Growth of axial SiGe heterostructures in nanowires using pulsed laser deposition

Axial heterojunctions between pure silicon and pure germanium in nanowires have been realized combining pulsed laser deposition, chemical vapor deposition and electron beam evaporation in a vapor-liquid-solid nanowire growth experiment using gold nanoparticles as catalyst for the 1D wire growth. Energy dispersive x-ray mappings and line scans show a compositional transition from pure silicon to pure germanium and vice versa with exponential and thus comparably sharp transition slopes. Based on these results not only Si-Ge heterojunctions seem to be possible using the vapor-liquid-solid growth process but also heterojunctions in optoelectronic III-V compounds such as InGaAs/GaAs or group III nitride compounds such as InGaN/GaN as well as axial p-n junctions in Si nanowires.     

Direct measurement of individual deep traps in single silicon nanowires

The potential of the metal nanocatalyst to contaminate vapor-liquid-solid (VLS) grown semiconductor nanowires has been a long-standing concern, since the most common catalyst material, Au, is known to induce deep gap states in several semiconductors. Here we use Kelvin probe force microscopy to image individual deep acceptor type trapping centers in single undoped Si nanowires grown with an Au catalyst. The switching between occupied and empty trap states is reversibly controlled by the back-gate potential in a nanowire transistor. The trap energy level, i.e., E(C) - E(T) = 0.65 ± 0.1 eV was extracted and the concentration was estimated to be ～2 × 10(16) cm(-3). The energy and concentration are consistent with traps resulting from the unintentional incorporation of Au atoms during the VLS growth.     

In‐situ Observations of Nanoscale Effects in Germanium Nanowire Growth with Ternary Eutectic Alloys

Vapour‐liquid‐solid (VLS) techniques are popular routes for the scalable synthesis of semiconductor nanowires. In this article, in‐situ electron microscopy is used to correlate the equilibrium content of ternary (Au_0.75Ag_0.25–Ge and Au_0.65Ag_0.35–Ge) metastable alloys with the kinetics, thermodynamics and diameter of Ge nanowires grown via a VLS mechanism. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires varies as a function of nanowire diameter and eutectic alloy composition. The behaviour of the faceted heterogeneous liquid–solid interface correlates with the growth kinetics of the nanowires, where the main growth facet at the solid nanowire–liquid catalyst drop contact line lengthens for faster nanowire growth kinetics. Pronounced diameter dependent growth kinetics, as inferred from liquid–solid interfacial behaviour, is apparent for the synthesised nanowires. Direct in‐situ microscopy observations facilitates the comparison between the nanowire growth behaviour from ternary (Au–Ag–Ge) and binary (Au–Ge) eutectic systems.     

Ultra-sharp pointed tip Si nanowires produced by very high frequency plasma enhanced chemical vapor deposition via VLS mechanism

Needle-like silicon nanowires have been grown using gold colloid as the catalyst and silane (SiH₄) as the precursor by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Si nanowires produced by this method were unique with sharpness below 3 nm. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction technique (XRD) confirmed the single crystalline growth of the Si nanowires with (111) crystalline structure. Raman spectroscopy also has revealed the presence of crystalline Si in the grown Si nanowire body. In this research, presence of a gold nanoparticle on tip of the nanowires proved vapor–liquid–solid growth mechanism.     

Silicon nanowire growth on glass substrates using hot wire chemical vapor deposition

We report here, the first observation of silicon nanowire growth via the VLS route at 400 °C using the HWCVD technique with gold (Au) as catalyst. The supersaturation of the alloy droplet, due to a large flux of atomic silicon generated due to efficient dissociation of the silane over the hot wire, leads to the precipitation of Si nanowires. The hot wire process plays a dual role in the entire nanowire growth. Firstly, the atomic hydrogen generated from the hot wire leads to the formation of the metal nanoclusters. Secondly, it offers a continuous supply of silicon atoms enabling efficient diffusion of Si into the Si-Au eutectic alloy leading to the growth of dense silicon nanowires as observed in the SEM. The Raman and TEM data show that the Si nanowires are amorphous in nature. Precise tuning of the hot wire CVD process parameters gives rise to a high density of silicon nanowires having diameters as small as 50 nm and lengths of about a few microns.     

