E. Gil

Université Blaise Pascal - Clermont-Ferrand II, Clermont, Auvergne, France

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Publications (22)71.7 Total impact

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    ABSTRACT: We report the Au catalyst-assisted synthesis of 20 μm long GaAs nanowires by the vapor-liquid-solid hydride vapor phase epitaxy (HVPE) exhibiting a polytypism-free zincblende phase for record radii lower than 15 nm down to 5 nm. HVPE makes use of GaCl gaseous growth precursors at high mass input, of which fast dechlorination at the usual process temperature of 715°C results in high planar growth rate (standard 30-40 µm/h). When it comes to the vapor-liquid-solid growth of nanowires, fast solidification at a rate higher than 100 µm/h is observed. Nanowire growth by HVPE only proceeds by introduction of precursors in the catalyst droplets from the vapor phase. This promotes almost pure axial growth leading to nanowires with a constant cylinder shape over unusual length. The question of the cubic zincblende structure observed in HVPE-grown GaAs nanowires regardless of their radius is at the heart of the paper. We demonstrate that the vapor-liquid-solid growth in our conditions takes place at high liquid chemical potential that originates from very high influxes of both As and Ga. This yields a Ga concentration systematically higher than 0.62 in the Au-Ga-As droplets. The high Ga concentration decreases the surface energy of the droplets, which disables nucleation at the triple phase line thus preventing the formation of wurtzite structure whatever the nanowire radius is.
    Nano Letters 05/2014; · 13.03 Impact Factor
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    ABSTRACT: High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy and micro-Raman spectroscopy revealed polytypism-free zinc blende (ZB) NWs over lengths of several tens of micrometers for a mean diameter of 50 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors GaCl of which high decomposition frequency after adsorption onto the liquid droplet catalysts, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the droplets. High influxes of Ga and As species then yield high axial growth rate of more than 100 μm/h. The diffusion of the Ga atoms in the liquid droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3 × 10(-9) m(2)/s. The fast diffusion of Ga in the droplet favors nucleation at the liquid-solid line interface at the center of the NW. This is further evidence, provided by an alternative epitaxial method with respect to metal-organic vapor phase epitaxy and molecular beam epitaxy, of the current assumption which states that this type of nucleation should always lead to the formation of the ZB cubic phase.
    The Journal of chemical physics. 05/2014; 140(19):194706.
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    ABSTRACT: GaN nanowires with exceptional lengths are synthesized by vapor-liquid-solid coupled with near-equilibrium hydride vapor phase epitaxy (HVPE) technique on c-plane sapphire substrates. Due to the high decomposition frequency of GaCl precursors and to a direct supply of Ga through the catalyst particle, the growth of GaN nanowires with constant diameters takes place at an exceptional growth rate of 130 µm/h. The chemical composition of the catalyst droplet is analyzed by Energy Dispersive X-ray spectroscopy (EDX). High resolution transmission electron microscopy and selective area diffraction show that the GaN nanowires crystallize in the hexagonal wurzite structure and are defect-free. GaN nanowires exhibit bare top facets without any droplet. Microphotoluminescence displays a narrow and intense emission line (1 meV linewidth) associated to the neutral-donor bound exciton revealing excellent optical properties of GaN nanowires.
    Nano Letters 01/2014; · 13.03 Impact Factor
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    ABSTRACT: Hydride Vapor Phase Epitaxy (HVPE) makes use of chloride III-Cl and hydride V-H3 gaseous growth precursors. It is known as a near-equilibrium process, providing the widest range of growth rates from 1 to more than 100 μm/h. When it comes to metal catalyst-assisted VLS (vapor-liquid-solid) growth, the physics of HVPE growth is maintained: high dechlorination frequency, high axial growth rate of nanowires (NWs) up to 170 μm/h. The remarkable features of NWs grown by HVPE are the untapered morphology with constant diameter and the stacking fault-free crystalline phase. Record pure zinc blende cubic phase for 20 μm long GaAs NWs with radii of 10 and 5 nm is shown. The absence of wurtzite phase in GaAs NWs grown by HVPE whatever the diameter is discussed with respect to surface energetic grounds and kinetics. Ni assisted, Ni-Au assisted and catalyst-free HVPE growth of wurtzite GaN NWs is also addressed. Micro-photoluminescence spectroscopy analysis revealed GaN nanowires of great optical quality, with a FWHM of 1 meV at 10 K for the neutral donor bound exciton transition.
