Publications by authors named "Martin Aagesen"

20 Publications

  • Page 1 of 1

Defect-Free Axially Stacked GaAs/GaAsP Nanowire Quantum Dots with Strong Carrier Confinement.

Nano Lett 2021 Jul 28;21(13):5722-5729. Epub 2021 Jun 28.

Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom.

Axially stacked quantum dots (QDs) in nanowires (NWs) have important applications in nanoscale quantum devices and lasers. However, there is lack of study of defect-free growth and structure optimization using the Au-free growth mode. We report a detailed study of self-catalyzed GaAsP NWs containing defect-free axial GaAs QDs (NWQDs). Sharp interfaces (1.8-3.6 nm) allow closely stack QDs with very similar structural properties. High structural quality is maintained when up to 50 GaAs QDs are placed in a single NW. The QDs maintain an emission line width of <10 meV at 140 K (comparable to the best III-V QDs, including nitrides) after having been stored in an ambient atmosphere for over 6 months and exhibit deep carrier confinement (∼90 meV) and the largest reported exciton-biexciton splitting (∼11 meV) for non-nitride III-V NWQDs. Our study provides a solid foundation to build high-performance axially stacked NWQD devices that are compatible with CMOS technologies.
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http://dx.doi.org/10.1021/acs.nanolett.1c01461DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8289304PMC
July 2021

Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids.

Adv Mater 2020 Jun 26;32(23):e1908411. Epub 2020 Apr 26.

Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Copenhagen, 2100, Denmark.

Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminum heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. This work introduces a crystal growth platform-based on 3D structuring of growth substrates-which enables synthesis of semiconductor nanowire hybrids with in situ patterned superconductor shells. The platform eliminates the need for etching, thereby enabling full freedom in the choice of hybrid constituents. All of the most frequently used superconducting hybrid device architectures are realized and characterized. These devices exhibit increased yield and electrostatic stability compared to etched devices, and evidence of ballistic superconductivity is observed. In addition to aluminum, hybrid structures based on tantalum, niobium, and vanadium are presented.
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http://dx.doi.org/10.1002/adma.201908411DOI Listing
June 2020

Highly Strained III-V-V Coaxial Nanowire Quantum Wells with Strong Carrier Confinement.

ACS Nano 2019 May 9;13(5):5931-5938. Epub 2019 May 9.

Department of Electronic and Electrical Engineering , University College London , London WC1E 7JE , United Kingdom.

Coaxial quantum wells (QWs) are ideal candidates for nanowire (NW) lasers, providing strong carrier confinement and allowing close matching of the cavity mode and gain medium. We report a detailed structural and optical study and the observation of lasing for a mixed group-V GaAsP NW with GaAs QWs. This system offers a number of potential advantages in comparison to previously studied common group-V structures ( e. g., AlGaAs/GaAs) including highly strained binary GaAs QWs, the absence of a lower band gap core region, and deep carrier potential wells. Despite the large lattice mismatch (∼1.7%), it is possible to grow defect-free GaAs coaxial QWs with high optical quality. The large band gap difference results in strong carrier confinement, and the ability to apply a high degree of compressive strain to the GaAs QWs is also expected to be beneficial for laser performance. For a non-fully optimized structure containing three QWs, we achieve low-temperature lasing with a low external (internal) threshold of 20 (0.9) μJ/cm/pulse. In addition, a very narrow lasing line width of ∼0.15 nm is observed. These results extend the NW laser structure to coaxial III-V-V QWs, which are highly suitable as the platform for NW emitters.
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http://dx.doi.org/10.1021/acsnano.9b01775DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7007272PMC
May 2019

Superconducting vanadium/indium-arsenide hybrid nanowires.

Nanotechnology 2019 Jul 4;30(29):294005. Epub 2019 Apr 4.

Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.

We report MBE synthesis of InAs/vanadium hybrid nanowires. The vanadium was deposited without breaking ultra-high vacuum after InAs nanowire growth, minimizing any effect of oxidation and contamination at the interface between the two materials. We investigated four different substrate temperatures during vanadium deposition, ranging from -150 °C to 250 °C. The structural relation between vanadium and InAs depended on the deposition temperature. The three lower temperature depositions gave vanadium shells with a polycrystalline, granular morphology and the highest temperature resulted in vanadium reacting with the InAs nanowire. We fabricated electronic devices from the hybrid nanowires and obtained a high out-of-plane critical magnetic field, exceeding the bulk value for vanadium. However, size effects arising from the nanoscale grains resulted in the absence of a well-defined critical temperature, as well as device-to-device variation in the resistivity versus temperature dependence during the transition to the superconducting state.
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http://dx.doi.org/10.1088/1361-6528/ab15fcDOI Listing
July 2019

Toward electrically driven semiconductor nanowire lasers.

