Session D31: Focus Session: Topological Insulators: Synthesis & Characterization - Thin Films

Epitaxial growth of high quality Bi$_{2}$Se$_{3}$ thin films on CdS

We report the experiment of high quality epitaxial growth of Bi$_{2}$Se$_{3}$ thin films on lattice-matched hexagonal CdS (0001) substrates using a solid source molecular-beam epitaxy system. Layer-by-layer growth of single crystal Bi2Se3 has been observed from the first quintuple layer with larger surface triangular terraces. The improved film quality facilitates the characterization of surface states during magneto-transport measurements, such as high Hall mobility of $\sim $6000 cm2/V?s, a distinct Shubnikov-de Haas (SdH) oscillations and weak anti-localization cusp in the magnetic field dependent longitudinal resistance. These characteristics of Bi$_{2}$Se$_{3}$ thin films promise a variety of potential applications in ultra-fast, low-power dissipation devices.   

Fabrication of High Quality Topological Insulator Thin Films and Heterostructures

In this talk, I will present a method of fabrication high quality topological insulator thin films and heterostructures with ferromagnet materials using MBE with a RF Selenium cracker cell and pulsed laser deposition. I will also show some preliminary results on the physical properties of those films, including topography, crystal structure and transport properties.   

MBE growth and transport of the topologically tunable (Bi$_{1-x}$ In $_{x})_{2}$Se$_{3}$ system

A current challenge in the field of topological insulators (TI) is identifying a clear transport signal of the surface conduction. The structural similarity between Bi$_{2}$Se$_{3}$ and In$_{2}$Se$_{3}$ allowed us to combine the two to obtain (Bi$_{1-x}$ In $_{x})_{2}$Se$_{3}$; Bi$_{2}$Se$_{3}$ has inverted bands, and thus is a non-trivial insulator. In$_{2}$Se$_{3}$ has no inverted bands and is therefore a trivial band insulator with energy gap 1.3-1.9eV. The mixing ratio x can be thought of as a knob to switch the system from a trivial to a non-trivial state. I will briefly discuss our scheme for producing atomically smooth molecular beam epitaxial grown thin films. I will also discuss our work on transport in the TI-to-non TI regime, and the metal to insulator regime, and compare these results with angle resolved photo emission spectroscopy data.   

Hybrid Physical-Chemical Vapor Deposition of Bi$_{2}$Se$_{3}$ Thin films on Sapphire

High quality thin films of topological insulators continue to garner much interest. We report on the growth of highly-oriented thin films of Bi$_{2}$Se$_{3}$ on c-plane sapphire using hybrid physical-chemical vapor deposition (HPCVD). The HPCVD process utilizes the thermal decomposition of trimethyl bismuth (TMBi) and evaporation of elemental selenium in a hydrogen ambient to deposit Bi$_{2}$Se$_{3}$. Growth parameters including TMBi flow rate and decomposition temperature and selenium evaporation temperature were optimized, effectively changing the Bi:Se ratio, to produce high quality films. Glancing angle x- ray diffraction measurements revealed that the films were c-axis oriented on sapphire. Trigonal crystal planes were observed in atomic force microscopy images with an RMS surface roughness of 1.24 nm over an area of 2$\mu $mx2$\mu $m. Variable temperature Hall effect measurements were also carried out on films that were nominally 50-70 nm thick. Over the temperature range from 300K down to 4.2K, the carrier concentration remained constant at approximately 6x10$^{18 }$cm$^{-3}$ while the mobility increased from 480 cm$^{2}$/Vs to 900 cm$^{2}$/Vs. These results demonstrate that the HPCVD technique can be used to deposit Bi$_{2}$Se$_{3}$ films with structural and electrical properties comparable to films produced by molecular beam epitaxy.   

