Semiconductor Nanowires Research Articles

Geometry-related optical properties in vertically aligned GaAs nanowires arrays: A study of sizes and embedding medium effects

We investigate the effects of the geometrical sizes and surrounding media on the optical properties of a periodic square array of GaAs nanowires (NWs) as candidates for applications on photovoltaic solar cells as well as sensors. As filling media, different insulator materials such as PMMA, Polycarbonate, Polystyrene and PVP are considered in this study. The simulated system is made of three stacked layers along with two semi-infinite air spaces. The stacked layer system is defined by a transparent layer of Indium Tin Oxide (ITO), the active region of GaAs nanowires array embedded into the filling medium and a substrate of Silicon (Si). The multilayer absorbance is simulated by transfer matrix method (TMM); while the GaAs nanowires surrounded by filling medium is treated as an homogeneous layer whose effective dielectric function is described by Bruggeman effective medium theory. For both s- and p-polarizations at normal and oblique incidences, we observe a typical oscillating behavior in the absorbance/reflectance spectra, regardless the type of embedding medium and sizes investigated. For all investigated sizes, we obtain that the absorbance values decrease for longer wavelength. Greater D/P ratio for longer NWs benefit the light trapping with absorbance values up to 80 per cent in the VIS range. For small variations of the refractive index (dielectric constant) of the filling media surrounding GaAs NWs, we observed a perceptible change in their reflectance spectral values at specific wavelengths. On the other hand, the Brewster angle, calculated at the interface of the ITO layer and the effective embedded nanowires composite, is seen to be sensitive to the D/P ratio size, as far as the wavelength is varied. The above numerical results are in good agreement with those reported in the literature by experimental research and sophisticated FDTD simulations in III–V semiconductor nanowires, which demonstrates that our TMM simulations are adequate approximations to account on optical properties in multilayered structures when homogenization of multilayers is required.

Short-wave infrared cavity resonances in a single GeSn nanowire

Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present an experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content ( ~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of short-wave infrared (SWIR).

Delocalized states in three-terminal superconductor-semiconductor nanowire devices

We fabricate three-terminal hybrid devices consisting of a semiconductor nanowire segment proximitized by a grounded superconductor and having tunnel probe contacts on both sides. By performing simultaneous tunneling measurements, we identify delocalized states, which can be observed from both ends, and states localized near one of the tunnel barriers. The delocalized states can be traced from zero magnetic field to fields beyond 0.5 T. Within the regime that supports delocalized states, we search for correlated low-energy features consistent with the presence of Majorana zero modes. While both sides of the device exhibit ubiquitous low-energy features at high fields, no correlation is inferred. Simulations using a one-dimensional effective model suggest that the delocalized states, which extend throughout the whole system, have large characteristic wave vectors, while the lower momentum states expected to give rise to Majorana physics are localized by disorder. To avoid such localization and realize Majorana zero modes, disorder needs to be reduced significantly. We propose a method for estimating the disorder strength based on analyzing the level spacing between delocalized states.

Stretchable Photodiodes with Polymer-Engineered Semiconductor Nanowires for Wearable Photoplethysmography.

Healthcare systems worldwide have been stressed to provide sufficient resources to serve the increasing and aging population in our society. The situation became more challenging at the time of pandemic. Technology advancement, especially the adoption of wearable health monitoring devices, has provided an important supplement to current clinical equipment. Most health monitoring devices are rigid, however, human tissues are soft. Such a difference has prohibited intimate contact between the two and jeopardized wearing comfortableness, which hurdles measurement accuracy especially during longtime usage. Here, we report a soft and stretchable photodiode that can conformally adhere onto the human body without any pressure and measure cardiovascular variables for an extended period with higher reliability than commercial devices. The photodiode used a composite light absorber consisting of an organic bulk heterojunction embedded into an elastic polymer matrix. It is discovered that the elastic polymer matrix not only improves the morphology of the bulk heterojunction for obtaining the desired mechanical properties but also alters its electronic band structure and improves the electrical properties that lead to a reduced dark current and enhanced photovoltage in the stretchable photodiode. The work has demonstrated high fidelity measurements and longtime monitoring of heat rate variability and oxygen saturation, potentially enabling next-generation wearable photoplethysmography devices for point-of-care diagnosis of cardiovascular diseases in a more accessible and affordable way.

