VTechWorks Repository :: Browsing by Author "Lester, Luke F."

Browsing by Author "Lester, Luke F."

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Advanced Energy-Efficient Devices for Ultra-Low Voltage System: Materials-to-Circuits
Liu, Jheng-Sin (Virginia Tech, 2018-01-18)
The overall energy consumption of portable devices has been projected to triple over the next decade, growing to match the total power generated by the European Union and Canada by 2025. The rise of the internet-of-things (IoT) and ubiquitous and embedded computing has resulted in an exponential increase in such devices, wherein projections estimate that 50 billion smart devices will be connected and online by 2020. In order to alleviate the associated stresses placed on power generation and distribution networks, a holistic approach must be taken to conserve energy usage in electronic devices from the component to the circuit level. An effective approach to reduce power dissipation has been a continual reduction in operating voltage, thereby quadratically down-scaling active power dissipation. However, as state-of-the-art silicon (Si) complimentary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) enter sub-threshold operation in the ultra-low supply voltage regime, their drive current is noticeable degraded. Therefore, new energy-efficient MOSFETs and circuit architectures must be introduced. In this work, tunnel FETs (TFETs), which operate leveraging quantum mechanical tunneling, are investigated. A comprehensive investigation detailing electronic materials, to novel TFET device designs, to memory and logic digital circuits based upon those TFETs is provided in this work. Combined, these advances offer a computing platform that could save considerable energy and reduce power consumption in next-generation, ultra-low voltage applications.
Bioimpedance spectroscopy of breast cancer cells: A microsystems approach
Srinivasaraghavan, Vaishnavi (Virginia Tech, 2015-11-04)
Bioimpedance presents a versatile, label-free means of monitoring biological cells and their responses to physical, chemical and biological stimuli. Breast cancer is the second most common type of cancer among women in the United States. Although significant progress has been made in diagnosis and treatment of this disease, there is a need for robust, easy-to-use technologies that can be used for the identification and discrimination of critical subtypes of breast cancer in biopsies obtained from patients. This dissertation makes contributions in three major areas towards addressing the goal. First, we developed miniaturized bioimpedance sensors using MEMS and microfluidics technology that have the requisite traits for clinical use including reliability, ease-of-use, low-cost and disposability. Here, we designed and fabricated two types of bioimpedance sensors. One was based on electric cell-substrate impedance sensing (ECIS) to monitor cell adhesion based events and the other was a microfluidic device with integrated microelectrodes to examine the biophysical properties of single cells. Second, we examined a panel of triple negative breast cancer (TNBC) cell lines and a hormone therapy resistant model of breast cancer in order to improve our understanding of the bioimpedance spectra of breast cancer subtypes. Third, we explored strategies to improve the sensitivity of the microelectrodes to bioimpedance measurements from breast cancer cells. We investigated nano-scale coatings on the surface of the electrode and geometrical variations in a branched electrode design to accomplish this. This work demonstrates the promise of bioimpedance technologies in monitoring diseased cells and their responses to pharmaceutical agents, and motivates further research in customization of this technique for use in personalized medicine.
Characterization of Proton and Sulfur Implanted GaSb Photovoltaics and Materials
Karimi, Ebrahim (Virginia Tech, 2021-01-25)
III-V compound Gallium Antimonide (GaSb), with a low bandgap of 0.72 eV at room temperature, is an attractive candidate for a variety of potential applications in optoelectronic devices. Ion implantation, among non-epitaxial methods, is a common and reliable doping technique to achieve local doping and obtain high-performance ohmic contacts in order to form a pn junction in such devices. An advantage of this technique over the diffusion method is the ability to perform a low-temperature process leading to accurate control of the dopant profile and avoiding Sb evaporation from GaSb surface occurring at 370 C. In this work, the effect of protons and sulfur ions as two implant species on the electrical behavior of MBE-grown undoped GaSb on semi-insulating (SI) GaAs was investigated via the Hall Effect. Protons and sulfur ions were implanted at room temperature (27 C) and 200 C, respectively, and rapid thermal annealing (RTA) was implemented at various temperatures and durations upon encapsulated GaSb. The damage induced by protons enhanced the hole density of GaSb up to around 10 times, whereas mobilities showed both increase and decrease compared to the un-implanted one, depending on the dose. While the activation of sulfur donors at an elevated temperature was anticipated after annealing sulfur implanted GaSb, instead it led to increase in p-type concentration, as the residual damage originated from sulfur implantation dominated substitutional doping. Furthermore, GaSb p/n photovoltaic devices were fabricated by applying sulfur implantation through silicon nitride layer at RT into an n-GaSb wafer (n-type base, p-type emitter). The device showed a rectifying current and photovoltaic characteristic. The J-V plot under AM1.5G illumination conditions, before and after an etch-back optimizing process, indicated lower short circuit current density J_sc, the same open circuit voltage V_oc, and higher fill factor FF, compared to the photovoltaic device with a p-type base. Also, both normalized series R_s and shunt R_p resistances in p/n diode indicated lower and higher values, respectively, as opposed to a GaSb p++/p diode, indicative of higher quality and lower manufacturing defects.
