Growth Direction Of Nanowires Research Articles

Controlled Self-Assembly of Nanoscale Superstructures in Phase-Change Ge-Sb-Te Nanowires.

Controlled growth of semiconductor nanowires with atomic precision offers the potential to tune the material properties for integration into scalable functional devices. Despite significant progress in understanding the nanowire growth mechanism, definitive control over atomic positions of its constituents, structure, and morphology via self-assembly remains challenging. Here, we demonstrate an exquisite control over synthesis of cation-ordered nanoscale superstructures in Ge-Sb-Te nanowires with the ability to deterministically vary the nanowire growth direction, crystal facets, and periodicity of cation ordering by tuning the relative precursor flux during synthesis. Furthermore, the role of anisotropy on material properties in cation-ordered nanowire superstructures is illustrated by fabricating phase-change memory (PCM) devices, which show significantly different growth direction dependent amorphization current density. This level of control in synthesizing chemically ordered nanoscale superstructures holds potential to precisely modulate fundamental material properties such as the electronic and thermal transport, which may have implications for PCM, thermoelectrics, and other nanoelectronic devices.

Hole subband dispersions in a cylindrical Ge nanowire: exact results based on the axial Luttinger–Kohn Hamiltonian

Based on the Luttinger–Kohn Hamiltonian in the axial approximation, the transcendental equations determining the hole subband dispersions in a cylindrical Ge nanowire are analytically derived. These equations are more general than that derived using the spherical approximation, and are suitable to study the growth direction dependence of the subband dispersions. The axial approximation almost gives rise to the accurate low-energy subband dispersions for high-symmetry nanowire growth directions [001] and [111]. The perturbation correction from the non-axial term is negligible for these two directions. The lowest two subband dispersions can be regarded as two shifted parabolic curves with an energy gap at kz=0 for both growth directions [001] and [111]. At the position of the energy gap, the eigenstates for growth direction [111] are inverted in comparison with the normal eigenstates for growth direction [001]. A nanowire growth direction where the energy gap closes at kz=0 is predicted to exist between directions [001] and [111].

Monte Carlo simulation of planar GaAs nanowire growth

The Monte Carlo model for the planar GaAs nanowire growth via the vapor–liquid-solid mechanism on GaAs substrate using a gallium droplet as a catalyst is realized. The effect of temperature, Ga and As2 deposition rates, substrate orientation and surface properties on the morphology of planar GaAs nanowires is considered. It is shown, that at the initial stages of nanowire growth, a 3D GaAs crystal is formed under a gallium droplet, which sets the nanowire growth direction. The range of temperatures and deposition rates of gallium and arsenic, which ensure the planar nanowire formation on the GaAs(111)A surface, is determined. The investigation of surface properties influence on the nanowire morphology showed that the arsenic diffusion influx into a gallium droplet is of critical importance for the planar GaAs nanowire growth. Ways for achieving a unidirectional growth of planar nanowires on singular and vicinal GaAs surfaces are proposed.

Low-energy hole subband dispersions in a cylindrical Ge nanowire: the effects of the nanowire growth direction

We examine the validity of the spherical approximation in the Luttinger–Kohn Hamiltonian in calculating the subband dispersions of the hole gas. We calculate the realistic hole subband dispersions (without the spherical approximation) in a cylindrical Ge nanowire by using quasi-degenerate perturbation theory. The realistic low-energy hole subband dispersions have a double-well anticrossing structure, that consists with the spherical approximation prediction. However, the realistic subband dispersions are also nanowire growth direction dependent. When the nanowire growth direction is restricted in the (100) crystal plane, the detailed growth direction dependences of the subband parameters are given. We find the spherical approximation is good approximation, it can nicely reproduce the real result in some special growth directions.

Synthesis of AlN Nanowires by Al-Sn Flux Method

This paper presents a recent study on the synthesis of AlN nanowires. AlN nanowires were successfully prepared on sapphire substrate by the Al-Sn flux method. The obtained nanowires were hundreds of nanometers in diameter and tens of microns in length. The results of transmission electron microscopy (TEM) show that the growth direction of AlN nanowires was perpendicular to the C axis. The photoluminescence (PL) spectrum of AlN nanowires shows a broad peak, which is ascribed to the defect levels in the AlN bandgap. This work provides a novel method for growing AlN nanowires, which offers a potential material for the application of photoelectron devices.

