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HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$ becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor ${v=-1}$ (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ${^4He}$ liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.

Transmission electron microscopy and photoluminescence investigations have been carried out on a series of quantum wells grown by molecular beam epitaxy and metal organic vapour phase epitaxy. In both cases, the emission wavelength and the peak width agree with the indium local composition as extracted from strain measurements and with the well width. As could be expected, the data is at small variance with the nominal values as set during growth, but the general trends are similar. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim