Individual InAs Research Articles

Automated Computer Vision-Enabled Manufacturing of Nanowire Devices.

We present a high-throughput method for identifying and characterizing individual nanowires and for automatically designing electrode patterns with high alignment accuracy. Central to our method is an optimized machine-readable, lithographically processable, and multi-scale fiducial marker system─dubbed LithoTag─which provides nanostructure position determination at the nanometer scale. A grid of uniquely defined LithoTag markers patterned across a substrate enables image alignment and mapping in 100% of a set of >9000 scanning electron microscopy (SEM) images (>7 gigapixels). Combining this automated SEM imaging with a computer vision algorithm yields location and property data for individual nanowires. Starting with a random arrangement of individual InAs nanowires with diameters of 30 ± 5 nm on a single chip, we automatically design and fabricate >200 single-nanowire devices. For >75% of devices, the positioning accuracy of the fabricated electrodes is within 2 pixels of the original microscopy image resolution. The presented LithoTag method enables automation of nanodevice processing and is agnostic to microscopy modality and nanostructure type. Such high-throughput experimental methodology coupled with data-extensive science can help overcome the characterization bottleneck and improve the yield of nanodevice fabrication, driving the development and applications of nanostructured materials.

Novel use of selectivity of resonance Raman spectroscopy to study polytypism and mixed to pure phase conversion in individual InAs NWs on laser irradiation

Conversion of Wurtzite to Zincblende phase of InAs NWs at high simulated temperature (>1300 K) is investigated using resonance Raman spectroscopy with specially designed two step heating–cooling experiment. For smaller nanowires (diameter < 700 nm), the certainty of determining the polytypism using polarized Raman spectroscopy, reduces significantly. Low signal to noise ratio for smaller diameter NWs is improved by employing the wavelength dependent unpolarized and polarized Raman spectroscopy to examine the sensitivity of resonance Raman spectroscopy as an alternate for probing polytypism. Innovative use of resonance Raman spectroscopy to probe phase purity of the III-V individual nanostructures, where, conventional methods may not be applicable is realized in this work.This work paves the way for online monitoring of the conversion of mixed phase to phase pure III-–V nanowires using laser irradiation for their fast electronic device applications. Further, this kind of surface modification study provides basis for the futuristic crystal structure engineering, which can facilitate the controlled tailoring of material properties in order to enhance their functionality.

Novel Use of Selectivity of Resonance Raman Spectroscopy to Study Polytypism and Mixed to Pure Phase Conversion in Individual InAs NWs on Laser Irradiation

Novel Use of Selectivity of Resonance Raman Spectroscopy to Study Polytypism and Mixed to Pure Phase Conversion in Individual InAs NWs on Laser Irradiation

Unveiling the detection dynamics of semiconductor nanowire photodetectors by terahertz near-field nanoscopy

Semiconductor nanowire field-effect transistors represent a promising platform for the development of room-temperature (RT) terahertz (THz) frequency light detectors due to the strong nonlinearity of their transfer characteristics and their remarkable combination of low noise-equivalent powers (<1 nW Hz−1/2) and high responsivities (>100 V/W). Nano-engineering an NW photodetector combining high sensitivity with high speed (sub-ns) in the THz regime at RT is highly desirable for many frontier applications in quantum optics and nanophotonics, but this requires a clear understanding of the origin of the photo-response. Conventional electrical and optical measurements, however, cannot unambiguously determine the dominant detection mechanism due to inherent device asymmetry that allows different processes to be simultaneously activated. Here, we innovatively capture snapshots of the photo-response of individual InAs nanowires via high spatial resolution (35 nm) THz photocurrent nanoscopy. By coupling a THz quantum cascade laser to scattering-type scanning near-field optical microscopy (s-SNOM) and monitoring both electrical and optical readouts, we simultaneously measure transport and scattering properties. The spatially resolved electric response provides unambiguous signatures of photo-thermoelectric and bolometric currents whose interplay is discussed as a function of photon density and material doping, therefore providing a route to engineer photo-responses by design.

