Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy

Improving the electrical properties of InAs nanowire field effect transistors by covering them with Y2O3/HfO2 layers

Quasi-one-dimensional semiconducting materials have attracted increasing attention due to their excellent ability to downscale the size of transistors. However, in quasi-one-dimensional nanowire (NW) transistors, their surface and interface properties play a very important role mainly due to the large surface-to-volume ratio of NWs and surface scattering, which degrade their carrier mobility. Herein, we developed a new method to cover the channel surface of InAs NW field effect transistors (FETs) with Y₂O₃/HfO₂ layers to improve their electrical properties. We successfully fabricated nine FETs and measured their electrical properties, which improve after depositing the Y₂O₃/HfO₂ layers, including an increase in on-state current, decrease in off-state current, increase in transconductance, increase in electron mobility and decrease in subthreshold swing. By comparing the properties of Y₂O₃/HfO₂-covered devices with that of the FETs fabricated without the Y₂O₃ covering or without annealing, we prove that it is the combined Y₂O₃/HfO₂ layers instead of only the Y₂O₃ or HfO₂ layer that improve the electrical properties of the FETs. The Cs-corrected high-resolution scanning transmission electron microscopy study demonstrates that Y can actually diffuse through the native oxide layer (confirmed to be InO_x) and reach the surface of the InAs NWs. Our results indicate that the desirable characteristics of Y₂O₃ and the surface passivation by HfO₂ improve the electrical properties of the InAs NW FETs, in which Y₂O₃ plays an important role to modify and stabilize the interface between the InAs NWs and the outside dielectric layer. Furthermore, this method should also be applicable to other III-V materials.

Suppressing the excess OFF-state current of short-channel InAs nanowire field-effect transistors by nanoscale partial-gate

The excess OFF-state current caused by band to band tunneling (BTBT) is a serious issue particularly in short-channel nanowire (NW) field-effect transistors (FETs), especially for narrow bandgap semiconductors such as InAs. Here, to clarify the components of the OFF-current and suppress the OFF-current, we for the first time fabricate and study InAs NW FETs with nanoscale partial-gate (PG). We fabricate a series of PGFETs and a normal full-gate (FG) FET on the same NW. Based on our results, the BTBT current component can reach tens of nanoamps in a typical 250 nm-channel InAs NW FGFET, and dominate the OFF-current. In contrast, there is almost no BTBT component in the PGFET, which provides a reference for other short-channel InAs NW FETs. Furthermore, the physical mechanism of the OFF-state carrier transport is discussed, and both electrons and holes currents are proven to be very important, based on our experimental results. Also, through statistic study, we find the BTBT effect can be more serious in the devices with better gate-control. Therefore, suppressing the BTBT effect is important to the future scaling-down.

Group-IVA element-doped SrIn2O4 as potential materials for hydrogen production from water splitting with solar energy

Band gap engineering can efficiently improve the photocatalytic activity of semiconductors for hydrogen generation from water splitting. Herein, we present a comprehensive investigation on the geometrical structures, electronic, optical, and potential photocatalytic properties and charge carrier mobility of pristine and group-IVA element-doped SrIn2O4 using first-principles density functional theory with the meta-GGA+MBJ potential. The calculated formation energies are moderate, indicating that the synthesis of the doped structures is experimentally feasible. In addition, the energy band gaps of the group-IVA element-doped SrIn2O4 range from 1.67 to 3.07 eV, which satisfy the requirements for photocatalytic water splitting, except for that of the Si mono-doped structure. Based on the deformation potential theory, a high charge carrier mobility of 2093 cm2 V-1 s-1 is obtained for the pristine SrIn2O4 and those of the doped-structures are also large, although a decrease in the values of some are observed. The optical absorption coefficient of the doped structures in the near ultraviolet (UV) and visible light range significantly increases. Therefore, group-IVA element-doped SrIn2O4 are potential candidates as photocatalysts for hydrogen generation from water splitting driven by visible light.

Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI₃ (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI₃. While MAPbI₃ has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI₃ vary smoothly over a range of 100-160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI₃ exhibits a clear T^-0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron-phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T^-1.5 dependence previously observed for MAPbI₃. Finally, we directly observe intraexcitonic transitions in FAPbI₃ at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

Nanowire network-based multifunctional all-optical logic gates

All-optical nanoscale logic components are highly desired for various applications because light may enable logic functions to be performed extremely quickly without the generation of heat and cross-talk. All-optical computing at nanoscale is therefore a promising alternative but requires the development of a complete toolbox capable of various logic functionalities. We demonstrate nanoscale all-optical switches by exploiting the polarization-dependent light emission property of crossbar InP and AlGaAs nanowire networks. These networks can perform various logic operations, such as AND, OR, NAND, and NOR binary logic functions. Furthermore, on the basis of these logic operations, our networks successfully enable all-optical arithmetic binary calculations, such as n-bit addition, to be conducted. Our results underscore the promise of assembled semiconductor nanowire networks as a building block of on-chip all-optical logic components for future nanophotonics.