Nanotechnology 2019 May 18;30(19):192002. Epub 2019 Jan 18.

Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom.

Semiconductor nanowire (NW) lasers are highly promising for making new-generation coherent light sources with the advantages of ultra-small size, high efficiency, easy integration and low cost. Over the past 15 years, this area of research has been developing rapidly, with extensive reports of optically pumped lasing in various inorganic and organic semiconductor NWs. Motivated by these developments, substantial efforts are being made to make NW lasers electrically pumped, which is necessary for their practical implementation. In this review, we first categorize NW lasers according to their lasing wavelength and wavelength tunability. Then, we summarize the methods used for achieving single-mode lasing in NWs. After that, we review reports on lasing threshold reduction and the realization of electrically pumped NW lasers. Finally, we offer our perspective on future improvements and trends.
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http://dx.doi.org/10.1088/1361-6528/ab000dDOI Listing
May 2019

Growth and Fabrication of High-Quality Single Nanowire Devices with Radial p-i-n Junctions.

Small 2019 Jan 17;15(3):e1803684. Epub 2018 Dec 17.

Department of Electronic and Electrical Engineering, University College London, London, WC1E 7JE, UK.

Nanowires (NWs) with radial p-i-n junction have advantages, such as large junction area and small influence from the surface states, which can lead to highly efficient material use and good device quantum efficiency. However, it is difficult to make high-quality core-shell NW devices, especially single NW devices. Here, the key factors during the growth and fabrication process that influence the quality of single core-shell p-i-n NW devices are studied using GaAs(P) NW photovoltaics as an example. By p-doping and annealing, good ohmic contact is achieved on NWs with a diameter as small as 50-60 nm. Single NW photovoltaics are subsequently developed and a record fill factor of 80.5% is shown. These results bring valuable information for making single NW devices, which can further benefit the development of high-density integration circuits.
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http://dx.doi.org/10.1002/smll.201803684DOI Listing
January 2019

Doping of Self-Catalyzed Nanowires under the Influence of Droplets.

Nano Lett 2018 01 5;18(1):81-87. Epub 2017 Dec 5.

Department of Electronic and Electrical Engineering, University College London , London WC1E 7JE, United Kingdom.

Controlled and reproducible doping is essential for nanowires (NWs) to realize their functions. However, for the widely used self-catalyzed vapor-liquid-solid (VLS) growth mode, the doping mechanism is far from clear, as the participation of the nanoscale liquid phase makes the doping environment highly complex and significantly different from that of the thin film growth. Here, the doping mechanism of self-catalyzed NWs and the influence of self-catalytic droplets on the doping process are systematically studied using beryllium (Be) doped GaAs NWs. Be atoms are found for the first time to be incorporated into NWs predominantly through the Ga droplet that is observed to be beneficial for setting up thermodynamic equilibrium at the growth front. Be dopants are thus substitutional on Ga sites and redundant Be atoms are accumulated inside the Ga droplets when NWs are saturated, leading to the change of the Ga droplet properties and causing the growth of phase-pure zincblende NWs. This study is an essential step toward the design and fabrication of nanowire devices.
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http://dx.doi.org/10.1021/acs.nanolett.7b03366DOI Listing
January 2018

Growth of Pure Zinc-Blende GaAs(P) Core-Shell Nanowires with Highly Regular Morphology.

Nano Lett 2017 08 31;17(8):4946-4950. Epub 2017 Jul 31.

Department of Electronic and Electrical Engineering, University College , London WC1E 7JE, United Kingdom.

The growth of self-catalyzed core-shell nanowires (NWs) is investigated systematically using GaAs(P) NWs. The defects in the core NW are found to be detrimental for the shell growth. These defects are effectively eliminated by introducing beryllium (Be) doping during the NW core growth and hence forming Be-Ga alloy droplets that can effectively suppress the WZ nucleation and facilitate the droplet consumption. Shells with pure zinc-blende crystal quality and highly regular morphology are successfully grown on the defect-free NW cores and demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective shells. These results provide useful information on guiding the growth of high-quality shell, which can greatly enhance the NW device performance.
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http://dx.doi.org/10.1021/acs.nanolett.7b02063DOI Listing
August 2017

Ten-Fold Enhancement of InAs Nanowire Photoluminescence Emission with an InP Passivation Layer.

