1,721,012 research outputs found
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Radio Frequency and Terahertz Plasmons in Two Dimensional Electron Gases
No full textAt the turn of the 21st century, the study of photonics, plasmonics and subwavelength phenomena became more and more intense as it became apparent that innovations in these fields could have important and widespread applications in miniaturizing electronic or photonic devices. Recent work in the Ham group have shown that the microwave to far infrared plasmons of 2D electron gases can achieve very small propagation velocities of <c/100, enabled by large kinetic inductances of the 2D electron gases, which promises size reduction factors of microwave circuits of 100 times or more.
We will first build and describe simple circuit models of 2D electron gas plasmons, by computing equivalent capacitance, inductance, and resistances. Building a transmission line model from these circuit elements allows us to calculate plasmonic wave dispersions. Modifications to the dispersions are studied, and in particular, we examine how a periodic geometry can result in a plasmonic crystal. We propose and demonstrate these far infrared plasmonic crystals using the 2D electron gas in graphene, and we show that concepts from photonic and electronic crystals such as band engineering and symmetry selection rules apply.
We also examine in great detail the origin and form of the kinetic inductance, which is key to the plasmonic response, and calculate the modifications to the Johnson-Nyquist noise that must necessarily result from any conductor with intrinsic inductance. An understanding of the high frequency noise spectrum is necessary to evaluate potential microwave and far infrared devices using these plasmons.
Lastly, we show how non-reciprocal plasmons can arise non-magnetically, by simply applying a drift current to the electrons. The plasmons carried along by the electrons also drift with the same velocity as the drift velocity, which modifies the plasmon dispersion non-reciprocally, due to the drift motion being non-reciprocal. This principle may enable novel devices based on this principle in the future.Engineering and Applied Sciences - Applied Physic
Statistical Electronics: Noise Processes in Integrated Communication Systems
Get PDFThis thesis presents a comprehensive investigation of noise and thermodynamics in electronic circuits and systems. This study of "statistical electronics" spans two disciplines, statistical thermodynamics and electronic circuit engineering, and leads to a general picture that bridges electronics and statistical thermodynamics.
Our work on statistical electronics has both scientific and engineering implications. Scientifically, this work is an extensive study of statistical thermodynamics in the context of electrical circuits, which has made several significant contributions to the understanding of noise processes in electrical circuits. The technological importance is a demonstration of how the fundamental physical considerations evolve to practical high-performance novel circuit design. The power of our fundamental approach is demonstrated through several practical circuit examples.
First, our investigation of fluctuations in nonlinear electrical circuits provides deep insight into the nonlinear fluctuation phenomena. Especially, the study of fluctuations in nonlinear active devices constitutes an important sector in this investigation; verifying the physical soundness of the contemporary active device noise modeling and leading to clear understanding of fluctuation-dissipation relations in nonlinear devices.
Second, we apply statistical electronics to noise problems involved in frequency conversion, an essential function in modern RF and microwave receivers. This study leads to two novel observations of noise figure degradation due to cyclostationary noise and conversion gain enhancement, both dependent on the size of energy storing elements. This novel behavior is experimentally verified with a direct measurement of integrated switching mixers. The results provide new insight into cyclostationary noise processes in frequency conversion and optimum deisgn for switching mixers.
Third, application of statistical electronics to noise in frequency generation by self-sustained oscillators leads to a new theory of oscillator noise. This study demonstrates the direct correspondence between the phase noise and the Einstein relation; revealing the underlying physics of oscillator noise. Our approach clarifies the fluctuation-dissipation relation in oscillator noise generation, establishing a link between currently available fluctuation-based and dissipation-based phase noise. models and leading to a clear definition of loaded quality factor of ail oscillator. The novel concepts of virtual damping and linewidth compression put resonators and oscillators in a unified framework, providing immediate design optimization insight. The power of this theoretical development is demonstrated through experimental measurements of various integrated oscillators.
