Subhash L. Shinde Sandia National Laboratories
Yujie J. Ding Lehigh University
Jacob Khurgin Johns Hopkins University
Gyaneshwar P. Srivastava University of Exeter
EE1: Phonon Transport
Subhash L. Shinde
Wednesday PM, November 28, 2007
Room 209 (Hynes)
9:30 AM - **EE1.1
Nanoscale Phonon Engineering: From Nanowire Transistors to Graphene Devices.
Alexander Balandin 1 Show Abstract
1 Electrical Engineering, University of California - Riverside, Riverside, California, United States
Acoustic and optical phonons manifest themselves practically in all electrical, thermal, optical and noise phenomena in semiconductors. Reduction of the feature size of electronic devices to the nanometer scale creates a new situation for the phonons propagation and interaction. From one side, it may complicate the heat removal from the downscaled electronic devices due to the increased diffuse acoustic phonon – boundary scattering and, correspondingly, degraded thermal conductivity. From the other side, it opens up an opportunity for the fine-tuning of the phonon dispersion in nanostructures with the high quality of interfaces, thus achieving what has been termed as phonon engineering . The concept of phonon engineering may have a major impact on the control of the lattice thermal conductivity and thermal management of the nanoscale electronic and optoelectronic devices. At the same time, a truly strong motivation for the phonon engineering is provided by the continuous quest for the electron mobility enhancement. In this talk I will discuss a possibility for improving the carrier mobility via the acoustic phonon engineering in the nanostructures with the “acoustically mismatched” boundaries. The attractive features of the proposed method is that it works at room temperature; and it does not require incorporation of the alloy layers with the low thermal conductivity, which are currently used in the mobility enhancement via strain engineering. Our theoretical and simulation results show that the electron mobility in the silicon nanowires with the proper “acoustically hard” coatings can be enhanced via partial suppression of the electron – acoustic phonon deformation potential scattering . As a second example of the usefulness of the phonon engineering concept, I will discuss our experimental results for the characterization of the graphene-based devices. In this work we used the knowledge of the phonon modes in graphene and their temperature dependence for assessment of the number of graphene layers and their quality . This work was supported, in part, by the DARPA – SRC MARCO Center on Functional Engineered Nano Architectonics (FENA) and DARPA – DMEA UCR – UCLA – UCSB Center for Nanoscience Innovation for Defense (CNID).  A. Balandin and K.L. Wang, Phys. Rev. B, 58, 1544 (1998); A.A. Balandin, J. Nanoscience and Nanotechnology, 5, 7 (2005). V.A. Fonoberov and A.A. Balandin, Nano Letters, 6, 2442 (2006). I. Calizo, A.A. Balandin, et al., Nano Letters, 2007; see also at http://www.ndl.ee.ucr.edu/
10:00 AM - EE1.2
Phonon Decay Bottlenecks in One-Dimensional Systems.
Jose Menendez 1 Show Abstract
1 Physics, Arizona State University, Tempe, Arizona, United States
Recent Raman work on carbon nanotubes reveals optical phonon lifetimes much longer than measured by tunneling techniques. It has been argued1 that the discrepancy is due to a phonon decay bottleneck caused by the one-dimensional nature of the phonon dispersion relations. To leading order, an optical phonon decays into pairs of phonons while conserving crystal momentum. In other words, the sum of the wave vectors of the decay products equals the wave vector of the original phonon. In three dimensional systems there is usually a continuum of possible decay channels satisfying the momentum selection rule, but in a one-dimensional system the number of possible decay channels is severely limited to just a few.
In this work we study in detail the conditions for the existence of an anharmonic decay bottleneck in one dimension. Decay bottlenecks may have a strong effect on the electronic and thermal transport properties of nanotubes and nanowires.We present numerical simulations of the phonon population under different experimental conditions. The decay process can be modeled in terms of systems of coupled non-linear differential equations. In previous work1
we make simplifying assumptions to linearize the equations. These assumptions are removed in the present simulations, which allow us to explore high-excitation regimes. The results confirm that decay bottlenecks are an important feature of one-dimensional phonon physics.
1R. Rao, A.M. Rao, C.D. Poweleit, and J. Menendez (submitted)
10:15 AM - EE1.3
Monte Carlo Modeling of Phonon Transport in Nanostructured Materials.
