Topological Field-effect Transistors Research Articles

Observation of topological phase transition by terahertz photoconductivity in HgTe‐based transistors

AbstractWe have found the possibility to probe the magnetic field‐driven topological phase transition in HgTe‐based transistors by measuring their Terahertz photoconductivity response. At the critical magnetic field to which zero‐mode Landau levels cross, we have observed a pronounced photoconductivity peak independent on incident frequency and carrier concentration. Our results pave the way towards terahertz topological field effect transistors. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Erratum: “Assessment of performance potential of MoS2-based topological insulator field-effect transistors” [J. Appl. Phys. 118, 124502 (2015)

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Monolayer Topological Insulators: Silicene, Germanene, and Stanene

We report the recent progress on the theoretical aspects of monolayer topological insulators including silicene, germanene and stanene, which are monolayer honeycomb structures of silicon, germanium and tin, respectively. They show quantum spin Hall effects in nature due to the spin orbit interaction. The band gap can be tuned by applying perpendicular electric field, which induces a topological phase transition. We also analyze the topological properties of generic honeycomb systems together with the classification of topological insulators. Phase diagram of topological insulators and superconductors in honeycomb systems are explicitly determined. We also investigate topological electronics including a topological field-effect transistor, the topological Kirchhoff's law and the topological spin-valleytronics.

Terahertz detection of magnetic field-driven topological phase transition in HgTe-based transistors

We report on terahertz photoconductivity under magnetic field up to 16 T of field effect transistor based on HgTe quantum well (QW) with an inverted band structure. We observe pronounced cyclotron resonance and Shubnikov-de Haas-like oscillations, indicating a high mobility electron gas in the transistor channel. We discover that nonlinearity of the transistor channel allows for observation of characteristic features in photoconductivity at critical magnetic field corresponding to the phase transition between topological quantum spin Hall and trivial quantum Hall states in HgTe QW. Our results pave the way towards terahertz topological field effect transistors.

Electric control of topological phase transitions in Dirac semimetal thin films.

Dirac semimetals host three-dimensional (3D) Dirac fermion states in the bulk of crystalline solids, which can be viewed as 3D analogs of graphene. Owing to their relativistic spectrum and unique topological character, these materials hold great promise for fundamental-physics exploration and practical applications. Particularly, they are expected to be ideal parent compounds for engineering various other topological states of matter. In this report, we investigate the possibility to induce and control the topological quantum spin Hall phase in a Dirac semimetal thin film by using a vertical electric field. We show that through the interplay between the quantum confinement effect and the field-induced coupling between sub-bands, the sub-band gap can be tuned and inverted. During this process, the system undergoes a topological phase transition between a trivial band insulator and a quantum spin Hall insulator. Consequently, one can switch the topological edge channels on and off by purely electrical means, making the system a promising platform for constructing topological field effect transistors.

Assessment of performance potential of MoS2-based topological insulator field-effect transistors

It was suggested that single-layer MoS2 at the 1T′ phase is a topological insulator whose electronic structure can be modulated by a vertical electric field for field-effect transistor (FET) applications [X. Qian, J. Liu, L. Fu, and J. Li, Science 346, 1344 (2014)]. In this work, performance potential of FETs based on vertical field modulation of the topological edge states is assessed by using quantum transport device simulations. To perform efficient device simulations, a phenomenological Hamiltonian is first proposed and validated to capture the effects of electric fields. Because the ON-state conductance is determined by transport through gapless edge states with a long scattering mean free path and the OFF-state conductance by transport through the gapped bulk states, the ON/OFF ratio is sensitive to the channel length, which is different from conventional FETs. Although a high vertical electric field is required to modulate the topological edge state, a reasonably small subthreshold swing of 131 mV/dec can still be achieved for a practical value of the gate insulator thickness.

Strain-tunable topological quantum phase transition in buckled honeycomb lattices

Low-buckled silicene is a prototypical quantum spin Hall insulator with the topological quantum phase transition controlled by an out-of-plane electric field. We show that this field-induced electronic transition can be further tuned by an in-plane biaxial strain ε, owing to the curvature-dependent spin-orbit coupling (SOC): There is a Z2 = 1 topological insulator phase for biaxial strain |ε| smaller than 0.07, and the band gap can be tuned from 0.7 meV for ε=+0.07 up to 3.0 meV for ε=−0.07. First-principles calculations also show that the critical field strength Ec can be tuned by more than 113%, with the absolute values nearly 10 times stronger than the theoretical predictions based on a tight-binding model. The buckling structure of the honeycomb lattice thus enhances the tunability of both the quantum phase transition and the SOC-induced band gap, which are crucial for the design of topological field-effect transistors based on two-dimensional materials.

A scheme for a topological insulator field effect transistor

We propose a scheme for a topological insulator field effect transistor. The idea is based on the gate voltage control of the Dirac fermions in a ferromagnetic topological insulator channel with perpendicular magnetization connecting to two metallic topological insulator leads. Our theoretical analysis shows that the proposed device displays a switching effect with high on/off current ratio and a negative differential conductance with a good peak to valley ratio.

