InAs Nanowires Research Articles

Proximity effect and interface transparency in Al/InAs-nanowire/Al diffusive junctions

We investigate the proximity effect in InAs nanowire (NW) junctions with superconducting contacts made of Al. The carrier density in InAs is tuned by means of the back gate voltage Vg. At high positive Vg the devices feature transport signatures characteristic of diffusive junctions with highly transparent interfaces—sizable excess current, re-entrant resistance effect and proximity gap values () close to the Al gap (). At decreasing Vg, we observe a reduction of the proximity gap down to at NW conductances , which is interpreted in terms of carrier density dependent reduction of the Al/InAs interface transparency. We demonstrate that the experimental behavior of is closely reproduced by a model with rectangular potential barrier at the Al/InAs interface.

Influence of growth parameters on In-droplet-assisted growth of InAs nanowires on silicon

The influence of growth parameters on the morphology and density of InAs nanowires (NWs) grown on bare Si substrates using Indium (In) droplets as catalyst is investigated. By tuning the growth temperature, V/III flux ratio, and growth rate, the diameter and yield of as-grown NWs were controllably manipulated. It is demonstrated that the In-droplet-assisted growth of InAs NWs can only be realized on bare Si within a relatively narrow growth window of 420–475 °C. Below 420 °C, NWs’ growth is kinetically limited, while the highest yield of vertically aligned NWs was obtained at ~450 °C. It is shown that In-catalyzed InAs NWs nucleation can only be realized on Si at highly As-rich conditions (V/III flux ratio >50), while the axial growth rate was found to be strongly dependent on the V/III flux ratio. The nucleation and axial growth of In-catalyzed InAs NWs are promoted by a low growth rate, while a high growth rate favors the formation of unwanted parasitic islands.

Critical Temperature for the Conversion from Wurtzite to Zincblende of the Optical Emission of InAs Nanowires

One hour annealing at 300 °C changes the optical emission characteristics of InAs nanowires (NWs) from the wurtzite (WZ) phase into that of zincblende (ZB). These results are accounted for by the conversion of a small fraction of the NW WZ metastable structure into the stable ZB structure. Several paths toward the polytype transformation in the configuration space are also demonstrated using first-principles calculations. For lower annealing temperatures, emission which is likely related to WZ polytypes is observed at energies that agree with theoretical predictions. These results demonstrate severe constraints on thermal processes to which devices made from InAs WZ NWs can be exposed.

Sensing Responses Based on Transfer Characteristics of InAs Nanowire Field-Effect Transistors.

Nanowire-based field-effect transistors (FETs) have demonstrated considerable promise for a new generation of chemical and biological sensors. Indium arsenide (InAs), by virtue of its high electron mobility and intrinsic surface accumulation layer of electrons, holds properties beneficial for creating high performance sensors that can be used in applications such as point-of-care testing for patients diagnosed with chronic diseases. Here, we propose devices based on a parallel configuration of InAs nanowires and investigate sensor responses from measurements of conductance over time and FET characteristics. The devices were tested in controlled concentrations of vapour containing acetic acid, 2-butanone and methanol. After adsorption of analyte molecules, trends in the transient current and transfer curves are correlated with the nature of the surface interaction. Specifically, we observed proportionality between acetic acid concentration and relative conductance change, off current and surface charge density extracted from subthreshold behaviour. We suggest the origin of the sensing response to acetic acid as a two-part, reversible acid-base and redox reaction between acetic acid, InAs and its native oxide that forms slow, donor-like states at the nanowire surface. We further describe a simple model that is able to distinguish the occurrence of physical versus chemical adsorption by comparing the values of the extracted surface charge density. These studies demonstrate that InAs nanowires can produce a multitude of sensor responses for the purpose of developing next generation, multi-dimensional sensor applications.

