Doctor of Philosophy, The Ohio State University, 2009, Electrical and Computer Engineering

GaN-based high electron mobility transistors (HEMTs) have been considered excellent candidates for high power, high speed and high temperature applications due to high breakdown voltage, high electron saturation velocity and high operation temperature. Such superiority of AlGaN/GaN HEMTs leads to power density several times higher than commercially available devices. AlGaN/GaN HEMT power amplifiers have applications for wireless base-station, and communication and military radar systems. Recently, impressive device performances in AlGaN/GaN HEMTs have been reported such as high output power of 40 W/mm at 4 GHz, current gain cut-off frequency (fT) of 190 GHz and power-gain cut-off frequency (fMAX) of 230 GHz. However, critical issues including current dispersion and device reliability have limited AlGaN/GaN HEMTs for practical applications. Current dispersion causing decrease in the actual output drain current and voltage swing at RF operations is attributed to surface/interface states in the AlGaN/GaN heterostructures created during material growth and device processing. Even though there have been remarkable improvements in growth/device fabrication technologies, trapping effects in AlGaN/GaN HEMTs cannot be removed perfectly, indicating that an accurate HEMT model including trapping effects is still needed for development of both novel HEMT processing/material growth techniques and implementation of AlGaN/GaN HEMT-based circuits. In this Ph.D research, we investigated distinct electrical behavior of AlGaN/GaN HEMTs through study on PGA effects using DC and pulsed I-V characterization under different pulse widths and quiescent biases to understand electron capture/emission phenomena. Through the electrical characterization at different measurement conditions Current collapse in AlGaN/GaN was analyzed in terms of trap activities, and impact of PGA effects was qualitatively evaluated as a method for device processing monitoring. Based on temperature-dependent drain (open full item for complete abstract)

Committee: Wu Lu (Advisor); Patrick Roblin (Committee Member); Siddharth Rajan (Committee Member); Steven A. Ringel (Committee Member) Subjects: Electrical Engineering

Master of Science (MS), Bowling Green State University, 2021, Biological Sciences

Using bacteria in cancer therapies is an emerging area of research. Certain bacteria can target tumors, and therapy can involve either the direct (colonize, invade, and deplete metabolic nutrients) or indirect action (deliver a therapeutic payload or "uncloak" the tumor to the immune system) of the bacteria. However, many of the best-suited bacterial species are pathogenic and require extensive genetic engineering to reduce or eliminate their pathogenicity before they can be used therapeutically. The facultative anoxygenic photoautotroph Rhodobacter sphaeroides is non-pathogenic, and has been shown to target tumors. We have been investigating its use as a vector for delivering 5-aminolevulinic acid (ALA) to tumors, where it functions as a prodrug in photodynamic therapies. ALA is a precursor in the formation of heme, and elevated concentrations delivered to tumor cells leads to overproduction of all products in the heme biosynthetic pathway, including precursor tetrapyrroles. In the presence of oxygen and therapeutic wavelengths of light, these molecules generate reactive oxygen species that destroy the tumor cells. R. sphaeroides naturally produces copious amounts of ALA for heme and bacteriochlorophyll synthesis needed to support photosynthetic growth. Prior studies have already shown that it is possible to engineer these bacteria to produce and excrete ALA in amounts that are suitable for photodynamic therapy (Zeilstra-Ryalls, 2013, unpublished results). A survey of wild type strains to identify which one grows best phototrophically under simulated intratumoral conditions was performed. By disrupting the genes in the optimal strain that code for ALA synthase enzymes a mutant was created that relies upon exogenous ALA for growth. Its minimal ALA requirement, as well as its ability to tolerate the presence of high concentrations of ALA under phototrophic conditions was then assessed. The latter was necessary in order to determine wheth (open full item for complete abstract)

Committee: Jill Zeilstra-Ryalls Ph.D (Advisor); Raymond Larsen Ph.D (Committee Member); Vipaporn Phuntumart Ph.D (Committee Member) Subjects: Biology; Biomedical Research; Microbiology; Molecular Biology; Oncology

