Denis Lefebvre

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (16)

  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The behavior of a high density radical source for the plasma assisted molecular beam epitaxy of GaN and AlGaN compounds is studied and compared with the one of a conventional plasma source. Plasma light emission correlates with the GaN growth rate. Both attest to the better efficiency of the new source for producing active nitrogen species with resulting growth rates well beyond 1 μm/h. The present study shows that GaN films with equivalent structural and optical quality can be grown even with a growth rate enhancement by a factor of 5. The purity of the grown films is investigated as well as point defects. Positron annihilation shows that plasma conditions can be tuned in order to limit the increase of the gallium-vacancy related complexes density by about 2x1016 cm-3 while reaching growth rates as high as 2.1 μm/h.
    Full-text Article · Oct 2015 · Journal of Crystal Growth
  • Y.Cordier · B.Damilano · P.Aing · [...] · M.Nemoz
    [Show abstract] [Hide abstract] ABSTRACT: The behavior of a high density radical source for the plasma assisted molecular beam epitaxy of GaN and AlGaN compounds is studied and compared with the one of a conventional plasma source. Plasma light emission correlates with the GaN growth rate. Both attest to the better efficiency of the new source for producing active nitrogen species with resulting growth rates well beyond 1 μm/h. The present study shows that GaN films with equivalent structural and optical quality can be grown even with a growth rate enhancement by a factor of 5. The purity of the grown films is investigated as well as point defects. Positron annihilation shows that plasma conditions can be tuned in order to limit the increase of the gallium-vacancy related complexes density by about 2x1016 cm-3 while reaching growth rates as high as 2.1 μm/h.
    Article · Jan 2015 · Journal of Crystal Growth
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: While the integration of graphene on semiconductor surfaces is important to develop new applications, epitaxial graphene has only been integrated on SiC substrates or 3C-SiC/Si templates. In this work, we explore the possibility of growing graphene on AlN/Si(111) templates. Using a chemical vapor deposition process with propane as the carbon source, we have obtained graphitic films (from 2 to 10 graphene layers) on AlN/Si(111) while preserving the morphology of the AlN layer beneath the graphitic film. This study is an important step for the integration of graphene with semiconductors other than SiC.
    Full-text Article · Jan 2014 · Applied Physics Letters
  • [Show abstract] [Hide abstract] ABSTRACT: Graphene growth from a propane flow in a hydrogen environment (propane-hydrogen chemical vapor deposition (CVD)) on SiC differentiates from other growth methods in that it offers the possibility to obtain various graphene structures on the Si-face depending on growth conditions. The different structures include the (6√3 × 6√3)-R30° reconstruction of the graphene/SiC interface, which is commonly observed on the Si-face, but also the rotational disorder which is generally observed on the C-face. In this work, growth mechanisms leading to the formation of the different structures are studied and discussed. For that purpose, we have grown graphene on SiC(0001) (Si-face) using propane-hydrogen CVD at various pressure and temperature and studied these samples extensively by means of low energy electron diffraction and atomic force microscopy. Pressure and temperature conditions leading to the formation of the different structures are identified and plotted in a pressure-temperature diagram. This diagram, together with other characterizations (X-ray photoemission and scanning tunneling microscopy), is the basis of further discussions on the carbon supply mechanisms and on the kinetics effects. The entire work underlines the important role of hydrogen during growth and its effects on the final graphene structure.
    Article · May 2013 · Journal of Applied Physics
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.
    Full-text Conference Paper · Jan 2013
  • [Show abstract] [Hide abstract] ABSTRACT: The present work is proposing a comparative analysis of the graphitization, achieved by argon–propane assisted chemical vapor deposition, of 6H–SiC(0001) bulk substrates and 3C–SiC heteroepilayers deposited on (111) and (100) silicon. We have investigated the influence of the experimental parameters of the graphitization (pressure, propane flow rate and duration) both on the structural and the electrical properties of the graphitic/graphene phases developed at the samples surface. In particular, the growth mode has been highlighted. It has been shown that, in our experimental conditions, the formation of graphene is only a transitory step followed by a stage of rapid over-deposition of the surface by a highly disordered graphitic phase. This can be understood by a surface chemical potential variation accompanied by a balance between some mass transport at the surface (which could include sublimation) and a deposition regime. It shows that the process time must be properly adjusted to conserve the graphene at the surface. Furthermore, it is shown that the graphene sheet resistance is significantly dependent on the surface uniformity and can be tuned by varying the process pressure.
    Article · Jun 2012 · Journal of Crystal Growth
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Structural and electrical properties of graphene elaborated on 3C-SiC(111)/Si and 3C-SiC(100)/Si templates, using propane-argon gas mixtures under CVD environment, are presented. On 3C-SiC(111), the graphitic phase is clearly attributable to graphene and presents good electrical conductivity at the macroscopic scale. The opposite case is observed on 3C-SiC(100), for which the graphitic phase develops more rapidly but with a high degree of disorientation. The graphitization, which can be coupled with 3C-SiC growth stage, is efficient over the whole surface of 2’’ wafer and allows to elaborate, in a single process, Graphene on Silicon wafers.
    Full-text Conference Paper · May 2012
  • [Show abstract] [Hide abstract] ABSTRACT: We have grown graphene on SiC(0001) using propane-hydrogen CVD. In this work, we present the effects of growth pressure and temperature on structural and electrical properties. Structural characterizations evidence the formation of graphene with in-plane rotational disorder, except for low growth pressure and high growth temperature which lead to the formation of a (6Ö3´6Ö3)-30° interface between graphene and SiC. Electrical properties of samples presenting different graphene/SiC stacking and interfaces are compared and discussed.
