Nanowire Ensembles Research Articles

Uniform large-area surface patterning achieved by metal dewetting for the top-down fabrication of GaN nanowire ensembles

The dewetting of thin Pt films on different surfaces is investigated as a means to provide the patterning for the top-down fabrication of GaN nanowire ensembles. The transformation from a thin film to an ensemble of nanoislands upon annealing proceeds in good agreement with the void growth model. With increasing annealing duration, the size and shape uniformity of the nanoislands improves. This improvement speeds up for higher annealing temperature. After an optimum annealing duration, the size uniformity deteriorates due to the coalescence of neighboring islands. By changing the Pt film thickness, the nanoisland diameter and density can be quantitatively controlled in a way predicted by a simple thermodynamic model. We demonstrate the uniformity of the nanoisland ensembles for an area larger than 1 cm2. GaN nanowires are fabricated by a sequence of dry and wet etching steps, and these nanowires inherit the diameters and density of the Pt nanoisland ensemble used as a mask. Our study achieves advancements in size uniformity and range of obtainable diameters compared to previous works. This simple, economical, and scalable approach to the top-down fabrication of nanowires is useful for applications requiring large and uniform nanowire ensembles with controllable dimensions.

Efficiency Limits in Coalesced AlGaN Nanowire Ultraviolet LEDs

Nanowire AlGaN III‐nitride LEDs are claimed as potential high‐efficiency solid‐state photon sources spanning to the short‐wavelength deep ultraviolet (UV). Nanowire LEDs (NWLEDs) emitting in the UV are compared with a transparent n‐AlGaN top electrode formed by coalescing the top region of nanowire–ensemble LEDs with commonly employed opaque conformal metallic electrodes used for nanowire‐based devices. The use of a transparent contact results in an increase in the wall plug efficiency of >25×, exceeding the expected increase due to enhanced photon‐extraction efficiency. Increased nanowire connectivity reduces the short‐circuit pathways, enabling higher device yields of relatively large‐area (>1 mm2) UV nanowire–ensemble LEDs. Despite these large relative improvements, the absolute output efficiency remains miniscule (<1 m%). Electroluminescence microscopy demonstrates that <0.1% of nanowires within the ensemble contribute to emission. The single‐nanowire efficiency is estimated and points toward improvement of the homogeneity of the injection current as a crucial step for realizing commercially viable UV NWLEDs.

Confined spin-wave characteristics in magnetic nanowire ensembles approaching the ultrathin regime

Confined spin-wave characteristics in magnetic nanowire ensembles approaching the ultrathin regime

Growth kinetics and substrate stability during high-temperature molecular beam epitaxy of AlN nanowires

We study the molecular beam epitaxy of AlN nanowires between 950 °C and 1215 °C, well above the usual growth temperatures, to identify optimal growth conditions. The nanowires are grown by self-assembly on TiN(111) films sputtered onto Al2O3. Above 1100 °C, the TiN film is seen to undergo grain growth and its surface exhibits {111} facets where AlN nucleation preferentially occurs. Modeling of the nanowire elongation rate measured at different temperatures shows that the Al adatom diffusion length maximizes at 1150 °C, which appears to be the optimum growth temperature. However, analysis of the nanowire luminescence shows a steep increase in the deep-level signal already above 1050 °C, associated with O incorporation from the Al2O3 substrate. Comparison with AlN nanowires grown on Si, MgO and SiC substrates suggests that heavy doping of Si and O by interdiffusion from the TiN/substrate interface increases the nanowire internal quantum efficiency, presumably due to the formation of a SiN x or AlO x passivation shell. The outdiffusion of Si and O would also cause the formation of the inversion domains observed in the nanowires. It follows that for optoelectronic and piezoelectric applications, optimal AlN nanowire ensembles should be prepared at 1150 °C on TiN/SiC substrates and will require an ex situ surface passivation.

GaN and AlGaN/AlN Nanowire Ensembles for Ultraviolet Photodetectors: Effects of Planarization with Hydrogen Silsesquioxane and Nanowire Architecture

GaN and AlGaN/AlN Nanowire Ensembles for Ultraviolet Photodetectors: Effects of Planarization with Hydrogen Silsesquioxane and Nanowire Architecture

Density control of GaN nanowires at the wafer scale using self-assembled SiN x patches on sputtered TiN(111)

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10 μm−2) or ultralow (<1 μm−2) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN x patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN x atoms prior to the GaN growth. By varying the amount of pre-deposited SiN x , the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN x patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III–V semiconductors nucleus grown on inert surfaces like 2D materials.

