A cathodoluminescence study of the influence of the seed particle preparation method on the optical properties of GaAs nanowires

Cathodoluminescence at 8?K is used to compare the optical properties of AlGaAs-capped GaAs nanowires, grown by metal-organic vapour phase epitaxy and seeded by gold particles prepared by different methods. Six different methods were used to fabricate and deposit gold seed particles onto GaAs substrates: colloid particles, aerosol particles and particles defined by electron beam lithography. The nanowires were grown with and without an in?situ annealing step prior to the nanowire growth. The morphology showed no significant differences between the nanowires. The emissions from ensembles of nanowires have the same peak position, irrespective of seed particle type. Without the in?situ annealing step prior to the nanowire growth, there are significant differences in the emission intensity and emission patterns from nanowires grown from different seed particles. When an in?situ annealing step is included, all the resulting nanowires show identical optical emission intensity and emission patterns. This shows the importance of using an in?situ annealing step prior to growth. This study demonstrates that different preparation methods for gold seed particles can be used to produce GaAs nanowires with highly similar optical properties. The choice of particle preparation method to be used can therefore be based on availability and cost.     

Influence of reaction time on growth of GaN nanowires fabricated by CVD method

GaN nanowires have been fabricated successfully on Si (111) substrates coated with NiCl₂ thin films by chemical vapor deposition method using Ga₂O₃ as raw material. The growth of GaN nanowires was investigated as a function of reaction times so as to study the influence of different durations on the components, microstructure, morphologies and optical properties of GaN samples in particular by X-ray diffraction, FT-IR spectrophotometer, scanning electron microscope, and photoluminescence. The results show that the samples after reaction are single crystal GaN with hexagonal wurtzite structure and high-quality crystalline after reaction at 1,100 °C for 60 min, which have good optical properties as revealed by PL spectra. Reaction time greatly influences the growth of GaN nanowires, that is, with the increase in reaction time, the crystalline quality of GaN nanowires is improved accordingly. The growth of the GaN nanowires follows the vapor–liquid-solid mechanism and Ni plays an important role as catalyst, which forms nucleation point in the growth of GaN nanowires.     

Photoluminescence of GaN nanowires of different crystallographic orientations

We utilized time-integrated and time-resolved photoluminescence of a-axis and c-axis gallium nitride nanowires to elucidate the origin of the blue-shifted ultraviolet photoluminescence in a-axis GaN nanowires relative to c-axis GaN nanowires. We attribute this blue-shifted ultraviolet photoluminescence to emission from surface trap states as opposed to previously proposed causes such as strain effects or built-in polarization. These results demonstrate the importance of accounting for surface effects when considering ultraviolet optoelectronic devices based on GaN nanowires.     

Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.     

Effect of the ammoniating time on microstructure and morphology of one-dimensional Mg-doped GaN nanowires catalysed with Au

Mg-doped GaN nanowires have been successfully synthesised on Si(1?1?1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Au thin films, and the effect of ammoniating time on microstructure and morphology were analysed in detail. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectrum were carried out to characterise the microstructure, morphology and optical properties of the GaN samples. The results demonstrate that the nanowires after ammonification at 900°C for 15?min are single crystal GaN with a hexagonal wurtzite structure and high crystalline quality, having the size of 50–80?nm in diameter, more than 10 microns in length and good emission properties. The growth direction of this nanowire is parallel to [0?0?1] direction of hexagonal unit cell. Ammoniating time has a great impact on the microstructure, morphology and optical properties of the GaN nanowires.     

Conversion between hexagonal GaN and beta-Ga(2)O(3) nanowires and their electrical transport properties

We have observed that the hexagonal GaN nanowires grown from a simple chemical vapor deposition method using gallium metal and ammonia gas are usually gallium-doped. By annealing in air, the gallium-doped hexagonal GaN nanowires could be completely converted to beta-Ga(2)O(3) nanowires. Annealing the beta-Ga(2)O(3) nanowires in ammonia could convert them back to undoped hexagonal GaN nanowires. Field effect transistors based on these three kinds of nanowires were fabricated, and their performances were studied. Because of gallium doping, the as-grown GaN nanowires show a weak gating effect. Through the conversion process of GaN nanowires (gallium-doped) --> Ga(2)O(3) nanowires --> GaN nanowires (undoped) via annealing, the final undoped GaN nanowires display different electrical properties than the initial gallium-doped GaN nanowires, show a pronounced n-type gating effect, and can be completely turned off.     

