Session J13: Focus Session: Topological Materials - Thin Films

Topological limit of ultrathin quasi-freestanding Bi$_2$Te$_3$ films grown on Si(111)

A fundamental issue for ultrathin topological films is the thickness limit below which the topological surface states become impacted by interfacial interactions. We show that for Bi$_{2}$Te$_{3}$ grown on Si(111) this limit is four quintuple layers (QLs) based on angle-resolved photoemission measurements, using optimized photon energies and polarizations, of the Dirac cone warping and interaction-induced gap as a function of film thickness. The results are close to theoretical predictions for free-standing films, despite the expected strong bonding of the film with the reactive Si(111) substrate. In-situ surface X-ray scattering (SXS) study shows that a buffer layer exist on the Si(111) surface, which effectively saturates all the Si(111) dangling bonds. These interfacial properties, revealed only by diffractions from deeply penetrating X-rays, are critical in understanding the topological surface states in ultrathin films, where electronic coupling is strongly enhanced. Our SXS measurement also yields new information regarding the internal structures of these topological thin films, including layer stacking, QL-by-QL growth, relaxations, etc.   

Transport and Capacitance Measurements of Bi2Se3 Devices

We report electronic transport and capacitance measurements on Bi2Se3 thin-film and exfoliated devices. Strong modulation of the charge carrier density is achieved via the electric field effect with a local top-gate electrode utilizing either high-k dielectric insulators or transferred hexagonal boron nitride. The understanding of ambipolarity due to the electric field effects in these systems is addressed by comparing the modulation of the quantum capacitance and resistance in different devices, accompanied by a model. Additionally, we report capacitance and resistance measurements on these devices at high magnetic fields.   

Photo-galvanic effect in Bi$_{2}$Se$_{3}$ thin films with ionic liquid gating

A key challenge in three dimensional (3D) topological insulators (TIs) is to reveal the helical spin-polarized surface states via electrical transport measurements. A recent study [Nature Nanotech. {\bf 7}, 96 (2012)] showed that circularly polarized light can be used to generate and control photocurrents in the 3D TI Bi$_2$Se$_3$, even at photon energies that are well above the bulk band gap. Symmetry considerations suggest that this ``photo-galvanic effect'' arises purely from photo-currents induced in the surface Dirac states. To gain insights into this phenomenon, we have carried out systematic measurements of the photo-galvanic effect in electrically gated MBE-grown Bi$_2$Se$_3$ thin films of varying thickness. By using an ionic liquid as an optically transparent gate, we map out the behavior of the photo-galvanic effect as a function of Fermi energy over a temperature range 5 K $\leq T \leq$ 300 K.   

Topological insulator engineering of Bi$_{2}$Se$_{3}$ through molecular beam epitaxy

Despite numerous reports proving the presence of the surface states on various topological insulator (TI) materials, all existing TI materials suffer from the bulk conductance problem at various levels. Therefore, achieving a truly insulating bulk state without degrading the surface state in their transport properties is one of the most important tasks of the TI materials research. In this talk, I will present how we address this problem by utilizing various molecular beam epitaxy (MBE) schemes with focus on Bi$_{2}$Se$_{3}$ family of materials. Considering that the bulk conductance problem originates mostly from the selenium vacancies in Bi$_{2}$Se$_{3}$, the typical MBE growth condition characterized by low growth temperature and high selenium vapor pressure is ideal for solving this bulk conductance problem. Moreover, thin films have another advantage of naturally reduced bulk effect due to the enhanced surface-to-bulk ratio. These intrinsic advantages of MBE-grown TI thin films recently led to a number of new findings. High quality Bi$_{2}$Se$_{3}$ thin films did show the expected dominant surface transport characters with negligible bulk conductance. However, the strong tendency toward downward band bending in undoped Bi$_{2}$Se$_{3}$ introduces trivial surface transport channels in addition to the topological surface states, leading to complications in the interpretations of transport results. Furthermore, even if reducing the thickness of TI samples helps reveal the surface transport channels by reducing the bulk contribution, it does not really solve the bulk conductance problem because regardless of how small it may be, the bulk state still remains metallic, shorting the top and bottom surfaces. According to the Mott-criterion of metal-insulator transition, in order to implement a truly insulating bulk state in the current generation TI materials, it is necessary to suppress the defect density below $\sim$ 10$^{14}$ cm$^{-3}$, which might be fundamentally impossible considering the weak Van der Waals bonding character of these materials. However, we have found that it is possible to overcome this limit and achieve a bulk-insulating topological insulator with fully decoupled surface states in thin film TIs.   