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires

Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires is studied. Based on the excitonic theory, the numerical method to calculate the photoconductivity spectrum in the nanowires is developed, which can simulate optical pump terahertz-probe spectroscopy measurements on real nanowires and thereby calculate the typical photoconductivity spectrum. With the help of the energetic structure deduced from the calculated linear absorption spectrum, the numerically observed shift of the resonant peak in the photoconductivity spectrum is found to result from the dominant exciton transition between excited or continuum states to the ground state, and the quantitative analysis is in good agreement with the quantum plasmon model. Besides, the dependence of the photoconductivity on the polarization of the terahertz field is also discussed. The numerical method and supporting theoretical analysis provide a new tool for experimentalists to understand the terahertz photoconductivity in intrinsic semiconductor nanowires at low temperatures or for nanowires subjected to below bandgap photoexcitation, where excitonic effects dominate.

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs_0.65Sb_0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs_0.65Sb_0.35 nanowires to date, exceeding 16,000 cm² V^-1 s^-1 at 10 K.

Photovoltaic Performance of a Nanowire/Quantum Dot Hybrid Nanostructure Array Solar Cell

An innovative solar cell based on a nanowire/quantum dot hybrid nanostructure array is designed and analyzed. By growing multilayer InAs quantum dots on the sidewalls of GaAs nanowires, not only the absorption spectrum of GaAs nanowires is extended by quantum dots but also the light absorption of quantum dots is dramatically enhanced due to the light-trapping effect of the nanowire array. By incorporating five layers of InAs quantum dots into a 500-nm high-GaAs nanowire array, the power conversion efficiency enhancement induced by the quantum dots is six times higher than the power conversion efficiency enhancement in thin-film solar cells which contain the same amount of quantum dots, indicating that the nanowire array structure can benefit the photovoltaic performance of quantum dot solar cells.

Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate

Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.

Self-Assembly Growth of In-Rich InGaAs Core-Shell Structured Nanowires with Remarkable Near-Infrared Photoresponsivity

Understanding the compositional distribution of ternary nanowires is essential to build the connection between nanowire structures and their potential applications. In this study, we grew epitaxial ternary InGaAs nanowires with high In concentration on GaAs {111}_B substrates. Our detailed electron microscopy characterizations suggest that the grown ternary InGaAs nanowires have an extraordinary core-shell structure with In-rich cores and Ga-enriched shells, in which both nanowire cores and shells showed compositional gradient. It was found that In-rich nanowire cores are formed due to the Ga-limited growth environment, caused by the competition with the spontaneous InGaAs planar layer growth on the substrate that consumes more Ga than the nominal Ga concentration during the growth. Moreover, the composition gradient in the nanowires cores and shells is a result of strain relaxation between them. Our optoelectronic property measurements from prototype nanowire devices show a remarkable photoresponsivity under the near-infrared illumination. This study provides a new approach for designing and realizing complex nanowire heterostructures for high-efficiency nanowire-based systems and devices.

In situ passivation of GaAsP nanowires

We report on the structural and optical properties of GaAsP nanowires (NWs) grown by molecular-beam epitaxy. By adjusting the alloy composition in the NWs, the transition energy was tuned to the optimal value required for tandem III-V/silicon solar cells. We discovered that an unintentional shell was also formed during the GaAsP NW growth. The NW surface was passivated by an in situ deposition of a radial Ga(As)P shell. Different shell compositions and thicknesses were investigated. We demonstrate that the optimal passivation conditions for GaAsP NWs (with a gap of 1.78 eV) are obtained with a 5 nm thick GaP shell. This passivation enhances the luminescence intensity of the NWs by 2 orders of magnitude and yields a longer luminescence decay. The luminescence dynamics changes from single exponential decay with a 4 ps characteristic time in non-passivated NWs to a bi-exponential decay with characteristic times of 85 and 540 ps in NWs with GaP shell passivation.

Quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires

A quantum plasmon model for the terahertz photoconductivity in intrinsic semiconductor nanowires is developed. The classical plasmon model assumes the excited electron in semiconductors feels a restoring force generated by a harmonic-oscillator potential. Although it is successfully applied to explain the terahertz photoconductivity in semiconductor nanowires, the classical treatment of the potential weakens accurate theoretical analysis. Here I treat the potential in a full quantum way and present an exact analytical formula for photoconductivity. The formula not only gives more reasonable photoconductivity, but also has the same conciseness when compared with that of the classical plasmon model. The validity of the quantum plasmon model is proved independently by numerical calculations in real space.

InP Nanowire Biosensor with Tailored Biofunctionalization: Ultrasensitive and Highly Selective Disease Biomarker Detection

Electrically active field-effect transistors (FET) based biosensors are of paramount importance in life science applications, as they offer direct, fast, and highly sensitive label-free detection capabilities of several biomolecules of specific interest. In this work, we report a detailed investigation on surface functionalization and covalent immobilization of biomarkers using biocompatible ethanolamine and poly(ethylene glycol) derivate coatings, as compared to the conventional approaches using silica monoliths, in order to substantially increase both the sensitivity and molecular selectivity of nanowire-based FET biosensor platforms. Quantitative fluorescence, atomic and Kelvin probe force microscopy allowed detailed investigation of the homogeneity and density of immobilized biomarkers on different biofunctionalized surfaces. Significantly enhanced binding specificity, biomarker density, and target biomolecule capture efficiency were thus achieved for DNA as well as for proteins from pathogens. This optimized functionalization methodology was applied to InP nanowires that due to their low surface recombination rates were used as new active transducers for biosensors. The developed devices provide ultrahigh label-free detection sensitivities ∼1 fM for specific DNA sequences, measured via the net change in device electrical resistance. Similar levels of ultrasensitive detection of ∼6 fM were achieved for a Chagas Disease protein marker (IBMP8-1). The developed InP nanowire biosensor provides thus a qualified tool for detection of the chronic infection stage of this disease, leading to improved diagnosis and control of spread. These methodological developments are expected to substantially enhance the chemical robustness, diagnostic reliability, detection sensitivity, and biomarker selectivity for current and future biosensing devices.

