Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session EC: Applied Physics I |
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Chair: Rosa Lukaszew, College of William and Mary Room: Holiday Inn Brownstone Lincoln |
Thursday, October 30, 2008 3:45PM - 3:57PM |
EC.00001: Information Extraction from Congested Molecular Spectra by Modulation Spectroscopy Mohammad Khan, Karan Mohan, Amin Dharamsi Wavelength Modulation Spectroscopy (WMS) of molecular species is quantified utilizing concepts of entropy and information content. The optimal harmonic detection order (i.e. the one for which maximum information can be extracted in a noise environment) is found by theoretical considerations and then verified by experiment. The method developed, which can be used for precise measurements of molecular collision dynamics encoded in the absorption lineshape profile, is applied to resolution of very weak (spin-forbidden, magnetic dipole driven) overlapping lines of disparate oscillator strengths in molecular Oxygen A-Band. The complexity of the structure (turning points and zero crossings) of WMS provides an ultra sensitive probe, sensitive to small perturbations in the lineshape profile. For particular experimental limitations and noise environments, finite amounts of information can be transmitted by the probe interacting with the information source (the lineshape) to the detection apparatus. This information reaches a maximum value at an optimum detection order. The theory developed is applied to experimental measurements of four overlapping transitions in Oxygen A-band. It is shown that detection at harmonics greater than the commonly-used second are optimal in this case. [Preview Abstract] |
Thursday, October 30, 2008 3:57PM - 4:09PM |
EC.00002: Regimes of resonance Raman utility: studies in diluted liquid benzene Christopher Chadwick, Hans Hallen, Adam Willitsford, C. Russell Philbrick Resonance Raman yields huge increases in cross section as the excitation laser is tuned through an absorption band. Unfortunately, the measured signal levels do not always correspondingly increase. We use a tunable laser source to investigate this phenomenon with three different concentrations of liquid benzene in heptane as well as pure liquid benzene. Resonant Raman signal enhancement of the ring-breathing mode ($992\,cm^{-1}$) has been confirmed. Additionally, species absorption limitations on this enhancement has been identified. Two different regimes have been identified where resonance Raman can be applied to enable spectrographic information where the signal level of non-resonant Raman is not measurable. While Raman scattering intensities are largely dependent on the number of scattering agents in the non-resonant case, when probing electronic absorption features in a species with the excitation light, enhancement yields a significant signal output even as the number of scattering agents is reduced. The enhancement gains of the signal to noise for resonant probing are superior in low concentrations and tiny interaction volumes. This enhancement characteristic has broad application for materials identification in trace species such as remote sensing and tiny sampling volumes such as near-field optical microscopy. [Preview Abstract] |
Thursday, October 30, 2008 4:09PM - 4:21PM |
EC.00003: EPR Study of SiC Defects Related to Device Processing Sarah Thomas, Mary Ellen Zvanut SiC is a promising replacement for Si in future high power, high temperature electronic devices. The surface of SiC is particularly important to MOSFETs, where the active region is on the surface. Previous research, which used electron paramagnetic resonance (EPR) spectroscopy to study electronically active defects in SiC, suggested that a defect, likely a broken C-Si bond, was created by oxidation. Our research focuses on identifying the cause and location of defects in as-grown SiC substrates using EPR. Samples underwent isochronal anneals from 400 to 1000 $^{\circ}$C in high purity dry (0.9 ppm) N$_{2}$ and O$_{2}$. Room temperature EPR spectra showed two signals, one broad (10 G) and one narrow (4 G). Because the results from the N$_{2}$ and O$_{2}$ anneals were similar, we conclude that the defects are not affected by the reaction with oxygen. That the heat of annealing decreases the broad EPR signal suggests the defect is removed, rather than passivated. During the talk we will discuss the heat treatment results, as well as the location of the defect, as discovered through reactive ion etching and forming gas anneals. [Preview Abstract] |
Thursday, October 30, 2008 4:21PM - 4:33PM |