Thomas Brown 1 , Edward Hensel 2 , Robert Stevens 2 Show Abstract
1 Microsystems Engineering, Rochester Institute of Technology, Rochester, New York, United States, 2 Mechanical Engineering, Rochester Institute of Technology, Rochester, New York, United States
10:30 AM - EE1.4
Phonon-mediated Thermal Transport in Microsystems.
Sylvie Aubry 1 , Christopher Kimmer 1 , Patrick Schelling 2 , Ed Piekos 3 , Leslie Phinney 3 Show Abstract
1 Mechanics of Materials, Sandia National Laboratories, Livermore, California, United States, 2 , University of Central Florida, Orlando, Florida, United States, 3 , Sandia National Laboratories, Albuquerque, New Mexico, United States
We provide a detailed characterization of phonon transport at the atomic scale using pre-existing and novel molecular dynamics techniques. We simulate explicitly phonon-microstructure interactions to produce subgrid models of phonon transport for use in our mesoscale models which, in turn, provide input to device-scale models. Microstructures considered are mass interface, dislocations, grain boundaries and impurities. Comparison against existing experimental data and further measurements of thermal conductivity over a broad range of temperatures in well-characterized polycrystalline samples are used to validate the new model.Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. This work has been supported by the Sandia Laboratory-Directed Research and Development (LDRD) program.
10:45 AM - EE1.5
Enhanced Phonon Scattering in Oxide Superlattices to Improve Laser Induced Thermoelectric Voltages.
Hanns-Ulrich Habermeier 1 2 , Peng Xiang Zhang 2 1 , Hui Zhang 2 Show Abstract
1 , MPI-FKF, Stuttgart Germany, 2 , Kunming University of Science and Technology, Kunming China
Optimizing the figure of merit for thermoelectric applications, ZT = S2σT/κ is currently at the core of materials oriented research in thermoelectricity. Here, one promising approach is to reduce ther thermal conductivity without sacrificing the electrical conductivity. Constructing superlattices of structurally compatible materials is one way to accomplish this goal. We report an enhanced laser induced thermoelectric voltage (LITV) effect observed in (YBa2Cu3O7/La1-xPbxMnO3)n multilayer thin films for the first time. Two groups of multilayer thin films grown on vicinal cut LaAlO3 substrates were prepared by pulsed laser deposition technique. The first group were grown on different substrates vicinal cut at different angles, and were used for checking the mechanism of the induced voltages. The second group samples were made at different period number n and for studying the number dependence of the peak values of LITV. The substrate angle dependence proved that this is a thermoelectric effect . It was found that the LITV signals were enhanced significantly for these multilayer thin films comparing with the single layer ones. It is natural that the conductivity is going to be anisotropic due to the layered structure, and the same holds for the Seebeck coefficients. The enlarged Seebeck anisotropy will lead to higher induced voltages. Another possible reason is the reduced thermal conductivity in the layered structure. The maximum enhancement of LITV signals takes place at period number of 7, which seems in agreement with the prediction of minimum thermal conductivity in superlattices by Simkin and Mahan .[ 1 ]H. Lengfellner, S. Zeuner, W. Prettl, and K. F. Renk, Europhys. Lett. 25, 375 (1994). M.V. Simkin, and G.D. Mahan, Phys. Rev. Lett., 84, 927(2000).
EE2: Phonon Engineered Systems
Subhash L. Shinde
Wednesday PM, November 28, 2007
Room 209 (Hynes)
11:30 AM - **EE2.1
Using Principal Phonons in Semiconductors for the Generation of Terahertz Radiation.
Sergey Pavlov 1 Show Abstract
1 , Institute of Planetary Research, Berlin Germany
Electron-phonon interactions employing principal phonons of the lattice belong to the fastest processes in semiconductors. Over last decade this fundamental feature has been used for generation of population inversion in far-infrared (terahertz frequency range) semiconductor lasers. In these lasers ultrafast, (sub-)picosecond interactions with principal phonons compete with fast thermal phonons which tend to support relaxation towards equilibrium. A proper design of electronic resonances with phonons can enable an inversed electron distribution as well as resonant Raman scattering. Different laser schemes realized for the terahertz frequency range will be discussed with emphasis on elemental semiconductor lasers, such as p-germanium and n-silicon lasers. These approaches have demonstrated for the first time the potential of principle phonons for intersubband and intracenter laser mechanisms. Later on this was used in terahertz quantum cascade heterostructure lasers. The realization of the intracenter Raman silicon laser has shown the possibility of using different principle phonons in semiconductors for the generation of stimulated emission in the mid- and far-infrared wavelength range.
