Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S1: Properties of Silica and other Inorganic Nanostructures |
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Sponsoring Units: DCMP Chair: Kristen Burson, Hamilton Room: 260 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S1.00001: Silicon transistors in reduced dimensions Felix Schupp, Muhammad Mirza, Donald MacLaren, Andrew Briggs, Douglas Paul, Jan Mol Junction-less nanowire transistors are being investigated for deeply scaled CMOS technology. To date almost all simulations to optimise the technology have been based on 3D or 2D modelling schemes. Here we will present measurements that unambiguously demonstrate 1D transport in sub-10 nm junction-less nanowires. The 1D nature of our device allows for excellent gate modulation despite doping concentrations being well above the metal-insulator transition. We find subthreshold slopes of 66 mV/dec, on- to off-current ratios above $10^8$ and on-currents of 1.15 mA/$\mu$m for a gate overdrive of 1.0 V from transistors with a gate-length of 150 nm at room temperature. We observe Universal Conductance Scaling as a function of voltage and temperature similar to previous reports of transport in 1D systems. In low temperature transport experiments Quantum interference effects including signatures of subband filling are observed. We extract a phase coherence length of 15.7 nm from universal conductance fluctuations, suggesting possible ballistic transport in shorter channels. Our experiments demonstrate the importance of dimensionality in ultra-scaled CMOS devices and highlight the need for 1D device modelling schemes. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S1.00002: Tuning the density and structure of amorphous Si thin films via growth parameters Hilary Jacks, Manel Molina-Ruiz, David Castells-Graells, Alejandro Ceballos, Frances Hellman The atomic density of electron beam evaporated amorphous silicon (a-Si) thin films is here shown to depend strongly upon deposition temperature, growth rate, and total film thickness. Previous work has shown that the density of two-level systems (TLS) -- a defect of interest in amorphous materials -- are tunable and inversely correlated with atomic density. Densities and structural qualities are measured via RBS, AFM, and Raman spectroscopy. We show that density increases steadily with growth temperatures up to 425 \textdegree C, with anomalous behavior at higher temperatures suggesting the nucleation of a different type of structure. Density also depends on growth rate; higher growth rates result in films with lower densities. Film density increases as the total thickness of a film is increased from 10 to 600 nm; additionally, and significantly, no gradient is observed in these films, which indicates that underlying film structure changes after subsequent layers are deposited. We describe physical processes to account for these trends, and implications to both the amorphous materials community and technologies that utilize these materials. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S1.00003: Structural refinement of vitreous silica bilayers Mahdi Sadjadi, Mark Wilson, M.F. Thorpe The importance of glasses resides not only in their applications but in fundamental questions that they put forth. The continuous random network model can successfully describe the glass structure, but determining details, like ring statistics, has always been difficult using only diffraction data. But recent atomic images of 2D vitreous silica bilayers can offer valuable new insights which are hard to be observed directly in 3D silica models/experiments [for references see (1)]. However, the experimental results are prone to uncertainty in atomic positions, systematic errors, and being finite. We employ special boundary conditions developed for such networks to refine the experimental structures. We show the best structure can be found by using various potentials to maximize information gained from the experimental samples. We find a range of densities, the so-called flexibility window, in which tetrahedra are perfect. We compare results from simulations using harmonic potentials, MD with atomic polarizabilities included and DFT. (1) Mark Wilson, Avishek Kumar, David Sherrington and M.F. Thorpe, Modeling vitreous silica bilayers, Phys. Rev. B 87, 214108, pages 1-9 (2013) [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S1.00004: Modification of 2D Silica Bilayer Structure via Strain and Al Doping Chao Zhou, Jin-Hao Jhang, Gregory Hutchings, Omur Dagdeviren, Andrei Malashevich, Udo Schwarz, Sohrab Ismail-Beigi, Eric Altman The preparation of 2D silicate