Modeling Catalyst-Free Growth of III-V Nanowires: Empirical and Rigorous Approaches

Catalyst-free growth of III-V and III-nitride nanowires (NWs) by the self-induced nucleation mechanism or selective area growth (SAG) on different substrates, including Si, show great promise for monolithic integration of III-V optoelectronics with Si electronic platform. The morphological design of NW ensembles requires advanced growth modeling, which is much less developed for catalyst-free NWs compared to vapor-liquid-solid (VLS) NWs of the same materials. Herein, we present an empirical approach for modeling simultaneous axial and radial growths of untapered catalyst-free III-V NWs and compare it to the rigorous approach based on the stationary diffusion equations for different populations of group III adatoms. We study in detail the step flow occurring simultaneously on the NW sidewalls and top and derive the general laws governing the evolution of NW length and radius versus the growth parameters. The rigorous approach is reduced to the empirical equations in particular cases. A good correlation of the model with the data on the growth kinetics of SAG GaAs NWs and self-induced GaN NWs obtained by different epitaxy techniques is demonstrated. Overall, the developed theory provides a basis for the growth modeling of catalyst-free NWs and can be further extended to more complex NW morphologies.

Gallium Phosphide Nanowires Grown on SiO2 by Gas-Source Molecular Beam Epitaxy

GaP as one of the III–V semiconductors has an indirect band gap in its natural zinc-blend (ZB) crystal phase, limiting its applications in optoelectronics. The atomic arrangements of the ZB GaP, however, can be changed by adding energy to the system, for example, using strain and defects. In such a way, GaP can be crystallized in the wurtzite (WZ) phase with a direct band gap in the yellow–green range and promising new optical properties. GaP nanostructures offer the great possibility to induce strain, and hence, one can expect to obtain the WZ phase by modifying the geometry and dimensionality of GaP. In this work, we present GaP nanowires (NWs) grown on SiO2 substrates by gas-source molecular beam epitaxy. Raman measurements on individual GaP NWs indicate that NWs are poly-type crystal structures with the starting growth of the WZ phase, transforming into the ZB phase, and ending as the WZ phase. Photoluminescence at 9 K from an ensemble of NWs shows emissions at 2.09–2.14 eV, which are related to the direct band gap of the WZ phase and peaks between 2.26 and 2.3 eV due to the ZB phase. The emission of the WZ GaP phase is observable up to 160 K. Cathodoluminescence at 83 K shows directly the emission between 2.09 and 2.14 eV along the single NWs, indicating the presence of the WZ phase. Our results demonstrate the realization of poly-type, ZB, and WZ GaP NWs on SiO2 by gas-source molecular beam epitaxy.

Optical properties of Ga-doped AlN nanowires

We show that intentional Ga doping of AlN nanowires in the 0.01%–0.5% range leads to the spontaneous formation of nanometric carrier localization centers. Accordingly, for single nanowires, we observed a collection of sharp cathodoluminescence lines in a wavelength range spanning from 220 to 300 nm. From temperature-dependent cathodoluminescence, a ratio between the intensity at room temperature and 5 K of 20–30% is measured. We found that an ensemble of Ga-doped AlN nanowires exhibits a wide-band cathodoluminescence emission, which opens the path to the realization of efficient UV-C light emitting diodes covering a wide part of DNA absorption band.

Ultrasensitive self-powered heterojunction ultraviolet photodetector of p-GaN nanowires on Si by halide chemical vapour deposition

In this study, we report the fabrication of quasi-aligned p-GaN nanowires (NWs) on n-Si (1 1 1) substrate by halide chemical vapour deposition (HCVD) using MgCl2 precursor and followed by low-energy electron beam irradiation to activate the Mg acceptor doping in GaN NWs. We aimed to attain a comprehensive understanding of p-doping in GaN NWs growth, extensive characterizations and fabrication of UV photodetector (PDs) based on p-GaN NWs/n-Si heterojunction. To realize the efficient UV photodetectors, we measure the current–voltage (I–V) characteristics of heterojunction PDs under dark and illuminated conditions and the I–V curve demonstrates good rectifying behaviours with 0.2 V turn-on voltage. At zero bias, the heterojunction PDs show a reverse photocurrent of 1.27 × 10−6 A with a very low dark current of 2.35 × 10−9 A under 325 nm UV illumination. Besides, the significance of the self-powered operation of UV PDs and the charge transfer mechanism are discussed with the aid of the energy band diagram. The substantial photocurrent increment with varying applied potential leads to narrowing the photo potential in the interface. The excitonic bound states present in p-GaN NWs/n-Si heterojunction is further elucidated. As a result, the heterojunction PDs demonstrate the high responsivity, detectivity, and external quantum efficiency of 134 mA W−1, 3.73 × 1013 Jones, and 51% respectively, at 0.1 V low applied potential under the reverse bias condition. The proposed work provides an archetype for Mg doping in GaN NWs ensembles, which will help to facilitate the heterojunction with n-Si to unleash the potential of self-powered UV PDs.

