Growth Of Semiconductor Nanowires Research Articles

Advances in semiconductor nanowire growth on graphene

Abstractmagnified imageHere we review the recent research activities on the epitaxial growth of semiconductor nanowires (NWs) on graphene substrates. Semiconductor NWs with quasi one‐dimensional structure have become an active research field due to their various interesting physical properties and potentials for future electronic and optoelectronic device applications, such as transistors, sensors, solar cells, light emitting diodes, and lasers. At almost the same time graphene, a two‐dimensional material made of carbon, was discovered and has gained an ever increasing interest during the last few years owing to its remarkable properties, including excellent electrical conductivity, optical transparency, and mechanical strength and flexibility. A hybrid structure by epitaxially growing semiconductor NWs on graphene could provide a new avenue for the development of future advanced NW‐based flexible electronic and optoelectronic devices. We address the challenges for the growth of semiconductor NWs on graphene, with a special focus on the III–V semiconductors, and highlight some potential applications of the NW/graphene hybrid system. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of twins on the electronic properties of GaAs

Generation of twin defects during the growth of semiconductor nanowires is a major concern, but their effects on electronic properties are not well understood. Here, combined quantum-mechanical and molecular-dynamics simulations reveal that the radiative decay time of an exciton increases due to twin. Furthermore, the twin-scattering contribution to electron mobility is found to be significant, in conformity with photoconductivity measurements. In addition to acting as a carrier-scattering source, twins in nanowires are found to modify the mobility by changing strain and thereby the effective mass. These effects should have profound impacts on the efficiency of nanowire-based devices.

Surface pinning of catalyst nanoparticles for enhanced size and position control of 1D nanostructures growth

Catalyst nanoparticles are required in most established synthetic routes for the growth of carbon nanotubes (CNTs) and semiconductor nanowires (NWs). A simple and widespread method to coat a substrate with metal nanoparticles is based on the dewetting of a thin metal film, often induced by a process of thermal annealing, and resulting in the formation of fine droplets. However, the control of the sole thermodynamic parameters affecting the dewetting results in a rather broad size distribution and random positioning of the catalyst. Efforts aiming at a better control of size and position of the catalytic seeds by top-down and bottom-up patterning techniques are well documented. However, patterning the catalyst may be a useless process step if, once approaching the temperatures at which the growth of nanowires and CNTs is catalyzed, nanoparticles become mobile on the substrates, displacing from their position and altering the initial size distribution due to coalescence. Moving from this observation, we propose a simple and general method to prevent the diffusion of the catalyst during annealing steps or the initial stage of the synthesis of NWs and CNTs. Its principle consists of confining the catalyst nanoparticles on top of vertical columnar structures. This can be achieved by etching the substrate all around the nanoparticles by dry anisotropic etching using the nanoparticles themselves as etch masks. Nanoparticles thus sitting on top of nanopillars result effectively confined, even at temperature at which they are found highly mobile on flat substrates of the same material, due to surface tension effects. Experimentally, we demonstrate this principle with iron nanodots obtained on silicon by lift-off starting from a P(MMA-b-S) diblock-copolymer template. The iron nanodots isolated on top of columnar nanostructures, were found to be stable at 600^oC, temperature suitable for the growth of CNTs.

A Time-Resolved Numerical Study of the Vapor–Liquid–Solid Growth Kinetics Describing the Initial Nucleation Phase as well as Pulsed Deposition Processes

Today, the vapor-liquid-solid (VLS) growth mechanism is a common process for the metal catalyzed bottom-up growth of semiconductor nanowires (NWs). Nevertheless, most of the literature only is concerned with the steady-state NW growth which applies when the amount of material supplied is equal to the amount consumed by the NW growth at the same time. While this description is suitable for chemical vapor deposition (CVD) or electron beam evaporation (EBE) processes after the initial nucleation time, problems arise when pulsed growth processes like pulsed laser deposition (PLD) are used since in this case the steady state growth condition cannot be applied. Moreover, the initial phase of NW growth cannot be described with steady state growth conditions, either. In this work, we present a modeling approach for VLS NW growth based on numerical simulations, which is capable of describing the nucleation phase of the VLS growth process as well as a pulsed deposition process.

