B. Jouault

Quantum Hall resistance standards from graphene grown by chemical vapour deposition on silicon carbide

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.

Current status of self-organized epitaxial graphene ribbons on the C face of 6H–SiC substrates

The current status of long, self-organized, epitaxial graphene ribbons grown on the (0 0 0 −1) face of 6H–SiC substrates is reviewed. First, starting from the early stage of growth it is shown that on the C face of 6H–SiC substrates the sublimation process is not homogeneous. Most of the time it starts from defective sites, dislocations or point defects, that define nearly circular flakes surrounded by bare SiC. These flakes have a volcano-like shape with a graphite chimney at the centre, where the original defect was located. At higher temperatures a complete conversion occurs, which is not yet homogeneous on the whole sample. This growth process can be modified by covering the sample with a graphite cap. It changes the physics of the surface reconstruction during the Si-sublimation process and, on the C face, makes more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of strongly step-bunched areas with, in between, long and large reconstructed terraces covered by graphitic material. Despite the low intrinsic optical absorption of a few graphene layers on SiC, micro-transmission experiments, complemented by micro-Raman spectroscopy, demonstrate that most of this graphitic coverage is made of one or two homogeneous graphene layers. We show also that most of the thermal stress between the graphene layer and the 6H–SiC substrate is relaxed by pleats or wrinkles which are clearly visible on the AFM images. Finally, the results of transport experiments performed on the graphitic ribbons reveal the p-type character of the ribbons.

Interplay between interferences and electron-electron interactions in epitaxial graphene

We separate localization and interaction effects in epitaxial graphene devices grown on the C-face of a 4H-SiC substrate by analyzing the low temperature conductivities. Weak localization and antilocalization are extracted at low magnetic fields, after elimination of a geometric magnetoresistance and subtraction of the magnetic field dependent Drude conductivity. The electron electron interaction correction is extracted at higher magnetic fields, where localization effects disappear. Both phenomena are weak but sizable and of the same order of magnitude. If compared to graphene on silicon dioxide, electron electron interaction on epitaxial graphene are not significantly reduced by the larger dielectric constant of the SiC substrate.

Charging and emission effects of multiwalled carbon nanotubes probed by electric force microscopy

Electrostatic properties of single-separated multiwalled carbon nanotubes (MWCNTs) deposited on a dielectric layer have been investigated by charge injection and electric force microscopy (EFM) experiments. We found that upon local injection from the biased EFM tip, charges delocalize over the whole nanotube length (i.e., 1–10μm), consistent with a capacitive charging of the MWCNT-substrate capacitance. In addition, the insulating layer supporting the nanotubes is shown to act as a charge-sensitive plate for electrons emitted from the MWCNTs at low electric fields, thus allowing the spatial mapping of MWCNT field-emission patterns.

Growth of monolayer graphene on 8° off-axis 4H–SiC (000–1) substrates with application to quantum transport devices

Using high temperature annealing conditions with a graphite cap covering the C-face of an 8° off-axis 4H–SiC sample, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magnetotransport measurements. We find a moderate p-type doping, high carrier mobility, and half integer quantum Hall effect typical of high quality graphene samples. This opens the way to a fully compatible integration of graphene with SiC devices on the wafers that constitute the standard in today’s SiC industry.

Quantum Hall effect in bottom-gated epitaxial graphene grown on the C-face of SiC

We demonstrate that the carrier concentration of epitaxial graphene devices grown on the C-face of a SiC substrate is efficiently modulated by a buried gate. The gate is fabricated via the implantation of nitrogen atoms in the SiC crystal. The charge neutrality point is observed close to gate voltage zero, and graphene can be populated by either holes or electrons down to low temperature (1.5 K). The hole concentration is hardly tuned by the gate voltage, possibly because of interface states below the Dirac point. A remarkably large quantum Hall plateau is observed for electrons.

Magnetoresistance of disordered graphene: From low to high temperatures

We present the magnetoresistance (MR) of highly doped monolayer graphene layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport studies are performed on a large temperature range, from T = 1.7 K up to room temperature. The MR exhibits a maximum in the temperature range 120–240 K. The maximum is observed at intermediate magnetic fields (B = 2–6 T), in between the weak localization and the Shubnikov-de Haas regimes. It results from the competition of two mechanisms. First, the low-field magnetoresistance increases continuously with T and has a purely classical origin. This positive MR is induced by thermal averaging and finds its physical origin in the energy dependence of the mobility around the Fermi energy. Second, the high-field negative MR originates from the electron-electron interaction (EEI). The transition from the diffusive to the ballistic regime is observed. The amplitude of the EEI correction points towards the coexistence of both longand short-range disorder in these samples.

Room-Temperature Transport of Indirect Excitons in (Al,Ga)N/GaN Quantum Wells

We report on the exciton propagation in polar (Al,Ga)N/GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells (QWs) grown on GaN template substrate. By comparing microphotoluminescence images of two identical QWs grown on sapphire and on GaN, we reveal the twofold role played by GaN substrate in the transport of excitons. First, the lower threading dislocation densities in such structures yield higher exciton radiative efficiency, thus limiting nonradiative losses of propagating excitons. Second, the absence of the dielectric mismatch between the substrate and the epilayer strongly limits the photon guiding effect in the plane of the structure,making exciton transport easier to distinguish from photon propagation. Our results pave the way towards room-temperature gate-controlled exciton transport in wide-bandgap polar heterostructures.

Investigation of AlGaN∕AlN∕GaN heterostructures for magnetic sensor application from liquid helium temperature to 300°C

We report a comparative investigation of the magnetic response of long channel AlGaN∕AlN∕GaN heterostructures (Hall-field effect transistor devices) grown on three different semi-insulating templates on silicon and sapphire. From Hall effect measurements conducted up to 573K (300°C), we find that some of these specific devices can be used as magnetic sensors in a large temperature range (∼600°C) with a magnetic sensitivity close to 60V∕AT and a small thermal drift. On the best sample, between liquid helium temperature and 300°C, the average value of the thermal drift is only −7ppm∕°C.

Spin exciton in a quantum dot with spin-orbit coupling at high magnetic field

Coulomb interactions of few (