Axial Nanowire Heterostructures Research Articles

Self-Consistent Model for the Compositional Profiles in Vapor-Liquid-Solid III-V Nanowire Heterostructures Based on Group V Interchange.

Due to the very efficient relaxation of elastic stress on strain-free sidewalls, III-V nanowires offer almost unlimited possibilities for bandgap engineering in nanowire heterostructures by using material combinations that are attainable in epilayers. However, axial nanowire heterostructures grown using the vapor-liquid-solid method often suffer from the reservoir effect in a catalyst droplet. Control over the interfacial abruptness in nanowire heterostructures based on the group V interchange is more difficult than for group-III-based materials, because the low concentrations of highly volatile group V atoms cannot be measured after or during growth. Here, we develop a self-consistent model for calculations of the coordinate-dependent compositional profiles in the solid and liquid phases during the vapor-liquid-solid growth of the axial nanowire heterostructure Ax0B1-x0C/Ax1B1-x1C with any stationary compositions x0 and x1. The only assumption of the model is that the growth rates of both binaries AC and BC are proportional to the concentrations of group V atoms A and B in a catalyst droplet, requiring high enough supersaturations in liquid phase. The model contains a minimum number of parameters and fits quite well the data on the interfacial abruptness across double heterostructures in GaP/GaAsxP1-x/GaP nanowires. It can be used for any axial III-V nanowire heterostructures obtained through the vapor-liquid-solid method. It forms a basis for further developments in modeling the complex growth process and suppression of the interfacial broadening caused by the reservoir effect.

Axial Growth Characteristics of Optically Active InGaAs Nanowire Heterostructures for Integrated Nanophotonic Devices.

III-V semiconductor nanowire (NW) heterostructures with axial InGaAs active regions hold large potential for diverse on-chip device applications, including site-selectively integrated quantum light sources, NW lasers with high material gain, as well as resonant tunneling diodes and avalanche photodiodes. Despite various promising efforts toward high-quality single or multiple axial InGaAs heterostacks using noncatalytic growth mechanisms, the important roles of facet-dependent shape evolution, crystal defects, and the applicability to more universal growth schemes have remained elusive. Here, we report the growth of optically active InGaAs axial NW heterostructures via completely catalyst-free, selective-area molecular beam epitaxy directly on silicon (Si) using GaAs(Sb) NW arrays as tunable, high-uniformity growth templates and highlight fundamental relationships between structural, morphological, and optical properties of the InGaAs region. Structural, compositional, and 3D-tomographic characterizations affirm the desired directional growth along the NW axis with no radial growth observed. Clearly distinct luminescence from the InGaAs active region is demonstrated, where tunable array-geometry parameters and In content up to 20% are further investigated. Based on the underlying twin-induced growth mode, we further describe the facet-dependent shape and interface evolution of the InGaAs segment and its direct correlation with emission energy.

A systematic study on self-catalyzed growth of InAs/GaSb axial heterostructured nanowires by MOCVD

Traditional Au-catalyzed growth method of nanowires will mix Au atoms into Si substrates and InAs/GaSb axial heterojunction materials to introduce deep energy levels, which will seriously degrade the electrical and photoelectric properties of nanodevices. In this work, we studied in detail the effects of multiple growth parameters on self-catalyzed growth of InAs/GaSb axial heterostructured NWs on Si substrate by MOCVD. It is found that in the switching process from InAs material precursor to GaSb material precursor, if growth temperature of GaSb is not appropriate and switching time is too long, it will cause the decomposition and disappearance of InAs NWs, and directly terminate the realization of InAs/GaSb axial heterojunction NWs. Both the parameter windows of the growth temperature and V/III ratio are very narrow, which directly affects the growth trend of GaSb material in axial or radial deposition. It provides an important and meaningful method for self-catalyzed high-quality nanowires by MOCVD that up high pure GaSb nanowires requires the catalyzed guidance of alloy droplets and buffer seed nanowire and bottom InAs nanowires just meet both requirements.

Hetero-radial MgO capped TiO2 nanowire arrays as a deep UV and self-powered photodetector

Nanowires acts as a potential component in photodetectors (PDs) owing to its superior capacity to generate morphology-dependent broad absorption spectrum. In this study, we have demonstrated an efficient self-powered PD based on the radial heterostructure of the MgO capping layer on top of TiO2 nanowires (RHNWs). RHNWs are highly sensitive under deep ultraviolet (UV) illumination when compared to TiO2-MgO axial heterostructure nanowires (AHNWs) PD. The finite-difference time-domain (FDTD) method is used to simulate electric field distribution, absorption spectra and generation rate profile of the structures. The RHNWs device possesses two times lower leakage current (∼ 1.78 × 10−9 A) than AHNWs (∼ 3.84 ×10−9 A) at – 1.5 V. Theoretical analysis shows a higher photoconductive gain for RHNWs (∼ 12) than the corresponding AHNWs device (∼ 9.9). Maximum photoresponsivity of ∼ 18 A/W has been observed for RHNWs with a sharp rise time (Tr) ∼ 0.12 s and fall time (Tf) ∼ 0.13 s at 200 nm (– 9 V) and the device also behaves like a self-powered PD under white light illumination.

Exploring Axial Organic Multiblock Heterostructure Nanowires: Advances in Molecular Design, Synthesis, and Functional Applications

AbstractOrganic multiblock heterostructure nanowires (OMHNs), integrating distinct components in the same axial structure, have attracted much attention due to their excellent properties such as intrinsic good light confinement and rectification characteristics. However, rationally incorporating such features into OMHNs with rigorous 1D morphology control remains a key synthetic objective. It is because that the single components possess fundamentally distinctive properties such as size‐dependent electronic, optical, and magnetic features as well as lattice parameters. In the last decade, researchers made their own efforts to this exciting field, especially from the controlled fabrication to the advanced applications of the multiblock nanowires. As a result, various OMHNs have been developed as very promising candidates for high‐performance optoelectronic applications such as light harvesting, logic gate devices, photonic transistors, and photonic anticounterfeiting. Herein, the recent advances of OMHNs from the aspects of molecular design, synthesis, and advanced applications are reviewed, and an outlook is given providing inspiration for the future development of OMHNs toward integrated optoelectronic at nanoscale.

Interfacial profile of axial nanowire heterostructures in the nucleation limited regime

We report thermodynamic modeling of the formation of axial III–V nanowire heterostructures grown by the self-catalyzed and Au-catalyzed vapor–liquid–solid methods.

On-wire axial perovskite heterostructures for monolithic dual-wavelength laser

All-inorganic lead halide perovskites have attracted tremendous attention for their tunable bandgaps, excellent photoluminescence efficiency and robust stability. Here, we report on a direct vapor-phase growth of high-quality CsPbCl3/CsPbI3 axial perovskite heterostructure and multi-heterojunction nanowires using a newly developed magnetic-pulling chemical vapor deposition approach. Microstructural characterization and optical investigations reveal that these structures are crystalline with abrupt heterojunctions. Micro-photoluminescence spectra and mapping at the heterojunctions exhibit dual-wavelength emissions at 417 nm and 698 nm, from the adjacent two disparate perovskites, respectively, further demonstrating the formation of unique heterostructures. Additionally, under a focused laser illumination, asymmetrical waveguide behavior along a single CsPbCl3/CsPbI3 wire is clearly observed. Taking a step further, we fabricated a monolithic dual-wavelength laser using an on-wire axial perovskite heterostructure and successfully realized blue and red emissions (425.5 nm and 687.4 nm). The capability to synthesize on-wire heterostructures represents a major step toward high-integration optoelectronic circuits and nanophotonics.

Thermally propagated Al contacts on SiGe nanowires characterized by electron beam induced current in a scanning transmission electron microscope

Here, we use electron beam induced current (EBIC) in a scanning transmission electron microscope to characterize the structure and electronic properties of Al/SiGe and Al/Si-rich/SiGe axial nanowire heterostructures fabricated by thermal propagation of Al in a SiGe nanowire. The two heterostructures behave as Schottky contacts with different barrier heights. From the sign of the beam induced current collected at the contacts, the intrinsic semiconductor doping is determined to be n-type. Furthermore, we find that the silicon-rich double interface presents a lower barrier height than the atomically sharp SiGe/Al interface. With an applied bias, the Si-rich region delays the propagation of the depletion region and presents a reduced free carrier diffusion length with respect to the SiGe nanowire. This behaviour could be explained by a higher residual doping in the Si-rich area. These results demonstrate that scanning transmission electron microscopy EBIC is a powerful method for mapping and quantifying electric fields in micrometer- and nanometer-scale devices.

Self-Catalyzed InSb/InAs Quantum Dot Nanowires.

The nanowire platform offers great opportunities for improving the quality and range of applications of semiconductor quantum wells and dots. Here, we present the self-catalyzed growth of InAs/InSb/InAs axial heterostructured nanowires with a single defect-free InSb quantum dot, on Si substrates, by chemical beam epitaxy. A systematic variation of the growth parameters for the InAs top segment has been investigated and the resulting nanowire morphology analyzed. We found that the growth temperature strongly influences the axial and radial growth rates of the top InAs segment. As a consequence, we can reduce the InAs shell thickness around the InSb quantum dot by increasing the InAs growth temperature. Moreover, we observed that both axial and radial growth rates are enhanced by the As line pressure as long as the In droplet on the top of the nanowire is preserved. Finally, the time evolution of the diameter along the entire length of the nanowires allowed us to understand that there are two In diffusion paths contributing to the radial InAs growth and that the interplay of these two mechanisms together with the total length of the nanowires determine the final shape of the nanowires. This study provides insights in understanding the growth mechanisms of self-catalyzed InSb/InAs quantum dot nanowires, and our results can be extended also to the growth of other self-catalyzed heterostructured nanowires, providing useful guidelines for the realization of quantum structures with the desired morphology and properties.

Nanowire-based telecom-band light-emitting diodes with efficient light extraction

Semiconductor nanowires grown by the gold-free, self-catalyzed, vapor–liquid–solid approach have great potential for realizing monolithically integrable light sources in the telecom band towards integrated optoelectronic circuits. However, light-emitting diodes based on such nanowires operating in the telecom band range remain challenging to achieve, due to difficulties in growing nanowires with suitable materials to cover the telecom band range, their geometrical shape to enhance light extraction efficiency, and their junction structure to suppress current leakage. Here we report telecom-band light-emitting diodes operating at room temperature using InP/InAs axial-heterostructure nanowires grown by self-catalyzed vapor–liquid–solid mode with efficient light extraction. Conically tapered nanowire structures with a single axial junction structure are advantageous for increasing light extraction and carrier injection efficiencies. We realize the structure by making use of a distinctive growth phenomenon in self-catalyzed vapor–liquid–solid growth; i.e. continuously tunable size of catalyst particles during growth.

Full characterization and modeling of graded interfaces in a high lattice-mismatch axial nanowire heterostructure

Controlling the strain level in nanowire heterostructures is critical for obtaining coherent interfaces of high crystalline quality and for the setting of functional properties such as photon emission, carrier mobility or piezoelectricity. In a nanowire axial heterostructure featuring a sharp interface, strain is set by the materials lattice mismatch and the nanowire radius. Here, we show that introducing a graded interface in nanowire heterostructures offers an additional parameter to control strain. For a given interface length and lattice mismatch, we first derive theoretically the maximum nanowire radius below which coherent growth is possible. We validate these findings by growing and characterizing various In(Ga)As/GaAs nanowire heterostructures with graded interfaces. Furthermore, we perform a complete chemical and structural characterization of the interface by combining energy-dispersive X-ray spectroscopy and high resolution transmission electron microscopy. In the case of coherent growth, we directly observe that the mismatch strain relaxes elastically on the side walls of the nanowire around the interface area, while the core of the nanowire remains partially strained. Moreover, our experimental data show good agreement with finite element calculations. This analysis confirms in particular that mechanical strain is largely reduced by interface grading. Overall, our work extends the parameter space for the design of nanowire heterostructures, thus opening new opportunities for nanowire optoelectronics.

Demonstration of InP/InAsP/InP axial heterostructure nanowire array vertical LEDs

Semiconductor nanowires (NWs), which have nanoscale footprints, enable us to realize various quantum structures with excellent position and size controllability, utilizing a wide range of materials for heterostructures. In addition, enhancing light extraction and controlling spontaneous emission by modifying their size and shape are possible. Thus, NWs are promising materials for nanoscale light sources applicable from visible to telecommunication bands. In this study, we grew InP/InAsP/InP axial heterostructure NWs, where the InAsP layer was embedded to serve as an active layer, by selective-area growth and demonstrated vertical NW array light-emitting diodes (LEDs) as a step towards realizing on-demand single photon sources. The NW array LEDs showed rectifying characteristics and electroluminescence originating from the embedded InAsP layer in the near-infrared region.

Growth of Self-Catalyzed InAs/InSb Axial Heterostructured Nanowires: Experiment and Theory.

The growth mechanisms of self-catalyzed InAs/InSb axial nanowire heterostructures are thoroughly investigated as a function of the In and Sb line pressures and growth time. Some interesting phenomena are observed and analyzed. In particular, the presence of In droplet on top of InSb segment is shown to be essential for forming axial heterostructures in the self-catalyzed vapor-liquid-solid mode. Axial versus radial growth rates of InSb segment are investigated under different growth conditions and described within a dedicated model containing no free parameters. It is shown that widening of InSb segment with respect to InAs stem is controlled by the vapor-solid growth on the nanowire sidewalls rather than by the droplet swelling. The In droplet can even shrink smaller than the nanowire facet under Sb-rich conditions. These results shed more light on the growth mechanisms of self-catalyzed heterostructures and give clear route for engineering the morphology of InAs/InSb axial nanowire heterostructures for different applications.

Elasticity of axial nanowire heterostructures with sharp and diffuse interfaces

We present a theoretical analysis of the elastic properties of lattice-mismatching axial nanowire heterostructures. In contrast to the previous works, we take into account non-ideal diffuse interfaces between the dissimilar materials, which are often observed in nanowire heterostructures. Another important result is the existence of a maximum of the elastic energy in a double nanowire heterostructure as a function of its height. The obtained results are applicable to a wide range of material combinations and can be used for understanding and controlling the crystal quality and physical properties of nanowire-based heterostructures for different applications.

Elasticity of Axial Nanowire Heterostructures with Sharp and Diffuse Interfaces

We present a theoretical analysis of the elastic properties of lattice-mismatching axial nanowire heterostructurues. In contrast to the previous works, we take into account non-ideal diffuse interfaces between the dissimilar materials, which are often observed in nanowire heterostructures. Another important result is the existence of a maximum of the elastic energy in a double nanowire heterostructure as a function of its height. The obtained results are applicable to a wide range of material combinations and can be used for understanding and controlling the crystal quality and physical properties of nanowire-based heterostructures for different applications.

Axial GaAs/Ga(As, Bi) nanowire heterostructures

Bi-containing III-V semiconductors constitute an exciting class of metastable compounds with wide-ranging potential optoelectronic and electronic applications. However, the growth of III-V-Bi alloys requires group-III-rich growth conditions, which pose severe challenges for planar growth. In this work, we exploit the naturally-Ga-rich environment present inside the metallic droplet of a self-catalyzed GaAs nanowire (NW) to synthesize metastable GaAs/GaAs1−xBix axial NW heterostructures with high Bi contents. The axial GaAs1−xBix segments are realized with molecular beam epitaxy by first enriching only the vapor–liquid–solid (VLS) Ga droplets with Bi, followed by exposing the resulting Ga-Bi droplets to As2 at temperatures ranging from 270 °C to 380 °C to precipitate GaAs1−xBix only under the NW droplets. Microstructural and elemental characterization reveals the presence of single crystal zincblende GaAs1−xBix axial NW segments with Bi contents up to (10 ± 2)%. This work illustrates how the unique local growth environment present during the VLS NW growth can be exploited to synthesize heterostructures with metastable compounds.

Tuning the orientation of the top-facets of GaN nanowires in molecular beam epitaxy by thermal decomposition

In this work, we demonstrate the possibility of controlling the orientation of the top-facets of GaN nanowires grown by plasma-assisted molecular beam epitaxy by means of an in situ thermal treatment at temperatures in the 800--910 ${}^{\ensuremath{\circ}}\text{C}$ range, at which the material decomposes with a non-negligible rate. Depending on whether the process is carried out under vacuum or active nitrogen exposure, the nanowires can develop either ${1\overline{1}0\overline{2}}$ or ${1\overline{1}0\overline{8}}$ semipolar top-facets. The shape transformation is reversible; namely, the original $(000\overline{1})$ polar facets of as-grown nanowires can be recovered after further GaN growth. Using reflection high-energy electron diffraction, the reshaping process is monitored in vivo by tracking the formation and subsequent evolution of chevrons in the diffraction pattern. This analysis reveals that the reshaping takes place after a certain delay time that can last up to several tens of minutes. It also evidences that the shape transformation is not abrupt; instead there is a continuous evolution from high- to low-index semipolar facets. We observe energy barriers higher than 3 eV along the reshaping path toward the ${1\overline{1}0\overline{2}}$ top-facets. The formation of ${1\overline{1}0\overline{8}}$ top-facets is assigned to an intermediate energy minimum along the reshaping path where the system freezes in the presence of active nitrogen. A change in the Ga chemical potential at the nanowire tip is proposed as the trigger for the reshaping, a process that could be either thermodynamically or kinetically driven. The formation of semipolar facets after thermal decomposition is also observed for $\mathrm{GaN}(000\overline{1})$ layers, demonstrating that the reshaping process is not related to the peculiar nanowire morphology. This unprecedented ability to form semipolar facets at the tip of N-polar GaN nanowires may pave the way for the fabrication of semipolar axial nanowire heterostructures along the $[000\overline{1}]$ direction.

Surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires

The surface-enhanced Raman scattering in graphene deposited on AlxGa1−xN/GaN axial heterostructure nanowires was investigated. It was found that the intensity of graphene Raman spectra is not correlated with aluminium content. Analysis of graphene Raman band parameters, Kelvin probe force microscopy, and electroreflectance showed a screening of polarisation charges. Theoretical calculations demonstrated that plasmon resonance in graphene is far beyond the Raman spectral range. This excludes the presence of an electromagnetic mechanism of surface-enhanced Raman scattering, hence suggesting a chemical mechanism of enhancement.

Catalyst-Assisted Solution-Liquid-Solid Synthesis of CdS/CuInSe2 and CuInTe2/CuInSe2 Nanorod Heterostructures.

Axial nanowire heterostructures composed of cadmium sulfide (CdS)/copper indium diselenide (CuInSe2) and copper indium telluride (CuInTe2)/copper indium diselenide (CuInSe2) were synthesized by a solution-liquid-solid (SLS) method with the catalyzer of bismuth nanocrystals. Electron microscopy and diffraction studies show CuInTe2 and CuInSe2 segments growing along the [112] direction with a clear epitaxial interface between them. In CdS/CuInSe2 nanorod heterostructures, CuInSe2 and CdS segments grow along the [112] and [111] direction, respectively, with an obvious epitaxial interface between them. Energy-dispersive X-ray spectrometry demonstrates the alloy-free composition modulation in two nanorod heterostructures. In CuInTe2/CuInSe2 nanorod heterostructures, Te and Se are localized in CuInTe2 and CuInSe2 segments, respectively. Cu/In/Se and Cd/S are localized in the CuInSe2 and CdS sections of the CdS/CuInSe2 nanorod heterostructures. This research confirms that the SLS mechanism provides a general alternate technique to prepare multicomponent axial 1D heterostructures that have been difficult to generate by using either catalyst-free solution-phase synthesis or vapor-liquid-solid growth.

Observing Solid-State Formation of Oriented Porous Functional Oxide Nanowire Heterostructures by in Situ TEM.

Transition metal oxide nanowires have attracted extensive attention because of their physical characteristics. Among them, ZnO nanowires have great potential. Due to the multifunctional properties of ZnO, devices built using ZnO-based heterostructures always perform well. In this study, interesting diffusion behavior between ZnO nanowires and Fe metal was observed by using in situ transmission electron microscopy. ZnO nanowires and Fe metal were annealed under ultrahigh vacuum (UHV) conditions at 800 K. By controlling the annealing time for the solid-state diffusion, porous Fe3O4 and unique ZnO/porous Fe3O4 nanowire heterostructures were formed. As-formed porous Fe3O4 nanowires with voids can be divided into two types by appearance: plate-like voids and zigzag-like hollow voids. From high-resolution transmission electron microscopy (HRTEM) images and fast Fourier transform (FFT) diffraction patterns, we found that plate-like voids formed along the {111} plane, which was the close-packed plane of Fe3O4, and that zigzag-like hollow voids formed along the {111}/{022} planes. Moreover, a transition region existed during diffusion, with a parallel relationship found between the Fe3O4 crystal with plate-like voids and the ZnO crystal. A sharp interface was determined to exist between the Fe3O4 crystal with zigzag-like hollow voids and ZnO. These oriented porous Fe3O4/ZnO axial nanowire heterostructures exhibited a unique appearance and interesting formation behavior. Furthermore, the structures had a high surface-area-to-volume ratio, which is promising for sensing applications.