Thin films of topological crystalline insulators in IV-VI semiconductors

Topological crystalline insulators (TCI) are new topological states of matter protected by crystalline symmetry of solids. The first example of TCI has been recently predicted and subsequently observed in the SnTe class of IV-VI semiconductors. In this work, we show that thin films of TCI realize a two-dimensional Dirac fermion system with a tunable band gap and host the quantum spin Hall state in an extended thickness range. We propose a ferromagnet-TCI device to measure the spin-dependent transport through helical edge states.   

What Limits Mobility and Carrier Concentration in Epitaxial Topological Insulator Films?

In order to investigate the predicted exotic behavior of topological insulators (TIs) epitaxial films with near ideal electronic properties are essential. Obtaining high quality TI films requires careful control of not only growth parameters but also a good understanding of the dynamics of film formation. We have developed methods to obtain consistently high mobility and low carrier density by carefully controlling the nucleation and growth process of Bi2Se3~epitaxial films. Such MBE grown epitaxial films have been well characterized by different diffraction based techniques and electrical transport to obtain a correlation between structural and electrical properties. This has allowed us to see their systematic dependence. For example, in thin films, carrier density in low 10$^{12}$/cm$^2$ range with bulk mobilities higher than 3000 cm$^2$/V-s are routinely seen which nicely compares very well with structural data.~Acknowledgements: NSF grant DMR 1207469 and NSF DMR 08-19762 (CMSE -- Initiative 2).   

Pairing of the spirals on epitaxially grown Bi2Se3 on Si(111)

Bi$_{2}$Se$_{3}$ is a 3D topological insulator that exhibits backscattering suppression and helical Dirac-like Quasiparticles, making it an ideal candidate for topological physics research. Molecular beam epitaxy (MBE) can control the Se stoichiometry and vacancy density by controlling the Se overpressure during growth, thereby producing bulk insulators, allowing access to the novel physics promised by these systems. We have prepared Bi$_{2}$Se$_{3}$ thin films on Si(111) substrates by MBE. Atomic force microscopy and scanning tunneling microscopy topographies of these films often show large (100 to 500nm in diameter) triangular wedding-cake-shaped islands with spirals on top. More interestingly, the spirals often come in pairs of a clockwise and a counter-clockwise spiral. The high density of spiral pairs suggests that, such a surface structure is thermodynamically more favorable during the MBE. Our ongoing study of the very early stages of the MBE growth is unveiling more information of the spiral pairs. The knowledge of this growth mode will help us improve the sample quality.   

Substrate-Independent Vapor-Solid Growth of Bi2Se3 Nanostructures

We describe a synthesis technique and low-temperature transport measurements of nanostructures of high-purity of topological insulator Bi2Se3. Our growth method is a catalyst-free atmospheric pressure vapor-solid growth, with the use of hydrogen as a carrier gas. It yields abundant amounts of a variety of nanostructures: nanowires, ribbons, platelets, and flakes of different sizes and shapes. Materials analysis shows highly ordered structures of bismuth selenide in all cases. The nanostructures can be used for electronic and optical applications including flexible ones: we show growth results on glass, silicon and flexible mica substrates. Low-temperature measurements of as-grown nanostructures indicate weak-antilocalization and tunable carrier density in all samples. With doping, the transport properties of the samples can be altered to exhibit superconductivity.   

Tunable topological electronic structures in Sb(111) bilayers: A first-principles study

Electronic structure and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations with the inclusion of spin-orbit coupling. While a trivial band insulator is predicted for the free-standing thin film, a band inversion at the Brillouin zone center can be induced by tensile strain, resulting in a topological insulator with a nontrivial topological invariant $Z_2=1$. Our study points at the possibility of realizing the quantum spin Hall state for an Sb(111) single bilayer on a suitable substrate. Moreover, the presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, which breaks the inversion symmetry and lifts the spin degeneracy. A topological phase transition driven by gating is demonstrated, and six spin-polarized Dirac cones are found at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications.   