    11/2013;
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    ABSTRACT: High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy (HRTEM) and micro-Raman spectroscopy revealed polytypism-free zinc blende NWs over lengths of several tens of micrometers for diameters ranging between 50 and 150 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors of which high decomposition frequency, after adsorption onto the catalyst particle, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the catalyst liquid droplet. This yields high axial growth rate (more than 100 micron/h) of NWs. The fast diffusion of the Ga atoms in the droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3x10-9 m2/s, which matches the experimental observations.
    10/2013;
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    ABSTRACT: Nanometric-width GaAs grating lines showing high aspect (height/width) ratio of up to 30, without roughness or shape deviation are reported. They were obtained by HVPE, capitalizing on the near-equilibrium nature of this epitaxial technique. HVPE rapid decomposition of GaCl growth precursors leads to immediate reactivity of the growth reaction to an increase or decrease of the supersaturation of the vapor phase. HVPE growth is also governed by surface kinetics: the morphologies are controlled by the intrinsic growth anisotropy of the crystals. By manipulating surface kinetics and exploiting the fast reactivity of the system, it is demonstrated that HVPE allows to benefit from an enhanced growth anisotropy favoring vertical growth of nanometer-scale grating lines. Unique high-aspect ratios are obtained in one-step short growth time (20 min), capitalizing upon the main advantage of the bottom-up approach: the structures are defined by their crystalline properties with no scattering defects as typically introduced by techniques involving dry etching. The role of surface kinetics and mass transport at the nanometer scale is discussed.
    Journal of Crystal Growth 10/2013; · 1.55 Impact Factor
  • E-MRS Spring Meeting, Strasbourg, France; 05/2013
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    ABSTRACT: The hydride vapor phase epitaxy (HVPE) process exhibits unexpected properties when growing GaN semiconductor nanowires (NWs). With respect to the classical well-known methods such as metal organic vapor phase epitaxy and molecular beam epitaxy, this near-equilibrium process based on hot wall reactor technology enables the synthesis of nanowires with a constant cylinder shape over unusual length. Catalyst-assisted HVPE shows a record short time process (less than 20 min) coupled to very low precursor consumption. NWs are grown at a fast solidification rate (50 μm h(-1)), facilitated by the high decomposition frequency of the chloride molecules involved in the HVPE process as element III precursors. In this work growth temperature and V/III ratio were investigated to determine the growth mechanism which led to such long NWs. Analysis based on the Ni-Ga phase diagram and the growth kinetics of near-equilibrium HVPE is proposed.
    Nanotechnology 09/2012; 23(40):405601. · 3.84 Impact Factor
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    ABSTRACT: The synthesis of GaN by selective area growth using Hydride Vapor Phase Epitaxy (SAG-HVPE) is reported for stripes patterned along 〈11¯00〉 and 〈112¯0〉GaN on c-plane sapphire substrates. A systematic control of the GaN morphologies was carried out by both cross-sectional and surface Scanning Electron Microscopy (SEM). A complete HVPE cartography of GaN-SAG revealed domains of zero growth rates at high concentration of hydrogen in the carrier gas. The determination of the mechanisms that govern the growth of GaN morphologies was particularly emphasized. A theoretical model based on thermodynamic and kinetic analyses of the grown (0001) GaN layers was discussed, in combination with experiments on unmasked (0001) GaN and patterned GaN/c-plane sapphire substrates. Long HVPE runs were performed to demonstrate that the prevailing growth mechanism, for high hydrogen concentration in the carrier gas, is a mechanism based on a dechlorination by GaCl2 gas species. This mechanism leads to growth blockade of GaN growth and constitutes a very interesting issue for shaping GaN material as an alternative to top-down micro- and nano-technologies.
    Journal of Crystal Growth 09/2012; 354(1):135–141. · 1.55 Impact Factor
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    ABSTRACT: GaN nanorods (NRs) with exceptional crystal-defined morphologies were grown by near-equilibrium hydride vapor phase epitaxy (HVPE) on c-plane sapphire (0001) and silicon (111) substrates. Hexagonal faceted individual, bunched, and hyperbunched GaN NRs were synthesized by using a catalyst-free process. GaN NRs with a diameter in the range of 125–700 nm and a length in the range of 0.9–3 μm were grown in a record short process time of 30 min. The influence of growth temperature and ammonia gas flow rate was investigated to establish the nucleation and the growth mechanisms of the GaN NRs. Shape and crystallographic facets of the NRs are discussed and a condensation growth mechanism is proposed for the formation of bunched GaN NRs. Emphasis is placed on the crystal symmetry of the nuclei in the first stage of growth. The morphology and the structural properties were analyzed by field emission scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD).
