High-quality Topological Insulator Research Articles

Magnetic switching driven by spin–orbit torque in topological-insulator-based (Bi0.5Sb0.5)2Te3/Ta/CoFe/Cu/CoFe/IrMn heterostructure

The giant spin–orbit torque (SOT) generated by topological surface states in topological insulators (TIs) provides an energy-efficient writing method for magnetic memory. In this study, we demonstrate a topological insulator/spin valve (TI/SV) device that operates at room temperature. An ultrathin, high-quality TI (Bi0.5Sb0.5)2Te3 (BST) thin film is epitaxially grown as a functional layer on a (0001)-Al2O3 substrate via molecular beam epitaxy in ultrahigh vacuum. Subsequently, Ta/CoFe/Cu/CoFe/IrMn layers are grown on BST/Al2O3 thin films using magnetron sputtering to form TI/SV devices via a subsequent standard lithography process. The resulting TI/SV devices exhibit a giant magnetoresistance of up to ∼1.1% at room temperature. Additionally, a low switching current density of approximately 1.25 × 105 A cm−2 is achieved, which implies high potential for further reducing the energy consumption of SOT-based devices. The SOT conversion efficiency and charge-spin conversion efficiency of the TI layer are approximately 4.74 × 10−6 Oe A−1 cm2 and 1.33, respectively, as extracted from the SOT-induced shift of the magnetic switching field. Moreover, the switching current density reduces steadily with the device size scaling down. This study can facilitate the realization of energy-efficient magnetic memory devices in the future.

Investigation of helicity dependent photocurrent of the surface states in a (Bi0.7Sb0.3)2Te3 nanoplate

Abstract Helicity dependent photocurrent (HDPC) of the surface states in a high-quality topological insulator (Bi$_{0.7}$Sb$_{0.3}$)$_2$Te$_3$ nanoplate grown by chemical vapor deposition (CVD) is investigated. By investigating the angle dependent HDPC, it is found that the HDPC is mainly contributed by the circular photogalvanic effect (CPGE) current when the incident plane is perpendicular to the connection of the two contacts, whereas the circular photon drag effect (CPDE) dominates the HDPC when the incident plane is parallel to the connection of the two contacts. In addition, the CPGE of the (Bi$_{0.7}$Sb$_{0.3}$)$_2$Te$_3$ nanoplate is regulated by temperature, light power, excitation wavelength, the source-drain and ionic liquid top-gate voltages, and the regulation mechanisms are discussed. It is demonstrated that (Bi$_{0.7}$Sb$_{0.3}$)$_2$Te$_3$ nanoplates may provide a good platform for novel opto-spintronics devices.

Spin pumping through nanocrystalline topological insulators

The topological surface states (TSSs) in topological insulators (TIs) offer exciting prospects for dissipationless spin transport. Common spin-based devices, such as spin valves, rely on trilayer structures in which a non-magnetic layer is sandwiched between two ferromagnetic (FM) layers. The major disadvantage of using high-quality single-crystalline TI films in this context is that a single pair of spin-momentum locked channels spans across the entire film, meaning that only a very small spin current can be pumped from one FM to the other, along the side walls of the film. On the other hand, using nanocrystalline TI films, in which the grains are large enough to avoid hybridization of the TSSs, will effectively increase the number of spin channels available for spin pumping. Here, we used an element-selective, x-ray based ferromagnetic resonance technique to demonstrate spin pumping from a FM layer at resonance through the TI layer and into the FM spin sink.

Controlled vapor phase epitaxial growth of high quality Bi2Se3 thin films and their photoresponse studies

Topological insulators are foreseen to offer promising applications in next generation electronic and optoelectronic devices. However, this is said to be tedious without understanding the underlying factors governing the fabrication and physicochemical properties of high-quality topological insulators. In this regard, this article reports on the two-dimensional (2D) growth of high quality Bi2Se3 thin films layered on sapphire by vapor phase epitaxy (VPE). The crystalline rhombohedral phase of the films was substantiated using X-ray diffraction results. The homogeneous distribution of Bi2Se3 thin films were approved through the microscopic images. The high quality in Bi2Se3 was examined through the selected area diffraction patterns and lattice fringes recorded using transmission electron microscopy. Finally, an infrared photo switching device was fabricated using the thin films processed in this work. The device responded linearly in accordance to the intensity of the laser irradiation subjected. The fabricated thin film based device also presented a high reliability through the lack of any degradation in its performance after consecutive switching cycles. Finally, we would like to augment with the fact that the findings observed this work could assist in opening up the possibility for novel optoelectronic applications using Bi2Se3 thin films.