12:00 PM - EE2.2
Evidence of Hot Phonons Generated by GaN-Based High Electron Mobility Transistor.
Guibao Xu 1 , Suvranta Tripathy 1 , Yujie Ding 1 , Kejia Wang 2 , Debdeep Jena 2 , Jacob Khurgin 3 Show Abstract
1 Electrical and Computer Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 2 Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, United States, 3 Electrical and Computer Engineering, The Johns Hopkins University, Baltimore, Maryland, United States
During this presentation, we will present our most recent result following our investigation of the effects of hot phonons generated from a GaN-based high electron mobility transistor. Such a device consists of a thin layer of AlN grown on the top of an unintentionally-doped GaN film by using molecular-beam epitaxy. We have then processed such a structure into a high electron mobility transistor. We have measured the photoluminescence and Raman spectra under different biases. After analyzing these spectra, we have confirmed the existence of the hot phonons generated by applying a bias to such a transistor.
EE3: Phonon Interactions
Yujie J. Ding
Gyaneshwar P. Srivastava
Wednesday PM, November 28, 2007
Room 209 (Hynes)
2:30 PM - **EE3.1
Phonon-phonon Interactions in Carbon Nanotubes, Graphene and Graphite.
Nicola Bonini 1 , Michele Lazzeri 2 , Francesco Mauri 2 , Nicola Marzari 1 Show Abstract
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Institut de Minéralogie et Physique des Milieux Condensés, Universités Paris 6 et 7, Paris France
3:00 PM - **EE3.2
Generation and Detection of Surface Acoustic Phonons using Ultrafast Optical Pulses.
David Hurley 1 , Ryan Lewis 1 , Stephen Reese 1 Show Abstract
1 , Idaho National Laboratory, Idaho Falls, Idaho, United States
3:30 PM - EE3.3
Phonon Interaction in InGaAs/GaAs Quantum Dots.
Axel Hoffmann 1 , P. Zimmer 1 , S. Werner 1 , A. Strittmatter 1 Show Abstract
1 Inst. f. Fstkoerperphysik, TU Berlin, Berlin Germany
In recent years carrier – phonon interaction in semiconductor quantum dots have considerable attention. They are important to understand the electronic properties of such system, like carrier relaxation processes. Some are convinced that carriers confirmed quantum dots are strongly coupled to the longitudinal optical (LO) vibrations of the semiconductor lattice. We report on exciton-phonon interactions in InGaAs/GaAs quantum dots. Photoluminescenc and time-resolved experiments were performed on different MBE and MOCVD grown samples to observe and to investigate varying phonon interactions. In our measurements we observed photoluminescence peaks constantly shifting with varied excitation energy. The energy gap between the laser peak and the observed two-peak structure remained unchanged The energy differences of 33.8 meV and 36. 9 meV precisely fit to QD LO phonon mode and to the interface mode, respectively. The very short radiative lifetime also points to inelastically scattered phonons, i.e. Raman scattering.
3:45 PM - EE3.4
Effective Phonon Spectra and Thermal Rectification in Two Anharmonic Lattices.
Jinghua Lan 1 2 , Baowen Li 2 , Jiansheng Wang 1 2 , Saikong Chin 1 Show Abstract
1 , Institute of High Performance Computing, Singapore Singapore, 2 , Department of Physics and Centre for Computational Science and Engineering, National University of Singapore, Singapore Singapore
4:30 PM - **EE3.5
Carrier-induced Softening of Atomic Vibrations.
David Emin 1 Show Abstract
1 Dept. of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico, United States
5:00 PM - EE3.6
Correlation Between Electron-phonon Coupling and Dielectric Breakdown Strength in Polymer-oxide Nanocomposites: An ab initio Computational Study.
