Compound Semiconductor Nanowires Research Articles

Enhanced Light Extraction of Nano‐Light‐Emitting Diodes with Metal‐Clad Structure Using Vertical GaAs/GaAsP Core–Multishell Nanowires on Si Platform

Nanometer‐scaled light sources using III–V compound semiconductor nanowires (NWs) on Si are expected as building blocks for next‐generation Si photonics, bioimaging, on‐chip microscopy, and light detection and ranging (LiDAR) techniques. This is, however, limited in a few materials systems due to complexity in integration of the vertical III–V NWs on Si and device process flow. Suppressing optical loss in the NW materials beyond the optical diffraction remains difficult in enhancing light extraction. Herein, the effect of the vertical metal‐clad architectures for the vertical nano‐light‐emitting diodes (LEDs) using GaAs/GaAsP‐related core–multishell NWs heterogeneously integrated on Si is investigated. The grown core–multishell NW is composed of a radial n‐GaAs/n‐GaAsP/p‐GaAs/p‐GaAsP double heterostructure. The vertical metal‐clad NW‐LEDs show suppression of carrier overflow effect and rapid enhancement of electrical luminescence.

Damage protection from focused ion beam process toward nanocavity-implemented compound semiconductor nanowire lasers

A focused ion beam (FIB) can precisely mill samples and freely form any nanostructure even on surfaces with curvature, like a nanowire surface, which are difficult to implement by using conventional fabrication techniques, e.g. electron beam lithography. Thus, this tool is promising for nanofabrication; however, fabrication damage and contamination are critical issues, which deteriorate optical properties. In this work, we investigated the protective performance of Al2O3 against the FIB process (especially by a gallium ion). Nanowires were coated with Al2O3 as a hard mask to protect them from damage during FIB nanofabrication. To estimate the protective performance, their emission properties by photoluminescence measurement and time-resolved spectroscopy were compared with and without Al2O3 coating conditions. From the results, we confirmed that the Al2O3 coating protects the nanowires. In addition, the nanowires also showed lasing behavior even after FIB processing had been carried out to implement nanostructures. This indicates that their optical properties are well maintained. Thus, our study proves the usefulness of FIBs for future nanofabrication.

A Self-Powered Portable Nanowire Array Gas Sensor for Dynamic NO2 Monitoring at Room Temperature.

The fast development of the Internet of Things (IoT) has driven an increasing consumer demand for self-powered gas sensors for real-time data collection and autonomous responses in industries such as environmental monitoring, workplace safety, smart cities, and personal healthcare. Despite intensive research and rapid progress in the field, most reported self-powered devices, specifically NO2 sensors for air pollution monitoring, have limited sensitivity, selectivity, and scalability. Here, a novel photovoltaic self-powered NO2 sensor is demonstrated based on axial p-i-n homojunction InP nanowire (NW) arrays, that overcome these limitations. The optimized innovative InP NW array device is designed by numerical simulation for insights into sensing mechanisms and performance enhancement. Without a power source, this InP NW sensor achieves an 84% sensing response to 1ppm NO2 and records a limit of detection down to the sub-ppb level, with little dependence on the incident light intensity, even under <5% of 1 sun illumination. Based on this great environmental fidelity, the sensor is integrated into a commercial microchip interface to evaluate its performance in the context of dynamic environmental monitoring of motor vehicle exhaust. The results show that compound semiconductor nanowires can form promising self-powered sensing platforms suitable for future mega-scale IoT systems.

Isolated conductance channels inside the bandgap of GaAs nanowires of zincblende and wurtzite heterostructures

Electronic properties of polytypes (i.e., the co-existence of zincblende and wurtzite structures in III–V compound semiconductor nanowires) are still highly controversial. In an effort to explore fundamental insights of this topic, we investigate the electronic conductance for GaAs nanowires having arbitrary sequence of zincblende and wurtzite heterostructures by a five-nearest-neighbor tight-binding model. The transmission function χ(E) of the zincblende–wurtzite heterostructure is calculated in the mixture from the complete zincblende to the complete wurtzite configuration by non-equilibrium Green’s function formulae. The properties of the ballistic conductance of the system are analyzed by convolving χ(E) with the thermal broadening function. Numerical results show that more inclusion of the wurtzite-type segments into the GaAs nanowires can establish isolated conductance channels in the neighborhood of the bandgap edge, providing potential optoelectronic applications for mid-infrared optics.

