A. T. J. van Helvoort

Norwegian University of Science and Technology, Nidaros, Sør-Trøndelag, Norway

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Publications (64)198.89 Total impact

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    ABSTRACT: Ternary semiconductor nanowire arrays enable scalable fabrication of nano-optoelectronic devices with tunable bandgap. However, the lack of insights into the effects of the incorporation of Vy element results in lack of control on the growth of ternary III-V1-yVy nanowires and hinders the development of high-performance nanowire devices based on such ternaries. Here, we report on the origins of Sb-induced effects affecting the morphology and crystal structure of self-catalyzed GaAsSb nanowire arrays. The nanowire growth by molecular beam epitaxy are changed both kinetically and thermodynamically by the introduction of Sb. An anomalous decrease of the axial growth rate with increased Sb2 flux is found to be due to both the indirect kinetic influence via the Ga adatom diffusion induced catalyst geometry evolution and the direct composition modulation. From the fundamental growth analyses and the crystal phase evolution mechanism proposed in this Letter, the phase transition/stability in catalyst-assisted ternary III-V-V nanowire growth can be well explained. Wavelength tunability with good homogeneity of the optical emission from the self-catalyzed GaAsSb nanowire arrays with high crystal phase purity is demonstrated by only adjusting the Sb2 flux.
    No preview · Article · Jan 2016 · Nano Letters
  • V T Fauske · M B Erlbeck · J Huh · D C Kim · A M Munshi · D L Dheeraj · H Weman · B O Fimland · A T J VAN Helvoort
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    ABSTRACT: For the development of electronic nanoscale structures, feedback on its electronic properties is crucial, but challenging. Here, we present a comparison of various in situ methods for electronically probing single, p-doped GaAs nanowires inside a scanning electron microscope. The methods used include (i) directly probing individual as-grown nanowires with a sharp nano-manipulator, (ii) contacting dispersed nanowires with two metal contacts and (iii) contacting dispersed nanowires with four metal contacts. For the last two cases, we compare the results obtained using conventional ex situ litho-graphy contacting techniques and by in situ, direct-write electron beam induced deposition of a metal (Pt). The comparison shows that 2-probe measurements gives consistent results also with contacts made by electron beam induced deposition, but that for 4-probe, stray deposition can be a problem for shorter nanowires. This comparative study demonstrates that the preferred in situ method depends on the required throughput and reliability.
    No preview · Article · Oct 2015 · Journal of Microscopy
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    ABSTRACT: GaAs nanowires (NWs) are seen as promising building blocks for future optoelectronic devices. To ensure reproducible properties, a high NW uniformity is required. Here, a substantial number of both position-controlled and randomly grown self-catalyzed GaAs/AlGaAs core-shell NWs are compared. Single NWs are characterized by correlated microphotoluminescence (µ-PL) spectroscopy and transmission electron microscopy (TEM). TEM is done in the 〈110〉- and 〈112〉-projections, and on the 〈111〉-cross-section of the NWs. The position-control grown NWs showed a higher degree of uniformity in morphology. All NWs on both samples had a predominantly stacking fault free zinc blende structure, with a main optical response around the GaAs free exciton energy. However, NW-to-NW structural variations in the tip region and radial compositional variations in the shell are present in both samples. These structural features could be the origin of variations in the optical response just below and above the free exciton energy. This correlated study demonstrates that the observed distinct, sharp PL peaks in the 1.6 - 1.8 eV energy range present in several NWs, are possibly related to radial compositional variations in the AlGaAs shell rather than the structural defects in the tip region.
    No preview · Article · Oct 2015 · Journal of Physics Conference Series
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    ABSTRACT: Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.
    No preview · Article · May 2015 · Nano Letters
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    H. Kauko · B. O. Fimland · T. Grieb · A. M. Munshi · K. Müller · A. Rosenauer · A. T. J. van Helvoort
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    ABSTRACT: There is no abstract available for this article.
    Preview · Article · Dec 2014 · Journal of Applied Physics
  • Lewys Jones · Katherine E MacArthur · Vidar T Fauske · Antonius T J van Helvoort · Peter D Nellist
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    ABSTRACT: Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticle's 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.
    No preview · Article · Oct 2014 · Nano Letters
  • H. Kauko · B. O. Fimland · T. Grieb · A. M. Munshi · K. Mueller · A. Rosenauer · A. T. J. van Helvoort
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    ABSTRACT: The near-surface reduction of the Sb mole fraction during the growth of GaAsSb nanowires (NWs) and GaAs NWs with GaAsSb inserts has been studied using quantitative high-angle annular dark field scanning transmission electron microscopy (STEM). A model for diffusion of Sb in the hexagonal NWs was developed and employed in combination with the quantitative STEM analysis. GaAsSb NWs grown by Ga-assisted molecular beam epitaxy (MBE) and GaAs/GaAsSb NWs grown by Ga- and Au-assisted MBE were investigated. At the high temperatures employed in the NW growth, As-Sb exchange at and outward diffusion of Sb towards the surface take place, resulting in reduction of the Sb concentration at and near the surface in the GaAsSb NWs and the GaAsSb inserts. In GaAsSb NWs, an increasing near-surface depletion of Sb was observed towards the bottom of the NW due to longer exposure to the As beam flux. In GaAsSb inserts, an increasing change in the Sb concentration profile was observed with increasing post-insert axial GaAs growth time, resulting from a combined effect of radial GaAs overgrowth and diffusion of Sb. The effect of growth temperature on the diffusion of Sb in the GaAsSb inserts was identified. The consequences of these findings for growth optimization and the optoelectronic properties of GaAsSb are discussed. (C) 2014 AIP Publishing LLC.
    No preview · Article · Oct 2014 · Journal of Applied Physics
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    ABSTRACT: Monodisperse iron nanoparticles of size range 5–19 nm with controlled crystallinity and magnetic properties are synthesized via thermal decomposition of iron pentacarbonyl in the presence of an appropriate ligand. Crystalline body-centered cubic iron nanoparticles synthesized in the presence of didodecyldimethylammonium bromide exhibit a high saturation magnetization and room temperature ferromagnetic behavior with a remarkable increased stability in the aqueous phase in comparison to amorphous nanoparticles synthesized in the presence of other ligands.
    Full-text · Article · Oct 2014 · Particle and Particle Systems Characterization
  • Lewys Jones · Vidar T. Fauske · Katherine E. MacArthur · Antonius T. J. van Helvoort · Peter D. Nellist

