NW-based Devices Research Articles

Ultra-thin size-controllable surface plasmon polariton laser by PDMS-assisted imprinting

Plasmonic laser has great potential to overcome the optical diffraction limit, playing a crucial role in advancing nanophotonics and nanoelectronics for on-chip integration. However, current plasmonic lasers face several challenges, such as the difficulty in controlling nanowire (NW) size, disordered arrangement, and complicated fabrication process. Herein, ultra-thin gain media for plasmonic lasers below the cutoff size of the photonic mode are prepared using the polydimethylsiloxane-assisted imprinting. This method enables precise control over the size of the perovskite NW, with the minimum size achievable being 60 nm. As a result, the plasmonic lasing is achieved from the CsPbBr3 NW-based device with a threshold as low as ∼49.13 μJ cm−2 and a Quality Factor (Q) of 1803 at room temperature, demonstrating its capability for achieving high-quality lasing. Meanwhile, a dual-pumping time-resolved fluorescence study suggests that the radiative recombination lifetime of CsPbBr3 NWs is shortened by a factor of 10 due to the Purcell effect, confirming the plasmonic effect exhibited by the device. Furthermore, a plasmonic laser array is developed using this method, demonstrating the applicability of the imprinting method in complex graphic fabrication. This breakthrough provides a solution for the application of plasmonic laser arrays in optoelectronic integration.

Heat-induced morphological changes in silver nanowires deposited on a patterned silicon substrate.

Metallic nanowires (NWs) are sensitive to heat treatment and can split into shorter fragments within minutes at temperatures far below the melting point. This process can hinder the functioning of NW-based devices that are subject to relatively mild temperatures. Commonly, heat-induced fragmentation of NWs is attributed to the interplay between heat-enhanced diffusion and Rayleigh instability. In this work, we demonstrated that contact with the substrate plays an important role in the fragmentation process and can strongly affect the outcome of the heat treatment. We deposited silver NWs onto specially patterned silicon wafers so that some NWs were partially suspended over the holes in the substrate. Then, we performed a series of heat-treatment experiments and found that adhered and suspended parts of NWs behave differently under the heat treatment. Moreover, depending on the heat-treatment process, fragmentation in either adhered or suspended parts can dominate. Experiments were supported by finite element method and molecular dynamics simulations.

The implementation of thermal and UV nanoimprint lithography for selective area epitaxy

Semiconductor nanowires (NWs) in horizontal configuration could provide a path for scalable NW-based devices. Bottom–up large-scale manufacturing of these nanostructures by selective area epitaxy (SAE) relies on precise nanopatterning of various shapes on the growth masks. Electron beam lithography offers an extraordinary accuracy suited for the purpose. However, this technique is not economically viable for large production as it has a low throughput and requires high investment and operational costs. Nanoimprint lithography (NIL) has the potential to reduce fabrication time and costs significantly while requiring less sophisticated equipment. In this work, we utilize both thermal and UV NIL for patterning substrates for SAE, elucidating the advantages and disadvantages of each lithography technique. We demonstrate the epitaxial growth of Ge and GaAs NWs on these substrates, where we observe high-quality mono-crystalline structures. Even though both processes can produce small uniform structures suitable for SAE, our results show that UV NIL proves to be superior and enables reliable and efficient patterning of sub-100 nm mask features at the wafer scale.

