ECE PhD Graduates

Photo: Jon Cameron Pouncey

Jon Cameron Pouncey [Spring 2020]
03-27-2020

Biography

Jon Cameron Pouncey defended his PhD thesis on Friday, March 27 at 10 am in room 118 of the ECE Building. Dr. Jane Lehr served as his committee chair. The title of Mr.Pouncey's dissertation is, "Enabling Compact Pulsed Power."  

Abstract

The first two decades of the 21st century have seen significant interest in expanding the application of pulsed power technology beyond traditional uses in physics and nuclear weapons research. Applications in the field of national defense, which present unique constraints on system size, have provided impetus to increase the exploration of compact pulsed power solutions. Innovations related to energy density, insulation, switching, and power conversion systems have been realized, bringing deployable compact pulsed power systems closer to realization than ever before. However, recent systems integration efforts have shown that research into tools and technologies is still needed to unlock the full potential of compact pulsed power. This thesis describes efforts to make contributions to three of these research needs.

The first need is for tools that improve predictive simulation of compact pulsed power systems. The principal challenge in satisfying this need is to develop simulation models of components unique to pulsed power systems for use in well-known simulation tools. In order to address this challenge, a predictive parametric model of a spark gap switch has been developed and validated for use in the ubiquitous SPICE circuit simulation software.

The second need is to formalize a set of design principles for compact pulsed power systems. The principal challenge in satisfying this need is to improve understanding of the behavior of pulsed power systems as they are made significantly more compact. This understanding can then be applied early in the development of systems to improve confidence that the system will not require significant redesign late in the development cycle. In order to address this challenge, a detailed analysis of the erection behavior of the compact Marx generator was completed, and translated into design improvement recommendations.

The third need is to leverage new technologies developed for commercial applications to advance compact pulsed power. The principal challenge in satisfying this need is to identify and assess technologies for applicability to compact pulsed power systems. In order to begin addressing this challenge, a series of novel experiments using a newly developed commercial infrared diode-pumped solid-state micro-laser to trigger gas switches were conducted.

Photo: Mitchell Martin

Mitchell Martin [Spring 2020]
04-08-2020

Biography

Mitchell Martin defended his PhD thesis on Wednesday, April 8 at 10 am in a Zoom video conference. Dr. James Plusquellic served as her committee chair. The title of Mr. Martin's dissertation is, "Physical Unclonable Functions Based on Delay Paths and an Interdigital Microstrip Notch Filter."  

Abstract

Physical Unclonable Functions Based on Delay Paths and an Interdigital Microstrip Notch Filter
Abstract: A physical unclonable function (PUF) is an integrated circuit hardware primitive that is designed to leverage naturally occurring variations to produce a random bitstring. The arbiter (ARB) PUF is one of the first to be described in the literature. It derives its entropy from variations that occur in the delays of identically configured logic paths. The ARB PUF uses a phase comparator to decide which path of a pair is faster under a given challenge and generates a 0 or 1 as a response indicator bit. Unfortunately, the ARB PUF is not reliable, requiring error correction in cases where the sequence of response bits (the bitstring) needs to be reproduced. In this proposal, a test structure is described, called a time-to-digital converter (TDC) that can measure the actual delays of the paths. This type of ’soft’ information can be used to improve the reliability of the ARB PUF. Data obtained from a set of chips fabricated in IBM’s 90 nm technology, and collected across 9 temperature-voltage corners, is used to demonstrate its effectiveness.

Current PUF designs are typically implemented in silicon like the ARB PUF or utilize variations found in commercial off-the-shelf (COTS) parts. Because of this, existing designs are insufficient for the authentication of Printed Circuit Boards (PCBs). In this thesis, we also propose a novel PUF design that leverages board variations in a manufactured PCB to generate unique and stable IDs for each PCB. In particular, a single copper trace is used as a source of randomness for bitstring generation. The trace connects three notch filter structures in series, each of which is designed to reject specific but separate frequencies. The bitstrings generated with both the ARB PUF and the PCB PUF are evaluated using statistical tests that measure randomness, uniqueness, and reliability.

