Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y16: Silicene, Germanene, and BeyondFocus
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Sponsoring Units: DMP Chair: Sufei Shi, Rensselaer Polytechnic Institute Room: 315 |
Friday, March 18, 2016 11:15AM - 11:51AM |
Y16.00001: Graphene challengers: silicene, germanene and stanene, group IV elemental synthetic electronic materials. Invited Speaker: Guy Le Lay Silicene, germanene and stanene, graphene's group IV elemental cousins, have attracted considerable interest since the birth of silicene in 2012 [1]. These novel synthetic two-dimensional (2D) Si, Ge and Sn allotropes are artificially created in situ under ultra high vacuum, since, at variance with graphene, which descents from graphite, they have no parent crystal in nature. They are considered as promising candidates for ultimate scaling of nanoelectronic devices [2,3]. Indeed, the recent fabrication of the first silicene field effect transistors with ambipolar characteristics operating at room temperature demonstrates their potential as emerging 2D electronic materials [4]. In this invited talk, I will present the archetype 3x3 silicene phase formed on a silver (111) substrate [1], its sister phases and the growth of multilayer silicene, which hosts Dirac fermions and which is stable in ambient air, protected by its ultra-thin native oxide [5]. The recent synthesis of single layer germanene [6,7] and stanene [8], near room temperature 2D topological insulators will be also presented, while multilayer germanene will be further addressed. Challenging graphene, silicene, germanene and stanene, which are directly compatible with the current semiconductor industry, could lead to the development of a new class of low energy consumption nanoelectronic devices. 1. P. Vogt et al., Phys. Rev. Lett., 108, 155501 (2012). 2. A. Dimoulas, Microelectronic Engineering, 131, 68 (2015). 3. G. Le Lay, Nature Nanotechnology, 10, 202 (2015). 4. Li Tao et al., Nature Nanotechnolgy, 10, 227 (2015). 5. P. De Padova et al., 2D Mater., 1, 021003 (2014). 6. M.E. Davila et al., New J. Phys., 16, 095002 (2014). 7. M. Derivaz et al. Nano Lett., 15, 2510 (2015). 8. Feng-feng Zhu et al., Nature Mater., 14, 1020 (2015). [Preview Abstract] |
(Author Not Attending)
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Y16.00002: Prediction of a quantum anomalous Hall state in Co-decorated silicene. Thaneshwor Kaloni, Georg Schreckenbach, Michael Freund Based on first-principles calculations, we demonstrate that Co-decorated silicene can host a quantum anomalous Hall state. The exchange field induced by the Co atoms combined with the strong spin-orbit coupling of the silicene opens a nontrivial band gap at the K point. As compared to other transition metals, Co-decorated silicene is unique in this respect, since usually hybridization and spin-polarization induced in the silicene suppress a quantum anomalous Hall state. [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y16.00003: Ordered structure upon deposition of Ge on the monolayer silicene on Ag(111) Han-De Chen, Dengsung Lin The growth of monolayer silicene on Ag (111) has been a hot research in recent years. The akin structure of the same group IV element: Germanene, has also been grown successfully on different metal substrates. In this investigation, Ge has been deposited by molecular beam epitaxy on the monolayer-thick silicene grown on Ag(111). Low-temperature scanning tunneling microscopy ( LT-STM ) has been employed to observed the surface morphology and atomic structure. On the ( 3 x 3 )Si phase, only one Ge adatom is found on each ( 3 x 3 )Si unit cell on two different sites, A and B. The deposited Ge adatoms prefer to settle around a unit cell that has already incorporated one Ge adatom, thereby forming two domains ( 3 x 3 )A and ( 3 x 3 )B. Results on ( r7 x r7 )Si superstructure showing local ordering will also be presented. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y16.00004: Germanene-like defects in Reverse Monte Carlo model of amorphous germanium revealed through new visualization method Al Rahemtulla, Bruno Tomberli, Edward Kim, Sjoerd Roorda, Stefan Kycia High Resolution x-ray diffraction experiments of amorphous germanium (a-Ge) revealed structural differences that cannot be reconciled with accepted theoretical models. An intuitive computational technique has been developed to construct 3D statistical density maps to directly resolve local atomic structure of a-Ge. A reverse monte carlo routine is used to compare the continuous random network model to the experimental model of a-Ge. Undercoordination in the refined model is shown to exist bimodally, as a $4$-coordinated tetrahedron and a buckled $3$-coordinated structure similar to germanene. These structures account for $95.7\%$ of the total atoms in an approximate $5$:$2$ ratio respectively. [Preview Abstract] |
Friday, March 18, 2016 12:27PM - 1:03PM |