Temperature-tunable one-dimensional plasmonic photonic crystals based on a single graphene layer and a semiconductor constituent

We first investigate a graphene based 1D plasmonic photonic crystal (PPC) composed of a graphene sheet deposited on an SiO₂ grating whose grooves are filled with air by using finite-element method (FEM) software (COMSOL Multiphysics). The dispersion effect of SiO₂ is considered in the simulation, and we show that this effect significantly affects the transmission spectrum of the proposed PPC. The transmission spectrum shows a stop band in the mid-infrared region, which is blueshifted by increasing the Fermi energy level of the graphene sheet. However, the transmission spectrum is not affected by variation of the ambient temperature. To achieve a temperature-tunable 1D graphene-based PPC, we propose that the graphene sheet be placed on a grating composed of InAs semiconductor material. Our results confirm that the stop band in the proposed structure can be easily tuned with temperature and moves to higher frequencies by increasing the ambient temperature. Moreover, we introduce a defect into the temperature-tunable PPC to obtain a temperature-tunable Fabry-Perot microcavity. It is demonstrated that the resonance defect mode is easily controllable by changing the temperature and the Fermi energy level.

Optical Pump Rectification Emission: Route to Terahertz Free-Standing Surface Potential Diagnostics

We introduce a method for diagnosing the electric surface potential of a semiconductor based on THz surface generation. In our scheme, that we name Optical Pump Rectification Emission, a THz field is generated directly on the surface via surface optical rectification of an ultrashort pulse after which the DC surface potential is screened with a second optical pump pulse. As the THz generation directly relates to the surface potential arising from the surface states, we can then observe the temporal dynamics of the static surface field induced by the screening effect of the photo-carriers. Such an approach is potentially insensitive to bulk carrier dynamics and does not require special illumination geometries.

Dielectric properties of semi-insulating Fe-doped InP in the terahertz spectral region

We report the values and the spectral dependence of the real and imaginary parts of the dielectric permittivity of semi-insulating Fe-doped InP crystalline wafers in the 2-700 cm-1 (0.06-21 THz) spectral region at room temperature. The data shows a number of absorption bands that are assigned to one- and two-phonon and impurity-related absorption processes. Unlike the previous studies of undoped or low-doped InP material, our data unveil the dielectric properties of InP that are not screened by strong free-carrier absorption and will be useful for designing a wide variety of InP-based electronic and photonic devices operating in the terahertz spectral range.

Carrier Recombination Processes in Gallium Indium Phosphide Nanowires

Understanding of recombination and photoconductivity dynamics of photogenerated charge carriers in Ga_xIn_1-xP NWs is essential for their optoelectronic applications. In this letter, we have studied a series of Ga_xIn_1-xP NWs with varied Ga composition. Time-resolved photoinduced luminescence, femtosecond transient absorption, and time-resolved THz transmission measurements were performed to assess radiative and nonradiative recombination and photoconductivity dynamics of photogenerated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping, whereas electrons are highly mobile until they recombine nonradiatively. We also resolve gradual decrease of mobility of photogenerated electrons assigned to electron trapping and detrapping in a distribution of trap states. We identify that the nonradiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as nonradiative recombination become faster with increasing Ga composition in Ga_xIn_1-xP NWs. We have estimated early time electron mobility in Ga_xIn_1-xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley.

Scalable Indium Phosphide Thin-Film Nanophotonics Platform for Photovoltaic and Photoelectrochemical Devices

Recent developments in nanophotonics have provided a clear roadmap for improving the efficiency of photonic devices through control over absorption and emission of devices. These advances could prove transformative for a wide variety of devices, such as photovoltaics, photoelectrochemical devices, photodetectors, and light-emitting diodes. However, it is often challenging to physically create the nanophotonic designs required to engineer the optical properties of devices. Here, we present a platform based on crystalline indium phosphide that enables thin-film nanophotonic structures with physical morphologies that are impossible to achieve through conventional state-of-the-art material growth techniques. Here, nanostructured InP thin films have been demonstrated on non-epitaxial alumina inverted nanocone (i-cone) substrates via a low-cost and scalable thin-film vapor-liquid-solid growth technique. In this process, indium films are first evaporated onto the i-cone structures in the desired morphology, followed by a high-temperature step that causes a phase transformation of the indium into indium phosphide, preserving the original morphology of the deposited indium. Through this approach, a wide variety of nanostructured film morphologies are accessible using only control over evaporation process variables. Critically, the as-grown nanotextured InP thin films demonstrate excellent optoelectronic properties, suggesting this platform is promising for future high-performance nanophotonic devices.

An Ultrafast Switchable Terahertz Polarization Modulator Based on III-V Semiconductor Nanowires

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.

