocking layer (EBL), is incorporated. The model is also in agreement with experiments wherein the electrons are cooled, eliminating hot electrons, inside a staircase electron injector (SEI) prior to their injection into the active …

Electrically And Magnetically Tunable Phase Shifters Based On A Barium Strontium Titanate-Yttrium Iron Garnet Layered Structure, J. H. Leach, H. Liu, V. Avrutin, E. Rowe, Ü. Özgür, H. Morkoç, Y.-Y. Song, M. Wu

Electrical and Computer Engineering Publications

We report on the tuning of permittivity and permeability of a ferroelectric/ferromagnetic bilayer structure which can be used as a microwavephase shifter with two degrees of tuning freedom. The structure was prepared by the growth of a yttrium iron garnet (YIG) layer on a gadolinium gallium garnet substrate by liquid phase epitaxy, the growth of a barium strontium titanate (BST) layer on the YIG layer through pulsed laser deposition, and then the fabrication of a coplanar waveguide on the top of BST through e-beam evaporation and trilayer liftoff techniques. The phase shifters exhibit a differential phase shift of 38°/cm at …

Generating System Requirements For A Mobile Digital Evidence Collection System: A Preliminary Step Towards Enhancing The Forensic Collection Of Digital Devices, Ibrahim Baggili

Electrical & Computer Engineering and Computer Science Faculty Publications

Collecting digital devices in a forensically sound manner is becoming more critical since 80% of all cases have some sort of digital evidence involved in them (Rogers, 2006, p. 1) .The process of documenting and tagging digital devices is cumbersome and involves details that might not apply to other types of evidence, since each evidence item has unique physical characteristics (Hesitis & Wilbon, 2005, p. 17). The process becomes less manageable when a large number of digital devices are seized. This paper examines the information and issues investigators should be aware of when collecting digital devices at crime scenes. Furthermore, …

Leadership, Libraries, Leed For The Future, Kathryn S. Ames, Greg Heid

Georgia Library Quarterly

The article discusses the use of Leadership in Energy and Environmental Design (LEED) principles in the design of the modern public library. It is stated that an LEED-certified library will have lower costs in energy, systems replacement and maintenance through the use of light-emitting diode (LED) lighting and efficient heating and air conditioning, but that due to special materials, the up-front expense is higher. The inspection of the facility throughout its construction is also noted.

Performance Analysis Of Srcp Image Based Sound Source Detection Algorithms, Praveen Reddy Nalavolu

University of Kentucky Master's Theses

Steered Response Power based algorithms are widely used for finding sound source location using microphone array systems. SRCP-PHAT is one such algorithm that has a robust performance under noisy and reverberant conditions. The algorithm creates a likelihood function over the field of view. This thesis employs image processing methods on SRCP-PHAT images, to exploit the difference in power levels and pixel patterns to discriminate between sound source and background pixels. Hough Transform based ellipse detection is used to identify the sound source locations by finding the centers of elliptical edge pixel regions typical of source patterns. Monte Carlo simulations of …

Design Of An Integrated Starter-Alternator For A Series Hybrid Electric Vehicle: A Case Study In Axial Flux Permanent Magnet Machine Design, Jessica L. Colton

Theses, Dissertations, and Student Research from Electrical & Computer Engineering

This thesis details the design of an electric machine to perform as both a starter and alternator in a series hybrid electric vehicle. The focus of the work is on practical design aspects specific to single-sided axial flux permanent magnet machines with non-overlapped windings. First, a characterization of the rotor losses in these machine types is presented through experimental validation of finite element analysis estimates. The approaches taken to model the axial flux geometry, especially in two-dimensions, are detailed, and the difficult issue of validating the finite element analysis estimates with experimental data is addressed with a prototype 24-slot, 20-pole …

Near-Field Radiative Transfer: Thermal Radiation, Thermophotovoltaic Power Generation And Optical Characterization, Mathieu Francoeur

University of Kentucky Doctoral Dissertations

This dissertation focuses on near-field radiative transfer, which can be defined as the discipline concerned with energy transfer via electromagnetic waves at sub-wavelength distances. Three specific subjects related to this discipline are investigated, namely nearfield thermal radiation, nanoscale-gap thermophotovoltaic (nano-TPV) power generation and optical characterization. An algorithm for the solution of near-field thermal radiation problems in one-dimensional layered media is developed, and several tests are performed showing the accuracy, consistency and versatility of the procedure. The possibility of tuning near-field radiative heat transfer via thin films supporting surface phononpolaritons (SPhPs) in the infrared is afterwards investigated via the computation of …

Fast Response Dual Frequency Liquid Crystal Materials, Qiong Song

Electronic Theses and Dissertations

Dual frequency liquid crystal (DFLC) exhibits a positive dielectric anisotropy at low frequencies and negative dielectric anisotropy at high frequencies. The frequency where dielectric anisotropy is zero is called crossover frequency. DFLC can achieve fast rise time and fast decay time with the assistance of applied voltage. However, one drawback of DFLC is that it has dielectric heating effect when driven at a high frequency. Thus, the first part of this dissertation is to develop low crossover frequency DFLC materials. The dielectric relaxation and physical properties of some single- and double-ester compounds were investigated. Experimental results indicate that the double-ester …

Vehicular Ad Hoc Networks, Hossein Pishro-Nik, Shahrokh Valaee, Maziar Nekovee

Hossein Pishro-Nik

No abstract provided.