Low frequency noise in β -Ga2O3 based nanoelectronic devices

In this work, low frequency noise in β-Ga2O3 nanowire-based (NW) electronic devices is analyzed, which exhibits different behaviors as the device size scales down. The noise spectrum for the narrower NW (∼80 nm) is closer to 1/f characteristics, whereas it starts to show evident 1/f2 components as the NW size gets thicker (∼200 nm), giving clear signs of distinctive features for the bunch of traps at the NW interface or in the bulk. Our results show that 1/f noise in these NW electronic devices seems predominantly originated from an aggregated effect of the intricate trap states close to the β-Ga2O3 NW surface or interface with a wide range distribution, while finite groups of active deep traps play a critical role in contributing 1/f2components via generation-recombination or random telegraph signal processes. Notably, as the bias voltage increases, the 1/f2 components in the noise spectra get more overwhelming and would shift toward lower frequencies, suggesting that electric ionization effects would screen the shallow traps close to the surface or interface based on the Poole–Frenkel model. The Hooge's constants extracted from the 1/f noise component for these β-Ga2O3 NW-based devices fall in the range of 0.008–0.019, which are comparable to those of the best reported devices based on other wide bandgap semiconductor with nanoscale structures, including GaN, ZnO, and SnO2. This work may give hints of revealing the sophisticated dynamic behaviors of traps in the surface/volume β-Ga2O3 materials and electronic devices in the nanoscale by low frequency noises.

Nanowire-based synaptic devices for neuromorphic computing

The traditional von Neumann structure computers cannot meet the demands of high-speed big data processing; therefore, neuromorphic computing has received a lot of interest in recent years. Brain-inspired neuromorphic computing has the advantages of low power consumption, high speed and high accuracy. In human brains, the data transmission and processing are realized through synapses. Artificial synaptic devices can be adopted to mimic the biological synaptic functionalities. Nanowire (NW) is an important building block for nanoelectronics and optoelectronics, and many efforts have been made to promote the application of NW-based synaptic devices for neuromorphic computing. Here, we will introduce the current progress of NW-based synaptic memristors and synaptic transistors. The applications of NW-based synaptic devices for neuromorphic computing will be discussed. The challenges faced by NW-based synaptic devices will be proposed. We hope this perspective will be beneficial for the application of NW-based synaptic devices in neuromorphic systems.

Electrical properties and chemiresistive response to 2,4,6 trinitrotoluene vapours of large area arrays of Ge nanowires

We study the electrical and morphological properties of random arrays of Ge nanowires (NW) deposited on sapphire substrates. NW-based devices were fabricated with the aim of developing chemiresistive-type sensors for the detection of explosive vapours. We present the results obtained on pristine and annealed NWs and, focusing on the different phenomenology observed, we discuss the critical role played by NW–NW junctions on the electrical conduction and sensing performances. A mechanism is proposed to explain the high efficiency of the annealed arrays of NWs in detecting 2,4,6 trinitrotoluene vapours. This study shows the promising potential of Ge NW-based sensors in the field of civil security.

Experimental and Modeling Study of Metal-Insulator Interfaces to Control the Electronic Transport in Single Nanowire Memristive Devices.

Memristive devices relying on redox-based resistive switching mechanisms represent promising candidates for the development of novel computing paradigms beyond von Neumann architecture. Recent advancements in understanding physicochemical phenomena underlying resistive switching have shed new light on the importance of an appropriate selection of material properties required to optimize the performance of devices. However, despite great attention has been devoted to unveiling the role of doping concentration, impurity type, adsorbed moisture, and catalytic activity at the interfaces, specific studies concerning the effect of the counter electrode in regulating the electronic flow in memristive cells are scarce. In this work, the influence of the metal-insulator Schottky interfaces in electrochemical metallization memory (ECM) memristive cell model systems based on single-crystalline ZnO nanowires (NWs) is investigated following a combined experimental and modeling approach. By comparing and simulating the electrical characteristics of single NW devices with different contact configurations and by considering Ag and Pt electrodes as representative of electrochemically active and inert electrodes, respectively, we highlight the importance of an appropriate choice of electrode materials by taking into account the Schottky barrier height and interface chemistry at the metal-insulator interfaces. In particular, we show that a clever choice of metal-insulator interfaces allows to reshape the hysteretic conduction characteristics of the device and to increase the device performance by tuning its resistance window. These results obtained from single NW-based devices provide new insights into the selection criteria for materials and interfaces in connection with the design of advanced ECM cells.

Low-Temperature Hydrothermal Growth of ZnO Nanowires on AZO Substrates for FACsPb(IBr)3 Perovskite Solar Cells.