Fabrication of GaN nanowires on Pd-coated sapphire substrates by magnetron sputtering technique

Large-scale GaN nanowires were successfully synthesized through ammoniating Ga₂O₃/Pd films sputtered on the sapphire(001) substrates. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, photoluminescence and Raman spectrum were used to characterize the specimens. The results demonstrate that nanowires are single crystal with hexagonal wurtzite structure and have good optical properties. Raman scattering appears broadened and asymmetric compared with those of bulk GaN due to its polycrystalline nature. In addition, the growth mechanism of GaN nanowires is briefly discussed.     

Single gallium nitride nanowire lasers

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices, including lasersand nonlinear optical frequency converters. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet-blue light-emitting diodes, lasers and photodetectors. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Here we report the observation of ultraviolet-blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry-Perot modes (Q approximately 10(3)) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV microJ x cm(-2)) supports the idea that the electron-hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet-blue coherent light sources.     

Catalyst-free synthesis of well-aligned ZnO nanowires on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates

Well-aligned ZnO nanowires have been synthesized vertically on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates, using a catalyst-free carbon thermal-reduction vapor phase deposition method for the first time. The as-synthesized nanowires are single crystalline wurtzite structure, and have a growth direction of [0001]. Each nanowire has a smooth surface, and uniform diameter along the growth direction. The average diameter and length of these nanowires are 120-150 nm, and 3-10 )m, respectively. We suggest that the growth mechanism follow a self-catalyzing growth model. Excitonic emission peaked around 385 nm dominates the room-temperature photoluminescence spectra of these nanowires. The room-temperature photoluminescence and Raman scattering spectra show that these nanowires have good optical quality with very less structural defects.     

GaN nanowires: CVD synthesis and properties

The growth of GaN nanowires from Ga and NH₃ sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH₃ flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO_x nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed.     

Synthesies and properties of Tb-doped GaN nanowires

The synthesis of Tb-doped GaN nanowires on Si (111) substrates through ammoniating Ga₂O₃ films doped with Tb was investigated. X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy and photoluminescence were used to characterize the composition, structure, morphology and optical properties of the products. The results show that the as-synthesized GaN nanowires doped with 3 at % Tb are of single-crystalline hexagonal wurtzite structure. The nanowires have diameters ranging from 30 to 50 nm and the lengths up to tens of micrometers. An f-f intra-atomic transition of rare earth at 545 nm corresponding to ⁵ D ₄–⁷ F ₅ of the Tb³⁺ and other two peaks related with doping are observed in PL spectrum, confirming the doping of Tb into GaN. The growth mechanism of GaN nanowires was discussed briefly.     

Three-dimensional GaN/AlN nanowire heterostructures by separating nucleation and growth processes

Bottom-up nanostructure assembly has been a central theme of materials synthesis over the past few decades. Semiconductor quantum dots and nanowires provide additional degrees of freedom for charge confinement, strain engineering, and surface sensitivity-properties that are useful to a wide range of solid state optical and electronic technologies. A central challenge is to understand and manipulate nanostructure assembly to reproducibly generate emergent structures with the desired properties. However, progress is hampered due to the interdependence of nucleation and growth phenomena. Here we show that by dynamically adjusting the growth kinetics, it is possible to separate the nucleation and growth processes in spontaneously formed GaN nanowires using a two-step molecular beam epitaxy technique. First, a growth phase diagram for these nanowires is systematically developed, which allows for control of nanowire density over three orders of magnitude. Next, we show that by first nucleating nanowires at a low temperature and then growing them at a higher temperature, height and density can be independently selected while maintaining the target density over long growth times. GaN nanowires prepared using this two-step procedure are overgrown with three-dimensionally layered and topologically complex heterostructures of (GaN/AlN). By adjusting the growth temperature in the second growth step either vertical or coaxial nanowire superlattices can be formed. These results indicate that a two-step method allows access to a variety of kinetics at which nanowire nucleation and adatom mobility are adjustable.     

