Topological Insulator Thin Films Research Articles

Electrical properties of a metal-germanium-topological insulator (metal/n-Ge/p-Bi2Te3) heterostructure devices

This work presents topological insulator (TI) heterojunction Metal/ $$n$$ -Ge/Bi2Te3, by growing a thin TI film of Bi2Te3 on an $$n$$ -type Germanium (Ge) substrate. The microstructure and morphology of the developed Bi2Te3 film are analyzed. The electrical behavior of the Metal/ $$n$$ -Ge/Bi2Te3 heterojunction is examined by current–voltage (I–V) and capacitance–voltage (C–V) measurements. The performance of the heterostructure was examined by depositing different metallic contacts. Metal contacts of aluminum (Al), silver (Ag), and platinum (Pt) were used, and they formed Schottky contact with Ge and an ohmic contact with Bi2Te3. Pt/ $$n$$ -Ge/Bi2Te3 heterostructure was found best in terms of rectification ratio $$(RR)$$ = 122.7, the figure of merit ( $$FOM)$$ = 49.5, and ideality factor $$(n)$$ = 7.33 with small series resistance ( $${R}_{s})$$ . The space-charge limited current (SLSC) and ohmic conduction are the transport mechanisms that govern these heterojunctions at a different voltage, affecting its performance. The influence of $$n$$ -Ge/ $$p$$ -Bi2Te3 junction on the performance of Metal/ $$n$$ -Ge/Bi2Te3 is analyzed. The experimental results are fitted by two-diode simulation and current density–voltage (J–V) and local ideality factor—voltage n–V plots of these heterojunctions are studied for the recombination current ( $${J}_{02})$$ . This study is significant in terms of the electrical performance of the Ge-TI-based, heterojunction devices.

Transitional Faraday and Kerr effect in hybridized topological insulator thin films

We theoretically investigate quantum phase transitions from topologically non-trivial to trivial states in three-dimensional hybridized topological insulator (TI) ultra-thin films. The interplay between the hybridization of the top and bottom surface states (SSs) and Zeeman energy gives rise to topological and normal insulating phases. By tuning the Zeeman energy, we can drive phase transition between these two phases by closing and reopening the band gap. Furthermore, we impinge a Gaussian beam on the surface of the 3D TI to study the Faraday rotation (FR) and magneto-optical Kerr effect (MOKE) in the presence of an external magnetic field, while explicitly taking into account the hybridization between the top and bottom Dirac SSs of the TI film. The FR and MOKE change sign when the polarization is changed from s to p. We demonstrate that giant FR and MOKE can be achieved by tuning the external magnetic field. Furthermore, the MOKE signal takes a sharp anti-phase peak at the charge neutrality point as the system goes from the quantum spin Hall insulator (QSHI) to the semimetallic state. Lastly, the MOKE and FR in the bottom and top SSs can be independently tuned for intra-band and inter-band transitions via n-type and p-type doping respectively.

High performing flexible optoelectronic devices using thin films of topological insulator

Topological insulators (TIs) possess exciting nonlinear optical properties due to presence of metallic surface states with the Dirac fermions and are predicted as a promising material for broadspectral phodotection ranging from UV (ultraviolet) to deep IR (infrared) or terahertz range. The recent experimental reports demonstrating nonlinear optical properties are mostly carried out on non-flexible substrates and there is a huge demand for the fabrication of high performing flexible optoelectronic devices using new exotic materials due to their potential applications in wearable devices, communications, sensors, imaging etc. Here first time we integrate the thin films of TIs (Bi2Te3) with the flexible PET (polyethylene terephthalate) substrate and report the strong light absorption properties in these devices. Owing to small band gap material, evolving bulk and gapless surface state conduction, we observe high responsivity and detectivity at NIR (near infrared) wavelengths (39 A/W, 6.1 × 108 Jones for 1064 nm and 58 A/W, 6.1 × 108 Jones for 1550 nm). TIs based flexible devices show that photocurrent is linearly dependent on the incident laser power and applied bias voltage. Devices also show very fast response and decay times. Thus we believe that the superior optoelectronic properties reported here pave the way for making TIs based flexible optoelectronic devices.