Radiation effects on GaAs/AlGaAs core/shell ensemble nanowires and nanowire infrared photodetectors

With the recent advances in nanowire (NW) growth and fabrication, there has been rapid development and application of GaAs NWs in optoelectronics. It is also of importance to study the radiation tolerance of optoelectronic nano-devices for atomic energy and space-based applications. Here, photoluminescence (PL) and time-resolved photoluminescence measurements were carried out on GaAs/AlGaAs core/shell NWs at room temperature before and after 1 MeV proton irradiation with fluences ranging from 1.0 × 10¹²-3.0 × 10¹³ cm^-2. It is found that the GaAs/AlGaAs core/shell NWs with smaller diameter show much less PL degradation compared with the ones with larger diameters. The increased radiation hardness is mainly attributed to the improvement of a room temperature dynamic-annealing mechanism near the surface of the NWs. We also found that the minority carrier lifetime is closely related to both the PL intensity and defect density induced by irradiation. Finally, GaAs/AlGaAs ensemble NW photodetectors operating in the near-infrared spectral regime have been demonstrated. The influence of proton irradiation on light and dark current characteristics also indicates that NW structures are a good potential candidate for radiation harsh-environment applications.

Bias-dependent spectral tuning in InP nanowire-based photodetectors

Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiO _x wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.

Synthesis and properties of ultra-long InP nanowires on glass

We report on the synthesis of Au-catalyzed InP nanowires (NWs) on low-cost glass substrates. Ultra-dense and ultra-long (up to ∼250 μm) InP NWs, with an exceptionally high growth rate of ∼25 μm min^-1, were grown directly on glass using metal organic vapor phase epitaxy (MOVPE). Structural properties of InP NWs grown on glass were similar to the ones grown typically on Si substrates showing many structural twin faults but the NWs on glass always exhibited a stronger photoluminescence (PL) intensity at room temperature. The PL measurements of NWs grown on glass reveal two additional prominent impurity related emission peaks at low temperature (10 K). In particular, the strongest unusual emission peak with an activation energy of 23.8 ± 2 meV was observed at 928 nm. Different possibilities including the role of native defects (phosphorus and/or indium vacancies) are discussed but most likely the origin of this PL peak is related to the impurity incorporation from the glass substrate. Furthermore, despite the presence of suspected impurities, the NWs on glass show outstanding light absorption in a wide spectral range (60%-95% for λ = 300-1600 nm). The optical properties and the NW growth mechanism on glass is discussed qualitatively. We attribute the exceptionally high growth rate mostly to the atmospheric pressure growth conditions of our MOVPE reactor and stronger PL intensity on glass due to the impurity doping. Overall, the III-V NWs grown on glass are similar to the ones grown on semiconductor substrates but offer additional advantages such as low-cost and light transparency.

Controlling the exciton emission of gold coated GaAs-AlGaAs core-shell nanowires with an organic spacer layer

Excitons are the most prominent optical excitations and controlling their emission is an important step towards new optical devices. We have investigated the exciton emission from uncoated and gold/aluminum quinoline (Alq₃) coated GaAs-AlGaAs-GaAs core-shell nanowires (NWs) using temperature-, intensity- and polarization dependent photoluminescence (PL). Plasmonic GaAs-AlGaAs-GaAs NWs with a ∼10 nm thick Au coating but without an Alq₃ spacer layer reveal a significant reduction of the PL intensity of the exciton emission compared with the uncoated NW sample. Plasmonic NW samples with the same nominal Au coverage and an additional Alq₃ interlayer of 3 or 6 nm thickness show a clearly stronger PL intensity which increases with rising Alq₃ spacer thickness. Time-resolved (TR) PL measurements reveal an increase of the exciton decay rate by a factor of up to two with decreasing Alq₃ spacer thickness suggesting the presence of Förster energy transfer from NW excitons to plasmon oscillations in the gold film. The weak change of the decay time, however, indicates that Förster energy-transfer is only partially responsible for the PL quenching in the gold coated NWs. The main reason for the reduction of the PL emission is attributed to a gold induced band-bending in the GaAs NW core which causes exciton dissociation. With increasing Alq₃ spacer thickness the band-bending decreases leading to a reduction of the exciton dissociation and PL quenching. Our interpretation is supported by electron energy loss spectroscopy measurements which show a signal reduction and blue shift of defect (possibly EL2) transitions when gold particles are deposited on NWs compared with bare or Alq₃ coated NWs.

Low temperature-grown GaAs carrier lifetime evaluation by double optical pump terahertz time-domain emission spectroscopy

We present the use of a "double optical pump" technique in terahertz time-domain emission spectroscopy as an alternative method to investigate the lifetime of photo-excited carriers in semiconductors. Compared to the commonly employed optical pump-probe transient photo-reflectance, this non-contact and room temperature characterization technique allows relative ease in achieving optical alignment. The technique was implemented to evaluate the carrier lifetime in low temperature-grown gallium arsenide (LT-GaAs). The carrier lifetime values deduced from "double optical pump" THz emission decay curves show good agreement with data obtained from standard transient photo-reflectance measurements on the same LT-GaAs samples grown at 250 °C and 310 °C.

