Significant reduction of thermal conductivity in Si/Ge core-shell nanowires

We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium molecular dynamics simulations. Our results show that a simple deposition of a Ge shell of only 1 to 2 unit cells in thickness on a single crystalline Si nanowire can lead to a dramatic 75% decrease in thermal conductivity at room temperature compared to an uncoated Si nanowire. By analyzing the vibrational density states of phonons and the participation ratio of each specific mode, we demonstrate that the reduction in the thermal conductivity of Si/Ge core-shell nanowire stems from the depression and localization of long-wavelength phonon modes at the Si/Ge interface and of high frequency nonpropagating diffusive modes.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Core/multishell nanowire heterostructures as multicolor, high-efficiency light-emitting diodes

We report the growth and characterization of core/multishell nanowire radial heterostructures, and their implementation as efficient and synthetically tunable multicolor nanophotonic sources. Core/multishell nanowires were prepared by metal-organic chemical vapor deposition with an n-GaN core and InxGa1-xN/GaN/p-AlGaN/p-GaN shells, where variation of indium mole fraction is used to tune emission wavelength. Cross-sectional transmission electron microscopy studies reveal that the core/multishell nanowires are dislocation-free single crystals with a triangular morphology. Energy-dispersive X-ray spectroscopy clearly shows shells with distinct chemical compositions, and quantitatively confirms that the thickness and composition of individual shells can be well controlled during synthesis. Electrical measurements show that the p-AlGaN/p-GaN shell structure yields reproducible hole conduction, and electroluminescence measurements demonstrate that in forward bias the core/multishell nanowires function as light-emitting diodes, with tunable emission from 365 to 600 nm and high quantum efficiencies. The ability to synthesize rationally III-nitride core/multishell nanowire heterostructures opens up significant potential for integrated nanoscale photonic systems, including multicolor lasers.     

几种Si、Ge纳米线导热系数的原子模拟

采用非平衡分子动力学方法模拟了Si纳米线、Ge纳米线、核-壳结构的Si/Ge纳米线及超晶格结构的Si/Ge纳米线的导热系数,给出了纳米线的温度与导热系数关系曲线,对比了几种纳米线导热特性的差异,研究结果表明,随着温度的升高,各纳米线的导热系数降低;相同温度下,纳米线导热系数的大小顺序为：核-壳结构的Si/Ge纳米线、超晶格结构的Si/Ge纳米线、Si纳米线、Ge纳米线。     

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

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.     

Band-gap modulation in single-crystalline Si1-xGex nanowires

We report the energy band-gap modulation of single-crystalline Si1-xGex (0 相似文献   

Performance analysis of a Ge/Si core/shell nanowire field-effect transistor

We ana/lyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3d5s* tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 and 85% of the ballistic limit. For this approximately 15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF = 100. To observe true one-dimensional transport in a 110 Ge nanowire transistor, the nanowire diameter would have to be less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on a common basis nanowire transistors of various materials and structures.     

Observation of hole accumulation in Ge/Si core/shell nanowires using off-axis electron holography

Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial [110]-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor-liquid-solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm(3) in the core regions.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

Misfit-guided self-organization of anticorrelated ge quantum dot arrays on si nanowires

Misfit-strain guided growth of periodic quantum dot (QD) arrays in planar thin film epitaxy has been a popular nanostructure fabrication method. Engineering misfit-guided QD growth on a nanoscale substrate such as the small curvature surface of a nanowire represents a new approach to self-organized nanostructure preparation. Perhaps more profoundly, the periodic stress underlying each QD and the resulting modulation of electro-optical properties inside the nanowire backbone promise to provide a new platform for novel mechano-electronic, thermoelectronic, and optoelectronic devices. Herein, we report a first experimental demonstration of self-organized and self-limited growth of coherent, periodic Ge QDs on a one-dimensional Si nanowire substrate. Systematic characterizations reveal several distinctively different modes of Ge QD ordering on the Si nanowire substrate depending on the core diameter. In particular, Ge QD arrays on Si nanowires of around 20 nm diameter predominantly exhibit an anticorrelated pattern whose wavelength agrees with theoretical predictions. The correlated pattern can be attributed to propagation and correlation of misfit strain across the diameter of the thin nanowire substrate. The QD array growth is self-limited as the wavelength of the QDs remains unchanged even after prolonged Ge deposition. Furthermore, we demonstrate a direct kinetic transformation from a uniform Ge shell layer to discrete QD arrays by a postgrowth annealing process.     

Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires

Finding a Si-based material with strong optical activity at the band-edge remains a challenge despite decades of research. The interest lies in combining optical and electronic functions on the same wafer, while retaining the extraordinary know-how developed for Si. However, Si is an indirect-gap material. The conservation of crystal momentum mandates that optical activity at the band-edge includes a phonon, on top of an electron-hole pair, and hence photon absorption and emission remain fairly unlikely events requiring optically rather thick samples. A promising avenue to convert Si-based materials to a strong light-absorber/emitter is to combine the effects on the band-structure of both nanostructuring and alloying. The number of possible configurations, however, shows a combinatorial explosion. Furthermore, whereas it is possible to readily identify the configurations that are formally direct in the momentum space (due to band-folding) yet do not have a dipole-allowed transition at threshold, the problem becomes not just calculation of band structure but also calculation of absorption strength. Using a combination of a genetic algorithm and a semiempirical pseudopotential Hamiltonian for describing the electronic structures, we have explored hundreds of thousands of possible coaxial core/multishell Si/Ge nanowires with the orientation of [001], [110], and [111], discovering some "magic sequences" of core followed by specific Si/Ge multishells, which can offer both a direct bandgap and a strong oscillator strength. The search has revealed a few simple design principles: (i) the Ge core is superior to the Si core in producing strong bandgap transition; (ii) [001] and [110] orientations have direct bandgap, whereas the [111] orientation does not; (iii) multishell nanowires can allow for greater optical activity by as much as an order of magnitude over plain nanowires; (iv) the main motif of the winning configurations giving direct allowed transitions involves rather thin Si shell embedded within wide Ge shells. We discuss the physical origin of the enhanced optical activity, as well as the effect of possible experimental structural imperfections on optical activity in our candidate core/multishell nanowires.     

Growth of nickel silicides in Si and Si/SiOx core/shell nanowires

We exploited the oxide shell structure to explore the structure confinement effect on the nickel silicide growth in one-dimensional nanowire template. The oxide confinement structure is similar to the contact structure (via hole) in the thin film system or nanodevices passivated by oxide or nitride film. Silicon nanowires in direct contact with nickel pads transform into two phases of nickel silicides, Ni31Si12 and NiSi2, after one-step annealing at 550 °C. In a bare Si nanowire during the annealing process, NiSi2 grows initially through the nanowire, followed by the transformation of NiSi2 into the nickel-rich phase, Ni31Si12 starting from near the nickel pad. Ni31Si12 is also observed under the nickel pads. Although the same phase transformations of Si to nickel silicides are observed in nanowires with oxide confinement structure, the growth rate of nickel silicides, Ni31Si12 and NiSi2, is retarded dramatically. With increasing oxide thickness from 5 to 50 nm, the retarding effect of the Ni31Si12 growth and the annihilation of Ni2Si into the oxide confined-Si is clearly observed. Ni31Si12 and Ni2Si phases are limited to grow into the Si/SiOx core-shell nanowire as the shell thickness reaches 50 nm. It is experimental evidence that phase transformation is influenced by the stressed structure at nanoscale.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Diffusion in Si(x)Ge(1-x)/Si nanowire heterostructures

Si0.48Ge0.52/Si tip/nanowire heterostructures were grown by pulsed laser vaporization (PLV) at a growth temperature of 1100 degrees C. Ge diffusion in [111]-growth Si nanowires was studied for different post-synthesis annealing temperatures from 200 degrees C to 800 degrees C. Ge composition profiles were quantified by energy-dispersive X-ray spectroscopy in a transmission electron microscope. The compositional profiles were modeled by a limited-source diffusion model to extract temperature-dependent diffusion coefficients. The Ge diffusion coefficients followed an Arrhenius relationship with an activation energy of 0.622 +/- 0.050 eV. This rather low activation energy barrier is similar to the previously reported activation energy barrier of 0.67 eV for Ge surface diffusion on Si, suggesting that surface diffusion may dominate in nanowires at this length scale.     