Ning Shi 1 , Rampi Ramprasad 1 Show Abstract
1 Deparment of Chemical, Materials & Biomolecular Engineering, Institue of Materials Science, University of Connecticut, Storrs, Connecticut, United States
The development of polymerics dielectrics with improved high voltage endurance is a major enabling technology for a wide range of high voltage applications from capacitor to transmission class cable. Nanocomposites based on a polymer matrix filled with inorganic dielectric nanoparticles have shown promise for such applications, motivated by recent observations that the incorporation of SiO2 nanoparticles into polyethylene can increase the breakdown strength, while incorporation of micron sized SiO2 particles does not. It thus appears that the interface between SiO2 and polymer plays an important role in improving the dielectric strength. Such interface states could act as potential electron traps, thereby scavenging “hot” electrons, and strong coupling between these interface states and phonons in SiO2 could provide a mechanism for cooling of hot electrons, thereby increasing dielectric strength. A systematic investigation of various types of inorganic oxide-polymer interfaces focusing on interface states and electron-phonon coupling will help identify trends related to dielectric breakdown strengths, and can aid in the rational design of polymeric systems with superior dielectric properties.We present density functional theory (DFT) based simulations of the interfaces between SiO2 and Al2O3 and polymers, including the effect of coupling agents. The extant to which interface effects modify the electron phonon coupling in these systems was explored in detail. The increase in dielectric strength has been correlated with increased levels of coupling between phonons in SiO2 and Al2O3 and defect states at the oxide-polymer interface. Our computational method for the estimation of the electron-phonon coupling involved two steps. First, the phonon band structure was determined, resulting in all the phonon frequencies and eigenmodes. Second, for each phonon eigenmode, the atoms of the systems were displaced as per that mode and the associated shifts in the electronic level computed. This two-step procedure allows for an estimate of the degree of coupling between a given electronic level and a given phonon mode, and solves some difficulties in the standard derivation of the electron phonon interaction. This study identifies some of the fundamental factors responsible for the high breakdown strength displayed by SiO2 based polymeric nanocomposites, and contrasts this with Al2O3-based systems.
5:15 PM - EE3.7
Phonon Engineering using Endohedral Nano Space in Clathrates.
Takeshi Rachi 1 , Ryotaro Kumashiro 1 , Hiroshi Fukuoka 2 , Shoji Yamanaka 2 , Katsumi Tanigaki 1 Show Abstract
1 Physics, Tohoku University, Sendai Japan, 2 Applied Chemistry, Hiroshima University, Higashi-Hiroshima Japan
Phonons as well as electrons and magnons play a very important role for controlling physical properties. Lattice phonons have been the main issues for electron-phonon interactions for long years in the past. However, intra-cluster phonons are also thought to be important to date and even more such phonons can provide the possibility for giving artificial control in phonons. Owing to the inner nano spaces in polyhedral network compounds that can accommodate atoms, the atomic phonons showing large motional freedom in time and space can be created and this is recently drawing much attention as rattling phonons. Clathrate compounds have the nano cage structure consisting of IVth group elements with aligned shared faces. Therefore, these materials can accommodate atomic elements to be confined. Because of the large inner spaces, the endohedral atoms move under the potentials made by the cages and give rise to such rattling phonons featured by anharmonic oscillations. These phonons are greatly different from the conventional lattice phonons and may produce unique electron-phonon interactions.In this talk, we would like to show one of such good examples of electron-phonon interactions which are considered to be related to rattling phonons, using silicon and germanium clathrate compounds. In the clathrate compounds Ba24Si100 and Ba24Ge100, Ba atoms are confined in the polyhedral cage consisting of Si and Ge respectively.These materials are crystallographically identical, however the space inside the cage is different in size because of the framework elements with different covalent bond lengths. This situation leads to the different phonon modes associated with Ba. We have exemplified such unique motions in terms of x-ray diffraction data using MEM/Rietveld analyses. Actually, quite different electron density maps were observed between these two compounds. The detailed electronic states have also been studied directly by soft x-ray photoelectron spectroscopy. Apparently, good causality between the change in the Ba electron density map as a function of temperature and the electronic states was observed.We will discuss these exotic systems especially focusing on the relation between superconductivity and the rattling phonons.
EE4: Poster Session
Thursday AM, November 29, 2007
Exhibition Hall D (Hynes)
9:00 PM - EE4.1
Spectroscopy of Chromium Doped Garnets in a Wide Temperature Range.
Humeyra Orucu 1 , Gonul Ozen 2 , John Collins 3 , Baldassare Di Bartolo 1 Show Abstract
1 Physics, Boston College, Chestnut Hill, Massachusetts, United States, 2 Physics, Istanbul Technical University , Istanbul Turkey, 3 Physics and Astronomy, Wheaton College, Norton, Massachusetts, United States
Chromium doped garnets are tunable solid state laser materials in the red and near infrared. We have studied the emission spectra of Y3Al5O12:Cr3+ (YAG:Cr), Gd3Ga5O12:Cr3+ (GGG:Cr) and Gd3Sc2Ga3O12:Cr3+ (GSGG:Cr) in a wide range of temperatures 25-800 K and we have related their spectroscopic properties to the interaction between the 2E and 4T2 energy levels of the chromium ion.