Quenching of the luminescence intensity of GaN nanowires under electron beam exposure: impact of C adsorption on the exciton lifetime

Electron irradiation of GaN nanowires in a scanning electron microscope strongly reduces their luminous efficiency as shown by cathodoluminescence imaging and spectroscopy. We demonstrate that this luminescence quenching originates from a combination of charge trapping at already existing surface states and the formation of new surface states induced by the adsorption of C on the nanowire sidewalls. The interplay of these effects leads to a complex temporal evolution of the quenching, which strongly depends on the incident electron dose per area. Time-resolved photoluminescence measurements on electron-irradiated samples reveal that the carbonaceous adlayer affects both the nonradiative and the radiative recombination dynamics.

Electromechanical Properties and Spontaneous Response of the Current in InAsP Nanowires

The electromechanical properties of ternary InAsP nanowires (NWs) were investigated by applying a uniaxial tensile strain in a transmission electron microscope (TEM). The electromechanical properties in our examined InAsP NWs were governed by the piezoresistive effect. We found that the electronic transport of the InAsP NWs is dominated by space-charge-limited transport, with a I ∞ V² relation. Upon increasing the tensile strain, the electrical current in the NWs increases linearly, and the piezoresistance gradually decreases nonlinearly. By analyzing the space-charge-limited I-V curves, we show that the electromechanical response is due to a mobility that increases with strain. Finally, we use dynamical measurements to establish an upper limit on the time scale for the electromechanical response.

GaAs nanowires with oxidation-proof arsenic capping for the growth of an epitaxial shell

We propose an arsenic-capping/decapping method, allowing the growth of an epitaxial shell around the GaAs nanowire (NW) core which is exposed to an ambient atmosphere, and without the introduction of impurities. Self-catalyzed GaAs NW arrays were firstly grown on Si(111) substrates by solid-source molecular beam epitaxy. Aiming for protecting the active surface of the GaAs NW core, the arsenic-capping/decapping method has been applied. To validate the effect of this method, different core/shell NWs have been fabricated. Analyses highlight the benefit of the As capping-decapping method for further epitaxial shell growth: an epitaxial shell with a smooth surface is achieved in the case of As-capped-decapped GaAs NWs, comparable to the in situ grown GaAs/AlGaAs NWs. This As capping method opens a way for the epitaxial growth of heterogeneous material shells such as functional oxides using different reactors.

Broadband Phase-Sensitive Single InP Nanowire Photoconductive Terahertz Detectors

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (∼1260 cm(2) V(-1) s(-1)) and low dark current (∼10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.

Review on photonic properties of nanowires for photovoltaics

III-V semiconductor nanowires behave as optical antennae because of their shape anisotropy and high refractive index. The antennae like behavior modifies the absorption and emission properties of nanowires compared to planar materials. Nanowires absorb light more efficiently compared to an equivalent volume planar material, leading to higher short circuit current densities. The modified emission from the nanowires has the potential to increase the open circuit voltage from nanowire solar cells compared to planar solar cells. In order to achieve high efficiency nanowire solar cells it is essential to control the surface state density and doping in nanowires. We review the physics of nanowire solar cells and progress made in addressing the surface recombination and doping of nanowires, with emphasis on GaAs and InP materials.

Doping-enhanced radiative efficiency enables lasing in unpassivated GaAs nanowires

Nanolasers hold promise for applications including integrated photonics, on-chip optical interconnects and optical sensing. Key to the realization of current cavity designs is the use of nanomaterials combining high gain with high radiative efficiency. Until now, efforts to enhance the performance of semiconductor nanomaterials have focused on reducing the rate of non-radiative recombination through improvements to material quality and complex passivation schemes. Here we employ controlled impurity doping to increase the rate of radiative recombination. This unique approach enables us to improve the radiative efficiency of unpassivated GaAs nanowires by a factor of several hundred times while also increasing differential gain and reducing the transparency carrier density. In this way, we demonstrate lasing from a nanomaterial that combines high radiative efficiency with a picosecond carrier lifetime ready for high speed applications.

High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO2/BiVO4 Core/Shell Nanorod-Array Photoanodes

BiVO4 has become the top-performing semiconductor among photoanodes for photoelectrochemical water oxidation. However, BiVO4 photoanodes are still limited to a fraction of the theoretically possible photocurrent at low applied voltages because of modest charge transport properties and a trade-off between light absorption and charge separation efficiencies. Here, we investigate photoanodes composed of thin layers of BiVO4 coated onto Sb-doped SnO2 (Sb:SnO2) nanorod-arrays (Sb:SnO2/BiVO4 NRAs) and demonstrate a high value for the product of light absorption and charge separation efficiencies (ηabs × ηsep) of ∼51% at an applied voltage of 0.6 V versus the reversible hydrogen electrode, as determined by integration of the quantum efficiency over the standard AM 1.5G spectrum. To the best of our knowledge, this is one of the highest ηabs × ηsep efficiencies achieved to date at this voltage for nanowire-core/BiVO4-shell photoanodes. Moreover, although WO3 has recently been extensively studied as a core nanowire material for core/shell BiVO4 photoanodes, the Sb:SnO2/BiVO4 NRAs generate larger photocurrents, especially at low applied voltages. In addition, we present control experiments on planar Sb:SnO2/BiVO4 and WO3/BiVO4 heterojunctions, which indicate that Sb:SnO2 is more favorable as a core material. These results indicate that integration of Sb:SnO2 nanorod cores with other successful strategies such as doping and coating with oxygen evolution catalysts can move the performance of BiVO4 and related semiconductors closer to their theoretical potential.