CMOS Receiver Design for Optical Communications over the Data-Rate of 20 Gb/s
Chong, Joseph (Virginia Tech, 2018-06-21)
Circuits to extend operation data-rate of a optical receiver is investigated in the dissertation. A new input-stage topology for a transimpedance amplifier (TIA) is designed to achieve 50% higher data-rate is presented, and a new architecture for clock recovery is proposed for 50% higher clock rate. The TIA is based on a gm-boosted common-gate amplifier. The input-resistance is reduced by modifying a transistor at input stage to be diode-connected, and therefore lowers R-C time constant at the input and yielding higher input pole frequency. It also allows removal of input inductor, which reduces design complexity. The proposed circuit was designed and fabricated in 32 nm CMOS SOI technology. Compared to TIAs which mostly operates at 50 GHz bandwidth or lower, the presented TIA stage achieves bandwidth of 74 GHz and gain of 37 dBohms while dissipating 16.5 mW under 1.5V supply voltage. For the clock recovery circuit, a phase-locked loop is designed consisting of a frequency doubling mechanism, a mixer-based phase detector and a 40 GHz voltage-controlled oscillator. The proposed frequency doubling mechanism is an all-analog architecture instead of the conventional digital XOR gate approach. This approach realizes clock-rate of 40 GHz, which is at least 50% higher than other circuits with mixer-based phase detector. Implemented with 0.13-μm CMOS technology, the clock recovery circuit presents peak-to-peak clock jitter of 2.38 ps while consuming 112 mW from a 1.8 V supply.
Design of a Highly Linear 24-GHz LNA
Elyasi, Hedieh (Virginia Tech, 2016-07-05)
The increasing demand for high data rate devices and many applications in short range high speed communication, attract many RF IC designers to work on 24-GHz transceiver design. The Federal Communication Commission (FCC) also dedicates the unlicensed 24-GHz band for industrial, science, and medical applications to overcome the interference in overcrowded communications and have higher output signal power. LNA is the first building of the receiver and is a very critical building block for the overall receiver performance. The total NF and sensitivity of the receiver mainly depends on the LNAs NF that mandates a very low NF LNA design. Depending on its gain, the noise figure of the next stages can relax. However, the high gain of an LNA enforces the next stages to be more linear since they suffer from larger signal at their input stage and can get saturated easily. Apparently, designing high gain, low noise, and highly linear LNA is very stimulating. In this thesis, a wideband LNA with low noise figure and high linearity has been designed in 8XP 0.13-um SiGe BiCMOS IBM technology. The highlight of this design is proposing the peaking technique, which results in considerable linearity improvement. Loading the LNA with class AB amplifier, power gain experiences a peaking in high input signal swing levels. The next stager after the LNA is the buffer to provide isolation between the LNA and mixer, and also avoid loading of the LNA from the mixer. Instead of using popular emitter follower architecture, another circuit is proposed to have higher gain and linearity. This buffer has two separate out of phase inputs, coming from the LNA and are combined constructively at the output of the buffer. Since the frequency of this design is high, electromagnetic (EM) simulation for pads, interconnects, transmission lines, inductors, and coplanar transmission lines has been completed using Sonnet cad tool to consider all the parasitic and coupling effects. Considering all the EM effects, the LNA has 15 dB gain with 2.9 dB NF and -8.8 dBm input 1-dB compression point. The designed LNA is wideband, covering the frequency range of 12-GHz to 31-GHz. However, the designed LNA, has the capability of having higher gain at the expense of lower linearity and narrower frequency band using different control voltage. As an example peak gain of 29.3 dB at the 3-dB frequency range of 23.8 to 25.8-GHz can be achieved, having 2.3 dB noise figure and -17 dBm linearity.
Design, Fabrication and Characterization of a GaAs/InxGa1-xAs/GaAs Heterojunction Bipolar Transistor
Lidsky, David (Virginia Tech, 2014-10-16)
Designs for PnP GaAs/InxGa1-xAs/GaAs heterojunction bipolar transistors (HBTs) are proposed and simulated with the aid of commercial software. Band diagrams, Gummel plots and common emitter characteristics are shown for the specific case of x=1, x=0.7, and x linearly graded from 0.75 to 0.7. Of the three designs, it is found that the linearly graded case has the lowest leakage current and the highest current gain. IV curves for all four possible classes of InAs/GaAs heterojunction (nN, nP, pN, pP) are calculated. A pN heterojunction is fabricated and characterized. In spite of the 7% lattice mismatch between InAs and GaAs, the diode has an ideality factor of 1.26 over three decades in the forward direction. In the reverse direction, the leakage current grows exponentially with the magnitude of the bias, and shows an effective ideality factor of 3.17, in stark disagreement with simulation. IV curves are taken over a temperature range of 105 K to 405 and activation energies are extracted. For benchmarking the device processing and the characterization apparatus, a conventional GaAs homojunction diode was fabricated and characterized, showing current rectification ratio of 109 between plus one volt and minus one volt. Because the PnP material for the optimal HBT design was not available, an Npn GaAs/InAs/InAs HBT structure was processed, characterized, and analyzed. The Npn device fails in both theory and in practice; however, by making a real structure, valuable lessons were learned for crystal growth, mask design, processing, and metal contacts.