Electron and hole transport layers (ETL and HTL) play an essential role in shaping the photovoltaic performance of perovskite solar cells. While compact metal oxide ETL have been largely explored in planar n-i-p device architectures, aligned nanowires or nanorods remain highly relevant for efficient charge extraction and directional transport. In this study, we have systematically grown ZnO nanowires (ZnO NWs) over aluminum-doped zinc oxide (AZO) substrates using a low-temperature method, hydrothermal growth (HTG). The main growth parameters were varied, such as hydrothermal precursors concentrations (zinc nitrate hexahydrate, hexamethylenetetramine, polyethylenimine) and growing time, in order to finely control NW properties (length, diameter, density, and void fraction). The results show that ZnO NWs grown on AZO substrates offer highly dense, well-aligned nanowires of high crystallinity compared to conventional substrates such as FTO, while demonstrating efficient FACsPb(IBr)3 perovskite device performance, without the requirement of conventional compact hole blocking layers. The device performances are discussed based on NW properties, including void fraction and aspect ratio (NW length over diameter). Finally, AZO/ZnO NW-based devices were fabricated with a recent HTL material based on a carbazole moiety (Cz–Pyr) and compared to the spiro-OMeTAD reference. Our study shows that the Cz–Pyr-based device provides similar performance to that of spiro-OMeTAD while demonstrating a promising stability in ambient conditions and under continuous illumination, as revealed by a preliminary aging test.

Characterization of Electrical Properties of Suspended ZnO Nanowires Using a Nanorobotic Manipulation System Inside a Scanning Electron Microscope for Nanoelectronic Applications

Electrical characterization of semiconducting oxide nanowires (NWs) is mostly performed using complex techniques, which necessitates a series of costly nanofabrication procedures. In this work, with the aim to provide a low-cost, feasible, facile, and reproducible approach for enabling the study of NW electrical properties, we report direct electrical measurements on individual and overlapped suspended zinc oxide NWs (ZnO NWs). We have succeeded in constructing both two- and three-terminal devices simply by employing tungsten (W) nanoprobes with the aid of a nanomanipulation system embedded inside a scanning electron microscope’s vacuum chamber. Stable contacts were established using the Joule heating effect and e-beam exposure at the junctions between the NW and the pre-cleaned W tips. P-channel field-effect transistor devices were achieved with an on–off current ratio of ∼101, a threshold voltage (>1.5 V), a transconductance of ∼16 μS, a sub-threshold swing of ∼220 mV/decade, and field-effect carrier mobility roughly estimated to be around 926.4 cm2/(V·s) after correction for contact resistances/optimization. The average resistivity of ZnO NWs was calculated to be ∼2.23 × 10–2 Ω·cm for NWs with diameters between 70 and 500 nm. Besides, we have demonstrated a contact resistance of ∼19.60 kΩ and a Schottky barrier height of ∼0.37 eV present at W/ZnO NW interfaces. The contact resistance between two overlapped ZnO NWs was estimated to be ∼283 kΩ, which is relatively higher than that offered between W/ZnO NWs. This work provides a solid experimental procedure to address true intrinsic electrical properties at metal/semiconductor interfaces, and our findings have potential applications in next-generation 3D suspended ZnO NW-based nanoelectronic devices.

Electromechanical conversion efficiency of GaN NWs: critical influence of the NW stiffness, the Schottky nano-contact and the surface charge effects.

The piezoelectric nanowires (NWs) are considered as promising nanomaterials to develop high-efficient piezoelectric generators. Establishing the relationship between their characteristics and their piezoelectric conversion properties is now essential to further improve the devices. However, due to their nanoscale dimensions, the NWs are characterized by new properties that are challenging to investigate. Here, we use an advanced nano-characterization tool derived from AFM to quantify the piezo-conversion properties of NWs axially compressed with a well-controlled applied force. This unique technique allows to establish the direct relation between the output signal generation and the NW stiffness and to quantify the electromechanical coupling coefficient of GaN NWs, which can reach up to 43.4%. We highlight that this coefficient is affected by the formation of the Schottky nano-contact harvesting the piezo-generated energy, and is extremely sensitive to the surface charge effects, strongly pronounced in sub-100 nm wide GaN NWs. These results constitute a new building block in the improvement of NW-based nanogenerator devices.