Doctor of Philosophy (Ph.D.), University of Dayton, 2021, Engineering

The planar and rigid nature of silicon-based electronics limit their reliability and integration into the next generation of electronics, like the Internet of Things (IoT) and wearable sensors. Unconventional electronics integrated with soft materials typically exhibit thermally limited performance due to low interfacial conductance and poor substrate thermal conductivity. To combat these issues, graphite nanoplatelets (GNPs) were used to increase the thermal conductivity of a flexible polydimethylsiloxane (PDMS) substrate by creating a percolating network of high thermal conductivity filler, increasing the substrate conductivity from 0.2 W-m-1K-1 to upwards of 1.8 W-m-1K-1, more than 9 times enhancement. This substrate material retained other useful properties including rheological behavior necessary for additive manufacturing, high temperature stability (upwards of 300C), flexibility (4 MPa compression modulus) and strong adhesion to device materials. This work is the first to demonstrate the direct transfer of the thinned AlGaN/GaN high electron mobility transistors (HEMTs) to the flexible polymeric nanocomposite substrate without an adhesive layer. The devices transferred to the PDMS composite substrates exhibited significantly lower self-heating temperatures experimentally (e.g., delta T = 24C at 30 mW) than those on PDMS when operated at comparable powers (15-50 mW), validating computational model results. These lower operating temperatures directly facilitate the operation of the devices at higher saturation currents and powers. The higher thermal conductivity of the PDMS composite substrate promotes heat conduction away from the device channel and effectively behaves as a flexible heat sink, which contributing to the high operating powers of 6 W-mm-1, especially compared to conventional flexible electronic substrates with low thermal conductivities (i.e. PDMS with no fillers). Additionally, the reduction of device temperatures at target operating powers resu (open full item for complete abstract)

Committee: Christopher Muratore PhD (Advisor) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2019, Physics

In this work, scanning probe microscopy (SPM) methods are developed and extended to spatially resolve performance-hampering electrically-active defects, known as traps, present in AlGaN/GaN Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs). Commercial devices used in these studies were cross-sectioned to expose electrically-active regions which are traditionally inaccessible to SPM techniques. Surface potential transients (SPTs) are collected over the cross-sectioned faces of devices using nanometer-scale scanning probe deep-level transient spectroscopy (SP-DLTS), a millisecond time-resolved derivative technique of scanning Kelvin probe microscopy (SKPM) that was implemented with a custom system designed to study SBDs and HEMTs in cross-section. Detected SPTs are indicative of carrier emission from bulk defect-related trap states. In conjunction with similar measurements of these trap states using macroscopic techniques, finite-element simulations provide strong, corroborating evidence that observable SPTs are produced by traps located in the bulk of these samples and are therefore not a result of surface states or surface-related phenomena. GaN-based materials offer advantages over many alternatives in high-frequency and high-voltage applications. Features including a wide bandgap and a large breakdown voltage often translate to improved efficiency, performance, and cost in many electronic systems. However, GaN-based material research is still maturing, and charge trapping may be a limiting factor in GaN electrical performance and therefore hinder its widespread application and adoption. Determining the signatures and spatial distributions of active traps in GaN devices is critical for understanding trap-related mechanisms of device failure as well as the growth or fabrication steps which may be responsible for introducing these defect states. Powerful techniques like deep-level transient spectroscopy (DLTS) exist for identifying specifi (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor); Ezekiel Johnston-Halperin (Committee Member); Richard Kass (Committee Member); Mohit Randeria (Committee Member) Subjects: Electrical Engineering; Physics