Tracing the two- to three-dimensional transition in InAs/GaAs sub-monolayer quantum dot growth

A two- to three-dimensional (2D to 3D) transition in InAs/GaAs sub-monolayer quantum dot (SML-QD) growth was investigated, and its evolution was traced. From the AFM results, the SML-QDs disappeared gradually and plateaus around each island formed with increasing GaAs capping layer thickness (tCL) from 1.5 ML to 4.5 ML. The SML-QDs become fully covered with a GaAs capping layer, leading to a planarization process caused by strain relaxation. In addition, when the individual InAs coverage was is insufficient to cause a 3D transition, similar plateaus formed when stacking with an increasing number of stacking layers (S) from 2 to 6. On the other hand, as the InAs coverage was increased from 0.3 to 0.5 ML, 3D dots with a comparable size to typical SK-QDs were observed beyond S = 4. This means that as the total InAs deposition thickness increases above a critical thickness (~2.0 ML), 2D-small islands transform spontaneously into 3D-pyramid QDs. Such structures then show similar optical characteristics to SK-QDs. Overall, the transition from 2D to 3D in InAs/GaAs SML-QD growth could be controlled by careful choice of the parameters tcl, θ, and S.

Conductometric Sensing with Individual InAs Nanowires.

In this work, we isolate individual wurtzite InAs nanowires and fabricate electrical contacts at both ends, exploiting the single nanostructures as building blocks to realize two different architectures of conductometric sensors: (a) the nanowire is drop-casted onto—supported by—a SiO2/Si substrate, and (b) the nanowire is suspended at approximately 250 nm from the substrate. We test the source-drain current upon changes in the concentration of humidity, ethanol, and NO2, using synthetic air as a gas carrier, moving a step forward towards mimicking operational environmental conditions. The supported architecture shows higher response in the mid humidity range (50% relative humidity), with shorter response and recovery times and lower detection limit with respect to the suspended nanowire. These experimental pieces of evidence indicate a minor role of the InAs/SiO2 contact area; hence, there is no need for suspended nanostructures to improve the sensing performance. Moreover, the sensing capability of single InAs nanowires for detection of NO2 and ethanol in the ambient atmosphere is reported and discussed.

Crossover from Coulomb blockade to ballistic transport in InAs nanowire devices

We report on the observation of a crossover from the single electron Coulomb blockade regime to the ballistic transport in individual InAs semiconducting nanowire devices. The InAs nanowires studied here were grown by molecular-beam epitaxy (MBE), which provides a clean system to study the intrinsic electrons transport in a quasi-one-dimensional system. Quantized conductance plateaus are observed for an InAs nanowire-based device by changing the Fermi level with a global back gate at low temperature, suggesting the ballistic transport of electrons. Further lowering the temperature, we observe the Coulomb blockade phenomenon with the formation of the quantum dot between the two normal metal contacts. By increasing the electron density, the characteristic Fabry–Pérot oscillations are observed, which further provides evidence for the ballistic nature of transport in the InAs nanowire device. Our observations indicate that high-quality InAs nanowires grown by MBE behave as clean quantum wires at low temperatures, which enables us to investigate novel phenomena in the quasi-one-dimensional system.

Correlation between Electrical Transport and Nanoscale Strain in InAs/In0.6Ga0.4As Core-Shell Nanowires.

Free-standing semiconductor nanowires constitute an ideal material system for the direct manipulation of electrical and optical properties by strain engineering. In this study, we present a direct quantitative correlation between electrical conductivity and nanoscale lattice strain of individual InAs nanowires passivated with a thin epitaxial In0.6Ga0.4As shell. With an in situ electron microscopy electromechanical testing technique, we show that the piezoresistive response of the nanowires is greatly enhanced compared to bulk InAs, and that uniaxial elastic strain leads to increased conductivity, which can be explained by a strain-induced reduction in the band gap. In addition, we observe inhomogeneity in strain distribution, which could have a reverse effect on the conductivity by increasing the scattering of charge carriers. These results provide a direct correlation of nanoscale mechanical strain and electrical transport properties in free-standing nanostructures.