Weak anti-localization properties of high-quality topological Bi2Te3 nanofilms prepared by home-built pulsed laser enhanced molecular epitaxy system

Efficient preparation of high-quality topological insulator nanofilms (TINFs) in large scale is demanding. Using a home-built pulsed laser enhanced molecular epitaxy system (PLEES), we efficiently fabricate TINFs with prefect stoichiometry using Bi2Te3 TINFs as an example. Instead of utilizing multiple beam source furnaces to provide essential elements, hybrid targets containing both pure Bi2Te3 and Te targets are applied in a single epitaxial chamber. Moreover, diffraction pattern of growing Bi2Te3 TINFs are monitored in live by a high-energy electron diffractometer, allowing fast optimization of growth conditions. Scanning electron microscopy and atomic force microscopy demonstrate that the surface morphology of the prepared Bi2Te3 nanofilms with a thickness of 70 nm are flat and dense, and energy-dispersive X-ray spectroscopy analysis suggests that the ratio of Bi to Te elements is 2:3. The grazing incidence X-ray diffractometer reveals that the X-ray diffraction peaks of the synthesized nanofilm matches those of the Bi2Te3 standard PDF card. The prepared nanofilm exhibits a weak anti-localization effect at low magnetic fields characterized by physical property measurement system, confirming its unique electrical transporting properties. This work demonstrates that using hybrid targets with optimized growth parameters enable PLEES to efficiently grow high-quality stoichiometric TINFs on a large scale.

Epitaxial growth of Bi(110) and Bi2Se3 thin films on a ferromagnetic insulator substrate of Cr2Ge2Te6

When a topological insulator (TI) is brought to the proximity of a ferromagnetic insulator (FMI), the breaking of the time-reversal symmetry may give rise to quantum anomalous Hall effect (QAHE). The physical properties of such TI–FMI systems are greatly affected by the interfacial structures of the components. Here, we report the growth and structural properties of Bi(110) and Bi2Se3 thin films on a FMI of Cr2Ge2Te6 (CGT) substrate by scanning tunneling microscopy. We observed various defects and impurities on the CGT surfaces, which serve as the preferential sites for initial nucleation and epitaxial growth of Bi(110) thin films. The exposure of the as-grown Bi(110) thin films to Se vapor leads to the formation of polycrystalline Bi2Se3 thin films with randomly distributed holes. The structure and composition of the as-prepared Bi2Se3 thin films were further confirmed by Raman spectroscopy and x-ray photoelectron spectroscopy. Our work shows that the quality of the CGT crystals is vital for the growth of high-quality TIs on CGT substrates for QAHE.

Ultralow power spin\u2013orbit torque magnetization switching induced by a non-epitaxial topological insulator on Si substrates

The large spin Hall effect in topological insulators (TIs) is very attractive for ultralow-power spintronic devices. However, evaluation of the spin Hall angle and spin–orbit torque (SOT) of TIs is usually performed on high-quality single-crystalline TI thin films grown on dedicated III-V semiconductor substrates. Here, we report on room-temperature ultralow power SOT magnetization switching of a ferrimagnetic layer by non-epitaxial BiSb TI thin films deposited on Si/SiO2 substrates. We show that non-epitaxial BiSb thin films outperform heavy metals and other epitaxial TI thin films in terms of the effective spin Hall angle and switching current density by one to nearly two orders of magnitude. The critical SOT switching current density in BiSb is as low as 7 × 104 A/cm2 at room temperature. The robustness of BiSb against crystal defects demonstrate its potential applications to SOT-based spintronic devices.

Near-infrared photodetection based on topological insulator P-N heterojunction of SnTe/Bi2Se3

Heterostructures constructed from topological insulator materials have aroused great research interests because of their various and unusual features and functions. Growth of topological insulator heterostructures can create a topological p-n junction at their interfaces with robust built-in potential barrier, which helps to improve the photodetection performance of topological insulators. Herein, the high-quality topological insulator SnTe/Bi2Se3 heterostructures were prepared via a two-step physical vapor deposition process. A robust topological p-n junction was confirmed to exist on the heterointerface with a large forward-to-reverse current ratio exceeding 700. Near-infrared photodetector was then fabricated by using of the heterostructure, which presents a ultrafast and steady response to the incident 1550 nm light even under self-powered mode. Note that the photodetector can exhibit a higher light responsivity of 145.74 mA W−1, a maximum detectivity of 1.15 × 1010 Jones, as well as a microsecond (6.90 μs) response speed. The demonstrated SnTe/Bi2Se3 heterostructure devices open up an exciting field for low-power and low-cost optical detection application and great potential of ultrafast memory devices. This novel heterostructure not only takes advantage of the efficient light absorption of topological insulators, but also provides an ideal platform to investigate the energy-band coupling effect between the p-type and n-type topological surface states.