Photo: Ran Luo

Ran Luo [Spring 2020]
03-16-2020

Biography

Ran Luo defended his PhD thesis on Monday, March 16 at 12:30 pm in room 118 of the ECE building. Dr. Yin Yang served as his committee chair. The title of Mr. Luo's dissertation is, "Advancing Elastic Solid Dynamics in Computer Graphics."  

Abstract

This dissertation proposes novel algorithms and applications and provides a real-time and easy-to-use simulator for realistic animation of the 3D solid model. The Finite Element Method (FEM) is a popular tool in the community because of its accurate result, however, the FEM is computationally expensive to handle a large number of DOFs. We present novel techniques to combine linear and nonlinear elasticity with model reduction to provide fast and realistic animation. On the other hand, one of the most important computation tasks of solid simulation is to evaluate the gradient vector and Hessian matrix of elastic energy function. We present a numerical routine to simplify the implementation of solid simulation with the complex-step finite difference (CSFD) that avoids subtractive cancellation. The complexity of nonlinearity is also an obstacle, and we provide a framework called NNWarp to combine the linear elasticity and neural network-based warping method to avoid expensive nonlinear optimization. We also propose an acoustic-VR system as the application. The system can convert acoustic signals of human language to realistic 3D tongue animation in real-time. The Deep Neural Networks (DNN) helps to convert the input speaking voice to positions of pre-defined EMA sensors. Then, a novel reduced physics-based solid simulator, introduced in previous, is used to synthesis the tongue animation.

 

Photo: Farhana Anwar

Farhana Anwar [Spring 2020]
03-10-2020

Biography

Farhana Anwar defended her PhD thesis on Tuesday, March 10 at 1 pm in the CHTM building. Dr. Ashwani Sharma served as her committee chair. The title of Ms. Anwar's dissertation is, "Tunneling and Transport Properties of Graphene."  

Abstract

This article provides a new method for computing electronic transport properties of graphene i.e. the peculiar tunneling properties of two-dimensional massless Dirac electrons. We consider a simple situation: a massless Dirac electron incident on a potential barrier which is tilted by applied bias and use finite difference method to obtain transmission probability(without involving transfer matrix). In the presence of an applied bias transmission coefficient and tunneling current were obtained and the effect of electric field which modulates the barrier profile therefore conductivity pattern were explained. Furthermore, this method can also be applied to investigate transport properties of disordered graphene as well as device characteristics of room temperature ballistic graphene field-effect transistors. Our study opens up a possibility for graphene-based device optimization by engineering barrier (gate) geometry for graphene MOSFETs as well as devices exploiting optics like behavior of Dirac fermions. In this work we show that finite difference method can be used as an effective approach in low dimensional semiconductor physics.

Photo: Joshua J. Trujillo, Sr.

Joshua J. Trujillo, Sr. [Fall 2019]
04-10-2019

Biography

Joshua J. Trujillo, Sr. defended his PhD thesis on Wednesday, April 10 at 11:30 am in room 118 of the ECE building. Dr. Payman Zarkesh-Ha served as his committee chair. The title of Mr. Trujillo's dissertation is, "Study of Distributed Versus Compressed Layouts for PUFs."  

Abstract

Society continues to depend on electronics for everything from smart systems in our homes to cellphones and tablets, which are more powerful today than many desktop computers that are still in use [1]. With increased consumption of these electronic products comes an increase in problems, such as counterfeit integrated circuits being sold as genuine integrated circuits. This is just one of many problems that corporations and end users are having to deal with in this digital age.

Software security has been accepted as a problem by both the media and general public, but only recently has hardware security begun to come out as a problem as important, if not more important, than software security. When there is an issue with software, a software patch can be released to fix this issue. Due to the nature of hardware, this is not possible with hardware security problems. Often times, frrmware can be updated to partially address the problem, as in the case with the Intel and AMD processor flaws in the news recently [2]. However, there are many times that the only solution is to take the hardware system offline and replace the questionable components.

Hardware security researchers have been trying to fmd ways to use security circuits in hardware design to help combat this problem. One type of security circuit being researched and used today is called a Physically Unclonable Circuit (PUF). PUF circuits can be added onto an existing integrated circuit design, interrogated at fabrication for challenge-response pairs which are stored in a database, then checked against that database at any time in the lifecycle of that integrated circuit [3].