ChemInform Abstract: Quantum Spin Hall Effect in Two‐Dimensional Transition Metal Dichalcogenides

Abstractfirst‐principles calculations

Quantum spin Hall effect in two-dimensional transition metal dichalcogenides

Quantum spin Hall (QSH) effect materials feature edge states that are topologically protected from backscattering. However, the small band gap in materials that have been identified as QSH insulators limits applications. We use first-principles calculations to predict a class of large-gap QSH insulators in two-dimensional transition metal dichalcogenides with 1T′ structure, namely, 1T′-MX 2 with M = (tungsten or molybdenum) and X = (tellurium, selenium, or sulfur). A structural distortion causes an intrinsic band inversion between chalcogenide- p and metal- d bands. Additionally, spin-orbit coupling opens a gap that is tunable by vertical electric field and strain. We propose a topological field effect transistor made of van der Waals heterostructures of 1T′-MX 2 and two-dimensional dielectric layers that can be rapidly switched off by electric field through a topological phase transition instead of carrier depletion.

Topological Insulator Bi2Se3 Nanowire Field Effect Transistors

Topological insulator is characterized as a new class of materials which have an insulating band gap in the bulk and gapless surface state which is protected by its intrinsic time-reversal symmetry. Here, we report high-performance topological insulator nanowire field effect transistors (FETs) with sharp turn-on, clear cutoff current, large On/Off current ratio (~ 108) and well-saturated, strong inversion-mode output current-voltage characteristics. The metallic electron transport at surface, with effective mobility of 200 cm2V-1s-1 ~ 1000 cm2V-1s-1, and bulk conduction, with activation energy close to band gap of bulk Bi2Se3 can be effectively separated and tuned by gated field effect. We also report the first experimental observation of anomalous Aharonov-Bohm oscillation in Bi2Se3 nanowire FET under strong surface disorder. The magnetotransport properties demonstrated here of topological insulator nanostructure provide effective approaches of surface states modulations, and can enable more spintronics device applications and sensing applications.

Photoinduced topological phase transition in epitaxial graphene

In epitaxial graphene irradiated by an off-resonance circularly polarized light, we demonstrate a phase transition taking place between the band insulator and Floquet topological insulator. Considering the competition between staggered sublattice potential and photon dressing, we derive the dynamical energy gap and phase diagram in the tight-binding approximation. It is found that a threshold value of light intensity is necessary to realize a Floquet topological insulator. At the phase boundary, for each set of parameters, there is a special state with only one valley that is Dirac cone gapless, but the other remains gapped; in the band insulating phase, only one valley provides low-energy electrons, and it could be switched to the other by reversing the polarization direction of light. From these results, two electronic devices are designed: one is an optical-sensing np junction, where the photodriven unusual intervalley tunneling exhibits a stronger detectable signal than the intravalley tunneling, and the other is a topological field-effect transistor, where polarized light is used to turn on or turn off a nonequilibrium current.

Electrically tunable conductance and edge modes in topological crystalline insulator thin films: minimal tight-binding model analysis

Topological crystalline insulators (TCI) have been experimentally manufactured and studied. We propose a minimal tight-binding model for thin films made of TCI on the basis of the mirror and discrete rotational symmetries. The basic term consists of the spin–orbit interaction describing a Weyl semimetal, where gapless Dirac cones emerge at all the high-symmetry points in the momentum space. We then introduce the mass term providing gaps to Dirac cones. They simulate the thin films made of the [001], [111] and [110] TCI surfaces. TCI thin films are two-dimensional topological insulators protected by mirror symmetry. The mirror symmetry is broken by introducing an electric field perpendicular to the film. We first note that the band structure can be controlled using the electric field. We then analyze the mirror-Chern number and the edge modes taking into consideration the bulk–edge correspondence, even for . We also calculate the conductance as a function of . We propose a multi-digit topological field-effect transistor by applying an electric field independently to the right and left edges of a nanoribbon. Our results will open up a new route to topological electronics.

Topological field-effect quantum transistors in HgTe nanoribbons

We propose practical designs to realize topological field-effect quantum transistors in an HgTe nanoribbon with an inverted band structure. Our theoretical calculations show that, as a strip-shape top gate is placed on the HgTe nanoribbon and with an increasing gate voltage, two new conductance channels develop in the HgTe nanoribbon and are localized to the lattice sites neighboring the boundaries of the gate, leading to an additional quantization of the conductance of 2e2/h. The quantum states in the new channels are not only robust against a short-range Anderson disorder, but can also couple with the intrinsic helical edge states in the boundaries of the HgTe nanoribbon to open a gap in the energy spectrum, indicating their topological characteristics. More importantly, the newly developed conductance channels can be turned on or off easily by adjusting the gate voltage. The proposal of controllable topological edge states produced by the gate voltage opens a new route for future topological field-effect quantum transistors in nanoelectronics and spintronics.

Conductance modulation in topological insulator Bi2Se3 thin films with ionic liquid gating

A Bi2Se3 topological insulator field effect transistor is investigated by using ionic liquid as an electric double layer gating material, leading to a conductance modulation of 365% at room temperature. We discuss the role of charged impurities on the transport properties. The conductance modulation with gate bias is due to a change in the carrier concentration, whereas the temperature dependent conductance change is originated from a change in mobility. Large conductance modulation at room temperature along with the transparent optical properties makes topological insulators as an interesting (opto)electronic material.

Atomic-layer-deposited Al2O3 on Bi2Te3 for topological insulator field-effect transistors

We report dual-gate modulation of topological insulator field-effect transistors (TI FETs) made on Bi2Te3 thin flakes with integration of atomic-layer-deposited (ALD) Al2O3 high-k dielectric. Atomic force microscopy study shows that ALD Al2O3 is uniformly grown on this layer-structured channel material. Electrical characterization reveals that the right selection of ALD precursors and the related surface chemistry play a critical role in device performance of Bi2Te3 based TI FETs. We realize both top-gate and bottom-gate control on these devices, and the highest modulation rate of 76.1% is achieved by using simultaneous dual gate control.