Increased Photoconductivity Lifetime in GaAs Nanowires by Controlled n-Type and p-Type Doping

Controlled doping of GaAs nanowires is crucial for the development of nanowire-based electronic and optoelectronic devices. Here, we present a noncontact method based on time-resolved terahertz photoconductivity for assessing n- and p-type doping efficiency in nanowires. Using this technique, we measure extrinsic electron and hole concentrations in excess of 10(18) cm(-3) for GaAs nanowires with n-type and p-type doped shells. Furthermore, we show that controlled doping can significantly increase the photoconductivity lifetime of GaAs nanowires by over an order of magnitude: from 0.13 ns in undoped nanowires to 3.8 and 2.5 ns in n-doped and p-doped nanowires, respectively. Thus, controlled doping can be used to reduce the effects of parasitic surface recombination in optoelectronic nanowire devices, which is promising for nanowire devices, such as solar cells and nanowire lasers.

Hyperbolic metamaterial-based near-field thermophotovoltaic system for hundreds of nanometer vacuum gap

Artificially designed hyperbolic metamaterial (HMM) possesses extraordinary electromagnetic features different from those of naturally existing materials. In particular, the dispersion relation of waves existing inside the HMM is hyperbolic rather than elliptical; thus, waves that are evanescent in isotropic media become propagating in the HMM. This characteristic of HMMs opens a novel way to spectrally control the near-field thermal radiation in which evanescent waves in the vacuum gap play a critical role. In this paper, we theoretically investigate the performance of a near-field thermophotovoltaic (TPV) energy conversion system in which a W/SiO₂-multilayer-based HMM serves as the emitter at 1000 K and InAs works as the TPV cell at 300 K. By carefully designing the thickness of constituent materials of the HMM emitter, the electric power of the near-field TPV devices can be increased by about 6 times at 100-nm vacuum gap as compared to the case of the plain W emitter. Alternatively, in regards to the electric power generation, HMM emitter at experimentally achievable 100-nm vacuum gap performs equivalently to the plain W emitter at 18-nm vacuum gap. We show that the enhancement mechanism of the HMM emitter is due to the coupled surface plasmon modes at multiple metal-dielectric interfaces inside the HMM emitter. With the minority carrier transport model, the optimal p-n junction depth of the TPV cell has also been determined at various vacuum gaps.

Transport in serial spinful multiple-dot systems: The role of electron-electron interactions and coherences

Quantum dots are nanoscopic systems, where carriers are confined in all three spatial directions. Such nanoscopic systems are suitable for fundamental studies of quantum mechanics and are candidates for applications such as quantum information processing. It was also proposed that linear arrangements of quantum dots could be used as quantum cascade laser. In this work we study the impact of electron-electron interactions on transport in a spinful serial triple quantum dot system weakly coupled to two leads. We find that due to electron-electron scattering processes the transport is enabled beyond the common single-particle transmission channels. This shows that the scenario in the serial quantum dots intrinsically deviates from layered structures such as quantum cascade lasers, where the presence of well-defined single-particle resonances between neighboring levels are crucial for device operation. Additionally, we check the validity of the Pauli master equation by comparing it with the first-order von Neumann approach. Here we demonstrate that coherences are of relevance if the energy spacing of the eigenstates is smaller than the lead transition rate multiplied by ħ.

Observation of Dielectrically Confined Excitons in Ultrathin GaN Nanowires up to Room Temperature

The realization of semiconductor structures with stable excitons at room temperature is crucial for the development of excitonics and polaritonics. Quantum confinement has commonly been employed for enhancing excitonic effects in semiconductor heterostructures. Dielectric confinement, which gives rises to much stronger enhancement, has proven to be more difficult to achieve because of the rapid nonradiative surface/interface recombination in hybrid dielectric-semiconductor structures. Here, we demonstrate intense excitonic emission from bare GaN nanowires with diameters down to 6 nm. The large dielectric mismatch between the nanowires and vacuum greatly enhances the Coulomb interaction, with the thinnest nanowires showing the strongest dielectric confinement and the highest radiative efficiency at room temperature. In situ monitoring of the fabrication of these structures allows one to accurately control the degree of dielectric enhancement. These ultrathin nanowires may constitute the basis for the fabrication of advanced low-dimensional structures with an unprecedented degree of confinement.

Thermal Delocalization of Excitons in GaAs/AlGaAs Quantum Well Tube Nanowires

We use temperature-dependent photoluminescence (PL), photoluminescence imaging, and time-resolved photoluminescence measurements to gain insights into the localization of excitons in single 2 nm GaAs/AlGaAs quantum well tube nanowires. PL spectra reveal the coexistence of localized and delocalized states at low temperatures, with narrow quantum dot-like emission lines on the high energy side of a broad emission band, and delocalized states on the low energy side. We find that the high energy QD-like emissions are metastable, disappearing at higher temperatures with only delocalized states (quantum well tube ground states) surviving. By comparing temperature- and time-dependent PL, we develop a theoretical model which provides insights into the confinement potentials and relaxation dynamics which localize the excitons in these quantum well tube nanowires.

