Showing papers on "Silicon published in 2008"

High-performance lithium battery anodes using silicon nanowires

Abstract: There is great interest in developing rechargeable lithium batteries with higher energy capacity and longer cycle life for applications in portable electronic devices, electric vehicles and implantable medical devices. Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh g(-1); ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials, silicon anodes have limited applications because silicon's volume changes by 400% upon insertion and extraction of lithium which results in pulverization and capacity fading. Here, we show that silicon nanowire battery electrodes circumvent these issues as they can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling.

Silicon nanowires as efficient thermoelectric materials

Abstract: Thermoelectric materials, capable of converting a thermal gradient to an electric field and vice versa, could be useful in power generation and refrigeration. But the fabrication of the available high-performance thermoelectric materials is not easily scaled up to the volumes needed for large-scale heat energy scavenging applications. Nanostructuring improves thermoelectric capabilities of some materials, but good thermoelectric materials tend not to take readily to nanostructuring. How about silicon? It can be processed on a large scale but has poor thermoelectric properties. Two groups now show that silicon's thermoelectric properties can be vastly improved by structuring it into arrays of nanowires and carefully controlling nanowire morphology and doping. So with more development, silicon may have potential as a thermoelectric material. Thermoelectric materials interconvert thermal gradients and electric fields for power generation or for refrigeration1,2. Thermoelectrics currently find only niche applications because of their limited efficiency, which is measured by the dimensionless parameter ZT—a function of the Seebeck coefficient or thermoelectric power, and of the electrical and thermal conductivities. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another3. Several groups have achieved significant improvements in ZT through multi-component nanostructured thermoelectrics4,5,6, such as Bi2Te3/Sb2Te3 thin-film superlattices, or embedded PbSeTe quantum dot superlattices. Here we report efficient thermoelectric performance from the single-component system of silicon nanowires for cross-sectional areas of 10 nm × 20 nm and 20 nm × 20 nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including ZT ≈ 1 at 200 K. Independent measurements of the Seebeck coefficient, the electrical conductivity and the thermal conductivity, combined with theory, indicate that the improved efficiency originates from phonon effects. These results are expected to apply to other classes of semiconductor nanomaterials.

Fundamentals of Power Semiconductor Devices

Abstract: Fundamentals of Power Semiconductor Devices provides an in-depth treatment of the physics of operation of power semiconductor devices that are commonly used by the power electronics industry. Analytical models for explaining the operation of all power semiconductor devices are shown. The treatment focuses on silicon devicesandincludes the unique attributes and design requirements for emerging silicon carbide devices.

Molecular doping of graphene

Abstract: Graphene is considered as one of the most promising materials for post silicon electronics, as it combines high electron mobility with atomic thickness [Novoselov et al. Science 2004, 306, 666−669. Novoselov et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453]. The possibility of chemical doping and related excellent chemical sensor properties of graphene have been demonstrated experimentally [Schedin et al. Nat. Mater. 2007, 6, 652−655], but a microscopic understanding of these effects has been lacking, so far. In this letter, we present the first joint experimental and theoretical investigation of adsorbate-induced doping of graphene. A general relation between the doping strength and whether adsorbates are open- or closed-shell systems is demonstrated with the NO2 system: The single, open shell NO2 molecule is found to be a strong acceptor, whereas its closed shell dimer N2O4 causes only weak doping. This effect is pronounced by graphene's peculiar density of states (DOS), which provides an id...

Silicon nanowire radial p-n junction solar cells.