Resistance fluctuation spectroscopy of thin films of 3D topological insulator BiSbTeSe1.6

Despite several years of studies, the origin of slow-kinetics of charge-carriers at the surface-states of strong topological insulators remains abstruse. In this article, we report on studies of charge dynamics of thin films of the 3-dimensional strong topological insulator material BiSbTeSe1.6 grown by pulsed laser deposition (PLD). The bulk of the films was insulating, making them suitable for transport studies of topological surface-states. Despite being disordered and granular, the films show definite signatures of the presence of topological surface-states with electronic transport coherence lengths comparable to those of high-quality grown films grown by molecular beam epitaxy (MBE). At high temperatures, the resistance fluctuations in these films were found to be dominated by trapping-detrapping of charge carriers from multiple defect-levels of the bulk. At low temperatures, fluctuations in the resistance of surface-states, arising due to the coupling of surface transport with defect dynamics in bulk, determine the noise. We thus confirm that the measured low-frequency fluctuations in these films, over the entire temperature range of 20 mK–300 K, are determined primarily by bulk defect density. The magnitude of noise was comparable to that measured on bulk-exfoliated films but was slightly higher than that in MBE grown films. Our studies establish PLD as a viable route to develop high-quality topological insulator materials.

Broadband photodetectors based on topological insulator Bi2Se3 nanowire with enhanced performance by strain modulation effect

Photoelectronic properties can be effectively modulated by external mechanical stimulation, which have been widely researched on materials with non-centrosymmetric wurtzite structure. Topological insulator (TI) materials with symmetric crystal structure have been theoretically predicted that its photoelectronic properties would also be greatly modulated through strain modulation effect. Here, for the first time, we experimentally demonstrate the strain modulation effect of TI Bi2Se3 nanowires (NWs), opening an innovative way to enhance the optoelectronic performance of TI materials. Single-crystal TI Bi2Se3 NWs were synthesized via a micro-environmental control chemical vapor deposition method. Based on these NWs, a high-performance photodetector (PD) with high responsivity, and broadband detection range from ultraviolet to near-infrared were fabricated. By introducing an external 0.54% compressive strain, the photocurrent and responsivity were enhanced by 97% and 503%, respectively, upon excitation by 442 nm light with an intensity of 17.6 mW/cm2. A theoretical model of the schematic energy band diagram was proposed to illustrate the enhancement mechanism. The results of theoretical calculation indicate that the conduction band of the TI can be modulated by strain, which can then influence the metal-semiconductor (M-S) junction potential for charges transport. This study reports a high-quality TI Bi2Se3 NWs based strain-modulated PDs, broadening the family of strain modulation materials system, and offering a promising material for high-performance broadband detectors.

Ultralow Saturation Intensity Topological Insulator Saturable Absorber for Gigahertz Mode-Locked Solid-State Lasers

The successful demonstration of Q-switched and mode-locked fiber-laser operations using a topological insulator (TI) as saturable absorber (SA) has opened an application window besides TI's originally expected features. However, to date, all-solid-state mode-locked lasers base on TISAs are still unavailable and became a desired goal not only due to their application as light sources, but also because of providing a way for deeper investigation of the nature of ultrafast dynamics present in TISAs. In this paper, the realization of a continuous-wave mode-locked all-solid-state laser with a repetition rate of around 1 GHz is reported using a high-quality TI SA mirror (TI-SAM) with ultralow saturation intensity, fabricated by a spin coating-co-reduction approach. An output power of 180 mW and pulse duration of 8 ps are observed. In addition, a 61 dB pulse-train quality from the radio frequency spectrum of mode-locked operation prove the feasibility of the proposed laser. To the best of our knowledge, this is the first experimental demonstration of a mode-locked solid-state laser based on TIs. In addition, this paper shows that the use of TISAs is a promising option for the realization of scaling solid-state mode-locked lasers with higher repetition rates, reaching order of GHz.