Prolonging lifetime of photogenerated carriers in WO3 nanowires by oxygen vacancies engineering for enhanced photoelectrocatalytic oxygen evolution reaction

WO3 nanowires (NWs) have emerged as a promising alternative electrocatalyst for the photoelectrochemical (PEC) oxygen evolution reaction (OER), due to their nontoxicity, low cost, good stability, and strong photocatalytic oxidation ability. However, a significant challenge is limited by the poor electrical conductivity and the rapid recombination rate of photogenerated carriers. This paper reports a facile and effective way to synthesize the WO3 NWs with oxygen vacancies (Ov-WO3 NWs), which have high PEC OER activity and good stability. Photoelectric measurements indicate that the Ov-WO3 NW-based device shows a good light-harvesting property under visible light and a prolonged photoresponse time. Electrochemical impedance spectra measurements reveal a reduced value of Rct denoting an improved electrical conductivity, which should be responsible for the superior PEC OER performance. Our work provides a strategy for fabricating efficient water-splitting electrodes to help establish rational design principles for future OER catalysts.

Nanoscale X-ray detectors based on individual CdS, SnO[formula omitted] and ZnO nanowires

The development of nanoscale X-ray sensors is of crucial importance to achieve higher spatial resolution in many X-ray-based techniques playing a key role in materials science, healthcare, and security. Here, we demonstrate X-ray detection using individual CdS, SnO2, and ZnO nanowires (NWs). The NWs were produced via vapor–liquid–solid technique and characterized using X-ray diffraction, scanning, and transmission electron microscopy. Electrical measurements were performed under ambient conditions while exposing two-terminal NW-based devices to X-rays generated by a conventional tungsten anode X-ray tube. Fast and stable nanoampere-range X-ray beam induced current (XBIC) in response to X-ray illumination was observed. The high XBIC measured in the NW devices could be attributed to the efficient transport and collection of generated charge carriers due to the single-crystalline nature of NWs and the short NW length. Such fast-response and high-sensitivity nanoscale X-ray detectors can find applications in sub-micron resolution imaging and nanofocused beam shape measurements.

Cost-Effective Fabrication of Fractal Silicon Nanowire Arrays.

Silicon nanowires (Si NWs) emerged in several application fields as a strategic element to surpass the bulk limits with a flat compatible architecture. The approaches used for the Si NW realization have a crucial impact on their final performances and their final cost. This makes the research on a novel and flexible approach for Si NW fabrication a crucial point for Si NW-based devices. In this work, the novelty is the study of the flexibility of thin film metal-assisted chemical etching (MACE) for the fabrication of Si NWs with the possibility of realizing different doped Si NWs, and even a longitudinal heterojunction p-n inside the same single wire. This point has never been reported by using thin metal film MACE. In particular, we will show how this approach permits one to obtain a high density of vertically aligned Si NWs with the same doping of the substrate and without any particular constraint on doping type and level. Fractal arrays of Si NWs can be fabricated without any type of mask thanks to the self-assembly of gold at percolative conditions. This Si NW fractal array can be used as a substrate to realize controllable artificial fractals, integrating other interesting elements with a cost-effective microelectronics compatible approach.