Abstract: We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n−p core−shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

Photonic Networks-on-Chip for Future Generations of Chip Multiprocessors

Abstract: The design and performance of next-generation chip multiprocessors (CMPs) will be bound by the limited amount of power that can be dissipated on a single die We present photonic networks-on-chip (NoC) as a solution to reduce the impact of intra-chip and off-chip communication on the overall power budget A photonic interconnection network can deliver higher bandwidth and lower latencies with significantly lower power dissipation We explain why on-chip photonic communication has recently become a feasible opportunity and explore the challenges that need to be addressed to realize its implementation We introduce a novel hybrid micro-architecture for NoCs combining a broadband photonic circuit-switched network with an electronic overlay packet-switched control network We address the critical design issues including: topology, routing algorithms, deadlock avoidance, and path-setup/tear-down procedures We present experimental results obtained with POINTS, an event-driven simulator specifically developed to analyze the proposed idea, as well as a comparative power analysis of a photonic versus an electronic NoC Overall, these results confirm the unique benefits for future generations of CMPs that can be achieved by bringing optics into the chip in the form of photonic NoCs

Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers.

Abstract: Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions. For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors and low-temperature quantum devices compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit. Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n=3, 13 and 26 and InGaN well thicknesses of 1-3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers.

Biocompatible luminescent silicon quantum dots for imaging of cancer cells.

Abstract: Luminescent silicon quantum dots (Si QDs) have great potential for use in biological imaging and diagnostic applications. To exploit this potential, they must remain luminescent and stably dispersed in water and biological fluids over a wide range of pH and salt concentration. There have been many challenges in creating such stable water-dispersible Si QDs, including instability of photoluminescence due their fast oxidation in aqueous environments and the difficulty of attaching hydrophilic molecules to Si QD surfaces. In this paper, we report the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained. These luminescent micelle-encapsulated Si QDs were used as luminescent labels for pancreatic cancer cells. This paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.

A map for phase-change materials.

Abstract: Phase-change materials are widely used as non-volatile memories, for example in optical data storage, but the search for improved phase-change materials has proved difficult. Based on a fundamental understanding of their bonding characteristics, a systematic prediction of phase-change properties has now become possible.

Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells

Abstract: Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.

Inorganic semiconductor nanostructures and their field-emission applications

Abstract: Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

Water modeled as an intermediate element between carbon and silicon

Abstract: Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, its anomalies and phase transitions with comparable or better accuracy than the most popular atomistic models of water, at less than 1% of the computational cost. We conclude that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water. The speedup in computing time provided by mW makes it particularly useful for the study of slow processes in deeply supercooled water, the mechanism of ice nucleation, wetting-drying transitions, and as a realistic water model for coarse-grained simulations of biomolecules and complex materials.

Hierarchical nanofabrication of microporous crystals with ordered mesoporosity.

Abstract: Zeolite nanocrystals with three-dimensionally ordered mesoporous structures are important for designing molecularly accessible and selective catalysts. With a single zeolite synthesis procedure, uniform nanocrystals and crystal zeolites with ordered imprinted mesoporosity can now be obtained.

Silicon microring resonators with 1.5-microm radius.

Abstract: We demonstrate a junction between a silicon strip waveguide and an ultra-compact silicon microring resonator that minimizes spurious light scattering and increases the critical dimensions of the geometry. We show cascaded silicon microring resonators with radii around 1.5 microm and effective mode volumes around 1.0 microm(3) that are critically coupled to a waveguide with coupled Q's up to 9,000. The radius of 1.5 microm is smaller than the operational wavelength, and is close to the theoretical size limit of the silicon microring ring resonator for the same Q. The device is fabricated with a widely-available SEM-based lithography system using a stitch-free design based on a U-shaped waveguide.

Waveguide-integrated, ultralow-energy GeSi electro-absorption modulators

Abstract: A waveguide–integrated GeSi electro-absorption modulator on silicon with an ultra-low energy consumption of 50 fJ–1bit is presented. Operating in the spectral range of 1539—1553 nm, the CMOS–compatible device has an active area of 30 µm2 and is anticipated to be useful for future communication systems based on large–scale electronic–photonic integration on silicon.