Direct growth of single-crystalline III-V semiconductors on amorphous substrates

The III-V compound semiconductors exhibit superb electronic and optoelectronic properties. Traditionally, closely lattice-matched epitaxial substrates have been required for the growth of high-quality single-crystal III-V thin films and patterned microstructures. To remove this materials constraint, here we introduce a growth mode that enables direct writing of single-crystalline III-V's on amorphous substrates, thus further expanding their utility for various applications. The process utilizes templated liquid-phase crystal growth that results in user-tunable, patterned micro and nanostructures of single-crystalline III-V's of up to tens of micrometres in lateral dimensions. InP is chosen as a model material system owing to its technological importance. The patterned InP single crystals are configured as high-performance transistors and photodetectors directly on amorphous SiO2 growth substrates, with performance matching state-of-the-art epitaxially grown devices. The work presents an important advance towards universal integration of III-V's on application-specific substrates by direct growth.

From Twinning to Pure Zincblende Catalyst-Free InAs(Sb) Nanowires

III-V nanowires are candidate building blocks for next generation electronic and optoelectronic platforms. Low bandgap semiconductors such as InAs and InSb are interesting because of their high electron mobility. Fine control of the structure, morphology, and composition are key to the control of their physical properties. In this work, we present how to grow catalyst-free InAs1-xSbx nanowires, which are stacking fault and twin defect-free over several hundreds of nanometers. We evaluate the impact of their crystal phase purity by probing their electrical properties in a transistor-like configuration and by measuring the phonon-plasmon interaction by Raman spectroscopy. We also highlight the importance of high-quality dielectric coating for the reduction of hysteresis in the electrical characteristics of the nanowire transistors. High channel carrier mobilities and reduced hysteresis open the path for high-frequency devices fabricated using InAs1-xSbx nanowires.

Comparing Hall Effect and Field Effect Measurements on the Same Single Nanowire

We compare and discuss the two most commonly used electrical characterization techniques for nanowires (NWs). In a novel single-NW device, we combine Hall effect and back-gated and top-gated field effect measurements and quantify the carrier concentrations in a series of sulfur-doped InP NWs. The carrier concentrations from Hall effect and field effect measurements are found to correlate well when using the analysis methods described in this work. This shows that NWs can be accurately characterized with available electrical methods, an important result toward better understanding of semiconductor NW doping.

Carrier Recombination Dynamics in Sulfur-Doped InP Nanowires

Measuring lifetime of photogenerated charges in semiconductor nanowires (NW) is important for understanding light-induced processes in these materials and is relevant for their photovoltaic and photodetector applications. In this paper, we investigate the dynamics of photogenerated charge carriers in a series of as-grown InP NW with different levels of sulfur (S) doping. We observe that photoluminescence (PL) decay time as well as integrated PL intensity decreases with increasing S doping. We attribute these observations to hole trapping with the trap density increased due to S-doping level followed by nonradiative recombination of trapped charges. This assignment is proven by observation of the trap saturation in three independent experiments: via excitation power and repetition rate PL lifetime dependencies and by PL pump-probe experiment.

Room temperature GaAsSb single nanowire infrared photodetectors

Antimonide-based ternary III-V nanowires (NWs) allow for a tunable bandgap over a wide range, which is highly interesting for optoelectronics applications, and in particular for infrared photodetection. Here we demonstrate room temperature operation of GaAs0.56Sb0.44 NW infrared photodetectors grown by metal organic vapor phase epitaxy. These GaAs0.56Sb0.44 NWs have uniform axial composition and show p-type conductivity with a peak field-effect mobility of ∼12 cm(2) V(-1) s(-1)). Under light illumination, single GaAs0.56Sb0.44 NW photodetectors exhibited typical photoconductor behavior with an increased photocurrent observed with the increase of temperature owing to thermal activation of carrier trap states. A broadband infrared photoresponse with a long wavelength cutoff at ∼1.66 μm was obtained at room temperature. At a low operating bias voltage of 0.15 V a responsivity of 2.37 (1.44) A/W with corresponding detectivity of 1.08 × 10(9) (6.55 × 10(8)) cm√Hz/W were achieved at the wavelength of 1.3 (1.55) μm, indicating that ternary GaAs0.56Sb0.44 NWs are promising photodetector candidates for small footprint integrated optical telecommunication systems.

The generalized Shockley-Queisser limit for nanostructured solar cells

The Shockley-Queisser limit describes the maximum solar energy conversion efficiency achievable for a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Nanostructured solar cells represent a novel class of photovoltaic devices, and questions have been raised about whether or not they can exceed the Shockley-Queisser limit. Here we show that single-junction nanostructured solar cells have a theoretical maximum efficiency of ∼42% under AM 1.5 solar illumination. While this exceeds the efficiency of a non-concentrating planar device, it does not exceed the Shockley-Queisser limit for a planar device with optical concentration. We consider the effect of diffuse illumination and find that with optical concentration from the nanostructures of only × 1,000, an efficiency of 35.5% is achievable even with 25% diffuse illumination. We conclude that nanostructured solar cells offer an important route towards higher efficiency photovoltaic devices through a built-in optical concentration.