Nano Lett 2017 06 1;17(6):3629-3633. Epub 2017 Jun 1.

Department of Electronic and Electrical Engineering, University College London , London WC1E 7JE, United Kingdom.

In this Letter, we demonstrate that a significant improvement of optical performance of InAs nanowires can be achieved by capping the core InAs nanowires with a thin InP shell, which successfully passivates the surface states reducing the rate of nonradiative recombination. The improvements have been confirmed by detailed photoluminescence measurements, which showed up to a 10-fold increase in the intensity of room-temperature photoluminescence from the capped InAs/InP nanowires compared to the sample with core-only InAs nanowires. Moreover, the nanowires exhibit a high stability of total photoluminescence emission strength across temperature range from 10 to 300 K as a result of strong quantum confinement. These findings could be the key to successful implementation of InAs nanowires into optoelectronic devices.
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http://dx.doi.org/10.1021/acs.nanolett.7b00803DOI Listing
June 2017

Influence of Droplet Size on the Growth of Self-Catalyzed Ternary GaAsP Nanowires.

Nano Lett 2016 Feb 6;16(2):1237-43. Epub 2016 Jan 6.

Department of Electronic and Electrical Engineering, University College London , London WC1E 7JE, United Kingdom.

The influences of droplet size on the growth of self-catalyzed ternary nanowires (NWs) were studied using GaAsP NWs. The size-induced Gibbs-Thomson (GT) effect makes the smaller catalytic droplets have lower effective supersaturations and hence slower nucleation rates than the larger ones. Large variation in droplet size thus led to the growth of NWs with low uniformity, while a good size uniformity of droplets resulted in the production of highly uniform NWs. Moreover, thinner NWs were observed to be richer in P, indicating that P is more resistant to the GT effect than As because of a higher chemical potential inside Ga droplets. These results provide useful information for understanding the mechanisms of self-catalyzed III-V NW nucleation and growth with the important ternary III-V material systems.
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http://dx.doi.org/10.1021/acs.nanolett.5b04554DOI Listing
February 2016

Defect-Free Self-Catalyzed GaAs/GaAsP Nanowire Quantum Dots Grown on Silicon Substrate.

Nano Lett 2016 Jan 17;16(1):504-11. Epub 2015 Dec 17.

Department of Electronic and Electrical Engineering, University College London , Torrington Place, London WC1E 7JE, United Kingdom.

The III-V nanowire quantum dots (NWQDs) monolithically grown on silicon substrates, combining the advantages of both one- and zero-dimensional materials, represent one of the most promising technologies for integrating advanced III-V photonic technologies on a silicon microelectronics platform. However, there are great challenges in the fabrication of high-quality III-V NWQDs by a bottom-up approach, that is, growth by the vapor-liquid-solid method, because of the potential contamination caused by external metal catalysts and the various types of interfacial defects introduced by self-catalyzed growth. Here, we report the defect-free self-catalyzed III-V NWQDs, GaAs quantum dots in GaAsP nanowires, on a silicon substrate with pure zinc blende structure for the first time. Well-resolved excitonic emission is observed with a narrow line width. These results pave the way toward on-chip III-V quantum information and photonic devices on silicon platform.
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http://dx.doi.org/10.1021/acs.nanolett.5b04142DOI Listing
January 2016

Polarity-Driven Quasi-3-Fold Composition Symmetry of Self-Catalyzed III-V-V Ternary Core-Shell Nanowires.

Nano Lett 2015 May 2;15(5):3128-33. Epub 2015 Apr 2.

†Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, United Kingdom.

A quasi-3-fold composition symmetry has for the first time been observed in self-catalyzed III-V-V core-shell nanowires. In GaAsP nanowires, phosphorus-rich sheets on radial {110} planes originating at the corners of the hexagonal core were observed. In a cross section, they appear as six radial P-rich bands that originate at the six outer corners of the hexagonal core, with three of them higher in P content along ⟨112⟩A direction and others along ⟨112⟩B, forming a quasi-3-fold composition symmetry. We propose that these P-rich bands are caused by a curvature-induced high surface chemical potential at the small corner facets, which drives As adatoms away more efficiently than P adatoms. Moreover, their polarity related P content difference can be explained by the different adatom bonding energies at these polar corner facets. These results provide important information on the further development of shell growth in the self-catalyzed core-shell NW structure and, hence, device structure for multicomponent material systems.
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http://dx.doi.org/10.1021/acs.nanolett.5b00188DOI Listing
May 2015

Self-catalyzed ternary core-shell GaAsP nanowire arrays grown on patterned Si substrates by molecular beam epitaxy.