Bioinspired Self‐Cleaning Antireflection Coatings

Abstract: Millions of years before we began to generate functional nanostructures, biological systems were using nanometer-scale architectures to produce unique functionalities. For instance, moths use hexagonal arrays of nonclose-packed (ncp) nipples as antireflection coatings (ARCs) to reduce reflectivity from their compound eyes. The outer surface of the corneal lenses of moths consists of ncp arrays of conical protuberances, termed corneal nipples, typically of sub-300 nm height and spacing. These arrays of subwavelength nipples generate a graded transition of refractive index, leading to minimized reflection over a broad range of wavelengths and angles of incidence. Similar periodic arrays of ncp pillars have also been observed on the wings of cicada to render superhydrophobic surfaces for self-cleaning functionality. In this Communication, we report a simple and scalable bioinspired templating technique for fabricating broadband and superhydrophobic ARCs on technologically important silicon and glass substrates. Crystalline silicon is the most important material for solar cells. Unfortunately, due to the high refractive index of silicon, more than 30% of incident light is reflected back from the surface of crystalline silicon. ARCs are therefore widely utilized to reduce the unwanted reflective losses. Quarterwavelength silicon nitride (SiNx) films deposited by plasmaenhanced chemical vapor deposition (PECVD) are the industrial standard for ARCs on crystalline silicon substrates. However, the PECVD-deposited SiNx films are expensive to fabricate. Additionally, commercial SiNx ARCs are typically designed to suppress reflection efficiently at wavelengths around 600 nm. The reflective loss is rapidly increased for near-infrared and other visible wavelengths, which contain a large portion of the incident solar energy. In contrast, subwavelength-structured moth-eye ARCs directly patterned in the substrates are broadband and intrinsically more stable and durable than multilayer ARCs since no foreign material is involved. Nevertheless, current topdown lithographic technologies in creating subwavelength

Crystal growth: Anatase shows its reactive side.

Abstract: Fluorine-containing species can cause titania to crystallize with an unusually large fraction of reactive {001} facets.

Silicon surface passivation by atomic layer deposited Al2O3

Abstract: Thin Al2O3 films with a thickness of 7–30 nm synthesized by plasma-assisted atomic layer deposition (ALD) were used for surface passivation of crystalline silicon (c-Si) of different doping concentrations. The level of surface passivation in this study was determined by techniques based on photoconductance, photoluminescence, and infrared emission. Effective surface recombination velocities of 2 and 6 cm/s were obtained on 1.9 Ω cm n-type and 2.0 Ω cm p-type c-Si, respectively. An effective surface recombination velocity below 1 cm/s was unambiguously obtained for nearly intrinsic c-Si passivated by Al2O3. A high density of negative fixed charges was detected in the Al2O3 films and its impact on the level of surface passivation was demonstrated experimentally. The negative fixed charge density results in a flat injection level dependence of the effective lifetime on p-type c-Si and explains the excellent passivation of highly B-doped c-Si by Al2O3. Furthermore, a brief comparison is presented between the ...

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Abstract: Atomic-layer-deposited aluminium oxide (Al2O3) is applied as rear-surface-passivating dielectric layer to passivated emitter and rear cell (PERC)-type crystalline silicon (c-Si) solar cells. The excellent passivation of low-resistivity p-type silicon by the negative-charge-dielectric Al2O3 is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al2O3 film covered by a 200 nm plasma-enhanced-chemical-vapour-deposited silicon oxide (SiOx) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single-layer of Al2O3, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.

Silicon nanowires for rechargeable lithium-ion battery anodes

Abstract: Large-area, wafer-scale silicon nanowire arrays prepared by metal-induced chemical etching are shown as promising scalable anode materials for rechargeable lithium battery. In addition to being low cost, large area, and easy to prepare, the electroless-etched silicon nanowires (SiNWs) have good conductivity and nanometer-scale rough surfaces; both features facilitate charge transport and insertion/extraction of Li ions. The electroless-etched SiNWs anode showed larger charge capacity and longer cycling stability than the conventional planar-polished Si wafer.