EC.00004: Thickness inhomogenities and growth mechanisms of GaP heteroepitaxy Xiang Liu, Inkyo Kim, David Aspnes We report comparative studies of the heteroepitaxy of GaP by organometallic chemical vapor deposition (OMCVD) using trimethylgallium (TMG) and phosphine (PH3) sources on (001) GaAs, thermally generated SiO2, (001) Si, and nanoscopically roughened Si surfaces. Inadvertent indirect but important data were also obtained from the polycrystalline GaP deposited on the Mo susceptor surrounding the 2 in. wafers. We found that the thicknesses of the deposited GaP films increases or decreases exponentially toward the edge of the wafers. This dependence is incompatible with the common explanation of gas-phase depletion of the precursors. Starting with the diffusion equation, we derive analytic expressions that describe the thickness variations in terms of the diffusion parameters, and evaluate the diffusion length quantitatively. We show that the cause is due to differences in chemical reactivities of the various surfaces, especially the different catalytic effects that they exert on PH3 decomposition. The results also show that different parts of the surface, including the susceptor, are in constant contact with each other during growth through gas-phase diffusion, and that deposition occurs via a precursor that involves both Ga and P. We propose a model for GaP growth based on the formation mechanism of this precursor. [Preview Abstract] |
Thursday, October 30, 2008 4:33PM - 4:45PM |
EC.00005: The Promotion of Au Adhesion on Polymer Surfaces Using Polyhedral Oligomeric Silsequioxane Chris Hughes, Brian Augustine, Alan Mo, Jonathan Wyrick, Bruno Caputo, Ethan Rosenthal The adhesion of Au on polymer surfaces is weak because of the inert nature of Au and the non-polarity of the hydrocarbon surface of the polymer. We seek to fabricate microfluidic devices in which vacuum deposited gold thin films will be used in electrical contacts and optical reflectors. Various fabrication steps involve the use of solvents which easily wash away the gold. To overcome this, we have explored the use of a thin layer of polyhedral oligomeric silsequioxane-methacrylate (POSS-MA) which is a nanocomposite having both polymer and inorganic silica glass characteristics. The POSS-MA is spun cast onto the surface of PMMA creating a film which is on the order of 100 nm thick. Au dots that are 1 mm in diameter were deposited onto both the virgin PMMA surface and the POSS-MA coated surface and the samples were covered with acetone, a known solvent for PMMA. Optical microscope video images of the dots revealed their delamination from the surface and image analysis was used to determine the time that it took for the dots to be undercut -- typically in the range of seconds to minutes. An obvious increase in the time required to undercut the Au was observed for the POSS-MA treated surface. A model explaining the improved adhesion will be discussed as will future plans for device fabrication. [Preview Abstract] |
Thursday, October 30, 2008 4:45PM - 4:57PM |
EC.00006: Characterization of polar molecular species adsorbed on LiNbO$_{3}$ surfaces Satyaveda Bharath, Thomas Pearl In order to explore the mechanisms of adsorption on ferroelectric surfaces, single crystalline lithium niobate (LiNbO$_{3}$: LN), `Z-cut' along the (0001) plane, has been prepared and characterized and subsequently exposed to a polar molecule. 4-$n$-octyl-4$\prime $-cyanobiphenyl (8CB) liquid crystal was chosen as our model system. Low-energy electron diffraction, atomic force microscopy, surface contact angle measurement, and X-ray photoelectron spectroscopy were used to characterize the surface of LN as well as the nature of the films grown on the surface. Atomically flat LN surfaces were prepared as a support for monolayer thick, 8CB molecular domains. Preferential attachment for positive domains was observed indicating an interaction between the polar end group of the molecule and the surface charge of the surface. Understanding anchoring mechanisms for polarizable molecules on uniformly poled surfaces allows for a fuller appreciation of how ferroelectric surfaces can be used for controlling molecular organization. [Preview Abstract] |
Thursday, October 30, 2008 4:57PM - 5:09PM |
EC.00007: Sequestration and selective oxidation of carbon monoxide on graphene edges Sujata Paul, Erik E. Santiso, Marco B. Nardelli The versatility of carbon nanostructures makes them attractive as possible catalytic materials, as they can be synthesized in various shapes and chemically modified by doping, functionalization, and the creation of defects in the nanostructure. In this work, we consider the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), an important problem in environmental chemistry and energy conversion. Using first principle calculations we study the key reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self sustained. Carbon-mediated CO sequestration produces half of the CO$_{2}$ compared to the direct oxidation of CO, which is used in the cleaning of automobile gas. Furthermore, the carbon-mediated oxidation of CO to CO$_{2}$ is selectively favored when hydrogen is present, and could be used to purify hydrogen for use in fuel cells. [Preview Abstract] |
Thursday, October 30, 2008 5:09PM - 5:21PM |