Development of an Automated Coin Grading System: Integrating Image Preprocessing, Feature Extraction, and ML Modeling
Chen, Jianzhu (Virginia Tech, 2024-12-20)
For more than 70 years, the Sheldon Coin Grading Scale has been essential in quantifying the value of coins within the coin collecting industry. Traditionally, coin grading has relied on human graders who may deliver inconsistent results. This inconsistency leads to variations in coin values. In this thesis, we present an automated coin grading system that uses image preprocessing, feature extraction, and advanced machine learning techniques to predict the grade across different coin types. Our system employs synthetic reference masks to identify "expected" regions, like the contours of reliefs, and "unexpected" regions, such as surface non-uniformities. All detected significant elements and tiny elements, extracted from these regions, will serve as one of the feature sets. Additionally, we extract color histograms as another feature set to analyze color and texture in detail. Both feature sets from the obverse and reverse sides of the coins are processed using a multi-layer perceptron (MLP) model and a random forest model. The best-performing model is then selected to grade the coins by analyzing their overall wear patterns and color characteristics. Our grading system has demonstrated an accuracy of up to 91.3% in predicting the Sheldon Grading Scale across five coin types, allowing for a grading tolerance of ±4. For a single coin type (Franklin Half Dollar), it has achieved an accuracy of up to 95.1% with a tolerance of ±1.
Diode Laser Spectroscopy for Measurements of Gas Parameters in Harsh Environments
Behera, Amiya Ranjan (Virginia Tech, 2017-03-06)
The detection and measurement of gas properties has become essential to meet rigorous criteria of environmental unfriendly emissions and to increase the energy production efficiency. Although low cost devices such as pellistors, semiconductor gas sensors or electrochemical gas sensors can be used for these applications, they offer a very limited lifetime and suffer from cross-response and drift. On the contrary, gas sensors based on optical absorption offer fast response, zero drift, and high sensitivity with zero cross response to other gases. Hence, over the last forty years, diode laser spectroscopy (DLS) has become an established method for non-intrusive measurement of gas properties in scientific as well as industrial applications. Wavelength modulation spectroscopy (WMS) is derivative form of DLS that has been increasingly applied for making self-calibrated measurements in harsh environments due to its improved sensitivity and noise rejection capability compared to direct absorption detection. But, the complexity in signal processing and higher scope of error (when certain restrictions on operating conditions are not met), have inhibited the widespread use of the technique. This dissertation presents a simple and novel strategy for practical implementation of WMS with commercial diode lasers. It eliminates the need for pre-characterization of laser intensity parameters or making any design changes to the conventional WMS system. Consequently, sensitivity and signal strength remain the same as that obtained from traditional WMS setup at low modulation amplitude. Like previously proposed calibration-free approaches, this new method also yields absolute gas absorption line shape or absorbance function. Residual Amplitude Modulation (RAM) contributions present in the first and second harmonic signals of WMS are recovered by exploiting their even or odd symmetric nature. These isolated RAM signals are then used to estimate the absolute line shape function and thus removing the impact of optical intensity fluctuations on measurement. Uncertainties and noises associated with the estimated absolute line shape function, and the applicability of this new method for detecting several important gases in the near infrared region are also discussed. Absorbance measurements from 1% and 8% methane-air mixtures in 60 to 100 kPa pressure range are used to demonstrate simultaneous recovery of gas concentration and pressure. The system is also proved to be self-calibrated by measuring the gas absorbance for 1% methane-air mixture while optical transmission loss changes by 12 dB. In addition to this, a novel method for diode laser absorption spectroscopy has been proposed to accomplish spatially distributed monitoring of gases. Emission frequency chirp exhibited by semiconductor diode lasers operating in pulsed current mode, is exploited to capture full absorption response spectrum from a target gas. This new technique is referred to as frequency chirped diode laser spectroscopy (FC-DLS). By applying an injection current pulse of nanosecond duration to the diode laser, both spectroscopic properties of the gas and spatial location of sensing probe can be recovered following traditional Optical Time Domain Reflectometry (OTDR) approach. Based on FC-DLS principle, calibration-free measurement of gas absorbance is experimentally demonstrated for two separate sets of gas mixtures of approximately 5% to 20% methane-air and 0.5% to 20% acetylene-air. Finally, distributed gas monitoring is shown by measuring acetylene absorbance from two sensor probes connected in series along a single mode fiber. Optical pulse width being 10 nanosecond or smaller in the sensing optical fiber, a spatial resolution better than 1 meter has been realized by this technique. These demonstrations prove that accurate, non-intrusive, single point, and spatially distributed measurements can be made in harsh environments using the diode laser spectroscopy technology. Consequently, it opens the door to practical implementation of optical gas sensors in a variety of new environments that were previously too difficult.