Master of Science, University of Toledo, 2018, Electrical Engineering

The recent development in the wide bandgap (WBG) semiconductor devices such as gallium nitride (GaN) has pushed the limit for the next generation power electronics in terms of high frequency switching applications with high power density. GaN devices have shown promising theoretical advantages such as large bandgap, breakdown field and electron saturation velocity, thereby presenting GaN as an effective alternative for Silicon in high power, temperature and frequency switching applications. Despite having numerous advantages over silicon, GaN technology has suffered with various device level as well as circuit level challenges. Although the very low inherent capacitance of the GaN is one of the most important attributes of the device, it can become disruptive in the presence of significant parasitic circuit inductance. Due to the high sensitivity of these capacitances and their interaction with the parasitic circuit components, undesirable transient events resulting in circuit deterioration can occur. In this thesis work, circuit level reliability issues of GaN due to high VGS stress and high frequency switching has been analyzed with emphasis on external circuit parasitics. The research study targets three important aspects of circuit level reliability issues in a GaN HEMT. It begins with 1. determination of degradation parameters, followed by 2. effect of external gate resistance over degradation parameters and finally 3. analysis of device degradation mechanism with respect to high VGS stress under zero input bias (VDS = 0). A simulation study is also developed to predict the VGS overshoot for a specific gate voltage with respect to parasitic inductance. For this purpose, a 100 V, “EPC-8010” normally o (open full item for complete abstract)

Committee: Raghav Khanna (Committee Chair); Mansoor Alam (Committee Member); Richard Molyet (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering

Electron-plasmonic devices are of strong interest for terahertz applications. In this work, we develop rigorous computational tools using finite difference time domain (FDTD) methods for accurate modeling of these devices. Existing full-wave-hydrodynamic models already combine Maxwell's and hydrodynamic electron-transport equation for multiphysical hybrid modeling. However, these multilevel methods are time-consuming as dense mesh is required for plasmonic modeling. Therefore, they are not suited for design and optimization. To address this issue, we propose new iterative ADI-FDTD-hydrodynamic hybrid coupled model. The new implementations provide time-efficient, yet accurate, modeling of these devices. It is demonstrated that for a typical simulation, up to 50% reduction in simulation-time is achieved with a nominal 3% error in calculations. Using the new tool-set, we investigate several devices that operate using the properties of 2D electron gas (2DEG). We provide one of the first multiphysical numerical analyses of these devices, giving accurate estimates of their terahertz performance. The developed tool allows simulation of arbitrary 2DEG based terahertz devices, providing useful and intuitive 2D field information. This has allowed understanding of the operation and radiation principles of these devices. Specifically, we examine the known plasma-wave instability in short-channel high electron mobility transistors (HEMTs) that leads to terahertz emissions at cryogenic temperatures. We also examine terahertz emitters that exploit resonant tunneling induced negative differential resistance (NDR) in HEMTs. Finally, using this tool we numerically demonstrate the existence of acoustic and optical-plasmonic modes within 2DEG bilayer systems in HEMTs. Methods for exciting and controlling these modes are also discussed enabling new physics among bilayer devices.

Committee: John Volakis (Advisor); Siddharth Rajan (Advisor); Kubilay Sertel (Committee Member); Teixeira Fernando (Committee Member); Niru Nahar (Committee Member); Karin Musier-Forsyth (Committee Member) Subjects: Electrical Engineering; Plasma Physics

Master of Science, The Ohio State University, 2017, Electrical and Computer Engineering

Gallium Nitride High Electron Mobility Transistor (GaN HEMT) has become one of the most attractive power transistors in recent years due to its superior electrical and thermal performance. While GaN devices have been used in more and more applications, short circuit capability of GaN HEMT and the method to qualify its reliability still worth discussion. This report presents the short circuit behavior of discrete 650 V/ 30 A large current rating Enhancement-mode (E-mode) GaN HEMT devices under single and repetitive short circuit operations. Firstly, structure and characteristics of GaN HEMT are introduced. Both cross-section structure of the lateral power transistor and gate structure of normally-off GaN HEMT are presented. Next, detailed test platform design is presented. The platform is established based on hard switching fault (HSF) circuit, and the turn-off transient is evaluated, which proved that soft turn-off is required for GaN HEMT short circuit tests. Besides, a system level thermal model based on FEA simulation and Cauer thermal network is established to have an accurate prediction of devices junction temperature. Then, short circuit roughness has been explored through designed tests, from these tests, maximum short circuit time, short circuit critical energy, as well as short circuit failure behavior and the mechanism is explored and analyzed. For the repetitive short circuit degradation tests, a series of experimental tests are carried out to determine the number of short circuit operations the devices can support before obvious degradation happens under different dissipated energies. More importantly, device static characteristics are explored and monitored during the degradation tests, and the characteristics shifting has been investigated and acts as the indicators of devices degradation. For different kinds of test, including failure condition, test waveforms are presented together with detailed analysis and mechanism discussion.