EC.00008: A Novel approach towards integration of VO$_{2}$ thin films on Si(100) for thermal switching devices applications Alok Gupta, Ravi Aggarwal, Jagdish Narayan VO$_{2}$ exhibits a very interesting semiconductor to metal transition (SMT) as the crystal structure changes from monoclinic to tetragonal upon heating close to 68C. Parameters associated with SMT in VO$_{2}$ thin films, such as, transition temperature (T$_{t})$, hysteresis ($\Delta $H), transition width ($\Delta $T) and the order of magnitude change ($\Delta $A) are a strong function of microstructure, orientation, and stoichiometry. We have developed a novel method to produce epitaxial VO$_{2}$ thin films with controlled SMT characteristics and its integration with Si(100) substrate which is of immense technological importance due to a variety of sensor- and memory-type applications. We have optimized the deposition conditions for the growth of epitaxial VO$_{2}$ films on Si substrate using a pulsed-laser deposition method. The integration of VO$_{2}$ with Si was accomplished via domain matching epitaxy of TiN and MgO intermediate layers on Si. XRD and HR-XTEM studies were carried out and resistance measurements were done to quantify the SMT parameters as a function of microstructure and composition. We have established structure-property correlations and related to our phenomenological model based upon thermodynamics. [Preview Abstract] |
Thursday, October 30, 2008 5:21PM - 5:33PM |
EC.00009: Plasmon polaritons in conducting-metal-oxide films A. Efremenko, C. Rhodes, M. Cerutti, M. Losego, D.E. Aspnes, J.-P. Maria, S. Franzen We report the evolution with thickness of p-polarized reflectance spectra of indium tin oxide (ITO) films deposited on BK-7 glass. ITO is one of the most common examples of the class of conducting metal oxides. Due to the low charge carrier density, $\sim $10-21/cm$^3$, the spectral features of the plasmon are observed in the near infrared. The spectra are dominated by two plasmon polariton structures, which we show are associated with the screened bulk plasmon (SBP) for very thin films and the surface plasmon for thick films. The conductor skin depth is the cut-off between the thin and thick film behavior. Remarkably, all features of these complex spectra are accurately described using only the three-phase model and the Drude free-electron representation of the dielectric function of the films. This first observation of the SBP is made possible by the unique features of these films, which include continuity for even the thinnest films and an absence of complications from bulk absorption in the spectral region of interest. The observation of the SBP is possible due to the fact that ITO behaves as a free electron conductor. Specifically, ITO has no band-to-band transitions that would obscure the intrinsic screed bulk plasmons. [Preview Abstract] |
Thursday, October 30, 2008 5:33PM - 5:45PM |
EC.00010: Multimedia ellipsometry of (aminomethylaminoethyl)phenethyltrimethoxysilane (PEDA) layers Jeremy Peters, Hans Hallen A novel ellipsometric characterization method is described for determining the layer thickness and refractive index of substrates functionalized using (aminomethylaminoethyl)phenethyltrimethoxysilane (PEDA) self assembled monolayers (SAMs). Since traditional ellipsometry cannot independently determine the thickness and index of very thin layers, we have developed an ellipsometric apparatus that measures films in liquid solvents, and an analysis procedure that combines the separate three-layer models in these solvent media to enable measurement of both values. Two types of ellipsometry analysis procedures are shown. We illustrate an analysis of thin layer deposition on a simple substrate using PEDA on Si growth vs. time and solution. For more complex substrates, which require measurement before and after SAM layer growth, we show results for a PEDA/oxide/Si multilayer system. Since the refractive index of the layer is a function of the density of the molecules on the substrate, multimedia ellipsometry can determine whether the changes of the ellipsometric parameters are due to density or overgrowth effects. [Preview Abstract] |
Thursday, October 30, 2008 5:45PM - 5:57PM |
EC.00011: Optimization of a Prototype Atomic Clock Based on Coherent Population Trapping Eugeniy Mikhailov, Nathan Belcher, Irina Novikova We developed and constructed a VCSEL-based laser system to study various configurations of coherent population trapping resonances (CPT) in hot Rb vapor, relevant for miniature CPT-based atomic clocks. We also locked an external crystal oscillator to CPT resonance; best observed fractional stability is $6\cdot10^{-12}$ at 400s. In addition to its research value, our apparatus can be easily adopted as a advance undergraduate experiment, since it uses primarily off-the-shelf electronic and optical components. [Preview Abstract] |
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