Ultralong zinc-blende ZnS nanowires grown on polar C face of 6H-SiC substrates at low temperatures by metalorganic chemical vapor deposition

Ultralong ZnS nanowires with high purity were grown on Au-coated polar C face of 6H-SiC substrates via metalorganic chemical vapor deposition at low temperatures. The ZnS nanowires have zinc-blende structure and the length is up to tens of micrometers. HRTEM investigations show that the nanowires are well crystalline single crystal grown along [1 1 1] and free of bulk defects. However, sparse straight and curved nanowires with poor crystalline nature are randomly grown on the Au-coated Si face of 6H-SiC substrates. We deduce that the growth of ZnS is related to the substrates and C face can enhance Au-catalytic VLS growth. The CL spectra of an individual nanowire grown on C and Si face reveal different optical properties. Intrinsic sulfur and zinc vacancies are the main reasons for the 458.1 nm and 459.2 nm blue emission detected in the nanowire grown on C face and Si face, respectively. Nevertheless, an unusual green emission at 565.1 nm is observed in the poor crystalline nanowire grown on Si face, which originates from the bulk defects.     

Manufacturing aspects of oxide nanowires

Exploring the mass manufacturing aspects of nanostructures can enable the transition from laboratory-based research into a commercial product. Among the several one-dimensional nanostructures, oxide nanomaterials have a wide variety of applications including energy harvesting, photonics and biosensing applications. In this article, mass manufacturing aspects of bottom-up grown silica nanowires on silicon (Si) by metal thin film catalysis have been detailed. The investigation reports on (a) a growth model derived from studying nanowire nucleation as a function of heating time, (b) nanowire growth rate estimation via weight differential of the Si substrate before and after growth, and (c) reusability of the Si substrate for nanowire growth.Silica nanowires were found to grow on Pd coated Si substrate in an open tube furnace at 1100 °C with Ar as a carrier gas and a Si support wafer. Nanowires nucleated following a combination of Vapor Liquid Solid (VLS) and Oxide Assisted Growth (OAG) mechanisms conducive for mass manufacturing. The role of SiO vapor was found to be critical in the growth of the wires. Further, five distinct growth regimes were identified while estimating the growth rate. Experimental observations indicated the non-reusability of the Si substrate after one time growth due to depletion of catalyst.     

Epitaxial growth of silicon nanowires using an aluminium catalyst

Silicon nanowires have been identified as important components for future electronic and sensor nanodevices. So far gold has dominated as the catalyst for growing Si nanowires via the vapour-liquid-solid (VLS) mechanism. Unfortunately, gold traps electrons and holes in Si and poses a serious contamination problem for Si complementary metal oxide semiconductor (CMOS) processing. Although there are some reports on the use of non-gold catalysts for Si nanowire growth, either the growth requires high temperatures and/or the catalysts are not compatible with CMOS requirements. From a technological standpoint, a much more attractive catalyst material would be aluminium, as it is a standard metal in Si process lines. Here we report for the first time the epitaxial growth of Al-catalysed Si nanowires and suggest that growth proceeds via a vapour-solid-solid (VSS) rather than a VLS mechanism. It is also found that the tapering of the nanowires can be strongly reduced by lowering the growth temperature.     

Morphology control of layer-structured gallium selenide nanowires

Layer-structured group III chalcogenides have highly anisotropic properties and are attractive materials for stable photocathodes and battery electrodes. We report the controlled synthesis and characterization of layer-structured GaSe nanowires via a catalyst-assisted vapor-liquid-solid (VLS) growth mechanism during GaSe powder evaporation. GaSe nanowires consist of Se-Ga-Ga-Se layers stacked together via van der Waals interactions to form belt-shaped nanowires with a growth direction along the [11-20], width along the [1-100], and height along the [0001] direction. Nanobelts exhibit a variety of morphologies including straight, zigzag, and saw-tooth shapes. These morphologies are realized by controlling the growth temperature and time so that the actual catalysts have a chemical composition of Au, Au-Ga alloy, or Ga. The participation of Ga in the VLS catalyst is important for achieving different morphologies of GaSe. In addition, GaSe nanotubes are also prepared by a slow growth process.     

The effect of substrate type on SiC nanowire orientation

beta-SiC nanowires were synthesized on different monocrystalline substrates: Si (001), Si (111), 3C-SiC (001), 4H-SiC (0001), 6H-SiC (0001). The SiC nanowire growth was carried out using a Chemical Vapor Deposition method, with silane and propane diluted in hydrogen (3%) as precursors. The deposition was performed at atmospheric pressure and at 1100 degrees C, after dewetting of the Ni catalyst, which had been previously evaporated onto the substrate, to induce 1D growth according to a VLS process. The crystal structure of the nanowires, as determined by X-ray diffraction and High Resolution Transmission Electron Microscopy, corresponds to 3C-SiC polytype growing along a (111) direction, irrespective of the substrate. The occurrence of (111) stacking faults was observed, partly reduced for samples grown on 3C-SiC substrate. The growth on (111) substrate allowed to achieve a good vertical alignment of the nanowires, as investigated by Scanning Electron Microscopy. High Angle Annular Dark Field imaging and Energy Dispersive X-Ray spectroscopy were performed to study the catalyst particle on top of the wires and showed the formation of a nickel-silicon alloy.     

Growth of silicon nanowires by electron beam evaporation using indium catalyst

For the first time silicon nanowires have been grown on indium (In) coated Si (100) substrates using e-beam evaporation at a low substrate temperature of 300 °C. Standard spectroscopic and microscopic techniques have been employed for the structural, morphological and compositional properties of as grown Si nanowires. The as grown Si nanowires have randomly oriented with an average length of 600 nm for a deposition time of 15 min. As grown Si nanowires have shown indium nanoparticle (capped) on top of it confirming the Vapor Liquid Solid (VLS) growth mechanism. Transmission Electron Microscope (TEM) measurements have revealed pure and single crystalline nature of Si nanowires. The obtained results have indicated good progress towards finding alternative catalyst to gold for the synthesis of Si nanowires.     

Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process

We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.     

Ultrafast growth of single-crystalline Si nanowires

Silicon nanowires (SiNWs) have been catalytically synthesized by heat treatment of Si nanopowder at 980 °C. The SiNWs comprise crystalline Si nanoparticles interconnected with metal catalyst. The formation mechanism of nanowires generally depends on the presence of Fe catalysts in the synthesis process of solid–liquid–solid (SLS). Although gas phase of vapor–liquid–solid (VLS) method can be used to produce various of different nanowire materials, growth model based on the SLS mechanism by heat treatment is more ascendant for providing ultrafast growth of single-crystalline Si nanowires and controlling the diameter of them easily. The growth of single-crystalline SiNWs and morphology were discussed.     

Selective growth of silica nanowires in silicon catalysed by Pt thin film

Selective growth of amorphous silica nanowires on a silicon wafer deposited with Pt thin film is reported. The mechanism of nanowire growth has been established to follow the vapour liquid solid (VLS) model via the PtSi phase acting as the catalyst. Nanowires grow with diameters ranging from 50 to 500?nm. These bottom-up grown nanowires exhibit photoluminescence with a stable emission of blue light at 430?nm under excitation. The effect of varying the seed layer thickness (Pt film) from 2 to 100?nm has been studied. It is observed that, above 10?nm thickness, a continuous layer of Pt(2)Si re-solidifies on the surface, inhibiting the growth of nanowires. The selectivity to the Pt thickness has been exploited to create regions of nanowires connected to conducting silicide (Pt(2)Si) simultaneously in a single furnace treatment. This novel approach has opened the gateways for realizing hybrid interconnects in silicon for various nano-optical applications such as the localization of light, low-dimensional waveguides for functional microphotonics, scanning near-field microscopy, and nanoantennae.     