9:00 PM - EE4.2
Polariton Band Structure in Quasiperiodic Photonic Crystals.
Eudenilson Albuquerque 1 , F. De Medeiros 1 , M. Vasconcelos 2 , P. Mauriz 2 Show Abstract
1 Fisica, UFRN, Natal, RN, Brazil, 2 Fisica, CEFET, São Luís, MA, Brazil
When the electromagnetic radiation propagating through a polarizable dielectric or magnetic crystal excites some internal degrees of freedom of the crystal, it gives rise to a hybrid (or mixed) modes called polaritons. Polaritons are quasi-particles consisting of a photon coupled to an elementary excitation (plasmon, phonon, exciton, etc.) which polarizes the crystal. In quasiperiodic structures, they exhibit collective properties not shared by their constituents. Therefore, the long range correlations induced by the construction of these structures are expected to be reflected someway in their excitation's spectra, defining a novel description of disorder. Indeed, theoretical transfer matrix treatments show that these spectra are fractals . The study of the fractal spectra generated by these quasiperiodic structure can help us to understand the global order and the rules that these systems obey at high generation order.The aim of this work is twofold: first, we want to show the polariton spectra of a photonic quasiperiodic multilayer structure comprised of alternating layers of both positive (SiO2) and negative refractive index (metamaterial) materials, using a theoretical model based on a transfer matrix treatment . The quasiperiodic structures are characterized by the nature of their Fourier spectrum, which can be dense pure point (Fibonacci sequences) or singular continuous (Thue-Morse and double-period sequences). These substitutional sequences are described in terms of a series of generations, and can be generated by the following inflation rules: A → AB, B → A (Fibonacci); A → AB, B → BA (Thue-Morse); and A → AB, B → AA (double-period), where A and B are the building blocks modeling the metamaterial and SiO2 respectively. We make use of the transfer matrix approach to analyze them, simplifying the algebra which would be otherwise quite involved. We discussed the polariton spectra for both the ideal case, where the negative refractive index material can be approximated as a constant in the frequency range considered, as well as the more realistic case, taking into account the frequency dependent electric permittivity ε and magnetic permeability μ. Second, we also present a quantitative analysis of the results, pointing out the distribution of the polariton band widths for high generations, which gives a good insight about their localization and power laws.We would like to thank partial financial support from CNPq-Rede Nanobioestruturas (Brazilian Research Agency) E.L. Albuquerque and M.G. Cottam, Phys. Rep. 376, 225 (2003). F.F. de Medeiros, E.L. Albuquerque and M.S. Vasconcelos, J. Phys.: Condens. Matter 18, 8737 (2006).
Subhash L. Shinde Sandia National Laboratories
Yujie J. Ding Lehigh University
Jacob Khurgin Johns Hopkins University
Gyaneshwar P. Srivastava University of Exeter
Yujie J. Ding
Subhash L. Shinde
Thursday AM, November 29, 2007
Room 209 (Hynes)
9:30 AM - **EE5.1
Phonon Engineered Nanostructures Studied by Picosecond Ultrasonics.
Bernard Perrin 1 , D. Lanzillotti-Kimura 1 , S. Zhang 1 , A. Huynh 1 , L. Belliard 1 , E. Peronne 1 , B. Jusserand 1 , A. Fainstein 2 , A. Lemaitre 3 , A. Michel 4 , G. Abadias 4 , C. Jaouen 4 Show Abstract
1 INSP, Université Pierre & Marie Curie, Paris France, 2 Instituto Balseiro, Centro-Atomico, San Carlos de Bariloche, Rio Negro, Argentina, 3 LPN, CNRS, Marcoussis France, 4 LMP, Univeristé de Poitiers, Poitiers France
10:00 AM - EE5.2
Temperature Dependence of the Graphene Phonon Modes.