Controllable Transfer of MOx Nanowires on Microhotplate-Based Gas Sensing Devices

Introduction SnO2 nanowires (NWs) have been synthesized and implemented on CMOS-based gas sensors devices with gold inter-digital electrode structures (IDES). For the implementation process of NWs a polydimethylsiloxane-based stamping transfer was employed with help of a Condor Sigma bond tester. The multi-nanowire gas sensor has demonstrated sensitivity for toluene concentrations as low as 100 ppb. The transfer method applied in this work is one step further from previous manual transfer [1] to the industrial adaptation of the process. Materials and substrates A two-step synthesis was applied to prepare SnO2 nanowires, as described elsewhere [2]. The gas sensing substrate was a commercial chemical sensor platform provided by ams AG, Austria - a 2x2 mm micro-hotplate chip with a Pt-based microheater and Pt-IDES with an electrode width and the distance between the electrode fingers both 5 µm. SnO2 NWs were transferred on the IDES-structure with a PDMS stamp, prepared by 1:10 standard recipe and cut into 3x3mm pieces. Transfer of SnO2 NWs A Condor Sigma bond tester tool was employed to transfer SnO2 NWs. The PDMS-stamp was glued on a specific tool of the bond tester, which is usually employed for testing wire bond connections. The PDMS stamp was pressed with well controlled parameters (force, distance and position) onto the substrate with MOx NWs in order to collect the NWs with the stamp. Afterwards the NWs were transferred onto the gas sensing substrate with maximal force of 0.7N and 10s time of contact between the stamp and the micro-hotplate substrate, which resulted in NW transfer and interconnection to the electrodes. Gas measurement Gas measurements were performed by an automatized setup, with synthetic air (80% N2, 20% O2) as a background gas and a constant flow rate of 1000 sccm. Three different humidity levels of 25, 50 and 75% were investigated in the presence of 5 different toluene concentrations – 0.05, 0.1, 0.5, 1 and 5 ppm. The temperature of the microhotplate was kept constant at 300°C. Response was calculated as follows:R = (Rair - Rgas)/Rair *100%where Rair is a resistance of the sensor in synthetic air before the gas pulse and Rgas is a resistance of the sensor in the presence of a test gas at the end of a gas pulse. Two independent sensors were measured, prepared by the same technique. Results and Conclusions The Condor Sigma bond tester enabled precise manipulation of the PDMS NW transfer stamp not only in all 3 axis but also by using force sensor. Due to the high precision of the machine the transfer process could be performed with high precision and control without damage to the micro-hotplate. This demonstrates that such a NW transfer process is suited for realization of NW-based chemical sensor devices. The response towards toluene is shown in Fig.1. The multi-nanowire based devices are able to sense a gas concentration as low as 100 ppb (where the threshold limit value for Switzerland is 50 ppm [3]). There is also visible dependence towards humidity – but it is not as high as for toluene – i.e. for 5 ppm of toluene the mean response for both sensors is 9%, 8% and 8% for 25, 50 and 75% rH levels, respectively. Presently the work is focused on the NWs transfer on CMOS-based microhotplate array chips. Acknowledgements This work was partly performed within the project “FunkyNano – Optimized Functionalization of Nanosensors for Gas Detection by Screening of Hybrid Nanoparticles” funded by the FFG - Austrian Research Promotion Agency (Project No. 858637). References Sosada-Ludwikowska, R. Wimmer-Teubenbacher, M. Sagmeister, A. Köck, Transfer Printing Technology as a Straightforward Method to Fabricate Chemical Sensors Based on Tin Dioxide Nanowires, Sensors (2019), 19, 3049; doi: 10.3390/s19143049Köck, A. Tischner, T. Maier, M. Kast, C. Edtmaier, C. Gspan, G. Kothleitner, Atmospheric pressure fabrication of SnO2-nanowires for highly sensitive CO and CH4 detection, Sensors and Actuators B: Chemical (2009), 138 no. 1, pp. 160–167 (2009); doi: 10.1016/j.snb.2009.02.055Schweizerische Unfallversicherungsanstalt (SUVA): Grenzwerte – Aktuelle MAK- und BAT-Werte für Toluol. Available online: https://www.suva.ch/de-CH/material/Richtlinien-Gesetzestexte/grenzwerte-am-arbeitsplatz-aktuelle-werte (accessed on 29.11.2019) Figure 1