Our work on statistical electronics combining circuit engineering and physical science has also resulted in other useful engineering methods, such as graphical optimization, noise simulations for computer-aided design (CAD), and time-varying filter theory.</p
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CMOS Nanoelectrode Array for High Throughput Electrocardiography
No full textIt has been a perpetual desire for human being to understand our own intelligence. Following the series of breakthroughs in the 20th century, neuroscience has enjoyed the spotlight being one of the most promising and rewarding field of science in the 21st century. Yet the steps of neuroscience have slowed down partly due to the lack of appropriate tools. Electrophysiology is one of the most fundamental aspects of brains sciences as neurons use electrical signals to perform computations in a network, many also believed these electrical signals are the underlying mechanism of higher functions like memories. Electrode based techniques are the most universal tools used in neuron electrophysiology, and they can be categorized into two large groups: intracellular electrodes and extracellular electrodes. Intracellular electrode, e.g. patch clamp electrode, can provide detailed information of a neuron’s full electrical signal but they are hard to operate and almost impossible to scale up. Extracellular electrode, e.g. Multi-electrode-array, can command thousands of electrodes in parallel yet their signals are often noisy and mixed up. Facing this challenge, this dissertation work strives to combing the intracellular power of nanoscale electrode with the multiplexing capability of Complimentary-Metal-Oxide-Semiconductor (CMOS) electronics and build a platform of high-throughput electrophysiology.
Chapter one gives a more detailed introduction of the development of electrophysiology for neuroscience and the challenges facing electrode-based techniques. It also provides a review of the state-of-the-art development for both intracellular techniques and extracellular techniques.
Chapter two describes our experimental efforts for the CMOS Nano Electrode Array (CNEA) device platforms. It starts with the inception of idea and the advantage of introducing CMOS electronics into nanoelectrode arrays. Then we discussed in detail the CMOS circuit design, fabrication and packaging of the device. Finally, we demonstrated the platform’s capability of massively intracellular recording on neonatal rat cardiomyocyte cells.
Chapter three continues on the same concept with the new generation of CMOS Neuro-Electrode Interface (CNEI) device system. In addition to basic CMOS circuit design and the fabrication process, this chapters emphasizes the improvements we made for this new generation. These improvements enable us to study the more delicate and yet more interesting neonatal rat neuron cultures and finally perform mapping of their synaptic connections.
Chapter four represents one of the later developments of the platform where we customize the electrode interface and use it for brain slice applications.Engineering and Applied Sciences - Applied Physic
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Locally Confined Chemistry at a CMOS Electrochemical Interface
No full textChemical reactions are closely linked to one another through a network of equilibrium reactions, whose rates depend directly on the concentrations of the involved chemical species. Locally confining a chemical species of interest at a target concentration in a miniscule volume via electrochemical means enables parallelized control of the relevant chemical reactions across an array of electrochemical cells. Namely, as pH controls a myriad of chemical and biochemical processes in water, densely arrayed confinement of pH would enable their high-throughput studies and applications.
Here, we present a 16 × 16 electrochemical cell array defined on and operated by a complementary metal-oxide-semiconductor (CMOS) integrated circuit. Each cell consists of a concentric pair of anode and cathode rings, where currents are injected with sub-nanoampere resolution to localize picoliters of acidic pH by making use of the quinone redox chemistry. The array also consists of open-circuit potential (OCP) sensors located within and in-between the cells for monitoring pH in real-time. To highlight the utility of arrayed pH localization, we parallelize pH-regulated enzymatic incorporation of nucleotides to single-stranded DNA molecules at any randomly selected set of cells. The site-specific enzymatic DNA elongation is enabled by local confinement of the pH-regulated DNA deprotection chemistry, which is directly controlled via electrochemical pH localization.
Then, we expand the utility of this CMOS electrochemical pH localizer-imager array to analog computing using ions and molecules in an aqueous environment. We reconfigure the circuitry of the 16 x 16 electrochemical cell array into a 16 × 16 ionic transistor array, where we change the role of the center OCP sensor to an input voltage terminal. The current output at the input voltage terminal is a multiplication of the voltage input at the disk electrode and the weight parameter tuned by local confinement of H+ and benzoquinone concentrations. The disk current outputs are summated at the global pseudo-reference electrode, completing the analog multiply-accumulate (MAC) operation in water. This aqueous ionic circuit is a step toward sophisticated, low-power analog computing in water, like the biological signal processing performed by neuronal networks in the brain.
