TI-based Devices Research Articles

Holographic Nano-Imaging of Terahertz Dirac Plasmon Polaritons in Topological Insulator Antenna Resonators.

Excitation of Dirac plasmon polaritons (DPPs) in bi-dimensional materials have attracted considerable interest in recent years, both from perspectives of understanding their physics and exploring their transformative potential for nanophotonic devices, including ultra-sensitive plasmonic sensors, ultrafast saturable absorbers, modulators, and switches. Topological insulators (TIs) represent an ideal technological platform in this respect because they can support plasmon polaritons formed by Dirac carriers in the topological surface states. Tracing propagation of DPPs is a very challenging task, particularly at terahertz (THz) frequencies, where the DPP wavelength becomes over one order of magnitude shorter than the free space photon wavelength. Furthermore, severe attenuation hinders the comprehensive analysis of their characteristics. Here, the properties of DPPs in real TI-based devices are revealed. Bi2Se3 rectangular antennas can efficiently confine the propagation of DPPs to a single dimension and, as a result, enhance the DPPs visibility despite the strong intrinsic attenuation. The plasmon dispersion and loss properties from plasmon profiles are experimentally determined, along the antennas, obtained using holographic near-field nano-imaging in a wide range of THz frequencies, from 2.05 to 4.3THz. The detailed investigation of the unveiled DPP properties can guide the design of novel topological quantum devices exploiting their directional propagation.

Improvement in the polishing characteristics of titanium-based materials using electrochemical mechanical polishing

Titanium (Ti) and its alloys are widely used in aerospace components and biomaterials. The smoothing of Ti materials is essential for realizing high-quality Ti-based devices and components to enhance their physical and chemical properties, such as fatigue strength and biocompatibility. However, with conventional polishing methods, obtaining sub-nanometer roughness in a large measurement area is difficult because of the generation of crystal grain steps on the polished surface. This paper presents an electrochemical mechanical polishing (ECMP) method that involves electrochemical oxidation and subsequent oxide removal using SiO2 particles to achieve a high material removal rate (MRR) and an ultra-smooth surface of pure Ti and Ti-6Al-4V. Under suitable electrolytic current and mechanical conditions, ECMP provides a higher MRR and smoother pure Ti surface than electroless polishing. X-ray and Raman analyses indicate that the mechanical condition affects the chemical states of polished surfaces; higher mechanical conditions retain less oxide film on the polished surface, resulting in a smoother surface. The highly efficient ECMP provides a defect-free and sub-nanometer roughness for pure Ti and Ti-6Al-4V surfaces without using chemicals. Furthermore, it is an environment-friendly and cost-effective approach to prepare Ti-based components.

Quantum pumping through the surface states of a topological insulator

We investigate quantum charge pumping through the surface states of a topological insulator (TI), i.e., ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. We consider a device with two oscillating potentials for the generation of a dc current in the adiabatic pumping regime. Applying the exchange magnetic field, we show that the device can work as a memory read-out. Also, we show that by applying a static gate voltage the pumping device can work as a field-effect transistor. In addition, the pumped current obtained in our proposed device is significantly (more than three orders of magnitude) greater than the pumped current obtained previously for TI-based devices. These properties show that ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ can be considered a good candidate for the fabrication of nanoelectronic devices based on TI as well as low-power and low-energy consumption devices in quantum computing.

Phase interference for probing topological fractional charge in a BiSbTeSe2-based Josephson junction array

Fractional charges can be induced by magnetic fluxes at the interface between a topological insulator (TI) and a type-II superconductor due to axion electrodynamics. In a Josephson junction array with a hole in the middle, these electronic states can have phase interference in an applied magnetic field with period, in addition to the 2π interference of the Cooper pairs. Here, we test an experimental configuration for probing the fractional charge and report the observation of phase interference effect in superconducting arrays with a hole in the middle in both Au- and TI-based devices. Our numerical simulations based on resistive shunted capacitive junction model are in good agreement with the experimental results. However, no clear sign of an axion charge-related interference effect was observed. We will discuss possible reasons and perspectives for future experiments.

