Browsing by Subject "Nanowire"

Now showing 1 - 12 of 12

Confined electron systems in Si-Ge nanowire heterostructures
(2011-08) Dillen, David Carl; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K.
Semiconductor nanowire field-effect transistors (NWFET) have been recognized as a possible alternative to silicon-based CMOS technology as traditional scaling limits are neared. The core-shell nanowire structure, in particular, also allows for the enhancement of carrier mobility through radial band engineering. In this thesis, we have evaluated the possibility of electron confinement in strained Si-Si1-xGex core-shell nanowire heterostructures. Cylindrical strain distribution was calculated analytically for structures of various dimensions and shell compositions. The strain-induced conduction band edge shift of each region was found using k•p theory coupled with a coordinate system shift to account for strain. A positive conduction band offset of up to 200 meV was found for a Si-Si0.2Ge0.8 structure. We have also designed and characterized a modulation doping scheme for p-type, Ge-SiGe core-shell NWFETs. Finite element simulations of hole density versus radial position were done for different combinations of dopant position and concentration. Three modulation doped nanowire samples, each with a different boron doping density in the shell, were grown using a combined vapor-liquid-solid and chemical vapor deposition process. Low temperature current-voltage measurements of bottom- and top-gate samples indicate that hole mobility is limited by the proximity of charged impurities.
Epitaxial germanium via Ge:C and its use in non-classical semiconductor devices
(2015-12) Mantey, Jason Christopher; Banerjee, Sanjay; Lee, Jack C; Register, Leonard F; Akinwande, Deji; Ferreira, Paulo J
The microelectronics industry has been using Silicon (Si) as the primary material for complementary metal-oxide-semiconductor (CMOS) chip fabrication for more than six decades. Throughout this time, these CMOS devices have gotten exponentially smaller, faster, and cheaper. While new materials and fabrication processes have been slowly added over the years, the CMOS device of today is largely the same as it was decades ago. However, field-effect transistors (FETs) have now scaled so far that Si is approaching physical limits. Thus, new channel materials and new fundamental device structures are being investigated to replace traditional CMOS. Germanium is one of the prime candidates to replace Si in the FET channel, with its increased electron and hole mobilities compared to Si. Perhaps more importantly, it is compatible with the existing Si manufacturing techniques by epitaxially growing thin layers of Ge crystal on the starting Si wafer. Because these two crystals do not share a lattice constant, there will inevitably be crystal defects in the thin Ge layer that can be catastrophic for device functionality. Several approaches have been introduced to reduce defects, but most of them are wastefully thick (>1 um) or require complex manufacturing methods. In this work, we utilize an extremely thin (~10 nm) buffer layer of carbon-doped Ge (Ge:C) to grow Ge and SiGe layers for FET and virtual substrate applications with improved crystalline quality and reduced surface roughnesses. These thin Ge layers not only offer new pathways for MOSFETs, but can also be used in non-classical structures. Semiconductor nanowires (NWs) and tunnel-FETs (TFETs) are two of the most promising device architectures, and both can be used with Ge. This dissertation presents a simulated Si/Ge heterostructure interface TFET that can be fabricated on a virtual substrate made with the Ge:C buffer layer. Detailed analysis on device operation is given. Also in this work is the fabrication process for individually addressable Ge NW-FETs. The NWs offer excellent electrostatic gate control through reduced dimensions and offer another potential pathway for Ge in a post-CMOS world.