A Study on the Effects of Gallium Droplet Consumption and Post Growth Annealing on Te-Doped GaAs Nanowire Properties Grown by Self-Catalyzed Molecular Beam Epitaxy

In this work, the effects of arsenic (As) flux used during gallium (Ga) seed droplet consumption and the post-growth annealing on the optical, electrical, and microstructural properties of self-catalyzed molecular beam epitaxially grown tellurium (Te)-doped GaAs nanowires (NWs) have been investigated using a variety of characterization techniques. NWs using the same amount of As flux for growth of the seed droplet consumption demonstrated reduced density of stacking faults at the NW tip, with four-fold enhancement in the 4K photoluminescence (PL) intensity and increased single nanowire photocurrent over their higher As flux droplet consumption counterparts. Post-growth annealed NWs exhibited an additional low-energy PL peak at 1.31 eV that significantly reduced the overall PL intensity. The origin of this lower energy peak is assigned to a photocarrier transition from the conduction band to the annealing assisted Te-induced complex acceptor state (TeAsVGa−). In addition, post-growth annealing demonstrated a detrimental impact on the electrical properties of the Te-doped GaAs NWs, as revealed by suppressed single nanowire (SNW) and ensemble NW photocurrent, with a consequent enhanced low-frequency noise level compared to as-grown doped NWs. This work demonstrates that each parameter in the growth space must be carefully examined to successfully grow self-catalyzed Te-doped NWs of high quality and is not a simple extension of the growth of corresponding intrinsic NWs.

Theory of MBE Growth of Nanowires on Adsorbing Substrates: The Role of the Shadowing Effect on the Diffusion Transport.

A new model for nanowire growth by molecular beam epitaxy is proposed which extends the earlier approaches treating an isolated nanowire to the case of ensembles of nanowires. I consider an adsorbing substrate on which the arriving growth species (group III adatoms for III-V nanowires) may diffuse to the nanowire base and subsequently to the top without desorption. Analytical solution for the nanowire length evolution at a constant radius shows that the shadowing of the substrate surface is efficient and affects the growth kinetics from the very beginning of growth in dense enough ensembles of nanowires. The model fits quite well the kinetic data on different Au-catalyzed and self-catalyzed III-V nanowires. This approach should work equally well for vapor-liquid-solid and catalyst-free nanowires grown by molecular beam epitaxy and related deposition techniques on unpatterned or masked substrates.

Shallow donor and DX state in Si doped AlN nanowires grown by molecular beam epitaxy

Si doping of AlN nanowires (NWs) grown by plasma assisted molecular beam epitaxy was investigated with the objective of fabricating efficient AlN based deep ultra-violet light-emitting-diodes. The Si concentration ranged from 1016 to 1.8 × 1021 cm−3. Current–voltage measurements performed on nanowire ensembles revealed an Ohmic regime at low bias (below 0.1 V) and a space charge limited regime for higher bias. From temperature dependent current–voltage measurements, the presence of Si donors is evidenced in both shallow and deep DX states with an ionization energy of 75 and 270 meV, respectively. The role of Fermi level pinning on NWs sidewalls is discussed in terms of near surface depletion, inducing a favorable formation of shallow Si donors.

Multiscale Modeling of Sintering-Driven Conductivity in Large Nanowire Ensembles.

Thermally driven sintering is widely used to enhance the conductivity of metal nanowire (NW) ensembles in printed electronics applications, with rapid nonisothermal sintering being increasingly employed to minimize substrate damage. The rational design of the sintering process and the NW morphology is hindered by a lack of mechanistically motivated and computationally efficient models that can predict sintering-driven neck growth between NWs and the resulting change in ensemble conductivity. We present a de novo modeling framework that, for the first time, links rotation-regulated nanoscale neck growth observed in atomistic simulations to continuum conductivity evolution in inch-scale NW ensembles via an analytical neck growth model and master curve formulations of neck growth and resistivity. This framework is experimentally validated against the emergent intense pulsed light-sintering process for Ag NWs. An ultralow computational effort of 0.2 CPU-h is achieved, 4-10 orders of magnitude reduction as compared to the state of the art. We show that the inherent local variation in the relative NW orientation within an ensemble drives significant junction-specific differences in neck growth kinetics and junction resistivity. This goes beyond the conventional assumption that the neck growth kinetics is the same at all the NW junctions in an ensemble, with significant implications on how nanoscale neck growth affects ensemble-scale conductivity. Through its low computational time, easy and rapid recalibration, and experimental relevance, our framework constitutes a much-needed foundational enabler for a priori design of the sintering process and the NWs.

