Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R7: Si and Other Semiconductors |
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Sponsoring Units: FIAP Chair: Alex Demkov, University of Texas at Austin Room: 303 |
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R7.00001: Crystalline phase-stability of tantalum pentoxide Santiago Walton, Antonio Claudio Padilha, Gustavo Dalpian, Jorge Guill\'en Memristive devices are attractive candidates to provide a paradigm change in memory devices fabrication. These new devices would be faster, denser and less power consuming than those available today. However, the mechanism of memristance is not yet well understood. It is believed that a voltage/current-driven phase transition occurs in the material, which leads to significant changes in the device's conductivity. In the particular case of tantalum-oxide-based devices the relevant crystalline phases are still a matter of debate. Some of these phases are not even completely known and there is no agreement about which model best explains the crystallographic results. In this work we have performed ab-initio DFT based calculations to study the structural properties of different phases (and models) of Ta$_2$O$_5$ - the structure which is believed to exist inside Tantalum Oxide based devices. The equations of state for this material were constructed through first principles total energy calculations and we have also calculated the phonon frequencies at $\Gamma$. These results show that the most stable phase of this oxide (B-Ta$_2$O$_5$) is in fact composed of octahedral, instead of pentagonal (as L-Ta$_2$O$_5$) or hexagonal (as $\delta$-Ta$_2$O$_5$) bipyramids. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R7.00002: Dynamical processes in semiconductor nanoclusters Peng Han, Gabriel Bester We study the electronic relaxation processes via electron-phonon interaction in colloidal semiconductor nanoclusters (NCs) using the Liouville-von Neumann equation including a phenomenological Lindblad decay term. The electron-phonon coupling matrix elements used in our study are obtained from frozen-phonon calculations based on \emph{ab initio} density functional theory (DFT). To estimate the phonon lifetime of NCs, which is used in the Lindblad decay term, we perform \emph{ab initio} molecular dynamics simulations of a Si$_{10}$H$_{16}$ cluster and extract the time evolution of the energy of selected vibrational modes from the energy auto-correlation functions. We find vibrational cooling times of around 0.1~ps for high frequency Si-H vibrations, and cooling time of around 1~ps for pure Si modes, which are close to the phonon lifetimes in bulk Si. Analyzing the electronic relaxation processes with the parameters from DFT calculations, we observe a decaying Rabi oscillation with a period of tens of femtoseconds corresponding to the emission/absorption of a phonon. We notice that the Rabi oscillation frequency is proportional to the electron-phonon coupling strength while the decay process is dominated by the phonon lifetime and the energy detuning. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R7.00003: Characterization of Silicon CMOS Quantum Well Field Effect Transistors Clint Naquin, Mark Lee, Hal Edwards, Tathagata Chatterjee Silicon CMOS field effect transistors (FETs) incorporating quantum wells (QWs) are of potential interest as advanced oscillators and sensors. We report on the design and electrical characterization of a set of Si CMOS QW FETs fabricated using industrial 45 nm processing. By using low doped drain and pocket implants, lateral QW potentials between 30 nm to 100 nm in length and approximately 0.1 to 0.5 eV in potential depth have been incorporated into the channel between source and drain. The potential depth can be modulated by the gate voltage. Measurements of drain current as a function of gate voltage for devices from 1.7 K up to room temperature will be reported, with the expectation of observing resonant conductance oscillations from transport through QW levels at temperatures where the QW energy levels are well formed. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R7.00004: Infrared spectroscopy of high purity Si for application in astronomy Berik Uzakbaiuly, Jian Ge, David Tanner High resolution infrared (IR) spectroscopy is an essential tool in astronomical studies and Silicon Immersion Gratings (SIGs) offer 3.4 times gain in spectral resolution over conventional echelle gratings of the same length and blaze angle. SIGs have Si as the host material, relying on its high transparency in much of the infrared region. Si is transparent below the multiphonon absorption for far-infrared ($\sim$20-300 microns) use as well as in the near infrared ($\sim$1-6 microns). We have measured high-purity silicon transmittance from far to near infrared. Narrow