Committee: Jin Wang (Advisor); Fang Luo (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2015, Electrical and Computer Engineering

This objective of this work is to develop predictive device modelling methodology to relate the physical behavior of AlGaN/GaN HEMTs with RF power amplifier parameters. The large signal performance of two 0.25µmx100µm GaN transistors –a standard HEMT with a recessed gate and a graded channel HEMT were analyzed at a frequency of 20 Ghz using the model. The devices are simulated in Silvaco Atlas TCAD with a velocity saturation model to obtain the high frequency two port network parameters. From these, the small signal circuit components of the device were extracted as a function of gate voltage, drain voltage and frequency. Harmonic balance simulations were performed in ADS to obtain the output power and linearity characteristics. The choice of bias points to maximize linearity from the device was explored.

Committee: Siddharth Rajan (Advisor); Wu Lu (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

This dissertation describes the design, fabrication and characterization of high-k dielectric enhanced Gallium Nitride (GaN)-based devices. Interface properties of atomic layer deposited (ALD) Aluminum Oxide (Al2O3) on GaN was initially investigated. The conduction band offset of Al2O3/GaN was experimentally found as 2.1 eV. High density of positive interface fixed charge (2.72x1013 cm-2) was observed in the Al2O3/GaN. These interface fixed charges can not only induce electrical field in the oxide which increases the reverse gate leakage, but shift the threshold voltage toward negative to prevent E-mode operation. A theoretical study using remote impurity scattering along with other scattering models showed that these interface fixed charges are able to degrade the electron mobility in the channel.

Committee: Siddharth Rajan (Advisor); Steven Ringel (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering

Master of Science, University of Toledo, 2015, Electrical Engineering

.The main goal of this work was to develop the necessary expertise and model various parameters and characteristics of gallium nitride power HEMT (High Electron Mobility Transistor) devices. These include modelling of voltage-current characteristics, high temperature reverse bias leakage current, some circuit simulation, as well as radiation related effects on the device performance. Before modelling gallium nitride transistors, several other types of devices were modelled as part of a process to better understand the underlying physics involved in simulating HEMTs. To better compare the performance of commercially available gallium nitride transistors with the modelled gallium nitride devices, different types of depletion and enhancement structures of gallium nitride transistors were modelled. The voltage-current characteristics are simulated for various physically modelled gallium nitride structures. The obtained results are compared with those for commercially available gallium nitride transistors. Furthermore, drain to source leakage currents (at high temperature) of physically modelled gallium nitride devices were also modelled and are compared with performance of commercially available gallium nitride devices such as from Efficient Power Conversion Corporation (EPC®), including with actual measurements and test performed in our lab. The resulting simulations and experiments gave insight towards the factors which were limiting the device performance due to self-heating, leakage currents and the device geometry. We reviewed the electrical behavior of AlGaN/GaN HEMTs using circuit simulation tools. Basic power circuits were modelled in PSpice® (Circuit simulator from OrCAD Cadence) and Silvaco TCAD® (Process & Device simulator framework from Silvaco, Inc.), in order to observe switching behavior and compare the obtained results. To do this, commercially available EPC 2012 devices were imported into PSpice® and physically modelled enhancement gallium nitri (open full item for complete abstract)

Committee: Daniel Georgiev Dr. (Committee Chair); Vijay Devabhaktuni Dr. (Committee Member); Roger King Dr. (Committee Member) Subjects: Electrical Engineering