    Conference Paper · May 2012
  • [Show abstract] [Hide abstract] ABSTRACT: Graphene growth on SiC using propane-hydrogen CVD has been recently demonstrated. In contrast to the annealing method, in-plane rotational disorder of graphene can be observed on SiC(0001). In this contribution, we study the effects of growth temperature on the interface between graphene and SiC. We show that growth temperature allows to control the formation of graphene with in-plane disorder or graphene on a (6√3×6√3)-R30° interface. Accordingly, grazing incidence X-ray diffraction evidences an effect of growth temperature on in-plane strain. These results are discussed on the basis of the hydrogenation of the SiC surface.
    Conference Paper · Feb 2012
  • Adrien Michon · Elodie Roudon · Marc Portail · [...] · Marcin Zielinski
    [Show abstract] [Hide abstract] ABSTRACT: We have grown graphene on 6H-SiC(0001) and 3C-SiC(111)/Si (111) using propane-hydrogen CVD. This contribution studies the effects of pressure on graphene growth and on its structural properties studied through low energy electron diffraction. We show that varying pressure allows to control the formation of graphene on a (6√3×6√3)-R30° interface reconstruction (low pressure) or graphene with in-plane rotational disorder (high pressure) on both 6H-SiC(0001) and 3C-SiC(111). The effects of the SiC morphology before graphene growth are discussed in order to explain the differences observed between polytypes.
    Conference Paper · Jan 2012
  • [Show abstract] [Hide abstract] ABSTRACT: It is shown that by using asymmetric barrier composition GaN - (Ga,In)N (GaInN) - (Al, Ga)N (AlGaN) multiple quantum wells it is possible to redshift the room temperature photoluminescence wavelength from 503 nm (blue-green) to 577 nm (yellow) when the Al composition in the AlGaN barrier increases from 0 to 0.15. This is obtained without modifying the GaInN QW composition and thickness. This effect is attributed to an increase of the internal electric field inside the GaInN QW originating to additional polarization charges at the AlGaN - GaN and GaInN - AlGaN interfaces. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Article · Jul 2010 · physica status solidi (c)
  • [Show abstract] [Hide abstract] ABSTRACT: We show that the shape of GaN nanostructures grown by molecular beam epitaxy on Al <sub>x</sub> Ga <sub>1-x</sub> N (0001) surfaces, for x≥0.4 , can be controlled via the ammonia pressure. The nanostructures are obtained from a two dimensional to three dimensional transition of a GaN layer occurring upon a growth interruption. Atomic force microscopy measurements show that depending on the ammonia pressure during the growth interruption, dot or dash-shaped nanostructures can be obtained. Low temperature photoluminescence measurements reveal a large redshift in the emission energy of the quantum dashes, as compared to the quantum dots. By simply adjusting the GaN deposited thickness, it is shown that quantum dashes enable to strongly extend the emission range of GaN / Al <sub>0.5</sub> Ga <sub>0.5</sub> N nanostructures from the violet-blue (∼400–470 nm ) to the green-orange range (∼500–600 nm ) .
    Article · Mar 2009 · Journal of Applied Physics
  • Article · Feb 2009
  • [Show abstract] [Hide abstract] ABSTRACT: GaN/Al0.5Ga0.5N nanostructures grown on c -plane sapphire by molecular beam epitaxy using ammonia as N source have been studied. Depending on the ammonia pressure during the two dimensional – three dimensional transition of the GaN layer, the shape of the islands is strongly modified: elongated or isotropic islands are observed, leading to the formation of quantum dashes or quantum dots, respectively. This shape transition is seen as a consequence of a change in surface energy. The change of morphology of the GaN layer is clearly evidenced by photoluminescence measurements, and a large redshift in the emission peak is observed for quantum dashes as compared to quantum dots. An electric field ∼3 MV/cm is estimated from the measurements at an excitation power ∼20 mW/cm2. Weak photoluminescence quenching between low and room temperature for both QDs and QDashes structures is observed, indicating a strong confinement of carriers into the nanostructures. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Article · Jan 2009 · physica status solidi (b)
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We have fabricated and characterized blue (Ga,In)N/GaN multiple quantum well light emitting diodes grown on a Si(l 10) substrate by molecular beam epitaxy. For a 20 mA current, we have found that the operating voltage and the series resistance are as low as 3.5 V and 17 Omega, respectively. A maximum light output power of 72 mu W is obtained as measured on the wafer. These characteristics are almost identical to those obtained on a reference sample grown on the commonly used Si(111) orientation. (C) 2008 The Japan Society of Applied Physics
    Full-text Article · Dec 2008 · Applied Physics Express
  • [Show abstract] [Hide abstract] ABSTRACT: The growth by molecular beam epitaxy and the optical properties of Ga N / Al <sub>0.5</sub> Ga <sub>0.5</sub> N quantum dots on (0001) sapphire substrates are reported. The quantum dots are spontaneously formed via a two dimensional to three dimensional transition upon growth interruption. Photoluminescence over the blue range (435–470 nm ) is obtained at room temperature by varying the GaN nominal thickness. A weak temperature dependence of the integrated photoluminescence intensity between low temperature and room temperature is observed indicating strong carrier localization in the quantum dots.
    Article · Mar 2008 · Applied Physics Letters