Growth Conditions Of Nanowires Research Articles

Evolution of Cu-In Catalyst Nanoparticles under Hydrogen Plasma Treatment and Silicon Nanowire Growth Conditions.

We report silicon nanowire (SiNW) growth with a novel Cu-In bimetallic catalyst using a plasma-enhanced chemical vapor deposition (PECVD) method. We study the structure of the catalyst nanoparticles (NPs) throughout a two-step process that includes a hydrogen plasma pre-treatment at 200 °C and the SiNW growth itself in a hydrogen-silane plasma at 420 °C. We show that the H2-plasma induces a coalescence of the Cu-rich cores of as-deposited thermally evaporated NPs that does not occur when the same annealing is applied without plasma. The SiNW growth process at 420 °C induces a phase transformation of the catalyst cores to Cu7In3; while a hydrogen plasma treatment at 420 °C without silane can lead to the formation of the Cu11In9 phase. In situ transmission electron microscopy experiments show that the SiNWs synthesis with Cu-In bimetallic catalyst NPs follows an essentially vapor-solid-solid process. By adjusting the catalyst composition, we manage to obtain small-diameter SiNWs-below 10 nm-among which we observe the metastable hexagonal diamond phase of Si, which is predicted to have a direct bandgap.

Effect of Si(111) Surface Modification by Ga Focused Ion Beam at 30 kV on GaAs Nanowire Growth

This paper presents the results of experimental studies of the effect of Si(111) surface modification by Ga-focused ion beam (FIB) at 30 kV accelerating voltage on the features of the epitaxial GaAs nanowire (NW) growth processes. We experimentally established the regularities of the Ga ions' dose effect during surface modification on the structural characteristics of GaAs NW arrays. Depending on the Ga ion dose value, there is one of three modes on the surface for subsequent GaAs NW growth. At low doses, the NW growth is almost completely suppressed. The growth mode of high-density (up to 6.56 µm-2) GaAs NW arrays with a maximum fraction (up to 70%) of nanowires normally oriented to the substrate is realized in the medium ion doses range. A continuous polycrystalline base with a dense array of misoriented short (up to 0.9 µm) and thin (up to 27 nm) GaAs NWs is formed at high doses. We assume that the key role is played by the interaction of the implanted Ga ions with the surface at various process stages and its influence on the surface structure in the modification region and on GaAs NW growth conditions.

Bi incorporation and segregation in the MBE-grown GaAs-(Ga,Al)As-Ga(As,Bi) core\u2013shell nanowires

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments.

Self‐Consistent Modeling of Nucleation and Growth of 2D Islands on the Top Facet of Self‐Catalyzed GaAs Nanowires

Self‐catalyzed GaAs nanowire (NW) growth via the vapor–liquid–solid mechanism is investigated by a theoretical model including the kinetics of material transport inside the catalyst droplet. The proposed model allows the description of nucleation and growth of 2D islands on the top facet of GaAs NWs. Analytical expressions for the growth rate of the disk‐shaped GaAs island due to the volume diffusion of species in the droplet and for the attachment rate of GaAs pairs to the critical island are derived. As a result, the duration of the droplet refilling stage and the island growth stage at typical growth conditions of self‐catalyzed GaAs NWs are obtained by a self‐consistent calculation. Also, the time evolution of the droplet composition and the island radius are found. The derived equations for the island growth rate can be applied for modeling of catalyst‐assisted growth of other III–V compounds. The results of the modeling are in good agreement with the experimental data on self‐catalyzed GaAs NW growth via molecular beam epitaxy and can be used for the optimization of the NW growth conditions.

Comparison of Triethylgallium and Trimethylgallium Precursors for GaInP Nanowire Growth

Nanowire (NW) arrays containing a top segment of GaxIn1–xP are investigated, comparing NWs grown using two different Ga precursors, trimethylgallium (TMGa) and triethylgallium (TEGa). TMGa is the precursor commonly used for the particle‐assisted vapor–liquid–solid (VLS) growth of GaxIn1–xP NWs. However, it shows inefficient pyrolysis at typical NW growth conditions. The use of the alternative precursor TEGa is investigated by making a direct comparison between NWs grown using TEGa and TMGa at otherwise identical growth conditions. Growth rates, resulting NW materials composition, and time‐resolved photoluminescence (TRPL) lifetimes are investigated. With increasing Ga content of the NWs, the TRPL lifetimes decrease, indicating trap states that are associated with GaP. Somewhat longer TRPL lifetimes for the samples grown using TEGa indicate a lower concentration of deep trap states. For doped NWs, it is found that the strong effect of the p‐type dopant diethylzinc (DEZn) on the NW composition, observed for GaxIn1–xP NWs grown using TMGa, is absent when using TEGa.