Master of Science (M.S.), University of Dayton, 2014, Electrical Engineering

The need for high power, highly efficient, multi-band and multi-mode radio frequency (RF) and microwave power amplifiers in the commercial and defense wireless industries continues to drive the research and development of gallium nitride (GaN) devices and their implementation in the receiver and transmitter lineups of modern microwave systems. Unlike silicon (Si) or gallium arsenide (GaAs), GaN is a direct wide bandgap semiconductor that permits usage in high voltage and therefore high power applications. Additionally, the increased saturation velocity of GaN allows for operation well into the super high frequency (SHF) portion of the RF spectrum. For the power amplifier designer, active devices utilizing GaN will exhibit power densities almost an order of magnitude greater than comparably sized GaAs devices and almost two orders of magnitude greater than Si devices. Not only does this mean an overall size reduction of an amplifier for a given output power, but it allows GaN to replace specialized components such as the traveling-wave tube (TWT) and other circuits once deemed impossible to realize using solid-state electronics. Designs utilizing GaN in amplifiers, switches, mixers, etc., are able to meet the continually shrinking size, increased power, stringent thermal, and cost requirements of a modern microwave system. There are two relatively straight forward methods used to investigate the intrinsic power scaling properties of a GaN high-electron-mobility transistor (HEMTs) configured as a common source amplifier. The first method involves sweeping the applied drain to source voltage bias and the second method involves scaling the physical size of the transistor. The prior method can be used to evaluate fixed sized transistors while the latter method requires an understanding of the obtainable power density for a given device technology prior to fabrication. Since the power density is also a function of the drain to source voltage bias, an initial iterative (open full item for complete abstract)

Committee: Guru Subramanyam Ph.D. (Committee Chair); Robert Penno Ph.D. (Committee Member); Weisong Wang Ph.D. (Committee Member) Subjects: Electrical Engineering

Master of Science in Engineering (MSEgr), Wright State University, 2007, Electrical Engineering

Investigation has been done on procedure, development, and verification of transistor topology for Aluminum-Gallium Nitride/Gallium Nitride (AlGaN/GaN) High-Electron Mobility Transistor (HEMTs). To date various models have been published that address modeling issues dealing with AlGaN/GaN HEMTs. Many models rely on analytic parameter solutions to help define model behavior. In order to find an optimum transistor at X-band frequency, 8-12 Gigahertz (GHz), layout, testing, modeling and simulation was conducted on various transistor topologies. Theoretical and experimental analysis has been completed for device operating point selection in measurement and modeling to account for self-heating and radio frequency (RF) dispersion effects. The model extraction allows for observations in parameter extraction changes that occur from growth imperfections to heating effects on the different topologies. The model extraction techniques allow for precise parameter extraction, resulting in predicted direct current (DC), scattering parameter (s-parameter), and large signal power performance verification at X-band.

Committee: Marian Kazimierczuk (Advisor) Subjects:

MS, University of Cincinnati, 2005, Engineering : Electrical Engineering

The increasing data rates demanded by third generation cellular communication systems and other high frequency applications require the use of power amplifiers operating at frequencies exceeding 1 GHz with output of the order of hundreds to thousands of watts. The area of very high frequency, very high power electronics is currently dominated by vacuum tube based devices as conventional semiconductor devices suffer from relatively low breakdown voltages precluding their operation at very high voltages and high powers. The vacuum tube based devices, however, suffer from the issues of high cost, large size and reliability issues. In recent years AlGaN/GaN HEMTs have demonstrated output power densities as high as 11 W/mm operating at microwave frequencies greater than 10 GHz. The extremely high output power density levels are achieved due to the high breakdown voltages of these wide bandgap devices and due to the large polarization induced charge leading to high output current densities. This work investigates the performance of the AlGaN/GaN HEMT using device modeling. Polarization effects have been incorporated using a highly doped AlGaN spacer layer. This thesis examines the effect of the device structure and doping profile on the AlGaN/GaN HEMT's microwave performance including the unilateral power gain and maximum frequency of oscillation.

