Commentaires: PROMIS – Postgraduate Research on Dilute Metamorphic Nanostructures and Metamaterials in Semiconductor Photonics, Marie Sklodowska-Curie Initial Training network (ITN), PROMIS Summer School, 31st August – 9th September, 2016, Montpellier

Résumé:

Excitons in quantum wells (QWs) based on group-III nitrides or ZnO are naturally indirect in real space, due to the strong internal electric field that exists along the growth axis. Within such excitons, the electron and hole are spatially separated, leading to (1) strong dipole moments and (2) long radiative lifetimes. Extensive studies of indirect excitons (IXs) in GaAs-based heterostructures have shown that a combination of these two features results in many interesting properties of IXs : they can propagate over large distances, they can be controlled in-situ by light or by external gate voltage, they can cool down to the lattice temperature before recombination, and form cold and dense gases of interacting bosons. Compared to traditional IXs in arsenide heterostrutures, IXs in GaN- or ZnO-based QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. Using spatially- and time-resolved photoluminescence experiments, we investigate the exciton transport in several QW samples based on GaN and ZnO. The IX emission is imaged, with spectral and temporal resolutions, along the sample plane. Therefore the corresponding spatial density profiles are obtained and monitored as a function of (i) temperature, (ii) the exciton density at the excitation spot, (iii) the substrate material and (iv) the excitation regime (continuous vs pulsed excitation). We provide a comprehensive analysis of the data combined with numerical modeling, and we show that the efficient propagation of IXs takes place in the high density regime where the in-plane disorder is efficiently screened by the dipole-dipole interaction. Under these latter conditions, exciton mobility is almost temperature-independent from 10K up to room temperature. This suggests that exciton-exciton interaction is by far the dominant scattering mechanism, compared to scattering by the interface disorder. However, exciton transport is maintained up to 300K only in the GaN/AlGaN QW that was epitaxially grown on a GaN substrate, since nonradiative processes dominate the transport in the case of a sapphire substrate, for which high densities of threading dislocations are present. We provide a detailed understanding of the physical mechanisms of IX transport and its temperature and density dependence, and we show that IXs in wide band-gap QWs constitute a promising system for the formation of collective bosonic states in semiconductors. Financial support : projects "INDEX" (FP7 PITN-GA-2011-289968) and "OBELIX" (ANR-15-CE30-0020-02).

Ref HAL: hal-01614018_v1
DOI: 10.1016/j.physe.2015.07.022
WoS: 000363564700002
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13 Citations

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DOI: 10.1109/CPEM.2016.7540650
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Résumé:

We report on measurements in large quantum Hall devices, made of high-quality graphene grown by propane/hydrogen chemical vapor deposition on SiC. These devices, having all the properties of an ideal quantum electrical resistance standard, surpass state-of-the-art GaAs/AlGaAs devices by considerable margins in their operational conditions. The Hall resistance can be found accurately quantized within one part in 109 over a 10-T range of magnetic fields with a lower bound at 3.5 T, temperatures as high as 10 K, or currents as high as 0.5 mA. This simplification sets the superiority of graphene for accessible and low-cost primary resistance standards.

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Résumé:

Graphene, known as one carbon layer material, holds attractive properties due to its hexagonal lattice, also called honeycomb structure. Since the seminal work on mechanically exfoliated few layer graphene, more growth processes were explored: Chemical Vapor Deposition (CVD) on metals or on silicon carbide (SiC); chemical reduction of graphene oxides and SiC sublimation. In contrast to the other graphene growth techniques, thermal decomposition of SiC provides wafer-scale homogeneous graphene spontaneously forming on semi-insulating substrate. SiC sublimation is the most promising option to achieve the transfer free and wafer-scale graphene. Furthermore, graphene/SiC is compatible with lithography techniques for further applications; thereby epitaxial graphene on SiC is a potential candidate for nanoelectronics. Until now, monolayer graphene growth by SiC sublimation at a pressure closed to the atmospheric pressure (around 900 mbar) and at high temperature (>1650°C) is well known . However, to obtain films with different and controlled characteristics such as the number of graphene layers or the type of doping by controlling the growth parameters remains challenging. We will present the initial growth stages from buffer layer to monolayer graphene on SiC (0001) as a function of the temperature at low pressure (10 mbar). A reproducible synthesis of low p-type doped monolayer graphene was optimized (few 10^11 cm-2). The p-type doping are obtained here on the bare samples without any post-growth process such as lithography. A prototypal HTA-100 furnace developed by Annealsys Company has allowed the synthesis of these original samples. All the samples were characterized by Raman spectroscopy and Atomic Force Microscopy (AFM). In addition, transport measurements (at room temperature and low temperature with high magnetic field) were carried out in the case of continuous monolayer graphene films showing especially characteristics of Quantum Hall Effect.

Ref HAL: hal-01440539_v1
DOI: 10.1063/1.4967301
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Résumé:

The effect of germanium (Ge) on n-type 4H-SiC is experimentally studied by electrical characterization of homoepitaxial layers grown by chemical vapor deposition (CVD). Measurements show that electrical properties of epitaxial layers can be changed by intentional incorporation of germane (GeH4) gas during the deposition process. On the nanoscale, two-dimensional mappings acquired with conductive atomic force microscopy show preferential conductive paths on the surface of Gedoped samples, which are related to the presence of this isoelectronic impurity. Hall effect measurements confirm that also macroscopic electrical properties of n-type 4H-SiC are improved due to incorporation of Ge into SiC during CVD growth. In particular, despite equal free electron concentration, enhanced mobility in a wide temperature range is measured in Ge-doped samples as compared to a pure 4H-SiC layer. Based on our results from Hall effect measurements as well as admittance spectroscopy and deep level transient spectroscopy, it is speculated that Ge influences the generation and annealing of other point defects and thus helps to reduce the total concentration of defects

Ref HAL: hal-01426431_v1
DOI: 10.1103/PhysRevB.94.054525
WoS: WOS:000383029200005
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6 Citations
Résumé:

Superconducting hybrid junctions are revealing a variety of effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi-ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical vapor deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in the presence of an incipient Berezinskii-Kosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean critical state model (CSM). We show that the CSM, with appropriate account for the low-dimensional geometry, can partly explain the odd features measured in these junctions.