Innovative design, assembling and actuation of arrays of nanoelectromechanical system (NEMS) devices using nanoscale building blocks
(2015-05) Kim, Kwanoh; Fan, Donglei; Bourell, David L; Chen, Ray; Manthiram, Arumugam; Mullins, Charles Buddie
Rotary nanomotors, a type of nanoelectromechanical system (NEMS) device that converts electric energy into mechanical motions, are critical for advancing NEMS technology in various fields but have been difficult to obtain using traditional techniques. As a consequence, it is highly desirable to investigate new mechanisms to develop large arrays of rotary NEMS devices with high efficiency, small size, and reliable performance at a low cost. In this dissertation, we report innovative designs and mechanisms for assembling and actuating arrays of rotary NEMS devices based on the electric tweezers and unique magnetic interactions among components. NEMS oscillators and motors were assembled from nanoscale building blocks including nanowires working as rotors, patterned nanomagnets as bearings, and quadrupole electrodes as stators. Multiple devices could be assembled in an ordered array and rotated either between two designated angles reciprocally or the full cycles continuously with controlled angle, speed (over 18,000 rpm) and chirality. Their fundamental electric, magnetic, and mechanical interactions were investigated, which provided an understanding of nanoscale dynamics critical to designing and actuating various metallic NEMS devices. We could reduce the size of the device with all its characteristic dimensions below 1 micron and continuously rotate a nanomotor for up to 15 hours, which is equivalent to more than 240,000 cycles in total. As an application, NEMS devices were used for controlled biochemical release and demonstrated the releasing rate of biochemical from the devices could be precisely tuned by mechanical rotation. Various magnetic configurations were purposely designed and successfully implemented, which resulted in distinct rotation behaviors including repeatable wobbling and self-rolling in addition to in-plane rotation. With the understandings of these rotation characteristics, high-performance micromotors have been rationally designed and successfully achieved, where the micromotors rotate at uniform speeds and position at desired angles, resembling step motors. Bioinspired micromotors comprised of three-dimensional porous diatom frustule rotors and patterned micromagnets were assembled and rotated in a microfluidic channel. Simulation results showed that they could stir and agitate liquid in a microchannel more efficiently than simple one-dimensional nanoentities and would be promising for microfluidic actuators. The innovations reported here, including concept, design, fabrication, actuation mechanisms, and applications, are expected to inspire various research areas including NEMS, nanorobotics, biomedical applications, microfluidics and lab-on-a-chip architectures.
Nanowire sharpening : application to field ionization
(2021-05-03) Leviyev, Alex; Raizen, Mark G.
Nanowires show potential for a wide range of fields, from developing next generation solar cells, to detecting viruses, to field ionizing gasses. Their uses in such disparate fields are due to the extreme flexibility in which nanowires can be manufactured and customized to order. The heart of a nanowire, however, is it’s tip. Here surface charges accumulates in extreme densities when the wire is biased, consequently producing large electric fields that are then used in creative ways for exciting applications. My aim for this thesis is three-fold. First, I intend to establish a context for nanowires. Why were these structures studied in the first place? What are some exciting application areas? What makes nanowires unique? Etc... This will set the stage for subsequent sections, and provide the salt and pepper that will make the main course more flavorful. The second is to present an overview of relevant results from the literature that I will later build upon. There are analytical models of varying complexity examining the type of protrusions we are interested in here, as well as numerous numerical studies. We will look at them to get a deeper intuition for whats happening, and use them as a basis for comparison later on. Finally, I will present my results and discuss their consequences. As we will see, sharpening a cylindrical-post nanowire of height H = 1um, starting radius of curvature r₀ = 100nm, and base size b = 2 · r₀ = 200nm can enhance the field further by an order of 100. In addition, the dielectrophoretic (DEP) force present due to strong electric field gradients can significantly alter cooled gas beam trajectories towards the nanowire tips. Non-cooled gas beams also display room temperature trajectory deviations for species with large polarizability to mass ratios. This leads to the conclusion that at lower temperatures (and even at room temperature) the field ionization cross section of a nanowire array is significantly increased under certain conditions when taking into account DEP coupling.
Novel 3-D IC technology
(2014-05) Zhai, Yujia; Banerjee, Sanjay; Willson, C. G. (C. Grant), 1939-
For many decades silicon based CMOS technology has made continual increase in drive current to achieve higher speed and lower power by scaling the gate length and the gate insulator thickness. The scaling becomes increasingly challenging because the devices are approaching physical quantum limits. Three-dimensional electronic devices, such as double gate, tri-gate and nanowire field-effect-transistors (FETs) provide an alternative solution because the ultra-thin fin or nanowire provides better electrostatic control of the device channel. Also high-[kappa] oxides lower the gate leakage current significantly, due to larger thickness for the same equivalent oxide thickness (EOT) compared with SiO₂ beyond the 22 nm node. Moreover, metal gate that avoids the poly-depletion effect in poly-Si gate has become mainstream semiconductor technology. The enabler technologies for high-[kappa] / metal gate 3D transistors include fabrication of high quality, vertical nanowire arrays, conformal metal and dielectric deposition and vertical patterning. One of the main focuses of this dissertation is developing a fabrication process flow to realize high performance MOSFETs with high-[kappa] oxide and metal gate on vertical silicon nanowire arrays. A variety of approaches to fabricating highly ordered silicon nanowire arrays have been achieved. Deep silicon etching process was developed and optimized for nanowire FETs. Process integration and patterning mythologies for high-[kappa] / metal gate were investigated and accomplished. 3-D electronic devices including nanowire capacitors, nanowire FETs and double gate MOSFETs for power applications were fabricated and characterized. The second part of this dissertation is about flexible electronics. Mechanically flexible integrated circuits (ICs) have gained increasing attention in recent years with emerging markets in portable electronics. Although a number of thin-film-transistor (TFT) IC solutions have been reported, challenges still remain for fabrication of inexpensive, high performance flexible devices. We report a simple and straightforward solution: mechanically exfoliating a thin Si film containing ICs. Transistors and circuits can be pre-fabricated on bulk silicon wafer with conventional CMOS process flow without additional temperature or process limitations. The short channel MOSFETs exhibit similar electrical performance before and after exfoliation. This exfoliation process also provides a fast and economical approach to produce thinned silicon wafers, which is a key enabler for three-dimensional (3D) silicon integration based on Through Silicon Vias (TSVs).