Realize low-power artificial photonic synapse based on (Al,Ga)N nanowire/graphene heterojunction for neuromorphic computing

The fast development of brain-inspired neuromorphic computing systems has stimulated urgent requirements for artificial synapses with low-power consumption. In this work, a photonic synaptic device based on (Al,Ga)N nanowire/graphene heterojunction has been proposed and demonstrated successfully. In the artificial synaptic device, the incident light, the nanowire/graphene heterojunction, and the light-generated carriers play the roles of action potential, pre-synaptic/post-synaptic membrane, and neurotransmitter in a biological synapse, respectively. As a key synaptic function, the paired pulse facilitation index of the photonic synapse can reach 202%, which can be modulated by the interval time between two adjacent light pulses. It is found that the graphene defects, the surface band bending, and the Al vacancies on the surface of (Al,Ga)N nanowires can be the key reasons contributing to the synaptic characteristics of artificial photonic devices. Hence, the dynamic “learning–forgetting” performance of the artificial synaptic device can resemble the “learning–forgetting” behavior of the human brain. Furthermore, the hand-written digits are set up to mimic a typical characteristic of human perceptual learning. After only three training epochs, the simulated network can achieve a high recognition rate of over 90% based on the experimental conductance for long-term potentiation and long-term depression. In supervised learning processes, such few training times are beneficial to reduce energy consumption significantly. Therefore, in the area of neuromorphic computing technology and artificial intelligence systems requiring low-power consumption, this work paves a potential way to develop the optoelectronic synapse based on semiconductor nanowires.

Investigation of hydrothermal growth time and temperature for optimized ZnO nanowire arrays toward photovoltaics

The challenge of meeting energy demands has led to the development of nanotechnology. Semiconductor nanowires are considered a promising option among various nanomaterials for efficiently capturing solar radiation in an axial geometry. This work presents the growth of ZnO nanowire arrays on seeded-coated glass substrates using a two-step hydrothermal method, by examining the effect of two crucial hydrothermal growth parameters: time and temperature of growth. X-ray diffraction (XRD) spectra showed the hexagonal wurtzite shape of the ZnO nanowires which displayed a preferred (002) axial orientation. To estimate the crystallite size, strain, stress, and energy density values, we used the Modified Williamson-Hall models. Scanning electron microscopy (SEM) observations showed that the growth time and temperature have a significant impact on the length and diameter of the nanowires, which increase with increasing time and temperature. A clear correlation was also observed between the diameter and density of the nanowires. Optical analysis showed that ZnO nanowires exhibited good transmittance of about 90% in the near ultraviolet and visible spectrum, but this decreased with increasing growth time and temperature. The measured bandgap energy showed a red shift, which confirmed its correlation with the structural characteristics. The improved optical and structural features of ZnO nanowires were analyzed with regards to their potential use in photovoltaics.

Quantized Spin Pumping in Topological Ferromagnetic-Superconducting Nanowires.

Semiconducting nanowires with strong spin-orbit coupling in the presence of induced superconductivity and ferromagnetism can support Majorana zero modes. We study the pumping due to the precession of the magnetization in single-subband nanowires and show that spin pumping is robustly quantized when the hybrid nanowire is in the topologically nontrivial phase, whereas charge pumping is not quantized. Moreover, there exists one-to-one correspondence between the quantized conductance, entropy change and spin pumping in long topologically nontrivial nanowires but these observables are uncorrelated in the case of accidental zero-energy Andreev bound states in the trivial phase. Thus, we conclude that observation of correlated and quantized peaks in the conductance, entropy change and spin pumping would provide strong evidence of Majorana zero modes, and we elaborate how topological Majorana zero modes can be distinguished from quasi-Majorana modes potentially created by a smooth tunnel barrier at the lead-nanowire interface. Finally, we discuss peculiar interference effects affecting the spin pumping in short nanowires at very low energies.

Subgap states and quantum phase transitions in one-dimensional superconductor–ferromagnetic insulator heterostructures