Lateral growth and shape of semiconductor nanowires

The lateral growth of semiconductor nanowires and its influence on the nanowire shape in the case of nanocrystal formation according to the diffusion mechanism is theoretically studied. Possible types of the dependence of the adatom concentration at the lateral faces of the nanowires on the vertical coordinate are found. A self-consistent model is developed within the linear approximation in the adatom concentration, which makes it possible to describe simultaneously both vertical and lateral nanowire growth. The possible shapes of the nanowires, depending on the growth conditions, are described within this model and compared with the experimental data on different III–V systems.

Controlled synthesis of ultrathin ZnO nanowires using micellar gold nanoparticles as catalyst templates

We demonstrate a simple and effective approach to control the diameter of ultrathin ZnO nanowires with high aspect ratios and high densities over large areas. Diblock copolymer-based nanoparticle arrays exhibiting a high degree of hexagonal order and offering easy control of particle size (typically 1-10 nm) and interparticle spacing (25-150 nm) are utilized as nanocatalysts for the subsequent growth of semiconductor nanowires. The as-grown ZnO nanowires exhibit a single crystal hexagonal wurtzite structure and grow along the [0002] direction. Facetted catalyst particles were observed at the tip of the nanowires after synthesis, thus suggesting a catalyst-assisted vapor-solid-solid (VSS) rather than a vapor-liquid-solid (VLS) growth mechanism, the latter being frequently used in semiconductor nanowire production. Such a growth process allows us to easily prepare ultrathin ZnO nanowires with tunable diameters well below 10 nm by taking advantage of the inherent size control of the micellar method during deposition of the catalyst nanoparticles. Raman spectroscopy reveals a phonon confinement effect as the diameter of nanowires decreases. Photoluminescence spectra of these ultrathin nanowires indicate a blue shift of the free excitons and their phonon replicas by 37 meV induced by quantum confinement.

Identification of an Intrinsic Source of Doping Inhomogeneity in Vapor–Liquid–Solid-Grown Nanowires

The vapor-liquid-solid (VLS) process of semiconductor nanowire growth is an attractive approach to low-dimensional materials and heterostructures because it provides a mechanism to modulate, in situ, nanowire composition and doping, but the ultimate limits on doping control are ultimately dictated by the growth process itself. Under widely used conditions for the chemical vapor deposition growth of Si and Ge nanowires from a Au catalyst droplet, we find that dopants incorporated from the liquid are not uniformly distributed. Specifically, atom probe tomographic analysis revealed up to 100-fold enhancements in dopant concentration near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. We hypothesize that radial and azimuthal inhomogeneities arise from a faceted liquid-solid interface present during nanowire growth, and we present a simple model to account for the distribution. As the same segregation behavior was observed in two distinct semiconductors with different dopants, the observed inhomogeneity is likely to be present in other VLS grown nanowires.

Vapour-phase growth of III-V semiconductor nanowires in situ in the TEM

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Using seed particle composition to control structural and optical properties of GaN nanowires

The morphology, structure, and optical properties of gallium nitride (GaN) nanowires grown using metal–organic chemical vapor deposition (MOCVD) on r-plane sapphire using gold and nickel seed particles were investigated. We found that different seed particles result in different growth rates and densities of structural defects in MOCVD-grown GaN nanowires. Ni-seeded GaN nanowires grow faster than Au-seeded ones, and they do not contain the basal plane stacking faults that are observed in Au-seeded GaN nanowires. We propose that stacking fault formation is related to the supersaturation and surface energies in different types of seed particles. Room temperature photoluminescence studies revealed a blue-shifted peak in Au-seeded GaN nanowires compared to the GaN near-bandgap emission. The blue-shifted peak evolves as a function of the growth time and originates from the nanowire base, likely due to strain and Al diffusion from the substrate. Our results demonstrate that seed particle composition has a direct impact on the growth, structure, and optical properties of GaN nanowires and reveal some general requirements for seed particle selection for the growth of compound semiconductor nanowires.

Controlling Silicon Nanowire Growth Direction via Surface Chemistry

We report on the first in situ chemical investigation of vapor-liquid-solid semiconductor nanowire growth and reveal the important, and previously unrecognized, role of transient surface chemistry near the triple-phase line. Real-time infrared spectroscopy measurements coupled with postgrowth electron microscopy demonstrate that covalently bonded hydrogen atoms are responsible for the (left angle bracket 111 right angle bracket) to (left angle bracket 112 right angle bracket) growth orientation transition commonly observed during Si nanowire growth. Our findings provide insight into the root cause of this well-known nanowire growth phenomenon and open a new route to rationally engineer the crystal structure of these nanoscale semi-conductors.