Hybrid Semiconductor Nanowire-Metallic Yagi-Uda Antennas

We demonstrate the directional emission of individual GaAs nanowires by coupling this emission to Yagi-Uda optical antennas. In particular, we have replaced the resonant metallic feed element of the nanoantenna by an individual nanowire and measured with the microscope the photoluminescence of the hybrid structure as a function of the emission angle by imaging the back focal plane of the objective. The precise tuning of the dimensions of the metallic elements of the nanoantenna leads to a strong variation of the directionality of the emission, being able to change this emission from backward to forward. We explain the mechanism leading to this directional emission by finite difference time domain simulations of the scattering efficiency of the antenna elements. These results cast the first step toward the realization of electrically driven optical Yagi-Uda antenna emitters based on semiconductors nanowires.

Short-wavelength infrared photodetector on Si employing strain-induced growth of very tall InAs nanowire arrays

One-dimensional crystal growth enables the epitaxial integration of III-V compound semiconductors onto a silicon (Si) substrate despite significant lattice mismatch. Here, we report a short-wavelength infrared (SWIR, 1.4-3 μm) photodetector that employs InAs nanowires (NWs) grown on Si. The wafer-scale epitaxial InAs NWs form on the Si substrate without a metal catalyst or pattern assistance; thus, the growth is free of metal-atom-induced contaminations, and is also cost-effective. InAs NW arrays with an average height of 50 μm provide excellent anti-reflective and light trapping properties over a wide wavelength range. The photodetector exhibits a peak detectivity of 1.9 × 10(8) cm · Hz(1/2)/W for the SWIR band at 77 K and operates at temperatures as high as 220 K. The SWIR photodetector on the Si platform demonstrated in this study is promising for future low-cost optical sensors and Si photonics.

Lattice-Matched InGaAs-InAlAs Core-Shell Nanowires with Improved Luminescence and Photoresponse Properties

Core-shell nanowires (NW) have become very prominent systems for band engineered NW heterostructures that effectively suppress detrimental surface states and improve performance of related devices. This concept is particularly attractive for material systems with high intrinsic surface state densities, such as the low-bandgap In-containing group-III arsenides, however selection of inappropriate, lattice-mismatched shell materials have frequently caused undesired strain accumulation, defect formation, and modifications of the electronic band structure. Here, we demonstrate the realization of closely lattice-matched radial InGaAs-InAlAs core-shell NWs tunable over large compositional ranges [x(Ga)∼y(Al) = 0.2-0.65] via completely catalyst-free selective-area molecular beam epitaxy. On the basis of high-resolution X-ray reciprocal space maps the strain in the NW core is found to be insignificant (ε < 0.1%), which is further reflected by the absence of strain-induced spectral shifts in luminescence spectra and nearly unmodified band structure. Remarkably, the lattice-matched InAlAs shell strongly enhances the optical efficiency by up to 2 orders of magnitude, where the efficiency enhancement scales directly with increasing band offset as both Ga- and Al-contents increase. Ultimately, we fabricated vertical InGaAs-InAlAs NW/Si photovoltaic cells and show that the enhanced internal quantum efficiency is directly translated to an energy conversion efficiency that is ∼3-4 times larger as compared to an unpassivated cell. These results highlight the promising performance of lattice-matched III-V core-shell NW heterostructures with significant impact on future development of related nanophotonic and electronic devices.

Cracking the Si Shell Growth in Hexagonal GaP-Si Core-Shell Nanowires

Semiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these, core-shell heterostructures are much more sensitive to strain than axial ones. It is now accepted that the formation of misfit dislocations depends both on the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core-shell nanowires: cracks. These defects do not originate from a lattice mismatch (we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I1 stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.

Correlation of electrical and structural properties of single as-grown GaAs nanowires on Si (111) substrates

We present the results of the study of the correlation between the electrical and structural properties of individual GaAs nanowires measured in their as-grown geometry. The resistance and the effective charge carrier mobility were extracted for several nanowires, and subsequently, the same nano-objects were investigated using X-ray nanodiffraction. This revealed a number of perfectly stacked zincblende and twinned zincblende units separated by axial interfaces. Our results suggest a correlation between the electrical parameters and the number of intrinsic interfaces.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Zn3As2 nanowires and nanoplatelets: highly efficient infrared emission and photodetection by an earth abundant material

The development of earth abundant materials for optoelectronics and photovoltaics promises improvements in sustainability and scalability. Recent studies have further demonstrated enhanced material efficiency through the superior light management of novel nanoscale geometries such as the nanowire. Here we show that an industry standard epitaxy technique can be used to fabricate high quality II-V nanowires (1D) and nanoplatelets (2D) of the earth abundant semiconductor Zn3As2. We go on to establish the optoelectronic potential of this material by demonstrating efficient photoemission and detection at 1.0 eV, an energy which is significant to the fields of both photovoltaics and optical telecommunications. Through dynamical spectroscopy this superior performance is found to arise from a low rate of surface recombination combined with a high rate of radiative recombination. These results introduce nanostructured Zn3As2 as a high quality optoelectronic material ready for device exploration.

Single nanowire photoconductive terahertz detectors

Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening, and nondestructive testing. However, compact optoelectronics suited to the most powerful terahertz technique, time-domain spectroscopy, are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus, nanowires are shown to be well suited for THz device applications and hold particular promise as near-field THz sensors.