Dynamic Intermode Beat Frequency Control of an Optical Frequency Comb Single Section Quantum Dot Laser by Dual-Cavity Optical Self-Injection
Stutz, Sebastian; Auth, Dominik; Weber, Christoph; Drzewietzki, Lukas; Nikiforov, Oleg; Rosales, Ricardo; Walther, Thomas; Lester, Luke F.; Breuer, Stefan (2019-10)
Dynamic frequency tuning of the 40.67 GHz intermode beat frequency of a 1255 nm emitting 1 mm long monolithic self mode-locked single section optical frequency comb InAs/InGaAs quantum dot laser across 70 MHz is experimentally demonstrated by fine-delay dual-cavity controlled all optical self-injection. Fiber-based macroscopic optical delay lengths are 9.4 m (round-trip time of 62.7 ns) and 16.5 m (round-trip time of 110.1 ns), the maximum studied microscopic delay tuning times are 40 ps and the optical self-injection strengths are below 0.02%. For selected delay times, the lowest intermode beat frequency line width amounts to 2 kHz indicating an improvement of carrier phase coherence by a factor of 700 as compared to the free-running laser. We validate these experimental results by a simple and universal stochastic time-domain model which is applied for the first time to model a self mode-locked quantum dot laser subject to optical self-injection. Modeling results are in good quantitative agreement.
Electrical Characterization of Gallium Nitride Drift Layers and Schottky Diodes
Allen, Noah P. (Virginia Tech, 2019-10-09)
Interest in wide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond has increased due to their ability to deliver high power, high switching frequency and low loss electronic devices for power conversion applications. To meet these requirements, semiconductor material defects, introduced during growth and fabrication, must be minimized. Otherwise, theoretical limits of operation cannot be achieved. In this dissertation, the non-ideal current- voltage (IV) behavior of GaN-based Schottky diodes is discussed first. Here, a new model is developed to explain better the temperature dependent performance typically associated with a multi-Gaussian distribution of barrier heights at the metal-semiconductor interface [Section 3.1]. Application of this model gives researches a means of understanding not only the effective barrier distribution at the MS interface but also its voltage dependence. With this information, the consequence that material growth and device fabrication methods have on the electrical characteristics can be better understood. To show its applicability, the new model is applied to Ru/GaN Schottky diodes annealed at increasing temperature under normal laboratory air, revealing that the origin of excess reverse leakage current is attributed to the low-side inhomogeneous barrier distribution tail [Section 3.2]. Secondly, challenges encountered during MOCVD growth of low-doped GaN drift layers for high-voltage operation are discussed with focus given to ongoing research characterizing deep-level defect incorporation by deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) [Section 3.3 and 3.4]. It is shown that simply increasing TMGa so that high growth rates (>4 µm/hr) can be achieved will cause the free carrier concentration and the electron mobilities in grown drift layers to decrease. Upon examination of the deep-level defect concentrations, it is found that this is likely caused by an increase in 4 deep level defects states located at E C - 2.30, 2.70, 2.90 and 3.20 eV. Finally, samples where the ammonia molar flow rate is increased while ensuring growth rate is kept at 2 µm/hr, the concentrations of the deep levels located at 0.62, 2.60, and 2.82 eV below the conduction band can be effectively lowered. This accomplishment marks an exciting new means by which the intrinsic impurity concentration in MOCVD-grown GaN films can be reduced so that >20 kV capable devices could be achieved.
Externally-Triggered Activation and Inhibition of Optical Pulsating Regimes in Quantum-Dot Mode-locked Lasers
Robertson, Joshua; Ackemann, Thorsten; Lester, Luke F.; Hurtado, Antonio (Springer Nature, 2018-08-21)
Controlled generation and inhibition of externally-triggered picosecond optical pulsating regimes are demonstrated experimentally in a quantum dot mode locked laser (QDMLL) subject to external injection of an amplitude modulated optical signal. This approach also allows full control and repeatability of the time windows of generated picosecond optical pulses; hence permitting to define precisely their temporal duration (from <1 ns spans) and repetition frequency (from sub-Hz to at least hundreds of MHz). The use of a monolithic QDMLL, operating at 1300 nm, provides a system with a very small footprint that is fully compatible with optical telecommunication networks. This offers excellent prospects for use in applications requiring the delivery of ultrashort optical pulses at precise time instants and at tunable rates, such as optical imaging, time-of-flight diagnostics and optical communication systems.
