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Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al deficient core and an Al rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies, but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature., This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2019), copyright (C) American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.9b01521, the supporting information is available (free of charge) under the same link

We demonstrate an all-epitaxial and scalable growth approach to fabricate single-crystalline GaN nanowires on graphene by plasma-assisted molecular beam epitaxy. As substrate, we explore several types of epitaxial graphene layer structures synthesized on SiC. The different structures differ mainly in their total number of graphene layers. Because graphene is found to be etched under active N exposure, the direct growth of GaN nanowires on graphene is only achieved on multilayer graphene structures. The analysis of the nanowire ensembles prepared on multilayer graphene by Raman spectroscopy and transmission electron microscopy reveals the presence of graphene underneath as well as in between nanowires, as desired for the use of this material as contact layer in nanowire-based devices. The nanowires nucleate preferentially at step edges, are vertical, well aligned, epitaxial, and of comparable structural quality as similar structures fabricated on conventional substrates.

Graphene nanoribbons (GNRs) are promising for applications in nanoelectronics due to their unique properties. Therefore, achieving the controlled and high-quality synthesis of GNRs is anticipated to be of great importance. One of the methods which shows great potential is the growth of GNRs on surface facets of SiC(0001) by the surface graphitization method. In this report we studied the dependency of the GNR width on growth temperature and SiC substrate miscut angle (or initial step height). While a linear growth rate best describes the growth in lower step heights, a nonlinear rate is observed for substrates with higher steps, which is also associated with the formation of few-layer graphene on the step edges. The structural characterization of the samples was performed by means of atomic force microscopy, scanning electron microscopy, and Raman spectroscopy.

To understand the behavior of a semiconductor device, you need to know how the material's $e\phantom{\rule{0}{0ex}}x\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}s$ (weakly bound electron-hole pairs) behave. The exciton diffusion length in GaN is often determined from the size of dark areas around dislocation outcroppings in cathodoluminescence maps. The present work shows that, contrary to common belief, the spatial extent of these areas is only weakly affected by diffusion, being set primarily by exciton dissociation in the piezoelectric field around the dislocation. In contrast, the local band-gap variation in the dislocation's strain field $d\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$ depend sensitively on exciton diffusion length, and hence enables its experimental determination.

Correction to Absence of Quantum-Confined Stark Effect in GaN Quantum Disks Embedded in (Al,Ga)N Nanowires Grown by Molecular Beam Epitaxy

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires (NWs), respectively, on different types of polycrystalline Ti foils. The NW orientation with respect to the substrate normal, which is affected by an in situ treatment of the foil surface before NW growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented NWs due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the NW growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiO x N y species. These compounds are more stable against chemical reactions and facilitate the growth of uniformly oriented GaN NW ensembles on the surface of the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented NWs are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of NW orientation and the understanding behind the achievement of vertically oriented NWs obtained in this study represent an important step towards the realization of GaN NW-based bendable devices on polycrystalline metal foils. Locked until 20.9.2018 due to copyright restrictions. This is an author-created, un-copyedited version of an article accepted for publication/published in [Nanotechnology]. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://iopscience.iop.org/article/10.1088/1361-6528/aa84a1/meta

We explore the Bi-surfactant-directed self-assembly and structure of InAs quantum dots grown on GaAs(110) by molecular beam epitaxy. The addition of a Bi flux during InAs deposition changes the InAs growth mode from two-dimensional (2D) Frank-van der Merwe to Stranski-Krastanov, resulting in the formation of three-dimensional (3D) InAs islands on the surface. Furthermore, exposing static InAs 2D layers to Bi induces a rearrangement of the strained layer into 3D islands. We explore the effect of varying the InAs thickness and Bi flux for these two growth approaches, observing a critical thickness for 3D island formation in both cases. Characterization of (110) InAs quantum dots with high-resolution transmission electron microscopy reveals that larger islands grown by the Stranski-Krastanov mode are plastically relaxed, while small islands grown by the on-demand approach are coherent. Strain relaxation along the [1-10] direction is achieved by 90 degree pure-edge dislocations with dislocation lines running along [001]. In contrast, strain relief along [001] is by 60 degree misfit dislocations. This behaviour is consistent with observations of planar (In,Ga)As/GaAs(110) layers. These results illustrate how surfactant Bi can provoke and control quantum dot formation where it normally does not occur.

We demonstrate the self-assembled growth of vertically aligned GaN nanowire ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The analysis of single nanowires by transmission electron microscopy reveals that they are single crystalline. Low-temperature photoluminescence spectroscopy demonstrates that, in comparison to standard GaN nanowires grown on Si, the nanowires prepared on the Ti foil exhibit a equivalent crystalline perfection, a higher density of basal-plane stacking faults, but a reduced density of inversion domain boundaries. The room-temperature photoluminescence spectrum of the nanowire ensemble is not influenced or degraded by the bending of the substrate. The present results pave the way for the fabrication of flexible optoelectronic devices based on GaN nanowires on metal foils., 4 pages, 3 figures

We theoretically analyze the contrast observed at the outcrop of a threading dislocation at the GaN(0001) surface in cathodoluminescence and electron-beam induced current maps. We consider exciton diffusion and recombination including finite recombination velocities both at the planar surface and at the dislocation. Formulating the reciprocity theorem for this general case enables us to provide a rigorous analytical solution of this diffusion-recombination problem. The results of the calculations are applied to an experimental example to determine both the exciton diffusion length and the recombination strength of threading dislocations in a free-standing GaN layer with a dislocation density of $6\times10^{5}$~cm$^{-2}$.