Near-field cavity optomechanical coupling in a compound semiconductor nanowire

A III-V compound semiconductor nanowire is an attractive material for a novel hybrid quantum interface that interconnects photons, electrons, and phonons through a wavelength-tunable quantum structure embedded in its free-standing structure. In such a nanomechanical element, however, a challenge is how to detect and manipulate a small number of phonons via its tiny mechanical motion. A solution would be to couple an optical cavity to a nanowire by introducing the ‘cavity optomechanics' framework, but the typical size difference between them becomes a barrier to achieving this. Here, we demonstrate near-field coupling of a silica microsphere cavity and an epitaxially grown InP/InAs free-standing nanowire. The evanescent optomechanical coupling enables not only fine probing of the nanowire’s mechanical motion by balanced homodyne interferometry but also tuning of the resonance frequency, linewidth, Duffing nonlinearity, and vibration axis in it. Combining this cavity optomechanics with epitaxial nanowire engineering opens the way to novel quantum metrology and information processing.

Solution Deposition and Patterning of Compound Semiconductor Metal Oxide Thin Films and Nanowire Networks

New types of transparent conducting materials (TCMs) are being sought due to the increase in demand for their use in next generation electronics and photonics. Methods to improve transparency be inferred by controlling porosity (graded refractive index), or from a change in the crystal structure of some materials that are not inherently transparent at visible frequencies. Transparent materials with tunable optical properties are important in conductive and capacitive displays, tandem solar cells, organic PVs and other devices. Transparent conducting oxides (TCO) have been extensively used in various technologically important applications including solar cells, flat panel displays, antireflective coatings, (organic)light emitting diodes and many other uses as advanced optical materials. Nanoporous and nanostructured films, assemblies and arrangements are important from an applied point of view in microelectronics, photonics and optical materials. The ability to minimize reflection, control light output and use contrast and variation of the refractive index to modify photonic characteristics can provide routes to enhanced photonic crystal devices, omnidirectional reflectors, antireflection coatings and broadband absorbing or reflecting materials.Here, we detail how interdiffusion processes can be used to modify the crystallinity and phase of solution processed semiconducting oxides, to dielectric complex oxides on glass as thin films or as oxide nanowire networks. In addition, we demonstrate how electrodeposition of mobile ionic species on TCOs and SiO2/Si can allow this process to happen from a top down process, enabling patterned optically transparent coatings with in-plane semiconductor-dielectric contrast. Last, we show how these processes can allow networks of oxide nanowire to form directly from solution processed oxide thin films on a range of substrate types. Antireflective properties and the onset of broadband transparency for interdiffusion-mediated oxide conversion processes are also shown.

(Invited) InGaAs/InP Quantum Well Nanowire Surface Emitting LEDs

III-V compound semiconductor nanowires (NWs) have drawn much attention as nanoscale light sources for integrated photonics due to their nanoscale size, good optical properties and strain relaxation feature enabling the monolithic growth on lattice mismatched substrates. In particular, NWs grown by selective area epitaxy (SAE) technique have many advantages such as controllability of their size and position, high uniformity in diameter and length, as well as complementary metal-oxide-semiconductor (CMOS) process compatibility, facilitating their integration with Si based electronic technologies. With suitable wavelength ranging from 1.3 to 1.6 μm and lattice match of constituent materials, InGaAs/InP quantum wells (QWs) have been being widely used for optical communication applications. However there has been limited understanding on the growth of InGaAs/InP QWs in nanowire architecture and their application for lasers or LEDs. In this work, we present the study of the SAE growth of InGaAs/InP QW NW array by metalorganic chemical vapour deposition (MOCVD) technique, and the demonstration of both single and array nanowire LEDs with an in-depth investigation of their geometry related device properties.

Synthesis and Applications of III-V Nanowires.

