Browsing by Subject "Heterostructures"
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Item Building fabrication-structure-application datacubes of 2D heterostructures(2020-11-30) Wang, Jimi; Akinwande, DejiResearches on graphene and other two-dimensional (2D) layered materials remain exponential growth, driven by the fundamental interests and potential applications. Isolated atomic layers are intrinsically the building blocks that can be reassembled into vertically stacked heterostructures. Those so-called van der Waals heterostructures exhibit intriguing, unique properties that cannot be found in their single-layer counterparts. Recent deterministic placement methods have further opened up new possibilities to fabricate even more complex heterostructures with high performance. Here, this report provides insights into the recent progress of 2D heterostructures with an emphasis on their fabrication-structure-performance datacubes. First, we introduce a detailed description of state-of-the-art deterministic assembly and fabrication methods. We then compare different approaches, summarize their advantages and limitations, alongside the recommendations on choosing suitable techniques. Next, we will discuss the supreme electrical properties of heterostructures and the electron transfer mechanisms that make them outstanding. Then, we present some typical examples of state-of-the-art high-performance electronic applications. Finally, the perspectives and challenges will be addressed for future developments of 2D heterostructures.Item Electronic properties and electron-electron interaction effects in transition metal dichalcogenides(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, LiTransition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.Item Graphene and hexagonal boron nitride heterostructures for beyond CMOS applications(2016-12) Kang, Sangwoo; Banerjee, Sanjay; Tutuc, Emanuel; Register, Leonard F; Akinwande, Deji; Demkov, AlexanderScaling limits of conventional complementary metal oxide semiconductor (CMOS) technology has motivated the research of numerous beyond CMOS device concepts. One such device is the interlayer tunnel FET (ITFET). This device is demonstrated using the two-dimensional (2D) materials bilayer graphene and hexagonal boron nitride (hBN). Stacking these materials together, we fabricate a double bilayer graphene and hBN heterostructure where the two graphene layers function as the top and bottom electrodes and the hBN as the tunnel barrier of the ITFET. Significant negative differential resistance (NDR) in the interlayer current-voltage characteristic is demonstrated at room temperature. Electrostatic analysis reveals that the multiple NDR peaks are due to energetic band alignment between the two sub-bands of the top and bottom bilayer graphene at the K-point of the Brillouin zone. Temperature dependent and parallel magnetic field measurements are conducted to further confirm that the conduction mechanism is momentum and energy conserving resonant tunneling. In addition, we demonstrate how the NDR can be used for implementing a one-transistor static random access memory element. Improvements in the transfer method which allowed rotationally aligned top and bottom electrode layers, made possible extensive experimental studies of 2D heterostructure based ITFETs. Utilizing such technique, we conducted experiments involving thicker multilayer graphene. We analyze how the graphene thickness and stacking order can influence the resonance condition and how this affects the overall device characteristic. The effects of interlayer hBN scaling such as increased tunneling current and peak position shifting is also briefly dealt with. Current-voltage simulations based on Bardeen transfer Hamiltonian approach were conducted for these devices, and it is shown that the peak positions predicted by theory match well with those obtained through measurements.Item Monte Carlo simulation of charge transport in Si-based heterostructure transistors(2002) Wang, Xin; Banerjee, SanjayStrain and bandgap engineering of strained materials has emerged as an important technique for improving the device performance other than conventional scaling method. The purpose of this work is to develop a Monte Carlo simulation tool to investigate properties of these strained materials and carrier transport in deep submicron novel devices with heterostructures and strained materials. A general full-band Monte Carlo simulation tool with high flexibility about device structure and material profiles is developed for the first time. The transport model is based on energy-dependent scattering rates including inelastic acoustic phonon scattering with longitudinal and transverse modes, optical phonon scattering, impact ionization, surface roughness scattering, impurity scattering and alloy scattering. The full-band treatment for strained material model substantially advances the state-of-the-art method relying on simpler valley model for the scattering rates. The multi-material profiles in devices are treated with parameterization of band