Irene Calizo 1 , Alexander Balandin 1 , Wenzhong Bao 2 , Feng Miao 2 , Chun Ning Lau 2 Show Abstract
1 Nano-Device Laboratory, Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States, 2 Department of Physics and Astronomy, University of California - Riverside, Riverside, California, United States
Graphene is a two-dimensional lattice of carbon atoms, which exhibits unusual energy dispersion relations, e.g., the low-lying electrons in the single layer graphene behave like massless relativistic Dirac fermions. The electron mobility above 15,000 cm2/Vs under ambient conditions was reported for graphene. One of the major hurdles in the graphene processing and device applications is the difficulty of identification and counting of the number of layers. Recently it was established that the Raman spectroscopy can be used as a convenient tool for the graphene characterization at room temperature. Here we report the results of our recent investigation, which allows one to perform the graphene “fingerprinting” at variable temperature. The latter is an important capability since the laser excitation or application of bias to the graphene-based devices may result in the local temperature change. Specifically, we obtained the temperature dependence of the G and 2D peaks in the Raman spectra of the single-layer and bi-layer graphene. The number of layers was independently confirmed by electrical measurements and atomic force microscopy. It was established that the extracted values of the temperature coefficients for the G and 2D-band frequencies in the spectra from the single-layer ad bi-layer graphene are different . In addition to the practical application for determining the number of layers, the obtained results shed light on the anharmonic properties of graphene. This work was supported, in part, by the DARPA – SRC MARCO Center on Functional Engineered Nano Architectonics (FENA) and DARPA – DMEA UCR – UCLA – UCSB Center for Nanoscience Innovation for Defense (CNID).  I. Calizo, A.A. Balandin, W. Bao, F. Miao and C.N. Lau, Appl. Phys. Lett., 2007; see also at http://www.ndl.ee.ucr.edu
10:15 AM - EE5.3
Investigations of the Thermal Transport Across the Interface Between Multiwalled Carbon Nanotube Arrays and Si/SiO2 and Inconel Substrates.
Youngsuk Son 1 , Sunil Pal 1 , Theodorian Borca-Tasciuc 1 , Pulickel Ajayan 2 , Richard Siegel 2 Show Abstract
1 Mechanical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Carbon nanotubes (CNTs) are ranked the most promising materials for interconnects and thermal management in nanodevices and nanoelectronics which demand a high degree of heat dissipation to preserve their integrity, reliability, and performance. Even though CNTs exhibit superior axial heat conduction property, the interface thermal resistance (ITR) between the tube and the growth substrate considerably limits the thermal transport. Therefore understanding the ITR is critical for thermal management and interconnect applications using CNTs. In this study, vertically aligned multiwalled carbon nanotube (MWCNT) arrays grown on dielectric (Si/SiO2) and metallic (Inconel) substrates are prepared and their thermal characterization is carried out by employing a photothermoelectric technique and a steady state method. The experimental results are compared with predictions of existing theoretical models, which reveal much smaller ITR values than experimentally measured. The theoretical models consider classical constriction effects and contributions due to diffuse mismatch thermal resistance and electron-phonon interactions. The observed discrepancy may be attributed to imperfect contact and/or existence of catalyst layer at the interface, producing high thermal resistance.
10:30 AM - EE5.4
Unusually Low Thermal Conductivity of Gallium Nitride Nanowires.
Csaba Guthy 1 , Chang-Yong Nam 1 , Aaron Stein 2 , James Misewich 2 , John Fischer 1 Show Abstract
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Despite significant interest and promising properties of GaN nanowires (NWs), to the best of our knowledge no experimental thermal conductivity data have been reported so far. In this study we measured the thermal conductivity κ of individual GaN NWs grown by thermal chemical vapor deposition (CVD) method with diameters ranging from 97 to 181 nm, using the “suspended islands” method. We observed an unexpectedly large κ reduction compared to the bulk, with room temperature values in the range of 13-19 W/m-K and weak diameter dependence. We also observe unusual T1.8 dependence at low temperature. The Callaway model of heat conduction suggests a moderate κ reduction due to increased boundary scattering for small cross-sections. TEM analysis revealed the presence of stacking faults (SFs), which are expected to limit the phonon mean free path. EELS measurements indicate high Si and O impurity concentrations that are estimated at 0.1-1 at% and 0.01-0.1 at%, respectively. Based on extensive numerical calculations we conclude that both the unexpectedly low κ as well as the T1.8 low-temperature dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that the mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems.Supported by DOE Grant No. DE-FG02-98ER45701.Research carried out (in whole or in part) at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.
10:45 AM - EE5.5
Growth and Processing of Diamond Films for Nanoscale Thermal Management Applications.