Nano Lett 2014 Aug 2;14(8):4542-7. Epub 2014 Jul 2.

Department of Electronic and Electrical Engineering, University College London , London WC1E 7JE, United Kingdom.

The growth of self-catalyzed ternary core-shell GaAsP nanowire (NW) arrays on SiO2 patterned Si(111) substrates has been demonstrated by using solid-source molecular beam epitaxy. A high-temperature deoxidization step up to ∼ 900 °C prior to NW growth was used to remove the native oxide and/or SiO2 residue from the patterned holes. To initiate the growth of GaAsP NW arrays, the Ga predeposition used for assisting the formation of Ga droplets in the patterned holes, was shown to be another essential step. The effects of the patterned-hole size on the NW morphology were also studied and explained using a simple growth model. A lattice-matched radial GaAsP core-shell NW structure has subsequently been developed with room-temperature photoluminescence emission around 740 nm. These results open up new perspectives for integrating position-controlled III-V NW photonic and electronic structures on a Si platform.
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http://dx.doi.org/10.1021/nl501565bDOI Listing
August 2014

Wafer-scale fabrication of self-catalyzed 1.7 eV GaAsP core-shell nanowire photocathode on silicon substrates.

Nano Lett 2014 1;14(4):2013-8. Epub 2014 Apr 1.

Department of Electronic and Electrical Engineering, University College London , Torrington Place, London WC1E 7JE, United Kingdom.

We present the wafer-scale fabrication of self-catalyzed p-n homojunction 1.7 eV GaAsP core-shell nanowire photocathodes grown on silicon substrates by molecular beam epitaxy with the incorporation of Pt nanoparticles as hydrogen evolution cocatalysts. Under AM 1.5G illumination, the GaAsP nanowire photocathode yielded a photocurrent density of 4.5 mA/cm(2) at 0 V versus a reversible hydrogen electrode and a solar-to-hydrogen conversion efficiency of 0.5%, which are much higher than the values previously reported for wafer-scale III-V nanowire photocathodes. In addition, GaAsP has been found to be more resistant to photocorrosion than InGaP. These results open up a new approach to develop efficient tandem photoelectrochemical devices via fabricating GaAsP nanowires on a silicon platform.
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http://dx.doi.org/10.1021/nl500170mDOI Listing
March 2015

Self-catalyzed GaAsP nanowires grown on silicon substrates by solid-source molecular beam epitaxy.

Nano Lett 2013 Aug 2;13(8):3897-902. Epub 2013 Aug 2.

Department of Electronic and Electrical Engineering, University College London, London, United Kingdom.

We realize the growth of self-catalyzed GaAsP nanowires (NWs) on silicon (111) substrates using solid-source molecular beam epitaxy. By optimizing the V/III and P/As flux ratios, as well as the Ga flux, high-crystal-quality GaAsP NWs have been demonstrated with almost pure zinc-blende phase. Comparing the growth of GaAsP NWs with that of the conventional GaAs NWs indicates that the incorporation of P has significant effects on catalyst nucleation energy, and hence the nanowire morphology and crystal quality. In addition, the incorporation ratio of P/As between vapor-liquid-solid NW growth and the vapor-solid thin film growth has been compared, and the difference between these two growth modes is explained through growth kinetics. The vapor-solid epitaxial growth of radial GaAsP shell on core GaAsP NWs is further demonstrated with room-temperature emission at ~710 nm. These results give valuable new information into the NW nucleation mechanisms and open up new perspectives for integrating III-V nanowire photovoltaics and visible light emitters on a silicon platform by using self-catalyzed GaAsP core-shell nanowires.
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http://dx.doi.org/10.1021/nl401981uDOI Listing
August 2013

Surface-passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon.

Nat Commun 2013 ;4:1498

SunFlake A/S, Universitetsparken 5, Copenhagen DK-2100, Denmark.

