We have shown that the thermal boundary conductance across solid interfaces can change based on whether the materials comprising the interfaces are amorphous or crystalline.  Via a series of MD simulations, we show that mode distribution in amorphous materials can lead to an increase in thermal boundary conductance as compared to their crystalline counterparts.  This work, in which Ash Giri was the first author, was recently published in Journal of Applied Physics (Journal of Applied Physics 118, 165303 (2015)), and was in collaboration with Dr. John Duda from Seagate Technology.

Abstract

We report on the thermal conductivities of amorphous Stillinger-Weber and Lennard-Jones superlattices as determined by non-equilibrium molecular dynamics simulations. Thermal conductivities decrease with increasing interface density, demonstrating that interfaces contribute a non-negligible thermal resistance. Interestingly, Kapitza resistances at interfaces between amorphous materials are lower than those at interfaces between the corresponding crystalline materials. We find that Kapitza resistances within the Stillinger-Weber based Si/Ge amorphous superlattices are not a function of interface density, counter to what has been observed in crystalline superlattices. Furthermore, the widely used thermal circuit model is able to correctly predict the interfacial resistance within the Stillinger-Weber based amorphous superlattices. However, we show that the applicability of this widely used thermal circuit model is invalid for Lennard-Jones based amorphous superlattices, suggesting that the assumptions made in the model do not hold for these systems.

We have shown conclusive evidence of diffusive thermal transport in a single molecule of a phosponic acid (henicosafluorododecyl-phosphonic acid (F21PA)) between various metals and a sapphire substrate.  thermal transport across single molecules is typically assumed ballistic, where the thermal resistance is dominated by the molecular contacts.  We show that the large molecular weight phosphonic acid F21PA, vibron scattering in the molecule can lead to additional thermal resistance at metal/PA/sapphire interfaces, indicative of diffusive thermal transport.  This work, in which Dr. John Gaskins was first author, was recently published in The Journal of Physical Chemistry C (The Journal of Physical Chemistry C 119, 20931-20939 (2015)), and was in collaboration with Dr. Sam Graham from Ga Tech.

Abstract

We have demonstrated the ability to independently control the electrical contract resistance and thermal boundary resistance at Au/graphene interfaces by functionalizing the graphene via an atmospheric plasma-based approach.   The modification of surface energy either oxygen, nitrogen or fluorine adsorbates leads to varying interactions at the Au/graphene interface.  Where the bonding at this interface affects thermal transport, the change in surface energy and resulting electrical interactions lead to modifications in electrical transport.  This work, in which Brian Foley was first author, was recently published in Nano Letters (Nano Letters 15, 4876-4882 (2015)), and was in collaboration with Dr. Scott Walton from the Naval Research Labs.

Abstract

The high mobility exhibited by both supported and suspended graphene, as well as its large in-plane thermal conductivity, has generated much excitement across a variety of applications. As exciting as these properties are, one of the principal issues inhibiting the development of graphene technologies pertains to difficulties in engineering high-quality metal contacts on graphene. As device dimensions decrease, the thermal and electrical resistance at the metal/graphene interface plays a dominant role in degrading overall performance. Here we demonstrate the use of a low energy, electron-beam plasma to functionalize graphene with oxygen, fluorine, and nitrogen groups, as a method to tune the thermal and electrical transport properties across gold-single layer graphene (Au/SLG) interfaces. We find that while oxygen and nitrogen groups improve the thermal boundary conductance (h_K) at the interface, their presence impairs electrical transport leading to increased contact resistance (rho_C). Conversely, functionalization with fluorine has no impact on h_K, yet rho_C decreases with increasing coverage densities. These findings indicate exciting possibilities using plasma-based chemical functionalization to tailor the thermal and electrical transport properties of metal/2D material contacts.

Acknowledgements

Congrats to the new doctor! Dr. Ramez Cheaito successfully defended his PhD, which is focused on size effects on the electron and phonon thermal conductivity of alloy, superlattice and multilayer thin films.  The title of his dissertation is “The Role of Size Effects on the Thermal Conductivity of Thin Film Alloys and Superlattices”.  At the time of his PhD defense, Dr. Cheaito’s work at U.Va. resulted in 13 journal publications and 123 citations.  Dr. Cheaito’s dissertation abstract is copied below.  CONGRATS DR. CHEAITO!!

Abstract

Advancements in modern technologies have relied primarily on the miniaturization of electronic devices. As the dimensions a these devices are reduced to hundreds of nanometers, thermal management becomes a challenge. Performances are now dictated by the amount of power a device can dissipate before surpassing the temperature set by reliability requirements. Understanding thermal transport in thin film nanostructures is a key element in manufacturing devices with long lifetimes and better energy efficiencies.
The role of size effects on the behavior of heat carriers in thin film structures and across interfaces have been the focus of numerous studies over the past few decades. However, discrepancies among studies on phonon behavior obstruct the understanding of the fundamental processes governing phonon transport. On the other hand, the lack of data on electron thermal transport across interfaces and in periodic structures motivates more research in this direction. This dissertation presents thermal conductivity measurement results on four different material systems of sample thicknesses spanning three orders of magnitude to provide a deep understanding into the processes of phonon and electron thermal transport in thin film alloys and superlattices. Measurements were performed using time-domain thermoreflectance, a non-contact, optical method for the thermal characterization of bulk and thin film materials.

The effect of boundary scattering of long mean free path phonons on the thermal conductivity of thin film SiGe alloys and AlAs-GaAs superlattices is thoroughly discussed in light of the spectral contribution of these phonons to thermal transport. The interplay between short and long range boundary scattering in AlAs-GaAs superlattices is studied by systematically varying the period and film thicknesses. Phonon coherence in epitaxially grown strontium titanate – calcium titanate superlattices is demonstrated by showing a minimum in the thermal conductivity as a function of period thickness. For electrons, the interplay between electron characteristic length and the materials’ intrinsic properties is studied via measurements of the thermal interface conductance in Cu-Nb multilayers.

A major result of this dissertation is demonstrating the possibility of achieving a desired thermal conductivity by prescribing both the period and sample thickness of a superlattice, a result that has important implications on thermal management and thermal engineering applications.

We have recently reported on the measurement of thermal conductivity in a series of ion irradiated materials via time domain thermoreflectance (TDTR), demonstrating TDTR as an effective means for the characterization of radiation induced damage.  Due to the relatively high modulation frequencies and time domain experimental data in a TDTR experiment, this measurement technique offers the unique ability to characterize radiation-induced damage of materials at varying depths under the surface via quantification of the thermal conductivity.  This work, in which Ramez Cheaito was the first author, was recently published in the Journal of Materials Research (J. Mat. Res. 30 1403 – 1412 (2015)) – Congrats Ramez!!

Abstract

We have recently demonstrated the ability to control phonon scattering rates at room temperature via the application of an electric field in lead zirconate titanate (PZT) thin films.  Upon application of the electric field, the ferroelastic domains reconfigure and influence the thermal conductivity with sub-second switching times.  This is the first demonstration of an active switch of phonon thermal conductivity at room temperature.  This work was recently published in Nano Letters (Nano Letters 15, 1791-1795 (2015)) – congrats Brian Foley on your exciting results and research that formed the core of this discovery.  This work was in collaboration with Dr. Jon Ihlefeld from Sandia National Laboratories.

Abstract

Chet Szwejkowski’s paper, “Size effects in the thermal conductivity of gallium oxide (beta-Ga₂O₃) films grown via open-atmosphere annealing of gallium nitride (GaN),” was recently published in Journal of Applied Physics (J. Appl. Phys. 117, 084308 (2015)). In this work, we demonstrated that the thermal conductivity of Beta-phase Ga₂O₃ that forms on the surface of GaN while annealing in open atmosphere is polycrystalline and has a thermal conductivity of 8.8 ± 3.4 W m⁻¹ K⁻¹. This relatively high thermal conductivity has major implications for annealing during the creation of GaN contacts.  Congrats Chet!!!

Abstract

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devicesdue to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (β-Ga2O3) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline β-Ga2O3 films of different thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these β-Ga2O3 films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the β-Ga2O3 grown via this technique (8.8 ± 3.4 W m⁻¹ K⁻¹) and large mean free paths compared to typical gate dielectrics commonly used in GaN device contacts. By conducting time domain thermoreflectance (TDTR) measurements with different metal transducers (Al, Au, and Au with a Ti wetting layer), we attribute this variation in effective thermal conductivity to a combination of size effects in the β-Ga2O3 film resulting from phonon scattering at the β-Ga2O3/GaN interface and thermal transport across the β-Ga2O3/GaN interface. The measured thermal properties of open atmosphere-grown β-Ga2O3 and its interface with GaN set the stage for thermal engineering of gate contacts in high frequency GaN-based devices.

Acknowledgements

The material is based upon work partially supported by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0067 (Subaward No. 5010-UV-AFOSR-0067), the National Science Foundation (CBET-1339436) and the Commonwealth Research Commercialization Fund (CRCF) of Virginia.

Ash Giri’s paper, “Experimental evidence of excited electron number density and temperature effects on electron-phonon coupling in gold films,” was recently published in Journal of Applied Physics (J. Appl. Phys. 117, 044305 (2015)). In this work, we studied the electron-phonon coupling processes in Au films after short pulsed laser heating using a range of laser fluences to control electron temperature while varying the lattice temperature via a LN2-cooled cryostat.  We provide quantitative experimental evidence of the transition from nonequilibrium-to-equilibrium dominated electron-phonon coupling relaxation in thin films. We show that the temperature dependence of the electron-phonon coupling factor in Au varies based on both the lattice and electron temperatures, in contrast to the two temperature model. This paper focuses on answering the fundamental question of scattering mechanisms and their relation to the experimental conditions that lead to different energy relaxation rates between the electronic and vibrational states in metals. In doing so, our measurements provide experimental evidence that elucidate novel transport processes in thin metal films along with resolving the discrepancies in nearly 20 years of experimental data.  Congrats Ash!!!

Abstract

Ramez Cheaito’s paper, “Thermal boundary conductance accumulation and interfacial phonon transmission across interfaces: measurements and theory,” has recently been published in Physical Review B (Phys. Rev. B 91, 035423 (2015)).  In this work, we develop the analytical formalism to mathematically model the accumulation of phonon energy across interfaces, and how this accumulation contributes the thermal boundary conductance and interfacial phonon transmission.  For example, this formalism gives the ability to determine what percentage of the phonon spectrum in a material adjacent to the interface contributes to a certain % of the thermal boundary conductance.  This metric is immensely important for thermal transport in nanosystems.  We validate our theory with a wide array of measurements across metal/nonmetal interfaces via TDTR measurements of thermal boundary conductance.  Congrats Ramez!!!

Abstract

The advances in phonon spectroscopy in homogeneous solids have unveiled extremely useful physics regarding the contribution of phonon energies and mean-free paths to the thermal transport in solids. However, as material systems decrease to length scales less than the phonon mean-free paths, thermal transport can become much more impacted by scattering and transmission across interfaces between two materials than the intrinsic relaxation in the homogeneous solid. To elucidate the fundamental interactions driving this thermally limiting interfacial phonon scattering process, we analytically derive and experimentally measure a thermal boundary conductance accumulation function. We develop a semiclassical theory to calculate the thermal boundary conductance accumulation function across interfaces using the diffuse mismatch model, and validate this derivation by measuring the interface conductance between eight different metals on native oxide/silicon substrates and four different metals on sapphire substrates. Measurements were performed at room temperature using time-domain thermoreflectance and represent the first-reported values for interface conductance across several metal/native oxide/silicon and metal/sapphire interfaces. The various metal films provide a variable bandwidth of phonons incident on the metal/substrate interface. This method of varying phonons’ cutoff frequency in the film while keeping the same substrate allows us to mimic the accumulation of thermal boundary conductance and thus provides a direct method to experimentally validate our theory. We show that the accumulation function can be written as the product of a weighted average of the interfacial phonon transmission function and the accumulation of the temperature derivative of the phonon flux incident on the interface; this provides the framework to extract an average, spectrally dependent phonon transmissivity from a series of thermal boundary conductance measurements. Our approach provides a platform for analyzing the spectral phononic contribution to interfacial thermal transport in our experimentally measured data of metal/substrate thermal boundary conductance. Based on the assumptions made in this work and the measurement results on different metals on native oxide/silicon and sapphire substrates, we demonstrate that high-frequency phonons dictate the transport across metal/Si interfaces, especially in low Debye temperature metals with low-cutoff frequencies.

Our work at U.Va. was supported through the Office of Naval Research, Young Investigator Program (Grant# N00014-13-4-0528).

Brian Donovan’s paper, “Thermal boundary conductance across metal-gallium nitride (GaN) interfaces from 80 – 450 K,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 203502 (2014)).    In this work, we show that the thermal boundary conductance across metal/GaN interfaces can impose a thermal resistance similar to that of GaN/substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.  Congrats Brian!!!

Abstract