J. Cibert

University of Grenoble, Grenoble, Rhône-Alpes, France

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Publications (272)453.47 Total impact

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    David Ferrand, Joel Cibert
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    ABSTRACT: The strain configuration induced by the lattice mismatch in a core-shell nanowire is calculated analytically, taking into account the crystal anisotropy and the difference in stiffness constants of the two materials. The method is applied to nanowires with the wurtzite structure or the zinc-blende structure with the hexagonal / trigonal axis along the nanowire, and the results are compared to available numerical calculations and experimental data. It is also applied to multishell nanowires, and to core-shell nanowires grown along the $<001>$ axis of cubic semiconductors.
    03/2014;
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    ABSTRACT: Gold-catalyzed ZnTe nanowires were grown at low-temperature by molecular beam epitaxy on a ZnTe(111) B buffer layer, under different II/VI flux ratios, including with CdTe insertions. High resolution electron microscopy and energy-dispersive x-ray spectroscopy (EDX) gave information about the crystal structure, polarity and growth mechanisms. We observe, under stoichiometric conditions, the simultaneous presence of zinc-blende and wurtzite nanowires spread homogeneously on the same sample. Wurtzite nanowires are cylinder-shaped with a pyramidal-structured base. Zinc-blende nanowires are cone-shaped with a crater at their base. Both nanowires and substrate show a Te-ended polarity. Te-rich conditions favor zinc-blende nanowires, while Zn-rich suppress nanowire growth. Using a diffusion-driven growth model, we present a criterion for the existence of a crater or a pyramid at the base of the nanowires. The difference in nanowire morphology indicates lateral growth only for zinc-blende nanowires. The role of the direct impinging flux on the nanowires sidewall is discussed.
    Nano Letters 02/2014; · 13.03 Impact Factor
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    ABSTRACT: Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer^{2}. The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are <111> oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires.
    Applied Physics Letters 06/2013; · 3.79 Impact Factor
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    ABSTRACT: X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L$_{2,3}$ edges of Mn in (Ge,Mn) compounds have been measured and are compared to the results of first principles calculation. Early \textit{ab initio} studies show that the Density Functional Theory (DFT) can very well describe the valence band electronic properties but fails to reproduce a characteristic change of sign in the L$_{3}$ XMCD spectrum of Mn in Ge$_3$Mn$_5$, which is observed in experiments. In this work we demonstrate that this disagreement is partially related to an underestimation of the exchange splitting of Mn 2$p$ core states within the local density approximation. It is shown that the change in sign experimentally observed is reproduced if the exchange splitting is accurately calculated within the Hartree-Fock approximation, while the final states can be still described by the DFT. This approach is further used to calculate the XMCD in different (Ge,Mn) compounds. It demonstrates that the agreement between experimental and theoretical spectra can be improved by combining state of the art calculations for the core and valence states respectively.
    submitted. 10/2012;
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    ABSTRACT: We use extensive first principle simulations to show the major role played by interfaces in the mechanism of phase separation observed in semiconductor multifunctional materials. We make an analogy with the precipitation sequence observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones in this new context. A new class of materials, the $\alpha$ phases, is proposed to understand the formation of the coherent precipitates observed in the GeMn system. The interplay between formation and interface energies is analyzed for these phases and for the structures usually considered in the literature. The existence of the alpha phases is assessed with both theoretical and experimental arguments.
    Physical Review B 03/2012; 85. · 3.77 Impact Factor
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    ABSTRACT: This work presents results of near-band gap magnetooptical studies on Zn1−xMnxO epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence that shift toward higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p−d exchange coupling to magnetic ions, by the electron-hole s−p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Γ7 and Γ9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data are described by an effective exchange energy N0(β(app)−α(app))=+0.2±0.1 eV, which points to small and positive N0β(app). It is shown that both the increase of the gap with x and the small positive value of the exchange energy N0β(app) corroborate recent theory describing the exchange splitting of the valence band in a nonperturbative way, suitable for the case of a strong p−d hybridization.
    Phys. Rev. B. 07/2011; 84(3).
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    ABSTRACT: We have performed numerical simulations using the finite elements method in order to investigate magnetotransport in manganese doped germanium thin films. Up to now, several groups have reported similar transport measurements on (Ge,Mn) films obtained either by low temperature molecular beam epitaxy or by manganese implantation of germanium wafers. However, different physical interpretations have been proposed, including the existence of a diluted carrier-mediated ferromagnetic phase. Here, by carefully taking into account the morphology of (Ge,Mn) films (Mn-rich ferromagnetic inclusions in a highly diluted germanium matrix), we can reproduce some experimental findings. In particular, in order to observe high positive magnetoresistance and Hall angles, there are two requirements: (i) a strong anomalous Hall effect within Mn-rich inclusions and (ii) a conductivity contrast between these inclusions and the germanium matrix. Finally, we conclude that (Ge,Mn) films behave as granular ferromagnetic systems.