This research contributes to PUF security circuits by showing how different ways of laying out a circuit design on an integrated circuit can change the performance of the circuit, as well as if PUF circuits designed on SiGe BiCMOS can be used in the same way that PUF circuits fabricated using standard CMOS processes are used today.

Photo: Vijay Saradhi Mangu

Vijay Saradhi Mangu [Fall 2019]
07-25-2019

Biography

Vijay Saradhi Mangu defended his PhD thesis on Thursday, July 25 at 9 am in room 101 of the CHTM building. Dr. Francesca Cavallo served as his committee chair. The title of Mr. Mangu's dissertation is, "Pixelated GaSb Membranes for Photovoltaics: Fabrication and Structure-Property Relationships."  

Abstract

The purpose of this thesis is to develop a reliable and efficient approach to heterogeneous integration of single-crystalline GaSb semiconductors with highly mismatched materials. The mismatch may refer to the crystalline structure and the thermal expansion coefficient of single-crystalline GaSb with respect to the other materials of interest. The strategy of hetero-integration relies on epitaxial lift-off (ELO). My approach prevents formation of extended structural defects that are detrimental to the performance of opto-electronic devices and preserves GaSb growth substrates for potential reuse.

Within my research work, I have overcome some outstanding challenges of epitaxial lift-off of lattice-matched GaSb epitaxial layers through pixelated approach and demonstrated the operation of single-crystalline GaSb photovoltaic devices with a unique architecture on single-crystalline Si substrates.

By leveraging release and transfer of GaSb membranes on Si, I have demonstrated operation of thin-film photovoltaic devices with areas of the of the order of 100sx100s mm2 (i.e., pixelated solar cells). The photo-conversion efficiency of ~ 340x340 mm2 pixelated devices amount to ~2.5%, i.e., a comparable efficiency to a 5 x 5 mm2 homo-epitaxial GaSb cell on a GaSb substrate.

A detailed structure-property relationships study is also performed to justify device characteristics of pixelated GaSb solar cells.

In conclusion, I have established a reliable and efficient process to isolate GaSb epilayers without the formation of any extended defects. I have demonstrated thin films and pixelated GaSb photovoltaic devices on single-crystalline Si substrates and performed a detailed structure-property relationship study to justify their properties and provide a path to improve their performance.

Photo: Joseph Gleason

Joseph Gleason [Fall 2019]
11-06-2019

Biography

Joseph Gleason defended his PhD thesis on Wednesday, November 6 at 4:30 pm in room 118 of the ECE building. Dr. Meeko Oishi served as his committee chair. The title of Mr. Gleason's dissertation is, "Software Design for Probabilistic Safety: Stochastic Reachability and Circadian Control."  

Abstract

Reachability is an important verification tool for that provides guarantees of safety while driving the system toward a target. For stochastic systems, in which there is some stochastic disturbance present in the dynamics, we seek controllers that provide a maximal likelihood of safety. Stochastic reachability analysis seeks to a) solves for controllers that maximize the likelihood that a system will reach a target state while staying in safe set, or b) determine the set of initial states for which there exists a controller that can reach a target while staying safe with at least a certain likeli­hood. Many domains are interested in solutions to stochastic reachability problems. An important example are biomedical systems. Because these systems interact with humans or are models that represent human biological functions, safety in control design is paramount. For biomedical systems, assurances of safety is a difficult be­cause many dynamical models of biological systems are often not well known and the health impacts of violations to safety constraints can have catastrophic consequences. Reachability analysis can provide a methodological framework that can work as the engineering equivalent of "do no harm".

This thesis covers topics in reachability and analysis of human circadian rhythms,i.e. the human sleep-wake cycle. We combine theoretical development with software implementation for both domains.

For stochastic reachability we propose methods for approximating stochastic reach­able sets using Lagrangian, set-based, methods. These allow for very fast computa­tion in low-dimensional systems when using polytopic sets, but provide conservative results. These methods utilize recursions that only use basic set operations. The theoretical development is applicable to nonlinear systems, but implementation is re­stricted to linear systems. We also describe SReachTools, a stochastic reachability toolbox which includes these Lagrangian methods as well as many other approximate and exact solutions for stochastic reachability problems.