    No preview · Article · Aug 2014 · Microscopy and Microanalysis
  • H. Kauko · T. Grieb · A. M. Munshi · K. Müller · A. Rosenauer · B. O. Fimland · A. T. J. van Helvoort

    No preview · Article · Aug 2014 · Microscopy and Microanalysis
  • V T Fauske · D C Kim · A M Munshi · D L Dheeraj · B-O Fimland · H Weman · A T J van Helvoort
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    ABSTRACT: A method for probing the electrical and structural characteristics of individual as-grown III-V nanowires was studied. In-situ electrical characterization was performed in a focused ion beam / scanning electron microscopy system by using a fine nano-manipulator and ion beam assisted deposition. Transmission electron microscopy specimens of probed nanowires are prepared afterwards. This method would potentially allow the correlation of electrical and structural characteristics (e.g. crystal faults such as twinning) of the nanowire-substrate system. The challenge is in contacting the nanowires so that the electrical characteristics of the nanowire-substrate system can be extracted correctly.
    No preview · Article · Jun 2014 · Journal of Physics Conference Series
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    ABSTRACT: We report on the epitaxial growth of large-area position controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2-inch Si wafers using nanoimprint lithography (NIL), for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs, the MBE growth parameter space is tuned, including Ga pre-deposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e. As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position controlled NW arrays for various optoelectronic device applications.
    Full-text · Article · Jan 2014 · Nano Letters
  • H. Kauko · C. L. Zheng · Y. Zhu · S. Glanvill · C. Dwyer · A. M. Munshi · B. O. Fimland · A. T. J. van Helvoort · J. Etheridge
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    ABSTRACT: We demonstrate a method for compositional mapping of AlxGa1-xAs heterostructures with high accuracy and unit cell spatial resolution using quantitative high angle annular dark field scanning transmission electron microscopy. The method is low dose relative to spectroscopic methods and insensitive to the effective source size and higher order lens aberrations. We apply the method to study the spatial variation in Al concentration in cross-sectioned GaAs/AlGaAs core-shell nanowires and quantify the concentration in the Al-rich radial band and the AlGaAs shell segments.
    No preview · Article · Dec 2013 · Applied Physics Letters
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    J Todorovic · H Kauko · L Ahtapodov · A F Moses · P Olk · D L Dheeraj · B O Fimland · H Weman · A T J Van Helvoort
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    ABSTRACT: In this work we have investigated the variation of Sb concentration among and within zinc blende (ZB) GaAsSb inserts in wurtzite (WZ) GaAs bare-core and WZ GaAs/AlGaAs core–shell nanowires (NWs) grown by Au-assisted molecular beam epitaxy. The Sb concentration variation was related to the optical properties as determined by photoluminescence (PL). The NW structure and the Sb concentration were studied by transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and quantitative high angle annular dark field scanning TEM (HAADF STEM). A clear trend relating the maximum Sb concentration with the insert length was observed: the longer the insert, the higher the Sb concentration. In addition, there are graded Sb concentration gradients both along and across the GaAsSb inserts. The influence of the Sb concentration variation on the PL emission from the GaAsSb inserts was investigated with correlated micro-PL and TEM-EDX on the same single NWs. Based on the PL results and the observed Sb concentration profiles, we propose a qualitative energy band diagram for a typical ZB GaAsSb insert in a WZ GaAs NW for the heterostructured NWs studied here. Type I transitions within the central region of the ZB GaAsSb inserts were found to dominate the insert-related PL emission. Weak type II transitions within the inserts due to the graded Sb concentration were observed as well. Using an existing empirical model, the Sb concentrations were additionally determined from the ground state PL energies (type I transition). For the average Sb concentration, the concentrations based on PL were in agreement with EDX and quantitative HAADF STEM results. (Some figures may appear in colour only in the online journal)
