Bi2Se3 Thin Films Research Articles

Changes of Magnetism in a Magnetic Insulator due to Proximity to a Topological Insulator.

We report the modification of magnetism in a magnetic insulator Y_{3}Fe_{5}O_{12} thin film by topological surface states (TSS) in an adjacent topological insulator Bi_{2}Se_{3} thin film. Ferromagnetic resonance measurements show that the TSS in Bi_{2}Se_{3} produces a perpendicular magnetic anisotropy, results in a decrease in the gyromagnetic ratio, and enhances the damping in Y_{3}Fe_{5}O_{12}. Such TSS-induced changes become more pronounced as the temperature decreases from 300 to 50K. These results suggest a completely new approach for control of magnetism in magnetic thin films.

Structurally and chemically compatible BiInSe3 substrate for topological insulator thin films

Quality of epitaxial films strongly depends on their structural and chemical match with the substrates: the more closely they match, the better the film quality is. Topological insulators (TI) such as Bi2Se3 thin films are of no exception. However, there do not exist commercial substrates that match with TI films both structurally and chemically, at the level commonly available for other electronic materials. Here, we introduce BiInSe3 bulk crystal as the best substrate for Bi2Se3 thin films. These films exhibit superior surface morphology, lower defect density and higher Hall mobility than those on other substrates, due to structural and chemical match provided by the BiInSe3 substrate. BiInSe3 substrate could accelerate the advance of TI research and applications.

Investigating the photovoltaic performance of surfactant-assisted MoBi2Se5 thin films

Molybdenum bismuth selenide (MoBi2Se5) has been proved to be a promising material for photoelectrochemical solar cell application. As MoBi2Se5 thin films are more corrosion resistant than bismuth (III) selenide (Bi2Se3) thin films, MoBi2Se5 can also be used as a buffer layer due to its distinctive optostructural properties. In this study, the authors synthesized surfactant-assisted nanostructured MoBi2Se5 thin films by using a cost-effective chemical route – namely, the arrested precipitation technique. The synthesized thin films were analyzed for optostructural, surface morphological, compositional and photovoltaic applications using the X-ray diffraction (XRD) technique, ultraviolet–visible spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy, and their photoelectrochemical solar cell performance was checked. The nanocrystalline nature of MoBi2Se5 thin films was confirmed from the XRD patterns. An increment in the optical bandgap energy was observed with surfactant mediation. The surface morphology of MoBi2Se5 changed from spherical to spongy-ball-like nanofibrous surface morphology after surfactant assistance. Furthermore, both the as-synthesized and surfactant-assisted MoBi2Se5 thin films showed photoelectrochemical solar cell applicability.

Anomalous Temperature Dependence of Quantum Correction to the Conductivity of Magnetic Topological Insulators.

Quantum transport in magnetic topological insulators reveals a strong interplay between magnetism and topology of electronic band structures. A recent experiment on magnetically doped topological insulator Bi_{2}Se_{3} thin films showed the anomalous temperature dependence of the magnetoconductivity while their field dependence presents a clear signature of weak antilocalization [Tkac etal., Phys. Rev. Lett. 123, 036406 (2019)PRLTAO0031-900710.1103/PhysRevLett.123.036406]. Here, we demonstrate that the tiny mass of the surface electrons induced by the bulk magnetization leads to a temperature-dependent correction to the π Berry phase and generates a decoherence mechanism to the phase coherence length of the surface electrons. As a consequence, the quantum correction to conductivity can exhibit nonmonotonic behavior by decreasing the temperature. This effect is attributed to the close relation of the Berry phase and quantum interference of the topological surface electrons in quantum topological materials.

Vapor growth of Bi2Se3 and Bi2O2Se crystals on mica

Thin Bi2Se3 films were deposited on mica substrates by physical vapor deposition without the use of the carrier gas. It was found that the films with high structural quality and high conductivity are grown at a source temperature of approximately 500 °C. The resistance of 20–300 nm thick films is in the range of 102-104 Ω/sq as compared with ∼10 Ω/sq for thicker films. Bi2O2Se crystals with a similar resistivity are revealed to grow at higher temperatures (600–700 °C). It was suggested that the decrease of the thin film resistance is due to the contribution of the surface channels. Low resistivity of the Bi2Se3 films expands the scope of their possible applications as infra-red transparent electrodes.

Acoustic plasmonics of Au grating/Bi2Se3 thin film/sapphire hybrid structures**Project supported by Carl-Zeiss-Stiftung

The surface plasmon polaritons of the topological insulator Bi2Se3 can be excited by using etched grating or grave structures to compensate the wave vector mismatch of the incident photon and plasmon. Here, we demonstrate novel gold grating/Bi2Se3 thin film/sapphire hybrid structures, which allow the excitation of surface plasmon polaritons propagating through nondestructive Bi2Se3 thin film with the help of gold diffractive gratings. Utilizing periodic Au surface structures, the momentum can be matched and the normal-incidence infrared reflectance spectra exhibit pronounced dips. When the width of the gold grating W (with a periodicity 2W) increases from 400 nm to 1500 nm, the resonant frequencies are tuned from about 7000 cm−1 to 2500 cm−1. In contrast to the expected dispersion for both massive and massless fermions, where q ∼ π/W is the wave vector, we observe a sound-like linear dispersion even at room temperature. This surface plasmon polaritons with linear dispersion are attributed to the unique noninvasive fabrication method and high mobility of topological surface electrons. This novel structure provides a promising application of Dirac plasmonics.

Crystalline Bi2Se3 topological insulator films prepared by dc magnetron sputtering

Topological insulator thin films of Bi2Se3 have been deposited on SrTiO3 (111) substrate by dc magnetron sputtering and annealed in Se environment. X-ray Diffraction (XRD) measurement shows (00l) (l = 3n, where n is an integer) type of reflections over 2θ range of 10–80° without any other peaks indicating that the films are of single crystalline quality along the [001] growth direction confirming that highly c-axis-preferred orientated Bi2Se3 thin films can be obtained by relatively simpler and easily scalable dc magnetron sputtering technique. The results are also confirmed by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDXS), Photo-electron Spectroscopy (PES) and X-ray absorption spectroscopy (XAS) measurements.

Ultrahigh Stability 3D TI Bi2Se3/MoO3 Thin Film Heterojunction Infrared Photodetector at Optical Communication Waveband

AbstractInfrared (IR) detection at 1300–1650 nm (optical communication waveband) is of great significance due to its wide range of applications in commerce and military. Three dimensional (3D) topological insulator (TI) Bi2Se3 is considered a promising candidate toward high‐performance IR applications. Nevertheless, the IR devices based on Bi2Se3 thin films are rarely reported. Here, a 3D TI Bi2Se3/MoO3 thin film heterojunction photodetector is shown that possesses ultrahigh responsivity (Ri), external quantum efficiency (EQE), and detectivity (D*) in the broadband spectrum (405–1550 nm). The highest on–off ratio of the optimized device can reach up to 5.32 × 104. Ri, D*, and the EQE can reach 1.6 × 104 A W−1, 5.79 × 1011 cm2 Hz1/2 W−1, and 4.9 × 104% (@ 405 nm), respectively. Surprisingly, the Ri can achieve 2.61 × 103 A W−1 at an optical communication wavelength (@ 1310 nm) with a fast response time (63 µs), which is two orders of magnitude faster than that of other TIs‐based devices. In addition, the device demonstrates brilliant long‐term (>100 days) environmental stability under environmental conditions without any protective measures. Excellent device photoelectric properties illustrate that the 3D TI/inorganic heterojunction is an appropriate way for manufacturing high‐performance photodetectors in the optical communication, military, and imaging fields.

Electrochemically exfoliated thin Bi2Se3 films and van der Waals heterostructures Bi2Se3/graphene

Thin Bi2Se3 flakes with few nanometer thicknesses and sized up to 350 μm were created by using electrochemical splitting from high-quality Bi2Se3 bulk monocrystals. The dependence of film resistance on the Bi2Se3 flake thickness demonstrates that, at room temperature, the bulk conductivity becomes negligible in comparison with the surface conductivity for films with thicknesses lower than 80 nm. Unexpectedly, all these films demonstrated p-type conductivity. The doping effect with sulfur or sulfur-related radicals during electrochemical exfoliation is suggested for the p-type conductivity of the exfoliated Bi2Se3 films. The formation of 2–8 nm films was predominantly found. Van der Waals (vdW) heterostructures of Bi2Se3/Graphene/SiO2/Si were created and their properties were compared with that of Bi2Se3 on the SiO2/Si substrate. The increase of the conductivity and carrier mobility in Bi2Se3 flakes of 3–5 times was found for vdW heterostructures with graphene. Thin Bi2Se3 films are potentially interesting for applications for spintronics, nano- and optoelectronics.