In the biomedical domain, we examine methods for determining circadian phase in traumatically brain injured subjects and describe the implementation of a testbed designed to facilitate experimentation in circadian control. For assessing circadian rhythms we use continuously gathered biological signals to allow for a continuous assessment of circadian phase. We additionally demonstrate a unique correlation between circadian power and subject outcome. The lighting testbed details many design considerations for implementing a circadian control facility and demonstrates its ability to operate and its efficacy on phase advancing subjects with a simple pilot study. Both of these work toward an ultimate goal of being able to integrate assurances of safety when using feedback-based lighting to regulate human circadian cycles. This is an area of ongoing and future work.

Photo: Michael Darling

Michael Darling [Fall 2019]
11-01-2019

Biography

Michael Darling defended his PhD thesis on Friday, November 1 at 11 am in room 310 of the ECE building. Dr. Don Hush served as his committee chair. The title of Mr. Darling's dissertation is, "Using Uncertainty To Interpret Supervised Machine Learning Predictions."  

Abstract

When a machine learning model generates a prediction, a decision maker needs to determine its validity. Currently, such judgments depend heavily on expert opinion, using a mix of domain and modeling expertise, and accuracy-based validation
metrics. While these methods evaluate a model’s performance relative to a set of validation data, they do not tell us the model’s certainty with respect to a particular prediction on an unseen, perhaps critical, data point.

In this dissertation we develop an uncertainty measure we call minimum prediction deviation which we use to assess the quality of the individual predictions made by supervised two-class classifiers. We show how minimum prediction deviation can be used to di↵erentiate between the samples that a model predicts credibly, and the samples on which an alternate analysis may be required.

Photo: Juan Briceno

Juan J. Faria-Briceno [Fall 2019]
11-04-2019

Biography

Juan J. Faria-Briceno defended his PhD thesis on Monday, November 4 at 9:30 am in room 101 of the CHTM building. Dr. Steven Brueck served as his committee chair. The title of Mr. Faria-Briceno's dissertation is, "Optical Angular Scatterometry: In-line Approach for Roll-2-roll and Nano-imprint Fabrication Systems."  

Abstract

As critical dimensions continue to shrink and structures become more complex, metrology processes are challenged to be implemented during in-line nanomanufacturing. Non-destructive, non-contact, and high-speed conditions are required to achieve proper metrology processes during in-line manufacturing. Optical scatterometry is a nanoscale metrology tool widely used in manufacturing. However, most applications of optical scatterometry operate off-line. A high-speed, in-line, non-contact, non-destructive scatterometry angular system has been demonstrated to scan pattern surfaces during real-time nano-fabrication.

Our system has demonstrated scanning capabilities using flat, 1D and 2D structures. The flat surface samples consist (commercially and grown) of thin film native oxide, grown oxide, and alumina. The 1D samples were made by using interferometric lithography with a thin deposited layer (~85 nm) of aluminum. The 2D complex samples are hollow silicon tubes fabricated using nano-imprint lithography. The inside diameter, outside diameter, and the period of the hollow pillars are respectively ~100 nm, ~135 nm and ~200 nm. These are test structures to establish the metrology capabilities. The applicability of the tool is not restricted to these samples.

Our current in-line scatterometer uses 45° off-axis parabolic mirrors which allows us to have an angular range up to ~50°. The system uses a high-speed scanner at 8 kHz to vary the angle of incident of the beam at the focal spot on the moving web. The system uses a 405 nm collimated diode laser. Our scan period is 125 µs which will allow us to scan 20-30 reflectance measurements before the web moves a distance comparable to the focal spot. A biased silicon detector is used to collect the high-speed reflection signal. The data collected is averaged with a digital scope and further processed on the computer for analysis. Our current system can be integrated with nano-imprint and other R2R real-time fabrication techniques. The goal is to improve quality control and monitor real-time high-speed nano fabrication processes. The angular range can be improved (up to 79°) by varying the focal length and the curvature of the parabolic mirrors. The scanning time can be reduced by increasing the frequency of the resonant scanner. Evidently, in-line angular scatterometry offers solutions to the future of R2R semiconductor nanomanufacturing.