Filter Design for Interference Cancellation for Wide and Narrow Band RF Systems
Zargarzadeh, MohammadReza (Virginia Tech, 2016-06-19)
In radio frequency (RF), filtering is an essential part of RF transceivers. They are employed for different purposes of band selection, channel selection, interference cancellation, image rejection, etc. These are all translated in selecting the wanted signal while mitigating the rest. This can be performed by either selecting the desired frequency range by a band pass filter or rejecting the unwanted part by a band stop filter. Although there has been tremendous effort to design RF tunable filters, there is still lack of designs with frequency and bandwidth software-tuning capability at frequencies above 4 GHz. This prevents the implementation of Software Defined Radios (SDR) where software tuning is a critical part in supporting multiple standards and frequency bands. Designing a tunable integrated filter will not only assist in realization of SDR, but it also causes an enormous shrinkage in the size of the circuit by replacing the current bulky off-chip filters. The main purpose of this research is to design integrated band pass and band stop filters aimed to perform interference cancellation. In order to do so, two systems are proposed for this thesis. The first system is a band pass filter capable of frequency and band with tuning for C band frequency range (4-8 GHz) and is implemented in 0.13 µm BiCMOS technology. Frequency tunability is accomplished by using a variable capacitor (varactor) and bandwidth tuning is carried out by employing a negative transconductance cell to compensate for the loss of the elements. Additional circuitry is added to the band pass filter to enhance the selectivity of the filter. The second system is a band stop filter (notch) with the same capability as the band pass filter in terms of tuning. This system is implemented in C band, similar to its band stop counterpart and is capable of tuning its depth by using a negative transconductance in an LC tank. A negative feedback is added to the circuit to improve the bandwidth. While implemented in the same process as the band pass filter, it only employs CMOS transistors since it is generally more attractive due to its lower cost and scalability. Both of the systems mentioned use a varactor for changing the center frequency which is a nonlinear element. Therefore, the nonlinearity of it is modelled using two different methods of nonlinear feedback and Volterra series in order to gain further understanding of the nonlinear process taking place in the LC tank. After the validation of the models proposed using Cadence Virtuoso simulator, two methods of design and tuning are suggested to improve the linearity of the system. After post layout-extraction, the band pass filter is capable of Q tuning in the range of 3 to 270 and higher. With the noise figure of 10 to 14 dB and input 1-dB compression point as high as 2 dBm, the system shows a reasonably good performance along its operating frequency of 4 to 8 GHz. The band stop filter which is designed in the same frequency band can achieve better than 55 dB of rejection with the noise figure of 6.7 to 8.8 dB and 1-dB compression point of -4 dBm. With the power consumption of 39 to 70 mW, the band stop filter can be used in a low power receiver to suppress unwanted signals. The technique used in the band stop filter can be applied to higher frequency ranges if the circuit is implemented in a more advanced silicon technology. Implementing the mentioned filters in a receiver along with other elements of low noise amplifiers, mixers, etc. would be a major step toward full implementation of SDR systems. Studying the linearity theory of varactors would help future designers identify the sources of nonlinearity and suggest more efficient tuning techniques to improve the linearity of RF electronic systems.
Germanium and GeSn based Quantum Well Lasers and Nanoscale Multi-gate FETs
Joshi, Rutwik S. (Virginia Tech, 2025-01-06)
The incredible technological advancements over the last century have been possible due to tiny trinkets designed using semiconducting crystalline materials, especially Silicon and III-V compounds. Silicon, a group IV element has become the first choice in developing microchips serving an ever-growing set of applications including, computation, RF communications, solar cells, power electronics, quantum computing and its periphery, optoelectronics, IOT sensors, and lately artificial intelligence. Billions of Si-based complementary transistors (CMOS) are present at the center of most computing devices used today such as HPC servers, compute farms, laptops, and smartphones. The astonishing rise in transistor count, performance, and functionality as well as the exponential reduction in cost has been possible over the past decades due to a singular idea: shrinking the device. However, this rule, also called Moore's Law has been slowing over the past two decades and has eventually come to a standstill in its traditional definition. Moore's law has since been sustained by ingenious innovations such as high-k gate dielectrics, vertical scaling, lattice strain engineering, novel material developments and, lately chiplets as well as multi-die vertical packaging. As conventional Si CMOS approaches a roadblock, this work presents research on Germanium-based multi-gate devices providing promise for faster and low-power operation. This work discusses how Ge grown on a GaAs substrate can be tuned and utilized to form a virtually defect-free channel for ultra-scaled multi-gate transistors. Calibrated solvers informed using in-house materials and devices as well as literature are used to predict device performance for advanced structures. Further, a hybrid CMOS system with the high hole mobility p-channel device formed using tensile strained Ge, and the high electron mobility n-channel device formed using the underlying InGaAs layer is proposed and simulated. As scaling approaches Gate-all-around Nanosheet FETs in 2024 and complementary-FETs (CFETs) around 2034, Ge-on-AlAs based transistors can offer unique process simplifications, defect reduction, yield improvement, and high-performance advantages showing promise for future IRDS nodes. The process design, material stack, device, and circuit performance for this novel Ge-based NSFET is presented in this work. The lack of large strain or strain relaxation in the NS multilayer starting stack is seen to be a great process advantage for the Ge-AlAs NSFET system. To a certain extent, Si seems omnipotent for all things electronics. However, one exception is on-chip light generation. A coherent electrically controllable on-chip light source is a central component critical for optoelectronics, quantum technologies, fiber communications, and sensing. Due to the indirect bandgap, Si cannot produce light hence direct bandgap materials such as GaAs and GaN have been the primary choice for off-chip light sources integrable on the platform. Interestingly, Ge has a pseudo-direct bandgap, i.e., unlike Silicon, it can be manipulated to produce light using heavy doping, tensile strain, and Sn alloying. Similar to conventional III-V light sources, reduction in the dimensionality of the gain medium, i.e., Ge can enable a drastic reduction in the current required to produce light, among other performance considerations. This reduced dimensionality can be achieved by forming quantum wells and quantum dots. In this work, two new types of Ge-based quantum well lasers are introduced and analyzed along with qualitative and quantitive benchmarking. The first QW laser uses a small epitaxial biaxial tensile strain to improve the direct-ness of the Ge gain medium. The internal quantum efficiency, net gain, and threshold current can be improved drastically by choosing the right tensile strain while staying within a certain critical thickness value. For the first time, the impact of biaxial tensile strain on the optical properties of Ge is analyzed and reported through a systematic study of the dielectric spectra and optical constant using VASE. The changes in the band structure due to tensile strain are correlated with the critical points to uncover various optical transitions. An even better QW laser architecture is possible by utilizing a GeSn QW. This QW laser uses Sn-alloying to form a GeSn active region which is further lattice matched to the waveguide (InGaAs) and the optical confinement layers (InAlAs) around it. This completely lattice-matched laser structure can offer unique advantages such as virtually defect-free active region, tunability as well as improved efficiency and threshold current density. The absence of strain and consequently strain relaxation in the laser stack enables one to steer away from the critical thickness limitation while opening doors to designing multiple quantum well lasers among other complex architectures. The impact of Sn alloying on the atomic structure, lattice coherence, and relaxation is analyzed through XRD reciprocal space maps and rocking curves as a function of Sn concentration. Further, this lattice-matched system, GeSn-InGaAs-InAlAs has the potential to mirror the benefits of the mature GaAs-AlGaAs system which led to many great technological innovations over the past decades such as lasers and LEDs.
Heteroepitaxial Germanium-on-Silicon Thin-Films for Electronic and Photovoltaic Applications
Ghosh, Aheli (Virginia Tech, 2017)
Developing high efficiency solar cells for lower manufacturing costs has been a key objective for photovoltaic researchers to drive down the levelized cost of energy for solar power. In this pursuit, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46% under concentrator and 32% under AM0. However, the expensive cost has made it challenging for III-V solar cells to compete with the mainstream Silicon (Si) technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high-efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch, polar on non-polar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to investigate heterogeneously integrated 1J GaAs solar cells on Si substrate using germanium (Ge) as an intermediate buffer layer that will address mitigation of defects and dislocations between GaAs active cell structure and Ge “virtual” substrate on Si. The all-epitaxial molecular beam epitaxy (MBE)-grown thin (<1 μm) hybrid GaAs/Ge “virtual” buffer approach provided 1J GaAs cell efficiency of ~10% on Si, as compared with cell structures with thick 3 μm GaAs buffers. Solar cell results were further corroborated with material analysis to provide a clear path for the reduction of performance limiting dislocations. The thin “Ge-on-Si” virtual buffer was then investigated comprehensively to understand the impact of the heterostructure on device performance. The growth, structural, morphological, and electrical transport properties of epitaxial thin-film Ge, grown by solid source MBE on Si using a two-step growth process, were investigated. High-resolution x-ray diffraction analysis demonstrated ~0.10% tensile strained Ge epilayer, owing to the thermal expansion coefficient mismatch between Ge and Si, and negligible epilayer lattice tilt due to misfit dislocations at the Ge/Si heterointerface. Micro-Raman spectroscopic analysis further corroborated the strain-state of the Ge thin-film on Si. Cross-sectional transmission electron microscopy revealed the formation of a 90° Lomer dislocation network at the Ge/Si heterointerface, suggesting the rapid and complete relaxation of the Ge epilayer during growth. Atomic force micrographs exhibited smooth surface morphologies with surface roughness < 2 nm. Hall mobility measurements, performed within a temperature range of 77 K to 315 K, and the modelling thereof indicated that ionized impurity scattering limited carrier mobility in the thin Ge epilayer. Additionally, capacitance- and conductance-voltage measurements were performed after fabricating the metal-oxide-semiconductor capacitors (MOS-Cs) in order to determine the effect of epilayer dislocation density on interfacial defect states (Dit), bulk trap density, and the energy distribution of Dit as a function of temperature for electronic device applications. Deep level transient spectroscopy was used to identify the location (within the Ge bandgap) of electrically active trap levels; however, no significant trap levels were detected. Finally, the extracted Dit values were benchmarked against previously reported Dit data for Ge MOS devices, as a function of threading dislocation density within the Ge layer. The results obtained in this work were found to be comparable with other Ge MOS devices integrated on Si via alternative buffer schemes. The understanding gained from this comprehensive study of Ge-on-Si will help optimize the 1J GaAs on Si via thin Ge buffer approach, to enable a future of high efficiency low cost solar cells for terrestrial applications.