Thomas Friedmann 1 , J. Sullivan 1 , S. Shinde 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Chemical vapor deposition (CVD) diamond films can be patterned and processed into simple microelectromechanical systems (MEMS) using standard micromachining techniques. For thermal management problems, this allows for the design of heat spreading structures that can be used to cool hot spots in devices. It is well known that the thermal conductivity of diamond films can vary due to defects introduced during the growth. What is less understood is how these defects will affect the thermal conductivity and mechanical dissipation of micro- to nanoscale structures. For example, the relative importance of grain boundary scattering may be suppressed in nanoscale structures made from microcrystalline diamond due to the reduced concentration of grain boundaries. Thus, by reducing device dimensions it may be possible to experimentally infer grain boundary thermal conductivity and compare this to theoretically calculated values.The focus of this presentation will be on growth and processing of diamond films into simple MEMS devices suitable for thermal conductivity measurements. From the growth side, control of stress and stress gradients is important for successful device fabrication. The magnitude of in-plane biaxial stress can be changed by altering the growth temperature and the methane to hydrogen ratio. Stress gradients through the film thickness can be caused by the evolution of the columnar grain structure during growth that can in turn be influenced by the nucleation conditions. To control nucleation, the diamond films were grown on oriented Ir(100) films grown on Si(100) surfaces with an yttria stabilized zirconium (YSZ) buffer layer. For device fabrication, simple cantilever beam structures were produced using e-beam lithography and lift-off to pattern films with an aluminum etch mask followed by etching in an oxygen plasma and a wet etch to undercut and release the diamond beams.‡ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
11:30 AM - **EE5.6
Photonic Functionalities Driven by Coherent and Monochromatic Surface Acoustic Phonons.
Mauricio deLima 1 , Markus Beck 2 , Paulo Santos 2 Show Abstract
1 Materials Science Institute, University of Valencia, Valencia Spain, 2 , Paul Drude Institute, Berlin Germany
The acoustic excitation of semiconductor-based photonic structures is an emerging field with great potential for new types of photonic manipulation . We will review recent experiments, where a coherent and monochromatic beam of acoustic phonons in the form of surface acoustic waves (SAWs) is used to modulate photons and polaritons in planar semiconductor microcavities. Furthermore, we will discuss novel approaches to use the phase coherence of these phonons to build extremely compact optical devices for light propagating in-plane. The strong interaction between a high population of non-thermal acoustic phonons and photons trapped in a semiconductor microcavity gives rise to a tunable optical superlattice with a folded dispersion relation and well-defined energy-gaps, which were probed by reflectivity and diffraction experiments . Placing a quantum-well with excitonic energy matched to the cavity mode allows for the investigation of the acoustic modulation of microcavity-polaritons in the strong-coupling regime. In this case, as the phonon population increases, one identifies the transition from a polariton superlattice to an array of weakly coupled polariton wires . These experimental results are in very good agreement with a comprehensive model that takes into account the phonon-induced strain as well as the elasto-optical and deformation potential interactions.Finally, we will introduce a novel device concept based on the modulation of the refractive index of ridge waveguides (WGs) by surface acoustic phonons. Recently, we have demonstrated an extremely compact (active region of approx. 15 microns) Mach-Zehnder modulator with 10 to 90 % peak to peak modulation and fundamental frequency above 500 MHz . This concept has been extended to structures with several WGs, which are acoustically modulated with a specific phase relationship. These devices allow for the implementation of complex optical functionalities such as optical switches, harmonic generators and pulse shapers.  M. M. de Lima, Jr. and P. V. Santos, Rep. Prog. Phys. 68, 1639 (2005). M. M. de Lima, Jr., R. Hey, P. V. Santos, and A. Cantarero, Phys. Rev. Lett. 94, 126805 (2005). M. M. de Lima, Jr., M. van der Poel, P. V. Santos, and J. M. Hvam, Phys.Rev.Lett. 97, 045501 (2006). M. M. de Lima, Jr., M. Beck, R. Hey, and P. V. Santos, Appl. Phys. Lett. 89, 121104 (2006).
12:00 PM - EE5.7
Pool Boiling Investigations of CNT based Nanofluids.