Large-Composition-Range Pure-Phase Homogeneous InAs1-xSbx Nanowires.

Narrow bandgap InAs1-xSbx nanowires show broad prospects for applications in wide spectrum infrared detectors, high-performance transistors, and quantum computation. Realizing such applications requires a fine control of the composition and crystal structure of nanowires. However, the fabrication of large-composition-range pure-phase homogeneous InAs1-xSbx nanowires remains a huge challenge. Here, we first report the growth of large-composition-range stemless InAs1-xSbx nanowires (0 ≤ x ≤ 0.63) on Si (111) substrates by molecular beam epitaxy. We find that pure-phase InAs1-xSbx nanowires can be successfully obtained by controlling the antimony content x, nanowire diameter, and nanowire growth direction. Detailed energy dispersive spectrum data show that the antimony is uniformly distributed along the axial and radial directions of InAs1-xSbx nanowires and no spontaneous core-shell nanostructures form in the nanowires. On the basis of field-effect measurements, we confirm that InAs1-xSbx nanowires exhibit good conductivity and their mobilities can reach 4200 cm2 V-1 s-1 at 7 K. Our work lays the foundation for the development of InAs1-xSbx nanowire optoelectronic, electronic, and quantum devices.

Kinking in Semiconductor Nanowires: A Review

The growth direction of nanowires (NWs) can change during synthesis as a result of stochastic processes or modulation of certain growth conditions. This phenomenon is known as kinking. Although deviations from a uniform vertical growth are typically considered to be undesirable, kinking opens a route for additional tweaking of the characteristics and functionalities of NWs in a controllable manner, thus extending the range of potential applications. In the present Review, we give an insight into the kinking mechanisms and summarize the most crucial factors that can lead to kinking of NWs during synthesis. Additionally, the properties and applications of kinked NWs are discussed.

Self-Catalytic Growth of Elementary Semiconductor Nanowires with Controlled Morphology and Crystallographic Orientation.

While the orientation-dependent properties of semiconductor nanowires have been theoretically predicted, their study has long been overlooked in many fields owing to the limits to controlling the crystallographic growth direction of nanowires (NWs). We present here the orientation-controlled growth of single-crystalline germanium (Ge) NWs using a self-catalytic low-pressure chemical vapor deposition process. By adjusting the growth temperature, the orientation of growth direction in GeNWs was selectively controlled to the ⟨110⟩, ⟨112⟩, or ⟨111⟩ directions on the same substrate. The NWs with different growth directions exhibit distinct morphological features, allowing control of the NW morphology from uniform NWs to nanoribbon structures. Significantly, the VLS-based self-catalytic growth of the ⟨111⟩ oriented GeNW suggests that NW growth is possible for single elementary materials even without an appropriate external catalyst. Furthermore, these findings could provide opportunities to investigate the orientation-dependent properties of semiconductor NWs.

Tuning Easy Magnetization Direction and Magnetostatic Interactions in High Aspect Ratio Nanowires.

Cobalt nanowires have been synthesized by electrochemical deposition using track-etched anodized aluminum oxide (AAO) templates. Nanowires with varying spacing-to-diameter ratios were prepared, and their magnetic properties were investigated. It is found that the nanowires’ easy magnetization direction switches from parallel to perpendicular to the nanowire growth direction when the nanowire’s spacing-to-diameter ratio is reduced below 0.7, or when the nanowires’ packing density is increased above 5%. Upon further reduction in the spacing-to-diameter ratio, nanowires’ magnetic properties exhibit an isotropic behavior. Apart from shape anisotropy, strong dipolar interactions among nanowires facilitate additional uniaxial anisotropy, favoring an easy magnetization direction perpendicular to their growth direction. The magnetic interactions among the nanowires were studied using the standard method of remanence curves. The demagnetization curves and Delta m (Δm) plots showed that the nanowires interact via dipolar interactions that act as an additional uniaxial anisotropy favoring an easy magnetization direction perpendicular to the nanowire growth direction. The broadening of the dipolar component of Δm plots indicate an increase in the switching field distribution with the increase in the nanowires’ diameter. Our findings provide an important insight into the magnetic behavior of cobalt nanowires, meaning that it is crucial to design them according to the specific requirements for the application purposes.