Growth of high-quality Bi2Se3 topological insulators using (Bi1-xInx)2Se3 buffer layers

In this article, the authors first report on the optimum growth parameters for (Bi1-xInx)2Se3 alloys of arbitrary composition using molecular beam epitaxy. It is found that smooth, single-phase films can only be obtained by using a sequential growth and annealing method to seed the film, after which normal codeposition growth can be used. The topological insulator Bi2Se3 is then grown on top of various (Bi1-xInx)2Se3 buffers and the electrical properties measured. For Bi2Se3 films grown on high-quality buffer layers, the mobility is greatly enhanced and the carrier density reduced compared to growth directly on sapphire substrates, indicating a significant improvement in film quality. The use of an almost lattice-matched trivially insulating buffer layer is therefore crucial to the growth of high-quality topological insulators on arbitrary substrates.

Van der Waals epitaxy of topological insulator Bi2Se3 on single layer transition metal dichalcogenide MoS2

We report the growth of high quality topological insulator Bi2Se3 thin films on a single layer, transitional metal dichalcogenide MoS2 film via van der Waals epitaxy in a planar geometry. In stark contrast to the reported growth of using 3-D crystalline substrates such as Al2O3(0001), Bi2Se3 thin films grown on a 2-D template made of single layer MoS2 showed excellent crystallinity starting immediately from the growth of the first quintuple layer. Excellent crystallinity of Bi2Se3 thin films is attained, with the increased size of the triangular shaped Bi2Se3 domains and 2–3 times enhancement in mobility, along with the observation of Shubnikov–de Haas oscillations in the magnetoresistance. Our approach of adopting a van der Waals type template may be extended to the thin film growth of other low dimensional layered materials.

Versatile Large-Area Custom-Feature van der Waals Epitaxy of Topological Insulators.

As the focus of applied research in topological insulators (TI) evolves, the need to synthesize large-area TI films for practical device applications takes center stage. However, constructing scalable and adaptable processes for high-quality TI compounds remains a challenge. To this end, a versatile van der Waals epitaxy (vdWE) process for custom-feature bismuth telluro-sulfide TI growth and fabrication is presented, achieved through selective-area fluorination and modification of surface free-energy on mica. The TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag crystalline edges highly oriented with the underlying mica lattice and only two preferred domain orientations mirrored at 180°. As-grown feature thickness dependence on lateral dimensions and denuded zones at boundaries are observed, as explained by a semiempirical two-species surface migration model with robust estimates of growth parameters and elucidating the role of selective-area surface modification. Topological surface states contribute up to 60% of device conductance at room temperature, indicating excellent electronic quality. High-yield microfabrication and the adaptable vdWE growth mechanism with readily alterable precursor and substrate combinations lend the process versatility to realize crystalline TI synthesis in arbitrary shapes and arrays suitable for facile integration with processes ranging from rapid prototyping to scalable manufacturing.

Observation of the magneto-transport property in a millimeter-long topological insulator Bi2Te3 thin-film Hall bar device

In this work, we report clean, surface protected and high quality topological insulator (TI) thin film Hall bar device of millimeter size long, without using any resist and lithography techniques. The pronounced effect of weak anti-localization (WAL) behavior has been observed in the magneto-transport measurements at low temperatures over the range T=4–10K and in the low field regions and as we go from 10K onwards to higher temperatures the WAL cusp disappears, also we find that the high field magenetoresistance (MR) is linear in field. The MR behavior with respect to magnetic field (B) seems to be symmetric. We also report thickness dependent weak-anti localization (WAL) behavior in topological insulator Bi2Te3 thin film Hall bar device. Our systematic magneto-transport measurements for varying thickness reveal WAL signals obtain in thicker films, where as a sudden diminishment of the surface transport below the critical thickness of ∼4nm has been observed by suppression of WAL behavior. The pronounced behavior of this effect is also found to be dependent on the temperatures, where the WAL cusps are observed at low temperatures in the low-field regions.

Molecular beam epitaxial growth and electronic transport properties of high quality topological insulator Bi2Se3 thin films on hexagonal boron nitride

We report the molecular beam epitaxial growth and characterization of high quality topological insulator Bi2Se3 thin films on hexagonal boron nitride (h-BN). A two-step growth was developed, enhancing both the surface coverage and crystallinity of the films on h-BN. High-resolution transmission electron microscopy study showed an atomically abrupt and epitaxial interface formation between the h-BN substrate and Bi2Se3. We performed gate tuned magnetotransport characterizations of the device fabricated on the thin film and confirmed a high mobility surface state at the Bi2Se3/h-BN interface. The Berry phase obtained from Shubnikov−de Haas oscillations suggested this interfacial electronic state is a topologically protected Dirac state.