 

Photo: Nicholas Tarasenko

Nicholas Tarasenko [Spring 2019]
02-21-2018

Biography

Nicholas Tarasenko defended his PhD thesis on Thursday, February 21, 2019 at 1 pm in room 118 of the ECE Building. Dr. Christos Christodoulou served as his committee chair. The title of Mr. Tarasenko's dissertation is, "DESIGN AND IMPLEMENTATION OF A 72 & 84 GHZ TERRESTRIAL PROPAGATION EXPERIMENT; EXPLOITATION OF NEXRAD DATA TO STATISTICALLY ESTIMATE RAIN ATTENUATION AT 72 GHZ."

Abstract

The wireless communication sector is rapidly approaching network capacities as a direct result of increasing mobile broadband data demands. In response, the Federal Communications Commission allocated 71-76 GHz “V-band” and 81-86 GHz “W-band” for terrestrial and satellite broadcasting services. Movement by the telecommunication industry towards W/V-band operations is encumbered by a lack of validated and verified propagation models, specifically models to predict attenuation due to rain. Additionally, there is insufficient data available at W/V-bands to develop or test propagation models. The first aim of this study was the successful installation and operation of a terrestrial link to collect propagation data at W/V-band frequencies. In September 2015, the University of New Mexico, in collaboration with the Air Force Research Laboratory’s Space Vehicle Directorate, NASA’s Glenn Research Center and industry partners including (ACME, Applied Technology Associates, and Quinstar Technologies, Inc.) established the W/V-band Terrestrial Link Experiment (WTLE). WTLE was installed in the Albuquerque metro area with persistent tonal transmissions at 72 GHz and 84 GHz on a 23.5 km slanted path.

The second aim of this study was the utilization of the National Weather Service’s Next Generation Weather Radar (NEXRAD) system data to statistically estimate attenuation due to rain at 72 GHz. NEXRAD data provides a distributed sense of rain rates along WTLE’s path and alleviates challenges associated with instrumenting the 23.5 km link. Furthermore, NEXRAD data alleviates the need to develop complicated routines using in-situ meteorological measurements to estimate the size of the rain cell affecting the link. Non-linear regression techniques were applied on 2017 monsoon season data to obtain rain rate power law model coefficients. Testing of these coefficients was conducted on 2018 monsoon season data with satisfactory results. The techniques employed in this analysis represent a significant advancement in the ability to predict attenuation due to rain at 72 GHz for terrestrial links by enabling the use of historical archives of publicly available National Weather Service NEXRAD data. The technique has promising potential for estimation of path attenuation due to rain for links other than WTLE because of the vast nationwide coverage provided by NEXRAD systems.  

 

Photo: Brock Roberts

Brock Roberts [Spring 2019]
04-15-2019

Biography

Brock Roberts defended his PhD thesis on Monday, April 15, 2019 at 1 pm in room 237 of the ECE Building. Dr. Edl Schamiloglu served as his committee chair. The title of Mr. Robert's dissertation is, "Noise and Gain Characterization of Interband Cascade Infrared Photodetectors."

Abstract

A cavity designed to have multiple harmonic TM0N0 modes can be used to accurately measure the longitudinal profile of a bunched charged particle beam passing through its bore, non-invasively, and in real time. 

Multi-harmonic TM0N0 cavities were designed, constructed, and beamline tested in a variety of experiments at the Thomas Jefferson National Accelerator Facility (TJNAF or Jlab).   Measurements with a sampling oscilloscope provided signals that resemble the profile of electron bunches passing through the cavity’s bore.  Straightforward signal processing techniques reduce distortion in the measurement and provide real time profiles of electron bunches with picosecond accuracy.  Subharmonic beams having bunch repetition rates of 1/3rd and 1/6th of Jlab’s 1497 MHz bunch frequency, and interleaved sub-harmonic beams were also measured.  Comparison between measurements made using a harmonic cavity were corroborated with an established invasive measurement method and with computer models.   A harmonic cavity from this effort has been installed within the CEBAF injector, allowing accelerator operators to view, in real time, the shape and duration of electron bunches entering the accelerator.  Another harmonic cavity has been installed within Jlab’s Upgraded Injector Test Facility (UITF), and two more are planned for installation there.   This effort was awarded the 2016 International Beam Instrumentation Conference’s Faraday Cup Award.


