Electronic transport properties of topological insulator films and low dimensional superconductors

In this review, we present a summary of some recent experiments on topological insulators （TIs） and superconducting nanowires and fihns. Electron electron interaction （EEI）, weak anti-localization （WAL） and anisotropic magneto-resistance （AMR） effect fbund in topological insulator fihns by transport measurements are reported. Then, transport properties of superconducting films, bridges and nanowires and proximity effect in non-superconducting nanowires are described. Finally, the interplay between topological insulators and superconductors （SCs） is also discussed.     

Bandgap opening in MoTe2 thin flakes induced by surface oxidation

Recently, the layered transition metal dichalcogenide 1T′-MoTe₂ has generated considerable interest due to their superconducting and non-trivial topological properties. Here, we present a systematic study on 1T′-MoTe₂ single-crystal and exfoliated thin-flakes by means of electrical transport, scanning tunnelling microscope (STM) measurements and band structure calculations. For a bulk sample, it exhibits large magneto-resistance (MR) and Shubnikov–de Hass oscillations in ρ_xx and a series of Hall plateaus in ρ_xy at low temperatures. Meanwhile, the MoTe₂ thin films were intensively investigated with thickness dependence. For samples, without encapsulation, an apparent transition from the intrinsic metallic to insulating state is observed by reducing thickness. In such thin films, we also observed a suppression of the MR and weak anti-localization (WAL) effects. We attributed these effects to disorders originated from the extrinsic surface chemical reaction, which is consistent with the density functional theory (DFT) calculations and in-situ STM results. In contrast to samples without encapsulated protection, we discovered an interesting superconducting transition for those samples with hexagonal Boron Nitride (h-BN) film protection. Our results indicate that the metallic or superconducting behavior is its intrinsic state, and the insulating behavior is likely caused by surface oxidation in few layer 1T′-MoTe₂ flakes.     

Dielectric and relaxation properties of thermally evaporated nanostructured bismuth sulfide thin films

Nanostructured bismuth sulfide thin films were prepared onto glass substrates with particle size of 21 nm by thermal evaporation using readily prepared bismuth sulfide nanocrystallite powder. The X-ray diffraction pattern revealed that bismuth sulfide thin films exhibit orthorhombic structure. The existence of quantum confinement effect was confirmed from the observed band gap energy of 1.86 eV. AC and DC electrical conductivity of Al/BiSnc/Al structures was investigated in the frequency range 0.5-100 kHz at different temperatures (303-463 K) under vacuum. The AC conductivity (σ_ac) is found to be proportional to angular frequency (ω^s). The obtained experimental result of the AC conductivity showed that the correlated barrier hopping model is the appropriate mechanism for the electron transport in the nanostructured bismuth sulfide thin films. DC conduction mechanism in these films was studied and possible conduction mechanism in the bismuth sulfide thin films was discussed.     

Mobility restriction of charge carriers in bismuth films due to film block structure

Electric properties of bismuth thin films on mica substrates depending on their structure and concentration of defects have been studied by atomic force microscopy. The obtained results allow optimization of the production of bismuth films with a structure close to single-crystal by thermal evaporation in vacuum.     

Topological Insulator State in Thin Bismuth Films Subjected to Plane Tensile Strain

The results of experimental examination of galvanomagnetic properties of thin bismuth films subjected to plane tensile strain resulting from the difference in thermal expansion coefficients of the substrate material and bismuth are presented. The resistivity, the magnetoresistance, and the Hall coefficient were studied at temperatures ranging from 5 to 300 K in magnetic fields as strong as 0.65 T. Carrier densities were calculated. A considerable increase in carrier density in films thinner than 30 nm was observed. This suggests that surface states are more prominent in thin bismuth films on mica substrates, while the films themselves may exhibit the properties of a topological insulator.     

Optical properties and adhesion of air oxidized vacuum evaporated bismuth thin films

Highly adhesive bismuth oxide thin films on glass have been prepared by air oxidation of vacuum evaporated bismuth thin films at various temperatures. The transmittance, optical band gap, refractive index and adhesion show temperature and oxidation time effects. The films show a direct band gap between 2 and 2.5 eV. The refractive indices are in the range 1.854-1.991. The transmittances of the bismuth oxide films are quite high in a large wavelength range. These bismuth oxide films can have potential use in optical waveguides.     

Oxidation temperature dependent optical properties of bismuth oxide thin films: Effect of vapour chopping and air exposure

The optical properties of vapour chopped and nonchopped bismuth oxide thin films of two thicknesses 1500 Å and 2000 Å have been studied. The films were prepared by thermal oxidation in air; of vacuum evaporated vapour chopped and nonchopped bismuth thin films. As revealed by XRD studies, multiphase and polycrystalline bismuth oxide thin films were obtained. The refractive index was found to increase with the thickness and exposure to air for 40 days. The vapour chopped films showed higher refractive index, band gap and lower grain size than those of nonchopped films. The films showed high transmittance in the visible spectrum. The ageing effect on the vapour chopped films was found low.     

