Philippe Caroff

Australian National University, Canberra, Australian Capital Territory, Australia

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Publications (159)535.82 Total impact

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    ABSTRACT: Ultra-narrow bandgap III-V semiconductor nanomaterials provide a unique platform for realizing advanced nanoelectronics, thermoelectrics, infra-red photodetection and quantum transport physics. In this work we employ molecular beam epitaxy to synthesize novel nanosheet-like InSb nanostructures exhibiting superior electronic performance. Through careful morphological and crystallographic characterization we show how this unique geometry is the result of a single twinning event in an otherwise pure zinc blende structure. Four-terminal electrical measurements performed in both the Hall and van der Pauw configurations reveal a room temperature electron mobility greater than 12,000 cm(2).V(-1).s(-1). Quantized conductance in a quantum point contact processed with a split-gate configuration is also demonstrated. We thus introduce InSb 'nanosails' as a versatile and convenient platform for realizing new device and physics experiments with a strong interplay between electronic and spin degrees of freedom.
    No preview · Article · Jan 2016 · Nano Letters
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    ABSTRACT: Antimonide-based ternary III-V nanowires (NWs) allow for a tunable bandgap over a wide range, which is highly interesting for optoelectronics applications, and in particular for infrared photodetection. Here we demonstrate room temperature operation of GaAs0.56Sb0.44 NW infrared photodetectors grown by metal organic vapor phase epitaxy. These GaAs0.56Sb0.44 NWs have uniform axial composition and show p-type conductivity with a peak field-effect mobility of ∼12 cm(2) V(-1) s(-1)). Under light illumination, single GaAs0.56Sb0.44 NW photodetectors exhibited typical photoconductor behavior with an increased photocurrent observed with the increase of temperature owing to thermal activation of carrier trap states. A broadband infrared photoresponse with a long wavelength cutoff at ∼1.66 μm was obtained at room temperature. At a low operating bias voltage of 0.15 V a responsivity of 2.37 (1.44) A/W with corresponding detectivity of 1.08 × 10(9) (6.55 × 10(8)) [Formula: see text] were achieved at the wavelength of 1.3 (1.55) μm, indicating that ternary GaAs0.56Sb0.44 NWs are promising photodetector candidates for small footprint integrated optical telecommunication systems.
    No preview · Article · Oct 2015 · Nanotechnology
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    ABSTRACT: InGaAs nanowires offer great promise in fundamental studies of ternary compound semiconductors with variable composition and opens up a wide range of applications due to their bandgap tunability and high carrier mobility. Here, we report a study on the growth of Au-seeded InGaAs nanowires by metal-organic vapour phase epitaxy and present a model to explain the mechanisms that govern the growth and composition evolution in ternary III-V nanowires. The model allows us to further understand the limitations on the growth rate and incorporation of the two group III species imposed by the deposition conditions and some intrinsic properties of the material transport and nucleation. Within the model, the evolution of InGaAs nanowire growth rate and composition with particle size, temperature and V/III ratio is described and correlates very well with experimental findings. The understanding gained in this study should be useful for the controlled fabrication of tunable ternary nanowires for various applications.
    No preview · Article · Sep 2015 · Nanoscale
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    ABSTRACT: The composition and band gap of the shell that formed during the growth of axial GaAs/GaAs81 Sb 19/ GaAs heterostructure nanowires have been investigated by transmission electron microscopy combined with energy dispersion spectroscopy, scanning tunneling spectroscopy, and density functional theory calculations. On the GaAs 81 Sb 19 intermediate segment, the shell is found to be free of Sb (pure GaAs shell) and transparent to the tunneling electrons, despite the (110) biaxial strain that affects its band gap. As a result, a direct measurement of the core band gap allows the quantitative determination of the band offset between the GaAs 81 Sb 19 core and the GaAs shell and identifies it as a type I band alignment.
    No preview · Article · Sep 2015 · Applied Physics Letters
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    ABSTRACT: Controllable axial switching of polarity in GaAs nanowires with minimal tapering and perfect twin-free ZB structure based on the fundamental understanding of nanowire growth and kinking mechanism is presented. The polarity of the bottom segment is confirmed to be (111)A by atomically resolved scanning transmission electron microscopy image.