    Crystal Growth & Design 04/2012; 12(5):2251–2256. · 4.69 Impact Factor
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    ABSTRACT: The tunnel photocurrent between a gold surface and a free-standing semiconducting thin film excited from the rear by above bandgap light has been measured as a function of applied bias, tunnel distance and excitation light power. The results are compared with the predictions of a model which includes the bias dependence of the tunnel barrier height and the bias-induced decrease of surface recombination velocity. It is found that i) the tunnel photocurrent from the conduction band dominates that from surface states. ii) At large tunnel distance the exponential bias dependence of the current is explained by that of the tunnel barrier height, while at small distance the change of surface recombination velocity is dominant.
    Physical review. B, Condensed matter 06/2010; · 3.77 Impact Factor
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    ABSTRACT: Local spin injectors using GaAs tips at the end of transparent cantilevers have been fabricated using a combination of epitaxial growth, etching processes and photolithographic techniques. The tip luminescence polarization is found to be small because of total internal light reflections of the luminescence inside the tip. However, measurements on planar films of similar doping along with a numerical solution of the spin and charge diffusion equations indicate that the injected spin polarization can be as high as 40% with corresponding electronic concentrations at the tip apex of the order of 10<sup>14</sup> cm <sup>-3</sup> .
    Journal of Applied Physics 06/2010; · 2.21 Impact Factor
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    ABSTRACT: We report the first synthesis of GaAs nanowires (NWs) by Au-assisted vapor-liquid-solid (VLS) growth in the novel hydride vapor phase epitaxy (HVPE) environment. Forty micrometer long rodlike <111> monocrystalline GaAs nanowires exhibiting a cubic zinc blende structure were grown in 15 min with a mean density of 10(6) cm(-2). The synthesis of such long figures in such a short duration could be explained by the growth physics of near-equilibrium HVPE. VLS-HVPE is mainly based on solidification after direct and continuous feeding of the arsenious and GaCl growth precursors through the Au-Ga liquid catalyst. Fast solidification (170 microm/h) is then assisted by the high decomposition frequency of GaCl. This predominant feeding through the liquid-solid interface with no mass and kinetic hindrance favors axial rather than radial growth, leading to twin-free nanowires with a constant cylinder shape over unusual length. The achievement of GaAs NWs several tens of micrometers long showing a high surface to volume ratio may open the field of III-V wires, as already addressed with ultralong Si nanowires.
    Nano Letters 04/2010; 10(5):1836-41. · 13.03 Impact Factor
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    ABSTRACT: GaN structures have been grown by selective area growth (SAG) in an atmospheric hydride vapour phase epitaxy (HVPE) reactor on c-plane sapphire and metal organic vapour phase epitaxy (MOVPE) grown GaN/c-plane sapphire substrates. The substrates were patterned in two perpendicular crystallographic directions 〈11−20〉 and 〈1−100〉. The growth morphologies of the GaN stripes were systematically investigated by scanning electron microscopy (SEM). A complete cartography of GaN SAG was established, after varying the growth temperature and the V/III ratio on both types of substrate. The cartography permits to fix the appropriate parameters for growing GaN quasi-substrates or GaN nano-structures with controlled morphology.
    Journal of Crystal Growth 01/2010; 312(12):1899-1907. · 1.55 Impact Factor
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    ABSTRACT: GaN structures have been fabricated by selective area growth by hydride vapour phase epitaxy (SAG-HVPE) on parallel stripes-patterned equivalent 〈11¯00〉 sapphire and 〈112¯0〉 GaN/sapphire substrates, and on parallel stripes-patterned 〈112¯0〉 sapphire and 〈11¯00〉 GaN/sapphire structures. The growth morphologies of the GaN stripes were systematically investigated by scanning electron microscopy (SEM). HVPE-GaN layers exhibit (0001), {11¯00} and {11¯01} or {22¯03} facets on 〈112¯0〉 GaN/sapphire substrates, while on the equivalent 〈11¯00〉 sapphire direction only (0001) and {11¯00} facets coexist. On the 〈11¯00〉 GaN/sapphire substrate direction, (0001), {112¯2} and {336¯2} facets compose the growth structures. Special emphasis was paid to the analysis of the crystal growth mechanisms. A crystallographic tool was developed to calculate the facets growth rates, the condensation surfaces and the surface recovered by GaN stripe in order to establish a correlation between the growth morphologies and the experimental conditions. It is shown that the growth rates of the facets, and so the morphologies, depend on the variations of the H2 ratio in the total flow rate.