    Full-text · Dataset · Sep 2013
  • Edita Garskaite · Andreas S. Flø · Antonius T.J. van Helvoort · Aivaras Kareiva · Espen Olsen
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    ABSTRACT: TiO2 and TiO2:Nd,Yb films were deposited by a doctor blade deposition technique from pastes prepared by a so-gel process, and characterized by electron microscopy and spectroscopic techniques. Near infrared (NIR) photoluminescence (PL) properties upon 808 nm excitation were also examined. The rutile TiO2:Nd,Yb samples exhibited the strongest NIR PL signal. The relationship between the morphological properties, annealing temperature and the optical behavior of TiO2:Nd,Yb films is discussed. Furthermore, the study showed that hyperspectral imaging spectroscopy can be used as a rapid and nondestructive macroscopic characterization technique for the identification of spectral features and evaluation of luminescent surfaces of oxides. (c) 2013 Elsevier B.V. All rights reserved.
    No preview · Article · Aug 2013 · Journal of Luminescence
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    ABSTRACT: Accomplishing control of the crystal phases in III–V semiconductor nanowires (NWs) is important for applications in future advanced nano-devices. In this work, we report on the growth of both zinc blende (ZB) and wurtzite (WZ) GaAs in self-catalyzed GaAs and GaAs/GaAsSb axial heterostructured NWs on Si(111) substrates by the vapor–liquid–solid technique using molecular beam epitaxy. The self-catalyzed GaAs NWs usually adopt the ZB phase. However, by growing GaAs NWs with short GaAsSb axial inserts, the crystal phase of GaAs can be changed from ZB below the GaAsSb insert, to WZ above. This crystal phase change in GaAs can be explained in terms of a change in the contact angle of the Ga droplet which changes the fraction of the triple-phase-line in contact with the edge of the NW top facet and therefore affects the probability of nucleation for ZB and WZ phases. In addition, by growing ZB GaAs after the insert using a flux interruption, we demonstrate the growth of all combinations of crystal phases of the GaAs segments on both sides of the GaAsSb insert. This understanding has also enabled us to achieve the growth of WZ phase in GaAs NWs as well as a ZB–WZ–ZB GaAs NW heterostructure, by changing the Ga droplet contact angle, without growing a GaAsSb insert. The contact angle was controlled by introducing different flux interruptions and tuning the V/III flux ratio.
    Full-text · Article · Jun 2013 · Journal of Crystal Growth
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    ABSTRACT: Accomplishing control of the crystal phases in III–V semiconductor nanowires (NWs) is important for applications in future advanced nano-devices. In this work, we report on the growth of both zinc blende (ZB) and wurtzite (WZ) GaAs in self-catalyzed GaAs and GaAs/GaAsSb axial heterostructured NWs on Si (111) substrates by the vapor–liquid–solid technique using molecular beam epitaxy. The self-catalyzed GaAs NWs usually adopt the ZB phase. However, by growing GaAs NWs with short GaAsSb axial inserts, the crystal phase of GaAs can be changed from ZB below the GaAsSb insert, to WZ above. This crystal phase change in GaAs can be explained in terms of a change in the contact angle of the Ga droplet which changes the fraction of the triple-phase-line in contact with the edge of the NW top facet and therefore affects the probability of nucleation for ZB and WZ phases. In addition, by growing ZB GaAs after the insert using a flux interruption, we demonstrate the growth of all combinations of crystal phases of the GaAs segments on both sides of the GaAsSb insert. This understanding has also enabled us to achieve the growth of WZ phase in GaAs NWs as well as a ZB–WZ–ZB GaAs NW heterostructure, by changing the Ga droplet contact angle, without growing a GaAsSb insert. The contact angle was controlled by introducing different flux interruptions and tuning the V/III flux ratio. & 2013 Elsevier B.V. All rights reserved.