We theoretically study the spectral properties of a one dimensional semiconductor-superconductor-ferromagnetic insulator (SE-SU-FMI) hybrid nanostructure, motivated by recent experiments where such devices have been fabricated using epitaxial growing techniques. We model the hybrid structure as a one-dimensional single-channel semiconductor nanowire under the simultaneous effect of two proximity-induced interactions: superconducting pairing and a (spatially inhomogeneous) Zeeman exchange field. The coexistence of these competing mechanisms generates a rich quantum phase diagram and a complex subgap Andreev bound state (ABS) spectrum. By exploiting the symmetries of the problem, we classify the solutions of the Bogoliubov-de Gennes equations into even and odd ABS with respect to the spatial inversion symmetry $x\ensuremath{\rightarrow}\ensuremath{-}x$. We find the ABS spectrum of the device as a function of the different parameters of the model: the length $L$ of the coexisting SU-FMI region, the induced Zeeman exchange field ${h}_{0}$, and the induced superconducting coherence length $\ensuremath{\xi}$. In particular we analyze the evolution of the subgap spectrum as a function of the length $L$. Interestingly, we generically find spin-polarized ABS emerging in the subgap region, which, depending on the ratio ${h}_{0}/\mathrm{\ensuremath{\Delta}}$, can eventually cross below the Fermi energy at certain critical values ${L}_{c}$, and induce spin- and fermion parity-changing quantum phase transitions. We argue that this type of device constitute a promising highly-tunable platform to engineer subgap ABS.

Humidity-dependent electrical performance of CuO nanowire networks studied by electrochemical impedance spectroscopy.

Electrochemical impedance spectroscopy was applied for studying copper oxide (CuO) nanowire networks assembled between metallic microelectrodes by dielectrophoresis. The influence of relative humidity (RH) on electrical characteristics of the CuO nanowire-based system was assessed by measurements of the impedance Z. A slight increase of Z with increasing RH at low humidity was followed by a three orders of magnitude decrease of Z at RH above 50-60%. The two opposite trends observed across the range of the examined RH of 5-97% can be caused by water chemisorption and physisorption at the nanowire interface, which suppress electronic transport inside the p-type semiconductor nanowire but enhance ionic transport in the water layers adsorbed on the nanowire surface. Possible physicochemical processes at the nanowire surface are discussed in line with equivalent circuit parameters obtained by fitting impedance spectra. The new investigation data can be useful to predict the behavior of nanostructured CuO in humid environments, which is favorable for advancing technology of nanowire-based systems suitable for sensor applications.

Focused ion beam lithography for position-controlled nanowire growth

To exploit the promising properties of semiconductor nanowires and ensure the uniformity required to achieve device integration, their position on the growth substrate must be controlled. This work demonstrates the direct patterning of a SiO2/Si substrate using focused ion beam (FIB) patterning to control self-catalyzed GaAsSb nanowire growth in molecular beam epitaxy (MBE). Besides position control, FIB patterning parameters influence nanowire yield, composition and structure. Total ion dose per hole is found to be the most important parameter. Yield of single nanowires ranges from ≈34% to ≈83%, with larger holes dominated by multiple nanowires per hole. Areas exposed to low ion beam doses are selectively etched by routine pre-MBE HF cleaning, enabling patterning and nanowire nucleation with minimal damage to the Si substrate. The optical and electronic properties of nanowires are found to depend on the ion dose used during patterning, indicating the potential for FIB patterning to tune nanowire properties. These findings demonstrate the possibility for a FIB lithography protocol which could provide a rapid and direct patterning process for flexible controlled nanowire growth.

Interplay between Crystal Structure and Optical Response in Plateau–Rayleigh Zn2GeO4/SnO2 Heterostructures

Hybrid semiconductor nanowire (NW) heterostructures are an ideal playground for cutting‐edge optoelectronic nanodevices. Among the several synthesis methods, Plateau–Rayleigh (PR) crystal growth is an effective route for producing decorated NWs with unprecedented properties. However, lateral variations in composition and/or crystal structure are postulated to play a central role in their optical response, but it is difficult to probe correlatively the elemental order, atomic organization, and light emission on length scales of tens to hundreds of nanometers. Usually, electron microscopies are applied to address the formation of clusters and imperfections in representative cross sections of the samples. Herein, a simultaneous spatially resolved nano‐analysis of the crystal symmetry, chemical composition, and optical properties of a whole Zn2GeO4/SnO2 NW heterostructure produced by PR instability is provided. The observations show the connection among Zn impurities, secondary phases, and asymmetrically distributed UV emissions present in the crystallites decorating the NW (Zn‐doped Sn1−xGexO2). The contributions of the elemental diffusion, crystal domains, and atomic site configurations to the light‐emission phenomena are disentangled in these hybrid architectures. The findings elucidate unknown underlying mechanisms that are critical to tailor emergent properties for rationally designing novel complex nanodevices based on 1D materials.

Impact of Junction Length on Supercurrent Resilience against Magnetic Field in InSb-Al Nanowire Josephson Junctions.