Low-dimensional semiconductor materials structures, where nanowires are needle-like one-dimensional examples, have developed into one of the most intensely studied fields of science and technology. The subarea described in this review is compound semiconductor nanowires, with the materials covered limited to III-V materials (like GaAs, InAs, GaP, InP,...) and III-nitride materials (GaN, InGaN, AlGaN,...). We review the way in which several innovative synthesis methods constitute the basis for the realization of highly controlled nanowires, and we combine this perspective with one of how the different families of nanowires can contribute to applications. One reason for the very intense research in this field is motivated by what they can offer to main-stream semiconductors, by which ultrahigh performing electronic (e.g., transistors) and photonic (e.g., photovoltaics, photodetectors or LEDs) technologies can be merged with silicon and CMOS. Other important aspects, also covered in the review, deals with synthesis methods that can lead to dramatic reduction of cost of fabrication and opportunities for up-scaling to mass production methods.

Single-photon sources with quantum dots in III–V nanowires

Abstract Single-photon sources are one of the key components in quantum photonics applications. These sources ideally emit a single photon at a time, are highly efficient, and could be integrated in photonic circuits for complex quantum system designs. Various platforms to realize such sources have been actively studied, among which semiconductor quantum dots have been found to be particularly attractive. Furthermore, quantum dots embedded in bottom-up-grown III–V compound semiconductor nanowires have been found to exhibit relatively high performance as well as beneficial flexibility in fabrication and integration. Here, we review fabrication and performance of these nanowire-based quantum sources and compare them to quantum dots in top-down-fabricated designs. The state of the art in single-photon sources with quantum dots in nanowires is discussed. We also present current challenges and possible future research directions.

Application of Thin Film Concepts to Explore Additional Compound Semiconductor Nanowires

Application of Thin Film Concepts to Explore Additional Compound Semiconductor Nanowires

(Invited) Patterning Transparent and Antireflective Compound Semiconductor Oxide Thin Films and Nanowire Networks from Solution

New types of transparent conducting materials (TCMs) are being sought due to the increase in demand for their use in next generation electronics and photonics. Methods to improve transparency be inferred by controlling porosity (graded refractive index), or from a change in the crystal structure of some materials that are not inherently transparent at visible frequencies. Transparent materials with tunable optical properties are important in conductive and capacitive displays, tandem solar cells, organic PVs and other devices. Transparent conducting oxides (TCO) have been extensively used in various technologically important applications including solar cells, flat panel displays, antireflective coatings, (organic)light emitting diodes and many other uses as advanced optical materials. Nanoporous and nanostructured films, assemblies and arrangements are important from an applied point of view in microelectronics, photonics and optical materials. The ability to minimize reflection, control light output and use contrast and variation of the refractive index to modify photonic characteristics can provide routes to enhanced photonic crystal devices, omnidirectional reflectors, antireflection coatings and broadband absorbing or reflecting materials. Here, we detail how interdiffusion processes can be used to modify the crystallinity and phase of solution processed semiconducting oxides, to dielectric complex oxides on glass as thin films or as oxide nanowire networks. In addition, we demonstrate how electrodeposition of mobile ionic species on TCOs and SiO2/Si can allow this process to happen from a top down process, enabling patterned optically transparent coatings with in-plane semiconductor-dielectric contrast. Last, we show how these processes can allow networks of oxide nanowire to form directly from solution processed oxide thin films on a range of substrate types. Antireflective properties and the onset of broadband transparency for interdiffusion-mediated oxide conversion processes are also shown. References M. Eliason and M. D. Shawkey, Opt. Express, 22, A642 (2014).C. Glynn and C. O'Dwyer, Adv. Mater. Interfaces, 4, 1600610 (2017).C. O’Dwyer, M. Szachowicz, G. Visimberga, V. Lavayen, S. B. Newcomb and C. M. S. Torres, Nature Nanotechology, 4, 239 (2009).Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu and J. A. Smart, Nature Photonics, 1, 176 (2007).C. O'Dwyer and C. M. S. Torres, Front. Physics, 1, 18 (2013).C. Glynn, D. Creedon, H. Geaney, J. O'Connell, J. D. Holmes and C. O'Dwyer, ACS Appl. Mater. Interfaces, 6, 2031 (2014).K. Banger, Y. Yamashita, K. Mori, R. L. Peterson, T. Leedham, J. Rickard and H. Sirringhaus, Nature Materials, 10, 45 (2011).C. Glynn, D. Creedon, H. Geaney, T. Collins, E. Armstrong, M. A. Morris and C. O'Dwyer, Sci. Rep., 5, 11574 (2015).C. Glynn, D. Aureau, G. Collins, S. O'Hanlon, A. Etcheberry and C. O'Dwyer, Nanoscale, 7, 20227 (2015).C. Glynn, H. Geaney, D. McNulty, J. O'Connell, J. D. Holmes and C. O’Dwyer, J. Vac. Sci. Technol. A, 35, 020602 (2017).C. Glynn, L. Balobaid, D. McNulty and C. O’Dwyer, ECS J. Solid State Sci. Technol., 6 (2017).

