L’obtention de films de graphène aux caractéristiques contrôlées, comme le nombre de couches ou le dopage, anime toujours une partie de la communauté. La décomposition thermique contrôlée du carbure de silicium (SiC) permet d’envisager la croissance homogène de graphène directement sur des substrats de grande surface. Ce mode de croissance du graphène a initialement été développé à des pressions proches de la pression atmosphérique sous argon et à haute température. Dans l’étude présentée, les croissances par sublimation ont été réalisées à basse pression d’argon (10 mbar). Dans tous les cas, lors de la sublimation du SiC du côté dit « face silicium », il se forme une première couche, dite tampon, qui deviendra, par la suite, la couche de graphène sous laquelle se formera une nouvelle couche tampon. Nous discuterons de la présence et des caractéristiques de cette couche tampon. Nous mettrons notamment en évidence l’inhomogénéité de sa signature Raman, et la nécessité d’une procédure du traitement des données robuste. Puis, nous discuterons, à partir d’une étude complémentaire Raman et AFM, les premiers instants de croissance de la monocouche de graphène, par un mécanisme en bord de marches du SiC. Enfin, des mesures électriques à basse température (1.7 K) et sous champ magnétique (jusqu’à 13 T) complèteront les études menées en confirmant la continuité et la qualité de la monocouche de graphène.

Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and AFM analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the BL on terraces with a step-flow growth mechanism, as demonstrated in Fig.1. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Regarding transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film.

T he main obstacle to the use of graphene on the industrial scale is the growth of a large and homogenous monolayer graphene Concerning this issue, it is worthnoting that our group has recently developed a reproducible and controlled growth process of a monolayer graphene on SiC 0001 by sublimation at low Ar pressure i e 10 mbar Still, the control of the electronic properties of the obtained graphene by this process is very challenging E g the mobility on our graphene on 4 H SiC (0001) is around 2000 cm 2 v 1 s 1 at RT which is in the range of the measured mobilities on similar substrates Yet, it is still very low when compared with the mobilities reported for suspended graphene. It is well acceptedthat the electronic properties of graphene on SiC are highly sensitive to the substrate underneath It was reported that the mobility of graphene on SiC 0001 increases with increasing SiC steps width, and its resistance increases with increasing SiC steps height. This means that the electronic properties of graphene on SiC 0001 can be tuned by controlling the height and width of the terraces that results from the surface reconstruction of SiC before the growth i e Step bunching phenomenon

We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f 1 MHz. Using several measurement configurations, we study the dependence of this inductance on the length of the edge channel and on the integer quantum Hall filling fraction. The experimental data are well described by a scattering model for edge magnetoplasmons taking into account effective long range Coulomb interactions within the sample. This demonstrates that the inductance's dependence on the filling fraction arises from the effective quantum inertia of charge carriers induced by Coulomb interactions within an ungated macroscopic quantum Hall bar.

We analyze the multilayer structure of sunflower leaves from Terahertz data measured in the time-domain at a ps scale. Thin film reverse engineering techniques are applied to the Fourier amplitude of the reflected and transmitted signals in the frequency range f < 1.5 Terahertz (THz). Validation is first performed with success on etalon samples. The optimal structure of the leaf is found to be a 8-layer stack, in good agreement with microscopy investigations. Results may open the door to a complementary classification of leaves.

We analyze Terahertz (THz) echoes by reflection on a sunflower leaf in order to evaluate the internal leaf structure (geometry, complex indices and thicknesses). The analysis is based on the thin film multilayer formalism in time and frequency domains. A high agreement is emphasized between experiment and theory, and we evaluate how realistic the multilayer solution can be in regard to our knowledge related to the sunflower leaf. A test campaign is performed in Charles Coulomb laboratory, which is equipped with the THz spectrometer.

We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a p-type HgTe/CdxHg1−xTe double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two i = 2 states (plateaus at ρxy = h/ie 2) separated by an intermediate i = 1 state, which looks like some anomalous peak on the extra-long i = 2 plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes ps and a high-field QH structure with a 2 − 1 plateau-plateau transition corresponding to much larger ps. We show that only a part of holes, residing in an additional light hole subband in the DQW, participate in QHE at weak fields while the rest of holes is excluded into the reservoir formed in the lateral maximum of the valence subband. All the holes come into play at high fields due to a peculiar behavior of the zero-mode levels.