Semiconducting nanowire Josephson junctions represent an attractive platform to investigate the anomalous Josephson effect and detect topological superconductivity. However, an external magnetic field generally suppresses the supercurrent through hybrid nanowire junctions and significantly limits the field range in which the supercurrent phenomena can be studied. In this work, we investigate the impact of the length of InSb-Al nanowire Josephson junctions on the supercurrent resilience against magnetic fields. We find that the critical parallel field of the supercurrent can be considerably enhanced by reducing the junction length. Particularly, in 30 nm long junctions supercurrent can persist up to 1.3 T parallel field─approaching the critical field of the superconducting film. Furthermore, we embed such short junctions into a superconducting loop and obtain the supercurrent interference at a parallel field of 1 T. Our findings are highly relevant for multiple experiments on hybrid nanowires requiring a magnetic-field-resilient supercurrent.

Tunnel Junction Engineered Photocarrier Dynamics in Epitaxial Semiconductor Nanowires for Efficient and Ultrafast Photoelectrochemical Photodetectors

Photodetection using photoelectrochemical (PEC) principles is an emerging field in photonics. In recent years, using epitaxial III-nitride nanowires as photoelectrodes, high-performance PEC-type photodetectors (PDs) have been demonstrated, making the epitaxial III-nitride nanowires a scalable, high-performance PEC-PD architecture. Despite the progress, the photodetection performance improvement mainly occurs through incorporating photocatalysts into the nanowire photoelectrodes. In this study, we show that a semiconductor tunnel junction (TJ), which can be a natural component in the epitaxy process of semiconductor nanowires, can drastically improve the photodetection performance of such nanowire-based PEC-PDs. By using a three-electrode PEC cell configuration, we clearly show that an n++-GaN/InGaN/p++-GaN TJ can lead to a factor of 9× improvement on the responsivity of the InGaN nanowire photoelectrode in the blue band due to the TJ-induced photocarrier dynamics tuning, compared to the InGaN nanowire photoelectrode without the TJ. More drastically, the TJ also improves the photoresponse speed of the nanowire photoelectrode by 2 orders of magnitude, and for the electrode with the TJ an ultrafast response time of less than 10 ms is estimated. This TJ concept can also be applied to other TJ structures for other band photodetections. This study therefore sheds new light on further improving the performance of emerging epitaxial nanowire-based PEC-PDs for a wide range of applications from sensing to information processing.

A type-II α-MnO2@Co3O4 architecture with superior photoelectrocatalytic performance toward water purification

Designing an environmentally friendly and cost-effective heterogeneous structure to treat refractory organic wastewater for long-term environmental benefits is a challenge in the photoelectrocatalytic (PEC) process, and one-dimensional (1D) semiconductor nanowires can be utilized as a suitable platform to interface with other components, constructing nanohybrids heterostructures with functionalities not found in the individual components. Herein, a two-step hydrothermal process prepared a necklace-like α-MnO2@Co3O4 (MC) heterostructure that integrated 1D α-MnO2 and Co3O4 octahedron with the exposed (111) facets. The optimized MC-2 with a suitable amount of Co3O4 exhibited superior PEC performance for removing reactive brilliant blue KN-R (KN-R) (removal rate of ∼ 88.44% and super long-term accelerated stability of ∼ 5000 s), which can be ascribed to its high oxygen evolution potential, large electrochemically active surface area, low charge transfer resistance, and low flat band potential. To gain insight into the significant photoelectric synergy, the type-II heterojunction between α-MnO2 and Co3O4 was proposed, effectively preventing photogenerated carriers' recombination and improving the dominant active species h+ and •O2− generation. This work uncovered the MC-2 heterostructure for the PEC degradation of KN-R, guiding the other heterogeneous materials to decrease industrial costs and realize large-scale water purification applications.

Waveguide-Integrated Light-Emitting Metal–Insulator–Graphene Tunnel Junctions

Ultrafast interfacing of electrical and optical signals at the nanoscale is highly desired for on-chip applications including optical interconnects and data processing devices. Here, we report electrically driven nanoscale optical sources based on metal-insulator-graphene tunnel junctions (MIG-TJs), featuring waveguided output with broadband spectral characteristics. Electrically driven inelastic tunneling in a MIG-TJ, realized by integrating a silver nanowire with graphene, provides broadband excitation of plasmonic modes in the junction with propagation lengths of several micrometers (∼10 times larger than that for metal-insulator-metal junctions), which therefore propagate toward the junction edge with low loss and couple to the nanowire waveguide with an efficiency of ∼70% (∼1000 times higher than that for metal-insulator-metal junctions). Alternatively, lateral coupling of the MIG-TJ to a semiconductor nanowire provides a platform for efficient outcoupling of electrically driven plasmonic signals to low-loss photonic waveguides, showing potential for applications at various integration levels.