bilayers on metal substrates in crystalline and amorphous forms paves the way to creating a multitude of 2D structures. We study strain effects on the bilayer structure using Ni$_{x}$Pd$_{1-x}$(111) films which allow continuous strain tuning from 6.0{\%} compressive to 3.8{\%} tensile. The interaction with the substrate and its atomic spacing and geometry determines the lattice strain exerted. Previously, a commensurate hexagonal crystalline bilayer was observed on Ru(0001) with a tensile mismatch of 2.2{\%}, while only amorphous bilayers were seen on Pt(111) (4.7{\%} tensile mismatch) and uniaxial strain on Pd(100) led to a commensurate crystalline phase with arrays of domain boundaries. Theory indicates that biaxial tensile strain above 2.5{\%} favors the introduction of 8-membered rings into the bilayer. Experiments show that SiO$_{2}$ bilayers grow in an incommensurate crystalline form on Pd(111) (x$=$0) which sets the maximum biaxial tensile strain to \textless 3.8{\%}. Meanwhile, AlSi$_{3}$O$_{8}$ thin films formed a commensurate crystalline film on Pd(111). Theory shows that the longer Al-O bonds reduce the strain energy by decreasing the mismatch to 1.9{\%}, explaining the transition from incommensurate to commensurate. Ongoing work on Ni$_{x}$Pd$_{1-x}$(111) is determining the maximum lattice strain that can be applied for controllable introduction of 8-membered rings into the structure. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S1.00005: Domain Boundaries and Defect Structures in 2D Insulating Silica Bilayers K. M. Burson, C B\"{u}chner, M Heyde, H.-J. Freund Two-dimensional (2D) materials present an exciting route towards tailored nanoelectronics. Bilayer silica, a wide band-gap two-dimensional insulator, has recently been added to the toolbox of 2D materials for van der Waals heterostructures [1]. Bilayer silica can be grown in crystalline and amorphous forms and successful transfer of a mm-scale film has been demonstrated. In their crystalline form, silica bilayers feature various defect structures. Here we present a scanning tunneling microscopy study with atomic resolution of grain boundaries in silica bilayers grown on Ru(0001) [2]. Tilt boundaries consisting of 5- and 7-membered rings and antiphase boundaries consisting of 5- and 8-membered rings are observed in addition to Stone-Wales defects and closed-loop boundaries. The influence of the Ru(0001) substrate is discussed and comparisons between grain boundary structures and amorphous silica structures are drawn based on ring-size statistics and local structural motifs. We conclude that the structure of domain boundaries in bilayer silica is distinct from the amorphous phase due to a high degree of order and periodicity. [1] C. B\"{u}chner, et al. ACS Nano 10, 7982 (2016). [2] K. M. Burson, et al. J. Phys. Condens. Matter (accepted). [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S1.00006: First principle study of SiO$_2$ bilayer on metallic substrates Xin Liang, Andrei Malashevich, Sohrab Ismail-Beigi, Eric I. Altman Silicates in zeolite form have wide applications in catalysis: they have porous structures with large internal surface areas. However, it is hard to characterize such internal surfaces with atomic resolution experimentally. Therefore, a 2D form of silica provides an interesting model system that can be probed using real-space methods such as scanning tunneling microscopy (STM). In fact, 2D SiO$_2$ bilayers can be created experimentally. Both crystalline and amorphous bilayers have been grown on various metallic substrates. Due to the fully saturated nature of the Si-O bonds, it has been argued that the SiO$_2$ bilayer and substrate interact weakly: however, the substrates can enforce epitaxial strain on the bilayer. In this work, we investigate to what extent metallic substrates (e.g. Ru and Pd) affect the stable morphologies of SiO$_2$ bilayers by using \textit{ab initio} density functional theory (DFT). We are particularly interested in seeing how strain in the bilayer competes with substrate interactions and the nature of the resulting bilayer structures. This work is supported by the National Science Foundation through grant NSF DMR-1506800. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S1.00007: Zinc Oxide Nanocolumns Periodically Grown on Silica Nanosphere Monolayers D Mateo, N Wright, A Ostoski, P Mukherjee, S Witanachchi ZnO nanocolumns (NCs) are promising building blocks for many existing and emerging applications owing to their unique optical, electrical, and piezoelectric properties. Specifically, the ZnO NCs could be used as seed layer for the growth of other oxide materials. Nanocolumnar ZnO is generally grown in randomly distributed arrays, where the entire substrate is covered and only through lithographic methods is selectivity of growth location achieved. We propose a method to be able to grow ZnO NCs in hexagonally closed packed structure with location tunability. Langmuir-Blodgett was used to construct a self-assembled monolayer of SNSs (3.5 $\mu $m, 1.18 $\mu $m, 850nm, 500nm and 250nm in diameter) on silicon substrates. Z-axis oriented ZnO NCs were grown on top of the spheres using the glancing angle pulsed laser deposition (GAPLD) technique. Column arrays grown in smaller SNSs diameter were vertical and grew in an hcp structure on top of each sphere. ZnO NCs aspect ratios were found to be dependent on underlying sphere size. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S1.00008: Two-Level Systems and Growth-Induced Thermodynamic Metastability in Hot-Wire Deposited Hydrogenated Amorphous Silicon Manel Molina-Ruiz, Hilary Jacks, Frances Hellman, Daniel Queen, Xiao Liu, Qi Wang, Richard Crandall Hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire chemical vapor deposition (HWCVD) shows a large specific heat C$_{\mathrm{P}}$ at low temperature T, despite low values of tunneling level states as measured by internal friction. C$_{\mathrm{P}}$ is significantly larger than the Debye specific heat calculated from the sound velocity, characteristic of glasses with two-level systems (TLS). The as-prepared films have an additional Schottky-like anomaly at low temperature that is associated with metastable hydrogen in the amorphous network. Annealing at 200 \textdegree C, well below the growth temperature, irreversibly reduces C$_{\mathrm{P}}$ by over an order of magnitude below 12 K, eliminating the Schottky-like anomaly. Based on the linear term in C$_{\mathrm{P}}$, the TLS density in this annealed state is orders of magnitude larger than expected based on internal friction Q$^{\mathrm{-1}}$ measurements, which are unchanged by the anneal. This large TLS density is suggested to result not from a local Si-H excitation, but instead from atomic scale regions best described as Si-H complexes in the a-Si network. Comparison of heat capacity to internal friction suggests that these TLS are decoupled from acoustic excitations. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S1.00009: Preparing TLS-Free Magnetron-Sputtered a-Si Matthew Abernathy, Thomas Metcalf, Xiao Liu Recent work has shown that e-beam evaporated a-Si films can be grown free of two-level tunneling states (TLS) through control of the substrate temperature during deposition. The decrease in TLS population in the evaporated films has been shown to be highly correlated with increased density of the films. The question remains as to whether the removal is due to the density increases, the deposition technique, or the chemical bonding conditions of silicon. Using magnetron sputtering, we are able to test these first two questions, as the particle energies in magnetron sputtering are significantly different from those in e-beam evaporation and generally result in higher-density films. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S1.00010: Deformation in amorphous-crystalline nanolaminates – an effective-temperature theory and interaction between defects Charles K. C. Lieou, Jason R. Mayeur, Irene J. Beyerlein Experiments and simulations suggest that the movement of plasticity carriers in deforming amorphous-crystalline nanolaminates is mediated by the interface between the amorphous and crystalline layers. We develop a unified theory of defects in both amorphous and crystalline materials that describes their interactions through the amorphous-crystalline interface (ACI) when nano-thick layers of the two materials are stacked upon one another. To this end, we will lay out the effective-temperature framework that describes the slow, configurational atomic rearrangements in a deforming solid driven out of equilibrium. We will show how the second law of thermodynamics constrains the defect density and the rate of configurational rearrangements, and apply this framework to dislocations in crystalline solids, as well as STZ's in amorphous materials. The effective-temperature formulation enables us to interpret the observed movement of dislocations to the ACI and the production of STZ's at the interface as a ``diffusion'' of configurational disorder across the material. We will conclude with some preliminary results that show agreement with experimental findings (Kim et al., Adv. Funct. Mater., 2011), and demonstrate how the ACI acts as a sink of dislocations and a source of STZ's. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S1.00011: MgO/Cu$_{\mathrm{2}}$O superlattices: growth of epitaxial two-dimensional nanostructures. P.V. Wadekar, M.J. Yang, W.C. Hsieh, Q.Y. Chen, C.W. Lin, J.W. Chou, C.F. Chang, S.T. You, L.W. Tu, I.K. Lo, C.H. Liao, H.W. Seo, H.C. Huang, N.J. Ho, S.W. Yeh, H.H. Liao, W.K. Chu Superlattices (SLS) created by alternating dissimilar materials produced novel materials of multi-functionalities. The majority of work reported in literature on epitaxial SLS has been on alternating layers of the same space group (SG) and crystal structures (CS), SLS with the same CS but different SG have not been studied as much. We have grown SLS with two well-known oxide materials cuprite (Cu$_{\mathrm{2}}$O, CS $=$ cubic and SG $=$ Pn m) and magnesium oxide (MgO, CS $=$ cubic, SG $=$ Fm m). The effects of an MgO buffer layer grown near 650\textdegree C at the film-substrate interface was found to be essential to reasonable long-range atomic orders. GIXRD, XRR, electron diffraction and TEM were used to investigate the interface abruptness, smoothness and the general crystallinity of the individual layers. Interdiffusion between the MgO and Cu$_{\mathrm{2}}$O at interfacial regions puts a limit of 250\textdegree C as growth temperature of superlattice of reasonably sharp interfaces. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S1.00012: Two level tunneling states in amorphous germanium thin films Thomas Metcalf, Xiao Liu, Glenn Jernigan, Matthew Abernathy In light of our recent work creating amorphous silicon (a-Si) thin films without the universal two-level tunneling states (TLS) that are found in every other amorphous solid that has been studied to date, the question arises whether it is possible to create other amorphous solids without TLS. To this end, we have measured the low-temperature internal friction of several amorphous germanium (a-Ge) films. Here we report preliminary measurements on a sequence of a-Ge films grown at different substrate temperatures. As with a-Si, we find a systematic decrease in TLS with increasing substrate growth temperature, and have observed an order-of-magnitude decrease in tunneling strength for a-Ge grown at $160^{\circ}$C. We address the question of whether the crystallization kinetics of a-Ge will permit the creation of a fully amorphous yet TLS-free film. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S1.00013: Understanding short range order in bulk metallic glasses: Resolving local atomic structure in amorphous copper zirconium through statistically based 3D visualizations Al Rahemtulla, Adam Hinkle, Bruno Tomberli, Michael Falk, Stefan Kycia Understanding the underlying local structure of metallic glasses has been a recent endeavor, incorporating a variety of computational methods such as, reverse monte carlo, molecular dynamics and ab-initio modeling. Interpreting similarities and differences between results is a great challenge. We dissect the underlying structures of 300k atom CuZr models, employing 3D statistical density maps through our recently established Local Atomic Motif (LAM) method. The use of LAMs revealed bimodal structures in amorphous germanium. It has since been expanded to handle more complex systems. An overview of the capabilities of the LAM method will be presented as well as a the resulting detailed description of the local order in metallic CuZr. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S1.00014: Shock wave dispersed nano beam epitaxy KSR Koteswara Rao, Atul Abhale, KPJ Reddy In this work, we introduce a new technique called "shock wave dispersed nano beam epitaxy (SD-NBE)" to create nano-layers of quantum dots towards development of ultra-thin-film devices. Though the technique is not limited to QDs, any other material in solution form such as conducting or insulating polymers, solution dispersed CNTs, fluorescents, photoresist etc., can be deposited on variety of substrates irrespective of the nature of substrate (silicon, glass, metal, polymer film etc.). It is based on the nano-dispersion technique that exploits the phenomena, which produce focused weak shock waves and disperse nanomaterials. Here, the solution dispersed PbS/CdTe QDs have been deposited on Si, SiO$_{2}$/Si substrates and found to have suitable physical, electrical and optical properties to be useful in the development of thin-film devices. This novel technique is inexpensive, economically use quite a small amount of material and minimizes the wastage; hence, it can be well implemented in various applications of material physics and device technology at nanoscale. [Preview Abstract] |
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