Hexagonal GaN microcrystals of a size between 1 to 3 μm are obtained by the pendeoepitaxial overgrowth of a GaN nanowire template on Si(111). The GaN microcrystals are free of threading dislocation...

On Si(1 1 1) and Si(0 0 1), GaN nanowires (NWs) form in a self-induced way without the need for any external material. On sapphire, NW growth is induced by Ni collectors. Both types of NWs exhibit the wurtzite crystal structure and grow in the Ga-polar C-direction perpendicular to the substrate. The NW sidewalls are M-plane facets, although on the Ni-induced NWs also A-plane segments form, if the growth temperature is low. Both self-induced and collector-induced NWs are free of strain and epitaxially aligned to the substrate, but in particular the former show a significant spread in tilt and twist caused by a mostly amorphous interfacial layer of Si-N. The self-induced NWs are virtually free of extended defects, but the collector-induced NWs contain many stacking faults. The photoluminescence of the former is significantly brighter and sharper. The spectra of single, dispersed, self-induced NWs contain extremely sharp excitonic lines. Significant emission is caused by excitons bound to donors close to the surface whose binding energy is reduced compared to the bulk value. In comparison, both the microstructure and optical properties of the self-induced NWs are superior. The limited material quality of the collector-induced NWs can be explained by detrimental effects of the collector.

GaN nanowires are grown as templates for pendeoepitaxial coalescence overgrowth at substrate temperatures between 720 and 820 °C on Si(1 1 1) by molecular beam epitaxy. The unintentional coalescence of nanowires is suppressed with increasing substrate temperature. Simultaneously, the width of the donor-bound-exciton transition at 3.472 eV decreases down to 1.6 meV. The length distribution of the GaN nanowires strongly influences the morphology of the pendeoepitaxial layer after coalescence.

Graphene, a two-dimensional carbon allotrope, exhibits excellent optoelectronic properties. The assembly of graphene into films provides a platform to deepen the study of its interaction with varying surfaces, to engineer devices, and to develop functional materials. A general approach to produce graphene films consists of preparing a dispersion and laying it on a substrate of choice, followed by solvent evaporation. Here, we report the preparation of stable suspensions of new types of graphene nanomaterials namely, graphene nanoflowers (GNFs) and multi-layer graphene (MLG) flakes, in ethanol, N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP). Sprayable suspensions of both GNFs and MLG were prepared in DMF/ethanol, which showed high stability, without addition of any surfactant. The stable suspensions were used to deposit micrometer-thick MLG/GNF films on glass substrates. Calculations of initial droplet size and of timescale of droplet evaporation are performed and possible thermophoretic effects on droplet deposition discussed as well. Coating glass substrates with a methacrylic acid–methyl methacrylate (MA) copolymer prior to the deposition significantly improved the adhesion of the nanomaterials to the substrate. With the MA coating, a substrate coverage of nearly 100% was achieved at 14-min spraying time for 0.05 wt% GNF and 0.1 wt% MLG suspensions. Raman spectra of the GNF and MLG films reveal that the films were made of MLG in which the individual graphene layers rotated from each other as in turbostratic graphene. This work provides a general approach to prepare graphene nanomaterial suspensions and to create films for a variety of applications. The spraying process applied in the current work is highly scalable and allows control of film characteristics through process parameters. Copyright 2015 American Association for Aerosol Research

Annealing of silicon-carbon nanoparticles was performed in argon at atmospheric pressure to enable formation of silicon carbide nanomaterials and/or carbon structures. Three precursor powders with increasing crystallinity and annealing temperatures from 1,900 to 2,600 °C were used to gain information about the effect of precursor properties (e.g. amorphous vs. nanocrystalline, carbon content) and annealing temperature on the produced materials. Three structures were found after annealing, i.e. silicon carbide crystals, carbon sheets and spherical carbon particles. The produced SiC crystals consisted of several polytypes. Low annealing temperature and increasing crystallinity of the precursor promoted the formation of the 3C-SiC polytype. Raman analysis indicated the presence of single-layer, undoped graphene in the sheets. The spherical carbon particles consisted of curved carbon layers growing from the amorphous Si–C core and forming a ‘nanoflower’ with a diameter below 60 nm. To our knowledge, the formation of this kind of structures has not been reported previously. The core was visible in transmission electron microscopy analysis at the annealing temperature of 1,900 °C, decreased in size with increasing temperature and disappeared above an annealing temperature of 2,200 °C. With increasing crystallinity of the precursor material, fewer layers (~5 with the most crystalline precursor) were detected in the carbon nanoflowers. The method presented opens up the possibility to produce new carbon nanostructures whose properties can be controlled by changing the properties of the precursor material or by adjusting an annealing temperature.

