Transport Properties of a 3D Topological Insulator based on a Strained High-Mobility HgTe Film

Abstract

We investigate the magnetotransport properties of strained 80 nm thick HgTe layers featuring a high mobility of μ ∼ 4 × 10 5 cm 2 =V · s. By means of a top gate, the Fermi energy is tuned from the valence band through the Dirac-type surface states into the conduction band. Magnetotransport measurements allow us to disentangle the different contributions of conduction band electrons, holes, and Dirac electrons to the conductivity. The results are in line with previous claims that strained HgTe is a topological insulator with a bulk gap of ≈ 15 meV and gapless surface states. DOI: 10.1103/PhysRevLett.112.196801 PACS numbers: 73.25.+i, 05.60.Gg, 73.20.At, 73.43.-f The discovery of two-(2D) and three-dimensional (3D) topological insulators (TIs), a new material class with insulating bulk and topologically protected conducting surface states, has opened an exciting research field in condensed matter physics [1][2][3][4][5][6][7][8][9][10]. Although quite a number of different, especially, Bi-based materials [11][12][13][14], belong to this category, materials which combine high charge carrier mobility and insulating bulk are still scarce. This is mostly due to the fact that Bi-based 3D TIs are heavily doped alloy films with a low mobility ≈1000 cm 2 =V · s and a high bulk carrier density of 10 17 -10 19 cm −3 . HgTe-based 2D TIs, on the other hand, are characterized by very high mobilities enabling the discovery of the quantum spin Hall effect [15]. A recent analysis of the sequence of quantum Hall plateaus suggests that also strained HgTe layers constitute a 3D TI. The strain opens a gap in the gapless semimetal HgTe so that the TI properties can be explored by tuning the Fermi energy E F into the bulk gap and probing the transport properties of the gapless surface states. Although the strained HgTe film has a much higher mobility μ ¼ ð3 − 4Þ × 10 4 cm 2 =V · s, the high bulk carrier density and the absence of a top gate have complicated the detection of 3D TIs so far [16,17].The strain in HgTe layers grown by molecular beam epitaxy stems from a 0.3% lattice mismatch between HgTe and CdTe. The corresponding critical film thickness is larger than 100 nm, meaning that thinner films adopt the substrate lattice constant. Because of this strain, a small gap of ∼15 meV opens (see below) in the bulk energy spectrum of the film. Within the bulk gap, the gapless surface states reside. The charge neutrality point of the corresponding Dirac cone is located in the valence band [16].In this Letter, we report on transport properties of highmobility, 80 nm wide, strained HgTe films equipped with a gate. The low disorder manifested in high charge carrier mobilities, together with the possibility to tune E F from the valence via the gap into the conduction band, enables us to probe the 2D Dirac surface states when E F is in the bulk energy gap. Since HgTe films grown on CdTe suffer from dislocations due to the lattice mismatch, our 80 nm thick HgTe films were separated from the CdTe substrate by a 20 nm thin Cd 0.7 Hg 0.3 Te buf...

Probing Quantum Capacitance in a 3D Topological Insulator

“…SdH oscillations have shown that σ xx oscillations at high B reflect the total carrier density in the TI, i.e., charge carrier densities in the bulk plus in top and bottom surfaces [7]. α total is directly proportional to C=A ¼ edn=dV g and corresponds to C=A ¼ eα total ¼ 1.22 × 10 −4 F=m 2 , a value close to the calculated capacitance C calc gt =A ¼ 1.45 × 10 −4 F=m 2 using thickness and dielectric constant of the layers [see Figs.…”

“…2(a). ρ xx displays a maximum near V g ¼ 1.5 V, whereas ρ xy changes sign; this occurs in the immediate vicinity of the charge neutrality point (CNP) [7]. The corresponding capacitance CðV g Þ at B ¼ 0 in Fig.…”

“…The properties of these surface states are of particular interest as they have a spin degenerate, linear Dirac-like dispersion with spins locked to their electrons' k vectors [4,5]. Strained HgTe, examined here, constitutes a 3D TI with high electron mobilities allowing the observation of Landau quantization and quantum Hall steps down to low magnetic fields [6,7]. While unstrained HgTe is a zero gap semiconductor with inverted band structure [8,9], the degenerate Γ8 states split and a gap opens at the Fermi energy E F if strained.…”

“…While unstrained HgTe is a zero gap semiconductor with inverted band structure [8,9], the degenerate Γ8 states split and a gap opens at the Fermi energy E F if strained. This system is a strong topological insulator [10], explored by transport [6,7,11], angleresolved photoemission spectroscopy [12], photoconductivity, and magneto-optical experiments [13][14][15][16]; also, the proximity effect has been investigated [17]. Since these two-dimensional electron states (2DES) have high electron mobilities of several 10 5 cm 2 =V s, pronounced Shubnikov-de Haas (SdH) oscillations of the resistivity and quantized Hall plateaus commence in quantizing magnetic fields [6,7,11], stemming from both top and bottom 2DES.…”

Split Dirac cones in HgTe/CdTe quantum wells due to symmetry-enforced level anticrossing at interfaces

“…While some TI compounds such as Bi 2 Se 3 and Bi 2 Te 3 show an insulating gap in their three-dimensional (3D) bulk band structure and so the Dirac cones can be studied at their 2D surface without modifying the material, a Mercury Telluride crystal does not have a 3D bulk band gap because the Fermi level resides within the four-fold degenerate Γ 8 band [5]. However, the topological insulation is realized by straining HgTe which opens a gap within the otherwise fourfold degenerate Γ 8 states [6,7] or growing the material in a HgTe/CdTe quantum well (QW) geometry [8][9][10]. In the latter case, CdTe (or Cd 1−x Hg x Te) barriers create quantum confinement within HgTe with the normal or inverted band structure depending on the well thickness, so that HgTe/CdTe QWs belong to the class of normal or topological insulators.…”