High-Mobility Ambipolar Magnetotransport in Topological Insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ Nanoribbons

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High-Mobility Ambipolar Magnetotransport in Topological Insulator ${Bi}_{2} {Se}_{3}$ Nanoribbons

Gunta Kunakova, Thilo Bauch, Xavier Palermo, Matteo Salvato, Jana Andzane, Donats Erts, and Floriana Lombardi

Phys. Rev. Applied 16, 024038 – Published 23 August 2021

Abstract

Nanoribbons of topological insulators (TIs) have been suggested for a variety of applications exploiting the properties of the topologically protected surface Dirac states. In these proposals it is crucial to achieve a high tunability of the Fermi energy, through the Dirac point while preserving a high mobility of the involved carriers. Tunable transport in TI nanoribbons has been achieved by chemical doping of the materials so to reduce the bulk carriers’ concentration, however at the expense of the mobility of the surface Dirac electrons, which is substantially reduced. Here we study bare ${Bi}_{2} {Se}_{3}$ nanoribbons transferred on a variety of oxide substrates and demonstrate that the use of a large relative permittivity ${Sr Ti O}_{3}$ substrate enables the Fermi energy to be tuned through the Dirac point and an ambipolar field effect to be obtained. Through magnetotransport and Hall conductance measurements, performed on single ${Bi}_{2} {Se}_{3}$ nanoribbons, we demonstrate that electron and hole carriers are exclusively high-mobility Dirac electrons, without any bulk contribution. The use of ${Sr Ti O}_{3}$ allows therefore an easy field effect gating in TI nanostructures providing an ideal platform to take advantage of the properties of topological surface states.

Received 30 March 2021
Revised 8 June 2021
Accepted 12 July 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.024038

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by Bibsam.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Hall effect Magnetotransport Topological insulators

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Gunta Kunakova ^1,2,*, Thilo Bauch¹, Xavier Palermo ¹, Matteo Salvato³, Jana Andzane ², Donats Erts ², and Floriana Lombardi^1,†

¹Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg SE-41296, Sweden
²Institute of Chemical Physics, University of Latvia, Raina Blvd. 19, Riga LV-1586, Latvia
³Dipartimento di Fisica, Università di Roma “Tor Vergata”, Roma 00133, Italy

^*gunta.kunakova@lu.lv
^†floriana.lombardi@chalmers.se

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Vol. 16, Iss. 2 — August 2021

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Figure 1
(a) SEM image of the ${Bi}_{2} {Se}_{3}$ nanoribbon with electrodes for Hall-effect measurement. (b) Hall carrier density $n_{3D}$ for ${Bi}_{2} {Se}_{3}$ nanoribbons within the thickness range of $7$ – $24$ nm and transferred on four different substrates. The inset shows a cross-section schematic of the nanoribbon device: the light blue dashed line highlights the interface between the nanoribbon bottom surface and the substrate where the accumulation layer is located.
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Figure 2
(a) $R_{x x}$ as a function of back-gate voltage $V_{g}$ . The red and gray curves represent two different scans and the arrows indicate sweep direction of $V_{g}$ . The first $R_{x x} (V_{g})$ scan is measured with $V_{g}$ applied as follows: 0 to $- 40$ ; $- 40$ to 40; 40 to 0 V (red arrows), while the second curve corresponds to the opposite scanning direction 0 to 40; 40 to $- 40$ ; $- 40$ to 0 V (gray arrows). (b) Hall carrier density $n_{2D}$ as a function of back-gate voltage. The carrier density values shown here are calculated from the linear fits of the antisymmetrized $R_{x y} (B)$ for $B$ in the range $- 2$ to 2 T. The positive sign of $n_{2D}$ indicates electronlike majority carrier concentrations. The inset shows the Hall coefficient as a function of the gate voltage. In both graphs the violet shaded region addresses a mixed region of $n$ - and $p$ -type carriers. The increase of $n_{2D}$ in the main graph, by going to negative $V_{g}$ is only “apparent”: it is a consequence of the $R_{H}$ changing sign (see inset), which gives an unrealistically low slope and as a consequence higher value of $n_{2D}$ . The values of $n_{2D}$ in the shaded violet regions therefore do not correspond to effective carriers. $R_{x y 1}$ and $R_{x y 2}$ are measured using different Hall contacts of the same nanoribbon device, and scan $A$ and scan $B$ represent two subsequent sweeps. (c) Antisymmetrized $R_{x y} (B)$ in the range of $B$ between $- 13$ to 13 T at back-gate voltage $V_{g} = 0$ V and $V_{g} = - 17$ V. The black dashed lines are the linear fit in the range of 0 to 2 T. The inset shows the $G_{x y} (B)$ dependences with the fit of the two-carrier model, Eq. (2), (dot dashed line). All the data refer to nanoribbon B51-10.
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Figure 3
(a) Schematic representation of the band-bending diagram of a 9-nm-thick ${Bi}_{2} {Se}_{3}$ nanoribbon at $V_{g} = 0$ ; $- 17$ V, constructed considering the carrier densities determined from the two-carrier model Eq. (2). BS and TS indicate the bottom (in contact with the substrate) and top surfaces, respectively. (b) $R_{x x}$ as a function of temperature for different applied back-gate voltages, nanoribbon B51-10. (c) Excess conductance $Δ G_{x x} (B) = G_{x x} (B) - G_{x x} (B = 0)$ as a function of magnetic field. The black solid curves are the fits using Eq. (4). The inset shows the phase coherence length extracted from the fitting of $Δ G_{x x} (B)$ as a function of back-gate voltage.
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Physical Review Applied

High-Mobility Ambipolar Magnetotransport in Topological Insulator Bi2Se3 Nanoribbons

Gunta Kunakova, Thilo Bauch, Xavier Palermo, Matteo Salvato, Jana Andzane, Donats Erts, and Floriana Lombardi

Phys. Rev. Applied 16, 024038 – Published 23 August 2021

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High-Mobility Ambipolar Magnetotransport in Topological Insulator ${Bi}_{2} {Se}_{3}$ Nanoribbons