Y16.00005: Room Temperature Silicene Field-Effect Transistors Invited Speaker: Deji Akinwande Silicene, a buckled Si analogue of graphene, holds significant promise for future electronics beyond traditional CMOS. In our predefined experiments via encapsulated delamination with native electrodes approach, silicene devices exhibit an ambipolar charge transport behavior, corroborating theories on Dirac band in Ag-free silicene. Monolayer silicene device has extracted field-effect mobility within the theoretical expectation and ON/OFF ratio greater than monolayer graphene, while multilayer silicene devices show decreased mobility and gate modulation. Air-stability of silicene devices depends on the number of layers of silicene and intrinsic material structure determined by growth temperature. Few or multi-layer silicene devices maintain their ambipolar behavior for days in contrast to minutes time scale for monolayer counterparts under similar conditions. Multilayer silicene grown at different temperatures below 300$^{\mathrm{o}}$C possess different intrinsic structures and yield different electrical property and air-stability. This work suggests a practical prospect to enable more air-stable silicene devices with layer and growth condition control, which can be leveraged for other air-sensitive 2D materials. In addition, we describe quantum and classical transistor device concepts based on silicene and related buckled materials that exploit the 2D topological insulating phenomenon. The transistor device physics offer the potential for ballistic transport that is robust against scattering and can be employed for both charge and spin transport. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y16.00006: Nearest Neighbor Hopping conduction observed with ionic liquid induced silicon surface states JJ Nelson, K. V. Reich, M. Sammon, B. I. Shklovskii, A. M. Goldman A two-dimensional hole gas can be created on the surface of a bulk Si wafer by using an ionic liquid in an electric double layer transistor configuration (EDLT). EDLTs are useful in observing metal to insulator and superconductor to insulator transitions due to record high carrier densities of 10$^{\mathrm{15\thinspace }}$cm$^{\mathrm{-2}}$ that can be achieved. In some cases the high carrier densities are due in part to oxidation of the sample surface. With an EDLT configuration we have observed a 2D insulator-to metal transition with low mobility Si at the highest reported critical carrier density. [J. Nelson and A. M. Goldman Phys. Rev. B 91, 241304(R) (2015)] The experiment reported here is designed to promote electrostatic carrier induction over electrochemical reactions and is focused on carrier densities near 10$^{\mathrm{11\thinspace }}$cm$^{\mathrm{-2}}$. At such a low densities we observe nearest neighbor hopping conduction on the surface of Si.[J. Nelson et al., Phys. Rev. B 92, 085424 (2015)] This observation suggests that the ionic liquid covering the surface should be treated as a series of discrete charges that can act as a platform to better understand EDLT physics at higher carrier densities. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y16.00007: Gapped Dirac cone in silicene and germanene on Al$_2$O$_3$(0001) Mingxing Chen, Michael Weinert Developing guidelines to find promising substrates that can stabilize the monolayer honeycomb structures of silicene and germanene while simultaneously preserving the Dirac-electron-driven properties is of practical importance for applications. From first-principles calculations, we find that silicene on Al-terminated Al$_2$O$_3$(0001) retains the main structural profile of the ideal low-buckled silicene with a binding energy comparable to that of silicene on Ag(111). Unfolded $k$-projected bands reveal that a gapped Dirac cone is formed at the K point. The underlying mechanism is that the substrate has a large energy gap and the workfunctions are such that there is little direct bonding of between the silicene Dirac states and the substrate, which further guides us to find gapped Dirac states in germanene on Al$_2$O$_3$(0001). [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y16.00008: Single and Few Layer Silicene: Structural, Electronic and Transport Properties J David Carey, Nathanael Roome Single layer silicene has weaker $\pi $ bonding that graphene resulting in buckling of the Si atoms in different sub-lattices. Despite the loss of planarity, a linear bandstructure emerges where we find a Fermi velocity of about 5.3 x 10$^{5}$ m/s. Determination of the phonon dispersion characteristics reveals a $\Gamma $ point optical phonon with an energy of 69 meV and a K point optical phonon with an energy of 62 meV. In graphene these phonons play important role in scattering electrons, and in Raman spectroscopy, but have larger energies of 194 and 166 meV, respectively. The lower phonon energies in silicene, arising from the higher atomic masses, would be expected to scatter carriers