Control of Si nanowire growth by oxygen

Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet volumes to remain constant for longer times and therefore permits the growth of untapered wires. This effect is observed for both gas-phase and surface-bound oxygen, so the source of oxygen is unimportant. The control of oxygen exposure during growth provides a new tool for the fabrication of long, uniform-diameter structures, as required for many applications of nanowires.     

Control of InAs nanowire growth directions on Si

We report on control of growth directions of InAs nanowires on Si substrate. We achieved to integrate vertical InAs nanowires on Si by modifying initial Si(111) surface in selective-area metal-organic vapor phase epitaxy with flow-rate modulation mode at low temperature. Cross-sectional transmission electron microscope and Raman scattering showed that misfit dislocation with local strains were accommodated in the interface.     

Realization of a linear germanium nanowire p-n junction

Germanium nanowires grown by chemical vapor deposition exhibit a peculiar dopant incorporation mechanism. The dopant atoms, such as boron and phosphorus, get incorporated through the wire surface, a mechanism which limits the doping modulation along the wire length, and therefore the fabrication of more elaborate structures that combine both n- and p-type doping. Using a novel device design that circumvents these constraints, we demonstrate here a linear Ge nanowire p-n junction.     

Template synthesis and growth mechanism of metal nanowire/carbon nanotube heterojunctions

Copper (Cu) and Cobalt (Co) with remarkable difference in the catalytic activity for the growth of carbon nanotubes (CNTs) have been used to prepare metal-nanowire/CNT heterojunctions. The ordered arrays of Cu nanowire/CNT (CuNW/CNT) and Co nanowire/CNT (CoNW/CNT) heterojunctions were prepared by combining electrochemical deposition and chemical vapor deposition. The interfaces between CNTs and Cu or Co nanowires have been examined and compared. At the interface of CuNW/CNT heterojunction, the tip of CuNW is encapsulated by carbon material (named "cap") and connected the CNT consisting of amorphous carbon (a-C). Two-segment CuNW/amorphous CNT (CuNW/a-CNT) hybrid nanostructure was obtained for the CuNW/CNT heterojunctions due to low catalytic activity of Cu. It is also interesting that a hollow gap was observed between the "cap" and the CuNW. By contrast with the case of Cu, multi-walled CNT (MWCNT) was achieved and no hollow gap was observed at the interface of CoNW/CNT heterojunctions. Three-segment CoNW/MWCNT/a-CNT hybrid nanostructure was observed for the CoNW/CNT heterojunctions because of high catalytic activity of Co. Because no stable copper carbides are observed, we infer that the growth mechanism of CuNW/CNT heterojunctions is different from that of CoNW/CNT. Possible growth models of CuNW/CNT and CoNW/CNT heterojunctions are proposed based on experimental results, respectively.     

Elementary processes in nanowire growth

We propose that many of the complex morpho-logical phenomena observed during nanowire growth arise from the interplay of just three elementary processes: facet growth, droplet statics, and the introduction of new facets. We incorporate these processes into an explicit model for the vapor-liquid-solid growth of fully faceted nanowires. In numerical simulations with this model, different conditions can lead to either growth of a free-standing wire or lateral growth where the catalyst droplet crawls along the surface. An external perturbation can cause the wire to kink into a different direction. Different growth conditions can also change the shape of the growth tip. All of these phenomena have been observed, and the model behavior is consistent with the experimental observations.     

Control of ZnO nanowire growth and optical properties in a vapor deposition process

A vapor deposition method was applied to synthesize zinc oxide(ZnO) nanowires and nanorods with diameter from 40 nm to 500 nm, length from 1 μm to 70 μm by adjusting the flow rate of argon, oxygen and the pressure during growth. Results of scanning electron microscopy(SEM) and high resolution transmission electron microscopy(TEM) proved the hexagonal wurtzite structure of the synthesized ZnO nanowires or nanorods, which grow along the 0001 direction. The results show that the growth conditions strongly impact the morphology, growth rate and optical properties of the ZnO nanostructures.The ZnO nanowires with small diameters tend to show stronger ultraviolet(UV) light emission from the electron-hole recombination near band edge in photoluminescence(PL), while those with larger diameters tend to exhibit PL spectra dominated by the broad green light emission due to the defects.     

Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes

We demonstrate the temperature-dependent growth of germanium oxide and silicon oxide based composite nanostructures (multiple nanojunctions of Ge nanowires and SiO(x) nanowires, Ge-filled SiO(2) nanotubes, Ge/SiO(2) coaxial nanocables, and a variety of interesting micrometer-sized structures), aligned SiO(x) nanowire assemblies, and SiO(x) microtubes. The structures were characterized by SEM, TEM, energy-dispersive X-ray spectroscopy, and electron diffraction. The combination of laser ablation of a germanium target and thermal evaoporation of silicon monoxide powders resulted in the formation of Ge and SiO(x) species in a carrier gas; the nano/micro-sized structures grow by either a Ge-catalyzed vapor-liquid-solid or a Ge-nanowire-templated vapor-solid process.     

New mode of vapor-liquid-solid nanowire growth

We report on the new mode of the vapor-liquid-solid nanowire growth with a droplet wetting the sidewalls and surrounding the nanowire rather than resting on its top. It is shown theoretically that such an unusual configuration happens when the growth is catalyzed by a lower surface energy metal. A model of a nonspherical elongated droplet shape in the wetting case is developed. Theoretical predictions are compared to the experimental data on the Ga-catalyzed growth of GaAs nanowires by molecular beam epitaxy. In particular, it is demonstrated that the experimentally observed droplet shape is indeed nonspherical. The new VLS mode has a major impact on the crystal structure of GaAs nanowires, helping to avoid the uncontrolled zinc blende-wurtzite polytylism under optimized growth conditions. Since the triple phase line nucleation is suppressed on surface energetic grounds, all nanowires acquire pure zinc blende phase along the entire length, as demonstrated by the structural studies of our GaAs nanowires.     