Compound Semiconductor Nanowire Laser Integrated in Silicon Photonic Crystal

Compound Semiconductor Nanowire Laser Integrated in Silicon Photonic Crystal

Byproduct-free mass production of compound semiconductor nanowires: zinc phosphide

A method for the mass production of compound semiconductor nanowires that involves the direct reaction of component elements in a chemical vapor deposition chamber (CVD) is presented. This method results in nanowires, without the associated production of any other byproducts such as nanoparticles or three-dimensional (3D) bulk crystals. Furthermore, no unreacted reactants remain mixed with the nanowire product in this method. This byproduct-free nanowire production thus circumvents the need to tediously purify and collect nanowires from a mixture of products/reactants after their synthesis. Demonstration made using zinc phosphide (Zn3P2) material system as an example indicated that the direct reaction of zinc microparticles with phosphorus supplied via the vapor phase results in the production of gram quantities of nanowires. To enhance thermal transport and achieve the complete reaction of zinc microparticles, while simultaneously ensuring that the microparticles do not agglomerate into macroscale zinc particles and partly remain unreacted (owing to diffusion limitations), pellets composed of mixtures of zinc and a sacrificial salt, NH4Cl, were employed as the starting material. The sublimation by decomposition of NH4Cl in the early stages of the reaction leaves a highly porous pellet of zinc composed of only zinc microparticles, which allows for inward diffusion of phosphorus/outward diffusion of zinc and the complete conversion of zinc into Zn3P2 nanowires. NH4Cl also aids in removal of any native oxide layer present on the zinc microparticles that may prevent their reaction with phosphorus. This method may be used to mass produce many other nanowires in a byproduct-free manner, besides Zn3P2.

Electronic properties of wurtzite GaAs: A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire

III-V compound semiconductor nanowires are generally characterized by the coexistence of zincblende and wurtzite structures. So far, this polytypism has impeded the determination of the electronic properties of the metastable wurtzite phase of GaAs, which thus remain highly controversial. In an effort to obtain new insights into this topic, we cross-correlate nanoscale spectral imaging by near-field scanning optical microscopy with a transmission electron microscopy analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer. Thus, spatially resolved photoluminescence spectra could be unambiguously assigned to nanowire segments whose structure is known with lattice-resolved accuracy. An emission energy of 1.528 eV was observed from extended zincblende segments, revealing that the dispersed nanowire was under uniaxial strain presumably due to interaction with its supporting substrate. These crucial information and the emission energy obtained for extended pure wurtzite segments were used to perform envelope function calculations of zincblende quantum disks in a wurtzite matrix as well as the inverse structure. In these calculations, we varied the fundamental bandgap, the electron mass, and the band offset between zincblende and wurtzite GaAs. From this multi-parameter comparison with the experimental data, we deduced that the bandgap between the Γ8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the Γ7–A bandgap.