Lastly, we review the semiconductor memory technologies and discuss them in terms of the tradeoff between speed and storage capacity. We propose how our arrayed pH localization technology can enable massively parallel enzymatic DNA synthesis for next-generation biomolecular data storage, whose theoretical data storage capacity is extremely high at the cost of being very slow
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Quantum Engineering of a Diamond Spin Qubit With Nanoelectromechanical Systems
No full textQuantum emitters are indispensable building blocks for quantum computers and networks. By entangling multiple individual quantum systems, it is possible to make overall system exponentially more powerful. Quantum emitters play a key role in this regard, since they offer an optical interface between a flying qubit (photon) and a stationary qubit (spin) for a long distance. Among those, solid-state emitters are an appealing candidate for its scalability. Among many kinds, we study color centers in diamond: nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers. Being trapped atom in a solid, a color center provides both unique opportunities and challenges. Using dynamic interaction between phonons and spin qubits, it is possible to build an on-chip universal quantum bus. On the other hand, the host material causes inhomogeneous distribution of emitters by its material strain and exposes color centers to thermal lattice vibrations. In this work, we use nanoelectromechanical systems (NEMS) to address both issues. First, we make nanocantilevers with embedded NV centers and use its flexural motion for parametric coupling. Both electron spin resonance and spin-echo measurements are performed. As a result, we deduce the single-phonon coupling rate of approximately 1.8 Hz, which is still many orders of magnitude smaller than the minimum requirement for a quantum node. Therefore, it is necessary to further scale down the device without deteriorating other parameters. In this context, we fabricate on-chip dynamic actuator that is compatible with cantilevers of small mode volume and high quality factor. We measure resonant frequencies of fundamental flexural modes on the order of tens of MHz, with mechanical quality factors on the order of thousands. Finally, we present electrostatically actuated diamond cantilever with implanted SiV centers. By deflecting beams, we control the electronic structure of SiV centers, which is revealed by taking optical spectra at different strain conditions. Furthermore, we probe the dynamics of the spin qubit while controlling strain. By applying strain on the order of ten-thousandths to SiV centers, we improve the spin coherence time by sixfold at 4K, until it is limited by a next dominant dephasing mechanism. We conclude with an outlook of phononic quantum nodes with SiV center.Engineering and Applied Sciences - Applied Physic
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Vapor Deposition of Halide Perovskites and Hole- Transport Materials for Use in Thin-Film Photovoltaics
No full textThe drive toward a sustainable, low-carbon energy future has created a considerable impetus to reduce the cost per watt of electricity produced from solar cells. Thin-film photovoltaics, specifically those produced with perovskite absorber layers, demonstrate great potential in this regard, given the high power conversion efficiencies that can be achieved with low material usage. However, perovskite solar cells are typically deposited by solution processing methods, which may be difficult to scale to the industrial level, particularly if an inert atmosphere is required to prevent the decomposition of these air-sensitive materials. Additionally, the most efficient perovskite cells employ an expensive and resistive p-type organic hole-transport material and a lead-based absorber layer. Use of the former increases cost and likely limits performance and use of the latter raises toxicity concerns that could restrict widespread commercial deployment of these devices. Chemical vapor deposition and atomic layer deposition provide scalable alternatives to solution processing that allow for the growth of materials in an inert atmosphere. Methods for the vapor deposition of the lead-free absorber layers CH3NH3SnX3 (X = I, Br) and the p-type inorganic hole-transport materials CuX are presented and the resultant films are evaluated for their use in thin-film photovoltaics.