Antisolvent-assisted one-step solution synthesis of defect-less 1D MAPbI3 nanowire networks with improved charge transport dynamics

Abstract In this study, both one-dimensional (1D) and three-dimensional (3D) methylammonium lead iodide (MAPbI3) were successfully synthesized from the same starting precursor solution using a one-step in situ spin-coating process assisted by antisolvent washing with different ethereal solvents. Antisolvent washing with diethyl ether (DE) induced isotropic growth, resulting in dense 3D MAPbI3 films. However, washing with dibutyl ether (DB) having complementary properties promoted 1D directional growth to avoid steric hindrance in the DB/perovskite complex. Antisolvent DB having a lower vapor pressure than DE retarded the vaporization of dimethylformamide and facilitated the growth of larger grains with fewer defects through the slow crystallization. It was also found that long-chain induced 1D directional growth creates an internal compressive strain in the 1D NW film, which plays an important role in charge carrier dynamics. To compare the power conversion efficiency (PCE) and charge transport dynamics, planar solar cells based on both 1D and 3D perovskites were fabricated. 1D NW perovskite based devices achieved a longer lifetime and faster carrier transport owing to the limited ion migration in the presence of compressive strain, although the PCE (15.73%) of the 1D NW-based device was lower than that (18.73%) of the 3D bulk-based device due to inevitable pore incorporation. The improved charge carrier dynamics of 1D NW MAPbI3 can offer potential advantages in the development of optoelectronic devices.

Polarity-controlled AlN/Si templates by in situ oxide desorption for variably arrayed MOVPE-GaN nanowires

In this paper, we present a comprehensive study of position-defined Al-polar AlN nucleation on lithographically patterned Si(111) substrates as a method to obtain ordered Ga-polar GaN nanowire arrays, with possible application in future nanowire-based devices such as LEDs and photoelectrochemical water-splitting cells. In a hydrogen processing step, ex situ prepared oxide on pre-structured Si-pillars could be selectively removed. This enabled Al-polar AlN nucleation on the Si-pillar’s sidewalls during the following metal–organic vapor phase epitaxy, while dominant N-polar AlN layer growth was observed on the still oxidized Si(111) horizontal substrate surface neighboring the pillars. 100% of the Ga-polar GaN wires are emerging on the Al-polar AlN growth sites, thus selective area epitaxy without any mask material could be realized. To gain a precise understanding of the growth mechanisms, the attainable Ga-adatom collection area per NW was varied by changing the Si-pillars’ placement pattern. The wire length and diameter increase with extended pitch. At a constant pitch, the size of the wires is adjustable by variation of the Si-pillars’ diameter, therefore growth of GaN wires of controllable dimension and pitch could be attained. Additionally, parasitic NW growth was completely suppressed for any pitch < 3.5 µm, while an increased pitch resulted in additional parasitic growth. Based on these results a model was derived, which includes the site-controlled removal of the oxide, the thus achieved local polarity control of AlN growth, and the influence of the collection area of each NW with respect to their size, whereas the collection area could be set in the experiment by adjustment of the lithographically controlled pitch and growth-parameter dependent Ga-adatom diffusion length. The mask-less polarity- and site-controlled growth of NWs with a height of 5.3 µm ± 0,33 µm and a diameter of 800 nm ± 160 nm at a pitch of 2.5 µm could be attained. Hence, a deep understanding of the growth mechanisms and the geometrical control of polarity- and site-controlled GaN NWs could be achieved, forming the base for development of NW-based devices on a conductive AlN/Si-template.

Epitaxial Growth of Ordered In-Plane Si and Ge Nanowires on Si (001).

Controllable growth of wafer-scale in-plane nanowires (NWs) is a prerequisite for achieving addressable and scalable NW-based quantum devices. Here, by introducing molecular beam epitaxy on patterned Si structures, we demonstrate the wafer-scale epitaxial growth of site-controlled in-plane Si, SiGe, and Ge/Si core/shell NW arrays on Si (001) substrate. The epitaxially grown Si, SiGe, and Ge/Si core/shell NW are highly homogeneous with well-defined facets. Suspended Si NWs with four {111} facets and a side width of about 25 nm are observed. Characterizations including high resolution transmission electron microscopy (HRTEM) confirm the high quality of these epitaxial NWs.