Growth of semiconductor nanowires at large diffusion lengths

A universal asymptotic expression for the law of nanowire (NW) growth in cases where the diffusion lengths for adatoms on the substrate surface are much greater than the NW radius and the diffusion lengths for adatoms on the side surface of the growing crystal are much greater than the NW length. The main stages of growth, which are characterized by different relations between the NW length and its radius and the growth time, are determined. Possible asymptotic regimes of NW epitaxy are considered, including the cases of exponential growth and limited growth to a certain critical thickness, which depend on the direction of the diffusion flux of adatoms on the side surface of the growing crystal.

Patterned gold-assisted growth of GaP nanowires on Si

The patterning of metal seed particles for the vapor–liquid–solid (VLS) growth of semiconductor nanowires is examined. We report the challenges of obtaining GaP nanowires grown by molecular beam epitaxy (MBE) from a patterned array of Au seed particles created by electron beam lithography. Transmission electron microscopy studies revealed the presence of a thin silicon oxide layer over the patterned Au seed particles. Au acted as a catalyst for silicon oxide growth at even moderate temperatures. In particular, the annealing of Au-patterned Si (1 1 1) substrates at low temperatures required for indium mounting (200–300 °C) was responsible for the formation of an oxide layer that was detrimental to Au-assisted growth. Removal of the oxide layer by a buffered HF etch prior to MBE sample loading enabled patterned VLS growth.

VLS Growth of III-V Semiconductor Nanowires on Graphene Layers

ABSTRACTGaP, GaAs, and InP nanowires were grown on graphitic layers by the vapor-liquid-solid method in a metalorganic vapor phase epitaxy chamber. On graphene/SiC(0001), Au particles as catalyst were formed at the steps by controlling the Au deposition rate and the annealing temperature in a low-energy electron microscopy system. GaP nanowires were grown on this substrate, and it was found that vertical nanowires were formed at the steps of the surface. We also performed GaP, GaAs, and InP nanowire growth on graphite substrates. Free-standing nanowires were obtained for all three materials, although they were vertically, diagonally, and laterally-oriented at the same time. The results suggested that the growth at the steps is the key to growing nanowires vertically on graphene surface.

Synthesis of hybrid polymethacrylate–noble metal (M = Au, Pd) nanoparticles for the growth of metal-oxide semiconductor nanowires

Metal-oxide semiconductor nanowires (NWs) such as ZnO, β-Ga2O3 and SnO2 with diameters of tens of nanometres and lengths of many micrometres have been grown using micellar nanohybrids consisting of methacrylate-based diblock copolymers and noble metal nanoparticles (MNPs). Micellar Au and Pd MNPs with diameters as small as 2.3 ± 0.3 nm were deposited on Si(001) by spin coating or drop casting and metal-oxide NWs were grown by reactive vapor transport. A high yield of β-Ga2O3 NWs with diameters of approximately 40 nm, lengths > 10 μm and a monoclinic crystal structure were obtained at 900 °C with the largest MNPs. These exhibited a broad, symmetric photoluminescence (PL) spectrum centred at 2.3 eV attributed to defect states situated energetically in the energy band gap of β-Ga2O3. We find that a reduction in the size of the MNPs below 10 nm leads to the formation of necklace like β-Ga2O3 NWs via the encapsulation of the MNPs which act as catalytic centres for the formation of branched nanostructures along the length of the β-Ga2O3 NWs that are also responsible for a blue shift in the PL at 2.8 eV as a result of quantum confinement. This was not observed upon reducing the density of MNPs or in the case of ZnO or SnO2 NWs grown with the smallest of MNPs probably due to differences in surface energy. We show that polymethacrylate-noble MNPs may be patterned directly by electron beam lithography and may be exploited for the selective location growth of semiconductor NWs while we also discuss the difference between the sizes of the hybrid polymer-MNPs and MO NWs which is attributed to agglomeration.