Epitaxial growth of β-Ga2O3 nanowires from horizontal to obliquely upward evolution

Changing the direction of horizontal nanowires to extend them in the z direction and grow obliquely has important practical significance for achieving higher density integration and making it have more free space in multi-dimension. However, how to guide the horizontal nanowires to extend in the z direction and grow obliquely is still a challenge. In this paper, we report the regular and horizontal β-Ga2O3 nanostructure grown on c-plane Al2O3 substrates, and the evolution of β-Ga2O3 nanowires from horizontal growth to obliquely upward growth through the chemical vapor deposition (CVD) method. Field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray diffraction, Raman spectrum and photoluminescence spectroscopy were used to investigate the morphology, structure, components, and optical properties of the as-synthesized β-Ga2O3 nanowires. Due to the change of the growth interface at the Au catalysts, there are four different growth modes for the growth of β-Ga2O3 nanowires. The preparation route reported in this study is more simple, general and low-cost compared to other common growth methods for Ga2O3 nanomaterials, and these studies about the evolution of nanowires growth can provide a reference for the change of growth direction of nanowires and simplify planar device processing technology.

Vapor–liquid–solid growth of silicon nanowires from surface nanoholes formed with metal-assisted chemical etching

The influence of shallow surface nanoholes on the growth direction of silicon nanowires (SiNWs) with the vapor–liquid–solid (VLS) process was studied in order to realize a single-step way to promote integration of high-density SiNWs along a specific direction. Shallow surface nanoholes were formed by the short-time metal-assisted chemical etching (MACE) process with catalytic Au nanoparticles of 60 nm also used for the VLS process, in order to shape nanoholes with a similar diameter to the nanoparticles. With an increase in MACE processing time to 5 min, the ratio of perpendicularly grown SiNWs to the SiNWs that appeared on the (111) silicon surface significantly increased in the initial growth phase, reaching higher than 80%. This ratio was more than 3 times higher than without the MACE process. On the other hand, the excess processing time brought about a decrease of the SiNWs detected. This result indicated that the formation of surface nanoholes with an appropriate depth could be an effective way of controlling SiNW growth direction.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Face-selective tungstate ions drive zinc oxide nanowire growth direction and dopant incorporation

Tailoring the elemental doping of inorganic nanowires remains an important challenge due to complex dopant incorporation pathways. Here we report that the face-selectivity of tungstate ions controls growth direction and dopant incorporation of hydrothermal zinc oxide nanowires. The introduction of tungstate ions on nanowire surface during synthesis unexpectedly enhances nucleation at sidewall left{ {10bar 10} right} planes, while dopant incorporation occurs only on (0001) planes. This conflicting face-selective behavior leads to inhomogeneous dopant distribution. Density functional theory calculations reveal that the face-selective behavior can be interpreted in terms of the effect of coordination structure of the tungstate ions on each zinc oxide crystal plane. In addition, we demonstrate a rational strategy to control the morphology and the elemental doping of tungsten-doped zinc oxide nanowires.

Epitaxial Growth of GaN Core and InGaN/GaN Multiple Quantum Well Core/Shell Nanowires on a Thermally Conductive Beryllium Oxide Substrate.

Beryllium oxide (BeO) belongs to a very unique material family that exhibits the divergent properties of high thermal conductivity and high electrical resistivity. BeO has the same crystal structure as GaN, and the absolute difference in the lattice constants is less than 17%. Here, the growth of GaN nanowires (NWs) on the polycrystalline BeO substrate is reported for the first time. The NWs are grown by a vapor–liquid–solid approach using a showerhead-based metal–organic chemical vapor deposition. The growth direction of NWs is along the m-axis on all planes of the substrate, and it is confirmed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns. The vertical and tilted growth of NWs is due to the different planes of the substrate such as the m-plane, a-plane, and semipolar planes and is confirmed by X-ray diffraction. Subsequently, the GaN shell and InGaN/GaN multiple quantum wells (MQWs) are coaxially grown using a vapor–solid approach in the same reactor. A very high crystal quality is verified by TEM and SAED and is also confirmed by measuring the photoluminescence. The optical emission is tuned for the entire visible spectrum by increasing the indium incorporation in InGaN quantum wells. The conformal growth of InGaN/GaN MQW shells and the defect-free nature of the structure are confirmed from spatially resolved cathodoluminescence. This study will provide a platform for researchers to grow GaN NWs on the BeO substrate for a range of optical and electrical applications.