Fermi energy dependence of the optical emission in core/shell InAs nanowire homostructures

InAs nanowires grown by vapor–liquid–solid (VLS) method are investigated by photoluminescence. We observe that the Fermi energy of all samples is reduced by ∼20 meV when the size of the Au nanoparticle used for catalysis is increased from 5 to 20 nm. Additional capping with a thin InP shell enhances the optical emission and does not affect the Fermi energy. The unexpected behavior of the Fermi energy is attributed to the differences in the residual donor (likely carbon) incorporation in the axial (low) and lateral (high incorporation) growth in the VLS and vapor–solid (VS) methods, respectively. The different impurity incorporation rate in these two regions leads to a core/shell InAs homostructure. In this case, the minority carriers (holes) diffuse to the core due to the built-in electric field created by the radial impurity distribution. As a result, the optical emission is dominated by the core region rather than by the more heavily doped InAs shell. Thus, the photoluminescence spectra and the Fermi energy become sensitive to the core diameter. These results are corroborated by a theoretical model using a self-consistent method to calculate the radial carrier distribution and Fermi energy for distinct diameters of Au nanoparticles.

Horizontal InAs nanowire transistors grown on patterned silicon-on-insulator substrate**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA02005003) and the National Natural Science Foundation of China (Grant Nos. 61376096 and 61327813).

High-density horizontal InAs nanowire transistors are fabricated on the interdigital silicon-on-insulator substrate. Hexagonal InAs nanowires are uniformly grown between face-to-face (111) vertical sidewalls of neighboring Si fingers by metal–organic chemical vapor deposition. The density of InAs nanowires is high up to 32 per group of silicon fingers, namely an average of 4 nanowires per micrometer. The electrical characteristics with a higher on/off current ratio of 2 × 105 are obtained at room temperature. The silicon-based horizontal InAs nanowire transistors are very promising for future high-performance circuits.

Features of electron gas in InAs nanowires imposed by interplay between nanowire geometry, doping and surface states

We present a study of electron gas properties in InAs nanowires determined by interaction between nanowire geometry, doping and surface states. The electron gas density and space distribution are calculated via self-consistent solution of coupled Schroedinger and Poisson equations in the nanowires with a hexagonal cross-section. We show that the density of surface states and the nanowire width define the spatial distribution of the electrons. Three configurations can be distinguished, namely the electrons are localized in the center of the wire, or they are arranged in a uniform tubular distribution, or finally in a tubular distribution with additional electron accumulation at the corners of the nanowire. The latter one is dominating for most experimentally obtained nanowires. N-type doping partly suppresses electron accumulation at the nanowire corners. The electron density calculated for both, various nanowire widths and different positions of the Fermi level at the nanowire surface, is compared with the experimental data for intrinsic InAs nanowires. Suitable agreement is obtained by assuming a Fermi level pinning at 60 to 100 meV above the conduction band edge, leading to a tubular electron distribution with accumulation along the corners of the nanowire.

Tilting Catalyst-Free InAs Nanowires by 3D-Twinning and Unusual Growth Directions

Controlling the growth direction of nanowires is of strategic importance both for applications where nanowire arrays are contacted in parallel and for the formation of more complex nanowire networks. We report on the existence of tilted InAs nanowires on (111)B GaAs. The tilted direction is predominantly the result of a three-dimensional twinning phenomenon at the initial stages of growth, so far only observed in VLS growth. We also find some nanowires growing in ⟨112⟩ and other directions. We further demonstrate how the tilting of nanowires can be engineered by modifying the growth conditions, and outline the procedures to achieve fully vertical or tilted nanowire ensembles. Conditions leading to a high density of tilted nanowires also provide a way to grow nanoscale crosses. This work opens the path toward achieving control over nanowire structures and related hierarchical structures.

Ten-Fold Enhancement of InAs Nanowire Photoluminescence Emission with an InP Passivation Layer.