efficiently and limit carrier mobility. We have calculated, however, that the electron-optical phonon coupling matrix elements are about a factor of 25 times smaller than in graphene and this important result will help with the further development of silicene based devices due to reduced phonon scattering. The two stable stacking configurations of bilayer silicene, AA and AB, now have to account of the position of the atomic buckling in the two layers, leading to four possible atomic configurations with the buckling between the layers being in- or out-of-phase with each other. We find that in contrast to graphene, the two stable configurations are based on AA type stacking being about 70 meV per atom more stable than AB stacking. The potential for elemental layered materials beyond graphene for device applications will also be discussed. [Preview Abstract] |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y16.00009: Two-dimensional group-IV monochalcogenides: structural, electronic and optical properties. Lidia Gomes, Alexandra Carvalho, A. H. Castro Neto Two-dimensional materials have attracted a massive attention of the scientific and industrial communities due to their unusual and interesting properties. The layered group-IV monochalcogenides-SnS, SnSe, GeS and GeSe- has gained attention as a promising group with potentially useful applications in diverse fields. The bulk SnS, a naturally occurring mineral, has been considered as an alternative to be used in film PV cells, due to its electronic and optical properties. We use first principles calculations to explore structural, electronic and optical properties of this group, with focus in their two-dimensional forms. We show that all those binary compounds are semiconducting, with bandgap energies covering most of the visible range. They have multiple valleys in the valence and conduction bands, with spin-orbit splitting of the order of 19-86 meV. An enhanced static dielectric permittivity is found for the monolayers. Structural analysis shows that the 2D form of these materials presents very high piezoelectric constants, exceeding values recently observed for other 2D-systems. The existence of a negative Poisson ratio is predicted for the GeS compound. [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y16.00010: Scattering mechanisms in shallow undoped Si/SiGe quantum wells Dominique Laroche, Shih-Hsien Huang, Erik Nielsen, Yen Chuang, Jiun-Yun Li, Chih-Wen Liu, Tzu-Ming Lu We report the magneto-transport and scattering mechanism analysis of a series of increasingly shallow Si/SiGe quantum wells with the shallowest 2DEG located only $\sim$ 10 nm away from the surface. The peak mobility increases with increasing depth, suggesting that charge centers near the oxide/semiconductor interface is the main source of disorder. The power-law exponent of the mobility versus density curve, $\mu \propto n^{\alpha}$ , is extracted as a function of the depth. At intermediate densities, the power-law dependence is characterized by $\alpha \sim 2.3$ while at the highest achievable densities for devices with intermediate depth, an exponent $ \alpha \sim$ 5 is observed. We propose, and show by simulations, that this increase in $\alpha$ is explained by a non-equilibrium model where electrons migrating to the surface smooth out the potential landscape seen by the 2DEG. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (DOE). Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE’s National Nuclear Security Administration under contract DE-AC04-94AL [Preview Abstract] |
Friday, March 18, 2016 2:03PM - 2:15PM |
Y16.00011: Silicene-type Surface Reconstruction on C40 Hexagonal Silicides Cameron Volders, Petra Reinke Silicene has emerged as the next two-dimensional material possessing a Dirac type electronic structure making it a prime candidate for integration in electronic devices. The study of silicene is relatively new and many aspects have yet to be fully understood. Here we present a scanning tunneling microscopy (STM) study of a Silicene-type surface reconstruction observed on nanometer scale hexagonal-MoSi2 crystallites. This surface reconstruction is specific to the C40 structure of h-MoSi2 and can initially be defined as a geometric silicene while the coupling between the silicene surface and the silicide bulk is under investigation. The lateral dimensions correspond to a superstructure where the silicene hexagons are slightly buckled and two of the six Si atoms are visible in the STM images creating a honeycomb pattern. The local electronic structure of the silicene is currently being studied with ST spectroscopy and the impact of confinement will be addressed. These results open an alternative route to Silicene growth by using surface reconstructions on metallic and semiconducting C40 silicide structures, which is promising for direct device integration on Si-platforms. [Preview Abstract] |
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