Observation of a charge delocalization from Se vacancies inBi2Se3: A positron annihilation study of native defects

By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects $({\text{V}}_{\text{Se1}})$ are present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both ${\text{V}}_{\text{Se1}}^{0}$ and ${\text{V}}_{\text{Se1}}^{+}$ charge states coexist. The observed charge delocalization determines the contribution of ${\text{V}}_{\text{Se1}}$ defects to the $n$-type conductivity of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. These findings are supported by theoretical calculations, which show that vacancies of nonequivalent Se1 and Se2 selenium atoms are clearly differentiated by positron annihilation lifetime spectroscopy, enabling us to directly detect and quantify the most favorable type of selenium vacancy. In addition to open-volume defects, experimental data indicate the presence of defects that act as shallow traps, suggesting that more than one type of native defects coexist in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. As will be discussed, the presence of a dislocation density around ${10}^{10}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ could be the source of the detected shallow traps. Understanding the one-dimensional defects and the origin of the charge delocalization that leads ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ to be an $n$-type semiconductor will help in the development of high-quality topological insulators based on this material.

High-Responsivity, High-Detectivity, Ultrafast Topological Insulator Bi2Se3/Silicon Heterostructure Broadband Photodetectors.

As an exotic state of quantum matter, topological insulators have promising applications in new-generation electronic and optoelectronic devices. The realization of these applications relies critically on the preparation and properties understanding of high-quality topological insulators, which however are mainly fabricated by high-cost methods like molecular beam epitaxy. We here report the successful preparation of high-quality topological insulator Bi2Se3/Si heterostructure having an atomically abrupt interface by van der Waals epitaxy growth of Bi2Se3 films on Si wafer. A simple, low-cost physical vapor deposition (PVD) method was employed to achieve the growth of the Bi2Se3 films. The Bi2Se3/Si heterostructure exhibited excellent diode characteristics with a pronounced photoresponse under light illumination. The built-in potential at the Bi2Se3/Si interface greatly facilitated the separation and transport of photogenerated carriers, enabling the photodetector to have a high light responsivity of 24.28 A W(-1), a high detectivity of 4.39 × 10(12) Jones (Jones = cm Hz(1/2) W(-1)), and a fast response speed of aproximately microseconds. These device parameters represent the highest values for topological insulator-based photodetectors. Additionally, the photodetector possessed broadband detection ranging from ultraviolet to optical telecommunication wavelengths. Given the simple device architecture and compatibility with silicon technology, the topological insulator Bi2Se3/Si heterostructure holds great promise for high-performance electronic and optoelectronic applications.

Sn-doped Bi1.1Sb0.9Te2S bulk crystal topological insulator with excellent properties.

A long-standing issue in topological insulator research has been to find a bulk single crystal material that provides a high-quality platform for characterizing topological surface states without interference from bulk electronic states. This material would ideally be a bulk insulator, have a surface state Dirac point energy well isolated from the bulk valence and conduction bands, display quantum oscillations from the surface state electrons and be growable as large, high-quality bulk single crystals. Here we show that this material obstacle is overcome by bulk crystals of lightly Sn-doped Bi1.1Sb0.9Te2S grown by the vertical Bridgman method. We characterize Sn-BSTS via angle-resolved photoemission spectroscopy, scanning tunnelling microscopy, transport studies, X-ray diffraction and Raman scattering. We present this material as a high-quality topological insulator that can be reliably grown as bulk single crystals and thus studied by many researchers interested in topological surface states.

Selective area growth of Bi2Te3 and Sb2Te3 topological insulator thin films

The intrinsic bulk behavior of topological insulators (TI) is a key issue for their employment in future device applications. State of the art TIs predominantly suffer from large bulk charge carrier concentrations that mask their extraordinary surface states. In this paper we present the selective area growth of Bi2Te3 and Sb2Te3 TI thin films on prestructured Si(111) Si on insulator (SOI) substrates, paving the way to high quality TI nanostructures in which access to surface states is enhanced. Therefore high quality Bi2Te3 and Sb2Te3 thin films were deposited by means of solid source molecular beam epitaxy (MBE) and subsequently investigated by energy dispersive x-ray spectroscopy (EDX). To investigate the transport properties of the selectively grown thin films, magnetotransport measurements were performed at low temperatures. Nucleation in the SiO2 valleys next to the prepatterned Si(111) mesa structures was not observed. The structural and morphological qualities of crystals deposited on untreated Si(111) SOI wafers are completely preserved by employing the selective area growth on prepatterned substrates. The transport characteristics of the selectively-grown TI systems are comparable to those of the analogous postpatterned films.