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DOI: 10.1109/IRMMW-THz.2018.8510039
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We report on the temperature-dependent magneto absorption and magnetotransport spectroscopy of HgTe/CdHgTe quantum wells above the critical well thickness d(c). Our results, obtained in magnetic fields up to 16 T and temperature range from 1.7 to 150 K, clearly indicate a change in the band-gap energy with temperature. A topological phase transition between quantum spin Hall and trivial insulator states, revealing appearance of single-valley Dirac fermions at T = 27 and 90 K for 6.5 and 8 nm QWs respectively, was clearly observed in our magnetospectroscopy measurements.

Commentaires: 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Nagoya, JAPAN, SEP 09-14, 2018

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A few years ago, we demonstrated that graphene can markedly surpass GaAs/AlGaAs semiconductor heterostructures for the implementation of the quantum Hall resistance standard in relaxed experimental conditions, while preserving a state-of-the-art accuracy [1]. This paper describes supplementary quantum transport experiments performed on a significant number of graphene based quantum Hall devices obtained from graphene grown by CVD on SiC, like the one used for the above-mentioned demonstration, and with similar process and preparation. The objectives were to investigate the sample-to-sample reproducibility of the electronic properties and device performance, the structural key control parameter and the underpinning physics.

Commentaires: Conference on Precision Electromagnetic Measurements (CPEM), Paris, FRANCE, JUL 08-13, 2018

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Structural and electronic properties of epitaxial graphene grown on SiC (0001) at low argon pressure

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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 thickness 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 ~1h30.First, we discuss the initial growth stages from buffer layer to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy analyses show that a buffer layer, 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 buffer layer on terraces with a step-flow growth mechanism. Eventually, reproducible syntheses of 1LG films were achieved by this temperature-controlled growth process. 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. Atomically resolved scanning tunnellingmicroscopy (STM) evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer[7]. Regarding the transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the1LG film. Moreover, we estimated a hole concentration of p~10^10 - 10^11 /(cm^2) at 1.7 K in helium atmosphere. We ascribe this unusual doping type to an atmosphere contamination effect since we regain the typical n-doped graphene under vacuum by transport measurement and STM analysis.