Room-Temperature Terahertz Detectors Based on Semiconductor Nanowire Field-Effect Transistors

The growth of semiconductor nanowires (NWs) has recently opened new paths to silicon integration of device families such as light-emitting diodes, high-efficiency photovoltaics, or high-responsivity photodetectors. It is also offering a wealth of new approaches for the development of a future generation of nanoelectronic devices. Here we demonstrate that semiconductor nanowires can also be used as building blocks for the realization of high-sensitivity terahertz detectors based on a 1D field-effect transistor configuration. In order to take advantage of the low effective mass and high mobilities achievable in III-V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly doped with selenium to control the charge density and to optimize source-drain and contact resistance. The detection mechanism exploits the nonlinearity of the transfer characteristics: the terahertz radiation field is fed at the gate-source electrodes with wide band antennas, and the rectified signal is then read at the output in the form of a DC drain voltage. Significant responsivity values (>1 V/W) at 0.3 THz have been obtained with noise equivalent powers (NEP) < 2 × 10(-9) W/(Hz)(1/2) at room temperature. The large existing margins for technology improvements, the scalability to higher frequencies, and the possibility of realizing multipixel arrays, make these devices highly competitive as a future solution for terahertz detection.

Surface Faceting Dependence of Thermal Transport in Silicon Nanowires

Surface faceting on sidewalls is ubiquitously observed during crystal growth of semiconductor nanowires. However, predicting the thermal transport characteristics of faceted nanowires relevant to thermoelectric applications remains challenging. Here, direct molecular dynamics simulations show that thermal conductivity is considerably reduced in crystalline <111> Si nanowires with periodic sawtooth faceting compared to nanowires of same size with smooth sidewalls. It is discovered that surface phonon scattering is particularly high with {100} facets, but less pronounced with {113} facets and remarkably low with {111} facets, which suggests a new means to optimize phonon dynamics for nanoscale thermoelectric devices. This anomaly is reconciled by showing that the contribution of each facet to surface phonons is due to diffuse scattering rather than to backward scattering. It is further shown that this property is not changed by addition of an amorphous shell to the crystalline core, similar to the structure of experimental nanowires.

Step flow and interface structure during the growth of semiconductor nanowires

Step flow and interface structure during the growth of semiconductor nanowires

Structure-Dependent Ferromagnetism in Mn-Doped III–V Nanowires

The electronic and magnetic properties of (Ga,Mn)As and (In,Mn)As nanowires are studied by ab initio methods. The results suggest that, in contrast to the bulk, in nanowires (In,Mn)As may exhibit better ferromagnetic behavior than (Ga,Mn)As. Moreover, the calculations show that in one-dimensional diluted magnetic semiconductors the distribution of Mn ions and the magnetic order depend crucially on the crystallographic structure. Since the growth of III-V nanowires of a given, either zinc blende or wurtzite, crystal structure is nowadays well controlled, these results can help to find the preferable material and conditions for the growth of ferromagnetic semiconductor nanowires.

Growth and Characterisation of ZnSe Semiconductor Nanowires

The growth and characteristics of wide gap II-VI semiconductor nanowires prepared by the so-called Vapour-Liquid-Solid (VLS) technique was presented ZnSe nanowires were prepared on Si (111) by using Au as catalyst. Vapor-Liquid-Solid (VLS) process under certain conditions to form the desired nanowires. The as-synthesized products were characterized by SEM and EDX. The SEM analysis of ZnSe nanowires indicated that nanowires grow randomly at angles widely different from the vertical.

Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires

The growth of semiconductor (SC) nanowires (NW) by CVD using Au-catalyzed VLS process has been widely studied over the past few years. Among others SC, it is possible to grow pure Si or SiGe NW thanks to these techniques. Nevertheless, Au could deteriorate the electric properties of SC and the use of other metal catalysts will be mandatory if NW are to be designed for innovating electronic. First, this article's focus will be on SiGe NW's growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGex NW has been successfully varied by modifying the gas flow ratio: R = GeH4/(SiH4 + GeH4). Characterization (by Raman spectroscopy and XRD) revealed concentrations varying from 0.2 to 0.46 on NW grown at 375°C, with R varying from 0.05 to 0.15. Second, the results of Si NW growths by CVD using alternatives catalysts such as platinum-, palladium- and nickel-silicides are presented. This study, carried out on a LPCVD furnace, aimed at defining Si NW growth conditions when using such catalysts. Since the growth temperatures investigated are lower than the eutectic temperatures of these Si-metal alloys, VSS growth is expected and observed. Different temperatures and HCl flow rates have been tested with the aim of minimizing 2D growth which induces an important tapering of the NW. Finally, mechanical characterization of single NW has been carried out using an AFM method developed at the LTM. It consists in measuring the deflection of an AFM tip while performing approach-retract curves at various positions along the length of a cantilevered NW. This approach allows the measurement of as-grown single NW's Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement.