931 research outputs found

    An Enhanced Statistical Phonon Transport Model for Nanoscale Thermal Transport and Design

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    Managing thermal energy generation and transfer within the nanoscale devices (transistors) of modern microelectronics is important as it limits speed, carrier mobility, and affects reliability. Application of Fourier’s Law of Heat Conduction to the small length and times scales associated with transistor geometries and switching frequencies doesn’t give accurate results due to the breakdown of the continuum assumption and the assumption of local thermodynamic equilibrium. Heat conduction at these length and time scales occurs via phonon transport, including both classical and quantum effects. Traditional methods for phonon transport modeling are lacking in the combination of computational efficiency, physical accuracy, and flexibility. The Statistical Phonon Transport Model (SPTM) is an engineering design tool for predicting non-equilibrium phonon transport. The goal of this work has been to enhance the models and computational algorithms of the SPTM to elevate it to have a high combination of accuracy and flexibility. Four physical models of the SPTM were enhanced. The lattice dynamics calculation of phonon dispersion relations was extended to use first and second nearest neighbor interactions, based on published interatomic force constants computed with first principles Density Functional Theory (DFT). The computation of three phonon scattering partners (that explicitly conserve energy and momentum) with the inclusion of the three optical phonon branches was applied using scattering rates computed from Fermi’s Golden Rule. The prediction of phonon drift was extended to three dimensions within the framework of the previously established methods of the SPTM. Joule heating as a result of electron-phonon scattering in nanoscale electronic devices was represented using a modal specific phonon source that can be varied in space and time. Results indicate the use of first and second nearest neighbor lattice dynamics better predicted dispersion when compared to experimental results and resulted in a higher fidelity representation of phonon group velocities and three phonon scattering partners in an anisotropic manner. Three phonon scattering improvements resulted in enhanced fidelity in the prediction of phonon modal decay rates across the wavevector space and thus better representation of non-equilibrium behavior. Comparisons to the range of phonon transport modeling approaches from literature verify that the SPTM has higher phonon fidelity than Boltzmann Transport Equation and Monte Carlo and higher length scale and time scale fidelity than Direct Atomic Simulation. Additional application of the SPTM to both a 1-d silicon nanowire transistor and a 3-d FinFET array transistor in a transient manner illustrate the design capabilities. Thus, the SPTM has been elevated to fill the gap between lower phonon fidelity Monte Carol (MC) models and high fidelity, inflexible direct quantum simulations (or Direct Atomic Simulations (DAS)) within the field of phonon transport modeling for nanoscale electronic devices. The SPTM has produced high fidelity device level non-equilibrium phonon information in a 3-d, transient manner where Joule heating occurs. This information is required due to the fact that effective lattice temperatures are not adequate to describe the local thermal conditions. Knowledge of local phonon distributions, which can’t be determined from application of Fourier’s law, is important because of effects on electron mobility, device speed, leakage, and reliability

    Electrons dynamics control by shaping femtosecond laser pulses in micro/nanofabrication: modeling, method, measurement and application

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    During femtosecond laser fabrication, photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions is of picosecond order. Hence, lattice motion is negligible within the femtosecond pulse duration, whereas femtosecond photon-electron interactions dominate the entire fabrication process. Therefore, femtosecond laser fabrication must be improved by controlling localized transient electron dynamics, which poses a challenge for measuring and controlling at the electron level during fabrication processes. Pump-probe spectroscopy presents a viable solution, which can be used to observe electron dynamics during a chemical reaction. In fact, femtosecond pulse durations are shorter than many physical/chemical characteristic times, which permits manipulating, adjusting, or interfering with electron dynamics. Hence, we proposed to control localized transient electron dynamics by temporally or spatially shaping femtosecond pulses, and further to modify localized transient materials properties, and then to adjust material phase change, and eventually to implement a novel fabrication method. This review covers our progresses over the past decade regarding electrons dynamics control (EDC) by shaping femtosecond laser pulses in micro/nanomanufacturing: (1) Theoretical models were developed to prove EDC feasibility and reveal its mechanisms; (2) on the basis of the theoretical predictions, many experiments are conducted to validate our EDC-based femtosecond laser fabrication method. Seven examples are reported, which proves that the proposed method can significantly improve fabrication precision, quality, throughput and repeatability and effectively control micro/nanoscale structures; (3) a multiscale measurement system was proposed and developed to study the fundamentals of EDC from the femtosecond scale to the nanosecond scale and to the millisecond scale; and (4) As an example of practical applications, our method was employed to fabricate some key structures in one of the 16 Chinese National S&T Major Projects, for which electron dynamics were measured using our multiscale measurement system