Topological surface states of Sb thin films adsorbed with impurities

An antimony film is known to exhibit topological surface states depending on the thickness of the film. If the thickness of the film is reduced to as low as four bilayers, for example, Dirac cones disappear as a result of quantum tunneling. We use density functional calculation to investigate the electronic structure of the four-bilayer Sb film and find that adsorptions of non-magnetic impurity atoms of hydrogen, copper, or zinc on the film actually facilitate the formation of Dirac cones that preserve time-reversal symmetry. But magnetic atoms such as iron and manganese do just the opposite. The results suggest the counterintuitive concept of achieving topological conduction by doping nonmagnetic foreign atoms on thin films of topological insulators.   

Medium Energy Ion Scattering investigation of In diffusion in In$_2$Se$_3$/Bi$_2$Se$_3$

In$_2$Se$_3$, a band insulator, and Bi$_2$Se$_3$, a three-dimensional topological insulator, have inherently good chemical and structural compatibility. This suggests possible promising applications of In$_2$Se$_3$/Bi$_2$Se$_3$ devices as tunnel barriers and gate dielectrics. Recently, it has been shown that the similar (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_2$Se$_3$ thin system undergoes a transition from topological insulator to band insulator as a function of In concentration [1]. It is therefore important to understand the extent of In diffusion in In$_2$Se$_3$/Bi$_2$Se$_3$ and its consequences for the transport properties. We have grown In$_2$Se$_3$/Bi$_2$Se$_3$ thin films on sapphire by Molecular Beam Epitaxy at three different temperatures. Medium Energy Ion Scattering measurements of those films showed that the higher growth temperature resulted in more In diffusion while our transport measurements showed that the Bi$_2$Se$_3$ mobility increases as the growth temperature decreases. We found that the trend of the mobility change of In$_2$Se$_3$/Bi$_2$Se$_3$ depending on the diffusion of In is similar with the trend of the mobility of (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_2$Se$_3$ as a function of In concentration [1].\\[4pt] [1] M. Brahlek, et al, Phys. Rev. Lett. 109, 186403 (2012)   

First-principles exploration of high-energy facets of bismuth chalcogenide nanocrystals

Binary bismuth chalcogenides Bi$_2$Se3, Bi$_2$Te$_3$, and derived materials are currently considered as the reference topological insulators (TIs) due to their simple surface-state band structures and relatively large bulk band gaps. Nanostructures of TIs are of particular interest as a large surface-to-volume ratio enhances the contribution of surfaces states. So far, the vast majority of research efforts have focused on the low-energy (111) surfaces which correspond to weak planes in the layered crystal structures. Low-dimensional nanostructures such as nanowires and nanoparticles will inevitably involve higher energy facets. We perform a systematic ab initio investigation of the high-energy surfaces of bismuth chalcogenide TIs characterized by different crystallographic orientations as well as surface reconstructions and stoichiometries. We find several stable surfaces which exist under varying thermodynamic equilibrium conditions. Surface orientation and stoichiometry are found to dramatically affect band dispersion and spin polarization of the topological surface-state charge carriers.   

Vapor-Liquid-Solid Synthesis of Bi$_2$Te$_3$ Nanowires via Metalorganic Chemical Vapor Deposition

Bi$_2$Te$_3$ is a topological insulator and high figure-of-merit thermoelectric material. In the context of thermoelectrics, the synthesis of ultra-thin nanowires could enable more efficient energy conversion technologies due to quantum confinement. We describe a route for the synthesis of Bi$_2$Te$_3$ nanowires using low-pressure metalorganic chemical vapor deposition (MOCVD). The combination of metalorganic precursors trimethyl bismuth and diisopropyl telluride allow a low 350$^\circ$C growth temperature that is conducive to nanowire formation. The nanowires form by VLS growth from gold nanoparticles deposited on a growth substrate. Structural and chemical characterizations of the growth products are presented, indicating that the nanowires are high quality, single crystals of Bi$_2$Te$_3$.