Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A35: Topological Insulators: Growth |
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Sponsoring Units: DCMP Chair: Joel Moore, University of California, Berkeley Room: C140 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A35.00001: Epitaxial Growth of Bi2Se3 Topological Insulator Thin Films on Si (111) Liang He, Faxian Xiu, Yong Wang, Alexei V. Fedorov, Guan Huang, Xufeng Kou, Ward P. Beyermann, Jin Zou, Kang L. Wang We report the studies of Bi2Se3 epitaxial films on Si(111) substrate using molecular beam epitaxial techniques. The structural properties of as-grown films have been investigated by AFM, STM and TEM, which exhibit good crystalline quality and terrace-like quintuple layers on the surfaces. Single-Dirac-cone-like surface states with a linear (E-K) dispersion have been observed through ARPES. Temperature- and thickness-dependent magneto-transport measurements indicate a combination of shallow impurity band hopping and surface-state electron conductions. More significantly, a very high surface contribution up to 50{\%} can be estimated in these ultrathin films, promising a potential applications in nanoelectornics and spintronics. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A35.00002: Topological insulator Bi2Se3 thin film growth by MBE Shuang Li, Yijie Huo, Dong Liang, Thorsten Hesjedal, James Harris Single crystalline high quality Bi2Se3 thin films were growth by molecular beam epitaxy (MBE) on sapphire c-plain substrate in UHV environment. X-ray diffraction (XRD) proved single crystal growth is achieved. Atomic ratio was measured by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. The growth parameters, including substrate temperatures ranging from room temperature to 400$^{\circ}$C, growth rate ranging from 0.5 nm/minute to 10 nm/minute and bismuth and selenium flux ratio, were optimized based on the results from scanning electron microscope (SEM), atomic force microscopy (AFM), XRD, and Raman spectroscopy. Triangle and hexagonal single crystals were preferred in the beginning of the growth at high temperature. More Bi2Se3 growth mechanisms will be discussed in the conference. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A35.00003: Crystal growth and physical property of Bi-Sb-Te-Se topological insulator materials Genda Gu, Zhijun Xu, Weidong Si, Zhihui Pan, Tonica Valla, John Tranquada The discovery of 3D topological insulator materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se and Bi-Sb-Te-Se topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A35.00004: MBE growth of topological insulator Bi2Se3 and Bi2Te3 films Tong Zhang, Niv Levy, Young Jae Song, Jungseok Chae, Joseph A. Stroscio Three-dimensional (3D) topological insulators are a new state of quantum matter with a band gap in bulk but gapless states on the surface. The surface states with spin helicity can be the host of many striking quantum phenomena. In this work, we use ultrahigh vacuum molecular beam epitaxy to grow atomically flat topological insulator (TI) Bi2Se3 and Bi2Te3 films. High quality TI films were obtained using epitaxial graphene on SiC as a substrate for TI growth. The growth dynamics was characterized by real time reflection high-energy electron diffraction (RHEED). The growth condition was optimized by adjusting for proper flux rate and substrate temperature while monitoring the RHEED patterns. In situ Auger spectroscopy and scanning tunneling microscopy (STM) measurements at 5K are used to study the as-grown films for their stoichiometry and defect density. We expect these MBE grown samples will provide a good candidate for studying the topological surface states and related phenomena, which will be studied using scanning tunneling spectroscopy at millikelvin temperatures [1]. 1. Y. J. Song et al., Nature 467, 185 [2010]. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A35.00005: Growth of the topological insulator Bi2Se3 on Al2O3 by molecular beam epitaxy Phillip Tabor, Cameron Keenan, David Lederman, Sergei Urazhdin We report the growth of single crystalline Bi$_2$Se$_3$ on Al$_2$O$_3$ (110) by molecular beam epitaxy. Previous studies utilizing silicon as a substrate demonstrate favorable structural, optical and transport properties, although this can include contributions from the substrate-film interface. In contrast, growth on Al$_2$O$_3$ may influence substrate-film interfacial contributions to structural and electronic properties. Films grown under a range of temperatures and relative selenium to bismuth deposition rates were characterized by ex-situ XPS, XRD, and Hall measurements and will be compared to previous measurements using silicon as a substrate. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A35.00006: Growth of topological insulator Bi2Se3 thin films by the van-der-Waals epitaxy on vicinal Si(111) substrate Z.Y. Wang, H.D. Li, X. Kan, X. Guo, H.T. He, Z. Wang, J.N. Wang, T.L. Wong, N. Wang, M.H. Xie Thin films of Bi2Se3, a three-dimensional topological insulator, have been synthesized by molecular-beam epitaxy with varying thicknesses. Their surface, structural and transport properties have been characterized. For the purpose of lowering the structural defects in film, van-der-Waals epitaxy (vdWe) was adopted in a ``two-step'' growth process, where the initial low-temperature seed layer is followed by a crystalline layer grown at elevated temperatures. Employing vicinal Si(111) substrates, the crystallinity and surface morphology of the epiflm is further improved. Relatively high magnetoresistance along with its linear dependence on the magnetic field at high fields have been observed in the vicinal samples. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A35.00007: Coherent heteroepitaxy of Bi$_2$Se$_3$ on GaAs and ZnSe A. Richardella, D.M. Zhang, J.S. Lee, A. Koser, N. Samarth, A. Yeats, B.B. Buckley, D.D. Awschalom Bi$_2$Se$_3$ is considered to be one to the most promising topological insulator candidate materials currently known because of its 0.3eV bandgap and mid-gap Dirac point. We use molecular beam epitaxy to deposit high quality c-axis oriented single crystal thin films of Bi$_2$Se$_3$ on (111) surfaces of GaAs after the growth of either GaAs or ZnSe buffer layers. Atomic force microscopy reveals films with large single quintuple layer terraces hundreds of nanometers wide. Transmission electron microscopy shows an atomically sharp interface at the heterostructure and narrow X-ray diffraction rocking curves indicate good quality single crystalline growth. We discuss the variation in carrier density, mobility and magnetoresistance with growth conditions. Spatially- and temporally-resolved Kerr spectroscopy allows us to explore coherent electron spin dynamics at the interface between this promising topological insulator and conventional semiconductor heterostructures. Supported by NSF and ONR. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A35.00008: MBE growth of topological insulator Bi$_{2}$Se$_{3}$ on epitaxial graphene on 6H-SiC(0001) Y. Liu, M. Weinert, L. Li In this work, we report results on the MBE growth of Bi$_{2}$Se$_{3}$, a prototypical topological insulator, on epitaxial graphene on 6H-SiC(0001). Step flow growth is observed, characterized by atomically smooth terraces that are 10 to 50 nm in width and separated by steps of 1-2 quintuple-layer in height. Two characteristic peaks at 130.21 and 171.48 cm$^{-1}$ are observed by Raman spectroscopy, corresponding to the in-plane E$_{g}^{2}$ and out-of-plane A$_{1g}^{2}$ vibrational modes, respectively. The close resemblance of the positions and line shapes of both peaks to that of bulk Bi$_{2}$Se$_{3}$ demonstrates the very high quality of the film. Oscillations are also observed near the steps in dI/dV imaging, attesting to the metallic nature of the surface states of the topological insulator Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A35.00009: Transition-metal impurities and intercalation in Bi$_2$Se$_3$ M. Weinert, Y. Liu, L. Li The prototype topological insulator Bi$_2$Se$_3$ consists of 5-layer (QL) units. Using first-principles calculations, we show that even for large (20\%) elongations along the c-axis, the in-plane lattice constant remains essential unchanged and the nearest neighbor bond lengths within a QL vary by only $\sim$0.02\,\AA. These results suggest that impurities may preferentially intercalate between the QLs, possibly leading to $\delta$-doped topological insulator superlattices. For Cu-intercalated Bi$_2$Se$_3$, the calculated separation between QLs slightly contracts ($\sim$2\%), and the Cu intercalation layer provides the internal surfaces necessary for the material to exhibit a Dirac cone. The competition between substitutional impurities and intercalation layers for Cu and Mn will be discussed and compared to experiment. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A35.00010: Epitaxial Bi$_{2}$Se$_{3}$ films on Si (111) with atomically sharp interface Eliav Edrey, Yong Seung Kim, Namrata Bansal, Yoichi Horibe, Seongshik Oh Atomically sharp epitaxial growth of Bi$_{2}$Se$_{3}$ films has been achieved on Si (111) substrate with MBE. The growth was self-limited; that is, growth rate was determined completely by Bi flux with excess Se species around. The Bi:Se flux ratio, measured by QCM, was kept $\sim $1:15. Two step growth temperatures were a key to achieving second-phase-free high quality Bi$_{2}$Se$_{3}$ films on Si substrates. With single-step high temperature growth, second phase, presumably SiSe$_{2}$ clusters, was formed at the early stage of growth. On the other hand, with low temperature growth, crystalline quality of the films was poor even if second phase was absent. With low temperature initial growth followed by high temperature growth, second-phase-free atomically sharp interface was obtained between Bi$_{2}$Se$_{3}$ and Si substrate, as verified by RHEED, TEM and XRD. The lattice constant of Bi$_{2}$Se$_{3}$ relaxed to its bulk value during the first quintuple layer based on the RHEED analysis, implying the absence of strain from the substrate. Single-crystalline XRD peaks of Bi$_{2}$Se$_{3}$ were observed in films as thin as 4 QL. TEM shows full epitaxial structure of Bi$_{2}$Se$_{3 }$film down to the first quintuple layer without any second phases. This growth method was used to grow high quality epitaxial Bi$_{2}$Se$_{3}$ films from 3 QL to 3600 QL. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A35.00011: Robust surface state and bulk carrier density in transport properties of Bi2Se3 films grown with MBE Yong Seung Kim, Namrata Bansal, Eliav Edrey, Mathew Brahlek, Gary A. Kapilevich, Sang-Wook Cheong, Seongshik Oh One of the main predictions of 3D topological insulators (TI) is the existence of a surface metallic state, independent of the sample thickness. However, so far this simple prediction has never been experimentally verified because of significant parallel bulk conduction. Here, we report observation of a robust 2D surface state for MBE-grown thin films in their magneto-transport properties. We also observed that volume carrier density tends to decrease as film gets thicker. Even if a robust 2D surface state exists, its topological protection seems to degrade in thin films due to interference with the bulk carriers, and thus this bulk carrier problem will be the most important next step to solve in order to implement the full topological protection on this surface state. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A35.00012: Controlling the topological states of Bi$_{2}$Se$_{3}$ by silver atom intercalation M. Ye, K. Kuroda, M. Nakatake, S. Kim, Y. Yamada, A. Kimura, K. Miyamoto, M. Arita, T. Okuda, K. Shimada, Y. Ueda, H. Namatame, M. Taniguchi Among the known topological insulators, the layered material, Bi$_{2}$Se$_{3}$, is one of the most promising candidates for potential applications to ultra-low power consumption quantum devices that can work stably at room temperature due to a sufficiently large energy gap in the bulk. The realization of quantum devices generally requires the exposure of the materials to ambient conditions, which significantly disturbs the topological properties through absorption. While intercalation of impurities into layered materials might be thought to be usually detrimental, we show here that that intercalation of Ag into Bi$_{2}$Se$_{3}$ has a benefit. After depositing silver atoms on the surface of Bi$_{2}$Se$_{3}$, massive electrons can be formed on the surface due to the decoupling of the layer-structure by silver intercalation. The newly formed massive electron observed on the surface serves as an evidence of an extremely weak interaction between the decoupled layers and the bulk TI crystal. These results strongly suggest the existence of a new non-trivial boundary state, which opens a pathway to realizing topological insulator-based electronic and spintronic devices, and fault tolerant quantum computation. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A35.00013: Magnetically doped nanoplate crystals of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$ Lukas Zhao, Lin Bo, Limin Huang, Alisa Agafonova, Simon Divilov, Stephen O'Brien, Myriam Sarachik, Lia Krusin-Elbaum The surface states of topological insulators are robustly protected by time-reversal symmetry. Introducing magnetic impurities should open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a \emph{magnetically ordered} insulating state will form, a state distinctly different from the conventional dilute magnetic semiconductors. In thin (quasi-2D) samples, this magnetic order gives rise to a topological electronic structure, with the quantized Hall conductance. Here we report synthesis and electrical and magnetic characterization of Fe and Cr doped \emph{thin} nanoplates of topological insulators Sb$_2$Te$_3$ and Bi$_2$Te$_3$. Nanoplate crystals were grown by catalyst-free vapor-liquid-solid method and were doped using the \textit{in situ} exchange of sources. Low-temperature magnetic, in-plane resistivity, and Hall measurements were performed in magnetic fields up to 9 T fields. The effects of magnetic dopant concentration on susceptibility and charge transport will be discussed. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A35.00014: Atmospheric Doping Affects on the Transport Properties of the Topological Insulator Bismuth Selenide (Bi$_{2}$Se$_{3}$) Grown By MBE Matthew Brahlek, Yong Seung Kim, Namrata Bansal, Eliav Edrey, Seongshik Oh During the last five years much experimental work has been done to determine if the theoretical prediction of topological insulting (TI) states truly exist. Angle resolved photo emission spectroscopy (ARPES) measurements have shown that a Dirac type linear dispersion does exist for a variety of materials, and the surface states have been observed by direct transport measurements. The next challenge is to isolate the surface electrons by removing the bulk conduction. This not trivial because bismuth selenide's Fermi energy sits in the conduction band, and most of the measured carriers are due to these bulk states. The prediction is that the surface states are robust under perturbation, but like standard semiconductors, Bi$_{2}$Se$_{3}$'s bulk states are sensitive to doping. I will report on our work done on how the transport properties of MBE grown Bi$_{2}$Se$_{3}$ thin films are affected by atmospheric dopants such as oxygen and water vapor. Future prospects for studying TIs such Bi$_{2}$Se$_{3}$ and ultimately building a device depend on being able to tune the Fermi level into the gap thereby isolating the surface states, and then passivating the surface against contamination due to atmospheric oxygen and water vapor. [Preview Abstract] |
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