Ref HAL: hal-01208523_v1
DOI: 10.1063/1.4813220
WoS: WOS:000321761600027
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We report magnetotransport measurements performed on AlGaN/GaN devices with different buffer layers. Standard samples with a 1 μm thick GaN buffer show a linear Hall resistance and an almost constant magnetoresistance, as expected from a single two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. Other samples, with an Al x Ga1– x N buffer (x = 5%) and a buried linear aluminium gradient, have an additional three-dimensional electron slab (3DES) close to the GaN substrate. In this case, the Hall resistance is strongly non-linear and presents an incorrect hole-type carrier signature, evidenced by low field mobility spectrum analysis. This effect is strengthened when the 3D layer, parallel to the mesa-etched 2DEG, is infinite. We suggest that the misplacement of the electrical contacts in the 3DES, i.e., far from the sample edges, could explain the wrong carrier type determination.

Commentaires: The authors would like to thank Dr. E. Litwin-Staszewska from the Institute of High Pressure Physics “Unipress,” for the Hall bar processing on the 3D electron slab. This work was supported by the GANEX program (ANR-11-LabEx-0014).

Ref HAL: hal-00911657_v1
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An indirect exciton is a bound pair of an electron and a hole confined in spatially separated semiconductor layers. Long lifetimes of indirect excitons allow them to cool down to low temperatures below the temperature of quantum degeneracy. In coherent exciton gases the spin relaxation mechanisms linked with the scattering of particles are strongly suppressed. Recent experiments evidenced spontaneous coherence and condensation, phase singularities, spatial ordering, and polarization textures in a cold excitons gas . We develop a theoretical model accounting for the spin-orbit interaction of electrons and holes, exchange splitting of exciton states and Zeeman effect. This model describes the polarization patterns observed experimentally, including helical patterns, four-leaf patterns, spiral patterns, and bell patterns, and confirms that the polarization textures at low temperature manifest ballistic propagation of cold excitons over the distances exceeding the exciton Bohr radius by 3 orders of magnitude and the exciton thermal De Broglie wavelength by more than an order of magnitude. The comparison of theory and experiment allows extracting the spin currents carried by electrons and holes bound to excitons. The spins tend to align along the effective spin-orbit field induced due to the Dresselhaus effect.

Ref HAL: hal-03037523_v1
DOI: 10.4028/www.scientific.net/MSF.711.235
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We present the growth and characterization of epitaxial Graphene on the (000-1) and (11-20) planes. In both cases, the growth was carried out in a RF furnace, by implementing our technique of confined atmosphere, covering the SiC substrate with a graphitic cap during the growth. The grown material was investigated by means of AFM, SEM, Raman spectroscopy and magneto transport. Contrary to the (0001) face, in both faces (000-1) and (11-20), almost free standing Graphene monolayers of very high quality are grown. These Graphene sheet are uniform, continuous, almost strain-free and lightly doped. In both faces, Hall bars were fabricated and Shubnikov-de Haas oscillations typical of Graphene, as well as the Half Integer Quantum Hall Effect are observed.

Commentaires: 4th Workshop on Advanced Semiconductor Materials and Devices for Power Electronics Applications (HeteroSiC & WASMPE 2011), Tours, FRANCE, JUN 27-30, 2011

Ref HAL: hal-03037521_v1
DOI: 10.4028/www.scientific.net/MSF.717-720.625
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We have grown graphene on SiC(0001) using propane-hydrogen CVD. In this work, we present the effects of growth pressure and temperature on structural and electrical properties. Structural characterizations evidence the formation of graphene with in-plane rotational disorder, except for low growth pressure and high growth temperature which lead to the formation of a (6 root 3x6 root 3)-30 degrees interface between graphene and SiC. Electrical properties of samples presenting different graphene/SiC stacking and interfaces are compared and discussed.

Commentaires: 14th International Conference on Silicon Carbide and Related Materials (ICSCRM 2011), Cleveland, OH, SEP 11-16, 2011

Ref HAL: hal-03037520_v1
DOI: 10.1063/1.3680564
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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. (C) 2012 American Institute of Physics. [doi:10.1063/1.3680564]

Ref HAL: hal-03037526_v1
DOI: 10.4028/www.scientific.net/AMR.324.269
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We report magnetotransport measurements in single epitaxial graphene layers grown on the C-face of an 8 degrees off-axis 4H-SiC substrate using high temperature annealing conditions with a graphite cap covering the sample. The graphene sheets were found p-type doped, with mobilities varying between 1000 and 11000 cm(2/)V.s from device to device at 1.6 K. We examine the signature of weak localization and universal conductance fluctuations at weak magnetic field and we show that the phase coherence lengths extracted from the two phenomena are in satisfactory agreement.

Commentaires: 1st Mediterranean Conference on Innovative Materials and Applications (CIMA 2011), Beirut, LEBANON, MAR 15-17, 2011

Ref HAL: hal-03037525_v1
DOI: 10.12693/APhysPolA.119.606
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Resonant tunneling in single-barrier GaAs/AlAs/GaAs junctions grown in [111] direction was studied for samples with different concentration of silicon delta-doping in AlAs. In the I(V) characteristics, measured at 4 K, two kinds of peaks were observed: related to resonant tunneling via donors states in the barrier, and through X-minimum quantum well subbands. The results are compared to those previously obtained for analogous samples grown along [001] direction. The investigations reveal different symmetry of donor states in both cases.

Commentaires: 39th Conference on the Physics of Semiconductors, Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010