Study of magnetic and transport properties of Bi2Se3/FeSe2 bilayer thin films

Thin films of topological insulator (TI) Bi2Se3 were grown onto the surfaces of FeSe2 layers of different thicknesses on Si (100) substrates by magnetron sputtering, forming bilayer films with smooth surface. Magnetic and transport measurements indicate ferromagnetism in these bilayer samples. Large coercive fields at low-temperatures and a room-temperature magnetic order were observed. Moreover, nonsaturated high-filed linear magnetoresistance (MR) and weak anti-localization effect were found in these bilayer thin films. These results indicate that the bilayer samples could have both strong spin–orbit coupling and ferromagnetic proximity effect, which are the desired features.

Optical properties of atomically thin films of the topological insulator Bi2Se3

Problem formulating. To study the transport properties of atomically thin films of topological insulators, it is very important to determine their thickness as accurately as possible, as well as the state of the surface. Goal. To obtain atomically thin films of the topological insulator Bi2Se3 and to determine their thickness and the degree of surface degradation via their optical properties. Result. A technique that allows one to obtain single-crystal films of large area and various thicknesses on optically transparent substrates has been developed. The dependence of the transparency depending on the number of atomic layers (Bi2Se3 quintiples) in the film is determined. The obtained exact dependence of transparency on the number of layers is in good agreement with theory. It was also possible to observe how the transparency of the films changes over time, which is associated with the degradation of its surface layers. Practical meaning.The results obtained can be used for optical quality control of atomically thin films of quasi-twodimensional compounds, as well as for the manufacture of various micro- and nanostructures based on them, which can bring closer to the creation of a new element base for electronic devices.

Phase Diagram and Edge States of Surface States of Topological Superconductors

Majorana fermions in two-dimensional systems satisfy non-Abelian statistics. They are possible to exist in topological superconductors as quasi particles, which is of great significance for topological quantum computing. In this paper, we study a new promising system of superconducting topological surface state topological insulator thin films. We also study the phase diagrams of the model by plotting the Majorana edge states and the density of states in different regions of the phase diagram. Due to the mirror symmetry of the topological surface states, the Hamiltonian can be block diagonalized into two spin-triplet p-wave superconductors, which are also confirmed by the phase diagrams. The chiral Majorana edge modes may provide a new route for realizing topological quantum computation.

The realization of quantum anomalous Hall effect in two dimensional electron gas

The quantum anomalous Hall effect (QAHE), carrying dissipationless chiral edge states, occurs without any magnetic field. Two main strategies were proposed to host QAHE: the magnetic topological insulator thin films and graphene systems. Only the former one was realized in experiment at low temperature. In this paper, by dealing with the two-dimensional electron gas with an anti-dot lattice, a realistic platform is proposed to host the QAHE with both Chern number and . Based on the calculation of the Berry curvature integral and spacial wave function, the topological nature of the QAH edge states is well demonstrated. In the QAH region, the conductance shows quantized plateaus and their values are robust against Anderson disorder. In addition, we have also studied the effects of the size and shape of the anti-dot lattice on QAHE and they provide extra manners to adjust the system parameters. Taking the advantages of the well developed micro-manufacture technique in semiconductors, the proposal is experimentally accessible in micro-scale.

Voltage-Controlled Magnetic Reversal in Orbital Chern Insulators.

Chern insulator ferromagnets are characterized by a quantized anomalous Hall effect and have so far been identified experimentally in magnetically doped topological insulator thin films and in bilayer graphene moiré superlattices. We classify Chern insulator ferromagnets as either spin or orbital, depending on whether the orbital magnetization results from spontaneous spin polarization combined with spin-orbit interactions, as in the magnetically doped topological insulator case, or directly from spontaneous orbital currents, as in the moiré superlattice case. We argue that, in a given magnetic state, characterized, for example, by the sign of the anomalous Hall effect, the magnetization of an orbital Chern insulator will often have opposite signs for weak n and weak p electrostatic or chemical doping. This property enables pure electrical switching of a magnetic state in the presence of a fixed magnetic field.

Plasmon coupling in topological insulator multilayers

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. We excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon-phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon near-zero mode in the top dielectric material ${({\mathrm{Bi}}_{0.5}{\mathrm{In}}_{0.5})}_{2}{\mathrm{Se}}_{3}$. Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.