Manipulation of polarization anisotropy in bare InAs and InAs/GaSb core-shell nanowires

In this article, we compare the excitation wavelength dependence of the polarization anisotropy (ρ) of an internal field induced Raman scattering signal in individual bare InAs and InAs/GaSb core-shell nanowires. The measured value of ρ of the Raman scattering intensity for InAs/GaSb core-shell nanowires has a minimum at ∼500 nm, while for the bare InAs nanowire, the value of ρ monotonically increases over the same range of wavelengths. We have modeled the scattering intensities of both systems by considering the joint role of Raman tensor components and confinement of electromagnetic radiation inside the nanowire at two orthogonal polarization configurations of the electromagnetic radiation. The theoretical results allow us to understand that the observed behavior of ρ is related to the nanowire geometry and to the difference in the wavelength dependence of the dielectric constants of InAs and GaSb. This work shows the possibility of manipulating the polarization anisotropy by selecting suitable diameters and materials for the core and the shell of the nanowire. We also report a six-fold increase in Raman scattering intensity due to the GaSb shell on InAs nanowires.

Static strain tuning of quantum dots embedded in a photonic wire

We use strain to statically tune the semiconductor band gap of individual InAs quantum dots (QDs) embedded in a GaAs photonic wire featuring very efficient single photon collection. Thanks to the geometry of the structure, we are able to shift the QD excitonic transition by more than 25 meV by using nano-manipulators to apply the stress. Moreover, owing to the strong transverse strain gradient generated in the structure, we can relatively tune two QDs located in the wire waveguide and bring them in resonance, opening the way to the observation of collective effects such as superradiance.

Micro-Raman spectroscopy for the detection of stacking fault density in InAs and GaAs nanowires

We investigate the relation between crystal stacking faults in individual wurtzite InAs and GaAs nanowires and the intensity of the forbidden longitudinal optical (LO) phonon mode in the Raman spectra. Micro-Raman spectroscopy and transmission electron microscopy are combined on the same individual nanowires to evaluate the LO mode intensity as a function of the stacking fault density. A clear increase in the LO mode intensity was observed when the stacking fault density was increased. Our results confirm the utility of Raman spectroscopy as a powerful tool for detecting crystal defects in nanowires.

Current–phase relations of few-mode InAs nanowire Josephson junctions

Gate-tunable semiconductor nanowires with superconducting leads have great potential for quantum computation and as model systems for mesoscopic Josephson junctions. The supercurrent, $I$, versus the phase, $\phi$, across the junction is called the current-phase relation (CPR). It can reveal not only the amplitude of the critical current, but also the number of modes and their transmission. We measured the CPR of many individual InAs nanowire Josephson junctions, one junction at a time. Both the amplitude and shape of the CPR varied between junctions, with small critical currents and skewed CPRs indicating few-mode junctions with high transmissions. In a gate-tunable junction, we found that the CPR varied with gate voltage: Near the onset of supercurrent, we observed behavior consistent with resonant tunneling through a single, highly transmitting mode. The gate dependence is consistent with modeled subband structure that includes an effective tunneling barrier due to an abrupt change in the Fermi level at the boundary of the gate-tuned region. These measurements of skewed, tunable, few-mode CPRs are promising both for applications that require anharmonic junctions and for Majorana readout proposals.

Coherent coupling of individual quantum dots measured with phase-referenced two-dimensional spectroscopy: Photon echo versus double quantum coherence

We employ two-dimensional (2D) coherent, nonlinear spectroscopy to investigate couplings within individual InAs quantum dots (QD) and QD molecules. Swapping pulse ordering in a two-beam sequence permits to distinguish between rephasing and non-rephasing four-wave mixing (FWM) configurations. We emphasize the non-rephasing case, allowing to monitor two-photon coherence dynamics. Respective Fourier transform yields a double quantum 2D FWM map, which is corroborated with its single quantum counterpart, originating from the rephasing sequence. We introduce referencing of the FWM phase with the one carried by the driving pulses, overcoming the necessity of its active-stabilization, as required in 2D spectroscopy. Combining single and double quantum 2D FWM, provides a pertinent tool in detecting and ascertaining coherent coupling mechanisms between individual quantum systems, as exemplified experimentally.

Switching from Negative to Positive Photoconductivity toward Intrinsic Photoelectric Response in InAs Nanowire

Negative photoconductivity (NPC) and positive photoconductivity (PPC) are observed in the same individual InAs nanowires grown by metal-organic chemical vapor deposition. NPC displays under weak light illumination due to photoexcitation scattering centers charged with hot carrier in the native oxide layer. PPC is observed under high light intensity. Through removing the native oxide layer and passivating the nanowire with HfO2, we eliminate the NPC effect and realize intrinsic photoelectric response in InAs nanowire.