Orientation‐Controlled Selective‐Area Epitaxy of III–V Nanowires on (001) Silicon for Silicon Photonics

AbstractMonolithic integration of III–V nanowires on silicon platforms has been regarded as a promising building block for many on‐chip optoelectronic, nanophotonic, and electronic applications. Although great advances have been made from fundamental material engineering to realizing functional devices, one of the remaining challenges for on‐chip applications is that the growth direction of nanowires on Si(001) substrates is difficult to control. Here, catalyst‐free selective‐area epitaxy of nanowires on (001)‐oriented silicon‐on‐insulator (SOI) substrates with the nanowires aligned to desired directions is proposed and demonstrated. This is enabled by exposing {111} planes on (001) substrates using wet chemical etching, followed by growing nanowires on the exposed planes. The formation of nanowire array‐based bottom‐up photonic crystal cavities on SOI(001) and their coupling to silicon waveguides and grating couplers, which support the feasibility for on‐chip photonic applications are demonstrated. The proposed method of integrating position‐ and orientation‐controllable nanowires on Si(001) provides a new degree of freedom in combining functional and ultracompact III–V devices with mature silicon platforms.

Tunable Shortwave Infrared and Midwave Infrared Optoelectronics in Germanium/Germanium Tin Core/Shell Nanowires

Semiconductor nanowire(NW)-based optoelectronic devices can exhibit superior performances in comparison to their thin film counterparts, as a result of their improved electrical and optical properties. For example, the light-matter interaction in nanowires is enhanced due to the high surface-volume ratio and the light confinement effect resulting from the resonant cavity formed by the nanowire. The enhanced light absorption when using a nanowire geometry can be exploited to increase the responsivity of nanowire-based photodetectors. Light detection and emission at near-IR wavelengths (NIR, 0.8-1.5 μm) can be tailored via strain engineering using Ge-based NW devices.[1] Furthermore, by incorporating Sn in the Ge lattice a direct band gap can be achieved across the short-wave-IR (SWIR, 1.5-3.0 μm) and mid-IR (MIR, 3-8 μm) wavelength range.[2] Despite the equilibrium solubility of Sn in Ge being limited to ~1at.%, non-equilibrium growth methods recently developed in a chemical vapor deposition (CVD) reactor demonstrated a Sn content of 18 at.% with a room-temperature photoluminescence emission up to 4.0 μm [3]. Similarly, when moving to the nanoscale a Sn incorporation well above 10 at.% with a direct band gap emission was demonstrated using Ge/GeSn core/shell NWs.[4]In this presentation, we will discuss the opto-electronic properties of Ge and Ge/GeSn core/shell NWs grown in a CVD reactor using a Ge or Si (111) substrate. The structural properties of the NWs were evaluated by combining Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) in order to understand the nanowire growth direction, crystalline properties as well as Sn distribution uniformity in the shell of the nanowires. Photoluminescence, absorption and Raman measurements will elucidate the effect of strain on the MIR direct bandgap emission/absorption of the GeSn shell.Finally, wewill discuss the electrical characterization of individual Ge and GeSn NW photodetectors. Our devices are fabricated by Electro-Beam Photolithography (EBL) based on metal-semiconductor-metal (M-S-M) structure to address the dependence of the photocurrent under illumination at different SWIR-MIR wavelengths. The time response and transport properties of Ge and Ge/GeSn core/shell NW Field Effect Transistor (FET) in back-gated structure are compared with their counterparts of conventional thin film heterostructures. References Pilon, FT Armand, et al. "Lasing in strained germanium microbridges." Nature communications1 (2019): 1-8.Assali, S., J. Nicolas, and O. Moutanabbir. "Enhanced Sn incorporation in GeSn epitaxial semiconductors via strain relaxation." Journal of Applied Physics2 (2019): 025304.Assali, S., et al. "Atomically uniform Sn-rich GeSn semiconductors with 3.0–3.5 μ m room-temperature optical emission." Applied Physics Letters25 (2018): 251903.Assali, S., et al. "Growth and optical properties of direct band gap Ge/Ge0. 87Sn0. 13 core/shell nanowire arrays." Nano letters3 (2017): 1538-1544.