Propagating Dirac plasmon polaritons in topological insulators

We investigate the excitation of propagating Dirac plasmon polaritons (DPPs) in topological insulator (TI) thin films. A series of TI thin films were grown by molecular beam epitaxy. Periodic gold grating couplers with different periodicity were fabricated on the TI films to excite the propagating DPPs. A series of absorption peaks that shift with the grating periodicity were observed in Fourier transform infrared spectroscopy transmission scans. We have ruled out any source other than the excitation of propagating DPPs that could have caused the peaks to appear. The peak dispersion is consistent with theoretical predictions for coupled DPPs in TI thin films.

Highly Tunable Spin-Orbit Torque and Anisotropic Magnetoresistance in a Topological Insulator Thin Film Attached to Ferromagnetic Layer.

We investigate spin-charge conversion phenomena in hybrid structures of topological insulator thin films and magnetic insulators. We find an anisotropic inverse spin-galvanic effect that yields a highly tunable spin-orbit torque. Concentrating on the quasiballistic limit, we also predict a giant anisotropic magnetoresistance at low dopings. These effects, which have no counterparts in thick topological insulators, depend on the simultaneous presence of the hybridization between the surface states and the in-plane magnetization. Both the inverse spin-galvanic effect and anisotropic magnetoresistance exhibit a strong dependence on the magnetization and the Fermi level position and can be used for spintronics and spin-orbit-torque-based applications at the nanoscale.

Multiple Majorana edge modes in magnetic topological insulator–superconductor heterostructures

We numerically investigate the electronic transport properties (i.e., electron tunneling and Andreev reflection) of a topological superconductor composed of a magnetic topological insulator and superconductors. A phase diagram is provided to distinguish various topological phases and their corresponding distinct Majorana edge modes. When superconductors are proximity coupled with the top and bottom surfaces of a magnetic topological insulator thin film, a quantum phase transition from topological insulator to quantum anomalous Hall effect passes through the regime possessing both chiral and helical edge modes. The hallmark feature is that the coefficient of electron tunneling is quantized to be $5/4$ and the remaining scattering processes exhibit an identical probability with a magnitude of $1/4$ in the coexisting states with a Chern number of $\mathcal{N}=\ifmmode\pm\else\textpm\fi{}1$. When the superconductors are proximity coupled with a nonmagnetic topological insulator thin film, we find that the perfectly quantized electron tunneling or crossed Andreev reflection can alternately appear via tuning the chemical potential.

Magnetized topological insulator multilayers

We discuss the magnetic and topological properties of bulk crystals and quasi-two-dimensional (quasi-2D) thin films formed by stacking intrinsic magnetized topological insulator (for example, Mn ([Formula: see text])2X4 with X = Se,Te) septuple layers and topological insulator quintuple layers in arbitrary order. Our analysis makes use of a simplified model that retains only Dirac cone degrees of freedom on both surfaces of each septuple or quintuple layer. We demonstrate the model's applicability and estimate its parameters by comparing with ab initio density-functional theory (DFT) calculations. We then employ the coupled Dirac cone model to provide an explanation for the dependence of thin-film properties, particularly the presence or absence of the quantum anomalous Hall effect, on film thickness, magnetic configuration, and stacking arrangement, and to comment on the design of Weyl superlattices.

Ultra-sensitive anomalous Hall effect sensors based on Cr-doped Bi2Te3 topological insulator thin films

The observation of anomalous Hall effect (AHE) in magnetically doped topological insulators brings a new candidate of Hall sensor with low power consumption. In this work, the transport properties and the sensitivity of AHE sensors based on Cr-doped Bi2Te3 thin films were studied. Obvious AHEs induced by ferromagnetic ordering were presented in all Cr x Bi2-x Te3 sensors. At the optimized doping concentration of x= 0.09, a high sensitivity of 6625 Ω T−1 was achieved, which has increased by 2.5 times compared to the highest reported one in Cr-doped Bi2Te3. More importantly, a considerable sensitivity of 4082 Ω T−1 can be obtained up to 20 K, which implies a higher working temperature than other reports. Our findings suggest Cr-doped Bi2Te3 sensor could be a good candidate for highly sensitive AHE sensors and reveal the extraordinary potential of magnetic TIs in the applications of field detection.

Challenges of Topological Insulator Research: Bi2Te3 Thin Films and Magnetic Heterostructures

Topological insulators (TIs) are of particular interest in the recent solid‐state research because of their exceptional features stemming from the conducting, topologically protected surface states. The exotic properties include the occurrence of novel quantum phenomena and make them promising materials for spintronics and quantum computing applications. Theoretical studies have provided a vast amount of valuable predictions and proposals, whose experimental observation and implementation, to date, are often hindered by an insufficient sample quality. The effect of even a relatively low concentration of defects can make the access to purely topological surface states impossible. This points out the need of high‐quality bulk‐insulating materials with ultra‐clean surfaces/interfaces, which requires sophisticated sample/device preparations as well as special precautions during the measurements. Herein, the challenging work on 3D TI thin films with a focus on is reported. It covers the optimization of the molecular beam epitaxy growth process, the in situ characterization of surface states and transport properties, the influence of exposure to ambient gases and of capping layers, as well as the effect of interfacing TI thin film with magnetic materials.

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.

Europium Doping Impact on the Properties of MBE Grown Bi2Te3 Thin Film.

The impact of europium doping on the electronic and structural properties of the topological insulator Bi2Te3 is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi2Te3 matrix remains divalent and substitutes bismuth in a Bi2Te3 matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi2Te3 topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.

Gapped Dirac cones and spin texture in thin film topological insulator

The protected surface states of topological insulators (TIs) form gapless Dirac cones corresponding non-degenerate eigenstates with helical spin polarisation. The presence of a warping term deforms the isotropic cone of the most simple model into snowflake Fermi surfaces as in Bi2Se3 and Bi2Te3. Their features have been identified in STM quasiparticle interference (QPI) experiments on isolated surfaces. Here we investigate the QPI spectrum for the TI thin-film geometry with finite tunnelling between the surface states. This leads to a dramatic change of spectrum due to gapping and a change in spin texture that should leave distinct signatures in the QPI pattern. We consider both normal and magnetic exchange scattering from the surface impurities and obtain the scattering t-matrix in Born approximation as well as the general closed solution. We show the expected systematic variation of QPI snowflake intensity features by varying film thickness and study, in particular, the influence on backscattering processes. We predict the variation of the QPI spectrum for Bi2Se3 thin films using the observed gap dependence from ARPES results.

Zitterbewegung-mediated RKKY coupling in topological insulator thin films

The dynamics of itinerant electrons in topological insulator (TI) thin films is investigated using a multi-band decomposition approach. We show that the electron trajectory in the 2D film is anisotropic and confined within a characteristic region. Remarkably, the confinement and anisotropy of the electron trajectory are associated with the topological phase transition of the TI system, which can be controlled by tuning the film thickness and/or applying an in-plane magnetic field. Moreover, persistent electron wavepacket oscillation can be achieved in the TI thin film system at the phase transition point, which may assist in the experimental detection of the jitter motion (Zitterbewegung). The implications of the microscopic picture of electron motion in explaining other transport-related effects, e.g., electron-mediated RKKY coupling in the TI thin film system, are also discussed.

On the bias voltage and staggered potential effects in tuning anisotropic temperature-dependent electrical conductivity of topological crystalline insulator thin films

Abstract Topological crystalline insulator thin films (TCITFs) possess a gap as the hybridization between the front and back surfaces. We demonstrate that in TCITFs described by standard continuum model, the gap survives in the presence of bias voltage and staggered potential, while the Fermi level alters. Therefore, conventional biased and staggered potential-induced TCITFs are associated with different phase transitions. We show this by analyzing the electronic band structure. This effect allows for revealing specific signatures of affected electrical conductivity (EC) with the aid of the Kubo–Greenwood formula qualitatively. In particular, we demonstrate that the temperature-dependent EC of TCITFs can be tuned by the bias voltage and staggered potential. A careful analysis of these processes, however, shows that the system has critical thermal energy for which tuning can be determined. This work may propose some ways of tuning of the topological phase and corresponding electronic practical applications of TCITFs.