    Full-text · Dataset · Mar 2013
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    D L Dheeraj · A M Munshi · M Scheffler · A T J van Helvoort · H Weman · B O Fimland
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    ABSTRACT: Control of the crystal phases of GaAs nanowires (NWs) is essential to eliminate the formation of stacking faults which deteriorate the optical and electronic properties of the NWs. In addition, the ability to control the crystal phase of NWs provides an opportunity to engineer the band gap without changing the crystal material. We show that the crystal phase of GaAs NWs grown on GaAs(111)B substrates by molecular beam epitaxy using the Au-assisted vapor-liquid-solid growth mechanism can be tuned between wurtzite (WZ) and zinc blende (ZB) by changing the V/III flux ratio. As an example we demonstrate the realization of WZ GaAs NWs with a ZB GaAs insert that has been grown without changing the substrate temperature.
    Full-text · Article · Jan 2013 · Nanotechnology
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    ABSTRACT: We report on the growth, structural and electrical characterizations of Be-doped GaAs nanowires (NWs) grown by the Au- and Ga-assisted vapour-liquid-solid techniques using molecular beam epitaxy. The growth rate of Be-doped GaAs NWs grown by the Au-assisted technique is observed to be lower as compared to the growth rate of undoped GaAs NWs grown under identical conditions. However, no effect on either the growth rate or the morphology of NWs has been observed for Be-doped GaAs NWs grown by the Ga-assisted technique with the same Be flux as used for the Au-assisted technique. Electrical characterization reveals that the Ga-assisted grown NWs show more consistent, symmetric current-voltage (I-V) characteristics with higher electrical current than the Au-assisted grown NWs. Finally, we show that ohmic contacts to Be-doped Ga-assisted NWs can be achieved either by post-annealing the metal-contacted NW or increasing the doping concentration during the NW growth.
    Full-text · Article · Jan 2013 · Journal of Crystal Growth
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    ABSTRACT: By utilizing the reduced contact area of nanowires, we show that epitaxial growth of a broad range of semiconductors on graphene can in principle be achieved. A generic atomic model is presented which describes the epitaxial growth configurations applicable to all conventional semiconductor materials. The model is experimentally verified by demonstrating the growth of vertically aligned GaAs nanowires on graphite and few-layer graphene by the self-catalyzed vapor-liquid-solid technique using molecular beam epitaxy. A two-temperature growth strategy was used to increase the nanowire density. Due to the self-catalyzed growth technique used, the nanowires were found to have a regular hexagonal cross-sectional shape, and are uniform in length and diameter. Electron microscopy studies reveal an epitaxial relationship of the grown nanowires with the underlying graphitic substrates. Two relative orientations of the nanowire side-facets were observed, which is well explained by the proposed atomic model. A prototype of a single GaAs nanowire photodetector demonstrates a high-quality material. With GaAs being a model system, as well as a very useful material for various optoelectronic applications, we anticipate this particular GaAs nanowire/graphene hybrid to be promising for flexible and low-cost solar cells.
    Full-text · Article · Aug 2012 · Nano Letters

Publication Stats

837 Citations
198.89 Total Impact Points

Institutions

  • 2004-2015
    • Norwegian University of Science and Technology
      • • Department of Physics
      • • Department of Materials Science and Engineering
      Nidaros, Sør-Trøndelag, Norway
  • 2011
    • NTNU Samfunnsforskning
      Nidaros, Sør-Trøndelag, Norway
  • 2003-2004
    • University of Cambridge
      • Department of Materials Science and Metallurgy
      Cambridge, England, United Kingdom