Effects of Density and Diameter on Surface Optical Phonon Modes in GaAs Nanowire Bundles.

We report the systematic investigation of the surface optical phonon modes in Au-catalyzed GaAs nanowires grown on an Au pre-patterned GaAs(111)B substrate using μ-Raman spectroscopy. We employed electron-beam dose rate as a control parameter during the substrate patterning step for adjusting the nanowire base diameter and coverage, which are independent from the nanowire growth conditions. We have experimentally studied the effect of the fill factor and average diameter on the surface optical phonon modes and explained the red-shift and broadening of the surface optical phonon frequencies by employing the dielectric continuum model. The surface optical phonon mode shift is exhibited to be sensitive to fill factor, rather than base diameter. The decrease in the average diameter from 280 nm to 180 nm results in the asymmetric broadening and red-shift of the surface optical phonon frequency (~1.83 cm-1) but the theoretical calculation from the isolated single nanowire-based dielectric continuum model cannot solely explain the behaviors of the surface optical phonon mode. In contrast, the change in the fill factor from 0.01 to 0.83 results in a shift of the surface optical phonon frequency (~6.5 cm-1) from the GaAs bulk value. The red-shift and asymmetric broadening of the surface optical phonons, in an agreement with the Maxwell-Garnett approximation, are consequences of dipolar interaction of randomly aligned neighboring nanowires and the polar nature of GaAs nanowire bundles. This work suggests the pre-patterning parameter dependent surface optical phonon characteristics of GaAs nanowire bundles which are of great importance in the nondestructive characterization of low-dimensional opto-electronic materials and devices.

Controlled synthesis of nonpolar GaInN/GaN multiple-quantum-shells on GaN nanowires by metal-organic chemical vapour deposition

Nonpolar GaInN/GaN multiple-quantum shells (MQSs) on nanowires (NWs) were investigated for high-efficiency light-emitting diodes (LEDs). The growth conditions of NWs were systematically optimized via a continuous growth mode in metal-organic chemical vapour deposition (MOCVD). The In incorporation rate on the m-planes decreased as the growth temperature elaborated, whereas the crystalline quality is improved. The cathodoluminescence (CL) results revealed that longer growth time of the GaInN well can induce additional In-rich droplets and degrade the emission properties of MQSs. The CL emission intensity and the peak wavelength increased as the number of MQS pairs increased from one to three pairs, which was attributed to the increased In incorporation as the diameter enhanced. The linearly enhanced CL emission intensity with barrier thickness was ascribed to the increase of the electron-hole states from the GaN barrier to the wells, resulting in a larger recombination probability. The scanning transmission electron microscopy (STEM) results demonstrated that a thicker barrier shell can suppress the formation of In-rich droplets. Overall, the feasibility of obtaining high-quality m-plane coaxial GaInN/GaN MQSs structures are promising for NW-based white and micro LEDs.

On the interpretation of cathodoluminescence intensity maps of wide band gap nanowires

It is commonly assumed in the spatially resolved cathodoluminescence (CL) studies of wide band gap (WBG) nanowires (NWs) that the CL intensity maps of deep-level (DL) and near band edge (NBE) emission reflect the spatial distribution of defects in these structures. On this basis, crucial conclusions about the technological growth conditions of NWs are drawn. However, here we showed using three-dimensional finite element analysis that in the case of WBG NWs, which exhibit surface band bending, the CL intensity maps of DL and NBE emission do not reflect the distribution of defects but, instead, the electric field strength in NWs. In particular, we found that independently of the defect concentration distribution, the DL emission intensity is always the highest in the areas where the electric field is the strongest and the lowest where the electric field is absent, while the NBE emission intensity exhibits the opposite trends. We explained this finding by the strong influence of the electric field on the spatial distribution of radiative recombination rates. Overall, our results indicate that (i) the frequently observed spatially inhomogeneous CL intensity distribution in WBG NWs can result from the presence of electric fields but not, as widely accepted, non-uniform defect distribution and (ii) spatially resolved CL spectroscopy measurements on the WBG NWs in most cases can not provide any quantitative information about the DL defect distribution.