The chemical vapor deposition of halide materials is shown to be complicated by the formation of nonvolatile salts produced by the reaction between the strongly acidic HI and HBr precursors and the basic ligands of the metalorganic Sn and Cu precursors. The atomic layer deposition of these same materials is also precluded by the lack of surface reactive sites provided by the hydrogen halides. To circumvent these difficulties, two-step conversion processes are developed with the goal of producing the selected materials CH3NH3SnX3 and CuX indirectly. It is first demonstrated that a post-deposition exposure of the contaminated (H3cyc)xSnBr(2+x) produced by chemical vapor deposition to CH3NH2 produces a film with atomic composition approaching that of the desired CH3NH3SnBr3. It is then shown that the surface of Cu(2-x)S grown by pulsed-chemical vapor deposition can be transformed to crystalline γ-CuBr upon exposure to anhydrous HBr. The produced CuBr initially forms as a continuous film, but becomes discontinuous as the conversion front approaches the interface with the SiO2 substrate. Al2O3 is predicted by contact angle measurements to have surface properties compatible with those of CuBr and does in fact demonstrate improved wetting of the converted material. Finally, the introduction of HI to Cu(2-x)S and Cu2O results in the formation of thin films of crystalline γ-CuI, with the conversion from the oxide proceeding to completion in less than 2 hours at room temperature. Exposure to wetting agents thiodiglycol and ethylene glycol prior to and during conversion is shown to improve the surface coverage of converted films. The quantification of surface properties by contact angle measurements allows for an optimization of substrate and wetting agent to produce films of CuI with high surface coverages at thicknesses of only 60 nanometers. The converted CuI demonstrates an optical bandgap of 3.1 eV, with resistivity values as low as 7.5 × 10-2 Ω×cm and hole mobilities of up to 6 cm2/V×s.Engineering and Applied Sciences - Applied Physicsphotovoltaics; atomic layer deposition; chemical vapor deposition; perovskite; halide; cuprous iodide; cuprous bromid
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Zero-Index Metamaterials: A New Phase in Integrated Optics
No full textMetamaterials with a refractive index of zero exhibit properties that are important for integrated optics. Possessing an infinite effective wavelength and zero spatial phase change, zero-index metamaterials may be especially useful for routing on-chip photonic processes and reducing the footprint of nonlinear interactions. Zero-index has only been achieved recently in an integrated platform through a Dirac-cone dispersion, enabling some of these more exciting applications in an integrated platform. This thesis presents an overview of Dirac-cone zero-index metamaterials (ZIMs), including the fundamental physics, history and demonstrations in the optical regime, as well as current challenges and future directions. Furthermore, it explores some of the applications and measurements enabled since the appearance of these integrated ZIMs and modifications to the metamaterial to eliminate characteristic radiative losses.Engineering and Applied Sciences - Applied PhysicsMetamaterials; Zero-Index; Integrated Photonics; Beam-Steering; Bound States; Nanophotonic
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Toward Ubiquitous Electronics with 2D Materials and 3D Printing: Atomically Thin Optoelectronic Machine Vision Processor and Gigahertz RF Electronics via Direct Ink Writing
No full textThe development of silicon semiconductor technology has produced breakthroughs in electronics by downscaling the physical size of devices and wires to the nanometer regime. However, silicon-based complementary metal–oxide–semiconductor (CMOS) technology is approaching its downscaling limit. Meanwhile, there is compelling demand for stretchable and flexible electronics— which are challenging to achieve with CMOS—, for numerous futuristic applications such as wearable systems, smart cities, and Internet-of-Things (IoT). Therefore, radically different types of materials and devices are needed to realize the potential of the next generation of electronics.
Leveraging two-dimensional (2D) materials and three-dimensional (3D) printing, I have worked on two projects at Harvard, to realize innovations in design, fabrication and functionality of electronics for ubiquitous applications. These two pieces of work comprise the present thesis. In the first work, we explore the potential of 3D printing for creating radio-frequency (RF) passive devices as well as their integration into active RF electronic circuits. Specifically, we produce a broad array of RF passives that operate at GHz frequencies via direct ink writing, including lumped devices and wave-based devices, whose maximum quality factors (Q) and operational frequencies exceed 40 and 45 GHz, respectively. Moreover, to demonstrate the utility of these printed RF passive structures in active RF electronic circuits, we combine them with discrete transistors to fabricate self-sustained oscillators, synchronized oscillator arrays, and wireless transmitters clocked by the oscillators. In the other work, we investigate two-dimensional (2D) semiconductors, in particular transition metal dichalcogenide (TMD) monolayers. We expand the functional complexity of 2D integrated circuits (ICs) through a tenfold increase in the device integration scale. Specifically, we have developed an analog optoelectronic processor comprised of 1,024 photo field-effect transistors (photo-FETs) arranged in a crossbar array structure. Through capturing of optical images into electrical data (like the eye and optic nerve), and subsequent recognition of this data (like the brain) via analog in-memory computing, our optoelectronic processor emulates the two core functions of human vision.Engineering and Applied Sciences - Engineering Science
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Development of Tin(II) Sulfide Solar Cells by Interface Engineering and Absorber Alloying
No full textTin(II) sulfide (SnS) is a promising candidate for alternative photovoltaic (PV) materials. Composed of cheap, non-toxic and earth-abundant elemental constituents, SnS has appropriate band gap and high absorption coefficient. The SnS films prepared by atomic layer deposition (ALD) demonstrate excellent phase purity, stoichiometry and crystal structure. The path toward high-efficiency SnS photovoltaic devices demands not only desired material properties, but also an optimized device stack. This thesis assesses the potentials of improving SnS PV device performance in both ways.