Novel Control Technique For Cadmium Sulfide, Rafael Ordonez

Open Access Theses & Dissertations

Cadmium sulfide Chemical-Bath-Deposition parameters are modified to allow, for the first time, real time monitoring of cadmium ion molarity using a commercially available cadmium ion Ion-Selective-Electreode at 60°C. This modification involves alterations to typical deposition parameters that eliminate the formation of cadmium hydroxide and keep cadmium ion molarity above 10-7M. The new or atypical parameters developed are pH values below 8.9 and lower moralities for all chemicals. In order to have complexation of cadmium ion driven mainly by ammonia, parameters that minimized complexation by other chemicals are found. Cadmium acetate concentration below 2mM and ammonium acetate concentration below 10mM minimize …

Novel Control Technique For Cadmium Sulfide Chemical Bath Deposition Using Real Time Monitoring Of Cadmium Ion Molarity, Rafael Ordonez

Open Access Theses & Dissertations

Cadmium sulfide Chemical-Bath-Deposition parameters are modified to allow, for the first time, real time monitoring of cadmium ion molarity using a commercially available cadmium ion Ion-Selective-Electreode at 60°C. This modification involves alterations to typical deposition parameters that eliminate the formation of cadmium hydroxide and keep cadmium ion molarity above 10-7M. The new or atypical parameters developed are pH values below 8.9 and lower moralities for all chemicals. In order to have complexation of cadmium ion driven mainly by ammonia, parameters that minimized complexation by other chemicals are found. Cadmium acetate concentration below 2mM and ammonium acetate concentration below 10mM minimize …

Investigation Of Selective Nanohetero-Epitaxy Growth On Si(100) Substrates For Hgcdte Infrared Detector Applications, Aryzbe Diaz

Open Access Theses & Dissertations

The selective CdTe Nanohetero-Epitaxy growth on Si(100) substrates has been achieved without the use of a mask in an attempt to reduce the dislocation density at the CdTe/Si interface to 10⁵/cm³ using the Close-Space Sublimation (CSS) technique, which is a low-cost effective technique for the deposition of polycrystalline CdTe films. Furthermore, CdTe is a compound semiconductor with a direct bandgap of 1.5 eV, which makes it an ideal material for the applications of infrared detectors and solar cells. It has been demonstrated that the selective growth of CdTe on patterned Si(100) substrates results in CdTe grains with …

Super-Resolution Using Adaptive Wiener Filters, Russell C. Hardie

Electrical and Computer Engineering Faculty Publications

The spatial sampling rate of an imaging system is determined by the spacing of the detectors in the focal plane array (FPA). The spatial frequencies present in the image on the focal plane are band-limited by the optics. This is due to diffraction through a finite aperture. To guarantee that there will be no aliasing during image acquisiton, the Nyquist criterion dictates that the sampling rate must be greater than twice the cut-off frequency of the optics. However, optical designs involve a number of trade-offs and typical imaging systems are designed with some level of aliasing. We will refer to …

Harvesting Single Ferroelectric Domain Stressed Nanoparticles For Optical And Ferroic Applications, Gary Cook, J. L. Barnes, S. A. Basun, Dean R. Evans, Ron F. Ziolo, Arturo Ponce, Victor Yu. Reshetnyak, Anatoliy Glushchenko, Partha P. Banerjee

Electrical and Computer Engineering Faculty Publications

We describe techniques to selectively harvest single ferroelectric domain nanoparticles of BaTiO3 as small as 9 nm from a plethora of nanoparticles produced by mechanical grinding. High resolution transmission electron microscopy imaging shows the unidomain atomic structure of the nanoparticles and reveals compressive and tensile surface strains which are attributed to the preservation of ferroelectric behavior in these particles.

We demonstrate the positive benefits of using harvested nanoparticles in disparate liquid crystal systems.

New Laser Technologies Analysis Of Quantum Dot And Lithographic Laser Diodes, Abdullah Demir

Electronic Theses and Dissertations

The first part of this dissertation presents a comprehensive study of quantum dot (QD) lasers threshold characteristics. The threshold temperature dependence of a QD laser diode is studied in different limits of p-doping, hole level spacing and inhomogeneous broadening. Theoretical analysis shows that the threshold current of a QD laser in the limit of uniform QDs is not temperature independent and actually more temperature sensitive than the quantum well laser. The results also explain the experimental trends of negative characteristic temperature observed in QD lasers and clarify how the carrier distribution mechanisms inside and among the QDs affect the threshold …