In this Letter, we demonstrate that a significant improvement of optical performance of InAs nanowires can be achieved by capping the core InAs nanowires with a thin InP shell, which successfully passivates the surface states reducing the rate of nonradiative recombination. The improvements have been confirmed by detailed photoluminescence measurements, which showed up to a 10-fold increase in the intensity of room-temperature photoluminescence from the capped InAs/InP nanowires compared to the sample with core-only InAs nanowires. Moreover, the nanowires exhibit a high stability of total photoluminescence emission strength across temperature range from 10 to 300 K as a result of strong quantum confinement. These findings could be the key to successful implementation of InAs nanowires into optoelectronic devices.

Manipulating III–V Nanowire Transistor Performance via Surface Decoration of Metal‐Oxide Nanoparticles

Recently, III–V semiconductor nanowires (NWs) are widely investigated as field‐effect transistors (FETs) for high‐performance electronics, optoelectronic, and others; nevertheless, effective control in their device performances, especially the threshold voltage is still not well attained, which can potentially limit their practical uses for technological applications. This study reports a simple but highly reliable metal‐oxide nanoparticle (NP) surface decoration approach onto the device channel in order to manipulate electrical characteristics of III–V NWFETs, such as the threshold voltage and transistor operation, through the manipulation of free electrons in the NW channel (i.e., InAs, InP, and In0.7Ga0.3As) via depositing various metal‐oxide NPs with different work functions. Without any passivation layer, this decoration approach can yield the stable NW device characteristics in ambient. Notably, the versatility of our decoration scheme has also been illustrated through the realization of high‐performance enhancement‐mode InAs NW‐paralleled‐arrayed devices as well as the configuration of highly efficient InAs NW NMOS inverters, comprising of both depletion and enhancement mode devices. All these results further elucidate the technological potential of this decoration approach for future high‐performance, low‐power nanoelectronic device fabrication, and circuit integration.

Space-charge-limited current in nanowires

Space-charge-limited current is often observed in semiconductor nanowires due to carrier depletion and reduced electrostatic screening. We present a numerical study on geometric scaling of the space-charge-limited current in nanowires, in comparison with the thin film and bulk geometries, using an n+-n-n+-model. The model highlights the effects of the surroundings for thin films and nanowires and shows that the dielectric properties of the semiconductor have a negligible effect on the space-charge-limited transport for small dimensions. The distribution of equilibrium and injected charge concentration vary as the semiconductor dimensionality is reduced. For low doping, the ohmic current is controlled by charge diffusion from degenerate contacts rather than by the nanowire impurity concentration. The results of numerical calculations agree with a simple capacitance formalism which assumes a uniform charge distribution along the nanowire, and experimental measurements for InAs nanowires confirm these results. The numerical model also predicts that an asymmetric nanowire contact geometry can enhance or limit charge injection.

An open-source platform to study uniaxial stress effects on nanoscale devices.

We present an automatic measurement platform that enables the characterization of nanodevices by electrical transport and optical spectroscopy as a function of the uniaxial stress. We provide insights into and detailed descriptions of the mechanical device, the substrate design and fabrication, and the instrument control software, which is provided under open-source license. The capability of the platform is demonstrated by characterizing the piezo-resistance of an InAs nanowire device using a combination of electrical transport and Raman spectroscopy. The advantages of this measurement platform are highlighted by comparison with state-of-the-art piezo-resistance measurements in InAs nanowires. We envision that the systematic application of this methodology will provide new insights into the physics of nanoscale devices and novel materials for electronics, and thus contribute to the assessment of the potential of strain as a technology booster for nanoscale electronics.

Focused ion beam supported growth of monocrystalline wurtzite InAs nanowires grown by molecular beam epitaxy

We investigate monocrystalline InAs nanowires (NWs) which are grown catalyst assisted by molecular beam epitaxy (MBE) and create the catalyst by focused ion beam (FIB) implanted Au spots. With this combination of methods an aspect ratio, i.e. the length to width ratio, of the grown NWs up to 300 was achieved. To control the morphology and crystalline structure of the NWs, the growth parameters like temperature, flux ratios and implantation fluence are varied and optimized. Furthermore, the influence of the used molecular arsenic species, in particular the As2 to As4 ratio, is investigated and adjusted. In addition to the high aspect ratio, this optimization results in the growth of monocrystalline InAs NWs with a negligible number of stacking faults. Single NWs were placed site-controlled by FIB implantation, which supplements the working field of area growth.