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.

Direct Observation of Liquid–Solid Two‐Phase Seed Particle‐Assisted Kinking in GaP Nanowire Growth

In the last decades, the metal‐assisted growth approach of semiconductor nanowires (NWs) has shown its potential in controlling crystal properties, such as crystal structure, composition, and morphology. Recently, literature reports have shown successful semiconductor NW growth with multiphase seed particles under growth conditions. Exploring alternative metal seeds and the mechanisms for growing semiconductor NWs is an exciting research field aiming to improve the control over the crystal growth process. Herein, the gallium phosphide (GaP) NW growth using Cu as seed particles inside an environmental transmission electron microscope is studied. In particular, the transformations of the Cu‐rich seed particles during the nucleation and growth of GaP NWs are observed. The supply of a relatively high amount of Ga atoms by the precursor mixture led to a solid Cu‐rich seed particle core covered by a liquid phase. Different growth dynamics within the two‐phase seed particle resulted in local competition in NW growth. As a result, the GaP NW kinked into another growth direction by forming a new interface at the NW growth front. The generated results enable insights into fundamental processes occurring in the seed particle during growth, creating leverage points for controlling the NW morphology.

Review on III–V Semiconductor Nanowire Array Infrared Photodetectors

AbstractIn recent years, III–V semiconductor nanowires have been widely investigated for infrared photodetector applications due to their direct and suitable bandgap, unique optical and electrical properties, flexibility in device design and to create heterostructures, and/or grow on a foreign substrate such as Si with more effective strain relaxation compared with planar structures. In particular, vertically aligned and ordered nanowire arrays have emerged as a promising photodetector platform, since their geometry‐related light absorption and carrier transport properties can be tailored to achieve high photodetector performance and new functionalities. In this article, the state‐of‐the‐art progress in the development of various types of infrared photodetectors based on III–V semiconductor nanowire arrays is reviewed. The nanowire synthesis/fabrication methods are introduced briefly at first, followed by discussions on the working principle and device performance of various types of nanowire array‐based photodetectors and their emerging applications. Finally, we analyze the challenges and present the perspectives for the development of future low‐cost, large‐scale, high‐performance nanowire array infrared photodetectors for practical applications.

About Some Fundamental Aspects of the Growth Mechanism Vapor-Liquid-Solid Nanowires

This study provides the formation of semiconductor nanowires (NWs) with a singular facet and a curved end surface by the vapor-liquid-solid (VLS) process that is analyzed and explained in details. Given the evidence, it is confirmed that the wettability of a liquid catalyst droplet on a crystal surface and the contact angle between the droplet and crystal play an essential role in the VLS process of NWs development. It is shown that for the VLS mechanism, the formation of NWs depends on the reduction in activation barrier to crystallization caused by the release of surplus-free energy by a spheroidizing drop in the region of the triple junction during the process of lowering surface area. This decreases the necessary supersaturation for the development of NW vertex facets at a fixed growth rate. The source of the extra free energy that drives the catalyst droplet movement during the steady-state development of NWs is the droplet’s outer surface. During the formation of NWs, those angles of inclination of the lateral surface NWs and droplet contact are obtained at which the solid/vapor, solid/liquid, and liquid/vapor interfaces experience the smallest increase in free energy. The wetting hysteresis is demonstrated to occur at the vertex of NWs, and the contact angle of a catalyst droplet may be regarded as an independent and fully-fledged thermodynamic parameter of the system’s state.

Terahertz nanofuse by a single nanowire-combined nanoantenna

Abstract We propose a terahertz nanofuse through irreversible modulations in transmitted terahertz using nanowires-combined nanoantenna structures. Semiconductor and metal nanowires show irreversible reconfiguration in their geometry at an incident field of 20 kV/cm. The concept can be explained by terahertz-field-induced ionization or electromigration. A strongly localized field due to geometrical conditions causes a floated metal nanowire from one side of a metal nanoantenna to touch the opposite side, bridging two separate metal plates and creating a junction. For the bridged nanoantenna, the highly enhanced field induced the breaking of the connection across the metal sides of a nanoantenna. In the bridging and breaking cases, permanent transformation occurs in opposite structural forms. It encompasses a potential application as an optical fuse to protect sensitive terahertz devices under excessive field focus.