Motility of metal nanoparticles in silicon and induced anisotropic silicon etching

Abstract: The autonomous motion behavior of metal particles in Si, and the consequential anisotropic etching of silicon and production of Si nanostructures, in particular, Si nanowire arrays in oxidizing hydrofluoric acid solution, has been systematically investigated. It is found that the autonomous motion of metal particles (Ag and Au) in Si is highly uniform, yet directional and preferential along the [100] crystallographic orientation of Si, rather than always being normal to the silicon surface. An electrokinetic model has been formulated, which, for the first time, satisfactorily explains the microscopic dynamic origin of motility of metal particles in Si. According to this model, the power generated in the bipolar electrochemical reaction at a metal particle's surface can be directly converted into mechanical work to propel the tunneling motion of metal particles in Si. The mechanism of pore and wire formation and their dependence on the crystal orientation are discussed. These models not only provide fundamental interpretation of metal-induced formation of pits, porous silicon, and silicon nanowires and nanopores, they also reveal that metal particles in the metal/Si system could work as a self-propelled nanomotor. Significantly, it provides a facile approach to produce various Si nanostructures, especially ordered Si nanowire arrays from Si wafers of desired properties.

Silicon/Graphite Composite Electrodes for High-Capacity Anodes: Influence of Binder Chemistry on Cycling Stability

Abstract: The binder's influence on the cycling stability of high-energy anodes for lithium-ion batteries is demonstrated. Varying the binder's nature strongly influences the composite electrode's performance on deep charging/discharging. If sodium-carboxymethylcellulose is used as binding agent, then a chemical bond between binder and silicon particles can be detected (attenuated total reflection-Fourier transform infrared spectroscopy). Consequently, a reaction mechanism that describes a condensation reaction between the binder and the silicon particles is verbalized. It is shown that, not necessarily the binder's physical flexibility, but its chemical interaction with the active masses is the major claim leading to long-lasting reversible lithium uptake/release.

Silicon nanowire-based solar cells.

Abstract: The fabrication of silicon nanowire-based solar cells on silicon wafers and on multicrystalline silicon thin films on glass is described. The nanowires show a strong broadband optical absorption, which makes them an interesting candidate to serve as an absorber in solar cells. The operation of a solar cell is demonstrated with n-doped nanowires grown on a p-doped silicon wafer. From a partially illuminated area of 0.6 cm2 open-circuit voltages in the range of 230–280 mV and a short-circuit current density of 2 mA cm−2 were obtained.

Insight into silicate-glass corrosion mechanisms

Abstract: The remarkable chemical durability of silicate glass makes it suitable for a wide range of applications. The slowdown of the aqueous glass corrosion kinetics that is frequently observed at long time is generally attributed to chemical affinity effects (saturation of the solution with respect to silica). Here, we demonstrate a new mechanism and highlight the impact of morphological transformations in the alteration layer on the leaching kinetics. A direct correlation between structure and reactivity is revealed by coupling the results of several structure-sensitive experiments with numerical simulations at mesoscopic scale. The sharp drop in the corrosion rate is shown to arise from densification of the outer layers of the alteration film, leading to pore closure. The presence of insoluble elements in the glass can inhibit the film restructuring responsible for this effect. This mechanism may be more broadly applicable to silicate minerals.

Theoretical and Experimental Studies of Bending of Inorganic Electronic Materials on Plastic Substrates

Abstract: This paper describes materials and mechanics aspects of bending in systems consisting of ribbons and bars of single crystalline silicon supported by sheets of plastic. The combined experimental and theoretical results provide an understanding for the essential behaviors and for mechanisms associated with layouts that achieve maximum bendability. Examples of highly bendable silicon devices on plastic illustrate some of these concepts. Although the studies presented here focus on ribbons and bars of silicon, the same basic considerations apply to other implementations of inorganic materials on plastic substrates, ranging from amorphous or polycrystalline thin films, to collections of nanowires and nanoparticles. The contents are, as a result, relevant to the growing community of researchers interested in the use of inorganic materials in flexible electronics.