John C. Duda

Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Manipulating Thermal Conductance at Metal−Graphene Contacts via Chemical Functionalization

Graphene-based devices have garnered tremendous attention due to the unique physical properties arising from this purely two-dimensional carbon sheet leading to tremendous efficiency in the transport of thermal carriers (i.e., phonons). However, it is necessary for this two-dimensional material to be able to efficiently transport heat into the surrounding 3D device architecture in order to fully capitalize on its intrinsic transport capabilities. Therefore, the thermal boundary conductance at graphene interfaces is a critical parameter in the realization of graphene electronics and thermal solutions. In this work, we examine the role of chemical functionalization on the thermal boundary conductance across metal/graphene interfaces. Specifically, we metalize graphene that has been plasma functionalized and then measure the thermal boundary conductance at Al/graphene/SiO(2) contacts with time domain thermoreflectance. The addition of adsorbates to the graphene surfaces are shown to influence the cross plane thermal conductance; this behavior is attributed to changes in the bonding between the metal and the graphene, as both the phonon flux and the vibrational mismatch between the materials are each subject to the interfacial bond strength. These results demonstrate plasma-based functionalization of graphene surfaces is a viable approach to manipulate the thermal boundary conductance.

Anharmonic Phonon Interactions at Interfaces and Contributions to Thermal Boundary Conductance

Continued reduction in characteristic dimensions in nanosystems has given rise to increasing importance of material interfaces on the overall system performance. With regard to thermal transport, this increases the need for a better fundamental understanding of the processes affecting interfacial thermal transport, as characterized by the thermal boundary conductance. When thermal boundary conductance is driven by phononic scattering events, accurate predictions of interfacial transport must account for anharmonic phononic coupling as this affects the thermal transmission. In this paper, a new model for phononic thermal boundary conductance is developed that takes into account anharmonic coupling, or inelastic scattering events, at the interface between two materials. Previous models for thermal boundary conductance are first reviewed, including the diffuse mismatch model, which only considers elastic phonon scattering events, and earlier attempts to account for inelastic phonon scattering, namely, the maximum transmission model and the higher harmonic inelastic model. A new model is derived, the anharmonic inelastic model, which provides a more physical consideration of the effects of inelastic scattering on thermal boundary conductance. This is accomplished by considering specific ranges of phonon frequency interactions and phonon number density conservation. Thus, this model considers the contributions of anharmonic, inelastically scattered phonons to thermal boundary conductance. This new anharmonic inelastic model shows improved agreement between the thermal boundary conductance predictions and experimental data at the Pb/diamond and Au/diamond interfaces due to its ability to account for the temperature dependent changing phonon population in diamond, which can couple anharmonically with multiple phonons in Pb and Au. We conclude by discussing phonon scattering selection rules at interfaces and the probability of occurrence of these higher order anharmonic interfacial phonon processes quantified in this work.

Extension of the diffuse mismatch model for thermal boundary conductance between isotropic and anisotropic materials

This model is an extension of the diffuse mismatch model (DMM), tailored to accurately predict thermal boundary conductance (hBD) at interfaces where one material comprising the interface is characterized by high elastic anisotropy. Temperature-dependent specific heat is calculated with this vibrational model and compared to published values. Modifications to the DMM that incorporate the vibrational model are presented with predictions of hBD at a metal-graphite interface. This model slightly underestimates experimental data, as expected, as the large acoustic mismatch between metals and graphite suggests inelastic scattering, something the DMM does not take into account.

Effect of dislocation density on thermal boundary conductance across GaSb/GaAs interfaces

We report on the thermal boundary conductance across structurally-variant GaSb/GaAs interfaces characterized by different dislocations densities, as well as variably-rough Al/GaSb interfaces. The GaSb/GaAs structures are epitaxially grown using both interfacial misfit (IMF) and non-IMF techniques. We measure the thermal boundary conductance from 100 to 450 K with time-domain thermoreflectance. The thermal boundary conductance across the GaSb/GaAs interfaces decreases with increasing strain dislocation density. We develop a model for interfacial transport at structurally-variant interfaces in which phonon propagation and scattering parallels photon attenuation. We find that this model describes the measured thermal boundary conductances well.

Role of dispersion on phononic thermal boundary conductance

The diffuse mismatch model (DMM) is one of the most widely implemented models for predicting thermal boundary conductance at interfaces where phonons dominate interfacial thermal transport. In the original presentation of the DMM, the materials comprising the interface were described as Debye solids. Such a treatment, while accurate in the low temperature regime for which the model was originally intended, is less accurate at higher temperatures. Here, the DMM is reformulated such that, in place of Debye dispersion, the materials on either side of the interface are described by an isotropic dispersion obtained from exact phonon dispersion diagrams in the [100] crystallographic direction. This reformulated model is applied to three interfaces of interest: Cr–Si, Cu–Ge, and Ge–Si. It is found that Debye dispersion leads to substantially higher predictions of thermal boundary conductance. Additionally, it is shown that optical phonons play a significant role in interfacial thermal transport, a notion not pre...

Thermal transport in organic semiconducting polymers

We report on the thermal conductivities of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), [6,6]-phenyl C61-butyric acid methyl ester (PCBM), poly(3-hexylthiophene-2,5-diyl) (P3HT), and P3HT:PCBM blend thin films as measured by time domain thermoreflectance. Thermal conductivities vary from 0.031±0.005 to 0.227 ± 0.014  W m−1 K−1 near room temperature and exhibit minimal temperature dependence across the range from 319 to 396 K. Thermal conductivities of blend films follow a rule of mixtures, and no percolation threshold is found. Thermal annealing of blend films has a variable effect on thermal conductivity. Finally, the thermal conductivities of P3HT films do not vary with changes in film thickness from 77 to 200 nm.

Systematically controlling Kapitza conductance via chemical etching

We measure the thermal interface conductance between thin aluminum films and silicon substrates via time-domain thermoreflectance from 100 to 300 K. The substrates are chemically etched prior to aluminum deposition, thereby offering a means of controlling interface roughness. We find that conductance can be systematically varied by manipulating roughness. In addition, transmission electron microscopy confirms the presence of a conformal oxide for all roughnesses, which is then taken into account via a thermal resistor network. This etching process provides a robust technique for tuning the efficiency of thermal transport while alleviating the need for laborious materials growth and/or processing.

Thermal conductivity of nano-grained SrTiO3 thin films

We measure the thermal conductivities of nano-grained strontium titanate (ng-SrTiO3) films deposited on sapphire substrates via time-domain thermoreflectance. The 170 nm thick oxide films of varying grain-size were prepared from a chemical solution deposition process. We find that the thermal conductivity of ng-SrTiO3 decreases with decreasing average grain size and attribute this to increased phonon scattering at grain boundaries. Our data are well described by a model that accounts for the spectral nature of anharmonic Umklapp scattering along with grain boundary scattering and scattering due to the film thickness.

On the Assumption of Detailed Balance in Prediction of Diffusive Transmission Probability During Interfacial Transport

Models intended to predict interfacial transport often rely on the principle of detailed balance when formulating the interfacial carrier transmission probability. However, assumptions invoked significantly impact predictions. Here, we present six derivations of the transmission probability, each subject to a different set of preliminary assumptions regarding the type of scattering at the interface. Application of each case to phonon flux and thermal boundary conductance allows for a final quantitative comparison. Depending on the preliminary assumptions, predictions for thermal boundary conductance span over two orders of magnitude, demonstrating the need for transparency when assessing the accuracy of any predictive model.