Impurity band assisted carrier relaxation in Cr doped topological insulator Bi2Se3

Topological insulators (TIs) with unique band structures have wide application prospects in the fields of ultrafast optical and spintronic devices. The dynamics of hot carriers plays a key role in these TI-based devices. In this work, using the time- and angle-resolved photoemission spectroscopy technique, the relaxation process of the hot carriers in Cr-doped Bi2Se3 has been systematically studied since the ferromagnetic TI is one of the key building blocks for next-generation spintronics. It is found that electronic temperature (Te) and chemical potential (μ) decrease faster with the increase in the Cr doping concentration. Similarly, the lifetime (τ) of the excited electrons also decreases with more Cr doped into Bi2Se3. The results suggest a mechanism of impurity band-assisted carrier relaxation, where the impurity band within the bulk bandgap introduced by Cr doping provides significant recombination channels for the excited electrons. This work directly illustrates the dynamic process of the photon-generated carriers in Cr-doped Bi2Se3, which is expected to promote the applications of (Bi1-xCrx)2Se3 in photoelectric devices.

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering**Supported by the National Key R&D Plan Program of China (Grant No. 2017YFF0206104), the National Key Scientific Research Projects of China (Grant No. 2015CB921502), the National Natural Science Foundation of China (Grant Nos. 61574169 and 51871018), Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, the Opening Project

Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure. Rapid, low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs. Here we show that wafer-sized Bi2Te3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO2/Si substrates by magnetron cosputtering. The SiO2/Si substrates enable us to electrically tune (Bi1–xSbx)2Te3 and Cr-doped (Bi1–xSbx)2Te3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states, such as the quantum anomalous Hall effect (QAHE). This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.

Quantum oscillations of robust topological surface states up to 50\u2009K in thick bulk-insulating topological insulator

As personal electronic devices increasingly rely on cloud computing for energy-intensive calculations, the power consumption associated with the information revolution is rapidly becoming an important environmental issue. Several approaches have been proposed to construct electronic devices with low-energy consumption. Among these, the low-dissipation surface states of topological insulators (TIs) are widely employed. To develop TI-based devices, a key factor is the maximum temperature at which the Dirac surface states dominate the transport behavior. Here, we employ Shubnikov-de Haas oscillations (SdH) as a means to study the surface state survival temperature in a high-quality vanadium doped Bi1.08Sn0.02Sb0.9Te2S single crystal system. The temperature and angle dependence of the SdH show that: (1) crystals with different vanadium (V) doping levels are insulating in the 3–300 K region; (2) the SdH oscillations show two-dimensional behavior, indicating that the oscillations arise from the pure surface states; and (3) at 50 K, the V0.04 single crystals (Vx:Bi1.08-xSn0.02Sb0.9Te2S, where x = 0.04) still show clear sign of SdH oscillations, which demonstrate that the surface dominant transport behavior can survive above 50 K. The robust surface states in our V doped single crystal systems provide an ideal platform to study the Dirac fermions and their interaction with other materials above 50 K.

Topological Insulator-Based van der Waals Heterostructures for Effective Control of Massless and Massive Dirac Fermions.

Three dimensional (3D) topological insulators (TIs) are an important class of materials with applications in electronics, spintronics and quantum computing. With the recent development of truly bulk insulating 3D TIs, it has become possible to realize surface dominated phenomena in electrical transport measurements e.g. the quantum Hall (QH) effect of massless Dirac fermions in topological surface states (TSS). However, to realize more advanced devices and phenomena, there is a need for a platform to tune the TSS or modify them e.g. gap them by proximity with magnetic insulators, in a clean manner. Here we introduce van der Waals (vdW) heterostructures in the form of topological insulator/insulator/graphite to effectively control chemical potential of the TSS. Two types of gate dielectrics, normal insulator hexagonal boron nitride (hBN) and ferromagnetic insulator Cr2Ge2Te6 (CGT) are utilized to tune charge density of TSS in the quaternary TI BiSbTeSe2. hBN/graphite gating in the QH regime shows improved quantization of TSS by suppression of magnetoconductivity of massless Dirac fermions. CGT/graphite gating of massive Dirac fermions in the QH regime yields half-quantized Hall conductance steps and a measure of the Dirac gap. Our work shows the promise of the vdW platform in creating advanced high-quality TI-based devices.