Laser Ablation Generation of Bismuth Selenide Clusters from Mixtures of Elements, Crystalline Bi2Se3, or Thin Films: Laser Desorption Ionization (LDI) and Surface Assisted LDI Time-of-Flight Mass Spectrometry using Graphene and Nanodiamonds.

A bismuth-selenium system from mixtures of the powdered elements in various molar ratios and from Bi2Se3 crystals and/or thin films was studied using laser desorption ionization and surface assisted laser desorption ionization. The BimSen clusters were observed in both positive and negative ion modes, but the mass spectra were more intense, and also a higher number of clusters was formed in the positive ion mode than in the negative mode. The BiSen+ (n = 1-8), Bi2Sen+ (n = 1-5), and Bi3Sen+ (n = 1-6) clusters were detected. Similarly, in the negative ion mode, BiSen- (n = 2-9) and Bi2Sen- (n = 1-2) clusters were observed. In addition, the formation of Bim+ (m = 1-5), Sen+ (n = 1-8), and Sen- (n = 1-7) clusters was also observed. In total, 33 clusters were generated, and 4 new bismuth selenide clusters that have not been reported before (namely, BiSe7+/-, BiSe8+/-, BiSe9-, and Bi2Se5+) were detected. The formation of similar clusters was also observed from bismuth-selenium mixtures and from crystalline Bi2Se3. Furthermore, the Bi2Se3 thin films prepared from a magnetron sputtering technique were also examined via laser desorption ionization. The generation of clusters from the surface of graphene and nanodiamonds was also studied, but no remarkable difference with comparison to the metal surface was observed.

Layer-dependent dielectric permittivity of topological insulator Bi2Se3 thin films

Dielectric properties and their evolutions are fundamental to reveal the optical responses of topological insulators and to guide the design of related devices. Here, we systematically investigate the layer-dependent dielectric permittivity of Bi2Se3 over an ultra-broad spectral range (0.73–6.43 eV) by combining the spectroscopic ellipsometry, energy-loss function (ELF), critical point (CP) analysis, and density function theory. Results demonstrate that the epsilon-near-zero point of the real permittivity and the ELF peak exhibit blue shifts, indicating the resonance frequencies of surface plasmon modes move towards the higher energy, due to the enhanced hybridization between the opposite surfaces. The joint density of states dominates the layer-dependent increase in imaginary permittivity as the thickness increases. We also find that the CP transition positions in band structures are layer-independent, and the CP center energies exhibit red shifts resulted by the reduced surface state gap with the thickness increasing.

Tuning of charge carrier density by deposition pressure in Sb-doped Bi2Se3 thin films

We report the tunability of charge carrier density in a pulsed laser deposition grown Bi1.95Sb0.05Se3 thin film system as a function of the deposition pressure. Dynamic partial pressure of argon maintained in the chamber during deposition suppresses Se escape, effectively regulating the stoichiometry, which in turn modulates the charge carrier density. With pressure variation, a striking three order change in carrier density is recorded and with additional control through the substrate temperature a carrier density value as low as 2 × 1018 cm−3 is found to be achievable. We observe a systematic evolution of structural, electronic, and magneto-transport properties of the system with variation of deposition pressure. The magneto-transport properties of the films reveal weak antilocalization behavior, analyzing which we have been able to determine the influence of deposition pressure on properties like the de-phasing mechanism involved in the system, the number of effective transport channels, and the coherence length.

Evidence for exchange Dirac gap in magnetotransport of topological insulator–magnetic insulator heterostructures

Transport signatures of exchange gap opening because of magnetic proximity effect (MPE) are reported for bilayer structures of Bi2Se3 thin films on yttrium iron garnet (YIG) and thulium iron garnet (TmIG) of perpendicular magnetic anisotropy (PMA). Pronounced negative magnetoresistance (MR) was detected, and attributed to an emergent weak localization (WL) effect superimposing on a weak antilocalization (WAL). Thickness-dependent study shows that the WL originates from the time-reversal-symmetry breaking of topological surface states by interfacial exchange coupling. The weight of WL declined when the interfacial magnetization was aligned toward the in-plane direction, which is understood as the effect of tuning the exchange gap size by varying the perpendicular magnetization component. Importantly, magnetotransport study revealed anomalous Hall effect (AHE) of square loops and anisotropic magnetoresistance (AMR) characteristic, typifying a ferromagnetic conductor in Bi2Se3/TmIG, and the presence of an interfacial ferromagnetism driven by MPE. Coexistence of MPE-induced ferromagnetism and the finite exchange gap provides an opportunity of realizing zero magnetic-field dissipation-less transport in topological insulator/ferromagnetic insulator heterostructures.

The coherent heteroepitaxy of high-index Bi2Se3 thin film on nanofaceted Si(211) substrate

In this paper, high-index Bi2Se3(2045) films are achieved on a Bi-modified Si(211) substrate with corrugated surface morphologies composed of striplike 19.5° tilted (001) facets by molecular beam epitaxy. The chemical bonds between Bi2Se3(001) faces and Bi-absorbed nanoscale faceted (111) terraces on Si(211) surfaces provide solid lattice constraints which favor the coherent epitaxy of Bi2Se3 films on the Bi-modified Si(211) substrate as compared to the H-passivated Si(211) substrate. This makes Bi2Se3(2045) grown on Bi-modified Si(211) more suitable for device applications. Temperature-dependent I-V measurements of the Bi2Se3(2045)/Si(211) heterojunction are performed which yield a Schottky barrier height of ФB = 0.22 eV at the heterointerface.

Temperature and excitation wavelength dependence of circular and linear photogalvanic effect in a three dimensional topological insulator Bi2Se3

The circular (CPGE) and linear photogalvanic effect (LPGE) of a three-dimensional topological insulator Bi2Se3 thin film of seven quintuple layers excited by near-infrared (1064 nm) and mid-infrared (10.6 m) radiations have been investigated. The comparison of the CPGE current measured parallel and perpendicular to the incident plane, together with the comparison of the CPGE current under front and back illuminations, indicates that the CPGE under front illumination of 1064 nm light is dominated by the top surface states of the Bi2Se3 thin film. The CPGE current excited by 10.6 m light is about one order larger than that excited by 1064 nm light, which may be attributed to the smaller cancelation effect of the CPGE generated in the two-dimensional electron gas when excited by 10.6 m light. Under the excitation of 1064 nm light, the LPGE current is dominated by the component which shows an even parity of incident angles, while the LPGE current excited by 10.6 m light is mainly contributed by the component which is an odd parity of incident angles. Both of the CPGE and LPGE currents excited by 1064 nm decrease with increasing temperature, which may be owing to the decrease of the momentum relaxation time and the stronger electron–electron scattering with increasing temperature, respectively.

First principles study of semihydrogenated graphene and topological insulator heterojunction

Based on first principles calculations, we study the electronic properties of heterostructures formed by a 2D ferromagnetic insulator semihydrogenated graphene (SG) and topological insulator Bi2Se3 thin films of a few quintuple layers (QLs). It is found that the unsaturated C atoms in SG form bonds with Se atoms in Bi2Se3 thin film and the top surface states (at the interface) are strongly hybridized with SG. Due to breaking of time-reversal symmetry, the surface states open gaps of 40 meV and 150 meV for SG/3QL-Bi2Se3 and SG/5QL-Bi2Se3 heterostructures, respectively. Furthermore, a giant Rashba spin splitting is found induced by the SG layer.

Dimensional Crossover and Topological Nature of the Thin Films of a Three-Dimensional Topological Insulator by Band Gap Engineering.