A soft lithographic approach to fabricate InAs nanowire field-effect transistors

The epitaxial layer transfer process was previously introduced to integrate high-quality and ultrathin III-V compound semiconductor layers on any substrate. However, this technique has limitation for fabrication of sub-micron nanoribbons due to the diffraction limit of photolithography. In order to overcome this limitation and scale down its width to sub-50 nm, we need either a costly short wavelength lithography system or a non-optical patterning method. In this work, high-quality III-V compound semiconductor nanowires were fabricated and integrated onto a Si/SiO2 substrate by a soft-lithography top-down approach and an epitaxial layer transfer process, using MBE-grown ultrathin InAs as a source wafer. The width of the InAs nanowires was controlled using solvent-assisted nanoscale embossing (SANE), descumming, and etching processes. By optimizing these processes, NWs with a width less than 50 nm were readily obtained. The InAs NWFETs prepared by our method demonstrate peak electron mobility of ~1600 cm2/Vs, indicating negligible material degradation during the SANE process.

(Invited) Transistor Applications Using Vertical III-V Nanowires on Si Platform

III-V compound semiconductor nanowires (NWs) have stimulated interest in recent years because of their unique properties and potential applications as building blocks for nanoscale electronic and photonic devices. For future electronics, inherent issue for low-power LSI is lowering supply voltage of transistor while avoiding physical limit of subthreshold slope (SS = 2.3 k BT/q = 60 mV/dec). Otherwise, this limitation will stop further scaling of the power consumption even if a multi-gate architecture and III-V/Ge channels are implemented. In this talk we report on new tunnel junction based on III-V/Si interface [1,2] which is formed by selective-area growth of III-V nanowires (NWs) on Si [3-5], and demonstrated vertical TFETs with steeper SS[6,7]. This new tunnel junctions can inherently forms abrupt heterojunction regardless of precise doping because the band discontinuity is determined only by the offset of each III-V and Si. Thus, good gate-electrostatic control and depletion-width control for the tunnel transport is defined only by the III-V channel region regardless of degeneration of source materials. For integration of III-V NWs on Si, selective-area growth has no thick buffer layer, thus the III-V NWs/Si interface is able to show unique band structures. The narrow gap III-Vs such as InAs and In0.7Ga0.3As NWs, for instance, exhibits staggered-Type II band structure when the n-type NWs are formed on p-Si substrates regardless precise doping. Thus, we integrated InGaAs NWs on Si substrate by utilizing specific growth sequence to align vertical NWs [6]. Sn was used for n-type dopant, and Zn-pulse doping technique [7] was used to make pseudo intrinsic layer as channel region. The formation of intrinsic layer in such small NW-volume would be very important to induce large internal electrical field at the InGaAs/Si heterojunction. The length of the Zn-pulse doped region correspond to channel-length (200 nm-long in this case). Next, we demonstrate electronic devices using III-V NWs on a Si substrate. Fabricated vertical transistors contained In0.7Ga0.3As NWs with modulation doped layer. We observed n-type FET behavior in ID-VDS and ID-VG characteristics. The performances are threshold voltage ~ 0.25 V, Gm,max = 1.45 µS/µm at VDS = 0.50 V, Ion / Ioff > 106, subthreshold slope, SS = 78 mV/decade. Besides the III-V NW-based vertical FETs, the heterointerface between the III-V NWs and Si can be used as tunnel FETs (TFETs). In this case, we use band discontinuity between III-V NWs and Si substrate for TFETs. The device fabricated by InAs NW on Si substrate showed switching behavior with an average subthreshold slope (SS) of 30 mV/dec, which is much lower than the physical limitation of metal-oxide-semiconductor field-effect transistors (MOSFETs, SS ~ 60 mV/dec.) [8,9]. Reducing the opening diameter of the masked substrate decreased the numbers of misfit dislocations. The TFETs made by using III-V NW/Si heterojunctions demonstrated steep-SS properties at room temperature. One of the key issues in the vertical TFETs is the formation of intrinsic channels inside the tiny NW-body and another is the reduction of NW-diameter. We also discuss several current-boosting technologies for the TFETs. The current-boosted TFET exhibits steep SS and high Ion, and the transconductance efficiency (Gm/ID), which is important parameter for low-power analog circuits, is a thousand times higher than that of conventional MOSFETs [10]. References 1) K. Tomioka and T. Fukui, Appl. Phys. Lett., 98, 083114-1 – 3 (2011). 2) K. Tomioka and T. Fukui, Appl. Phys. Lett., 104, 073507-1 – 4 (2014). 3) K. Tomioka et al., Nano Lett., 8, 3475 – 3480 (2008). 4) K. Tomioka et al., Nanotechnology, 20, 145302 – 145309 (2009) 5) K. Tomioka et al., Nature, 488, 189 – 192 (2012). 6) K. Tomioka et al., Nano Lett., 13, 5822 – 5826 (2013) 7) E. Nakai et al., Jpn. J. Appl. Phys., 54, 015201-1 – 3 (2015). 8) K. Tomioka et al., IEEE IEDM Tech. Dig. 88 – 91 (2013). 9) K. Tomioka et al., IEEE ESSDERC Tech. Dig. 397 – 402 (2016). 10) K. Tomioka et al., paper submitted