structure. The tunneling across a potential barrier is treated with Feynman’s effective potential scheme. An orthorhombically-strained silicon (OS-Si) is reported in this work. The six degenerate valleys in OS-Si near X points break into three pairs with different energy minima due to the orthorhombic strain. Thus the drift velocity is enhanced under an electric field transverse to the longitudinal-axis of the lowest valleys. The OS-Si grown on a compressively-strained Si0.6Ge0.4 sidewall has a mobility almost twice that of bulk Si and electron saturation velocity approximately 20% higher. For homogenous strained silicon on Si0.7Ge0.3 (001), in-plane mobility of 2670 cm2 /(Vs) for electrons is obtained, with enhancement by a factor of 1.8 compared to the unstrained case. Electron transport in a strained Si nMOSFET with 50 nm channel length is also investigated by full-band Monte Carlo. Strained silicon devices exhibit around 60% increase of drain current compared to unstrained silicon. Strained SiGe is also studied with full-band Monte Carol tool. A 90% enhancement in hole mobility is obtained for strained Si0.7Ge0.3 compared with bulk Si. The preliminary investigation of hole transport in vertical pMOSFETs with graded SiGe channel is also reported in this work.Item Nanostructuring silicon and germanium for high capacity anodes in lithium ion batteries(2012-12) Harris, Justin Thomas; Korgel, Brian Allan, 1969-; Ekerdt, John G.; Hwang, Gyeong S.; Mullins, Charles B.; Stevenson, Keith A.Colloidally synthesized silicon (Si) and germanium (Ge) were explored as high capacity anode materials in lithium ion batteries. a-Si:H particles were synthesized through the thermal decomposition of trisilane in supercritical n-hexane. Precise control over particle size and hydrogen content was demonstrated. Particles ranged in size from 240-1500 nm with hydrogen contents from 10-60 atomic%. Particles with low hydrogen content had some degree of local ordering and were easily crystallized during Raman spectroscopy. The as-synthesized particles did not perform well as an anode material due to low conductivity. Increasing surface conductivity led to enhanced lithiation potential. Cu nanoparticles were deposited on the surface of the a-Si:H particles through a hydrogen facilitated reduction of Cu salts. The resulting Cu coated particles had a lithiation capacity seven times that of pristine a-Si:H particles. Monophenylsilane (MPS) grown Si nanowire paper was annealed under forming gas to reduce a polyphenylsilane shell into conductive carbon. The resulting paper required no binder or carbon additive and achieved capacities of 804 mA h/g vs 8 mA h/g for unannealed wires. Si and Ge heterostructures were explored to take advantage of the higher inherent conductivity of Ge. Ge nanowires were successfully coated with a-Si by thermal decomposition of trisilane on their surface, forming Ge@a-Si core shell structures. The capacity increased with increasing Si loading. The peak lithiation capacity was 1850 mA h/g after 20 cycles – higher than the theoretical capacity of pure Ge. MPS additives created a thin amorphous shell on the wire surfaces. By incubating the wires after MPS addition the shell was partially reduced, conductivity increased, and a 75% increase in lithiation capacity was observed for the nanowire paper. The syntheses of Bi and Au nanoparticles were also explored. Highly monodisperse Bi nanocrystals were produced with size control from 6-18 nm. The Bi was utilized as seeds for the SLS synthesis of Ge nanorods and copper indium diselenide (CuInSe2) nanowires. Sub 2 nm Au nanocrystals were synthesized. A SQUID magnetometer probed their magnetic behavior. Though bulk Au is diamagnetic, the Au particles were paramagnetic. Magnetic susceptibility increased with decreasing particle diameter.Item Scanning tunneling microscopy of compound semiconductor heterostructures : from alloy ordering to composition determination(2001-12) Liu, Ning, 1962-; Shih, Chih-kangItem Tuning of core-shell heterostructured nanoparticles generated by laser ablation of microparticles(2009-05) Gallardo, Ignacio Francisco; Keto, John W.We investigate the temperature and size distribution of Ag, Ge, CdSe and ZnS nanoparticles undergoing UV excimer laser pulses. A two laser pulse experiment is designed to monitor nanoparticle size before and after laser interaction. We study HRTEM images and measure the ablation and fluorescence spectra of particles before and after evaporation. Results show that the nanoparticle mean radius decreases from 3.4 ± 0.2 nm to 2.6 ± 0.2 nm, from 4.3 ± 0.1 nm to 3.5 ± 0.1 nm, and from 3.1 ± 0.2 nm to 2.6 ± 0.2 nm for Ag, Ge and CdSe, respectively. No ZnS nanoparticle size reduction was observed. Theoretical models for nanoparticles undergoing laser heating show that temperatures above the bulk and nanoparticle material melting point reduce the nanoparticles size by a factor of 0.3 and suggest recondensation before collection. For CdSe nanoparticles collected on dry substrates and solvents, blue shifted fluorescence (PL) peaks support the size reduction.