 

Photo: Alvaro Cerna

Alvaro Ulloa Cerna [Summer 2019]
07-03-2019

Biography

Alvaro Ulloa Cerna defended his PhD thesis on Wednesday, July 3 at 8 am in room 118 of the ECE building. Dr. Marios Pattichis served as his committee chair. The title of Mr. Cerna's dissertation is, "Large Scale Electronic Health Record Data and Echocardiography Video Analysis for Mortality Risk Prediction."  

Abstract

Electronic health records contain the clinical history of patients. The enormous potential for discovery in such a rich dataset is hampered by their complexity. We hypothesize that machine learning models trained on EHR data can predict future clinical events significantly better than current models. We analyze an EHR database of 594,862 Echocardiography studies from 272,280 unique patients with both unsupervised and supervised machine learning techniques.    

In the unsupervised approach, we first develop a simulation framework to evaluate a family of different clustering pipelines. We apply the optimized approach to 41,645 patients with heart failure without providing any survival information to the underlying clustering approach. The model separates patients with significantly different survival characteristics. For example, in a 10-cluster model, the minimum and maximum risk clusters had a median survival of 22 and 53 months respectively.

In the supervised approach, with 723,754 videos available from 27,028 unique patients, we assess the predictive capacity of Echocardiography video data for one- year mortality. Also, we hold out a balanced dataset of 600 patients to compare the model performance against cardiologists. We found that the best model, among four candidate architectures, is a 3D dyadic CNN model with an average AUC of 0.78 for a single parasternal long axis view. The model yields an accuracy of 75% (AUC of 0.8) on the held-out dataset while the cardiologists achieve 56% and 61%. The model performance was significantly higher than that of the cardiologists.

Finally, we develop a multi-modal supervised approach that enables interpretability. The model provides interpretations through polynomial transformations that describe the individual feature contribution and weights the transformed features to determine their importance. We validate our proposed approach using 31,278 videos from 26,793 patients. We test our proposed approach against logistic regression and non-linear and non-interpretable models based on Random Forests and XGBoost. Our results show that the proposed neural network architecture always outperforms logistic regression models while its performance approximates the other non-linear models. Overall, our multi-modal classifier based on 3D dyadic CNN and the interpretable neural network outperforms all other classifiers (AUC=0.83).

 

Photo: Francisco German Perez Venegas

Francisco German Perez Venegas [Spring 2019]
04-12-2019

Biography

Francisco German Perez Venegas defended his PhD thesis on Friday, April 12 at 2:30 pm in room 118 of the ECE Building. Dr. Balu Santhanam served as his committee chair. The title of Mr. Venegas' dissertation is, "Detection and classification of vibrating objects in SAR images."

Abstract

The vibratory response of buildings and machines contains key information that can be exploited to infer their operating conditions and to diagnose failures. Further­more, since vibration signatures observed from the exterior surfaces of structures are intrinsically linked to the type of machinery operating inside of them, the ability to monitor vibrations remotely can enable the detection and identification of concealed machinery.

This dissertation focuses on developing novel detection schemes for performing detection and classification of vibrating objects in SAR images. Three are the cen­tral claims of this dissertation. First, the non-linear transformation that the micro­doppler return of a vibrating object suffers through SAR sensing does not destroy its information. Second, the instantaneous frequency (IF) of the SAR signal has sufficient information to characterize vibrating objects. Third, it is possible to de­velop a detection model that encompasses multiple scenarios including both mono­component and multi-component vibrating objects immersed in noise and clutter.