Agglomeration of Ensembles of Silver Nanowires, Obtained by the Method of Template Synthesis

Agglomeration of Ensembles of Silver Nanowires, Obtained by the Method of Template Synthesis

Dynamics of Monolayer Growth in Vapor-Liquid-Solid GaAs Nanowires Based on Surface Energy Minimization.

The vapor–liquid–solid growth of III-V nanowires proceeds via the mononuclear regime, where only one island nucleates in each nanowire monolayer. The expansion of the monolayer is governed by the surface energetics depending on the monolayer size. Here, we study theoretically the role of surface energy in determining the monolayer morphology at a given coverage. The optimal monolayer configuration is obtained by minimizing the surface energy at different coverages for a set of energetic constants relevant for GaAs nanowires. In contrast to what has been assumed so far in the growth modeling of III-V nanowires, we find that the monolayer expansion may not be a continuous process. Rather, some portions of the already formed monolayer may dissolve on one of its sides, with simultaneous growth proceeding on the other side. These results are important for fundamental understanding of vapor–liquid–solid growth at the atomic level and have potential impacts on the statistics within the nanowire ensembles, crystal phase, and doping properties of III-V nanowires.

Ensembles of Layered Nanowires, Obtained by Matrix Synthesis Technique, for Generation of THz Irradiation

Ensembles of parallel nanowires, consists of the parts from different ferro-magnetic metals (for example-FeNi/Co/FeNi) were obtained by method of template synthesis. It was shown that such samples could be the sources of electromagnetic emission – irradiation of Terahertz range. It could be detected when current passed through the contacts at the sample with these wires. The methods of obtaining nanowires was proposed (two-bath galvanic technique), formation of contact layers at the surface (parallel narrow strips of metal) were proposed. The features of these radiation were investigated–it‘s threshold character, it‘s non-thermal origin and spatial distribution. The calculation was also presented, which demonstrated possibility of detecting of Terahertz irradiation, using ensemble of such nanowires, included ferro-magnetic metal and anti-ferromagnetic metal.

Semiconductor nanowire arrays for optical sensing: a numerical insight on the impact of array periodicity and density

Recent advances in the nanofabrication and modeling of metasurfaces have shown the potential of these systems in providing unprecedented control over light–matter interactions at the nanoscale, enabling immediate and tangible improvement of features and specifications of photonic devices that are becoming always more crucial in enhancing everyday life quality. In this work, we theoretically demonstrate that metasurfaces made of periodic and non-periodic deterministic assemblies of vertically aligned semiconductor nanowires can be engineered to display a tailored effective optical response and provide a suitable route to realize advanced systems with controlled photonic properties particularly interesting for sensing applications. The metasurfaces investigated in this paper correspond to nanowire arrays that can be experimentally realized exploiting nanolithography and bottom-up nanowire growth methods: the combination of these techniques allow to finely control the position and the physical properties of each individual nanowire in complex arrays. By resorting to numerical simulations, we address the near- and far-field behavior of a nanowire ensemble and we show that the controlled design and arrangement of the nanowires on the substrate may introduce unprecedented oscillations of light reflectance, yielding a metasurface which displays an electromagnetic behavior with great potential for sensing. Finite-difference time-domain numerical simulations are carried out to tailor the nanostructure parameters and systematically engineer the optical response in the VIS-NIR spectral range. By exploiting our computational-methods we set-up a complete procedure to design and test metasurfaces able to behave as functional sensors. These results are especially encouraging in the perspective of developing arrays of epitaxially grown semiconductor nanowires, where the suggested design can be easily implemented during the nanostructure growth, opening the way to fully engineered nanowire-based optical metamaterials.

Reducing Conductivity Variability in Si Nanowires via Surface Passivation for Nanoelectronics

Ensembles of semiconductor nanowires grown via the bottom-up vapor–liquid–solid (VLS) mechanism, especially those that are lightly doped or nominally undoped, can exhibit large nanowire-to-nanowire...

Broadband optical spin dependent reflection in self-assembled GaAs-based nanowires asymmetrically hybridized with Au

Hybridization of semiconductor nanostructures with asymmetric metallic layers offers new paths to circular polarization control and chiral properties. Here we study, both experimentally and numerically, chiral properties of GaAs-based nanowires (NWs) which have two out of six sidewalls covered by Au. Sparse ensembles of vertical, free-standing NWs were fabricated by means of lithography-free self-assembled technique on Si substrates and subsequently covered by Au using tilted evaporation. We report on optical spin-dependent specular reflection in the 680–1000 nm spectral range when the orientation of the golden layers follows the rule of extrinsic chirality. The analysis shows reflection peaks of the chiral medium whose intensity is dependent on the light handedness. We further propose a novel, time-efficient numerical method that enables a better insight into the far-field intensity and distribution of the scattered light from a sparse NW ensembles. The measurements done on three different samples in various orientations show good agreement with theoretical predictions over a broad wavelength range.