    Journal of Applied Physics 06/2011; 109(12):123906-123906-9. · 2.21 Impact Factor
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    ABSTRACT: This work presents results of near-band gap magnetooptical studies on Zn1-xMnxO epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence that shift toward higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma7 and Gamma9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data are described by an effective exchange energy N0(beta(app)-alpha(app))=+0.2±0.1 eV, which points to small and positive N0beta(app). It is shown that both the increase of the gap with x and the small positive value of the exchange energy N0beta(app) corroborate recent theory describing the exchange splitting of the valence band in a nonperturbative way, suitable for the case of a strong p-d hybridization.
    Physical review. B, Condensed matter 01/2011; 84.
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    ABSTRACT: We theoretically describe the spin excitation spectrum of a two dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work, we introduce equations that allow to consider the interplay between the Coulomb interaction of delocalized electrons and the $sd$ exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found : mixed waves propagate below the single particle continuum, an anticrossing gap is open at a specific wavevector and the kinetic damping due to the electron motion strongly influences the coupling strength between electrons and impurities spins.
    Physical review. B, Condensed matter 11/2010;
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    ABSTRACT: We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE| ∼ 100 Oe and a coercivity enhancement ΔHc ∼ 70 Oe have been obtained upon cooling (300–5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.
    Applied Physics Letters 08/2010; 97(6):062501-062501-3. · 3.79 Impact Factor
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    ABSTRACT: An optical spin orientation is achieved for a Mn atom localized in a semiconductor quantum dot using quasiresonant excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy.The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. Relaxation times exceeding the micro-second range are measured (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2010; 7(6):1651 - 1654.
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    ABSTRACT: We demonstrate a high degree of an optical spin preparation of a single Mn atom embedded in a CdTe/ZnTe quantum dot (QD). Due to the strong exchange interaction of the manganese atom with an exciton injected into the QD the spin orientation can be achieved by quasi-resonant or fully-resonant optical creation of the polarized electron-hole pairs. A measured spin memory of the isolated Mn atom, in most of the cases, is in the microsecond range, and depends on the built-in strain in the quantum dot. During the resonant optical pumping process exciton spin-flip can occur without a change of the Mn spin providing a way to directly read the dynamics of the pumped spin state. The manganese spin orientation is achieved in a few tens of ns.
    Journal of Physics Conference Series 03/2010; 210(1):012038.
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    ABSTRACT: The field of ferromagnetic semiconductors evolves very fast nowadays for their potential use in spintronic devices. Up to now, efforts have mainly focused on Diluted Magnetic Semiconductors but Curie temperatures in these materials still remain modest. One possible route to increase at least locally transition temperatures is to use spinodal decomposition leading to transition metal-rich high TC nanostructures. We focus here on (Ge,Mn) considered as a model system for spinodal decomposition and compatible with Si-based microelectronics. While the growth of (Ge,Mn) films on Ge substrates leads systematically to Mn-rich self-assembled nanocolumns exhibiting high TC, we demonstrate the fine control of spinodal decomposition in (Ge,Mn) films grown on GaAs. Using different surface preparations, we clearly identify the role of surface morphology and impurity diffusion from the substrate (Ga or As) on the nanocolumns growth and the electrical properties (MR and AHE).
    03/2010;
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    ABSTRACT: Time-resolved Kerr rotation experiments in CdMnTe quantum wells provide the evidence of mixed spin excitations of the two-dimensional electron gas and magnetic ions. The onset of strong coupling between electron and Mn spin modes reveals the collective (spin-wave) nature of electronic spin excitations probed by this method. We show that resonant exchange coupling between electron-spin waves and magnetic ions spin-flip excitations provide insights in the many-body physics of the two-dimensional electron gas.
    Physical review. B, Condensed matter 01/2010;
  • ChemInform 01/2010; 33(22).
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    ABSTRACT: We describe selected results obtained on diluted magnetic semiconductors, from the growth by molecular beam epitaxy to the study of the magnetic, magneto-optical and magneto-transport prop-erties associated to carrier induced ferromagnetism in layers, quantum wells, nanocolumns, and to the coherent manipulation of a single Mn spin in a quantum dot. from the Ecole Supérieure de Physique et Chimie de Paris, he got his PhD from the Universié Paris VI in 1981 on 'Band structure and disor-der effects in III-V semiconductors alloys layers prepared by epitaxy'. In 1986, he became a CNRS researcher at the Laboratoire de Spectrométrie Physique in Grenoble, after postdoctoral stays at the Max Planck In-stitute in Stuttgart and at the IBM Yorktown-Heights research labora-tories. Since 2005, he is the head of the CEA-CNRS 'Nanophysique et Semiconducteurs' group in Grenoble. He has published more than 150 articles in periodic journals and 50 in-vited papers in international conferences. He advised 12 PhD theses at the Université Joseph Fourier. His research focusses today on semicon-ducting nanostructures.
    Int. J. of Nanotechnology. 01/2010;