lines, due to residual impurities and with interesting temperature dependences, appear in the far IR region. We present the transmittance of high purity bulk Si from the band edge (9000 cm$^{-1})$ to far infrared (20 cm$^{-1})$ using FT-IR spectrometer and modified Perkin Elmer grating spectrometer. Impurities have been identified and their temperature dependence will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R7.00005: Dynamic Formation of NiSi$_2$ in Porous Amorphous Si Nanorods Observed by In Situ TEM Jianguo Fan We investigated the dynamic formation of NiSi$_{2}$ nanocrystals in a porous amorphous silicon nanorod with in situ heating TEM. The nanorod was prepared by sequential electron beam depositions of Si, Ni, and Si at an oblique angle. Due to the nature of atomic shadowing and limited diffusion at low deposition temeperature, the structure was porous and amorphous. Ni diffusion started at 300 $^{\circ}$C and oxides in the porous structure greatly surpressed the formation of early silicide phases such as NiSi, Ni$_{3}$Si$_{2}$, and Ni$_{31}$Si$_{12}$. At 400-500 $^{\circ}$C, NiSi$_{2}$ crystallites formed along the nanorod and were defined by the porous template. These structural evolutions were identified and confirmed by electron diffraction, X-ray analysis, and high-resolution TEM. The formation mechanism and possible applications will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R7.00006: Enrichment and growth of enriched $^{28}$Si films Joshua Pomeroy, Kevin Dwyer In support of quantum information and spintronics efforts, we are producing enriched $^{28}$Si films that are 99.9{\%} $^{28}$Si according to secondary ion mass spectrometry assessment. We use an ionization source to crack and ionize natural abundance silane gas, then extract the ions through a magnetic sector analyzer to isolate the major isotope $^{28}$Si. We have presently demonstrated \textgreater\ 100 nm thick films of silicon and carbon, which was enriched to 99.996{\%} $^{12}$C. With ongoing improvements, we expect to produce $^{28}$Si enriched to better than 99.99{\%} at thicknesses \textgreater\ 1 $\mu $m grown epitaxially on Si(100) substrates. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R7.00007: Atomic and Electronic Processes During the Formation of an Ionic NaCl Monolayer on Si(100) Deng-Sung Lin, Chan-Yuen Chang, Hong-Dao Li, Shiow-Fon Tsay An atomic layer of stoichiometric NaCl was formed on a covalent Si(100) surface after two successive half-reactions at room temperature. The first half-reaction due to Cl$_{\mathrm{2}}$ exposure generates a square array of Cl adatoms with a distance close to that in a NaCl(100) surface plane. By utilizing scanning tunneling microscopy, core-level photoemission spectroscopy and ab initio density functional theory calculations, it was found that progressive deposition of Na in the second-half reaction results in surface-supported Na$_{\mathrm{3}}$Cl clusters, one dimensional cluster chains and (2x2) patches, and eventually turns the Cl-adlayer into a single-terrace, wavy NaCl layer at one monolayer Na coverage. The grown NaCl monolayer rolls over atomic steps like a carpet and covers the entire surface. The atomic and electronic structure of the topmost Si layer underneath the NaCl layer resembles that of the initial silicon surface layer with buckled dimers. Results of the comprehensive investigation together suggest that an ionic NaCl monolayer is very weakly bonded to the covalent substrate and appears nearly free standing. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R7.00008: Electronic structure of the Sr/Si(001) Zintl template from density functional theory and photoemission Hosung Seo, Miri Choi, Richard Hatch, Agham Posadas, Alexander Demkov Since the first demonstration of epitaxial growth of crystalline SrTiO3 on Si(001) by Mckee and co-workers, sub-monolayer Sr on Si(001) has been extensively investigated. Charge transfer induced by half-monolayer of Sr has been shown to be a key element enabling wetting of Si by SrTiO3. However, a detailed understanding of the electronic structure reconstruction is not complete. Such knowledge could be extended and applied to the other epitaxial crystalline oxides on semiconductors. Recently, using in-situ x-ray core-level spectroscopy, we have studied the change in electronic structure of Si(001) induced by sub-monolayer Sr deposition in terms of surface core level shift. One of the interesting features is shift of the Si 2p level toward the higher binding energy by 0.49eV after Sr deposition. In this talk, we present a detailed theoretical investigation of the surface core level shifts in sub-monolayer Sr/Si(001). Using the final state theory, we calculate the bulk 2p binding energy to be increased by 0.42eV when half-monolayer of Sr is deposited in excellent agreement with experiment. We are able to compare the calculated evolution of the surface band structure in sub-monolayer Sr/Si(001) to angle-resolved photoemission spectroscopy (ARPES) data. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R7.00009: Purification of germanium crystal by zone-refining technique Gang Yang, Jayesh Govani, Hao Mei, Guojian Wang, Yutong Guan, Chaoyang Jiang, Dongming Mei Zone refining is a purification technique of metal materials, which was developed at Bell Telephone Laboratories in the early of 1950s. In zone-refining of high-purity germanium crystals, the influential factors include vacuum level, container of germanium ingot, ambient gases, speed of zone travel, the ratio of ingot length to molten zone length, etc. In the present work, we have investigated the influences of the following factors on the purification of germanium crystals: graphite/quartz boats, hydrogen/argon gas, speed of zone travel and the ratio of ingot length to molten zone length. Additionally, we have also analyzed the influences of segregation of three main impurities, such as boron, aluminum and phosphor on the electrical properties of the zone-refined crystals. In this paper, we report the results from the zone-refined germanium ingots produced at the University of South Dakota. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R7.00010: Optical Microscopic and Spectroscopic Study of the High-Purity Germanium (HPGe) Single Crystals Jayesh Govani, Gang Yang, Guojian Wang, Muhammad Khizar, Chaoyang Jiang, Dongming Mei High-purity germanium (HPGe) single crystals are required for the fabrication of radiation detectors. Before grown HPGe crystals can be effectively utilized, they need to be characterized for their purity, identification of impurities and dislocation density. These characterizations help to determine if the grown crystal is qualified for making detector and provide the feedback for crystal growth, so the crystals with the required qualities can be grown consistently. In the present study, we have performed optical microscopic analysis of the grown HPGe crystals. Our experimental results indicated that the crystals exhibit dislocation density in a range of 3000/cm$^2$ to 8000/cm$^2$ demonstrating that the dislocation density is within the required range ($\sim$ 10$^2$ -10$^4$ dislocations/cm$^2)$ to avoid the formation of undesired di-vacancy hydrogen (V$_2$H) complexes. Photo-thermal ionization spectroscopic (PTIS) analysis indicated that aluminum; boron and phosphorus are the dominant impurities in the grown crystals. We also performed the Van-der Pauw hall measurement for the determination of carrier concentration, resistivity and mobility of the charge carrier. In this paper, we show some characterization results from the grown crystals at USD. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R7.00011: Tracing the phosphorus contamination sources and reducing the phosphorus contamination in HPGe crystal growth Guojian Wang, Yutong Guan, Gang Yang, Jayesh Govani, Muhammad Khizar, Hao Mei, Dongming Mei The net impurity concentration and the dislocation density for the grown crystals must be controlled within a narrow range of values to produce crystals acceptable for large-volume coaxial germanium detector fabrication. Phosphorus is the main shallow level donor in high purity germanium crystal. The phosphorus contamination is a disaster for growing p-type high-purity germanium crystal. The phosphorus contamination mainly comes from crucible, insulation, ambient gas or crystal growth chamber. Regrowth method was used to trace the phosphorus contamination sources. The contamination level from sources was discussed in detail in this paper. For different contamination source, targeted approaches were used to reduce the contamination. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R7.00012: Variation of electron-phonon coupling in group IV elemental semiconductors Nandan Tandon, Lisa Pugsley, L. R. Ram-Mohan Electron-phonon (e-ph) coupling determines the transfer of energy from hot electrons to the lattice, resulting in the heating of devices. In the current treatments, the e-ph coupling is determined within the long-wavelength phonon approximation. In this work, we consider the e-ph coupling and its variation over the entire Brillouin zone (BZ). The electronic structure and the full phonon dispersion are evaluated with the phonon dispersion calculated using the density functional perturbation theory (DFPT). The e-ph coupling is evaluated using maximally localized Wannier functions and generalized Fourier interpolation to generate e-ph matrix elements on arbitrary grids. Examples of specific initial electron momentum both in the valence and in the conduction bands are presented, together with the variation of the e-ph coupling over the entire BZ associated with the specific initial carrier momenta. We observe variations of up to about $400$meV in Diamond and $50-100$meV in Silicon and Germanium for the evaluated e-ph matrix element. We comment on the consequence of this variation on the carrier lifetimes in these materials. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R7.00013: Morphology control of WS2 monolayer islands: triangles, stars, and snowflakes Yuanxi Wang, Cheng-Ing Chia, Ana Elias, Nestor Perea-Lopez, A. C. Beltran, A. Berkdemir, Humberto Gutierrez, Florentino Lopez-Urias, Humberto Terrones, Mauricio Terrones, Vincent Crespi Interfaces play an important role in determining the electronic structure and equilibrium morphologies of monolayer nanoclusters. An additional difficulty for polar materials is that a conventional edge energy calculation using a nanoribbon exposes two different types of edge terminations, making the energy of each edge inextricable. Based on density functional theory, we report the energies of different types of edge terminations of monolayer WS$_{2}$ at different experimental environments in terms of varying chemical potentials of the W and S species. The Wulff construction is then applied to show that triangular shapes are most favorable at higher S chemical potential, where bulk sulfur start to become present in the system. Our results are in agreement with recent experiments that triangular islands of WS$_{2}$ are synthesized by CVD method using vaporized sulfur. Stacking energetics and kinetic growth factors will also be discussed to explain the formation of six-pointed star shapes and edge irregularities. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R7.00014: Chemical sensing with ultra-thin MoS2 Adam Friedman, Keith Perkins, Enrique Cobas, Paul Campbell, Glenn Jernigan, Berend Jonker Although the majority of focus and excitement in recent years has been on studying the remarkable properties of single atomic-layer graphene, there exists a whole class of materials called dichalcogenides that are relatively easily fabricated in single-crystal mono- or few-layer format. Graphene, being chemically inert, does not lend itself to chemical sensing applications. However, MoS2, a dichalcogenide of recent interest because of its potential for transistor applications, possesses many advantageous properties for chemical sensing. Two primary examples include a sizable bandgap, which is necessary for fabricating transistors with large on/off current ratios, and a chemically reactive surface, which is necessary for easy surface functionalization. In this talk, we discuss our current research effort on MoS2 chemical sensors. We discuss aspects of transistor device fabrication and chemical sensing experiments. We expose MoS2 chemical sensors to a variety of analytes, finding the best response to triethylamine, a nerve gas by product, and explain our results based on a donor-acceptor model. MoS2 sensors are compared to other similar low-dimensional sensors and found to be of comparable quality. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R7.00015: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 5:30PM - 5:42PM |
R7.00016: Design, fabrication and performance optimization of bi-polar blocking planar HPGe radiation detector Muhammad Khizar, Guojian Wang, Dongming Mei A prototype planar radiation detector is designed, fabricated and characterized using bi-polar contact deposited on high purity single crystal germanium (HPGe). Performances of planar and semi-planar detectors are carried out for their low background counting and high absolute efficiency for high-energy photons applications. For this study, 40mm ? 15mm (diameter to vertical height) p-type HPGe samples with dislocation density EPD \textless 3000 cm-2 are taken from HPGe ingots grown by Czochralski method. After a successful mechanical preparation, and standard cleaning and polishing procedure, samples are chemically etched by using a mixture of highly concentrated acids HF:HNO3 (1:4) in order to remove the surface oxides. A bi-polar blocking layer of amorphous germanium (a-Ge) is deposited on both the samples using low temperature RF sputtering plasma in a pre-mix precursor of H2 (15{\%}) and Ar. For this, an optimized dose of the plasma power and chamber pressure is used for a controlled low temperature. The process was completed with the evaporation of Ohmic contacts using electron beam evaporator. This is worth noticing that special care is introduced during the handling of these samples, especially for the bi-polar blocking and metal contact layers deposition. Finally, the fabricated detectors are characterized at 77K temperature. In this paper, we show the results from the first prototype detector made of home grown crystals at USD. [Preview Abstract] |
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