Structure and electrical transport properties of bismuth thin films prepared by RF magnetron sputtering

Bismuth thin films were prepared on glass substrates with RF magnetron sputtering and the effects of deposition temperature on surface morphology and their electrical transport properties were investigated. Grain growth of bismuth and the coalescence of grains were observed above 393 K with field emission secondary electron microscopy. Continuous thin films could not be obtained above 448 K because of the segregation of grains. Hall effect measurements showed that substrate heating yields the decrease of carrier density and the increase of mobility in exponential ways until 403 K. Resistivity of sputter deposited bismuth films has its minimum (about 0.7 × 10⁻³ Ω cm) in range of 403-433 K. Annealing of bismuth films deposited at room temperature was carried out in a radiation furnace with flowing hydrogen gas. The change of resistivity was not significant due to the cancellation of the decrease of carrier density and the increase of mobility. However, the abrupt change of electrical properties of film annealed above 523 K was observed, which is caused by the oxidation of bismuth layer.     

Classical in-plane negative magnetoresistance and quantum positive magnetoresistance in undoped InSb thin films on GaAs (1 0 0) substrates

Magnetoresistance (MR) effects have been investigated in perpendicular and parallel magnetic fields at 300, 80 K and liquid He temperatures for undoped InSb thin films 0.1–2.3 μm thick grown on GaAs(1 0 0) substrates by MBE. At high temperatures, the intrinsic carriers show the parabolic negative MR observable only in magnetic fields parallel to the film. The skipping-orbit effect due to surface boundary scattering in the classical orbits in the plane vertical to the film has been argued to be responsible for the negative MR. At low temperatures (T=80 K), the transport is dominated by the two-dimensional (2D) electrons in the accumulation layers at the InSb/GaAs(1 0 0) hetero interface; MR is positive and shows a logarithmic increase with anisotropy between parallel and perpendicular field orientation, arising from the 2D weak anti-localization (WAL) that reflects the interplay between the spin-Zeeman effect and strong spin–orbit interaction caused by the asymmetric potential at the interface (Rashba term). The zero-field spin splitting energy of Δ₀13 meV, the electron effective mass of m^*0.10m₀ seven times of the band edge mass in bulk InSb and the effective g-factor of |g^*|15 in the accumulation layer have been inferred from fits of MR for the 0.1 μm thick film to the 2D WL theory.     

基底对光学薄膜弱吸收测量的影响

 对表面热透镜技术测量光学薄膜弱吸收低频调制时不同基底对测量的影响进行了理论分析。用Lambda—900分光光度计测量了K9和石英基底的Ti₃O₅单层膜的吸收值,将该组样品作为定标片;用表面热透镜装置分别测量了BK7和石英空白基底及HfO₂,ZnO两组不同基底不同厚度单层膜样品的吸收。通过分析比较同一工艺条件下镀制的不同基底薄膜样品用与其同种和不同种基底定标片定标测量的结果,表明在低频测量时需要用与测量样品同种基底的定标片定标;不同厚度样品的测量结果表明,在不能严格满足热薄条件时,测量结果需引入修正值。     

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi₂Se₃ thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi₂Se₃ grew on amorphous Al₂O₃ even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi₂Se₃ channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.     

Properties of NiO thin films deposited by intermittent spray pyrolysis process

NiO thin films have been grown on glass substrates by intermittent spray pyrolysis deposition of NiCl₂·6H₂O diluted in distilled water, using a simple “perfume atomizer”. The effect of the solution molarity on their properties was studied and compared to those of NiO thin films deposited with a classical spray system. It is shown that NiO thin films crystallized in the NiO structure are achieved after deposition. Whatever the precursor molarity, the grain size is around 25-30 nm. The crystallites are preferentially oriented along the (1 1 1) direction. All the films are p-type. However, the thickness and the conductivity of the NiO films depend on the precursor contraction. By comparison with the properties of films deposited by classical spray technique, it is shown that the critical precursor concentration, which induces strong thin films properties perturbations, is higher when a perfume atomizer is used. This broader stability domain can be attributed to better chlorides decomposition during the rest time used in the perfume atomizer technique.     

Ferromagnetism in carbon-doped In2O3 thin films

Carbon-doped In₂O₃ thin films exhibiting ferromagnetism at room temperature were prepared on Si (100) substrates by the rf-magnetron co-sputtering technique. The effects of carbon concentration as well as oxygen atmosphere on the ferromagnetic property of the thin films were investigated. The saturated magnetizations of thin films varied from 1.23 to 4.86 emu/cm³ with different carbon concentrations. The ferromagnetic signal was found stronger in samples with higher oxygen vacancy concentrations. In addition, deposition temperature and different types of substrates also affect the ferromagnetic properties of carbon-doped In₂O₃ thin films. This may be related to the oxygen vacancies in the thin film system. The experiment suggests that oxygen vacancies play an important role in introducing ferromagnetism in thin films.     

Effect of a Substrate on the Magnetoelectric Effect in Rare-Earth-Doped Bismuth Iron Garnet

JETP Letters - The mechanism of relaxation of the electric polarization in thin films of rare-earth-doped bismuth iron garnet on glass and gallium gadolinium garnet substrates is determined in...     