Committee: Dr. Kenneth Roenker (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2010, Electrical and Computer Engineering

Demands for high-frequency signal amplification have been addressed over past decades with high electron mobility transistors (HEMTs) based on III-V semiconductors which can operate in mm-wave frequency bands (30-300 GHz). The high dielectric breakdown strength, saturated electron velocity, and carrier mobility in AlGaN/GaN two-dimensional electron gas (2DEG) provides a platform for high output power density for amplification over a similar frequency range compared to more mature material systems such as GaAs. Nitride semiconductors present unique challenges in terms of their strong polarization fields and sensitivity of 2DEG to fluctuations in surface potential. Aggressive geometry scaling is required in order to extend the frequency response of GaN-based HEMTs into the mm-wave regime, and short-channel effects (SCEs) thus far have resulted in diminishing returns as gate lengths (lg) are reduced below 300 nm. Successful field-effect transistor scaling requires that not only lg be reduced for low gate capacitance, but that channel aspect ratio defined as lg/d, where d is the gate-channel distance, remain large. To overcome processing challenges associated with aggressive scaling of GaN-based HEMTs we have developed technology enabling us to construct gates shorter than 30 nm and to precisely remove GaN and AlGaN from the gate area of (GaN/)AlGaN/GaN HEMTs to reduce d. The optical transparency of GaN has made consistent sub-100-nm gate definition challenging for electron beam lithography tools that employ optical height detection, and our transparent substrate electron beam focusing strategy eliminates such difficulty. Recessed gate GaN-based HEMTs have historically been plagued with high gate leakage and DC-RF current dispersion due to ion damage incurred during recess etching, but the N2/Cl2/O2 inductively-coupled plasma etch process described herein can provide very high GaN over AlGaN etch selectivity and does not require energetic ions. We compare two AlGaN/ (open full item for complete abstract)

Committee: Wu Lu (Advisor); Steven Ringel (Committee Member); Siddarth Rajan (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2009, Electrical and Computer Engineering

Despite numerous advances in the growth, fabrication, and characterization of AlGaN/GaN HEMT devices, there remain a number of unknowns related to the impact of deep levels on HEMT performance. Of specific interest to ongoing development of HEMT technology is the development of techniques which can not only detect the specific energy levels of deep levels in operating devices, but can also relate the presence of these defects to changes in specific device parameters. By examining more established techniques and developing new on-device characterization methods, the impact of defects on AlGaN/GaN HEMTs was quantitatively studied. Constant-voltage measurements of current and conductance transients are applied to AlGaN/GaN HEMT devices. Current deep level transient spectroscopy revealed two levels, one with an apparent activation energy that ranged from 0.16 eV to 0.31 eV, and one with an activation energy of 0.52 eV. The manifestation of both of these levels was shown to be affected by the magnitude of the gate-drain electric field. Conductance deep level transient spectroscopy reveled a number of traps with energies between EC- 0.20 eV and EC-0.42 eV, with the presence of a complex field profile making it difficult to determine which peaks were due to unique defects and which were due to the same species of defect emitting under multiple field conditions. Current deep level optical spectroscopy and conductance deep level optical spectroscopy were used to identify various deep levels with onsets at EC-1.55 eV, EC-2.55 eV, EC-2.9 eV, EC-3.25 eV, and EC-3.8 eV. These levels were similar to deep levels previously identified by capacitance measurements on similar material. None of these measurements yielded either trap concentrations or the impact of deep levels on parameters. To facilitate quantitative examination of the effect of deep levels on device parameters, the theory of constant-current measurements is developed. By regulating either the drain or gate voltage (open full item for complete abstract)

Committee: Steven A. Ringel PhD (Advisor); Siddharth Rajan PhD (Committee Member) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2009, Electrical and Computer Engineering

Although nitride electronics have matured rapidly, the performance and reliability of nitride high electron mobility transistors (HEMT) and other electronic devices have been hampered by electrically active defects that manifest as deep levels in the bandgap and/or as trap states. To alleviate these problems, not only is a fundamental understanding of the defects in GaN and AlGaN necessary for the continued development of nitride electronics, but also correlations of these defects to the performance and reliability limiting problems is required. Using a multipronged effort, defects were quantitatively studied at the component layer level (i.e. GaN and AlGaN) and also in operational HEMTs using techniques uniquely designed to quantitatively characterize defects as deep levels and traps in these devices.Deep level optical spectroscopy and related methods of trap spectroscopy are applied to several sets of systematically varied GaN and AlGaN materials. Traps in GaN were typically located at EC-0.25, EC-0.60, EC-0.90, EC-(1.28-1.35), EC-2.6, and EC-3.22/3.28 and for AlGaN at EC-0.87, EC-1.5, EC-3.10, and EC-3.93. It was determined that several traps showed specific dependencies on variations in growth parameters, substrate orientation, dislocation density, and growth method. Physical sources were attributed to most of these states for the first time, and this taxonomy is essential for analysis of trap effects in working AlGaN/GaN transistors, which constitutes the second focus of this research. To relate defect incorporation with HEMT performance and reliability, constant drain current deep level optical/transient spectroscopies using gate or drain voltage as the feedback mechanism are developed. This enables simultaneous and quantitative measurement of defect energies and concentrations of individual defects throughout the bandgap in HEMTs, measurement of device relevant parameters (threshold voltage shift and the change in gate-drain access resistance), separation of (open full item for complete abstract)