    Semiconductor Nanowires: Epitaxy and Applications

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    Semiconductor nanowires are nanoscale objects formed by bottom-up synthesis. In recent years their unique properties have been exploited in fields such as electronics, photonics, sensors and the life sciences. In this work, the epitaxial growth of nanowires and their applications were studied. Heteroepitaxial growth of III-V nanowires on silicon substrates was demonstrated. This may enable direct band gap materials for optoelectronic devices, as well as high-mobility, low-contact resistance materials for electronics, to be integrated directly on the Si platform. Furthermore, gold-free nanowire synthesis on Si was demonstrated, which offers an advantage in terms of compatibility with established Si processing. Controlled nanowire synthesis by employing lithography was demonstrated. This combination of established "top-down" planar processing, and "bottom-up" nanowire growth, enables deterministic synthesis with individual nanowire site control. The process was first demonstrated with electron beam lithography and later extended to nanoimprint lithography, which is a parallel, high-throughput method, suitable for commercial volumes. Nanowire applications were demonstrated by three examples: (i) Vertical light-emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates. LED functionality was established on both kinds of substrates. This provided a direct demonstration of light-emitting devices on Si made possible by heteroepitaxial III-V nanowire growth on Si. (ii) A single-electron transistor constructed from a heterostructured nanowire with an InAs island sandwiched between two InP barriers. The narrow diameter of the nanowire provides the lateral confinement, and the tunnel barrier resistances are tunable by varying the InP barrier thickness. Coulomb oscillations and Coulomb blockade with a charging energy of approx. 4 meV were observed. (iii) Sensory nerve cell interactions with nanowires. Substrates covered with 2.5 um long and 50 nm diameter nanowires supported cell adhesion and axonal outgrowth. The cells interacted closely with the nanostructures, and viable cells penetrated by wires were observed, as well as wire bending due to forces exerted by the cells

    Quantum properties of atomic-sized conductors

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    Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to single atom upon further stretching. For some metals it is even possible to form a chain of individual atoms in this fashion. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. The properties of the contact are then dominantly determined by the nature of this atom. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics. Properties investigated include multiple Andreev reflection, shot noise, conductance quantization, conductance fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum effects show up in the mechanical properties of the contacts, as seen in the force and cohesion energy of the nanowires. We review this reseach, which has been performed mainly during the past decade, and we discuss the results in the context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have been compressed. A higher-resolution version can be found at: http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB zip PostScript

    Theory and Simulation of Semiconducting Nanowires for Thermoelectric Applications

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    In this Thesis we present novel, robust and general algorithms for combining planewave density-functional theory with the Landauer-Buttiker transport formalism. The method automates this process with minimal user input to allow a high throughput of calculations. We make use of a maximally-localised Wannier function basis to describe systems using short-ranged Hamiltonians. Further, these Hamiltonians may be used as "building-blocks" to create model Hamiltonians of much larger (10,000+ atom) systems, thus allowing electronic transport properties of structurally complex systems to be determined with first-principles accuracy. A similar building-block method is applied to construct model dynamical matrices from those of smaller systems, from which the lattice thermal conductivity Kl may be inferred. The methods were applied to investigate the thermoelectric properties of (110), (111) and (211) Si nanowires (SiNWs) that contain axial heterostructures of Ge. Their performance is measured by the figure of merit, [equation included here], where S, G , Ke and T are the Seebeck co-efficient, electronic conductance, electronic contribution to the thermal conductance and average temperature between the sample's contacts, respectively. We find the thermoelectric power factor S2G is reduced by the presence of heterostructures, however, as a result of the differences between phonon density of states in the Si and Ge regions, low Kl values (< 0.1 nWK-1) are reported. Thus greater values of zT are found compared to the pristine SiNW case. Of the growth directions studied, the (111) direction is found to display the greatest values of zT, with values as large as three in systems with periodic arrangements of heterostructures. More modest values of 1.6 are found in structures that model disorder in the heterostructure length, which may occur experimentally; this is still a factor of four greater than the pristine case. In addition, we observe that trends in S2G, KI and zT that are predicted for systems containing a single heterostructure can often be used to predict trends in systems with many heterostructures