Rational fabrication, assembling and actuation of nanowire multi-mer nanomotors
(2015-08) Hong, Ki-Pyo, M.S. in Engineering; Fan, Donglei; Li, Wei
Direct field induced manipulations of nanowires have been recognized as a possible alternative to conventional chemical based assembling techniques. In particular, manipulation of nanowires with an external electric field allows the facile and precision assembly of nanowires into various nanoscale devices. In this study, we have rationally synthesized multisegment Au/Ni nanowires and assembled them into a unique type of rotary nanomotors made of nanowire multi-mers with designed geometric configurations by the electric tweezers. The electric tweezers are a recent invention developed by Prof. Fan’s group, which are based on the combined electrophoretic and dielectrophoretic forces to transport and align nanowires independently in low Reynolds number suspensions. The Au/Ni multi-segmented nanowires are rationally designed and fabricated by electrodeposition into nanoporous templates. By employing the ferromagnetic properties of the nickel segments in the nanowires, we precisely transported and assembled randomly disperse nanowires into multi-mer nanowire devices with designed configuration and further assembled them as the rotors of nanomotors. The magnetic attraction between the Ni segments in the nanowires holds the joints of dimers, trimers and tetramers tightly. The rotary nanomotors made of multiple assembled nanowires with designed configuration are the first to the best of our knowledge. Our study of their rotation behaviors as functions of voltage and frequency shows that the rotational speed of the nanomotors linearly increases with the square of the applied AC voltages and depends on the AC frequencies. The voltage square dependence is highly desirable for achieving ultrahigh speed rotation. This research could generate interest and impact multiple research fields including nanoelectromechanical system (NEMS) devices, nanomotors, microfluidic architectures and single-cell biology.
Silicon and germanium battery materials : exploring new structures, surface treatments, and full cell applications
(2018-05) Adkins, Emily Renee; Korgel, Brian Allan, 1969-; Mullins, Charles B; Manthiram, Arumugam; Hwang, Gyeong S; Yu, Guihua
Lithium ion batteries (LIBs) with higher energy and power density are needed to meet the increasing demands of portable electronic devices, extended-range electric vehicles, and renewable energy storage. Silicon (Si) and germanium (Ge) are attractive anode materials for next generation batteries because they have significantly higher capacities compared with current graphite anodes. One of the challenges Si and Ge face during battery cycling is high volume expansion upon lithiation, which can be accommodated by nanostructuring. LIBs made using Si and Si-Ge type II clathrates exhibited superior reversible cycling performance. This high capacity and stability is due to the type II phase purity of the samples which is a unique feature of the synthetic method used in this study. During cycling, the anode will react with the electrolyte, forming a passivating solid electrolyte interphase (SEI) layer on the surface, which is crucial to stable battery function. The formation of this layer is influenced by the surface chemistry of the active material. Ge NWs with different surface passivations exhibited different battery performance and rate capability. One strategy used to improve the performance of nanostructured Si, is the addition of a surface coating layer. Si nanowires coated with an SiO[subscript x] shell examined using in situ transmission electron microscopy during battery cycling showed reduced volume expansion, at the expense of complete lithiation. When the nanowire is delithiated, pores are observed to form in the amorphized Si due to the SiO[subscript x] shell, which prevents the migration of vacancies formed during delithiation to the nanowire surface. To increase the performance of the LIB, both the anode and cathode capacities must increase. Prelithiation of the Si anode was crucial to improve the capacity and stability of battery cycling for both lithium iron phosphate and sulfur cathodes, and the prelithiation process used strongly influenced battery performance. In a full cell with a sulfur cathode, no sulfides were observed in the Si SEI layer, due to the use of a carbon interlayer. Si-S batteries fully consumed the lithium nitrate electrolyte additive during cycling, resulting in high levels of electrolyte degradation that contaminated the anode and reduced battery stability