Pulsed Laser Deposition of Epitaxial Topological Insulator Thin Films: Bi$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se

While high quality epitaxial thin films of topological insulators have been achieved by molecular beam epitaxy, there has been little progress using other thin film growth techniques. Here, we report the growth of high quality epitaxial Bi$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se thin films on silicon (111) and YSZ (111) substrates by pulsed laser deposition (PLD). Systematic structural characterization of the films using x-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving high quality epitaxial films. Rutherford backscattering spectrometry measurements suggest that the film composition is strongly influenced by the growth temperature and background gas pressure. The electrical transport properties of the films grown at the optimal conditions will also be discussed. Since PLD is an excellent tool to grow a variety of functional oxides, including multiferroics, magnetic semiconductors and high temperature superconductors, the growth of epitaxial topological insulator thin films by the same technique paves the way to synthesize multi-layered heterostructures of the above materials and search for novel physics arising from the resulting interfacial couplings.   

Growth and properties of half-Heusler DyPdBi

Some half-Heusler phases have been predicted by Chadov, \textit{et al}, to exhibit topological electronic structure. In addition to providing an exciting new topological system, the breadth of the elemental parameter space for this system opens the door for investigation of the interplay between many novel physical states with the topological system. We have grown thin films of one of these phases, the cubic half-Heusler material DyPdBi, using carefully flux matched molecular beam epitaxy. Crystalline quality was monitored via \textit{in situ} RHEED and verified by \textit{ex situ} x-ray diffraction measurements. Transport measurements indicate the emergence of interesting correlated behavior at low temperature.   

Vacancy Reduction, Structural and Electronic Studies of Epitaxial Films of Topological Insulators

We have developed methods for controlling the carrier concentration via a vacancy concentration reduction procedure in the MBE grown epitaxial topological insulator (TI) thin film on various substrates to reach the intrinsic features of TI. Our single crystalline TI thin films allowed us to systematically investigate the nature of coherent transport in this system. For structural characterization of TI thin films, various non-distractive methods, such as x-ray and electron based diffraction techniques, were used as a local probe to understand the long-, short-range atomic ordering and also lattice site occupation. Besides the improved electronic properties of the layers, as grown crystalline films density increased by 20{\%} due to controlled vacancy reduction, determined by in-situ x-ray diffraction. Furthermore, correlation of vacancies and Se ion migration was observed to be the likely reason for lowering the carrier concentration. Our study also shows the dependence of carrier mobility and the vacancy concentration which has been optimized.   

Growth and in-situ ultra-high resolution ARPES studies of the Bi-Te family of topological insulators

Topological insulators have received intense focus in the condensed matter community due to their academic and technical potential. The Quantum Anomalous Hall state is an example of the exotic physics that could have a major industry impact if it can be realized and controlled. While the topologically protected states live at interfaces between insulators of two topological classes, investigations of the underlying electronic structure via angle resolved photoemission spectroscopy requires pristine surfaces. Here we present results from in situ ultra-high resolution laser ARPES investigations at low temperatures of the doped Bi-Te family of topological insulator thin films grown via molecular beam epitaxy. Electronic structure evolution as a function of dopant, dopant level and thickness will be presented and compared to theoretical predictions.   

Fabrication of Bismuth Selenide Topological Insulating Samples

In this talk, I will discuss fabrication of nanometric topological insulator Bi$_{2}$Se$_{3}$ devices. Our group uses two fabrication methodologies: Epitaxial thin films and single crystal exfoliation. I will discuss the benefits and drawbacks of each methodology. I will also address the effects on device performance by various steps of the fabrication process.   

Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer on Bi$_2$Te$_3$

Topological insulators (TI) are insulating materials but have metallic edge states that carry spin currents and are robust against nonmagnetic impurities [1]. While there have been a large number of reports on three-dimensional (3D) TI, only few works have been done in terms of two-dimensional (2D) TI. In the present paper, we report the successful formation of bilayer Bi, which was theoretically predicted to be a 2D TI [2]. We deposited bilayer Bi on a 3D TI $\mathrm{Bi_2Te_3}$, which the lattice mismatch is very small. From angle-resolved photoemission spectroscopy measurements and {\it ab initio} calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and $\mathrm{Bi_2Te_3}$. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface [3]. \\[0pt] [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. {\bf 82}, 3045 (2010).\\[0pt] [2] S. Murakami, Phys. Rev. Lett. {\bf 97}, 236805 (2006).\\[0pt] [3] T. Hirahara {\it et al.,} Phys. Rev. Lett. {\bf 107}, 166801 (2011).   