Titanium-enriched medium drives low profile of ECM remodeling as a pre-requisite to pre-osteoblast viability and proliferative phenotype

Titanium is widely used for biomedical applications, but little information is being delivered regarding the cellular/molecular mechanisms explaining their efficacy, mainly considering the effects of the Ti-released trace elements on pre-osteoblasts. We addressed this issue by investigating decisive intracellular signal transduction able to modulate cytoskeleton rearrangement, proliferative phenotype and extracellular matrix (ECM) remodeling. We considered titanium grades IV and V, submitted or not to dual acid-etching (w/DAE or wo/DAE, respectively). Our results showed there is no cytotoxicity, preserving AKT involvement. Additionally, Ti-enriched medium promoted a diminution of the downstream signaling upon integrin activation (phosphorylating Rac1 and cofilin), guaranteeing a dynamic cytoskeleton rearrangement. Moreover, the low profile of ECM remodeling obtained in response to trace molecules released by Ti-based devices seems contributing to the osteoblast performance in mediating extracellular support to cell anchorage. This hypothesis was validated by the up-expression of ß1-integrin, src and Focal adhesion kinase (fak) genes, mainly in response to titanium grade V. Proliferative phenotype showed an unbalance between cyclin-dependent kinases (CDKs) and p15INK4b/p21Cip1. In conjunction, we showed for the first time that trace elements from Ti-based biomedical devices provoke important modulation of the osteoblast biology, driving cell anchoring, viability, and proliferative phenotype. Certainly, these biological outcomes compromise implant osseointegration.

The Property, Preparation and Application of Topological Insulators: A Review.

Topological insulator (TI), a promising quantum and semiconductor material, has gapless surface state and narrow bulk band gap. Firstly, the properties, classifications and compounds of TI are introduced. Secondly, the preparation and doping of TI are assessed. Some results are listed. (1) Although various preparation methods are used to improve the crystal quality of the TI, it cannot reach the industrialization. Fermi level regulation still faces challenges; (2) The carrier type and lattice of TI are affected by non-magnetic impurities. The most promising property is the superconductivity at low temperature; (3) Magnetic impurities can destroy the time-reversal symmetry of the TI surface, which opens the band gap on the TI surface resulting in some novel physical effects such as quantum anomalous Hall effect (QAHE). Thirdly, this paper summarizes various applications of TI including photodetector, magnetic device, field-effect transistor (FET), laser, and so on. Furthermore, many of their parameters are compared based on TI and some common materials. It is found that TI-based devices exhibit excellent performance, but some parameters such as signal to noise ratio (S/N) are still lower than other materials. Finally, its advantages, challenges and future prospects are discussed. Overall, this paper provides an opportunity to improve crystal quality, doping regulation and application of TI.

Observation of hidden atomic order at the interface between Fe and topological insulator Bi2Te3.

To realize spintronic devices based on topological insulators (TIs), well-defined interfaces between magnetic metals and TIs are required. Here, we characterize atomically precisely the interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. For both sites, our core-level photoemission spectra and density-functional theory calculations demonstrate simultaneous chemical bonding of Fe to both Te and Bi atoms. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm interface layer containing FeTe. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of ∼25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work will be of critical importance when optimizing the efficiency of spin injection in TI-based devices.

Work function of bulk-insulating topological insulator Bi2–xSbxTe3–ySey

Recent discovery of bulk insulating topological insulator (TI) Bi2–xSbxTe3–ySey paved a pathway toward practical device application of TIs. For realizing TI-based devices, it is necessary to contact TIs with a metal. Since the band-bending at the interface dominates the character of devices, knowledge of TIs' work function is of essential importance. We have determined the compositional dependence of the work function in Bi2–xSbxTe3–ySey by high-resolution photoemission spectroscopy. The obtained work-function values (4.95–5.20 eV) track the energy shift of the surface chemical potential seen by angle-resolved photoemission spectroscopy. The present result serves as a useful guide for developing TI-based electronic devices.

Magnetotransport study of (Sb1−xBix)2Te3 thin films on mica substrate for ideal topological insulator

We deposited high quality (Sb1−xBix)2Te3 on mica substrate by molecular beam epitaxy and investigated their magnetotransport properties. It is found that the average surface roughness of thin films is lower than 2 nm. Moreover, a local maxima on the sheet resistance is obtained with x = 0.043, indicating a minimization of bulk conductivity at this composition. For (Sb0.957Bi0.043)2Te3, weak antilocalization with coefficient of -0.43 is observed, confirming the existence of 2D surface states. Moreover Shubnikov-de Hass oscillation behavior occurs under high magnetic field. The 2D carrier density is then determined as 0.81 × 1016 m−2, which is lower than that of most TIs reported previously, indicating that (Sb0.957Bi0.043)2Te3 is close to ideal TI composition of which the Dirac point and Fermi surface cross within the bulk bandgap. Our results thus demonstrate the best estimated composition for ideal TI is close to (Sb0.957Bi0.043)2Te3 and will be helpful for designing TI-based devices.