Study on fracture behavior of individual InAs nanowires using an electron-beam-drilled notch

The mechanical properties and fracture behavior of individual InAs nanowires (NWs) were investigated under uniaxial tensile loading in a transmission electron microscope.

Anisotropic nature of hole g‐factor in individual InAs quantum rings

AbstractThe in‐plane g‐factors of electron and hole spins (, ) confined in the individual InAs/GaAs quantum rings (QRs) were investigated by using experimental and theoretical approaches. From the measurements, we found that the experimentally obtained varies largely from QR to QR, while the variation in is small. In addition, the in‐plane () and the out‐of‐plane () anisotropies in hole g‐factor were obviously confirmed while the electron g‐factor exhibits isotropic natures in both cases. From the model calculations, the effects of the shape anisotropies and the uniaxial stress were examined. The shape anisotropy in QRs modifies the spatial distributions of hole wavefunctions. Thus, it brings the resultant changes in the degree of valence‐band mixing and , and combined with uniaxial stress, a larger modulation in was achieved. Although more detailed discussions are necessary at this stage, our findings will give valuable information for the g‐factor control in semiconductor nanostructures.

(Invited) The Structure-Dependent Properties of InAs Nanowires and Their Devices

InAs nanowire is an important III-V group semiconducting nanomaterial with direct and narrow bandgap and small effective electron mass. It has great potential in high-performance electronic nanodevices and infrared detectors. In this talk, I will introduce our recent progresses on the structure-dependent properties of InAs nanowires and their devices. Various properties of individual InAs nanowires are studied and correlated with their structures through a comprehensive in situ platform we have developed in a scanning electron microscope. Remarkable piezoelectric and piezoresistive effects are observed for the first time in single-crystalline <0001> WZ InAs nanowires, but negligible effect is found in WZ nanowires with other orientations and nanowires with ZB structures. An ultrafast and reversible electrochemical lithiation of InAs nanowires are observed for the first time. Field effect transistors (FETs) based on MBE-grown InAs nanowires are studied. Importantly, based on nano-manipulation and microlithography, we develop a new method to characterize the same individual InAs nanowires in the FETs by transmission electron microscopy to obtain the atomic-level structures. Our study shows that not only the diameter, but also the crystal structure and the orientation of the InAs nanowires remarkably affect the electronic properties of the FETs, such as the threshold voltage, On/OFF current ratio, on-state current, subthreshold swing and electron mobility, etc. Ohmic contact are obtained for the nanowires with different diameters down to 7 nm.

Dynamics of excitons in individual InAs quantum dots revealed in four-wave mixing spectroscopy

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.

Crystal Phase Transformation in Self-Assembled InAs Nanowire Junctions on Patterned Si Substrates.

We demonstrate the growth and structural characteristics of InAs nanowire junctions evidencing a transformation of the crystalline structure. The junctions are obtained without the use of catalyst particles. Morphological investigations of the junctions reveal three structures having an L-, T-, and X-shape. The formation mechanisms of these structures have been identified. The NW junctions reveal large sections of zinc blende crystal structure free of extended defects, despite the high stacking fault density obtained in individual InAs nanowires. This segment of zinc blende crystal structure in the junction is associated with a crystal phase transformation involving sets of Shockley partial dislocations; the transformation takes place solely in the crystal phase. A model is developed to demonstrate that only the zinc blende phase with the same orientation as the substrate can result in monocrystalline junctions. The suitability of the junctions to be used in nanoelectronic devices is confirmed by room-temperature electrical experiments.

Diamagnetic and paramagnetic shifts in self-assembled InAs lateral quantum dot molecules

We uncover the underlying physics that explains the energy shifts of discrete states of individual InAs lateral quantum dot molecules (LQDMs) as a function of magnetic fields applied in the Faraday geometry. We observe that ground states of the LQDM exhibit a diamagnetic shift while excited states exhibit a paramagnetic shift. We explain the physical origin of the transition between these two behaviors by analyzing the molecular exciton states with effective mass calculations. We find that charge carriers in delocalized molecular states can become localized in single QDs with increasing magnetic field. We further show that the net effects of broken symmetry of the molecule and Coulomb correlation lead to the paramagnetic response.