Fabrication of Bi$_{2}$Te$_{3}$ Nanodots by Droplet Epitaxy on GaAs substrates

Bi$_{2}$Te$_{3}$, as a three-dimensional topological insulator, causes wide attention. Here, we report the fabrication of Bi$_{2}$Te$_{3}$ nanodots on GaAs substrate by droplet epitaxy using molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and Raman measurement revealed the existence of Bi$_{2}$Te$_{3}$ nanodots. Several approaches were developed to control the size and density of as-grown Bi$_{2}$Te$_{3}$ nanodots. Temperature and density dependent magneto-transport measurements were investigated. This may provide a platform for the interaction investigation among topological insulators, semiconductors, ferromagnets and superconductors.   

AFM, Raman and electrical transport studies of topological insulating materials subjected to argon plasma etching

Plasma etching is an important tool in nano-device fabrication. We report a study on argon plasma etching of exfoliated flakes of topological insulator materials Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se. We present data from atomic force microscopy (AFM), Raman spectroscopy and low-temperature magneto-transport measurements. The thickness of our samples measured by AFM is observed to decrease approximately linearly with plasma exposure time. We extract an etching rate for each type of material. For the initial increase in plasma exposure time, we observe increasing intensity and width of the characteristic E$_{g}$$^{2}$ Raman peak with little change in peak position. The width of this peak for etched flakes becomes larger than those of unetched samples of the same thickness. Additionally, we find that even moderate etching can significantly reduce the conductivity and hall mobility. These results indicate disorder is generated by plasma etching and impedes both phonon and carrier transport. Our findings are valuable for understanding the effects of argon plasma etching on topological insulator materials and using irradiation as a potential method to introduce controlled disorder in such materials.   

Tuning the spatial location of topological surface states via proximity effects

In order to exploit promising applications of topological insulators in quantum computing, spintronics, and catalysis, one prerequisite is to gain effective manipulation of the spatial distribution of the topological surface states (TSS). We use first-principles calculations to investigate the interfacial proximity effects on the TSS for hybrid systems consisting of semiconducting thin films with different bandgaps, spin-orbital coupling (SOC) strengths,~and lattice mismatches grown on the TI substrate of Bi$_{2}$Se$_{3}$. Our results show that the spatial location of the robust TSS can be tuned by the interplay of the effects associated with the SOC strength and the band gap size of the semiconductor. Potential experimental confirmations of these strong predictions are also discussed.   

Metal-Supported High Crystalline Bi$_{2}$Se$_{3}$ Quintuple Layers

Atomically flat thin films of Bi$_{2}$Se$_{3}$ were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple-layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by X-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using X-ray photoemission spectroscopy. Moir\'e patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi$_{2}$Se$_{3}$ and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi$_{2}$Se$_{3}$ devices.   

Ab initio study of epitaxial graphene on top of Sb$_{2}$Te$_{3}$ topological insulator

Understanding topological phase as observed in Dirac materials such as graphene and topological insulator (TI) has been a central issue in the field of condensed matter physics. Graphene and TI exhibit unique 2D electronic structures that attract great attention for potential application to spintronic devices. Heterostructures of graphene and TI provide interesting platforms to explore exotic electronic and transport properties of Dirac materials. Electronic structures of graphene in contact with TI were investigated using first-principles methods and tight-binding models. The Dirac cones of graphene on top of TI surface show several interesting features including band-gap opening and band splitting. By fitting first-principles calculations to tight-binding models, we analyzed the origin of the changes in the Dirac cones. We found that both intrinsic and extrinsic spin-orbit couplings are enhanced significantly due to proximity to topological insulator Sb$_{2}$Te$_{3}$ and that graphene turns into quantum spin-Hall phase. Our results suggest that graphene is also useful as a probe of TI surface states.