Time-Dependent Transport in Gated Topological Insulators

The surface states of topological insulators (TI) possess a linear dispersion in the low-energy approximation, and are protected against time-reversal-invariant perturbations such as non-magnetic impurities, defects [1-3]. The chiral nature of TI surface states can induce perfect quantum tunneling for Dirac fermions incident in the normal direction, which can be utilized in some novel TI-based devices. Recently, the effect of the time-dependent perturbation has been paid much more attention, especially the optical response of the TI surface states [4-8]. However, the modulation of the time-dependent potential on the transport properties has obtained much less attention. Here we apply the scattering matrix method based on the low-energy Hamiltonian to analyze the transport characteristic of a gated topological insulator in the presence of a time-dependent potential.

Experimental evidence and control of the bulk-mediated intersurface coupling in topological insulatorBi2Te2Senanoribbons

Nearly a decade after the discovery of topological insulators (TIs), the important task of identifying and characterizing their topological surface states through electrical transport experiments remains incomplete. The interpretation of these experiments is made difficult by the presence of residual bulk carriers and their coupling to surface states, which is not yet well understood. In this work, we present the first evidence for the existence and control of bulk-surface coupling in Bi2Te2Se nanoribbons, which are promising platforms for future TI-based devices. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way.

Universal conductance fluctuations of topological insulators

As an exotic quantum condensed matter, the topological insulator (TI) is a bulk-insulating material with a Diractype conducting surface state. Such a dissipationless transport of topological surface state (TSS) is protected by the timereversal symmetry, which leads to the potential applications in spintronics and quantum computations. Understanding the topological symplectic transport of the Dirac fermions is a key issue to the study and design of the TI-based devices. There are many transport properties about Dirac fermions. And universal conductance fluctuation (UCF) is one of the most important transport manifestations of mesoscopic electronic interference. So the UCF effect in TI is a very meaningful research field It can provide an intriguing and special perspective to reveal the quantum transport of TSSs In this review, we introduce the research progress on the UCF of TSSs in a pedagogical way We review the achievements and the existing problems in order to inspire future research work.#br#We start this review with the basic UCF theory and the experimental observation. The UCF has been observed in TI earlier, but weather it originates from TSS has not been further studied. Then a series of work is carried out to prove the topological nature of UCF in TI Firstly, the UCF phenomenon in TIs is demonstrated to be from two-dimensional (2D) interference by magnetoconductance measurements. But the residual bulk state and the 2D electron gas (2DEG) on the surface can also bring about the 2D UCF The field-tilting regulation helps us exclude the distribution from the bulk And the classic self-averaging of UCF is investigated then to obtain the intrinsic UCF amplitude. By comparing with the theoretical prediction, the possibility has been ruled out that the 2D UCF may originate from the 2DEG So its topological nature is demonstrated. Secondly, we discuss the UCF effect in TI by a macroscopic perspective, i.e. the statistical symmetry of UCF, which should be more concise and reflect its universality. For a single TSS, the applied magnetic field will drive the system from a Gaussian symplectic ensemble into a Gaussian unitary ensemble. It results in a √2 fold increase of the UCF amplitude. However, the experiment reveals that the UCF amplitude is reduced by 1/√2. This is contradictory to the theoretical prediction. Actually, there are two TSSs and they are coherently coupled to each other in TIs since the sample’s thickness is smaller than its bulk dephasing length. This leads to a Gaussian orthogonal ensemble of the intersurface coupling system without an external field. In such a case, the UCF amplitude will be reduced by 1/√2 with field increasing. It is consistent with the experimental result. Finally, the other progress on UCFs is discussed, and the general outlook is also mentioned briefly.