Optimization of self-catalyzed InAs Nanowires on flexible graphite for photovoltaic infrared photodetectors

The recent discovery of flexible graphene monolayers has triggered extensive research interest for the development of III-V/graphene functional hybrid heterostructures. In order to fully exploit their enormous potential in device applications, it is essential to optimize epitaxial growth for the precise control of nanowire geometry and density. Herein, we present a comprehensive growth study of InAs nanowires on graphitic substrates by molecular beam epitaxy. Vertically well-aligned and thin InAs nanowires with high yield were obtained in a narrow growth temperature window of 420–450 °C within a restricted domain of growth rate and V/III flux ratio. The graphitic substrates enable high nanowire growth rates, which is favourable for cost-effective device fabrication. A relatively low density of defects was observed. We have also demonstrated InAs-NWs/graphite heterojunction devices exhibiting rectifying behaviour. Room temperature photovoltaic response with a cut-off wavelength of 3.4 μm was demonstrated. This elucidates a promising route towards the monolithic integration of InAs nanowires with graphite for flexible and functional hybrid devices.

Manipulating Surface States of III-V Nanowires with Uniaxial Stress.

III-V compound semiconductors are indispensable materials for today's high-end electronic and optoelectronic devices and are being explored for next-generation transistor logic and quantum technologies. III-V surfaces and interfaces play the leading role in determining device performance, and therefore, methods to control their electronic properties have been developed. Typically, surface passivation studies demonstrated how to limit the density of surface states. Strain has been widely used to improve the electronic transport properties and optoelectronic properties of III-Vs, but the potential of this technology to modify the surface properties still remains to be explored. Here we show that uniaxial stress induces a shift in the energy of the surface states of III-V nanowires, modifying their electronic properties. We demonstrate this phenomenon by modulating the conductivity of InAs nanowires over 4 orders of magnitude with axial strain ranging between -2.5% in compression and 2.1% in tension. The band bending at the surface of the nanostructure is modified from accumulation to depletion reversibly and reproducibly. We provide evidence of this physical effect using a combination of electrical transport measurement, Raman spectroscopy, band-structure modeling, and technology computer aided design (TCAD) simulations. With this methodology, the deformation potentials for the surface states are quantified. These results reveal that strain technology can be used to shift surface states away from energy ranges in which device performance is negatively affected and represent a novel route to engineer the electronic properties of III-V devices.

Magnetically-driven colossal supercurrent enhancement in InAs nanowire Josephson junctions

The Josephson effect is a fundamental quantum phenomenon where a dissipationless supercurrent is introduced in a weak link between two superconducting electrodes by Andreev reflections. The physical details and topology of the junction drastically modify the properties of the supercurrent and a strong enhancement of the critical supercurrent is expected to occur when the topology of the junction allows an emergence of Majorana bound states. Here we report charge transport measurements in mesoscopic Josephson junctions formed by InAs nanowires and Ti/Al superconducting leads. Our main observation is a colossal enhancement of the critical supercurrent induced by an external magnetic field applied perpendicular to the substrate. This striking and anomalous supercurrent enhancement cannot be described by any known conventional phenomenon of Josephson junctions. We consider these results in the context of topological superconductivity, and show that the observed critical supercurrent enhancement is compatible with a magnetic field-induced topological transition.