Committee: Steve Ringel (Advisor); Leonard Brillson (Committee Member); Siddharth Rajan (Committee Member) Subjects: Engineering

Doctor of Philosophy, The Ohio State University, 2006, Electrical Engineering

The AlGaN/GaN material system is ideally suited for UV detectors, light sources, and high performance, high power transistors. Through an understanding of the physics and device properties associated with defects, engineered solutions can allow the utilization of the full potential of AlGaN/GaN device properties. Auger Electron Spectroscopy (AES) and secondary electron threshold (SET) techniques allow the characterization of band bending and work function at semiconductor surfaces. Using these techniques with ultra-high vacuum (UHV) sample cleaving and metal deposition, Schottky barrier formation to non-polar GaN was investigated revealing cases of both ideal band-bending and Fermi level pinning. Cathodoluminescence spectroscopy (CL) allows the investigation of luminescent defect levels with depth-resolving capability by controlling the incident beam voltage and associated electron beam penetration into the sample. High electron mobility transistors (HEMTs) exhibiting current collapse were investigated using CL and CL mapping and specific defects were found in the GaN channel and buffer regions that may help explain the current collapse phenomena. Coupling a novel gate mask into a typical HEMT fabrication sequence and utilizing three, independent UHV sample cleaning techniques including thermal desorption of contaminants, Ga-reflux, and N2 ion sputtering, and metallization of the gates on AlGaN/GaN HEMTs, correlations in defect levels, surface cleaning technique, and finished device performance were found. In analyzing the CL data for this sample, however, a specific feature located just below the GaN near band edge was observed to accumulate near the Ohmic contacts prompting a further investigation of both the effects of the RIE etch used in producing the UHV-compatible mask as well as four different Ohmic contact structures on both defect levels determined by CL and on final device performance. Finally, a bulk GaN sample was processed with Ohmic contacts to determ (open full item for complete abstract)

Committee: Leonard Brillson (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2006, Electrical Engineering

Spatially-resolved cathodoluminescence spectroscopy (CLS) has been used to identify the presence of radiative point and extended defects in the semiconductor band gap produced by irradiation and processing conditions for Si and GaN-based devices. Changes in deep level emission in Al-SiO 2 -Si capacitor structures revealed a gradient in relative defect concentrations across the SiO 2 film after x-ray irradiation, indicating interface-specific defect creation. CLS measurements also revealed changes in the near-band edge signatures of AlGaN-GaN high-electron mobility transistor (HEMT) structures subjected to 1.8 MeV proton irradiation. These changes were indicative of alloying of AlGaN and GaN at the charge confinement interface and relaxation of piezoelectric strain in the AlGaN film. Alloying was confirmed with secondary-ion mass spectrometry, and each mechanism contributed to the measured degradation in HEMT channel transport properties. Ni-GaN Schottky barrier height decreases were also observed at lower fluences. 1.0 MeV protons were ~1.5 times more damaging than 1.8 MeV protons, which is consistent with simulations of total non-ionizing energy loss. Schottky contacts on x Al ~0.4 AlGaN were also investigated versus pre-deposition cleaning procedure. Two inductively-coupled plasma reactive-ion etching (ICP-RIE) procedures were compared with a standard HCl etch. The ICP-RIE treated samples exhibited higher uniformity than the HCl-etched surface, from electrical and CLS measurements. The presence of a spectral emission at in the HCl-etched piece correlated with the presence of a secondary Schottky barrier at ~1 eV. The emergence of a second spectral peak after ICP treatment also resulted in pinned barriers near 1 eV. A pre-metallization rapid-thermal annealing process after the ICP-RIE treatment resulted in the disappearance of both peaks, and correlated with the best diode electrical properties. The degree of Fermi level pinning from interface states, inferred from (open full item for complete abstract)