Improved Polarization Retention of BiFeO_3 Thin Films Using GdScO_3 (110) Substrates

Epitaxial ferroelectric thin films on single-crystal substrates generally show a preferred domain orientation in one direction over the other in demonstration of a poor polarization retention. This behavior will affect their application in nonvolatile ferroelectric random access memories where bipolar polarization states are used to store the logic 0 and 1 data. Here the retention characteristics of BiFeO_3 thin films with SrRuO_3 bottom electrodes on both GdScO_3(110) and SrTiO_3(100) substrates are studied and compared, and the results of piezoresponse force microscopy provide a long time retention property of the films on two substrates. It is found that bismuth ferrite thin films grown on GdScO_3 substrates show no preferred domain variants in comparison with the preferred downward polarization orientation toward bottom electrodes on SrTiO_3 substrates. The retention test from a positive-up domain to a negative-down domain using a signal generator and an oscilloscope coincidentally shows bistable polarization states on the GdScO_3 substrate over a measuring time of 500 s, unlike the preferred domain orientation on SrTiO_3, where more than 65% of upward domains disappear after 1 s. In addition, different sizes of domains have been written and read by using the scanning tip of piezoresponse force microscopy; where the polarization can stabilize over one month. This study paves one route to improve the polarization retention property through the optimization of the lattice-mismatched stresses between films and substrates.     

Interaction effects in magnetic oxide nanoparticle systems

The interaction effects in magnetic nanoparticle system were studied through a Monte Carlo simulation. The results of simulations were compared with two different magnetic systems, namely, iron oxide polymer nanocomposites prepared by polymerization over core and nanocrystalline cobalt ferrite thin films prepared by sol-gel process. The size of the particles in the nanocomposites were estimated to be ∼15 nm with very little agglomeration. The low values of the coercivity obtained from the hysteresis measurements performed confirm that the system is superparamagnetic. SEM studies showed the cobalt ferrite films to have a nanocrystalline character, with particle sizes in the nanometer range. Hysteresis measurements performed on the thin films coated on silicon do not give evidence of the superparamagnetic transition up to room temperature and the coercivity is found to increase with decreasing film thickness. Comparison with simulations indicate that the nanocomposites behave like a strongly interacting array where exchange interactions lead to high blocking temperatures, whereas the films are representative of a semi-infinite array of magnetic clusters with weak interactions and thickness-dependent magnetic properties.     

三维拓扑绝缘体antidot阵列结构中的磁致输运研究

利用聚焦离子束刻蚀技术在拓扑绝缘体Bi_2Se_3薄膜中刻蚀了纳米尺度的反点(antidot)阵列,并对制作的三个器件进行了系统的电学输运测量研究.低温下,所有器件中都观察到明显的弱反局域化效应.通过对弱反局域化效应的分析,发现器件一(Dev-1,不含有antidot阵列)和器件二(Dev-2,含有周期较大的antidot阵列)是始终由一个导电通道主导的量子输运系统,但在器件三(Dev-3,含有周期较小的antidot阵列)中能明确观察到较低温度下存在两个独立的导电通道,而在较高温度下Dev-3表现为由一个导电通道主导的量子输运系统.     

Deposition of highly oriented Bi12SiO20 thin films on Y-stabilized zirconia and SiO2 by pulsed-laser deposition

We describe the deposition and characterization of Bi₁₂SiO₂₀ (bismuth silicon oxide; BSO) thin films on Y-stabilized zirconia (YSZ) and SiO₂ glass substrates by pulsed-laser deposition (PLD) for the application of an electric field sensor. It was found that all films deposited on YSZ substrates heated at 400 °C and more were crystallized and the (310) plane was perpendicular to the substrate normal. The highly (310) oriented crystallized films were even deposited on SiO₂ glass substrates, and this will make it possible to grow the crystallized films on the end surface of a SiO₂ glass fiber.     

Transport properties of nanoperforated Nb thin films

Porous silicon, obtained by electrochemical etching, has been used as a substrate for the growth of nanoperforated Nb thin films. The films, deposited by UHV magnetron sputtering, inherited from the Si substrates their structure, made of holes of 10 nm diameter and of 20 and 40 nm spacing, which provide an artificial pinning lattice. Commensurability effects between the Abrikosov vortex lattice and the artificial array of holes were investigated by transport measurements.     

Electrochemically deposited bismuth telluride thin films

Thin films of bismuth telluride grown by electrochemical deposition technique on conducting glass and Mo sheet substrates, were characterized for their structural, morphological, optical and compositional analysis. These studies revealed polycrystalline anisotropic and layered structure of these films with different compositional stoichiometry. In the present work electrochemical deposition of bismuth telluride thin films is studied as a dopant material in II–VI group absorber materials for photovoltaic application since it has a narrow optical energy band gap of 0.13 eV. In this deposition process different film growth parameters were optimized to get good quality of compositionally uniform bismuth telluride thin film. XRD analysis revealed a hexagonal symmetry with large c-axis lattice constants (Bi₂Te₃, Bi_2+XTe_3−X).