Heterogeneous Integration of III-V Multijunction Solar Cells on Si Substrate: Cell Design and Modeling, Epitaxial Growth and Fabrication
Jain, Nikhil (Virginia Tech, 2015-05-07)
Achieving high efficiency solar cells and concurrently driving down the cell cost has been among the key objectives for photovoltaic researchers to attain a lower levelized cost of energy (LCOE). While the performance of silicon (Si) based solar cells have almost saturated at an efficiency of ~25%, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46%. However, the expensive cost has made it challenging for the high efficiency III-V solar cells to compete with the mainstream Si technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high-efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch polar-on-nonpolar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to systematically investigate heterogeneously integrated III-V multijunction solar cells on Si substrate. Utilizing a combination of comprehensive solar cell modeling and experimental techniques, we seek to better understand the material properties and correlate them to improve the device performance, with simulation providing a very valuable feedback loop. Key technical design considerations and optimal performance projections are discussed for integrating metamorphic III-V multijunction solar cells on Si substrates for 1-sun and concentrated photovoltaics. Key factors limiting the “GaAs-on-Si” cell performance are identified, and novel approaches focused on minimizing threading dislocation density are discussed. Finally, we discuss a novel epitaxial growth path utilizing high-quality and thin epitaxial Ge layers directly grown on Si substrate to create virtual “Ge-on-Si” substrate for III-V-on-Si multijunction photovoltaics. With the plummeting price of Si solar cells accompanied with the tremendous headroom available for improving the III-V solar cell efficiencies, the future prospects for successful integration of III-V solar cell technology with Si substrate looks very promising to unlock an era of next generation of high-efficiency and low-cost photovoltaics.
Improving the Accessibility of Smartwatches as Research Tools by Developing a Software Library
Wanjara, Dhwan Devendra (Virginia Tech, 2022-06-13)
Over the past 10 years, smartwatches have become increasingly popular for commercial use. Their ever-increasing capabilities, accuracy, and sophistication of smartwatches is making them increasingly appealing to physical activity researchers as a valuable research tool. The non-invasive nature, prevalence, and versatility of smartwatches is being utilized to track heart rate, blood-oxygen levels, activity and movement, and sleep. However, the current state of the art lacks a uniform method to extract, organize, and analyze data collected from these devices. The objective of this research was to develop a Python software library that is widely available, highly capable, and easy to use with the data collected by the Apple Watch. The library was designed to offer data science, visualization, and mining features that help physical activity research find and communicate patterns in the Apple Health data. The custom-built caching system of the library provides near-instant runtime to parse and analyze large files without trading off on memory usage. The Wanjara Smartwatch Library has significantly better performance, proven reliability and robustness, and improved usability than the alternatives discovered in the review of the literature.
MicroGC: Of Detectors and their Integration
Sreedharan Nair, Shree Narayanan (Virginia Tech, 2014-04-29)
Gaseous phase is a critical state of matter around us. It mediates between the solid crust on earth and inter-stellar vacuum. Apart from the atmosphere surrounding us where compounds are present, natively, in a gaseous phase, they are also trapped within soil and dissolved in oceanic water. Further, those that are less volatile do enter the gaseous phase at high temperatures. It is this gaseous phase that we inhale every second. It is thus critical that we possess the tools to analyze a mixture of gaseous compounds. One such method is to separate the components in time and then identify, primarily based on the retention times, also known as gas chromatography. This research focuses on the development of gas detectors and their integration, in different styles, primarily for gas chromatography. Utilizing fabrication techniques used in semiconductor industry and exploiting scaling laws we investigate the ability to improve on conventional gas separation and identification techniques. Specifically, we have provided a new spin to the age-old thermal conductivity detector enabling its monolithic integration with a separation column. A reference-less, two-port integration architecture and a one-of-its-kind released resistor on glass are some of its salient features. The operation of this integrated device with a preconcentrator and in a matrix array was investigated. The more unique contribution of this research lies in the innovative discharge ionization detector. An ultra-low power, sensitive, easy to fabricate detector, it requires more investigation for a thorough understanding and will likely mature to replace the thermal conductivity detector, as the detector of choice for universal detection, in time to come.