Thermodynamics of oxidation and reduction during the growth of metal catalyzed silicon nanowires

We have studied the thermodynamics of oxide formation, and the recovery of silicon and the metal catalysts from the dissociation of their respective oxides during the growth process of Si nanowires (NWs) under the Vapor → Liquid Droplet → Solid (VLS) mechanism. It is demonstrated that Si NWs under growth temperatures are thermodynamically unstable in any gas phase containing even a small concentration of O2, and under favorable kinetic conditions Si NWs should entirely convert into dioxide (SiO2). Thermal dissociation of SiO2 in the growth conditions for NWs is practically impossible as it would require either lowering the pressure of O2 to an unachievable, critically low value or raising the temperature to a point which is impossible to reach in ordinary conditions. Decreasing the diameter, i.e., increasing the Si NW dispersity, leads to an increase in the interaction with O2 and this O2 will be held more tightly by Si. Thus, thinner NWs should oxidize stronger than thicker ones. The main metal catalysts for NW growth, as well as Si, are characterized by significant affinity with O2 and inconvertible oxide formation reactions. The most stable Si NW growth is observed with metals (Au, Pt, Cu, Ni, etc.) whose oxides are less stable than SiO2. Only metals with a lower affinity to O2 than to H2 or any other reducing agent are applicable to guarantee the recovery process when used as a catalyst for the growth of NWs.

Lithography-free variation of the number density of self-catalyzed GaAs nanowires and its impact on polytypism

Abstract

Understanding InP Nanowire Array Solar Cell Performance by Nanoprobe-Enabled Single Nanowire Measurements.

III-V solar cells in the nanowire geometry might hold significant synthesis-cost and device-design advantages as compared to thin films and have shown impressive performance improvements in recent years. To continue this development there is a need for characterization techniques giving quick and reliable feedback for growth development. Further, characterization techniques which can improve understanding of the link between nanowire growth conditions, subsequent processing, and solar cell performance are desired. Here, we present the use of a nanoprobe system inside a scanning electron microscope to efficiently contact single nanowires and characterize them in terms of key parameters for solar cell performance. Specifically, we study single as-grown InP nanowires and use electron beam induced current characterization to understand the charge carrier collection properties, and dark current-voltage characteristics to understand the diode recombination characteristics. By correlating the single nanowire measurements to performance of fully processed nanowire array solar cells, we identify how the performance limiting parameters are related to growth and/or processing conditions. We use this understanding to achieve a more than 7-fold improvement in efficiency of our InP nanowire solar cells, grown from a different seed particle pattern than previously reported from our group. The best cell shows a certified efficiency of 15.0%; the highest reported value for a bottom-up synthesized InP nanowire solar cell. We believe the presented approach have significant potential to speed-up the development of nanowire solar cells, as well as other nanowire-based electronic/optoelectronic devices.

AlGaAs and AlGaAs/GaAs/AlGaAs nanowires grown by molecular beam epitaxy on silicon substrates

The data on growth peculiarities and physical properties of GaAs insertions embedded in AlGaAs nanowires grown on different (1 1 1) substrates by Au-assisted molecular beam epitaxy are presented. The influence of nanowires growth conditions on structural and optical properties is studied in detail. It is shown that by varying the growth parameters it is possible to form structures like quantum dots that emit in a wide wavelengths range. These quantum dots show sharp and intense emission lines when an optical signal is collected from a single nanowire. The technology proposed opens new possibilities for integration of direct-band AIIIBV materials on silicon platform.

Magnetic and structural properties of Fe–Co nanowires fabricated by matrix synthesis in the pores of track membranes

Fe1-x Co x nanowires are obtained by electrochemical deposition into the pores of track-etched membranes. The characteristics of the growth process that allow controlling the length and aspect ratio of the nanowires are established. The elemental composition and magnetic properties of the nanowires depend on the diameter of the track-etched pores, which varies from 30 to 200 nm, and the electrochemical potential U (650–850 mV), which determines the nanowire growth rate. According to the results of elemental analysis and the Mossbauer spectroscopy data, the Co content in Fe1-x Co x lies in the range of x=0.20−0.25. It is found that the orientation of the magnetic moment of Fe–Co nanoparticles in the wires depends both on the track pore size d and on the nanowire growth rate. Thus, the magnetic moments in nanowires grown in 50-nm-diameter pores are oriented within 0°–40° with respect to the nanowire axis. The magnetic properties of the nanowires are explained in the framework of a theoretical model describing the magnetic dynamics of nanocomposites, which was extended to include the relaxation of the magnetization vector and to take into account interaction between the particles. The key physical parameters important for the technological applications of the nanowires are determined, their dependence on the nanowire growth conditions is traced, and the possibility of controlling them is established.

Static Finite Element Modeling for Sensor Design and Processing of an Individually Contacted Laterally Bent Piezoelectric Nanowire

We report on the static finite element simulations of the representative elementary pixel of an arrayed force-sensing device. Our goal is to provide quantified guidelines for adjusting pixel geometry and tuning technological parameters in a view to on-chip device fabrication, according to the targeted device performance or usage. The force sensitive pixel consists of an individual piezoelectric nanowire (NW), selectively grown on a material stack representative of the targeted process, and with side electrodes. The model takes into account the full pixel environment but makes the assumption of purely insulating materials (no free charges). We found that the piezopotential collection factor was strongly dependent on the stack characteristics. A 69% collection factor was obtained when a 5-nm-thick growth-seeding ZnO layer was introduced directly below the NW. We also simulated micro-fabrication related defects, such as the loss of physical contact between the NW and the electrodes, and found that the collection factor dropped to 33% for a 3 nm gap before stabilizing. Our results provide important guidelines for the optimization of the overall sensor response and calibration, with resulting constraints on NW growth conditions and substrate patterning, as well as collected data dispersion sources for posttreatment purposes.

Size-dependent optical properties of InGaN quantum dots in GaN nanowires grown by MBE

Quantum dots in nanowires (DINWs) are considered as important building blocks for novel nanoscale semiconductor optoelectronic devices. In this paper, pure axial heterojunction InGaN/GaN DINWs are grown by using plasma-assisted molecular beam epitaxy (PA-MBE) system. The InGaN quantum dots (QDs) are disk-like observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The diameter of QDs can be controlled by the growth conditions of nanowires (NWs), while the thickness of QDs can be controlled by the growth time of InGaN. Temperature-dependent photoluminescence (TDPL) measurements demonstrate that the PL peak of DINWs with small and uniform sizes shows a general red shift with increasing temperature. However, the PL peak of DINWs with non-uniform sizes shows an abnormal blue shift with increasing temperature, which is due to different internal quantum efficiencies of the DINWs with different sizes.

Growth Conditions Control the Elastic and Electrical Properties of ZnO Nanowires.

Great efforts have been made to synthesize ZnO nanowires (NWs) as building blocks for a broad range of applications because of their unique mechanical and mechanoelectrical properties. However, little attention has been paid to the correlation between the NWs synthesis condition and these properties. Here we demonstrate that by slightly adjusting the NW growth conditions, the cross-sectional shape of the NWs can be tuned from hexagonal to circular. Room temperature photoluminescence spectra suggested that NWs with cylindrical geometry have a higher density of point defects. In situ transmission electron microscopy (TEM) uniaxial tensile-electrical coupling tests revealed that for similar diameter, the Young's modulus and electrical resistivity of hexagonal NWs is always larger than that of cylindrical NWs, whereas the piezoresistive coefficient of cylindrical NWs is generally higher. With decreasing diameter, the Young's modulus and the resistivity of NWs increase, whereas their piezoresistive coefficient decreases, regardless of the sample geometry. Our findings shed new light on understanding and advancing the performance of ZnO-NW-based devices through optimizing the synthesis conditions of the NWs.