Growth of InGaAsP Alloy Nanowires for Emission from Visible to Mid-Infrared Wavelengths

III-V compound semiconductor nanowires (NWs) have attracted considerable attention during the past decades as the building block of next generation of optoelectronic devices. Their tunable direct band gap, high carrier mobility and excellent mechanical properties make them superior candidates to other semiconductor materials (‎1). Having direct band gaps, most of the III-V NWs can be rationally designed and synthesized to cover a wide range of the band gap energy by tuning the composition. Full composition growth of InxGa1-xN on a single substrate covering the band gap from near UV to mid-IR was achieved for the first time through composition gradient of source materials by Kuykendall et al.(2). Recently we have been able to grow CdSSe and ZnCdSSe NWs alloys using dual gradient method (3,4). Even though alloy nanowires with tunable optical properties for visible wavelengths have received extensive attention, similar NWs for infrared applications have received much less attention. In this contribution, we present successful Au-catalyzed Vapor-Liquid-Solid (VLS) growth of InxGa1-xAs, GaxAs1-xP and InxGa1-xAsyP1-yNWs on a single substrate. Figure 1(a) is 2D composition plane demonstrating different paths for (In,Ga)(As,P) alloy systems. The four corners represent the four binaries, InAs, GaAs, InP, and GaP, while the four edges represent the corresponding four possible ternaries with varying composition, and each given (x,y) point in the interior represents a quaternary alloy of given composition. We report a chemical vapor deposition (CVD) of spatially composition-graded ternary and quaternary alloy semiconductor NWs based on (In,Ga)(As,P) material system with potential applications in optoelectronics, e.g. solar cells and photodetectors. Using the dual gradient method (DMG) we have achieved the continuous spatial composition gradient over a single substrate with a band gap spanning from InAs (0.35 eV) to GaP (2.26 eV) for different (In,Ga)(As,P) alloyed system. The current systematic study based on DMG growth methodology opens a new horizon on structural-optical study of different alloys in III-V systems for various optoelectronic device applications. Different alloys were grown on Si(111) coated with 2-3 nm Au film as catalyst for VLS growth. The growth time varies between 30-60 min for different samples, and also growth temperature was adjusted between 550-680◦C for various materials. For example, highly crystalline GaP is preferred to be grown at temperature around 600◦ C while GaAs around 650-680◦C. 40-45 sccm Ar+5%H2 was used as carrier gas and the pressure of the reactor was kept 1-10 Torr during the process. Based on the difference in enthalpy of formation for different binary materials, Au-coated substrate was positioned downstream facing the opening of the minitubes, each of which is loaded with different source materials. By tilting the substrate along the axial direction of hot-wall furnace tube it was possible to achieve spatially composition gradient growth of NWs over the single substrate. Figure 1(b) shows the representative SEM image of InGaAsP NWs grown on a Si(111) substrate. Optical properties of the as-grown NWs were investigated by photoluminescence (PL) measurements at room temperature using a Ti-Sapphire laser (810 nm) as an excitation source for NIR range and Nd-YLF (349 nm) for visible ranges. PL spectrum of InGaAs NWs shows a red shift from 1100 to 1900 nm from InAs to GaAs rich along the substrate (Fig 1(b)). NW size dependence on time evolution and temperature is also studied. Photoluminescence (PL) emission results agree with both EDS and XRD results on alloy compositions. M. Fang, N. Han, F Wang, Z. X. Yang, S. Yip, G. Dong, J. J. Hou, Y. Chuneh, and J. C. Ho, J. Nanomaterials 2014 (2014).T. Kuykendall, P. Ulrich, S. Aloni and P. Yang, Nature Materials 6, 951(2007) D. Caselli, Z. Liu, D. Shelhammer, and C.Z. Ning, Nano letters14(10), 5772 (2014)A. Pan , R. Liu , M. Sun, and C. Z. Ning, ACS Nano, 4 (2), 671 (2010) Figure 1