    No preview · Article · Sep 2015 · Advanced Materials
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    Dingxun Fan · Sen Li · N. Kang · Philippe Caroff · L. B. Wang · Y. Q. Huang · M. T. Deng · C. L. Yu · H. Q. Xu
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    ABSTRACT: We report on realization and transport spectroscopy study of single quantum dots (QDs) made from InSb nanowires grown by molecular beam epitaxy (MBE). The nanowires employed are 50-80 nm in diameter and the QDs are defined in the nanowires between the source and drain contacts on a Si/SiO$_2$ substrate. We show that highly tunable QD devices can be realized with the MBE-grown InSb nanowires and the gate-to-dot capacitance extracted in the many-electron regimes is scaled linearly with the longitudinal dot size, demonstrating that the devices are of single InSb nanowire QDs even with a longitudinal size of ~700 nm. In the few-electron regime, the quantum levels in the QDs are resolved and the Land\'e g-factors extracted for the quantum levels from the magnetotransport measurements are found to be strongly level-dependent and fluctuated in a range of 18-48. A spin-orbit coupling strength is extracted from the magnetic field evolutions of a ground state and its neighboring excited state in an InSb nanowire QD and is on the order of ~300 $\mu$eV. Our results establish that the MBE-grown InSb nanowires are of high crystal quality and are promising for the use in constructing novel quantum devices, such as entangled spin qubits, one-dimensional Wigner crystals and topological quantum computing devices.
    Full-text · Article · Aug 2015 · Nanoscale
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    ABSTRACT: Designing strategies to reach monodispersity in fabrication of semiconductor nanowire ensembles is essential for numerous applications. When Ga-catalyzed GaAs nanowire arrays are grown by molecular beam epitaxy with help of droplet-engineering, we observe a significant narrowing of the diameter distribution of the final nanowire array with respect to the size distribution of the initial Ga droplets. Considering that the droplet serves as a non-equilibrium reservoir of a group III metal, we develop a model that demonstrates a self-equilibration effect on the droplet size in self-catalyzed III-V nanowires. This effect leads to arrays of nanowires with a high degree of uniformity regardless of the initial conditions, while the stationary diameter can be further finely tuned by varying the spacing of the array pitch on patterned Si substrates.
    No preview · Article · Jul 2015 · Nano Letters
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    ABSTRACT: Mid-infrared GaAs1−xSbx/InP core/shell nanowires are grown coherently with perfectly twin-free zinc blende crystal structure. An unusual triangular InP shell with predominantly 112A facets instead of 112B facets is reported. It is found that this polarity preference is due to the surfactant role of Sb, which inhibits InP shell growth rate in the 〈112〉A directions. This behavior reveals a new degree of control and tunability allowed in manipulating nanowire facet geometry and polarity in radial heterostructures by a simple means. Tuning the Sb composition in the core yields controllable intense photoluminescence emission in both the 1.3 and 1.5 μm optical telecommunication windows, up to room temperature for single nanowires. The internal quantum efficiency of the core/shell nanowires is experimentally determined to be as high as 56% at room temperature. Transient Rayleigh scattering analysis brings complementary information, revealing the photoexcited carrier lifetime in the core/shell nanowire to be ≈100 ps at 300 K and ≈800 ps at 10 K. In comparison, the carrier lifetime of core-only nanowire is below the detection limit of the system (25 ps). The demonstrated superior optical quality of the core/shell nanowires and their ideal emission wavelength range makes them highly relevant candidates for near-infrared optoelectronic applications.
    No preview · Article · Jul 2015 · Advanced Functional Materials
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    ABSTRACT: Obtaining compositional homogeneity without compromising morphological or structural quality is one of the biggest challenges in growing ternary alloy compound semiconductor nanowires. Here we report growth of Au-seeded InxGa1-xAs nanowires via metal-organic vapour phase epitaxy with uniform composition, morphology and pure wurtzite (WZ) crystal phase by carefully optimizing growth temperature and V/III ratio. We find that high growth temperatures allow the InxGa1-xAs composition to be more uniform by suppressing the formation of typically observed spontaneous In-rich shells. A low V/III ratio results in the growth of pure WZ phase InxGa1-xAs nanowires with uniform composition and morphology while a high V/III ratio allows pure zinc-blende (ZB) phase to form. Ga incorporation is found to be dependent on the crystal phase favouring higher Ga concentration in ZB phase compared to the WZ phase. Tapering is also found to be more prominent in defective nanowires hence it is critical to maintain the highest crystal structure purity in order to minimize tapering and inhomogeneity. The InP capped pure WZ In0.65Ga0.35As core-shell nanowire heterostructures show 1.54 μm photoluminescence, close to the technologically important optical fibre telecommunication wavelength, which is promising for application in photodetectors and nanoscale lasers.
    Full-text · Article · May 2015 · Nanotechnology
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    ABSTRACT: We demonstrate a simple way of fabricating high performance tunnel devices from p-doped InAs nanowires by tailoring the n-doped surface accumulation layer inherent to InAs surfaces. By using appropriate ammonium sulfide based surface passivation before metallization without any further thermal treatment, we demonstrate characteristics of tunnel p-n junctions, namely Esaki and backward diodes, with figures of merit better than previously published for InAs homojunctions. The further optimization of both the surface doping, in a quantitative way, and the device geometry allows us to demonstrate that these nanowire-based technologically-simple diodes have promising direct current characteristics for integrated high frequency detection or generation.
