Ref HAL: hal-01615243_v1
DOI: 10.1038/nnano.2015.192
WoS: 000364528300014
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92 Citations

Ref HAL: hal-03037518_v1
DOI: 10.1038/ncomms7806
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Replacing GaAs by graphene to realize more practical quantum Hall resistance standards (QHRS), accurate to within 10(-9) in relative value, but operating at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date, the required accuracy has been reported, only few times, in graphene grown on SiC by Si sublimation, under higher magnetic fields. Here, we report on a graphene device grown by chemical vapour deposition on SiC, which demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4 K. This is explained by a quantum Hall effect with low dissipation, resulting from strongly localized bulk states at the magnetic length scale, over a wide magnetic field range. Our results show that graphene-based QHRS can replace their GaAs counterparts by operating in as-convenient cryomagnetic conditions, but over an extended magnetic field range. They rely on a promising hybrid and scalable growth method and a fabrication process achieving low-electron-density devices.

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Commentaires: Ecole Internationale. Cours présenté par Masha Vladimirova.

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Counting number of graphene layers on copper ? (OK for SiO2/Si, SiC, glass)Graphene source in Montpellier by CVD on copper and on SiC and by SiC sublimation (development of prototype furnace)

Ref HAL: hal-01305122_v1
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An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (µ-PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100µm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 µm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-dipole repulsion in high-density regions and for disorder along the sample plane.[1]Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard. Optics Letters 35 (10), 1587 (2010).[2]A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard. Science 321, 229 (2008).[3]A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard. Optics Letters 32, 2466 (2007).[4]F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre. Phys. Rev. B 91, 205424 (2015).

Ref HAL: hal-01154792_v1
DOI: 10.1103/PhysRevB.91.205424
WoS: 000354972700012
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19 Citations
Résumé:

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density,measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons.We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μm is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the excitonemission pattern on the scale up to 100 μm. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

Ref HAL: hal-03037519_v1
DOI: 10.1088/0022-3727/47/9/094009
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We review some of the electric properties of self-organized graphene monolayers on the carbon face of SiC. From sparse surface defects acting as nucleation centres, isolated graphene layers grow in the shape of triangles or ribbons on the step bunched SiC surface. Using e-beam lithography, standard Hall bars have been made. At low magnetic fields, conductance fluctuations, weak localization, electron-electron interactions are usually observed. At higher magnetic fields, the anomalous quantum Hall (QHE) effect typical of monolayer graphene is also observed. In this regime, the breakdown of the QHE appears at moderate currents, which we attribute to the persistence of impurities in the vicinity of the graphene layer. Moderate heating (150 degrees C) is not sufficient to overcome this issue, and moreover, the carrier concentration cannot be controlled. In order to control the carrier concentration, bottom-gated samples are also presented. In these devices, the carrier concentration can be modulated, but the breakdown current remains very small.