Mixed As/Sb and tensile strained Ge/InGaAs heterostructures for low-power tunnel field effect transistors
Zhu, Yan (Virginia Tech, 2014-05-02)
Reducing supply voltage is a promising way to address the power dissipation in nano-electronic circuits. However, the fundamental lower limit of subthreshold slope (SS) within metal-oxide-semiconductor field-effect transistors (MOSFETs) is a major obstacle to further scaling the operation voltage without degrading ON/OFF-ratio in today's integrated circuits. Tunnel field-effect transistors (TFETs) benefit from steep switching characteristics due to the quantum-mechanical tunneling injection of carriers from source to channel, rather than by conventional thermionic emission in MOSFETs. TFETs based on group III-V compound semiconductor and Ge heterostructures further improve the ON-state current and reduce SS due to the low bandgap energies and smaller carrier tunneling mass. The mixed arsenide/antimonide (As/Sb) InxGa1-xAs/GaAsySb1-y and Ge/InxGa1-xAs heterostructures allow a wide range of bandgap energies and various band alignments depending on the alloy compositions in the source and channel materials. Band alignments at source/channel heterointerface can be well modulated by carefully controlling the compositions of the InxGa1-xAs or GaAsySb1-y. In particular, this research systematically investigate the development and optimization of low-power TFETs using mixed As/Sb and Ge/InxGa1-xAs based heterostructures including: basic working principles, design considerations, material growth, interface engineering, material characterization, band alignment determination, device fabrication, device performance investigation, and high-temperature reliability. A comprehensive study of TFETs using mixed As/Sb and Ge/InxGa1-xAs based heterostructures shows superior structural properties and distinguished device performances, both of which indicate the mixed As/Sb and Ge/InxGa1-xAs based TFET as a promising option for high performance, low standby power and energy efficient logic circuit application.
Multi-Material Fiber Fabrication and Applications in Distributed Sensing
Yu, Li (Virginia Tech, 2019-01-25)
Distributed sensing has been an attractive alternative to the traditional single-point sensing technology when measurement at multiple locations is required. Traditional distributed sensing methods based on silica optical fiber and electric coaxial cables have some limitations for specific applications, such as in smart textiles and wearable sensors. By adopting the fiber thermal drawing technique, we have designed and fabricated multi-material electrode-embedded polymer fibers with distributed sensing capabilities. Polymers sensitive to temperature and pressure have been incorporated into the fiber structure, and thin metal electrodes placed inside fiber by convergence drawing have enabled detection of local impedance change with electrical reflectometry. We have demonstrated that these fibers can detect temperature and pressure change with high spatial resolution. We have also explored the possibility of using polymer optical fiber in a Raman scattering based distributed temperature sensing system. Stokes and Anti-Stokes signals of a PMMA fiber illuminated by a 532 nm pulsed laser was recorded, and the ratio was used to indicate local temperature change. We have also developed a unique way to fabricate porous polymer by thermal drawing polymer materials with controlled water content in the polymer. The porous fibers were loaded with a fluorescent dye, and its release in tissue phantoms and murine tumors was observed. The work has broadened the scope of multi-material, multi-functional fiber and may shed light on the development of novel smart textile devices.
Multiresonant Plasmonics with Spatial Mode Overlap
Safiabadi Tali, Seied Ali (Virginia Tech, 2022-02-03)
Plasmonic nanostructures can enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices are optimized to operate in a single wavelength band, which is not efficient for wavelength-multiplexed operations and quantum optical applications involving multi-photon nonlinear processes at multiple wavelength bands. Overcoming the limitations of single-resonant plasmonics requires development of plasmonic devices that can enhance the optical interactions at the same locations but at different resonance wavelengths. This dissertation comprehensively studies the theory, design, and applications of such devices, called "multiresonant plasmonic systems with spatial mode overlap". We start by a literature review to elucidate the importance of this topic as well as its current and potential applications. Then, we briefly discuss the fundamentals of plasmonic resonances and mode hybridization to thoroughly explore, classify, and compare the different architectures of the multiresonant plasmonic systems with spatial mode overlap. Also, we establish the black-box coupled mode theory to quantify the coupling of optical modes and analyze the complicated dynamics of optical interactions in multiresonant plasmonic systems. Next, we introduce the nanolaminate plasmonic crystals (NPCs), wafer-scale metamaterials structures that support many (>10) highly-excitable plasmonic modes with spatial overlap across the visible and near-infrared optical bands. The enabling factors behind the NPC's superior performance as multiresonant systems are also theoretically and experimentally investigated. After that, we experimentally demonstrate the NPCs application in simultaneous second harmonic generation and anti-Stokes photoluminescence (ASPL) with controllable nonlinear emission properties. By designing specific non-linear optical experiments and developing advanced ASPL models, this work addresses some important but previously unresolved questions on the ASPL mechanism as well. Finally, we conclude the dissertation by discussing the potential applications of out-of-plane plasmonic systems with spatial mode overlap in wavelength-multiplexed devices and presenting some preliminary results.

Browsing by Author "Lester, Luke F."

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