We investigate the 3.45-eV luminescence band of spontaneously formed GaN nanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy. This band is found to be particularly prominent for samples synthesized at comparatively low temperatures. At the same time, these samples exhibit a peculiar morphology, namely, isolated long nanowires are interspersed within a dense matrix of short ones. Cathodoluminescence intensity maps reveal the 3.45-eV band to originate primarily from the long nanowires. Transmission electron microscopy shows that these long nanowires are either Ga polar and are joined by an inversion domain boundary with their short N-polar neighbors, or exhibit a Ga-polar core surrounded by a N-polar shell with a tubular inversion domain boundary at the core/shell interface. For samples grown at high temperatures, which exhibit a uniform nanowire morphology, the 3.45-eV band is also found to originate from particular nanowires in the ensemble and thus presumably from inversion domain boundaries stemming from the coexistence of N- and Ga-polar nanowires. For several of the investigated samples, the 3.45-eV band splits into a doublet. We demonstrate that the higher-energy component of this doublet arises from the recombination of two-dimensional excitons free to move in the plane of the inversion domain boundary. In contrast, the lower-energy component of the doublet originates from excitons localized in the plane of the inversion domain boundary. We propose that this in-plane localization is due to shallow donors in the vicinity of the inversion domain boundaries., Comment: 24 pages, 12 figures, 1 table

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E(g,ZB) are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E(g,WZ)-E(g,ZB)., 8 pages, 10 figures

We present a universal approach for determining the spontaneous polarization Psp of a wurtzite semiconductor from the emission energies of excitons bound to the different types of stacking faults in these crystals. Employing micro-photoluminescence and cathodoluminescence spectroscopy, we observe emission lines from the intrinsic and extrinsic stacking faults in strain-free GaN micro-crystals. By treating the polarization sheet charges associated with these stacking faults as a plate capacitor, Psp can be obtained from the observed transition energies with no additional assumptions. Self-consistent Poisson-Schroedinger calculations, aided by the microscopic electrostatic potential computed using density-functional theory, lead to nearly identical values for Psp. Our recommended value for Psp of GaN is -0.022+/-0.007 C/m^{2}., 5 pages, 5 figures

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires (NWs) transferred to a SAW beam line on a LiNbO3 crystal. We show that carriers generated in the NW by a focused light spot can be acoustically transported to a second location, where they recombine emitting short light pulses. The results presented here demonstrate the high-frequency manipulation of carriers in NWs without the use of electrical contacts, which opens new perspectives for applications in opto-electronic devices operating at GHz frequencies.

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.

The incorporation of Au during vapor−liquid−solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free grown GaAs/(Al,Ga)As core−shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures.

This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.

We investigate the low-temperature photoluminescence of GaN nanowires grown catalyst free on Si(111). For single nanowires dispersed on Si(111), we observe excitonic transitions with linewidths below $300\text{ }\ensuremath{\mu}\text{eV}$ and at energies clearly above the donor-bound exciton in the bulk. We show that these transitions are due to donor-bound excitons close to the surface. The broadening of about 3 meV observed for the nanowire ensemble is shown to be a natural consequence of the energy dispersion of bound-exciton states as a function of their distance from the surface.

International audience; We observe a significant increase in the photoluminescence intensity of GaN nanowires under continuous ultraviolet irradiation on a time scale of minutes. Experiments carried out under different ambient conditions demonstrate that this increase is caused by the photoinduced desorption of oxygen from the nanowire side-walls. The slow, highly nonexponential temporal evolution of the photoluminescence signal is modeled by a random-walk approach. The model reveals that already desorbed oxygen molecules are likely to be readsorbed at adjacent nanowires. Time-resolved photoluminescence measurements are performed to unravel the correlation between the oxygen desorption and the increase in the photoluminescence intensity. We find that the oxygen desorption unpins the Fermi level, which in turn leads to an increase in quantum efficiency by enhancing the radiative decay of excitons.

We use Raman scattering to investigate how light penetrates into and escapes out of GaN nanowires with diameters and interspacings smaller than the effective wavelength of the light. Quantitatively, the efficiencies for light coupling into and out of the nanowires can be explained by considering the nanowire ensemble as a homogeneous effective medium. The observed phonon modes in the Raman spectra, however, suggest that light enters the nanowires to a considerable extent through their side facets even for normal incidence. This finding is supported by rigorous calculations of the spatially distributed light intensity within a periodic nanowire structure.

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport.

Single, free-standing GaN nanowires grown by plasma-assisted molecular-beam epitaxy have been investigated with low temperature micro-photoluminescence. The quantitative analysis of the luminescence spectra of around 100 nanowires revealed that each nanowire exhibits its own individual spectrum. A significant fraction of nanowires exclusively emits at energies corresponding to either surface-donor-bound or free excitons, demonstrating that optical properties of individual nanowires are determined by a few impurity atoms alone. The number of impurities per nanowire and their location within the nanowires varies according to Poissonian statistics.