Committee: Leonard Brillson (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2004, Electrical Engineering

AlGaN/GaN heterostructures and high Al mole fraction AlGaN films are used in a wide variety of applications, such as high power/high frequency transistors, UV photodetectors, solar-blind detectors, light-emitting diodes, and laser diodes. However, there are several important issues that need to be addressed in AlGaN/GaN heterostructures, such as the impact of defect states on electronic properties such as mobility and two-dimensional electron gas (2DEG) sheet charge density as well as the role of surface processing on the Schottky barrier height. Also, Si doping of AlGaN with high Al mole fraction has been shown to be difficult and may be restricted by non-intentional impurities and their associated deep levels (such as O), as well as an increasing dopant donor energy with higher Al mole fraction. Correlations have been made between deep level defects and the 2DEG sheet charge density, interface broadening, surface roughness, and Ga-N ratios. Depth-dependent cathodoluminescence spectroscopy (CLS) and secondary ion mass spectrometry (SIMS) reveal the nature of deep level defects and their effect on Si doping of high Al mole fraction (25%-100%) AlGaN. SIMS results provide correlations between AlGaN deep level emissions from CLS and elemental impurities, such as oxygen, distributed through the epitaxial bulk films. Cross-sectional CLS measurements of the AlGaN/sapphire interface reveal luminescence signatures which correlate with oxygen diffusing from the sapphire into the AlGaN. Internal photoemission spectroscopy (IPE) reveals changes in the Schottky barrier height of Ni on AlGaN/GaN heterojunction field effect transistor structures (HFETs) with pre-metallization processing conditions and post-metallization ultra-high vacuum annealing. These variations in the IPE Schottky barrier height are correlated with AlGaN near band edge emissions from low energy electron-excited nanoluminescence spectroscopy (LEEN) and Ni/AlGaN interface impurities by SIMS. It is shown that ch (open full item for complete abstract)

Committee: Leonard Brillson (Advisor) Subjects:

Doctor of Philosophy, The Ohio State University, 2002, Electrical Engineering

The III-Nitride material system has proven extremely valuable for semiconductor device applications. The ability to grow high quality AlGaN/GaN that can be used for RF device applications is largely due to the commercial success of the implementation of p-type doping in GaN for optical devices. Even high quality GaN has relatively large defect densities. GaN devices are still able to achieve impressive performance, but not consistently. The variation in material quality, including deep-level defects and non-uniformities introduced by processing and growth, have deleterious effects on microwave device performance. These variations and the inability to control them reduce yield and reliability thus making AlGaN/GaN devices difficult to produce commercially. The purpose of this work is to characterize and contribute to the understanding of defects in AlGaN/GaN device systems and their effects on microwave device performance both DC and RF. The effects of device fabrication and surface processing on these defects have also been characterized. Low Energy Electron-Excited Nano-luminescence (LEEN) Spectroscopy has been used to characterize radiative defects in the AlGaN/GaN material system on a microscopic scale and compare them with electrical measurements on HEMT's and TLM structures. Salient features commonly observed in the LEEN spectra include donor-bound excitons in GaN at ~3.43 eV, donor-acceptor pair transitions (DAP) at ~3.30 eV, yellow luminescence (YL) centered at ~2.20 eV, AlGaN donor-bound exciton emission, and associated phonon replicas. These measurements have been used to successfully correlate contact and sheet resistance with DAP, YL, and AlGaN near-band edge emission spectral features within a given wafer and between wafers. The effects of ultra-high vacuum processing with Argon sputtering and rapid thermal annealing on defects observed with LEEN spectra have been documented. Microscopic LEEN analysis has also been performed on working microwave devices (open full item for complete abstract)

Committee: Leonard Brillson (Advisor) Subjects: