Ivana Savic, Tyndall National Institute
Olivier Delaire, Duke University
Keivan Esfarjani, University of Virginia
Richard Wilson, University of California, Riverside
QN04.01/QN05.03: Joint Session: Nanoscale and Nonequilibrium Thermal Transport
Tuesday AM, April 23, 2019
PCC North, 100 Level, Room 124 B
10:30 AM - *QN04.01.01/QN05.03.01
Nanoscale Thermal Metrology Using SEM, TEM and Confocal Microscopy
University of California, Berkeley1,Lawrence Berkeley National Laboratory2Show Abstract
I will review several collaborative efforts at developing new non-contact methods for heating and thermometry at the nanometer scale. Examples include techniques based on SEM (e-beam as a point heater; secondary electron yield as a thermometer), TEM (thermometry using the Debye-Waller effect), and confocal microscopy (luminescence thermometry of individual nanoparticles).
11:00 AM - QN04.01.02/QN05.03.02
A Multi-Temperature Model for Non-Equilibrium Thermal Transport
Purdue University1Show Abstract
Conventionally, the two-temperature model has been widely used for electron-phonon coupled non-equilibrium thermal transport. However, many recent applications have shown that different phonon branches can be in strong thermal non-equilibrium. Therefore, assuming a local equilibrium lattice can lead to misleading or wrong results. Here, we present a multi-temperature model to capture the non-equilibrium among different phonon branches, and demonstrate its advantages over the conventional two-temperature model for bulk materials and across interfaces.
11:15 AM - QN04.01.03/QN05.03.03
Specular Reflection Creates Lowest Thermal Phonon Conductivity
Georgia Institute of Technology1Show Abstract
The thermal conductivity of materials is not a fixed physical property but can be manipulated by controlling the transport properties of thermal phonons. Recently, a large number of experiments have been introduced where thermal conduction is reduced by orders of magnitude via phonon mean free path reduction through diffuse surface scattering. In contrast to established work that use the diffuse surface scattering of phonons as the physical mechanism to reduce the thermal conductivities, in this talk we show that the largest reduction of thin film heat conduction is achieved via specular scattering. Our results thus create a new paradigm for heat conduction manipulation since smooth surfaces – in contrast to rough surfaces – can be more effective on suppressing thin film phonon heat conduction.
11:30 AM - *QN04.01.04/QN05.03.04
Phonon Heat Conduction and Nanoscale Disorder—From Scatterings to Localizations
CNRS–University of Tokyo1Show Abstract
Effect of structural disorder on phonon thermal conduction remains an open question with a large spectrum of physical effects, as the plane-wave description of atomic vibrations is expected to become irrelevant. In a first stage, atomic scale disorder will be investigated in various systems –silicon , SiGe nanowires , partial-crystal partial-liquid , 2D [4,5] - with atomic scale simulations. Secondly, thermal properties of nanoscale random materials [6,7] will be presented to raise the question of the eventual impact of localization on heat conduction.
 K. Sääskilahti, J. Oksanen, J. Tulkki, A. J. H. McGaughey, and S. Volz, Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations, AIP Advances.
 Honggang Zhang, Haoxue Han, Shiyun Xiong, Hongyan Wang, Sebastian Volz, and Yuxiang Ni, Impeded thermal transport in composition graded SiGe nanowires, App. Phys. Lett., 111, 121907, (2017).
 Y. Zhou, S. Xiong, X.L. Zhang, S. Volz, M. Hu, Thermal Transport Crossover from Crystalline to Partial-crystalline Partial-liquid State, Nat. Comm., accepted.
 Van-Truong Tran, Jérôme Saint-Martin, Philippe Dollfus & Sebastian Volz, Optimizing the thermoelectric performance of graphene nano- ribbons without degrading the electronic properties, Sc. Rep., 7: 2313 | DOI:10.1038/s41598-017-02230-0.
 Van-Truong Tran, Jérôme Saint-Martin, Philippe Dollfus and Sebastian Volz, High thermoelectric performance of graphite nanofibers, Nanoscale, DOI: 10.1039/C7NR07817J
 J. Maire, R. Anufriev, R. Yanagisawa, A. Ramiere, S. Volz, and M. Nomura, Heat conduction tuning by wave nature of phonons, Science Advances, 3(8), e1700027 (2017).
 S Hu, Z Zhang, P Jiang, J Chen, S Volz, M Nomura, B Li, Randomness-Induced Phonon Localization in Graphene Heat Conduction, The journal of physical chemistry letters 9 (14), 3959-3968
QN04.02: Nanostructures and Interfaces I
Tuesday PM, April 23, 2019
PCC North, 100 Level, Room 124 A
1:30 PM - *QN04.02.01
Far-Field Submicron Thermoreflectance Imaging
Ali Shakouri1,Amirkoushyar Ziabari1,Sami Alajlouni1
Purdue University1Show Abstract
Self-heating and localized hotspots can limit the performance of electronic and optoelectronic devices. Contact-related artifacts and diffraction limit the accurate temperature measurements at deep submicron scale. Here we combine thermoreflectance (TR) imaging with a robust image reconstruction technique to achieve accurate temperature maps of nanoscale metal interconnects. Reconstructing accurate temperature profiles from far-field diffraction-limited images is an ill-posed inverse problem. To formulate the inverse problem, we developed a model based image processing technique based on a Bayesian framework that consists of a maximum-a-posteriori (MAP) cost function with Generalized Gaussian Markov random field priors (GGMRF). The imaging system was approximated by a Gaussian blurring kernel using the information about the pixel dimensions, numerical aperture of the lens used in the system as well as the wavelength of the light. The iterative coordinate descent (ICD) optimization is then used to minimize the corresponding cost function and extract the temperature profiles.
We performed the reconstruction both for numerically designed experiments as well as experimental thermoreflectance thermal images. Additionally, statistical analysis of the resolution limit shows that image reconstruction is a strong function of signal-to-noise ratio. We carried out detailed analysis to investigate the impact of the signal-to-noise ratio on extracting the accurate temperature of nanoscale features. Theoretical and experimental results demonstrate accurate thermal mapping of objects down to 100nm. The proposed non-contact technique provides an alternative to techniques such as scanning probe that are often slower and quite sensitive to surface roughness. Additionally, the technique can be applied to other far field imaging experiments to study the performance of nanoscale optoelectronic and plasmonic devices.
Since quasi ballistic phonons dominate submicron thermal transport in many semiconductors at room temperature, there are questions about the definition of temperature and what actually a thermometer measures. Transient hyperspectral imaging as well as resistance thermometry are used to study the apparent temperatures of various metals and nearby semiconductors and to verify our submicron TR imaging results. Indications for superdiffusive and hydrodynamic heat flow are briefly described.
2:00 PM - QN04.02.02
Pre-Interface Phonon Scattering Effect in Thermal Transport Across Solid Interfaces
Ruiyang Li1,Eungkyu Lee1,Tengfei Luo1
University of Notre Dame1Show Abstract
Understanding phonon transport across solid-solid interfaces can greatly impact thermal engineering in many solid-state applications. Conventionally, phonon scattering are considered impedance to thermal transport. In this study, we present systematic study of the effect of pre-interface phonon in thermal boundary conductance using non-equilibrium molecular dynamics (NEMD) simulations. We first examine the fundamental role of anharmonic phonon scattering in thermal transport across an artificial solid junction, which consists of a monoatomic lattice and a diatomic lattice. Using interatomic force parameters expanding up to the third-order, we investigate how inelastic phonon scattering processes in the diatomic lattice layer influence thermal boundary conductance. It is found that the anharmonicity inside the diatomic lattice layer promotes phonon-phonon energy conversion between optical and acoustic phonon modes, which eventually lead to enhanced thermal transport across the solid junction by populating more acoustic phonon modes with higher transmission probability through the pre-interface phonon scattering. Importantly, we find that anharmonic scattering effect in the pre-interfacial region is more important than that right at the interfaces in thermal transport. Next, we explore pre-interface phonon scattering effect in thermal transport across SiC/GaN interfaces by doping isotopes in the GaN, where the SiC/GaN interface is of technological importance in power electronics. We use 15N or 35Ga as isotopes in NEMD simulations. By tuning various isotope-doping characteristics (i.e., the isotope concentration, skin depth of the isotope region, and its distance from the interface), we investigate an enhancement of thermal conductance across SiC/GaN interfaces. We find that 10% of 15N isotope in GaN layer can increase TBC by as much as 23% compared to the isotopically pure cases. In addition, when the isotope-doped region is located away from the SiC/GaN interface, the TBC enhancement still exist, showing pre-interface phonon scattering by isotopes is mainly responsible for the enhanced TBC. Moreover, analysis of spectral temperatures of phonon modes reveal that heat transfer rates by low frequency phonons (< 20 THz) across SiC/GaN interfaces increase after isotopes are introduced.
Our studies demonstrate that pre-interface phonon scatterings can help redistribute phonon energy, so that phonon modes with higher transmission probability can be more populated, which leads to TBC enhancements across solid-solid interfaces. Development of new theoretical models should consider this pre-interface phonon scattering effect. Practical materials design can also take advantage the results of this work to enhance interfacial thermal transport.
2:15 PM - QN04.02.03
Predicting the Phonon Mode-Resolved Specularity Parameter Using the Atomistic S-Matrix Method
Institute of High Performance Computing1Show Abstract
The phonon specularity parameter represents the probability of the phonon being specularly scattered by a surface and is crucial for understanding phonon-mediated thermal transport in low-dimensional semiconducting or insulating materials such as thin films and nanowires. However, there exists no reliable and accurate method to predict the relationship between the mode-dependent specularity parameter and the atomistic structure of a non-ideal edge or interface.  In addition, the dependence of the specularity parameter on wavelength, direction and polarization cannot be determined reliably with current theories.
To treat this problem, we introduce an S-matrix approach  that is grounded conceptually in conventional quantum mechanical scattering theory and developed from our earlier extension of the Atomistic Green’s Function (AGF) method,  allowing us to compute the scattering amplitude between an incident phonon and a reflected or transmitted phonon. The utility of our approach is illustrated through two examples. In the first example, we study the specularity dependence on edge chirality and explain how specularity is reduced for the ideal armchair edge in graphene, shedding light on the findings by Wei, Chen and Dames . In the second example, we investigate grain boundary phonon scattering in graphene and provide direct evidence that the the specularity is significantly different between reflected and transmitted phonons.  Our S-matrix method resolves some of the existing difficulties in predicting the mode-dependent specularity parameter and opens the way for analyzing how edges and interfaces can be modified to control phonon scattering with atomistic precision.
1. G. Chen, Nanoscale Energy Transport and Conversion (Oxford University Press, Oxford, 2005).
2. Z.-Y. Ong, “Atomistic S-matrix method for numerical simulation of phonon reflection, transmission and boundary scattering,” Phys. Rev. B (in press).
3. Z.-Y. Ong and G. Zhang, “Efficient approach for modeling phonon transmission probability in nanoscale interfacial thermal transport,” Phys. Rev. B 91, 174302 (2015); Z.-Y.Ong, “Tutorial: Concepts and numerical techniques for modeling individual phonon transmission at interfaces,” J. Appl. Phys. 124, 151101 (2018).
4. Z. Wei, Y. Chen, and C. Dames, “Wave packet simulations of phonon boundary scattering at graphene edges,” J. Appl. Phys. 112, 024328 (2012).
5. D. Li and A. J. H. McGaughey, “Phonon dynamics at surfaces and interfaces and its implications in energy transport in nanostructured materials — an opinion paper,” Nanoscale Microsc. Thermophys. Eng. 19, 166 (2015).
2:30 PM - *QN04.02.04
High-Throughput Thermal Conductivity Predictions and Spatial-Temporal Imaging
Kedar Hippalgaonkar2,3,Ding Ding1
Singapore Institute of Manufacturing Technology1,Institute of Materials Research and Engineeringn2,Nanyang Technological University3Show Abstract
With nanostructures in solid state materials reaching the size of the phonon mean free path and lower, quasiballistic and anisotropic effects play significant roles in their thermal properties. Many new experimental techniques have been developed to probe phonon physics at such lengthscales, however ascertaining spatially dependent thermal properties typically requires control of parameters in the spatial or frequency domain. Here, I will present our proposal for a one-shot optical pump-probe methodology that can potentially directly image both spatial and temporal temperature profiles. Coupled with a multi-parameter optimization of the heat diffusion equation, this technique can prove powerful in measuring anisotropic and quasiballistic heat transport directly, serving as a high-throughput tool for screening of thermal properties. Coupled with first principles DFT calculations of the gruneisen parameter and debye temperature of a large number of compounds extracted from Materialsproject.org, I will illustrate a vision of how machine learning can be used for prediction of desirable thermal properties.
QN04.03: Phonons, Magnons and Magnetic Phenomena
Tuesday PM, April 23, 2019
PCC North, 100 Level, Room 124 A
3:30 PM - *QN04.03.01
Time-Resolved Magneto-Thermal Microscopy—High-Resolution Dynamic Imaging of Magnetic Materials Using Picosecond Heat Pulses
Cornell University1Show Abstract
Research into magnetic devices – emerging memory, logic, and oscillator technologies – is enabled by magnetic imaging techniques that possess simultaneous picosecond temporal resolution and 10 – 100 nm spatial resolution. Conventionally, this combination is available only at facility-based research centers using e.g., pulsed x-ray dichroism techniques. In addition, many of the most exciting magnetic material systems, including ultrathin ferromagnetic or antiferromagnets insulators buried beneath heavy metals are difficult to image with any method. To address these challenges in an accessible way, we have developed a table-top spatiotemporal magnetic microscope based on nanoscale, picosecond thermal pulses. Our method takes advantage of magneto-thermal interactions that couple heat flow to electron or spin transport, including the anomalous Nernst effect  and the longitudinal spin Seebeck effect . Using focused light as a picosecond heating source, we demonstrate that these imaging modalities have time resolution on the order of 10 ps and sensitivities to magnetization angle of 0.1 – 0.3°/√Hz for ferromagnetic metals and insulators. In combination with phase-sensitive microwave current imaging, phase-sensitive ferromagnetic resonance imaging  enables direct imaging of the gigahertz-frequency magnetic driving torque vector, which is valuable for understanding spin-orbit interactions . We also demonstrate magneto-thermal imaging of antiferromagnetic order in FeRh and NiO, offering a simple and accessible method to study spin-orbit torque switching of antiferromagnetic materials . Finally, I will describe how time-resolved magnetic imaging can be extended to greatly exceed the optical diffraction limit, both theoretically  and experimentally. We demonstrate scanning a sharp gold tip illuminated by picosecond laser pulses as the basis of a nanoscale spatiotemporal magnetic microscope.
 J. M. Bartell, D. H. Ngai, Z. Leng, and G. D. Fuchs, Nat. Commun. 6, 8460 (2015).
 J. M. Bartell, C. L. Jermain, S. V. Aradhya, J. T. Brangham, F. Yang, D. C. Ralph, and G. D. Fuchs, Phys. Rev. Appl. 7, 044004 (2017).
 F. Guo, J. M. Bartell, D. H. Ngai, and G. D. Fuchs, Phys. Rev. Appl. 4, 044004 (2015).
 F. Guo, J. M. Bartell, and G. D. Fuchs, Phys. Rev. B 93, 144415 (2016).
 I. Gray, T. Moriyama, N. Sivadas, R. Need, B. J. Kirby, D. H. Low, G. M. Stiehl, J. T. Heron, D. C. Ralph, K. C. Nowack, T. Ono, and G. D. Fuchs, arXiv:1810.03997 (2018).
 J. C. Karsch, J. M. Bartell, and G. D. Fuchs, APL Photonics 2, 086103 (2017).
4:00 PM - QN04.03.02
Spin-Lattice Dynamics Calculations of Phonon-Magnon Coupling in Bulk Magnetic Materials
Joseph Cooke1,Jennifer Lukes1
Univeristy of Pennsylvania1Show Abstract
We present calculations of phonon-magnon coupling in bulk iron and nickel using spin-lattice dynamics [1,2], an increasingly popular simulation technique in which atomic spins and positions are computed simultaneously. It has been shown recently that a position-dependent magnetic anisotropy term is required to allow for full two-way coupling of thermal energy between the spin and lattice subsystems [3,4], but the parameters in this term are not known for many materials. Here we describe a procedure to obtain these parameters from experimental data on iron and nickel. We find that simulations including our parameters yield physically meaningful thermalization of spin and lattice systems and produce coupling times on the order of 100 ps, which agrees with the order of magnitude for coupling times reported by others [3,5].
 P. W. Ma, C. H. Woo, and S. L. Dudarev, “Large-scale simulation of the spin-lattice dynamics in ferromagnetic iron,” Phys. Rev. B, 78, 024434 (2008).
 J. Tranchida, S. J. Plimpton, P. Thibaudeau and A. P. Thompson, “Massively parallel symplectic algorithm for coupled magnetic spin dynamics and molecular dynamics”, J. Comp. Phys. 372 p 406-425(2018)
 D. Beaujouan, P. Thibaudeau, and C. Barreteau, “Anisotropic magnetic molecular dynamics of cobalt nanowires,” Phys. Rev. B, 86, 174408 (2012)
 D. Perera, M. Eisenbach, D. M. Bicholson, G. M. Stocks, and D. P. Landau, “Reinventing atomistic magnetic simulations with spin-orbit coupling,” Phys. Rev. B, 93, 060402(R) (2016)
 A. Vaterlaus, T. Beutlet, and F. Meier, “Spin-lattice relaxation time of ferromagnetic gadolinium determined with time-resolved spin-polarized photoemissions,” Phys. Rev. Let. 67 p 3313-2217 (1991)
4:15 PM - QN04.03.03
Effect of External Magnetic Field on Electron-Phonon Coupling and Transport Properties
Jiayue Yang1,Wenjie Zhang1,Linhua Liu1
Shandong University1Show Abstract
With its merit of being controllable, nondestructive and easy-to-apply, magnetic field demonstrates great advantages in engineering electrical and thermal transport for practical applications. Yet, the scientific challenge is to better understand how the quantum behaviors of electrons and phonons that dominate the electrical and thermal transport, respectively, are modulated by the external magnetic field. In this work, we apply the all-electron first-principles simulations to calculate the influence of external magnetic field on the intrinsic electron-phonon coupling, and then investigate its effect on the electronic thermal conductivity of representative metals (Al, Ni and Nb), superconducting transition temperature of hydrogen sulfide and magnetoresistance of monolayer WTe2. For the electronic thermal conductivity study, the crucial issue is that electron-phonon coupling dominantly determines electron’s lifetime and external magnetic field can modulate electronic thermal transport by altering the electron-phonon coupling. We observe opposite change trend of electronic thermal conductivity with varying external magnetic field in Al, Ni and Nb, which is closely related to its electron localization function. Moreover, we find that magnetic field can slightly reduce the superconducting transition temperature by decreasing electron-phonon coupling. As for the magnetoresistance, the key issue is to calculate the magnetic field-dependent electron-phonon coupling and electron’s lifetime. This work sheds light on how external magnetic field alters the quantum interactions between electrons and phonons, and then modulates the macroscopic electrical and thermal transport properties.
4:30 PM - *QN04.03.04
Quasiparticle Thermometry in Nonequilibrium Systems
University of Texas at Austin1Show Abstract
In non-equilibrium phenomena, different quasiparticles such as phonons and magnons may exhibit different temperatures with the caveat that temperature is only strictly defined for an equilibrium system. Spin caloritronics, as an emerging field, investigates the interplay between the transport of spin and heat. In the representative spin Seebeck effect , a thermal gradient across a magnetic material generates a spin current. Many theories hypothesize that a temperature difference between the energy carriers of the spin and lattice subsystems, namely the magnons and phonons, is necessary for such thermal non-equilibrium generation of spin current. The experimental evidence for such phonon-magnon non-equilibrium has been critically lacking  because of the difficulty to characterize phonon and magnon temperatures independently.
We use Brillouin light scattering (BLS) to investigate thermally driven spin current in a prototypical magnetic insulator yttrium iron garnet (YIG). BLS, as an inelastic light scattering technique, is conceptually the same as Raman scattering. It is optimized to probe low-frequency excitations, e.g., acoustic phonons and magnons. These quasiparticles are the main energy and spin carriers in magnetic insulators. Three different quantities (i.e., central frequency, linewidth, and integrated intensity) associated with the BLS spectra all vary with temperature systematically and can be used as a temperature sensor for probing phonons and magnons independently . We show how BLS spectra can be used to probe phonon-magnon non-equilibriums , measure pure spin current, and evaluate magnon chemical potential in a magnetic insulator thermally driven out of equilibrium.
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 M. Agrawal, V.I. Vasyuchka, A.A. Serga, A.D. Karenowska, G.A. Melkov, and B. Hillebrands, Phys. Rev. Lett. 111, 107204 (2013)
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QN04.04: Poster Session: Nanoscale Heat Transport
Tuesday PM, April 23, 2019
PCC North, 300 Level, Exhibit Hall C-E
5:00 PM - QN04.04.01
Thermal Conductivity Characterization by Means of Scanning Thermal Microscopy—Impact of Sample Properties
Pierre-Olivier Chapuis1,Eloise Guen1,Axel Pic1,2,Sebastien Gallois-Garreignot2,Ali Alkurdi1,Nupinder Jeet Kaur3,Petr Klapetek3,Séverine Gomès1
Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon1,STMicroelectronics2,Czech Metrology Institute3Show Abstract
Scanning Thermal Microscopy (SThM) allows the thermal characterization of materials with a submicrometric spatial resolution . While obtaining qualitative trends, e.g. by means of maps, is straightforward, determining reliable quantitative thermal data from the experiments is challenging because the probe-sample heat exchange depends strongly on parameters such as the size, geometry and surface states of probe and sample [2-3]. There is a need for a complete and accurate thermal measurement methodology, which is the aim of this work. We focus on heated probes dissipating heat flux into samples initially at ambient temperature. Simultaneous heating and measurement of the SThM probe temperature provide information on the sample effective thermal conductivity.
For three different electrically-resistive and Joule-heated SThM probes, involving different sizes and sensor materials (etched Wollaston-wire microprobe, palladium nanoprobe and doped silicon nanoprobe), an improved methodology for thermal conductivity local-point measurement was developed. Studies in ambient and vacuum conditions were considered. The methodology enables to eliminate the impact of thermal drifts occurring in such experiments, to deal with the variation in the laser irradiation on the cantilever and probe, and to detect any change at the tip apex due to a deformation or a contamination. As a consequence, results are reproducible with a temperature resolution of the order of few millikevins.
Specimens of well-known thermal conductivity spanning between 0.1 and 150 W.m-1.K-1 and surfaces controlled in terms of roughness and nanomechanical properties were used as reference materials in order to obtain a calibration curve for each probe. In the three cases, their applications allow us to clearly confirm that thermal-conductivity measurements with SThM are limited to low thermal conductivity materials (k < few W.m-1.K-1). This is due to the designs of the cantilever of the three probes and may not easily be improved. As expected, reducing the probe size and placing it in vacuum improves the spatial resolution, but it also induces unfortunately an increase of sensitivity to the tip-sample contact physical parameters (e.g. roughness, thermal boundary resistance, surface state), which is detrimental to the accuracy.
The impact of roughness on the thermal conductance at the contact was studied in detail. Samples consisting of several sets of silicon surfaces with out-of-plane Sq roughness parameters of ~0, 0.5, 4, 7 and 12 nm were prepared by anodic oxidation . Our results show that roughness induces a thermal conductance decrease at the contact up to 35% compared to a flat silicon sample, depending on the probe and environment. Precise knowledge of the surface state is therefore key to accurate measurements.
The calibration and methodology were applied to characterize thin films of oxides and materials involved in electronics. The experimental results are backed by finite-element simulations (FEM) including effective thermal conductivities when the size requires it, local heat sinks
representing thermal constrictions of typical sizes smaller than the energy carrier mean free paths (when FEM is not able to capture the correct physics), and thermal boundary conductances in the Acoustic or Diffuse Mismatch Models (AMM-DMM). Advantages and drawbacks of SThM are discussed in light of the results.
 S. Gomès et al. Phys. Status Solidi Appl. Mater. Sci., 212, 3, 477-494 (2015)
 Y. Ge et al. Nanotechnology, 27, 32, 325503 (2016)
 F. Menges et al. Rev. Sci. Instrum., 87, 7, 74902 (2016)
 A. Pic et al. THERMINIC 2017 – 23rd Int. Work. Therm. Investig. ICs Syst., Sept (2017)
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The research leading to these results has received funding from the EU FP7 Programme under GA n°604668. and French ANR Programme under project TIPTOP.
5:00 PM - QN04.04.02
Characterization of 2D Surface Acoustic Waves in Silicon Gratings via Time-Domain Thermoreflectance (TDTR)
Yee Rui Koh1,John Gaskins1,Jeffrey Braun1,Patrick Hopkins1
University of Virginia1Show Abstract
Picosecond acoustics have long been used to probe the response of material properties  and, more recently, structural excitations such as surface acoustic waves   . Optical excitation via pulsed lasers offers the ability to use picosecond acoustics to provide non-contact, non-destructive, and ultrafine temporal resolution of these phenomena. We demonstrate the generation and characterization of the two-dimensional Rayleigh surface acoustic wave (2D SAW) on a set of silicon line grating samples with varying line widths (400 nm, 800 nm, 1600 nm, and 3200 nm) fabricated via electron beam lithography. The structures were patterned with line spacing equal to the grating width, etched to a depth of 100 nm, and then conformally coated with an 80 nm aluminum film that acts as an optical transducer. The existence of this 2D SAW in patterned structures was previously studied by Li et al.  In this work, we use time-domain thermoreflectance (TDTR) to investigate the propagation of 2D SAWs (v ~ 0-20 GHz) in the silicon line gratings. Prior studies  have shown that most of the acoustic oscillations die out in a few hundreds of picoseconds. Our TDTR measurements show SAW oscillations up to ~5000 ps in the 800 and 1600 nm silicon line grating samples. The time periods of the acoustic oscillations are approximately 90 ps, 300 ps, and 600 ps for the silicon line grating widths of 400 nm, 800 nm, and 1600 nm, respectively. We employed two variations of TDTR, two color (400 nm pump/800 nm probe) and two tint (spectrally split 800 nm pump/800 nm probe) in order to study the variations in excitation based on pump and probe excitation and sensing. Fast Fourier transforms (FFT) of the resultant data were used to obtain the SAW frequencies. The two tint (800 nm pump) measurements show a typical simple single SAW frequency f0, which is caused by the periodicity of the silicon line grating. A fundamental SAW frequency f0 and a secondary SAW with a frequency 20.5f0 are excited in the two color (400 nm pump) measurements. This 20.5f0 SAW is attributed to the acoustic wave in the diagonal direction of the silicon line grating. This diagonal wave is known as a 2D SAW . In addition to measurement and quantification of the 2D SAWs, we discuss the effects of the silicon grating periodicity as it relates to excitation, or lack of excitation, as a function of the TDTR system used to probe the SAW.
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5:00 PM - QN04.04.03
Effects of Ultrafast Structural Dynamics on the Accuracy of Transient Debye-Waller Temperature Measurements
Elisah VandenBussche1,David Flannigan1
University of Minnesota1Show Abstract
The demand for smaller, faster, and more efficient electronic architectures and devices has produced an urgent need to understand and mitigate the associated deleterious thermal effects. As a result, significant efforts have been devoted to the development of experimental methods that quantify nanoscale thermal transport with both high spatial and temporal resolutions, bringing about new insights into a variety of phenomena; including thermal conductivity, thermal strain, and phonon behaviors . This illustrates the value of extending time-averaged metrology techniques into the combined ultrafast and nanoscale regimes, which are the native scales on which thermal-energy carriers operate. In light of this, we are exploring ultrafast electron microscopy (UEM) as a method to access this challenging parameter space. In a typical UEM experiment, a femtosecond (fs) laser pulse is used to excite (pump) the specimen in situ, upon which a fs-duration discrete packet of photoelectrons generated from the TEM electron source is used to probe the response . By varying the relative arrival times of the pump laser pulses and the probe electron packets at the specimen, nanometer-scale ultrafast structural responses can be directly imaged, while angstrom-scale responses can be tracked in reciprocal space. In UEM, as in other ultrafast electron and X-ray experiments, the Debye-Waller (DW) effect is often invoked when explaining photoinduced scattering intensity changes and when determining transient temperatures by relating the changes to atomic thermal vibrations. While this enables direct probing of the intrinsic lattice response to thermal effects with high spatial and temporal resolutions, reliance on transient intensity changes of diffracted beams poses significant challenges. This is because factors other than mean-square atomic displacements can produce signal changes similar to those generated by thermal effects [3-5]. Therefore, in order to accurately and precisely determine nanoscale transient temperatures using ultrafast scattering methods, deleterious effects that obfuscate intensity changes arising from atomic thermal vibrations must be identified, quantified, and deconvolved.
To address this, we have systematically studied and quantified a number of potential effects that can arise during in situ fs photoexcitation that may impact DW-type responses and the resulting interpretations . Using small-grained polycrystalline aluminum films as a well-characterized test system, we explicitly illustrate the impact of specimen tilting and translation via rigorous statistical analysis of Debye-Scherrer-ring intensities obtained from numerous individual specimens and measurements. Despite having more than 105 individual grains within the selected areas, we find that tilting by as little as a fraction of a degree, or translating a 22.5-μm diameter illuminated region by only 20 nm, yields statistically-significant changes in the diffracted-beam intensity. This result is at first surprising, as one may have expected a uniform distribution of random zone-axis orientations under such conditions, thus circumventing any negative effects due to specimen tilting and translation. We explicitly show this is not the case, and we explore and quantify several sources of error and artifacts, such as slight texturing of the polycrystalline films due to surface-energy minimization. Possible approaches to mitigating inaccuracies in the DW-calculated temperatures are also addressed, including effective methods of data normalization and consideration of ideal specimen geometries for such studies.
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5:00 PM - QN04.04.04
Correlating Coherent Structural Dynamics to Photoexcited Charge-Carrier Behaviors Using Femtosecond Electron Imaging
Daniel Du1,Daniel Cremons1,2,David Flannigan1
University of Minnesota Twin Cities, CEMS1,NASA Goddard Space Flight Center2Show Abstract
Intense femtosecond (fs) photoexcitation of semiconducting materials can lead to the generation of high densities of charge carriers, shifting the Fermi-Dirac distribution into a highly non-equilibrium state. The relaxation pathways available to this new state are numerous, and the associated mechanisms can be intermingled and highly non-linear . One particular pathway is via the excitation of highly-coherent, low-frequency acoustic phonons that propagate outward from the photoexcited zone . For initially high carrier concentrations, this behavior is attributed either to a time-varying deformation potential or a time-varying thermoelastic effect, depending upon the band structure and the nature of photoexcitation. Importantly, these two regimes can be differentiated temporally, depending upon the carrier densities generated; at relatively high concentrations, Auger recombination begins to further augment the time-varying thermoelastic effect. Indeed, a material as simple as undoped germanium (Ge) displays several intriguing transient behaviors within the context just described. For example, the excited carrier-density lifetimes can be quite long and can significantly overlap with the electron-phonon coupling times, thus heavily interweaving the various relaxation pathways [3,4]. Further, intense fs photoexcitation can lead to the generation of hypersonic electron-hole plasma waves, the nature of which may evolve into coherent acoustic-type oscillatory behaviors . This suggests such behaviors may manifest as coherent, transient lattice-strain effects, the properties of which are directly linked to the hypersonic plasma waves .
Here, we describe our efforts to link the photoexcited charge-carrier dynamics in single-crystal Ge to the lattice degrees of freedom using fs electron imaging in an ultrafast electron microscope [7,8]. By leveraging the extreme sensitivity of local diffraction contrast to changes in reciprocal-lattice orientation, we are able to directly image the generation and the evolution of highly-coherent acoustic phonons initially propagating at hypersonic velocities in the plane of the crystal . While each phonon wavefront propagates with a constant velocity, the entire wave train displays a time-varying phase-velocity dispersion, relaxing from initial velocities greater than 35 nm/ps to the Ge longitudinal speed of sound (5 nm/ps) within one nanosecond. Analysis of the dispersion curves expected for symmetric dilatational and asymmetric flexural modes indicates the dynamics match most closely to a single, low-order symmetric mode. Interestingly, this symmetric mode appears to give rise only to increased scattering for each wavefront, indicating the reciprocal-lattice shapes (approximated here as rods) are only brought further onto the Ewald sphere rather than being equally distributed during the oscillation. Further, by using a plasma lensing technique , we find that the onset of the first phonon wavefronts are delayed by tens of picoseconds or more, relative to the moment of fs photoexcitation. Combined, these anomalous behaviors provide tantalizing hints of charge-carrier-dominant dynamics, rather than lattice deformations caused by coherent phonons, as producing the observed ultrafast contrast behaviors. Those possibilities are explored here.
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5:00 PM - QN04.04.05
Thermal Transport in Holey Silicon Membranes Investigated with Optically-Induced Transient Thermal Gratings
Ryan Duncan1,Giuseppe Romano1,Marianna Sledzinska2,Alexei Maznev1,Jean-Philippe Peraud3,Olle Hellman4,5,6,Clivia Sotomayor Torres2,Keith Nelson1
Massachusetts Institute of Technology1,Catalan Institute of Nanoscience and Nanotechnology2,Lawrence Berkeley National Laboratory3,California Institute of Technology4,Linköping University5,Boston College6Show Abstract
In semiconductor nanostructures with feature sizes on the order of 100 nm, thermal transport is expected to be well-described by the phonon Boltzmann transport equation (BTE) with diffuse boundary scattering. However, over the past several years there have been reports of anomalously low effective thermal conductivity values in one- and two-dimensional semiconductor nanostructures. In this study, we investigate thermal transport in nanostructured holey silicon membranes using the non-contact optical transient thermal grating (TTG) technique. We compare the experimental results with two ab-initio BTE numerical techniques. We obtain excellent agreement between theory and experiment, indicating that semiclassical Boltzmann transport theory for phonons is adequate for describing room-temperature thermal transport in semiconductor nanostructures with feature sizes on the order of 100 nm.
5:00 PM - QN04.04.06
Theory of Anisotropic Thermal Interface Resistance in Nanocomposite Materials
Iorwerth Thomas1,Gyaneshwar Srivastava1
University of Exeter1Show Abstract
We present a theory of anisotropic thermal interface (Kapitza) resistance for rough interfaces in nanocomposite materials. This is based on an extension  of a modified effective medium theory [2,3] that includes anisotropically resistive interface regions in a model of the lattice thermal conductivity of anisotropic nano insertions in anisotropically conductive hosts. The thermal conductivities of the host and nanodot insertions have been evaluated using a semi ab-initio theory  based on the solution of the linearised phonon Boltzmann transport equation within a generalized  Callaway effective relaxation time scheme . Phonon boundary scattering and the Kapitza resistance at the insertion-host interface have been treated by taking into account both specular and diffuse contributions [7,8]. The theory has been applied to a transition metal dichalcogenide (TMD) nanocomposite consisting of 2H WS2 inserts in a 2H MoS2 host. In general, it is found that the effect of specular scattering due to interface roughness is more pronounced for inserts smaller than 100 nm. Analysis of the results allows us to identify key physical parameters that should prove effective in controlling (i.e. obtaining minimum) lattice thermal conductivity of TMD nanocomposites.
 I. O. Thomas and G. P. Srivastava, Phys. Rev. B 98, 094201 (2018).
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 I. O. Thomas and G. P. Srivastava, J. Phys. CM 29, 505703 (2017).
 G. P. Srivastava, Rep. Prog. Phys. 78, 026501 (2015).
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 Y. K. Koh et al, Adv. Func. Mater. 19, 610 (2009).
 G. Chen, Phys. Rev. B 57, 14958 (1998).
5:00 PM - QN04.04.07
Impact of Irradiation Induced Nanoscale Defects on Thermal Conductivity of Cerium Dioxide
Vinay Chauhan1,Lingfeng He2,Janne Pakarinen3,David Hurley2,Marat Khafizov1
The Ohio State University1,Idaho National Laboratory2,Belgian Nuclear Research Center (SCK-CEN)3Show Abstract
Thermal conductivity is a critical physical property of ceramic nuclear fuels such as uranium dioxide. While cerium dioxide is considered as a solid electrolyte in solid oxide fuel cells (SOFC), in this study it is used as surrogate material to study the properties of nuclear fuel. In nuclear fuel, these oxides are exposed to extreme environments such as high temperature and bombardment with heavy particles. The damage introduced by such conditions in the form of defects generated inside the material can be detrimental to the structural stability of the material and ability to transport heat efficiently. Also, thermal conductivity degradation impacts fuel performance negatively. As a result, thermal conductivity of ceria has been widely investigated as one of the critical properties. This study is aimed at understanding the interplay between these nanoscale defects and the thermal conductivity of ceria.
Polycrystalline ceria samples were irradiated at 600 deg C to the same dose but at different rates using protons accelerated to 2 MeV. These irradiation conditions were chosen to promote the generation of nanoscale defects and to investigate their impact on thermal conductivity of ceria. SRIM simulations were done to identify the peak and plateau damage region inside the sample. The calculations estimated the plateau damage to be ~0.14 dpa. The quantitative analysis of radiation induced dislocation loops including size and density was performed using transmission electron microscope (TEM) for which the samples were prepared using focused ion beam (FIB) system.
X-ray diffraction was used to confirm the stability of crystal structure and also revealed detectable lattice expansion caused by accumulation of nanoscale defects. In addition to this, thermal conductivity was measured using modulated thermoreflectance methods and showed a notable reduction in irradiated samples. In order to isolate the impact of different defects on thermal conductivity, measurements were done for ambient temperature range of 100 to 300 K. The changes in thermal conductivity were analyzed quantitatively using the classical thermal transport model based on Klemens-Callaway formalism that considers reduction of thermal conductivity by irradiation induced nanoscale defects.
5:00 PM - QN04.04.08
Multiscale Thermal and Electrical Modeling of CMOS Devices and Circuits
Robin Daugherty1,Dragica Vasileska1
Arizona State University1Show Abstract
This simulation work explores the thermal effects on electrical characteristics of CMOS devices and circuits using a multiscale dual-carrier approach. Simulating for electron and hole transport simultaneously allows for complementary logic gates to be simulated at the device level, while current and voltage continuity are maintained at the circuit level. Further, the electrical model couples with a multiscale thermal solver, which solves for electron-phonon and hole-phonon interactions at the device level and phonon-phonon thermal transport in the packaging level. This methodology allows for the study of package level thermal transport without sacrificing the nuances of device self-heating, ultimately providing a more comprehensive understanding of how these interactions affect power consumption in CMOS systems.
The electrical model is comprised of an ensemble Monte Carlo simulator coupled with a Poisson solver. This framework provides accurate electrical characteristics in quasi-static regimes by iteratively solving for the potential profile and the electric fields then simulating the effect of the electric field on charge carriers. The Monte Carlo simulator solves the Boltzmann Transport by balancing each particle’s movement in real and momentum space with the collision integral through probabilistic scattering mechanisms. This framework provides current and voltage characteristics for each device; current and voltage continuity are maintained by solving at the circuit level.
Similarly, the methodology for simulating thermal characteristics includes two scales. At the device scale, the energy balance equation determines the transfer of energy from charge carriers to phonons. High-energy electrons or holes relinquish energy to optical and acoustic phonons through scattering and optical phonons decay into acoustic phonons. At the package level, a Fourier law solver simulates the subsequent conduction of heat in the form of lattice vibrations.
This framework proved effective in previous simulations for the electro-thermal characteristics in NMOS devices. This work demonstrates the effectiveness of the dual-carrier electrical solver in simulating CMOS circuits. Future work requires the coupling the dual-carrier electrical solver with the previously proven thermal solver to provide comprehensive electro-thermal simulations of CMOS systems.
5:00 PM - QN04.04.09
Graphene Composites for Thermal and Electromagnetic Shielding Applications—Performance Below and Above Percolation Thresholds
Fariborz Kargar1,Zahra Barani1,Jacob Lewis1,Ruben Salgado1,Sahar Naghibi1,Ece Aytan1,Alexander Balandin1
Phonon Optimized Engineered Materials (POEM) Center, Department of Electrical and Computer Engineering, Materials Science and Engineering Program, Bourns College of Engineering, University of California, Riverside1Show Abstract
Excellent heat conduction properties of graphene and the progress in the large-scale few-layer graphene exfoliation make the prospects of graphene composite applications particularly promising [1-2]. We investigated thermal properties of the epoxy-based composites with a high loading fraction – up to f=45 vol.% – of the randomly oriented electrically conductive graphene fillers and electrically insulating boron nitride fillers . It was found that both types of the composites revealed a distinctive thermal percolation threshold at the loading fraction above 20 vol.%. The graphene loading required for achieving the thermal percolation was substantially higher than the loading for the electrical percolation. Graphene fillers outperformed boron nitride fillers in the thermal conductivity enhancement. It was established that thermal transport in composites with the high filler loading is dominated by heat conduction via the network of percolating fillers. Unexpectedly, we determined that the thermal transport properties of the high loading composites were influenced strongly by the cross-plane thermal conductivity of the quasi-two-dimensional fillers. It was also found that composites with the certain types of few-layer graphene fillers reveal an efficient total electromagnetic interference shielding in the important X-band frequency range, while simultaneously providing the high thermal conductivity . The efficiency of the dual functional application depends on the filler characteristics: thickness, lateral dimensions, aspect ratio and concentration. Graphene loading fractions above the electrical and thermal percolation thresholds allow for strong enhancement of both the electromagnetic interference shielding and heat conduction properties. Interestingly, graphene composites can block the electromagnetic energy even below the electrical percolation threshold, remaining electrically insulating. The dual functionality of the graphene composites can substantially improve the electromagnetic shielding and thermal management of the airborne systems while simultaneously reducing their weight and cost.
This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE award 1433395, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237.
 A.A. Balandin, “Thermal properties of graphene and nanostructured carbon materials,” Nature Materials, 10 (8), 569–581 (2011).
 D. L. Nika and A. A. Balandin, “Phonons and thermal transport in graphene and graphene-based materials,” Reports on Progress in Physics, 80, 36502 (2017).
 F. Kargar, Z. Barani, R. Salgado, B. Debnath, J.S. Lewis, E. Aytan, R.K. Lake, A.A. Balandin, “Thermal percolation threshold and thermal properties of composites with high loading of graphene and boron nitride fillers,” ACS Applied Materials and Interfaces, (2018) DOI:10.1021/acsami.8b16616.
 F. Kargar, Z. Barani, M.G. Balinskiy, A.S. Magana, J.S. Lewis, A.A. Balandin, “Graphene composites with dual functionality: electromagnetic shielding and thermal management,” Advanced Electronic Materials (in print, 2018) arXiv:1808.03401.
5:00 PM - QN04.04.10
Fine-Tuning the Acoustic Phonon Spectrum in Bulk Crystals via Incorporation of the Size-Dissimilar Substitutional Dopant Atoms—Brillouin—Mandelstam Spectroscopy Study
Fariborz Kargar1,Elias Penilla2,Chun-Yu Huang1,Ece Aytan1,Javier Garay2,Alexander Balandin1
Phonon Optimized Engineered Materials (POEM) Center, Department of Electrical and Computer Engineering, Materials Science and Engineering Program, Bourns College of Engineering, University of California, Riverside1,Advanced Materials Processing and Synthesis (AMPS) Laboratory, Department of Mechanical and Aerospace Engineering, University of California, San Diego2Show Abstract
Acoustic phonons make a dominant contribution to thermal transport in insulators and semiconductors, scatter electrons and holes, and participate in the non-radiative carrier recombination processes. Acoustic phonons are important in certain types of the Auger recombination processes where they are needed to satisfy the momentum conservation. A possibility of engineering the acoustic phonon spectrum provides a tuning capability for changing thermal conductivity and electron – phonon interactions . Until recently, the tuning of the phonon spectrum has been associated with the nanostructured materials, where the phonon dispersion undergoes modification due to the periodic or stationary boundary conditions. In this talk, we describe a drastically different approach for changing the acoustic phonon spectrum of the materials, which does not rely on nanostructuring . We change the phonon spectrum in bulk crystalline materials via introduction of a small concentration of dopant atoms that have a substantially different size and mass from those of the host atoms. We report results of Brillouin – Mandelstam spectroscopy (BMS) of transparent Al2O3 crystals with Nd, Cr and other atoms used as substitutional dopants. The ionic radius and atomic mass of Nd atoms are distinctively different from those of the host Al atoms. Our results show that even a small concentration of Nd atoms incorporated into the Al2O3 samples produces a profound change in the acoustic phonon spectrum. The frequency and velocity of the transverse acoustic phonons decrease by ~4 GHz and ~600 m/s, respectively, at the Nd density of only ~0.1 %. In contrast to Nd dopants, both the ionic radius and atomic mass of Cr atoms are closer to those of the host Al atoms. The BMS results show that the phonon group velocity does not significantly change when the substitutional dopants are similar in size and mass to the host atoms. Our findings confirm that even a small concentration of dopants with strongly dissimilar size and mass can result in a profound change in the bulk phonon spectrum. The difference in atomic size can result in the crystal lattice distortion, i.e. increased inter-atomic plane distance associated with the incorporation of larger atoms. The obtained results, demonstrating a possibility of fine-tuning the phonon spectrum in bulk materials, have important implications for a range of electronic and optoelectronic devices.
This work was supported, in part, by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES) under Award # SC0012670. AAB acknowledges support from the Defense Advanced Research Projects Agency (DARPA) project W911NF18-1-0041 Phonon Engineered Materials for Fine-Tuning the G-R Center and Auger Recombination. JEG acknowledges support from the High Energy Laser - Joint Technology Office (HEL-JTO) administered by the Army Research Office for development of over-equilibrium doped alumina.
 A. A. Balandin, D.L. Nika, “Phononics in low-dimensional materials,” Materials Today, 15, 266 (2012).
 F. Kargar, E.H. Penilla, E. Aytan, J.S. Lewis, J.E. Garay, A.A. Balandin, “Acoustic phonon spectrum engineering in bulk crystals via incorporation of dopant atoms,” Applied Physics Letters, 112, 191902 (2018).
5:00 PM - QN04.04.11
Electron-Phonon Coupling in Metal Contacts—Two-Temperature Molecular Dynamics Simulations
Henry Aller1,Jonathan Malen1,Alan McGaughey1
Carnegie Mellon University1Show Abstract
The interface between a metal and a semiconductor that have dissimilar phonon frequency spectra typically exhibits a low thermal boundary conductance, as found for many Au/semiconductor interfaces. A thin metal layer (i.e., a contact or adhesion layer) placed between the gold (i.e., the capping layer) and the semiconductor can increase the thermal boundary conductance, an effect that has been attributed to the contact serving as a vibrational bridge. The impact of a change in the electron-phonon coupling due to the presence of the contact on the thermal boundary conductance, however, is not fully understood. To assess the roles played by vibrational bridging and electron-phonon coupling, we apply a two-temperature non-equilibrium molecular dynamics simulation approach. By specifying the heat flow through a structure, we can predict the temperature profiles in the electronic and phononic subsystems and the resulting thermal boundary conductance(s). Four types of structure built from metals (M) and semiconductors (SC) are considered: M/SC, M/M/SC, M/SC/SC, and SC/SC/SC. Increasing the contact layer thickness for the M/M/SC systems results in a monotonic increase in thermal boundary conductance, matching trends from previous experimental studies. The phonon density of states in the contact layer identifies the existence of non-bulk phonon modes for small thicknesses. Increasing thickness results in contact layer phonons gradually shifting to bulk frequencies, which explains the occurrence of the plateau in thermal boundary conductance for large thicknesses. A comparison between the M/M/SC, M/SC/SC, and SC/SC/SC systems provides insight to how the electron-phonon coupling in each metal layer impacts thermal boundary conductance.
5:00 PM - QN04.04.12
Thermal Conductivity of Cu3Sn
Scott Schiffres1,Matthias Daeumer1,Arad Azizi1,Sitaram Panta2,Faramarz Hadian2,Eric Cotts2
Binghamton University1,Binghamton University, The State University of New York2Show Abstract
In order to realize vertical integration for next generation electronics packaging, purely intermetallic bonding is a promising contender to replace solder. The thermal properties of these industrially important intermetallics are relatively unstudied and poorly understood. Due to their high melting temperatures intermetallic bonds provide stability through several reflows enabling vertical packaging. However, in vertical packaging the interconnects must carry a significant portion of the heat away from the chip. In an effort to better understand the thermal capability of fully intermetallic bonding Frequency Domain Frequency Domain Thermoreflectance mapping was used to determine thermal conductivity of the arc-melted Cu3Sn with respect to crystallographic orientation. This will be complemented by electron backscatter diffraction mapping of the crystal orientation.
5:00 PM - QN04.04.13
Uncovering Phonon Transport Mechanisms Underneath Nanoscale Heat Sources
Hossein Honarvar1,Joshua Knobloch1,Travis Frazer1,Jorge Nicolas Hernandez Charpak1,Begona Abad Mayor1,Mahmoud Hussein1,Henry Kapteyn1,Margaret Murnane1
University of Colorado Boulder1Show Abstract
Nanostructured and quantum materials are enabling revolutionary advances in nanoscience and nanotechnology. These engineered materials promise to go beyond what is offered by natural materials. Advances in materials growth now make it possible to synthesize 3D nanostructured materials with nanometer resolution creating nanosystems with tunable electronic, photonic, magnetic, and thermal properties for a variety of applications including nanoelectronics, thermoelectrics, photovoltaics, and sensors. However, understanding of the fundamental mechanism behind many of these applications is still incomplete, making the development of powerful theoretical and experimental tools a priority for many material research communities.
Understanding thermal transport in nanostructured systems has long been a challenge. In the last few years, experimental techniques have pushed the measurement limits on systems with characteristic dimensions and geometries down to tens of nanometers, often times finding new surprising physical behaviors . However, it is still challenging to apply appropriate models that connect novel data to meaningful thermal properties and fundamental mechanisms. Most researchers rely on using effective or phenomenological diffusive models or the linearized Boltzmann transport equation that are not able to capture all of the observed nanoscale effects and nonlinear interactions [1-3]. This is because it is very difficult to perform atomistic simulations over experimentally-relevant length scales, due to prohibitive computational requirements. As a result, there is a large gap in understanding of how the nanoscale phonon physics unfolds for different size and geometries to modify the transport properties. Moreover, possible coherent effects are often not adequately included in current models.
The focus of this work is understanding thermal transport in nanostructured materials by bridging the gap between theory and experiment. In particular, previous work done in our research group uncovered a new thermal transport regime named “collectively-diffusive” by tracking the heating and cooling of periodic arrays of nanoscale heat sources on bulk crystalline silicon and sapphire substrates . This regime emerges from the interaction between the nanoscale geometries and energy carriers in materials. In particular, when nanoscale heat sources are placed closer, they cool down faster than when farther apart—the opposite of heat dissipation dynamics of the macroscale. The observed increase in heat dissipation efficiency has large consequences in thermal management in microelectronic devices, and is currently not captured by models available to both researchers in academia and industry.
In this work, we present the results of steady-state molecular dynamics (MD) simulations that can model the experimentally-explored geometries, and how these results provide a global understanding of non-diffusive phonon dynamics that appear to dominate energy transport in the deep nanoscale regime. Our MD results of nanoscale heat sources indicate that we are able to successfully capture the novel collectively-diffusive thermal dynamics observed recently.
Hoogeboom-Pot et al. PNAS doi:10.1073/pnas.1503449112 (2015).
Hu et al. Nature nanotechnology, 10 (8): 701 (2015).
Ziabari et al. Nature communications, 9(1):255, (2018).
5:00 PM - QN04.04.14
Thermal Conductivity of Perovskite-Structured Superlattices from First-Principles Calculations
Qi Zhang1,Xue Xiong2,Eugene Rasaga2,Simon Phillpot2,Aleksandr Chernatynskiy1
Missouri University of Science and Technology1,University of Florida2Show Abstract
Low thermal conductivity materials are important for a variety of applications, including thermal barrier coatings and thermoelectric devices. Superlattices are particularly interesting due to the possibility of lowering thermal conductivity. Numbers of explanations have been proffered for the low thermal conductivity of superlattice structures. A full explanation, however, has yet to be developed.
Here, we are presenting the thermal-transport properties of natural perovskite-structured superlattices, the Ruddlesden-Popper (RP) series of phases of the Sr-Ti-O system, formed by the interleaving of SrTiO3 perovskite layers with SrO rocksalt layers. We have computed their thermal conductivity from first principles via the Boltzmann-transport equations (BTE) approach encoded in the PhonTS software package. In short, the thermal conductivity is determined by computing the heat current using the nonequilibrium phonon density distribution function, which in turn is found as a solution of the linearized BTE for phonons. The required input for the BTE are the second and third spatial derivatives of the total energy with respect to atomic positions which we have obtained from the DFT calculations performed using the Vienna Ab initio Simulation Package (VASP) computational package. A clear minimum in the thermal conductivity as a function of a number of STO layers is observed. Results are compared with the recent experimental data.
5:00 PM - QN04.04.15
Controlling Thermal and Electrical Properties of Composites Using Percolating Network of Nanowires with Fusible Tips
Konrad Rykaczewski1,Robert Wang1
Arizona State University1Show Abstract
Development of polymeric composites with high thermal conductivity is necessary for thermal management of microelectronics. Percolation of metallic filler particles and reduction of the thermal contact resistance between individual particles (e.g. via thermal or electromagnetic fusing) can significantly improve thermal conductivity of composites. However, composite materials with connected metallic particles can also conduct electricity, creating a risk of short-circuiting in chip-board and the inter-chip gaps. In this presentation we theoretically show that this problem can be resolved with the application of fusible metallic coatings to the tips of nanowires with thermally conductive, but electrically insulating cores . Specifically, we use Monte Carlo simulation-validated analytical models that relate the ratio of the coated and total nanowire lengths to the fraction of fused, and thus conductive, bonds within percolating networks of these structures to show that thermally conductive, but electrically insulating composites can be achieved using these novel nanostructures. We discuss silver-like coatings, which only form conductive bonds when contacting the silver-like coating of another nanowire as well as liquid metal-like coatings, which form conductive bonds regardless of whether they contact a coated or uncoated segment of another nanowire. We show that use of the liquid metal-like coatings will yield twice as many thermally conductive bonds as silver-like coatings while maintaining a negligible risk of electrical short-circuiting.
 K. Rykaczewski and R. Wang, Applied Physics Letters, 112, (13), 131904, 2018.
5:00 PM - QN04.04.16
Magnon and Phonon Dispersion, Lifetime and Thermal Conductivity of Iron from Spin-Lattice Dynamics Simulations
Zeyu Liu1,Xufei Wu1,Tengfei Luo1
University of Notre Dame1Show Abstract
In recent years, the fundamental physics of spin-lattice (e.g., magnon-phonon) interaction has attracted significant experimental and theoretical interests given its potential paradigm-shifting impacts in areas like spin-thermoelectrics, spin-caloritronics, and spintronics. Modelling studies of the transport of magnons and phonons in magnetic crystals are very rare. In this paper, we use spin-lattice dynamics (SLD) simulations to model ferromagnetic crystalline iron, where the spin and lattice systems are coupled through the atomic position-dependent exchange function, and thus the interaction between magnons and phonons is naturally considered. We then present a method combining SLD simulations with spectral energy analysis to calculate the magnon and phonon harmonic (e.g., dispersion, specific heat, and group velocity) and anharmonic (e.g., scattering rate) properties, based on which their thermal conductivity values are calculated. This work represents an example of using SLD simulations to understand the transport properties involving coupled magnon and phonon dynamics.
5:00 PM - QN04.04.17
Normal Modes for Thermal Transport
Anant Raj1,Jacob Eapen1
North Carolina State University1Show Abstract
Normal modes of vibrations can be extracted from atomistic simulations by projecting the real space trajectories onto the reciprocal space. Normal modes coordinates are not unique and their primary function is to transform the coupled Hamiltonian to a set of independent harmonic oscillators. Several descriptors of normal modes exist but not all of them are appropriate for analyzing thermal transport. In most normal mode analyses (NMA), complex normal mode coordinates are employed, which combine the modal contributions of waves moving in opposite directions. The wave-vector q that is represented in the complex normal modes does not uniquely represent a wave traveling in the +q or –q direction, instead it denotes an average of both directions. Thus the popular complex normal modes have a theoretical inability to resolve a real heat current along a specific direction-dependent wave-vector q.
In this work, we employ a set of real asymmetric normal mode coordinates, which can distinguish lattice waves moving in opposite directions – a virtue that immediately endows the ability to discriminate a heat current in a certain direction. These normal mode coordinates have a real amplitude A(q,p) that is not equal to A(−q,p). We then derive an expression for heat current that is real, and which can be expressed as a difference between the squares of the amplitudes in +q and –q directions. Finally, we drive a correction term for phonon lifetime that arises from the correlation between modes moving along opposite directions.
5:00 PM - QN04.04.18
Implementation of the Hydrodynamic Heat Transport Model for Complex Geometries Using Finite Elements
Albert Beardo Ricol1,Juan Camacho1,Lluc Sendra1,Javier Bafaluy1,F. Xavier Alvarez1
Universitat Autònoma de Barcelona1Show Abstract
Recent experiments have evidenced that effective Fourier models are unable to predict heat transfer at the nanoscale [1,2]. This has a big impact on electronic engineering that relies on this law to study the thermal behavior of their devices.
One of the last proposals to better describe thermal transport at the nanoscale has been phonon hydrodynamics. In this line, the Guyer-Krumhansl equation has been proposed as the required generalization of the Fourier law to describe heat transport at this scale . This equation is combined with the Kinetic Collective Model (KCM) to obtain the included parameters from ab initio calculations [4,5]. One of the main advantages of this approach is that its simplicity allows to obtain solutions for arbitrary geometries using Finite Element Methods. Therefore, this combination offers a full predictive framework to describe thermal conductivity in semiconductors in general geometries with characteristic sizes up to the order of the hydrodynamic characteristic length.
Validation of the model has been done with experimental data for different systems such as semiconductor porous membranes with different periodic alignments, thin membranes with different constrictions or 2D materials. In parallel, the tool offers also the interpretation of the results in terms of new phenomena like vorticity and viscosity, giving an insight on the reason for the reduction of the effective thermal conductivity at reduced scales.
 A. Ziabari et.al., Nat. Comm. 9, 255 (2018).
 K. M. Hoogeboom-Pot et. al., PNAS 112 16 4851 (2015)
 P. Torres et al. Phys Rev. Mat. 2, 076001 (2018)
 Y. Guo et. al., Phys. Rev. B 93, 035421 (2018)
 R.A. Guyer et. al. Phys. Rev. 2, 148 (1966)
5:00 PM - QN04.04.19
Effect of Intrinsic and Extrinsic Defects on Phonon Heat Transfer in Nanostructured Metals
Peter Sushko1,Richard Williams1,Christopher Barrett1,Marvin Warner1
Pacific Northwest National Laboratory1Show Abstract
Ongoing efforts in low temperature experimental research, including the work on quantum materials and high energy physics experiments, underscores the need to achieve and maintain ultra-low temperatures. Of particular interest are metal/non-metal systems, in which the dominant mechanism of heat transfer switches at the interface, and nanomaterials, in which structural and chemical deviations from idealities may lead to significant deviations from the Fourier law.
We investigate how phonon component of the heat flux across nanoscale metals is affected by point defects, dislocations and model two-dimensional defects. Gaseous models for the heat source and heat sink are adopted and the computations are conducted using classical molecular dynamics approach. Analysis of the heat transfer coefficients, as derived via Green-Kubo relation, suggests that surface nanopatterning and chemical defects have larger an effect on the heat propagation than intrinsic defects.
5:00 PM - QN04.04.20
Thermal Transport Across Rough Interfaces—A Finite-Difference Time-Domain Study
Laleh Avazpour1,Sina Soleimanikahnoj1,Suraj Suri1,Irena Knezevic1
University of Wisconsin-Madison1Show Abstract
Heat transport across interfaces in nanostructures is an important open problem that arises into play in the design and operation of electronic and thermoelectric devices. In particular, lattice mismatch across the interface and the interface profile strongly influence thermal transport. In this work, we study elastic-wave scattering at the junction between two different materials. We solve the elastic-wave equations with the finite-difference time domain (FDTD) technique. We investigate the effects of mode conversion and energy trapping at the interface and how they are affected by interface roughness and lattice mismatch. The results provide a unique outlook on the interfacial heat transport and its effect on the operation of nanodevices.
5:00 PM - QN04.04.21
Thermal Conductivity of Small-Angle Misoriented Bilayer Graphene
Chenyang Li1,Bishwajit Debnath1,Roger Lake1
University of California, Riverside1Show Abstract
The properties of misoriented bilayer graphene have recently received renewed interest after the discovery of superconductivity at very small rotation angles. Compared to the electronic properties, the effect of misorientation on the phonon and thermal properties has received less attention. For the larger misorientation angles, the in-plane thermal conductivity depends not on the angle, but on the commensurate lattice constant, and the thermal conductivity decreases approximately linearly as the commensurate lattice constant increases . This trend is qualitatively consistent with the hypothesis that the zone folding gives rise to an increase in Umklapp scattering that reduces the low-energy phonon lifetimes and thus the thermal conductivity. However, our recent calculations show that as the misorientation angle falls below 13.2o, this trend reverses itself, and the thermal conductivity starts increasing back towards the value of the unrotated structure. For angles below 13.2o, the thermal conductivity initially increases rapidly as the angle decreases from 13.2o to 7.3o, even though the commensurate lattice constant is monotonically increasing. As the angle continues to decrease down to 1.9o, which is the smallest angle simulated, the thermal conductivity gradually returns to its unrotated value.
For small angles with minimal commensurate unit cells, the commensurate lattice constants monotonically increase as the angles decrease, so that it is not clear what determines the functional dependence of the thermal conductivity in the small angle regime. Is it the misorientation angle or the commensurate lattice constant? To answer this question, we investigated two very different angles, 3.9o and 20.3o. Both of these angles which have exactly the same commensurate lattice constant of 3.6 nm. The thermal conductivities of the two structures are identical indicating that the functional dependence of the thermal conductivity in the low angle regime continues to be on the lattice constant rather than on the misorientation angle.
As the lattice constant increases, the reciprocal lattice constant decreases, so that Umklapp scattering should become more accessible to the low-energy, small-wavevector phonons that determine the thermal conductivity. All else being equal, the increased scattering should decrease the thermal conductivity. While this picture is consistent with the trends in the large angle regime, in the small angle regime (< 13o), this is the opposite of the trend that we observe. However, as the misorientation angle returns to zero degrees, the thermal conductivity must return to that of the AB aligned structure, and this is what we observe. In this talk, we will describe the thermal transport in the low angle regime, and we will present our analysis of the thermal transport that includes the average phonon velocities and density of modes, and we will also present a spectral decomposition of the lattice thermal conductivity determined from the force-velocity cross correlation function.
The investigation uses both nonequilibrium molecular dynamics (NEMD) and ab-initio density functional theory (DFT) combined with the phonon Boltzmann transport equation. For the NEMD direct calculations of the thermal conductivity, the width of the simulated bilayer graphene structures is approximately 10 nm. To ensure that the results do not depend on the sample width, multiple increasing widths are simulated until there is no longer any width dependence. The sample lengths are varied from 20 nm to 426 nm. The largest structure contains 317,600 atoms.
 C. Li, et al., Carbon, 138, 451 (2018).
This work was supported by the National Science Foundation under Award NSF EFRI-1433395. The ab initio simulations used the Extreme Science and Engineering Discovery Environment (XSEDE), supported by National Science Foundation (NSF) grant No. ACI-1548562 and allocation ID TG-DMR130081.
5:00 PM - QN04.04.22
Spatial Mapping of Thermal Boundary Conductance at Interfaces of Metal and 2D Materials
Satish Kumar1,David Brown1,Wenqing Shen1,Diego Vaca1,Xufan Li2,Kai Xiao2,David Geohegan2
Georgia Institute of Technology1,Oak Ridge National Laboratory2Show Abstract
Improving the thermal transport across interfaces is a necessary consideration for micro- and nano-electronic devices and necessitates accurate measurement of the thermal boundary conductance (TBC) and understanding of transport mechanisms. Two-dimensional transition metal dichalcogenides (TMDs) have been studied extensively for their electrical properties, including the metal-TMD electrical contact resistance, but the thermal properties of these interfaces are significantly less explored irrespective of their high importance in their electronic devices. We isolate individual islands of MoSe2 grown by chemical vapor deposition using photolithography and correlate the 2D variation of TBC with optical microscope images of the MoSe2 islands. We measure the 2D spatial variation of the TBC at metal-MoSe2-SiO2 interfaces using a modified time-domain thermoreflectance (TDTR) technique which requires much less time than full TDTR scans. The thermoreflectance signal at a single probe delay time is compared with a correlation curve which enables us to estimate the change in the signal with respect to the TBC at metal-MoSe2-SiO2 interface as opposed to recording the decay of the thermoreflectance signal over delay times of several nanoseconds. The results show higher TBC across Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2. An image analysis method is developed to differentiate the TBC for different number of MoSe2 layers, which reveals the TBC in single-layer regions is higher than bilayer. We perform traditional TDTR measurements over a range of delay times and verify TBC is higher at Ti-MoSe2-SiO2 interface compared to Al-MoSe2-SiO2 highlighting the importance of the choice of metal to heat dissipation at electrical contacts in TMD devices.
5:00 PM - QN04.04.23
The Influence of Interfacial Structure and Strain Energy on Phonon Transport
Riley Hanus1,G. Snyder1
Northwestern University1Show Abstract
Phonon transport across interfaces is an inherently complex topic of great scientific and technological importance to fields ranging from microelectronics to energy materials. Here, I will present several experimental and theoretical results which aim to establish a fundamental understanding of heat transfer across interfaces, specifically grain boundaries (GBs). First, I will demonstrate how phonon diffraction and dimensionality crossover effects arise when the nanoscale structure of interfaces and GBs is considered. An expression for the relaxation time of phonon interacting with GB strain fields (τgbs) is presented and is shown to effectively describe the temperature dependence of the lattice thermal conductivity (κL) of polycrystals at low temperatures (< 100 K). At these temperatures the total phonon relaxation time is dominated by interactions with GBs, and the temperature dependence of κL reveals important information about the nature of the phonon-GB interaction. Next, an experimental study is presented where the thermal boundary resistance is measured on individual Si-Si twist GBs at different twist angle. The thermal boundary resistance at GBs again seems to be dominated by the interfacial strain field. Finally, it is shown how the thermal boundary resistance can be controlled by modifying the GB complexion with 2D materials. Specifically, several layers of graphene can be introduced into the GBs of skutterudite materials which dramatically increases the materials thermal boundary resistance, while negligibly effecting electronic transport. This results in a significant improvement in zT and a 24% improvement in device efficiency which was measured experimentally.
5:00 PM - QN04.04.24
Study of Phonon Transport in GaN Thin Films Using Boltzmann Transport Equations
Nitish Kumar1,Ajit Vallabhaneni2,Satish Kumar1
Georgia Tech1,Qualcomm2Show Abstract
GaN is a promising wide bandgap material for radio frequency and power electronics applications due to higher breakdown voltage and efficiency. However, the high-power dissipation could result in the formation of hot-spots and high temperature in localized regions which could significantly affect the performance and reliability of these high-power devices. Therefore, it’s necessary to understand the thermal transport mechanism in GaN based electronics to develop efficient thermal solutions. In this study, we develop a phonon transport model based on the first-principles Density Functional Theory (DFT) along with the non-gray Boltzmann Transport Equations (BTE) to predict the steady-state and transient spatial temperature distribution in a GaN thin film with localized power dissipation. First-principles DFT calculations are conducted to calculate the phonon properties of the GaN, such as phonon life time, mean free path and group velocity. The full non-gray BTE is solved across the entire domain to calculate the temperature distribution near the hot spot in the thin film where ballistic effects dominate and the results are compared with the Fourier model. We observed that the BTE model is needed to obtain accurate temperature distribution as the Fourier model would significantly under predict the hot spot temperature. However, solving BTE in the entire domain might be time-consuming and expensive. Therefore, we developed acceleration techniques to reduce computation time of solving the BTEs without significant loss of accuracy in temperature estimation. BTEs for phonon modes with Knudsen number less than the cut-off are simplified into Fourier like equations to improve computation time. These equations are selected based on a cut-off Knudsen number, which can be adjusted to get desired accuracy. The effect of diffusive-ballistic effects on hot spot temperature rise in time has been investigated. The results from this work will help us understand the mechanism of phonon transport in the GaN thin film and provide insights for the future design of GaN based electronic devices.
5:00 PM - QN04.04.25
Thermometry with Sub-Nanometer Resolution in the Electron Microscope Using Phonon Scattering
Maureen Joel Lagos1,Philip Batson2
McMaster University1,Rutgers, The State University of New Jersey2Show Abstract
Nanoscale phonon properties have recently received attention because new material properties can be designed by tailoring vibrational behaviour. This has triggered significant efforts towards a better nanoscale control and manipulation of thermal processes in materials. In this context, the reduction of the physical size of materials also resulted in the need of improved spectroscopic techniques and methods to characterize phonon and thermal properties of materials with higher spatial resolution. Recently, aberration-corrected electron microscopes were equipped with monochromators, thus opening the doors for phonon spectroscopy studies in nanomaterials using an electron probe of the size of a hydrogen atom. For instance, this advancement allowed us to obtain spatially-resolved maps of phonon scattering in a single nanoparticle (< 100 nm) with nanometer resolution . Also, due the nature of the inelastic electron scattering by phonons, the scattering amplitude exhibit a dependence on temperature and the spatial distribution of the scattering has several types of localization. In this work, exploiting the characteristic of the phonon scattering, we present a non-invasive method to measure the local temperature of a single nanostructure using an electron probe. We determined the local temperature of nanostructures with high precision (down to 1K) and sub-nanometer spatial resolution (down to 2Å) .
We obtained energy-loss/energy gain spectra from nanostructures (nanoparticles and interfaces) using a Nion STEM microscope equipped with a monochromator to study the the phonon response in the infrared range using a ~ 1.5 Å probe with an energy spread of 9 meV. Our instrument allows the detection of acoustic bulk phonon excitations down to 20 meV . The amplitudes of the energy-loss and energy-gain probabilities are linked by the Boltzmann factor, as established by the Principle of Detailed Balancing (PDB). We measured the nanostructure temperature by plotting the logarithm of the ratio of the scattering amplitude on the gain and loss sides of zero energy. The obtained curves are linear for several nanomaterials, supporting the use of PDB on the nanoscale. Also, this method yields measurements without reference to the nanomaterial morphology, or scattering conditions. Our experimental results obtained at room temperature indicate that the probed object indeed is close to 290 K with a precision of ~ 7-10K with ~ 99% of confidence level (3 standard deviations) and agreed very well with the accepted values for room temperature conditions. Similar results were obtained for measurements at high temperatures (up to 1000 K). We found also that temperatures obtained from bulk phonon scattering compared well with those obtained using surface phonon polariton scattering, typically present in nanoparticles , suggesting that temperature gradients from the inside towards the surface should be detectable. We also investigated the localization degree of our measurements and we found that highly-localized scattering, associated with the excitation of short-wavelength acoustic phonons, can be used to perform measurements with atomic resolution, because bulk scattering can vary drastically within neighboring atom columns . Meanwhile, surface phonon polaritons can yield average values over a region of interest (several nanometers).
We show that the temperature of a n