Using transmission line method (TLM), the electrical properties of metal-SnS interfaces are investigated. It is found that the contact resistivity between annealed SnS films and Mo substrates under light illumination is as low as 0.1 Ω cm2. Temperature-dependent TLM measurements suggest a strong Fermi level pinning effect at such interfaces. The heterojunction between absorber and buffer layer is optimized by tuning conduction band offset and carrier concentration. Furthermore, by comparing various SnS surface oxidation treatments, the influence of interface passivation is studied.
Alloying of two or more materials is a powerful way to design and control materials properties. Here we seek to investigate the alloying of ALD-grown SnS and CaS films using novel precursors. Theories predict that (Sn,Ca)S films have a tunable direct band gap and cubic structural phase. We explore the kinetic stabilization of the metastable structures and composition-induced transition between different crystal structures. Structural, electrical and optical properties of (Sn,Ca)S alloys are analyzed. It is shown that a recently-discovered cubic structure, which contains 64 atoms in each unit cell, is obtained in the as-deposited alloy films. Upon annealing, however, the cubic phase is separated into orthorhombic SnS and amorphous CaS.Engineering and Applied Sciences - Applied PhysicsAtomic Layer Deposition; Tin Sulfide; Solar Cells; Alloyin
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Complementary Metal-Oxide-Semiconductor (CMOS) Bio-electronic Interface for Cell-based Phenotypic Drug Screening
No full textMicroelectrode Array (MEA) has been widely researched and with some commercial usage in measuring electrical properties and behavior of cardiac and neuronal networks, thanks to its low cost and parallelism compared to traditional patch clamp. Integration of Complementary Metal-Oxide-Semiconductor (CMOS) technology with MEA allows further miniaturization of the electrodes, improvement on the spatial resolution and the signal-to-noise ratio and enabling highly-paralleled real-time measurement and stimulation. CMOS-MEA, therefore, becomes an excellent research tool for in vitro electrophysiology studies, usually with a single-well device to measure and perform manual electrical stimulation on one cell culture at a time. Furthermore, CMOS-MEA also improves the multi-parametric label-free readouts on general cells from commercially available MEAs, opening the potential applications into phenotypic drug discovery.
This dissertation discusses the design and development of two CMOS-MEA Integrated Circuit (IC) systems. First, an extension of two previously published CMOS nanoelectrode array (CNEA) systems, the 3rd generation of CNEA, is presented. The 3rd generation of CNEA features 1,024 pixels capable of simultaneously recording, on-chip action potential detection, on-chip inter-pixel feedback decision, and arbitrary stimulation pattern generation. The on-chip action potential detection and inter-pixel feedback decision making allow a stimulation pattern to be generated and applied to an arbitrary pixel within a microsecond from an action potential detected on another pixel. This capability makes the 3rd generation CNEA a perfect candidate for Spike-Timing-Dependent-Plasticity (STDP) studies in neuronal networks.
The majority focus of this dissertation would be on the multiwell version of the CNEA (Multiwell Platform). The Multiwell Platform contains 24 custom design ICs interlinked onto a custom interposer printed circuit board (PCB), creating 96 identical wells and their associated circuitries into a standard form factor wellplate. Each well contains 4,096 pixels and 256 readout channels capable of scanning through the entire pixel array with arbitrary square patterns. The unique design and functionalities of the Multiwell Platform enable parallel measurements on multiple biological-relevant parameters on general cells and therefore enable high-throughput phenotypic drug screening applications
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