For answering these claims, two different schemes are studied for both the de­tection and classification of vibrating objects in SAR images. The first scheme is data-driven and utilizes features extracted with the discrete fractional Fourier transform (DFRF T) to feed machine-learning algorithms (MLAs). Specifically, the DFRFT is applied to the IF of the slow-time SAR data, which is reconstructed using techniques of time-frequency analysis. The second scheme is model-based and employs a probabilistic model of the SAR slow-time signal, the Karhunen-Loeve transform (KLT), and a likelihood-based decision function. The performance of the proposed schemes is characterized using simulated data as well as real SAR data co­llected with the Lynx SAR. The suitability of SAR for sensing vibrations is demons­trated by showing that the separability of different classes of vibrating objects is preserved even after non-linear SAR processing.

Finally, the proposed algorithms are studied in the presence of signal noise and terrain clutter. The results show that the proposed schemes produce high-precision classifiers capable of dealing with noise and clutter of moderate intensity. In order to loosen these requirements, the Hankel rank reduction (HRR) method, previously used for suppressing ocean clutter in ground-wave radar, is adapted to suppress clutter-noise in SAR images.

Photo: Eli Garduno

Eli Garduno [Spring 2019]
09-06-2018

Biography

Eli Garduno defended his PhD thesis on Thursday Sept. 6, 2018 at 10 am in the CHTM Building. Dr. Ganesh Balakrishnan served as his committee chair. The title of Mr. Garduno's dissertation is, "Noise and Gain Characterization of Interband Cascade Infrared Photodetectors."

Abstract

Infrared (IR) detectors are an enabling technology for a broad and growing list of applications including gas detection, night vision, and space-based missile warning. There are ongoing efforts in IR detector research to explore the potential of new material systems and energy band structures in addition to continuously improving their sensitivity through increasing their quantum efficiency and lowering their dark current and noise. This dissertation examines an emerging class of IR detectors known as Interband Cascade Infrared Photodetectors (ICIPs).

ICIPs contain multiple regions to facilitate the collection of photogenerated elec­trons and to limit unwanted dark current. Theory regarding their performance also indicates that multi-stage ICIPs may have lower noise than single-stage ICIPs and may provide improved detectivity in cases where the absorption coefficient of a ma­terial system is small and/ or where the diffusion length in the material is short or degraded.

In this work, four long-wavelength infrared ICIP devices with one, four, six, and eight stages were characterized at varying temperatures from 80 to 300 K and at biases up to one volt in both forward and reverse polarities. Noise spectra were collected on the four devices and show significant 1 / f noise that prevented direct measurement of the ICIP noise gain. The 1/ f noise in the ICIPs was linked to generation-recombination current. The devices were found to cause circuit instability when operated in bias regions with negative differential conductance (NDC) due to bias-dependent resonant tunneling. Additionally, bias-dependent photocurrent gain was observed using illumination of the devices with 632 nm and 1550 nm lasers which peaked near the NDC regions. This photocurrent gain was experimentally shown to be caused by current-mismatch between device stages, verifying theories regarding its origin.

Photo: Lilian Casias

Lilian Casias [Spring 2019]
03-20-2019

Biography

Lilian Casias defended her PhD thesis on Wednesday, March 20 at 10 am at the CHTM building. Dr. Ganesh Balakrishnan served as her committee chair. The title of Ms. Casias' dissertation is, "Transport in Mid-Wavelength Infrared (MWIR) p- and n- type InAsSb and InAs/InAsSb Type-II Strained Layer Superlattices (T2SLs) for infrared detection."

Abstract

III-V materials such as InAsSb ternaries and InAs/InAsSb Type-II Strained Layer Superlattices (T2SLs) have significant potential for infrared (IR) detector applications, including space-based detection, when utilized in a unipolar barrier detector architecture (nBn). However, recent studies revealed the quantum efficiency in nBn detectors degrades significantly faster from proton-irradiation induced displacement damage as compared to HgCdTe photodiodes. Improving the quantum efficiency radiation-tolerance is theoretically possible by enhancing vertical hole mobility and thereby the vertical hole diffusion length. The vertical hole mobility of T2SLs materials differs significantly from the lateral mobility and measuring it is much less straightforward.

