Abstract
HgTe quantum wells (QWs) are two-dimensional semiconductor systems that change their properties at the critical thickness dc, corresponding to the band inversion and topological phase transition. The motivation of this work was to study magnetotransport properties of HgTe QWs with thickness approaching dc, and examine them as potential candidates for quantum Hall effect (QHE) resistance standards. We show that in the case of d > dc (inverted QWs), the quantization is influenced by coexistence of topological helical edge states and QHE chiral states. However, at d ≈ dc, where QW states exhibit a graphene-like band structure, an accurate Hall resistance quantization in low magnetic fields (B ≤ 1.4 T) and at relatively high temperatures (T ≥ 1.3 K) may be achieved. We observe wider and more robust quantized QHE plateaus for holes, which suggests—in accordance with the “charge reservoir” model—a pinning of the Fermi level in the valence band region. Our analysis exhibits advantages and drawbacks of HgTe QWs for quantum metrology applications, as compared to graphene and GaAs counterparts.
Highlights
Mercury cadmium telluride (Hg1−xCdxTe) zinc-blende compounds are an example of rare semiconductor materials that form alloys over the whole composition range x while keeping the same crystal structure and the virtually unaltered lattice parameters.[1,2]it is possible to tune the band structure by changing x and grow bulk films, two-dimensional (2D) quantum wells (QWs) or superlattices without strain-related material degradation
quantum Hall effect (QHE) in HgTe QWs with graphene-like band structures[14] was observed even at liquid nitrogen temperatures.[35,36]. These findings show that HgTe/(Cd,Hg)Te QWs are promising candidates for use in QHE metrology
Such high growth quality is crucial for obtaining the appropriate carrier mobility of HgTe QWs for QHE standards and can be reached only by the molecular beam epitaxy (MBE) growth technique
Summary
It is possible to tune the band structure by changing x and grow bulk films, two-dimensional (2D) quantum wells (QWs) or superlattices without strain-related material degradation. In this sense, Hg1−xCdxTe crystals are similar to the well-known Ga1−xAlxAs semiconductors, but show a much larger energy bandgap tunability, with band gaps ranging from Eg ≡ EΓ6–EΓ8 = 1.6 eV for CdTe to the inverted band ordering, with Eg = −0.30 eV for HgTe at 4.2 K.1. HgTe/(Cd,Hg)Te QWs have allowed the demonstration of the existence of various topological phases in condensed matter.[9–11] By changing the QW widths, the barrier alloy composition and the number of coupled QWs, it has been possible to demonstrate 2D topological insulators with 1D edge conducting channels[12,13] as well as structures with band dispersions similar to single layer[14] or bilayer graphene.[15]
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