Simultaneous growth mechanisms for Cu-seeded InP nanowires

We report on epitaxial growth of InP nanowires (NWs) from Cu seed particles by metal-organic vapor phase epitaxy (MOVPE). Vertically-aligned straight nanowires can be achieved in a limited temperature range between 340 °C and 370 °C as reported earlier. In this paper we present the effect of the V/III ratio on nanowire morphology, growth rate, and particle configuration at a growth temperature of 350 °C. Two regimes can be observed in the investigated range of molar fractions. At high V/III ratios nanowires grow from a solid Cu₂In particle. At low V/III ratios, nanowire growth from two particle types occurs simultaneously: Growth from solid Cu₂In particles, and significantly faster growth from In-rich particles. We discuss a possible growth mechanism relying on a dynamic interplay between vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) growth. Our results bring us one step closer to the replacement of Au as seed particle material as well as towards a deeper understanding of particle-assisted nanowire growth.      

Self-assembled GaN nanowires on diamond

We demonstrate the nucleation of self-assembled, epitaxial GaN nanowires (NWs) on (111) single-crystalline diamond without using a catalyst or buffer layer. The NWs show an excellent crystalline quality of the wurtzite crystal structure with m-plane faceting, a low defect density, and axial growth along the c-axis with N-face polarity, as shown by aberration corrected annular bright-field scanning transmission electron microscopy. X-ray diffraction confirms single domain growth with an in-plane epitaxial relationship of (10 ?10)(GaN) [parallel] (01 ?1)(Diamond) as well as some biaxial tensile strain induced by thermal expansion mismatch. In photoluminescence, a strong and sharp excitonic emission reveals excellent optical properties superior to state-of-the-art GaN NWs on silicon substrates. In combination with the high-quality diamond/NW interface, confirmed by high-resolution transmission electron microscopy measurements, these results underline the potential of p-type diamond/n-type nitride heterojunctions for efficient UV optoelectronic devices.     

Control of GaAs nanowire morphology and crystal structure

The morphology and crystal structure of Au-seeded GaAs nanowires (NWs) grown by molecular beam epitaxy were investigated as a function of the temperature, V/III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400 and 500?°C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1?nm(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600?°C, a V/III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07?nm?s(-1). Comparison of nanowire densities on the various post-growth surfaces suggests a possible incubation time between the moment the Ga shutter is opened and when nanowire growth is initiated. Increasing the flux ratio favored uniform sidewall growth, making the process suitable for the fabrication of core-shell structures.     

Synthesis of GaN nanowires by CVD method: effect of reaction temperature

GaN nanowires are fabricated on Si substrates by ammoniating Ga₂O₃/NiCl₂ thin films using chemical vapour deposition method. The influence of reaction temperature on microstructure, morphology and optical properties of GaN nanowires is characterised by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrophotometer, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence. The results demonstrate that the GaN nanowires are single crystalline and exhibit hexagonal wurtzite symmetry. The best crystalline quality was achieved for an reaction temperature of 1150°C for 15?min. The growth process follows vapour–liquid–solid mechanism and Ni plays an important role as the nucleation point and as a catalyst.     