    No preview · Article · Mar 2015 · Nano Research
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    ABSTRACT: While III-V binary nanowires are now well controlled and their growth mechanisms reasonably well understood, growing ternary nanowires, including controlling their morphology, composition and crystal structure remains a challenge. However, understanding and control of ternary alloys is of fundamental interest and critical to enable a new class of nanowire devices. Here, we report on the progress in understanding the complex growth behaviour of gold-seeded GaAs1-xSbx nanowires grown by metalorganic vapour phase epitaxy. The competition between As and Sb atoms for incorporation into the growing crystal leads to a tunability of the Sb over a broad range (x varies from 0.09 to 0.6), solely by changing the AsH3 flow. In contrast, changing TMSb flow is more effective in affecting the morphology and crystal structure of the nanowires. Inclined faults are found in some of these nanowires, directly related to the kinking of the nanowires and controlled by TMSb flow. Combined with the observed sharp increase of wetting angle between the Au seed and nanowire, the formation of inclined faults are attributed to the Au seed being dislodged from the growth front to wet the sidewalls of the nanowires, and are ascribed to the surfactant role of Sb. The insights provided by this study should benefit future device applications relying on taper- and twin-free ternary antimonides III-V nanowire alloys and their heterostructures.
    No preview · Article · Feb 2015 · Nanoscale
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    ABSTRACT: InP-InxGa(1-x)As-InP quantum well tube (QWT) structures are grown on InP nanowires that are [100] oriented. The In mole fraction, x is varied between 0 and 1. The QWTs grown on the facets of the [100] nanowires that have {100} and {011} side facets forming an octagonal cross-section, are found to be highly non-uniform. Bright emission is observed at room temperature from these QWTs. Band-gap tunability in the near to mid-infrared region is achieved by controlling the thickness and composition of the quantum well.
    No preview · Article · Feb 2015
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    ABSTRACT: The development of earth abundant materials for optoelectronics and photovoltaics promises improvements in sustainability and scalability. Recent studies have further demonstrated enhanced material efficiency through the superior light management of novel nanoscale geometries such as the nanowire. Here we show that an industry standard epitaxy technique can be used to fabricate high quality II-V nanowires (1D) and nanoplatelets (2D) of the earth abundant semiconductor Zn3As2. We go on to establish the optoelectronic potential of this material by demonstrating efficient photoemission and detection at 1.0 eV, an energy significant to both photovoltaics and optical telecommunications. Dynamical spectroscopy reveals this superior performance as resulting from a low rate of surface recombination combined with a high rate of radiative recombination. These results introduce nanostructured Zn3As2 as a high quality optoelectronic material ready for device exploration.
    No preview · Article · Nov 2014 · Nano Letters
  • María de la Mata · Cesar Magen · Philippe Caroff · Jordi Arbiol
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    ABSTRACT: Combination of mismatched materials in semiconductor nanowire heterostructures offers a freedom of bandstructure engineering that is impossible in standard planar epitaxy. Nevertheless, the presence of strain and/or structural defects directly control the optoelectronic properties of these nanomaterials. Understanding with atomic accuracy how mismatched heterostructures release or accommodate strain is therefore highly desirable. By using atomic resolution high angle annular dark field scanning transmission electron microscopy combined with geometrical phase analyses and computer simulations we are able to establish the relaxation mechanism (including both elastic and plastic deformations) involved in order to release the mismatch strain in axial nanowire heterostructures. Formation of misfit dislocations, diffusion of atomic species, polarity transfer and induced structural transformations are studied at atomic resolution at the intermediate ternary interfaces. Two nanowire heterostructure systems with promising applications (InAs/InSb and GaAs/GaSb) have been selected as key examples.
    No preview · Article · Oct 2014 · Nano Letters
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    ABSTRACT: We report the growth of stacking-fault-free and taper-free wurtzite InP nanowires with diameters ranging from 80 to 600 nm using selective-area metalorganic vapor-phase epitaxy and experimentally determine a quantum efficiency of similar to 50%, which is on par with InP epilayers. We also demonstrate room-temperature, photonic mode lasing from these nanowires. Their excellent structural and optical quality opens up new possibilities for both fundamental quantum optics and optoelectronic devices.
    Full-text · Article · Aug 2014 · Nano Letters
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    ABSTRACT: Growth of III-V nanowires on the [100]-oriented industry standard substrates is critical for future integrated nanowire device development. Here we present an in-depth analysis of the seemingly complex ensembles of epitaxial nanowires grown on InP (100) substrates. The nanowires are categorized into 3 types as vertical, non-vertical and planar, and the growth directions, facets and crystal structure of each type are investigated. The non-vertical growth directions are mathematically modelled using a 3-dimensional multiple order twinning concept. The non-vertical nanowires can be further classified into two different types, with one type growing in the <111> directions and the other in the <100> directions after initial multiple three dimensional twinning. We find that 99% of the total nanowires are grown either along <100>, <111> or <110> growth directions by {100} or {111} growth facets. We also demonstrate relative control of yield of these different types of nanowires, by tuning pre-growth annealing conditions and growth parameters. Together, the knowledge and controllability of the types of nanowires provide an ideal foundation to explore novel geometries that combine different crystal structures, with potential for both fundamental science research as well as device applications.