Unified Compact Model for Nanowire Transistors Including Quantum Effects and Quasi-Ballistic Transport

We present a surface potential-based compact model for nanowire FETs, which considers 1-D electrostatics along with the effect of multiple energy subbands. The model is valid for any semiconductor material, cross-sectional geometry, and any channel length with transport regimes varying from drift-diffusive to quasi-ballistic. The model captures the phenomenon of quantum capacitance and the effect of temperature. We have validated it with numerical simulations and experimental data for Si, Ge, and InAs nanowires of different geometries. Circuit simulation has also been performed with the model. The physics-based model is accurate and can be used as a tool for analysis and prediction of the effects of geometry scaling, material dependence, and temperature variation on device and circuit characteristics. To the best of our knowledge, this is the first time a compact model for nanowire FETs is being presented, which includes multiple subbands along with geometry scaling while being valid for different degenerate and nondegenerate semiconductor materials.

Ballistic One-Dimensional InAs Nanowire Cross-Junction Interconnects.

Coherent interconnection of quantum bits remains an ongoing challenge in quantum information technology. Envisioned hardware to achieve this goal is based on semiconductor nanowire (NW) circuits, comprising individual NW devices that are linked through ballistic interconnects. However, maintaining the sensitive ballistic conduction and confinement conditions across NW intersections is a nontrivial problem. Here, we go beyond the characterization of a single NW device and demonstrate ballistic one-dimensional (1D) quantum transport in InAs NW cross-junctions, monolithically integrated on Si. Characteristic 1D conductance plateaus are resolved in field-effect measurements across up to four NW-junctions in series. The 1D ballistic transport and sub-band splitting is preserved for both crossing-directions. We show that the 1D modes of a single injection terminal can be distributed into multiple NW branches. We believe that NW cross-junctions are well-suited as cross-directional communication links for the reliable transfer of quantum information as required for quantum computational systems.

Dynamic properties of III–V polytypes from density-functional theory

The recently discovered hexagonal wurtzite phase of several III–V nanowires opens up strong opportunity to engineer optoelectronic and transport properties of III–V materials. Herein, we explore the dynamical and dielectric properties of cubic (3C) and wurtzite (2H) III–V compounds (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, and InSb). For cubic III–V compounds, our calculated phonon frequencies agree well with neutron diffraction and Raman-scattering measurements. In the case of 2H III–V materials, our calculated phonon modes at the zone-center Γ point are in distinguished agreement with available Raman-spectroscopy measurements of wurtzite GaAs, InP, GaP, and InAs nanowires. Particularly, the “fingerprint” of the wurtzite phase, which is our predicted E2(high) phonon mode, at 261 cm−1(GaAs), 308 cm−1(InP), 358 cm−1(GaP), and 214 cm−1(InAs) matches perfectly the respective Raman values of 258 cm−1, 306.4 cm−1, 353 cm−1, and 213.7 cm−1 for GaAs, InP, GaP, and InAs. Moreover, the dynamic charges and high-frequency dielectric constants are predicted for III–V materials in both cubic (3C) and hexagonal (2H) crystal polytypes. It is found that the dielectric properties of InAs and InSb contrast markedly from those of other 2H III–V compounds. Furthermore, InAs and InSb evidence relative strong anisotropy in their dielectric constants and Born effective charges, whereas GaP evinces the higher Born effective charge anisotropy of 2H III–V compounds.

Signatures of interaction-induced helical gaps in nanowire quantum point contacts

Spin-momentum locking in a semiconductor device with strong spin-orbit coupling (SOC) is a fundamental goal of nanoscale spintronics and an important prerequisite for the formation of Majorana bound states. Such a helical state is predicted in one-dimensional (1D) nanowires subject to strong Rashba SOC and spin-mixing, its hallmark being a characteristic reentrant behaviour in the conductance. Here, we report the first direct experimental observations of the reentrant conductance feature, which reveals the formation of a helical liquid, in the lowest 1D subband of an InAs nanowire. Surprisingly, the feature is very prominent also in the absence of magnetic fields. This behaviour suggests that exchange interaction exhibits substantial impact on transport in our device. We attribute the opening of the pseudogap to spin-flipping two-particle backscattering. The all-electric origin of the ideal helical transport bears momentous implications for topological quantum computing.