Abhishek Jain 1 , Theodorian Borca-Tasciuc 1 , Michael Jensen 1 Show Abstract
1 Deparment of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Nanofluids (dispersions of nanostructures in liquids) are reported to have higher values of heat transfer coefficients, thermal conductivities and critical heat flux as compared with the host liquid. These properties make nanofluids a potential solution for challenging thermal management problems such as those encountered in high heat flux electronic device applications. In prior reports, nanoparticles (Al2O3) based nanofluids were found to increase critical heat flux values by 200%, however, the nucleate boiling heat transfer coefficients were not found to change by significant values. The objective of the present work is to test the pool boiling characteristics of carbon nanotubes (CNTs) based nanofluid solutions. Due to their high thermal conductivity (highest reported to date is ~3000 W/m-K) carbon nanotubes nanofluids may exhibit enhanced heat transfer coefficients. A microfabricated heater with integrated temperature sensors are prepared for the pool boiling experiments using the conventional micro fabrication techniques. The heater is made of a gold film 2000 A° thick deposited on a glass substrate. Thermistor based temperature sensors are distributed along 2 mm sections of the heater to measure local heat transfer coefficients. The resistive temperature sensors are calibrated before experiment to determine the temperature coefficient resistance (TCR) for each section. The low value of thermal conductivity of the glass substrate minimizes conduction losses and improves the accuracy of the measured heat transfer coefficients. De-ionized water is used as the base fluid. Different type (single wall versus multiwall tubes) and concentrations of CNTs are studied and their effect on heat transfer coefficients and critical heat flux will be reported. Preliminary results were performed on nanofluids containing single walled (SW, 0.00067 mg/l) and multi-walled (MW, 0.00113 mg/l) carbon nanotubes. As compared to water, an increase of 57.8% with SWCNTs and 70.71% with MWCNTs in the local heat transfer coefficients was found. The mechanisms responsible for the observed enhancement are under investigation and will be reported.
12:15 PM - EE5.8
Thermal Conductivity of Thermal Interface Materials.
Travis Fullem 1 , Eric Cotts 1 Show Abstract
1 Materials Science program and Department of Physics, Binghamton University, Binghamton, New York, United States
While detailed theories exist for thermal conduction due to electrons and phonons in crystalline solids, phonon scattering and transmission at solid/solid interfaces is not as well understood. Steady increases in the power density of devices have resulted in an increasing need in the electronics industry for an understanding of thermal conduction in multilayered structures. The materials of interest in this study consist of a polymer matrix in which small (approximately ten microns in diameter) highly conductive filler particles (such as Ag or alumina) are suspended. These materials are used to form a thermal interface material bondline (a fifty to two hundred micron bonding layer) between a power device and a heat spreader. Such a bondline contains many polymer/filler interfaces. Using a micro Fourier apparatus, the thermal conductivities of such thermal interface material (TIM) bondlines of various thicknesses, ranging from fifty microns to several hundred microns, have been measured. The microstructure of these bondlines has been investigated using optical microscopy and electron microscopy. Measured values of thermal conductivity are compared to values for bulk samples, and considered in terms of microstructural features such as filler particle depleted regions. The influence of polymer/filler particle interfaces in the TIM bondline on phonon transport through the bondline is also considered.
12:30 PM - EE5.9
Computational Investigation of Thermal Property Measurement Techniques.
Edward Piekos 1 , Leslie Phinney 1 Show Abstract
1 Microscale Science and Technology, Sandia National Laboratories, Albuquerque, New Mexico, United States
A necessary step in the quest to create nanostructured materials with tailored physical properties is to understand the effect of various features on phonon transport. While detailed modeling is a useful probative tool, measurements are a critical component to these investigations due to the complex phenomena involved, as well as the need to validate the models. Due to constraints on the fabrication of tightly-controlled structures, however, samples available for measurement may contain only a small quantity of the feature under study. The measurement must therefore be very sensitive in order to discern the effect. One may therefore argue that these investigations demand a detailed understanding of the measurements themselves.Toward this end, a computational investigation of common thermal property measurement techniques is performed. In this study, test structures typical of those employed by the steady state and 3ω techniques are constructed and simulations are run that emulate test procedures using a Fourier's Law-based (continuum) thermal analysis package. Data reduction techniques commonly used in these measurements, which depend on analytical models with varying levels of idealization, are then applied to data extracted from the simulations and the resultant thermal properties are compared to the known values. In this manner, errors introduced by effects neglected in the data reduction models are quantified. It is demonstrated that the systematic, rather than random, nature of these errors can produce a large change in the derived quantity from a small deviation from the ideal case. By modifying the simulation to exclude effects in turn, a detailed breakdown of error sources, and their corrections, is constructed.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.