    No preview · Article · Jun 2014 · ACS Nano
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    ABSTRACT: We report on magneto-transport measurements in InAs nanowires under large magnetic field (up to 55T), providing a direct spectroscopy of the 1D electronic band structure. Large modulations of the magneto-conductance mediated by an accurate control of the Fermi energy reveal the Landau fragmentation, carrying the fingerprints of the confined InAs material. Our numerical simulations of the magnetic band structure consistently support the experimental results and reveal key parameters of the electronic confinement.
    Full-text · Article · Feb 2014 · Physical Review Letters
  • Kimberly A Dick · Philippe Caroff
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    ABSTRACT: Semiconductor nanowires composed of III-V materials have enormous potential to add new functionality to electronics and optical applications. However, integration of these promising structures into applications is severely limited by the current near-universal reliance on gold nanoparticles as seeds for nanowire fabrication. Although highly controlled fabrication is achieved, this metal is entirely incompatible with the Si-based electronics industry. In this Feature we review the progress towards developing gold-free bottom-up synthesis techniques for III-V semiconductor nanowires. Three main categories of nanowire synthesis are discussed: selective-area epitaxy, self-seeding and foreign metal seeding, with main focus on the metal-seeded techniques. For comparison, we also review the development of foreign metal seeded synthesis of silicon and germanium nanowires. Finally, directions for future development and anticipated important trends are discussed. We anticipate significant development in the use of foreign metal seeding in particular. In addition, we speculate that multiple different techniques must be developed in order to replace gold and to provide a variety of nanowire structures and properties suited to a diverse range of applications.
    No preview · Article · Feb 2014 · Nanoscale
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    ABSTRACT: We explore the signatures of Majorana fermions in a nanowire based topological superconductor-quantum dot-topological superconductor hybrid device by charge transport measurements. The device is made from an epitaxially grown InSb nanowire with two superconductor Nb contacts on a Si/SiO$_2$ substrate. At low temperatures, a quantum dot is formed in the segment of the InSb nanowire between the two Nb contacts and the two Nb contacted segments of the InSb nanowire show superconductivity due to the proximity effect. At zero magnetic field, well defined Coulomb diamonds and the Kondo effect are observed in the charge stability diagram measurements in the Coulomb blockade regime of the quantum dot. Under the application of a finite, sufficiently strong magnetic field, a zero-bias conductance peak structure is observed in the same Coulomb blockade regime. It is found that the zero-bias conductance peak is present in many consecutive Coulomb diamonds, irrespective of the even-odd parity of the quasi-particle occupation number in the quantum dot. In addition, we find that the zero-bias conductance peak is in most cases accompanied by two differential conductance peaks, forming a triple-peak structure, and the separation between the two side peaks in bias voltage shows oscillations closely correlated to the background Coulomb conductance oscillations of the device. The observed zero-bias conductance peak and the associated triple-peak structure are in line with the signatures of Majorana fermion physics in a nanowire based topological superconductor-quantum dot-topological superconductor system, in which the two Majorana bound states adjacent to the quantum dot are hybridized into a pair of quasi-particle states with finite energies and the other two Majorana bound states remain as the zero-energy modes located at the two ends of the entire InSb nanowire.
    Full-text · Article · Jan 2014 · Scientific Reports
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    ABSTRACT: The modified phonon dispersion is of importance for understanding the origin of the reduced heat conductivity in nanowires. We have measured the phonon dispersion for 50 nm diameter InSb (111) nanowires using time-resolved X-ray diffraction. By comparing the sound speed of the bulk (3880 m/s) and that of a classical thin rod (3600 m/s) to our measurement (2880 m/s), we conclude that the origin of the reduced sound speed and thereby to the reduced heat conductivity is that the C44 elastic constant is reduced by 35% compared to the bulk material.
    Preview · Article · Jan 2014 · Nano Letters

Publication Stats

3k Citations
535.82 Total Impact Points


  • 2013-2015
    • Australian National University
      • Department of Electronic Materials Engineering (EME)
      Canberra, Australian Capital Territory, Australia
  • 2007-2015
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2010-2014
    • Lund University
      • Division of Solid State Physics
      Lund, Skåne, Sweden
  • 2005-2009
    • Institut National des Sciences Appliquées de Rennes
      Roazhon, Brittany, France
    • INSA
      Альтамира, Tamaulipas, Mexico