Selective Growth of Stacking Fault Free ⟨100⟩ Nanowires on a Polycrystalline Substrate for Energy Conversion Application.

Cubic semiconductor nanowires grown along ⟨100⟩ directions have been reported to be promising for optoelectronics and energy conversion applications, owing to their pure zinc-blende structure without any stacking fault. But, until date, only limited success has been achieved in growing ⟨100⟩ oriented nanowires. Here we report the selective growth of stacking fault free ⟨100⟩ nanowires on a commercial transparent conductive polycrystalline fluorine-doped SnO2 (FTO) glass substrate via a simple and cost-effective chemical vapor deposition (CVD) method. By means of crystallographic analysis and density functional theory calculation, we prove that the orientation relationship between the Au catalyst and the FTO substrate play a vital role in inducing the selective growth of ⟨100⟩ nanowires, which opens a new pathway for controlling the growth directions of nanowires via the elaborate selection of the catalyst and substrate couples during the vapor-solid-liquid (VLS) growth process. The ZnSe nanowires grown on the FTO substrate are further applied as a photoanode in photoelectrochemical (PEC) water splitting. It exhibits a higher photocurrent than the ZnSe nanowires do without preferential orientations on a Sn-doped In2O3 (ITO) glass substrate, which we believe to be correlated with the smooth transport of charge carriers in ZnSe ⟨100⟩ nanowires with pure zinc-blende structures, in distinct contrast with the severe electron scattering happened at the stacking faults in ZnSe nanowires on the ITO substrate, as well as the efficient charge transfer across the intensively interacting nanowire-substrate interfaces.

Effect of Temperature on the Morphology of Planar GaAs Nanowires (Simulation)

The self-catalyzed growth of planar GaAs nanowires is analyzed using the lattice kinetic Monte Carlo model. Vapor–liquid–solid nanowire growth on (111)A and (111)B substrates is investigated. The effect of temperature and the location of gallium droplets on the morphology and growth direction of planar GaAs nanowires is examined. The temperature range of the stable growth of planar GaAs nanowires on the GaAs(111)A surface is revealed. Special asymmetric arrangement of the droplets is shown to ensure unidirectional nanowire growth.

Foreign-catalyst-free GaSb nanowires directly grown on cleaved Si substrates by molecular-beam epitaxy

We have successfully fabricated foreign-catalyst-free GaSb nanowires directly on cleaved Si (111) substrates by molecular-beam epitaxy. We find that GaSb nanowires with the absence and presence of Ga droplets at the tip can be simultaneously obtained on cleaved Si substrates without Ga pre-deposition. Systematic morphological and structural studies verify that the two kinds of nanowires presented have different growth mechanisms, which are vapor-solid and vapor-liquid-solid mechanisms. The growth of GaSb nanowires can also be achieved on cleaved Si (110) and Si (100) substrates. The cleavage plane of the Si substrate has an obvious influence on the growth of the GaSb nanowires. The growth direction and crystal quality of catalyst-free nanowires are independent of the cleavage plane of the substrate. Our results may facilitate the understanding of the growth mechanism of III–V nanowires and the integration of foreign-catalyst-free GaSb nanowire-based devices with mature semiconductor technology.

Влияние температуры на морфологию планарных нанопроволок GaAs (моделирование)

Using a kinetic lattice Monte Carlo model, the self-catalyzed growth of planar GaAs nanowires was analyzed. The nanowire growth via the vapor-liquid-crystal mechanism was considered. The effect of temperature and the catalyst droplet location on the morphology and growth direction of planar GaAs nanowires was studied. For GaAs(111)A and GaAs(111)B substrates, a temperature range corresponding to stable growth of planar GaAs nanowires was revealed. The special asymmetric arrangement of droplets allows the one-directional nanowire growth.