Kinetics of Guided Growth of Horizontal GaN Nanowires on Flat and Faceted Sapphire Surfaces.

The bottom-up assembly of nanowires facilitates the control of their dimensions, structure, orientation and physical properties. Surface-guided growth of planar nanowires has been shown to enable their assembly and alignment on substrates during growth, thus eliminating the need for additional post-growth processes. However, accurate control and understanding of the growth of the planar nanowires were achieved only recently, and only for ZnSe and ZnS nanowires. Here, we study the growth kinetics of surface-guided planar GaN nanowires on flat and faceted sapphire surfaces, based on the previous growth model. The data are fully consistent with the same model, presenting two limiting regimes—either the Gibbs–Thomson effect controlling the growth of the thinner nanowires or surface diffusion controlling the growth of thicker ones. The results are qualitatively compared with other semiconductors surface-guided planar nanowires materials, demonstrating the generality of the growth mechanism. The rational approach enabled by this general model provides better control of the nanowire (NW) dimensions and expands the range of materials systems and possible application of NW-based devices in nanotechnology.

Quantitative Assessment of Carrier Density by Cathodoluminescence. II. GaAs Nanowires

Precise control of doping in single nanowires (NWs) is essential for the development of NW-based devices. Here, we investigate a series of MBE-grown GaAs NWs with Be (p-type) and Si (n-type) doping using high-resolution cathodoluminescence (CL) mapping at low- and room-temperature. CL spectra are analyzed selectively in different regions of the NWs. Room-temperature luminescence is fitted with the generalized Planck law and an absorption model, and the bandgap and band tail width are extracted. For Be-doped GaAs NWs, the bandgap narrowing provides a quantitative determination of the hole concentration ranging from about $1\times 10^{18}$ to $2\times 10^{19}$~cm$^{-3}$, in good agreement with the targeted doping levels. High-resolution maps of the hole concentration demonstrate the homogeneous doping in the pure zinc-blende segment. For Si-doped GaAs NWs, the electron Fermi level and the full-width at half maximum of low-temperature CL spectra are used to assess the electron concentration to approximately $3\times 10^{17}$ to $6\times 10^{17}$~cm$^{-3}$. These findings confirm the difficulty to obtain highly-doped n-type GaAs NWs, maybe due to doping compensation. Notably, signatures of high concentration (5--9$\times 10^{18}$~cm$^{-3}$) at the very top of NWs are unveiled.

Doping challenges and pathways to industrial scalability of III–V nanowire arrays

Semiconductor nanowires (NWs) have been investigated for decades, but their application into commercial products is still difficult to achieve, with triggering causes related to the fabrication cost and structure complexity. Dopant control at the nanoscale greatly narrows their exploitation as components for device integration. In this context, doping appears the truly last missing piece of the puzzle for III–V NWs, for them to become commercially exploitable. In this paper, we review the doping of bottom up III–V NW arrays grown by molecular beam epitaxy and metal-organic vapor phase epitaxy, aiming to link materials science challenges with the critical aspect of device design. First, the doping methods and mechanisms are described, highlighting the differences between self-assembled and ordered NW arrays. Then, a brief overview of the available tools for investigating the doping is offered to understand the common strategies used for doping characterization. Both aspects are crucial to discuss the recent advancements in reproducibility and up-scalability, which are discussed in view of large area fabrication for industrial production. Finally, the most common doping-related challenges are presented together with the latest solutions to achieve high performing NW-based devices. On this basis, we believe that new insights and innovative findings discussed herein will open the low dimensional materials era, on the premise of multidisciplinary collaborative works of all the sectors involved in the design and optimization of commercial products.