In order to tackle vertical transport, in-plane or lateral transport must be better understood. There are added challenges to determining the in-plane bulk carrier concentration in narrow bandgap materials due to the potential for electron accumulation at the surface of the material and at its interface with the layer grown directly below it. Electron accumulation layers form high conductance electron channels that can dominate both resistivity and Hall-effect transport measurements. Therefore, to correctly determine the in-plane bulk concentration and mobility, temperature- and magnetic-field-dependent transport measurements in conjunction with Multi-Carrier Fit (MCF) analysis were utilized on a series of p-doped InAs0.91Sb0.09 samples on GaSb substrates. The samples are etched to different thicknesses and variable-field measurements are utilized to assist in confirming whether a carrier species represents bulk, interface or surface conduction.

Secondly, n-type temperature- and magnetic-field dependent measurements on InAsSb and InAs/InAsSb T2SLs materials were performed to extract the in-plane transport properties for all the carriers present in each sample under two different doping concentrations (undoped and Silicon-doped). Lastly, substrate-removed, metal-semiconductor-metal (MSM) devices were fabricated to attempt vertical measurements, while standard van der Pauw structures were used for in-plane measurements. The MSM processing serves as a potential fabrication technique to measure vertical transport, that can be improved in the future. The goal of this dissertation is to accurately determine the lateral and vertical transport properties in the presence of multiple carrier species, Multi-Carrier Fit (MCF) and High-Resolution Mobility Spectrum Analysis (HR-MSA) were employed.



Photo: Saadat M. Mishkat-Ul-Masabih

Saadat M. Mishkat-Ul-Masabih [Summer 2019]
04-24-2019

Biography

Saadat M. Mishkat-Ul-Masabih defended his PhD thesis on Wed, April 24 at 10 am in room 101 of the CHTM Building. Dr. Daniel Feezell served as a committee chair. The title of Mr Saadat M. Mishkat-Ul-Masabih's dissertation is, "Nonpolar GaN-Based VCSELs with Lattice-Matched Nanoporous Distributed Bragg Reflector Mirrors."

Abstract

Wide-bandgap optoelectronic devices have undergone significant advancements with the advent of commercial light emitting diodes and edge-emitting lasers in the violet-blue spectral region. They are now ubiquitous in several lighting, communication, data storage, display, and sensing applications. Among the III-nitride emitters, vertical-cavity surface-emitting lasers (VCSELs) have attracted significant attention in recent years due to their inherent advantages over edge-emitting lasers. The small active volume enables single-mode operation with low threshold currents and high modulation bandwidths. Their surface-normal device geometry is conducive to the cost-effective formation of high-density 2D arrays while simplifying on-chip wafer testing. Furthermore, the low beam divergence and circular beam profiles in VCSELs allow efficient fiber coupling.

Nevertheless, GaN-based VCSELs are still in the early stages of development. Several challenges need to be addressed before high-performance devices can be commercially realized. One such challenge is the lack of high-quality distributed Bragg reflector (DBR) mirrors. Conventionally, epitaxial and dielectric DBRs are used which often involve complex growth and fabrication techniques. This dissertation provides an alternative approach where subwavelength air-voids (nanopores) are introduced in alternating layers of doped/undoped GaN to form the DBR structure. Selective electrochemical etching creates nanopores in the doped layers, reducing the effective refractive index relative to the surrounding undoped GaN. Using only 16-pairs, DBR reflectance >99.9% could be achieved. Several research groups have shown optically pumped VCSELs using nanoporous DBRs on c-plane. However, there are no reports of electrically injected nanoporous VCSELs. Using m-plane GaN substrates, we have demonstrated the first ever electrically injected GaN-based VCSEL using a lattice-matched nanoporous DBR. The nonpolar m-plane orientation is beneficial for leveraging the higher per-pass gain and polarization-pinning properties absent in c-plane. Lasing under pulsed operation at room temperature was observed at 409 nm with a linewidth of ~0.6 nm and a maximum output power of ~1.5 mW. This is the highest output power from m-plane VCSELs to date with relatively stable operation at elevated temperatures. All tested devices were linearly polarization-pinned in the a-direction with high polarization ratios >0.9. Overall, the nanoporous DBRs help in mitigating some of the issues that limit the performance of III-nitride VCSELs.