Study of iridium/germanium interaction in a lateral diffusion couple

Ion beam analysis using micro-Rutherford backscattering spectrometry has been used to investigate the interaction between germanium and iridium in a lateral diffusion couple. Optical microscopy, scanning electron microscopy and atomic force microscopy have also been employed. When samples of germanium islands on iridium films are annealed within a range of temperatures between 600 to 800 °C, substantial lateral diffusion is observed, resulting in a number of reaction regions. Micro-Rutherford backscattering analysis indicates that the phase Ir₃Ge₇ stretches across the original island interface at all temperatures, with the phase Ir₄Ge₅ forming in the reaction region with unreacted iridium. The phase IrGe₄ is observed to nucleate in the middle of the island at temperatures above 800 °C. Depth information is readily obtainable from micro-Rutherford backscattering spectrometry which is used in conjunction with atomic force microscopy data to estimate the densities of the phases formed. The results demonstrate the complementary nature of the techniques used for studying lateral diffusion couples.     

ZnO纳米线阵列的图形化生长

采用光刻和射频磁控溅射技术在Si衬底上制备了图形化的ZnO种子层薄膜。分别采用气相榆运和水热合成法，制备了最小单元为30μm的图形化的ZnO纳米线阵列。X射线衍射（XRD）分析显示单晶纳米线阵列具有高度的c轴[001]择优取向生长性质，从扫描电子显微镜（SEM）照片看出，阵列图形完整清晰，边缘整齐，纳米线阵列在室温下光致发光（PL）谱线中在380hm左右具有强烈的紫外发射峰，可见光区域发射峰得到了抑制，证明ZnO纳米线阵列氧空位缺陷少，晶体质量高。     

Effect of ZnO nanowire morphology on the interfacial strength of nanowire coated carbon fibers

The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength [1]. Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200 nm and lengths up to 4 μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64 MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.     

Multiplicity of steady modes of nanowire growth

For nanowire growth by the vapor-liquid-solid process, we examine whether there is a unique steady-state growth morphology. Applying a continuum model for faceted nanowire evolution to a model crystal structure, we enumerate the possible growth morphologies and calculate their dynamical stability. We find that even for a single set of experimental conditions there can be multiple distinct modes of steady-state growth. The actual growth mode occurring in experiment thus depends on the initial conditions and growth history. Relevant experiments are discussed.     

Stranski-Krastanow growth of germanium on silicon nanowires

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.     

Epitaxial growth of InP nanowires on germanium

The growth of III-V semiconductors on silicon would allow the integration of their superior (opto-)electronic properties with silicon technology. But fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains have prevented the epitaxial integration of III-V with group IV semiconductors. Here we demonstrate the principle of epitaxial growth of III-V nanowires on a group IV substrate. We have grown InP nanowires on germanium substrates by a vapour-liquid-solid method. Although the crystal lattice mismatch is large (3.7%), the as-grown wires are monocrystalline and virtually free of dislocations. X-ray diffraction unambiguously demonstrates the heteroepitaxial growth of the nanowires. In addition, we show that a low-resistance electrical contact can be obtained between the wires and the substrate.     

Realizing lateral wrap-gated nanowire FETs: controlling gate length with chemistry rather than lithography

An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the subthreshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 and 4 K, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.     

单晶铋纳米线阵列的可控生长

采用脉冲电化学沉积技术,利用同一直径的氧化铝模板,通过调节脉冲参数制备出了不同直径的单晶铋纳米线阵列,同时实现了纳米线取向的可控生长.保持脉冲弛豫时间不变,纳米线的直径随着脉冲沉积时间的增加而变大,纳米线的取向随着脉冲占空比的变化发生移动.     

Large-scale solution-phase growth of Cu-doped ZnO nanowire networks

Film-like networks of Cu-doped (0.8-2.5 at.%) ZnO nanowires were successfully synthesized through a facile solution process at a low temperature (<100 degrees C). The pH value of solution plays a key role in controlling the density and quality of the Cu-doped ZnO nanowires and the dopant concentration of ZnO nanowires was controlled by adjusting the Cu2+/Zn2+ concentration ratio during the synthesis. The structural study showed that the as-prepared Cu-doped ZnO nanowires with a narrow diameter range of 20-30 nm were single crystal and grew along [0001] direction. Photoluminescence and electrical conductivity measurements showed that Cu doping can lead to a redshift in bandgap energy and an increase in the resistivity of ZnO. The thermal annealing of the as-grown nanowires at a low temperature (300 degrees C) decreased the defect-related emission within the visible range and increased the electrical conductivity. The high-quality ZnO nanowire network with controlled doping will enable further application to flexible and transparent electronics.     

Ag-assisted CBE growth of ordered InSb nanowire arrays

We present growth studies of InSb nanowires grown directly on [Formula: see text] and [Formula: see text] substrates. The nanowires were synthesized in a chemical beam epitaxy (CBE) system and are of cubic zinc blende structure. To initiate nanowire nucleation we used lithographically positioned silver (Ag) seed particles. Up to 87% of the nanowires nucleate at the lithographically pre-defined positions. Transmission electron microscopy (TEM) investigations furthermore showed that, typically, a parasitic InSb thin film forms on the substrates. This thin film is more pronounced for InSb((111)B) substrates than for InAs((111)B) substrates, where it is completely absent at low growth temperatures. Thus, using InAs((111)B) substrates and growth temperatures below 360?°C free-standing InSb nanowires can be synthesized.