Electrical tuning of transport properties of topological insulator ultrathin films

Considering that topological insulator (TI) ultrathin films (UTFs) provide an ideal platform for the transport measurement of topologically protected surface states, we have investigated the transport properties of the three-dimensional (3D) TI UTFs through an array of potential barriers. The 3D TI UTF was considered to be thin enough (5 nm) that the top and bottom surface states of the UTF can hybridize to create an energy gap at the Dirac point, which results in a hyperbola-like energy dispersion. It was found that the Klein tunneling effect disappears due to the interaction between the top and bottom surface states. By tuning the barrier strength or the incident energy, three kinds of transport processes can be realized, and the conditions of the transport processes were determined. The oscillatory characters of the transmission (conductance) spectra without a decaying envelope are due to the novel surface states of TIs, which are quite different from that observed for a conventional two-dimensional electron gas. For the structure consisting of two anti-parallel potential barriers, the conductance spectra exhibit a perfect on/off switching effect by tuning the barrier strength, which is favorable for electrically controllable device applications. In the case of a superlattice (SL) structure, due to the mini-gaps induced by the SL geometry, some additional resonant peaks and valleys can be observed in the transmission spectra, and similar characters are also reflected in the conductance spectra. Owing to the Dirac characters of the charge carriers therein, the transmission (conductance) spectra never decay with increasing barrier strength, which is distinguished from that observed for semiconductor SLs. These findings were not only meaningful for understanding the basic physical processes in the transport of TIs, but also useful for developing nanoscaled TI-based devices.

Interplay between Different Magnetisms in Cr-Doped Topological Insulators

Breaking the time-reversal-symmetry of topological insulators through magnetic doping has led to exotic physical discoveries. Here, we report the gate-dependent magneto-transport measurements on the Cr-doped (BixSb1-x)2Te3 thin films. With effective top-gate modulations, we demonstrate the presence of both the hole-mediated RKKY coupling and carrier-independent van Vleck magnetism in the magnetic TI systems. Most importantly, by varying the Cr doping concentrations from 2% to 20%, we unveil the interplay between the two magnetic orders and establish the valid approach to either enhance or suppress each individual contribution. The electric-field-controlled ferromagnetisms identified in the Cr-doped TI materials will serve as the fundamental step to further explore the TRS-breaking TI systems, and it may also help to expand the functionality of TI-based device for spintronics applications.

Topological insulator Bi2Te3 films synthesized by metal organic chemical vapor deposition

Topological insulator (TI) materials such as Bi2Te3 and Bi2Se3 have attracted strong recent interests. Large scale, high quality TI thin films are important for developing TI-based device applications. In this work, structural and electronic properties of Bi2Te3 thin films deposited by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrates were characterized via x-ray diffraction (XRD), Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and electronic transport measurements. The characteristic topological surface states with a single Dirac cone have been clearly revealed in the electronic band structure measured by ARPES, confirming the TI nature of the MOCVD Bi2Te3 films. Resistivity and Hall effect measurements have demonstrated relatively high bulk carrier mobility of ∼350 cm2/Vs at 300 K and ∼7400 cm2/Vs at 15 K. We have also measured the Seebeck coefficient of the films. Our demonstration of high quality topological insulator films grown by a simple and scalable method is of interests for both fundamental research and practical applications of thermoelectric and TI materials.

Resistive switching of HfO2-based Metal–Insulator–Metal diodes: Impact of the top electrode material

This paper deals with the impact of the top metal electrode on the resistive switching properties of HfO2-based Metal-Insulator-Metal diodes. By screening five different metals as top electrode, Al–Cu–Hf–Pt–Ti, we have demonstrated the feasibility of the resistive switching effect on HfO2. Metals with a low enthalpy of formation of oxides ΔHf0 (Pt and Cu) lead to uni-polar switching whereas easily oxidizable metals with a higher ΔHf0 (Al, Hf and Ti) lead to bipolar switching. Cu-, Hf- and Pt-based devices show a degradation of the top electrode after the forming step by the formation of bubbles whereas such phenomenon was not observed in Al- and Ti-based devices. 200 switching cycles were performed on each device in order to extract the main parameters of the resistive switching effect: ION and IOFF currents in the mA range, ROFF/RON resistance ratio up to 5, Vset and Vreset, voltage levels around 1V, and powers dissipated during read and write operations in the μW and mW range, respectively. For all systems, the reset process dissipates higher power than the set process. From these results, the Ti top adlayer shows the best performance in terms of stability and resistive switching characteristics.