Continued development of high-efficiency multi-junction solar cells requires growth of lattice-mismatched materials. Today, the need for lattice matching both restricts the bandgap combinations available for multi-junctions solar cells and prohibits monolithic integration of high-efficiency III-V materials with low-cost silicon solar cells. The use of III-V nanowires is the only known method for circumventing this lattice-matching constraint, and therefore it is necessary to develop growth of nanowires with bandgaps >1.4 eV. Here we present the first gold-free gallium arsenide phosphide nanowires grown on silicon by means of direct epitaxial growth. We demonstrate that their bandgap can be controlled during growth and fabricate core-shell nanowire solar cells. We further demonstrate that surface passivation is of crucial importance to reach high efficiencies, and present a record efficiency of 10.2% for a core-shell single-nanowire solar cell.
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http://dx.doi.org/10.1038/ncomms2510DOI Listing
June 2013

Influence of the oxide layer for growth of self-assisted InAs nanowires on Si(111).

Nanoscale Res Lett 2011 Aug 31;6:516. Epub 2011 Aug 31.

Nano-Science Center, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.

The growth of self-assisted InAs nanowires (NWs) by molecular beam epitaxy (MBE) on Si(111) is studied for different growth parameters and substrate preparations. The thickness of the oxide layer present on the Si(111) surface is observed to play a dominant role. Systematic use of different pre-treatment methods provides information on the influence of the oxide on the NW morphology and growth rates, which can be used for optimizing the growth conditions. We show that it is possible to obtain 100% growth of vertical NWs and no parasitic bulk structures between the NWs by optimizing the oxide thickness. For a growth temperature of 460°C and a V/III ratio of 320 an optimum oxide thickness of 9 ± 3 Å is found.
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http://dx.doi.org/10.1186/1556-276X-6-516DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3212055PMC
August 2011

Impact of the liquid phase shape on the structure of III-V nanowires.

Phys Rev Lett 2011 Mar 22;106(12):125505. Epub 2011 Mar 22.

Nano-Science Center, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.

The III-V nanowire structure (zinc blende or wurtzite) grown by the vapor-liquid-solid process is shown to be highly dependent on the parameters which shape the droplet at the top of the nanowire. Under conditions that the droplet volume does not exceed a certain value, it is demonstrated that when the nucleation of the solid starts at the solid-liquid-vapor triple line, a relatively large droplet volume and low wetting angle favor the formation of the wurtzite structure. We show that the effective V/III flux ratio is the primary parameter controlling the structure.
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http://dx.doi.org/10.1103/PhysRevLett.106.125505DOI Listing
March 2011

Junctions in axial III-V heterostructure nanowires obtained via an interchange of group III elements.

Nano Lett 2009 Nov;9(11):3689-93

Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Denmark.

We present an investigation of the morphology and composition of novel types of axial nanowire heterostructures where Ga(x)In(1-x)As is used as barrier material in InAs nanowires. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray analysis we demonstrate that it is possible to grow junctions by changing the group III elements, and we find that a substantial fraction of Ga can be incorporated in axial InAs/Ga(x)In(1-x)As/InAs, retaining straight nanowire configurations. We explain how the adatoms are transferred to the incorporation site at the growth interface via two different routes, (1) interface diffusion and (2) volume diffusion through the catalyst particle.
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http://dx.doi.org/10.1021/nl901348dDOI Listing
November 2009

Molecular beam epitaxy growth of free-standing plane-parallel InAs nanoplates.

Nat Nanotechnol 2007 Dec 25;2(12):761-4. Epub 2007 Nov 25.

Free-standing nanostructures such as suspended carbon nanotubes, graphene layers, III-V nanorod photonic crystals and three-dimensional structures have recently attracted attention because they could form the basis of devices with unique electronic, optoelectronic and electromechanical characteristics. Here we report the growth by molecular beam epitaxy of free-standing nanoplates of InAs that are close to being atomically plane. The structural and transport properties of these semiconducting nanoplates have been examined with scanning electron microscopy, transmission electron microscopy, X-ray diffraction and low-temperature electron transport measurements. The carrier density of the nanoplates can be reduced to zero by applying a voltage to a nearby gate electrode, creating a new type of suspended quantum well that can be used to explore low-dimensional electron transport. The electronic and optical properties of such systems also make them potentially attractive for photovoltaic and sensing applications.
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http://dx.doi.org/10.1038/nnano.2007.378DOI Listing
December 2007
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