Obtaining uniform dopant distributions in VLS-grown Si nanowires

Semiconducting nanowires grown by the vapor-liquid-solid method commonly develop nonuniform doping profiles both along the growth axis and radially due to unintentional surface doping and diffusion of the dopants from the nanowire surface to core during synthesis. We demonstrate two approaches to mitigate nonuniform doping in phosphorus-doped Si nanowires grown by the vapor-liquid-solid process. First, the growth conditions can be modified to suppress active surface doping. Second, thermal annealing following growth can be used to produce more uniform doping profiles. Kelvin probe force microscopy and scanning photocurrent microscopy were used to measure the radial and the longitudinal active dopant distribution, respectively. Doping concentration variations were reduced by 2 orders of magnitude in both annealed nanowires and those for which surface doping was suppressed.     

Interface charge induced p-type characteristics of aligned Si(1-x)Gex nanowires

This study reports the electrical transport characteristics of Si(1-x)Gex (x=0-0.3) nanowires. Nanowires with diameters of 50-100 nm were grown on Si substrates. The valence band spectra from the nanowires indicate that energy band gap modulation is readily achievable using the Ge content. The structural characterization showed that the native oxide of the Si(1-x)Gex nanowires was dominated by SiO2; however, the interfaces between the nanowire and the SiO2 layer consisted of a mixture of Si and Ge oxides. The electrical characterization of a nanowire field effect transistor showed p-type behavior in all Si(1-x)Gex compositions due to the Ge-O and Si-O-Ge bonds at the interface and, accordingly, the accumulation of holes in the level filled with electrons. The interfacial bonds also dominate the mobility and on- and off-current ratio. The large interfacial area of the nanowire, together with the trapped negative interface charge, creates an appearance of p-type characteristics in the Si(1-x)Gex alloy system. Surface or interface structural control, as well as compositional modulation, would be critical in realizing high-performance Si(1-x)Gex nanowire devices.     

Ultralong single-crystal metallic Ni2Si nanowires with low resistivity

Ultralong, single-crystal Ni2Si nanowires sheathed with amorphous silicon oxide were synthesized on a large scale by a chemical vapor transport (CVT) method, using iodine as the transport reagent and Ni2Si powder as the source material. Structural characterization using powder X-ray diffraction, electron microscopy, and energy-dispersive spectroscopy shows that the nanowires have Ni2Si-SiOx core-shell structure with single-crystal Ni2Si core and amorphous silicon oxide shell. The oxide shell is electrically insulating and can be removed by HF etching. Four-terminal electrical measurements show that the single-crystal nanowire has extremely low resistivity of 21 muOmega.cm and is capable of supporting remarkably high failure current density >108 A/cm2. These unique Ni2Si nanowires are very attractive nanoscale building blocks for interconnects and fully silicided (FUSI) gate applications in nanoelectronics.     

Vertically aligned ZnO/amorphous-Si core-shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.     

The dependence of structural stability and tunable gap on the Si components of ZnSe/Si bi-coaxial nanowire heterostructures

The bare and hydrogen-passivated ZnSe/Si bi-coaxial nanowire heterostructures along [110] direction have been investigated by using the first-principle calculations within density functional theory. The structural stability and electronic property of ZnSe/Si bi-coaxial nanowire heterostructures have been shown by changing the Si components. It is found that the ZnSe/Si nanowires have zero gaps at lower Si components, and then they have the increasing gap at higher Si components. It is seen clearly that there is the transition of band gap form zero to nonzero. With increasing Si components, the ZnSe/Si nanowires can be also achieved as n-type or p-type, in agreement qualitatively with the experimental observations. In addition, the structural stabilities and the cohesive energies of ZnSe/Si bi-coaxial nanowires are changed obviously with the different Si components.