GaAs nanowires grown by Ga-assisted chemical beam epitaxy: Substrate preparation and growth kinetics

Growth kinetics of GaAs nanowires (NWs) on Si(111) substrates by Ga-assisted chemical beam epitaxy is studied as a function of growth conditions such as substrate temperature (Ts), V/III flux ratio and catalyst dimension. The preparation method for Si(111) substrates is optimized in order to obtain a thin surface oxide with a thickness around 0.5nm, allowing both the decomposition of metalorganic precursors and GaAs nucleation at oxide pinholes. The use of thinner oxides enables the growth of a GaAs layer whereas the utilization of thicker oxides could even inhibit GaAs nucleation. The successful self-formation of Ga droplets over this slightly oxidized Si surface has been observed by scanning electron microscopy (SEM), whose initial size is demonstrated to affect both the NW growth rate and the resultant NW aspect ratio. The formation of these droplets is crucial to enable the catalytic growth of NWs whose morphology is thoroughly analyzed by SEM, showing a self-organized array of vertically aligned match shaped GaAs NWs with a hexagonal footprint. In addition, the crystalline structure of NWs is monitored in-situ by reflection high energy diffraction, showing pure zincblende phase along the whole NW stem.In terms of better NW aspect ratio, higher crystalline quality and faster growth rates, the best NW growth conditions are found at Ts=580°C, using an effective flux ratio V/III≈0.8. Moreover, NW growth kinetics is demonstrated to be improved when using a pre-deposited Ga coverage of 7.5 monolayers, stabilized for 90s prior to the NW growth.

Photoluminescence evolution in GaAs/AlGaAs core/shell nanowires grown by MOCVD: Effects of core growth temperature and substrate orientation

We explore the growth temperature dependence of GaAs/AlGaAs core/shell nanowires (NW) grown by metal–organic chemical-vapor deposition (MOCVD) on (100) and (111)B GaAs substrates. In-situ calibration of the growth temperature allows for a systematic variation with unprecedented resolution. Characterization by photoluminescence (PL) indicates that, at low growth temperatures, nanowires grown on (111)B substrates incorporate higher density of defects than wires on (100). On either substrate, nanowire core growth temperatures above 450°C result in rising defect luminescence in the PL spectra concomitant with the increased epitaxial growth on the nanowire sidewalls at these higher temperatures. This work presents a systematic study of nanowire growth conditions that reveals the correlation between the growth temperature of the GaAs core, the chosen substrate surface orientation, and the resulting optical properties of GaAs/AlGaAs nanowires.

On the diffusion growth mechanism of semiconductor nanowires with the participation of hot atoms

A kinetic model of semiconductor nanowire growth by gas phase adsorption is developed taking into account, along with the surface equilibrium diffusion of adatoms, the nonequilibrium diffusion of excited (hot) atoms generated by acts of adsorption and their relaxation as a result of excitation-energy accommodation via an electron channel at catalyst droplets. The processes that occur on the surface are simulated using the stochastic Monte Carlo method. It is shown that hot-adatom relaxation can determine the nanowire growth rate. The conditions for nanowire growth by the equilibrium or nonequilibrium diffusion of adatoms are established. It is demonstrated that the nanowire growth rate depends on the diameter of the nanodroplets, the distance between them, and the mean free path of atoms excited by the act of adsorption.

Metastable Ge1–xCx Alloy Nanowires

Carbon-containing alloy materials such as Ge(1-x)C(x) are attractive candidates for replacing silicon (Si) in the semiconductor industry. The addition of carbon to diamond lattice not only allows control over the lattice dimensions, but also enhances the electrical properties by enabling variations in strain and compositions. However, extremely low carbon solubility in bulk germanium (Ge) and thermodynamically unfavorable Ge-C bond have hampered the production of crystalline Ge(1-x)C(x) alloy materials in an equilibrium growth system. Here we successfully synthesized high-quality Ge(1-x)C(x) alloy nanowires (NWs) by a nonequilibrium vapor-liquid-solid (VLS) method. The carbon incorporation was controlled by NW growth conditions and the position of carbon atoms in the Ge matrix (at substitutional or interstitial sites) was determined by the carbon concentration. Furthermore, the shrinking of lattice spacing caused by substitutional carbon offered the promising possibility of band gap engineering for photovoltaic and optoelectronic applications.