Silicon nanowires : synthesis and use as lithium-ion battery anodes
(2014-12) Bogart, Timothy Daniel; Korgel, Brian Allan, 1969-; Mullins, C. Buddie; Ekerdt, John G.; Chelikowsky, James R.; Manthiram, Arumugam
As the power demands of mobile technologies continue to increase, lithium-ion batteries are needed with greater power and energy density. Silicon anodes offer an alternative to commercial graphite with much greater gravimetric and volumetric Li storage. Si nanowires are particularly compelling anode materials because they provide short Li diffusion paths due to their narrow diameter combined with long continuous paths for electron transport down their length. To achieve reasonable battery performance in Si nanowire anodes, conductive carbon particles must be added to provide sufficient electrical conductivity through the anode layer. This lowers the capacity of the anode, but more importantly the carbon particles can segregate in the electrode layer during processing or as a result of mechanical stresses during cycling, leading to unreliable performance. Better performance can be achieved by altering the structure of the Si nanowire to improve electrical conductivity. Si nanowires with a conductive carbon coating were synthesized in a supercritical organic solvent using an organometallic tin precursor to seed growth. The coating eliminated the need for additional conductive additives and improved Si nanowire anode performance. In situ TEM experiments showed that the coated nanowires exhibit higher lithiation rates than bare Si nanowires, but the coating restricts volume expansion limiting the amount of Li storage. Nanowires with a crystalline Si core and amorphous Si shell were also synthesized. The thickness of the core and shell were controlled by altering the Si:Sn precursor ratio. Sn was found to incorporate strongly within the crystalline core, but not at all in the amorphous shell, creating nanowires with varying conductivity. The addition of tin improved Si nanowire performance in Li-ion batteries, eliminating the need for conductive additives. Lastly, the low-temperature limit on the solution synthesis of Si nanowires via in situ seeding was explored using tin, gallium, and indium seeds.
Simulation of a plasmonic nanowire waveguide
(2009-05) Malcolm, Nathan Patrick; Howell, John R.; Shi, Li
In this work a Finite Difference Time Domain (FDTD) simulation is employed to explore local field enhancement, plasmonic coupling, and charge distribution patterns. This 3D simulation calculates the magnetic and electric field components in a large matrix of Yee cells using Maxwell’s equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber, and a sample system of cells is checked for convergence. In the specific simulations considered here, a laser pulse of single wavelength is incident on a silicon substrate, travels through an embedded ZnO nanowire (NW) waveguide only (due to an Ag filter), then incites plasmonic coupling at the gap between an Au nanoparticle tip and an Au substrate, an Au nanoparticle (NP), or a trio of Au nanoparticles incident on an angled Si substrate. The angle between the axis of the NW and the normal of the substrate is varied from 0-60°. The NP perpendicular deflection with respect to the NW axis is also varied from -115 - 75 nm. The enhancement patterns reveal superior signal to noise ratio compared to Near Field Scanning Optical Microscopy (NSOM), three times smaller than the NP diameter 100 nm, as well as resolution and spot size of less than 50 nm. This method of Apertureless NSOM (ANSOM) using a NW waveguide grown on a transparent microcantilever therefore shows promise for imaging of single molecules incident on a substrate and NP-labeled cell membrane.
Strain and modulation doping in epitaxial Si/Ge core-shell nanowire heterostructures
(2015-12) Dillen, David Carl; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; Dodabalapur, Ananth; Yu, Edward T; Korgel, Brian A
For over five decades, silicon based electronics relied on scaling of individual field-effect transistors (FETs) for improvements in integrated circuit performance. Recently, however, further enhancement of packing density and switching speed was limited by the increase in power consumption of short channel devices. New materials and device geometries were introduced to help expand CPU performance while also decreasing power dissipation. Semiconducting nanowires have also been recognized for potential applications as channel material in highly scaled FETs. These structures present opportunities for strain and energy band engineering through the use of radial, or core-shell, heterostructures. To fully exploit the benefits of radial heterostructures, however, requires knowledge of elastic strain distributions and energy band alignments, necessitating the development of new characterization methods. This is especially true in Si/Ge material systems, where a large lattice mismatch over 4% is possible. In this thesis, we grow Si/Ge core-shell nanowires and demonstrate multiple techniques to characterize the nanoscale heterostructure, including strain measurements and extraction of valence band offsets. We grow Ge-SixGe1-x core-shell nanowires and measure the elastic strain using Raman spectroscopy. The Ge core’s Raman spectrum is consistent with a compressive strain in this region due to lattice mismatch with the SixGe1-x shell. The strain distribution and expected Raman peak positions are calculated using continuum elasticity models and lattice dynamic theory, finding excellent agreement to experimental data. We also demonstrate radial modulation doping in Ge-SixGe1-x core-shell nanowire heterostructures by doping a portion of the SixGe1-x shell with boron during growth. The modulation doped nanowire FETs show an enhanced low temperature hole mobility and also a decoupling of transport between core and shell. Through comparison to finite-element calculations, we extract the valence band offset at the core-shell interface. Lastly, we grow coherently strained Si-SixGe1-x core-shell nanowires and characterize the structure using Raman spectroscopy. We first optimize the Si nanowire growth process to favor the diamond crystal structure and to minimize sidewall coverage by Au catalyst, followed by epitaxial growth of the SixGe1-x shell using the Si nanowire as substrate. Raman measurements on core-shell samples indicate a tensile strain in the Si core and a compressive strain in the SixGe1-x shell, both consistent with calculations of the strain and the strain-induced shift of the Raman peaks in this structure.
Thermal and thermoelectric transport in organic and inorganic nanostructures
(2012-08) Weathers, Annie C.; Shi, Li, Ph. D.; Tutuc, Emanuel
Thermal transport in nanowires and nanotubes has attached much attention due to their use in various functional devices and their use as a model system for low dimensional transport phenomena. The precise control of the crystal structure, defects, characteristic size, and electronic properties of nanowires has allowed for fundamental studies of phonon and electron transport in a variety of nanoscale systems. The thermal conductivity in nanostructured materials can vary greatly compared to bulk values owing to classical and quantum size effects. In this work, two model systems for investigating fundamental phonon transport were investigated for potential use in thermoelectric and thermal management applications. The thermoelectric properties of twin defect indium arsenide nanowires and the thermal conductivity of polythiophene nanofibers with improved polymer chain crystallinity were measured with a microfabricated measurement device. The effects of twin planes on reducing the mean free path of phonons in indium arsenide and the effects of improved chain alignment in increasing the thermal conductivity in polymer fibers is discussed.
Tip-enhanced Raman spectroscopy of strained semiconductor nanostructures
(2018-06-25) Zhang, Zhongjian, Ph. D.; Yu, Edward T.; Tutuc, Emanuel; Banerjee, Sanjay; Akinwande, Deji; Lai, Keji
Raman spectroscopy can serve as a powerful tool to probe the vibrational modes of solid state materials. By taking advantage of the enhanced electric fields caused by the surface-enhanced plasmon resonance of a noble metal coated atomic force microscopy tip, tip-enhanced Raman spectroscopy can dramatically increase local signal intensity and measurement spatial resolution. In this dissertation, work is presented on conventional and tip-enhanced Raman measurements of various semiconductor nanostructures with a specific focus on analyzing strain and strain related properties in these material systems. We use tip-enhanced Raman to study Ge-Si₀.₅Ge₀.₅ core-shell nanowires where we observe two distinct Ge-Ge mode Raman peaks that are affected by strain in the core-shell structure. Tip-enhanced measurements show dramatically increased sensitivity to the modes at the interface between the core and shell and a shift in position of this mode due to plasmonic field localization at the tip apex and the resulting change in phonon self-energy caused by increased coupling between phonons and intervalence-band carrier transitions. We also use tip-enhanced Raman spectroscopy to characterize unstrained and strained MoS₂ and show spatial resolution of approximately 100 nm in the measurements. The strain dependence of the second order Raman modes in MoS₂ reveals changes in the electronic band structure in strained MoS₂ that are manifested through changes in the Raman peak positions and peak area ratios, which are corroborated through density functional theory calculations. Finally, we use conventional Raman spectroscopy to probe uniaxially strained monolayer and three-layer WSe₂. Using mechanical modeling of strain in atomically thin WSe₂ on a stretched elastic substrate, we confirm complete transfer of strain from the substrate to the WSe₂ flakes and analyze the evolution of the Raman spectra with applied uniaxial strain above 1 percent. These studies enable us to experimentally determine the strain induced Raman shift for various Raman modes and to calculate the Grüneisen parameter and strain deformation potential for the first order in-plane Raman mode, with experimental values confirmed with theoretical values calculated using density functional theory.