Identification and control of topological phases in topological thin films offer great opportunities for fundamental research and the fabrication of topology-based devices. Here, combining molecular beam epitaxy, angle-resolved photoemission spectroscopy, and ab initio calculations, we investigate the electronic structure evolution in (Bi1-xInx)2Se3 films (0 ≤ x ≤ 1) with thickness from 2 to 13 quintuple layers. By employing both thickness and In substitution as two independent "knobs" to control the gap change, we identify the evolution between several topological phases, i.e., dimensional crossover from a three-dimensional topological insulator to its two-dimensional counterpart with gapped surface state, and topological phase transition from a topological insulator to a normal semiconductor with increasing In concentration. Furthermore, by introducing In substitution, we experimentally demonstrated the trivial topological nature of Bi2Se3 thin films (below 6 quintuple layers) as two-dimensional gapped systems, consistent with our theoretical calculations. Our results provide not only a comprehensive phase diagram of (Bi1-xInx)2Se3 and a route to control its phase evolution but also a practical way to experimentally determine the topological properties of a gapped compound by a topological phase transition and band gap engineering.

Linear magnetoresistance in Cr-doped Bi2Se3 thin film

This paper reports the magnetotransport property of a Cr-doped Bi2Se3 thin film. A linear magnetoresistance (MR) is observed in a high field region, while the weak antilocalization effect appears up near zero magnetic fields below 15 K, and it is transformed into a positive parabolic MR at high temperatures. It is found that the slope of the observed linear MR is proportional to the Hall mobility. Meanwhile, a crossover field from the parabolic MR to the linear MR is proportional to the inverse Hall mobility. These indicate that the observed linear MR can be explained by the classical mechanism. Most importantly, we find that the observed linear MR is suppressed with increasing time when the film is sealed in vacuum after growth. This indicates that the observed linear MR does not originate from a two-dimensional electron gas formed on the film surface. It is found that the observed linear MR is likely to arise from the bulk state.

Thermoelectric properties including thermal conductivity of electrodeposited bismuth selenide thin films fabricated using different acid solutions

Using electrodeposition to prepare thermoelectric thin films enables the fabrication of energy-harvesting devices with low manufacturing cost. However, it is difficult to compute the thermal conductivity of these electrodeposited thin films owing to their large surface roughness. In this work, we prepared electrodeposited bismuth selenide (Bi2Se3) thin films using hydrochloric acid (HCl) and nitric acid (HNO3) solutions and measured the thermal conductivity using the 3ω method. The Bi2Se3 thin films prepared using the HCl solution consisted of round grains with relatively large pores that reached the substrate appearing between the grains. In contrast, the Bi2Se3 thin films prepared using the HNO3 solution were composed of flake-like grains with the appearance of relatively small and shallow pores. As a result, the thermal conductivity could only be measured for the Bi2Se3 thin films synthesized utilizing HNO3 solution; however, the measured thermal conductivity was about 0.14 W/(m⋅K). The low value of thermal conductivity was attributed to their amorphous structure. To improve the electrical properties of Bi2Se3 thin films, the electrodeposited samples were subjected to a thermal annealing treatment. An improvement in the electrical properties of the thin films was noticed with an annealing temperature of 300 °C; however, the thermal conductivity also increased to 0.76 W/(m⋅K). Therefore, the dimensionless quantity which represents the performance of the Bi2Se3 thin film, ZT, was determined to be 0.08–0.09. Therefore, the effectiveness of the proposed method for preparing electrodeposited thin films while estimating the thermal conductivity and ZT was successfully demonstrated.

Effect of graphene substrate type on formation of Bi2Se3 nanoplates

Knowledge of nucleation and further growth of Bi2Se3 nanoplates on different substrates is crucial for obtaining ultrathin nanostructures and films of this material by physical vapour deposition technique. In this work, Bi2Se3 nanoplates were deposited under the same experimental conditions on different types of graphene substrates (as-transferred and post-annealed chemical vapour deposition grown monolayer graphene, monolayer graphene grown on silicon carbide substrate). Dimensions of the nanoplates deposited on graphene substrates were compared with the dimensions of the nanoplates deposited on mechanically exfoliated mica and highly ordered pyrolytic graphite flakes used as reference substrates. The influence of different graphene substrates on nucleation and further lateral and vertical growth of the Bi2Se3 nanoplates is analysed. Possibility to obtain ultrathin Bi2Se3 thin films on these substrates is evaluated. Between the substrates considered in this work, graphene grown on silicon carbide is found to be the most promising substrate for obtaining of 1–5 nm thick Bi2Se3 films.