    Journal of Crystal Growth 01/2009; 311(6):1460-1465. · 1.55 Impact Factor
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    ABSTRACT: GaN layers have been grown on patterned c-plane (0 0 0 1) sapphire and (0 0 0 1) GaN Metal Organic Vapour-Phase Epitaxy (MOVPE)/sapphire substrates by Hydride Vapour-Phase Epitaxy (HVPE) using the epitaxial lateral overgrowth (ELO) process in a single HVPE reactor. The crystal growth mechanisms via the lateral and vertical extensions were analysed with respect to the theoretical results based on the phenomenological model. The best experimental conditions supporting a high lateral extension were deduced. Coalescence of growth structures was favoured and high-quality uniform GaN layers about 10 μm thick were successfully grown directly on low-cost sapphire substrates. The properties of GaN layers were systematically investigated by scanning electron microscopy (SEM). GaN films exhibited a mirror-like surface without defects through the thickness.
    Journal of Crystal Growth 03/2008; 310(5):924–929. · 1.55 Impact Factor
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    ABSTRACT: Thick high quality gallium nitride (GaN) layers presenting a dislocation density reduced to 6×106 cm−2 were grown by hydride vapour phase epitaxy (HVPE). Scanning electron microscopy (SEM) characterizations, X-ray double diffraction (XRD) measurements, photoluminescence and reflectivity experiments, both at 4.5 K have been carried out to reveal the optical and the structural properties of the GaN epilayers. The strain relaxation was studied as a function of increasing the thickness. Special emphasis has been placed to the control of the parasitic nucleation on the reactor wall upstream the substrate, which consistently appears for long-time growth experiments. The amount of the parasitic deposit was quantitatively assessed by theoretical modelling. Attention was paid to accurate characterizations of 450 μm thick GaN layers obtained via thickening through a regrowth step after growth interruption and strain relaxation on cooling the templates.
    Journal of Crystal Growth 01/2007; · 1.55 Impact Factor
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    ABSTRACT: Selective area growth experiments were carried out by hydride vapour phase epitaxy (HVPE) on (1 0 0) GaAs-patterned substrates for the making of GaAs tips to be used as spin injectors. The tips exhibited various morphological profiles bounded by low and high index faces. Sharp apices were formed by the intersection of four perfectly crystal-defined {1 1 0} facets. The tip height and the apex sharpness have been optimized through the control of the crystal intrinsic growth anisotropy as a function of the experimental growth parameters. A good agreement was obtained between the morphologies of the selectively grown tips and kinetic Wulff constructions based on knowledge of the growth rate anisotropy that was independently determined on low index substrates. This demonstrates that a strictly surface kinetics dependent growth regime was established. HVPE appears to be a rapid, reliable tool for the making of morphologically controlled microtips on large area wafers.
    Journal of Crystal Growth 01/2007; · 1.55 Impact Factor
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    ABSTRACT: Strained InAs/InP single quantum wells grown by hydride vapour phase epitaxy have been investigated by photoluminescence. The evolution of the spectra when the temperature is varied above 5 K is analysed in particular. At temperatures larger than approximately 150 K, a signal is generally detected at an energy higher than that of the heavy exciton (e1hh1) peak which is still observed. It is interpreted as being due to the recombination of conduction band electrons in the InP barriers with heavy holes in the InAs well (echh1). A value of 0.48+or-0.02 eV is determined for the strained valence band offset from the fit of a simple single-band model to both the e1hh1 and echh1 transition energies.
    Semiconductor Science and Technology 12/1998; 8(8):1666. · 1.92 Impact Factor
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    ABSTRACT: A study of the growth copnditions and the relaxation processes of mismatched frsol|GaxIn1−xAs / InP structures (x = 0.3 to 0.5) grown by hydride vapour phase epitaxy is presented. Using double crystal X-ray diffraction and photoluminescence spectroscopy results, the bulk composition and the strain of the layers have been determined. Strained was relieved at thicknesses well above the Matthews-Blakeslee critical thickness; however, small residual strain (≈ 10%) always remained. A kinetic model applied to the growth of strained ternary alloys is developed; with the appropriate choice of the activation energy (Δϵ∗ = 1250 cal/mol) and the interaction parameter (ω = 2800 cal/mol), good agreement between experiemental and theoretical results has been obtained.
    Journal of Crystal Growth 01/1994; 135:11-22. · 1.55 Impact Factor