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    ABSTRACT: Changing the morphology of the growing surface and the nature of residual impurities in (Ge,Mn) layers dramatically changes nanospinodal decomposition, i.e., the morphology of ferromagnetic Mn-rich inclusions. By this way, we are able to control the magnetotransport properties of (Ge,Mn) films. By using different substrates and substrate preparation, we have indeed obtained p-type layers with nanocolumns, either parallel or entangled, and n-type layers with spherical clusters. Holes exhibit an anomalous Hall effect and electrons exhibit a tunneling magnetoresistance, both with a clear dependence on the magnetization of the Mn-rich inclusions; holes exhibit orbital MR and electrons show only the normal Hall effect, and an additional component of magnetoresistance due to weak localization, all three being independent of the magnetic state of the Mn-rich inclusions. Identified mechanisms point to the position of the Fermi level of the Mn-rich material with respect to the valence band of germanium as a crucial parameter for the control and the optimization of magnetotransport in such hybrid layers.
    Physical review. B, Condensed matter 01/2010; 82(3).
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    ABSTRACT: We have shown recently that the magnetic properties of excitons change significantly as the excitons acquire kinetic energy. In particular, the exciton magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectroscopic studies of excitons confined in quantum wells of large width (greater than five times the exciton Bohr radius) and these motion-induced changes in the magnetic properties have now been observed for CdTe, ZnSe, ZnTe and GaAs. The present paper summarises these phenomena, with particular focus on CdTe and ZnSe, and shows that the changes can be accounted for by motion-induced mixing between the exciton ground and higher lying states. The mixing is caused by the gamma3 term in the Luttinger Hamiltonian which describes the dispersion curves for the valence band and, as a result, the form of the exciton wavefunction becomes motion-dependent. For both materials, excellent agreement is obtained between experiment and the results predicted by this mechanism.
    physica status solidi (b) 01/2010; 9999:NA. · 1.49 Impact Factor
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    ABSTRACT: Spin injection in semiconductors has been a long-standing issue in the field of spintronics for nearly 10 years. Only at the end of the 1990s, electrical spin injection in III-V semiconductors could be demonstrated using spin-light emitting diodes as spin detectors. Although silicon is the key material of microelectronics, spin injection in silicon could be achieved only recently in 2007. For this purpose, we have developed a new ferromagnetic semiconductor (Ge,Mn) that may be suitable for spin injection in silicon. Indeed this material is compatible with mainstream silicon technology and is predicted as a half-metallic ferromagnet. We have used low temperature molecular beam epitaxy to grow germanium films doped with manganese. Growth temperatures as well as Mn concentrations were kept low in order to avoid phase separation due to the low solubility of Mn in Ge. Careful structural and chemical analyses showed that (Ge,Mn) films are not diluted magnetic semiconductors. We rather observe high-TC ferromagnetic self-assembled nanocolumns in the germanium film with magnetic properties strongly dependent on growth parameters. We could clearly identify four different magnetic phases in Ge1−xMnx films: diluted paramagnetic Mn atoms in the germanium matrix, low TC (textless200 K) nanocolumns, high TC nanocolumns (textgreater400 K) and Ge3Mn5 clusters. The relative weight of each phase mainly depends on the growth temperature and to a lesser extent to Mn concentration. In parallel, we have developed methods to detect spin injection in silicon. Using a ferromagnetic metal with perpendicular magnetisation we could perform spin injection in silicon in the tunnelling regime through an alumina barrier. Spin detection was optically achieved using a SiGe/Si spin-light emitting diode without applying a magnetic field.
    International Journal of Nanotechnology 01/2010; 7(4/5/6/7/8):575 - 590. · 1.09 Impact Factor
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    ABSTRACT: Changing the morphology of the growing surface and the nature of residual impurities in (Ge,Mn) layers - by using different substrates - dramatically changes the morphology of the ferromagnetic Mn-rich inclusions and the magnetotransport properties. We obtained p-type layers with nanocolumns, either parallel or entangled, and n-type layers with spherical clusters. Holes exhibit an anomalous Hall effect, and electrons exhibit a tunneling magnetoresistance, both with a clear dependence on the magnetization of the Mn-rich inclusions; holes exhibit orbital MR, and electrons show only the normal Hall effect, and an additional component of magnetoresistance due to weak localization, all three being independent of the magnetic state of the Mn rich inclusions. Identified mechanisms point to the position of the Fermi level of the Mn-rich material with respect to the valence band of germanium as a crucial parameter in such hybrid layers.
    10/2009;

Publication Stats

1k Citations
673 Downloads
453.47 Total Impact Points

Institutions

  • 2012
    • University of Grenoble
      Grenoble, Rhône-Alpes, France
  • 2011
    • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 1990–2011
    • French National Centre for Scientific Research
      • Institut Néel
      Paris, Ile-de-France, France
  • 1989–2010
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhone-Alpes, France
  • 1998–2009
    • University of Warsaw
      • Institute of Experimental Physics
      Warsaw, Masovian Voivodeship, Poland
  • 2006
    • Université Paris-Sud 11
      Orsay, Île-de-France, France
    • University of Bath
      • Department of Physics
      Bath, ENG, United Kingdom
  • 2004
    • Université de Technologie de Belfort-Montbéliard
      Belfort, Franche-Comté, France
  • 1999–2002
    • Polish Academy of Sciences
      • Instytut Fizyki
      Warsaw, Masovian Voivodeship, Poland
  • 2001
    • University of Wuerzburg
      • Institute of Physics
      Würzburg, Bavaria, Germany