InxGa1−xAs nanowires with uniform composition, pure wurtzite crystal phase and taper-free morphology

Obtaining compositional homogeneity without compromising morphological or structural quality is one of the biggest challenges in growing ternary alloy compound semiconductor nanowires. Here we report growth of Au-seeded InxGa1−xAs nanowires via metal-organic vapour phase epitaxy with uniform composition, morphology and pure wurtzite (WZ) crystal phase by carefully optimizing growth temperature and V/III ratio. We find that high growth temperatures allow the InxGa1−xAs composition to be more uniform by suppressing the formation of typically observed spontaneous In-rich shells. A low V/III ratio results in the growth of pure WZ phase InxGa1−xAs nanowires with uniform composition and morphology while a high V/III ratio allows pure zinc-blende (ZB) phase to form. Ga incorporation is found to be dependent on the crystal phase favouring higher Ga concentration in ZB phase compared to the WZ phase. Tapering is also found to be more prominent in defective nanowires hence it is critical to maintain the highest crystal structure purity in order to minimize tapering and inhomogeneity. The InP capped pure WZ In0.65Ga0.35As core–shell nanowire heterostructures show 1.54 μm photoluminescence, close to the technologically important optical fibre telecommunication wavelength, which is promising for application in photodetectors and nanoscale lasers.

Modulating the morphology and electrical properties of GaAs nanowires via catalyst stabilization by oxygen.

Nowadays, III-V compound semiconductor nanowires (NWs) have attracted extensive research interest because of their high carrier mobility favorable for next-generation electronics. However, it is still a great challenge for the large-scale synthesis of III-V NWs with well-controlled and uniform morphology as well as reliable electrical properties, especially on the low-cost noncrystalline substrates for practical utilization. In this study, high-density GaAs NWs with lengths >10 μm and uniform diameter distribution (relative standard deviation σ ∼ 20%) have been successfully prepared by annealing the Au catalyst films (4-12 nm) in air right before GaAs NW growth, which is in distinct contrast to the ones of 2-3 μm length and widely distributed of σ ∼ 20-60% of the conventional NWs grown by the H2-annealed film. This air-annealing process is found to stabilize the Au nanoparticle seeds and to minimize Ostwald ripening during NW growth. Importantly, the obtained GaAs NWs exhibit uniform p-type conductivity when fabricated into NW-arrayed thin-film field-effect transistors (FETs). Moreover, they can be integrated with an n-type InP NW FET into effective complementary metal oxide semiconductor inverters, capable of working at low voltages of 0.5-1.5 V. All of these results explicitly demonstrate the promise of these NW morphology and electrical property controls through the catalyst engineering for next-generation electronics.

Application of free-standing InP nanowire arrays and their optical properties for resource-saving solar cells

III–V compound semiconductor nanowire (NW) arrays have exhibited remarkable behavior in photovoltaic applications. We embedded an orderly vertical InP NW array in poly(dimethylsiloxane) (PDMS) and peeled it off from the substrate to form a AuZn contact. The sample with the substrate exhibited a very high average absorptance of 92%. However, when the array was peeled off, the optical absorptance degraded, particularly in the longer-wavelength region. After the AuZn was deposited on the back side of the NW array, the absorptance increased. This technology could enable a new approach for NW-based photovoltaics with a lower fabrication cost.