The influences of technological conditions and Au cluster islands on morphology of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires grown by VLS method on GaAs substrate

The synthesis of semiconductor nanowires is more and more interested to the applications for building blocks of the innovative nano-sized devices and circuits, but the research and fabrication of these nanowires are also holding a number of difficulties and challenges. Among many different kinds of semiconductor nanowires, Ga₂O₃ is increasingly grown for many promising applications in nano-device production, namely nanowire LED and Laser. So far there are many synthesizing methods of semiconductor nanowires, among them the vapor–liquid–solid (VLS) method is simple, cheap and popular. However, when we use the VLS method for nanowire growth, various technological problems exist. This paper aims at investigating some influences of the growth technological conditions and Au metal catalyst on the morphology of Ga₂O₃ nanowire grown by VLS on GaAs substrate. The main considering factors include the different growing temperatures and times, the effects of Au diffusion, Au droplets formation, Au cluster islands formation, and gas volume of the growing tube/ampoule at the 10⁻¹ torr low air pressure. The obtained experimental results regarding the structural properties of nanowires under these effects investigated by scanning electron microscopy, field emission scanning electron microscopy, high angle annular dark field and bright field, scanning transmission electron microscopy, energy-dispersive X-ray techniques, and focus ion beam are presented and discussed.     

Tunable light emission from quantum-confined excitons in TiSi2-catalyzed silicon nanowires

Visible and near-infrared photoluminescence (PL) at room temperature is reported from Si nanowires (NWs) grown by chemical vapor deposition from TiSi2 catalyst sites. NWs grown with average diameter of 20 nm were etched and oxidized to thin and passivate the wires. The PL emission blue shifted continuously with decreasing nanowire diameter. Slowed oxidation was observed for small nanowire diameters and provides a high degree of control over the emission wavelength. Transmission electron microscopy, PL, and time-resolved PL data are fully consistent with quantum confinement of charge carriers in the Si nanowire core being the source of luminescence. These light emitting nanowires could find application in future CMOS-compatible photonic devices.     

Hyperbranched PbS and PbSe nanowires and the effect of hydrogen gas on their synthesis

We report a chemical vapor deposition (CVD) synthesis of hyperbranched single-crystal nanowires of both PbS and PbSe using PbCl2 and S/Se as precursors under hydrogen flow. Multiple generations of nanowires grow perpendicularly from the previous generation of nanowires in an epitaxial fashion to produce dense clusters of a complex nanowire network structure. The flow rate and duration of the hydrogen co-flow in the argon carrier gas during the CVD reactions are found to have a significant effect on the morphology of the PbS/PbSe grown, from hyperbranched nanowires to micrometer-sized cubes. No intentional catalyst was employed for the nanowire synthesis, but it is suggested that elemental lead that has been reduced from the vapor by the hydrogen might serve as a vapor-liquid-solid (VLS) catalyst for the anisotropic growth of PbS/PbSe. The nanowires were also investigated with Raman spectroscopy. These PbS and PbSe nanostructures can have applications in photovoltaics because multiple exciton generation has been demonstrated in nanocrystals of both materials.     

Formation of Single Tiers of Bridging Silicon Nanowires for Transistor Applications Using Vapor–Liquid–Solid Growth from Short Silicon‐on‐Insulator Sidewalls

Single tiers of silicon nanowires that bridge the gap between the short sidewalls of silicon‐on‐insulator (SOI) source/drain pads are formed. The formation of a single tier of bridging nanowires is enabled by the attachment of a single tier of Au catalyst nanoparticles to short SOI sidewalls and the subsequent growth of epitaxial nanowires via the vapor–liquid–solid (VLS) process. The growth of unobstructed nanowire material occurs due to the attachment of catalyst nanoparticles on silicon surfaces and the removal of catalyst nanoparticles from the SOI‐buried oxide (BOX). Three‐terminal current–voltage measurements of the structure using the substrate as a planar backgate after VLS nanowire growth reveal transistor behaviour characteristics.     

Diameter dependent growth rate and interfacial abruptness in vapor-liquid-solid Si/Si1-xGex heterostructure nanowires

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.     

Growth of Ge-Si nanowire heterostructures via chemical vapor deposition

The growth of Ge-Si and Ge-Si nanowire (NW) heterostructures was demonstrated via chemical vapor deposition. Due to the influence of interface energy, differing topographies of the heterostructures were observed. On initially grown Ge NWs, numerous Si NW branches were grown near the tip due to Au migration. However, on initially grown Si NWs, high-density Ge nanodots were observed.