    Molybdenum chalcohalide nanowires as building blocks of nanodevices

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    Molybdenum chalcohalide nanowires are systems, which structural, electronic and optical properties have been analyzed in detail. However, their potential as building blocks for electronic devices has not been investigated so far. This question is raised in Dissertation, focusing on unique electronic transport properties of these systems, and comparing them with those of the popular carbon nanotubes

    Silicon-germanium nanowire heterojunctions: Optical and electrical properties

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    Semiconductor nanowires are quasi-one-dimensional objects with unique physical properties and strong potential in nanophotonics, nanoelectronics, biosensing, and solar cell devices. The next challenge in the development of nanowire functional structures is the nanowire axial heterojunctions, especially lattice mismatched heterojunctions. Si and Ge have a considerable lattice mismatch of ~ 4.2% as well as a mismatch in the coefficient of thermal expansion, and the formation of a Si1-xGex transition layer at the heterointerface creates a non-uniform strain and modifies the band structures of the adjacent Si and Ge nanowire segments. These nanostructures are produced by catalytic chemical vapor deposition employing vapor-liquid-solid mechanism on (111) oriented p-type Si substrate, and they exhibit unique structural properties including highly localized strain, and short-range interdiffusion/intermixing revealed by transmission electron microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy. Our studies of the structural properties of axial Si-Ge nanowire heterojunctions show that despite the 4.2% lattice mismatch between Si and Ge they can be grown without a significant density of structural defects. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface during growth and lateral expansion of the Ge segment of the nanowire, which is in part due to a higher solubility of Ge in metal precursors. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si/Ge nanowire heterojunctions are proposed to be responsible for the thermally induced mechanical stress detected under intense laser radiation. The performed electrical measurements include current-voltage, conductance-voltage, transient electrical measurements under various applied voltages at temperatures ranging from 20 to 400K. We find that Si-Ge nanowire heterojunctions exhibit strong current instabilities associated with flicker noise and damped oscillations with frequencies close to 10-30 MHz. Flicker (or 1/f ) noise is characterized and analyzed on carrier number fluctuation model and mobility fluctuation model noise mechanism, respectively. The proposed explanation is based on a carrier transport mechanism involving electron transitions from Ge to Si segments of the NWs, which requires momentum scattering, causes electron deceleration at the Ge-Si heterointerface and disrupts current flow. Both Si/Ge heterojunctions and NW surface states are demonstrated to be the two dominant elements that strongly influence the electrical characteristics of nanowires

    RAMAN SPECTROSCOPIC EVIDENCE FOR ANHARMONIC PHONON LIFETIMES AND BLUESHIFTS IN 1D STRUCTURES

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    Anharmonic effects in two different quasi-1D systems were probed via micro-Raman spectroscopy. In the first system, we observed upshifts of peaks in the Raman spectra for b-Ga2O3 nanowires grown along the [110] growth direction compared those present in bulk b-Ga2O3. Contrary to our Raman studies on -Ga2O3 nanowires, downshifts in the Raman spectrum for b-Ga2O3 nanowires grown along [401] direction has also been reported by other research groups. We attribute these Raman shifts to the growth direction-induced lattice strains (compressive and tensile) present in the nanowires, and present a model based on the quasi-harmonic density functional theory to support our hypothesis. In the second study, the anharmonic phonon lifetime in suspended single-walled carbon nanotubes was measured using high-resolution micro-Raman spectroscopy. Previous studies on suspended nanotubes performed with scanning tunneling microscopy (STM) reported phonon lifetimes of the order of nanoseconds for the radial breathing mode (RBM). However, the longest phonon lifetimes measured from Raman spectroscopy is of the order of picoseconds. Our study also showed the RBM lifetime to be in the picosecond regime, and we sought to explain this discrepancy with the STM study by invoking an anharmonic model for phonon decay in carbon nanotubes
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