High quality GaAs nanowires grown on glass substrates

We report for the first time the growth of GaAs nanowires directly on low-cost glass substrates using atmospheric pressure metal organic vapor phase epitaxy via a vapor-liquid-solid mechanism with gold as catalyst. Substrates used in this work were of float glass type typically seen in household window glasses. Growth of GaAs nanowires on glass were investigated for growth temperatures between 410 and 580 °C. Perfectly cylindrical nontapered nanowires with a growth rate of ~33 nm/s were observed at growth temperatures of 450 and 470 °C, whereas highly tapered pillar-like wires were observed at 580 °C. Nanowires grew horizontally on the glass surface at 410 °C with a tendency to grow in vertically from the substrate as the growth temperature was increased. X-ray diffraction and transmission electron microscopy revealed that the nanowires have a perfect zinc blende structure with no planar structural defects or stacking faults. Strong photoluminescence emission was observed both at low temperature and room temperature indicating a high optical quality of GaAs nanowires. Growth comparison on impurity free fused silica substrate suggests unintentional doping of the nanowires from the glass substrate.     

Catalyst and its diameter dependent growth kinetics of CVD grown GaN nanowires

GaN nanowires were grown using chemical vapor deposition with controlled aspect ratio. The catalyst and catalyst-diameter dependent growth kinetics is investigated in detail. We first discuss gold catalyst diameter dependent growth kinetics and subsequently compare with nickel and palladium catalyst. For different diameters of gold catalyst there was hardly any variation in the length of the nanowires but for other catalysts with different diameter a strong length variation of the nanowires was observed. We calculated the critical diameter dependence on adatoms pressure inside the reactor and inside the catalytic particle. This gives an increasing trend in critical diameter as per the order gold, nickel and palladium for the current set of experimental conditions. Based on the critical diameter, with gold and nickel catalyst the nanowire growth was understood to be governed by limited surface diffusion of adatoms and by Gibbs–Thomson effect for the palladium catalyst.     

Titanium-assisted growth of silica nanowires: from surface-matched to free-standing morphologies

We report on an oxide-assisted growth technique for silica nanowires which allows tuning the growth from surface-matched nanowires to free-standing morphologies based on growth control by Ti in the role of a catalyst and surfactant. Using an adjustable Ti concentration, we grew silica nanowires with lengths ranging from 100 nm up to several millimetres whose defect chemistry was analysed by electron microscopy tools, monochromatic cathodoluminescence imaging and time resolved photoluminescence spectroscopy. The knowledge of the luminescence properties and the related defect occurrence along with their spatial distribution is pivotal for advancing silica nanowire growth in order to realize successful device designs based on self-assembled Si/SiO(x) nanostructures. We demonstrate a core-shell structure of the grown nanowires with a highly luminescent 150 nm thick shell and outstandingly fast decaying dynamics (≈1 ns) for glass-like materials. The conjunction of the observed efficient and stable luminescences with their attributed decaying behaviours suggests applications for silica nanowires such as active and passive optical interconnectors and white light phosphors. The identification of a time domain difference for the spectral regime from 2.3 to 3.3 eV, within the confined spatial dimensions of a single nanowire, is very promising for future, e.g. data transmission applications, employing silica nanowires which exhibit achievable compatibility with commonly applied silicon-based electronics. A qualitative growth model based on silica particle diffusion and Ti-assisted seed formation is developed for the various types of segregated silica nanowires which extends commonly assumed oxide-assisted growth mechanisms.     

Defect transfer from nanoparticles to nanowires

Metal-seeded growth of one-dimensional (1D) semiconductor nanostructures is still a very active field of research, despite the huge progress which has been made in understanding this fundamental phenomenon. Liquid growth promoters allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However the transfer of crystallographic information from a catalytic nanoparticle seed to a growing nanowire has not been described in the literature. Here we define the theoretical requirements for transferring defects from nanoparticle seeds to growing semiconductor nanowires and describe why Ag nanoparticles are ideal candidates for this purpose. We detail in this paper the influence of solid Ag growth seeds on the crystal quality of Ge nanowires, synthesized using a supercritical fluid growth process. Significantly, under certain reaction conditions {111} stacking faults in the Ag seeds can be directly transferred to a high percentage of <112>-oriented Ge nanowires, in the form of radial twins in the semiconductor crystals. Defect transfer from nanoparticles to nanowires could open up the possibility of engineering 1D nanostructures with new and tunable physical properties and morphologies.