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As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between “high-entropy” and “entropy-stabilized” phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10−6 K−1. Mechanical softening is also observed across the transition.

The thermal properties of semiconductors following exposure to ion irradiation are of great interest for the cooling of electronic devices; however, gradients in composition and structure due to irradiation often make the measurement difficult. Furthermore, the nature of spatial variations in thermal resistances due to spatially varying ion irradiation damage is not well understood. In this work, we develop an advancement in the analysis of time-domain thermoreflectance to account for spatially varying thermal conductivity in a material resulting from a spatial distribution of defects. We then use this method to measure the near-surface ([Formula: see text]1 [Formula: see text]m) thermal conductivity of silicon wafers irradiated with Kr+ ions, which has an approximate Gaussian distribution centered 260 nm into the sample. Our numerical analysis presented here allows for the spatial gradient of thermal conductivity to be extracted via what is fundamentally a volumetric measurement technique. We validate our findings via transmission electron microscopy, which is able to confirm the spatial variation of the sub-surface silicon structure, and provide additional insight into the local structure resulting from the effects of ion bombardment. Thermal measurements found the ion stopping region to have a nearly 50[Formula: see text] reduction in thermal conductivity as compared to pristine silicon, while TEM showed the region was not fully amorphized. Our results suggest this drastic reduction in silicon thermal conductivity is primarily driven by structural defects in crystalline regions along with boundary scattering between amorphous and crystalline regions, with a negligible contribution being due to implanted krypton ions themselves.

The detailed understanding of energy transfer between hot electrons and lattice vibrations at non-cryogenic temperatures relies primarily upon the interpretation of ultrafast pump–probe experiments, where thermo-optical models provide insight into the relationship between optical response and temperature of the respective sub-systems; in one of the more studied materials, gold, the Drude model provides this relationship. In this work, we investigate the role of intra- and interband contributions applied to transient optical responses in ultrafast pump–probe experiments using both experiments and first-principle calculations, with probe wavelengths spanning from UV wavelengths into the infrared. We find that during conditions of electron–phonon equilibrium, the Drude model is not applicable to visible wavelengths due to interband transitions. Instead, at probe wavelengths far from these interband transitions (e.g., infrared wavelengths), the optical response is linearly proportional to the temperature of the phonon sub-system and is no longer obfuscated by Fermi-smearing, thus greatly simplifying the extraction of the electron–phonon coupling factor. Our intraband-probe measurements on the electron–phonon coupling factor of Au are in excellent agreement with analytical models and ab initio calculations; we observe a constant electron–phonon coupling factor up to electron temperatures of at least ∼ 2000 K.

Studies of size effects on thermal conductivity typically necessitate the fabrication of a comprehensive film thickness series. In this Letter, we demonstrate how material fabricated in a wedged geometry can enable similar, yet higher-throughput measurements to accelerate experimental analysis. Frequency domain thermoreflectance (FDTR) is used to simultaneously determine the thermal conductivity and thickness of a wedged silicon film for thicknesses between 100 nm and 17 μm by considering these features as fitting parameters in a thermal model. FDTR-deduced thicknesses are compared to values obtained from cross-sectional scanning electron microscopy, and corresponding thermal conductivity measurements are compared against several thickness-dependent analytical models based upon solutions to the Boltzmann transport equation. Our results demonstrate how the insight gained from a series of thin films can be obtained via fabrication of a single sample.

We report on the experimental determination of electron–electron conductance at Au/TiOx interfacial regions and electron–phonon coupling of thin TiOx layers for x = 0–2.62. Our study demonstrates that the electronic energy transport mechanisms at metal/metal oxide interfaces are enhanced through metallic defects that lead to electronic band alignment between the metal and metal oxide (in our case, Au and TiOx). Electronic heat transport processes are interrogated via a pump/probe technique, utilizing sub-picosecond laser pulses to monitor the ultrafast thermoreflectance responses of Au/TiOx systems, which were analyzed using a two-temperature model to extract electron–electron conductances at Au/TiOx interfaces and the electron–phonon coupling in TiOx layers. We find that TiOx stoichiometries near TiO2 have ultrahigh electron–phonon coupling factors similar to that of pure Ti and that electronic energy transmission from Au to TiOx layers is comparable to that of Au to Ti due to the presence of Ti0 defects. For x = 2.62 in TiOx, electron–phonon coupling is reduced by more than a factor of 5. Our experimental data are corroborated by real-time time-dependent density functional theory calculations, which show that excited electrons in Au do not participate in the TiOx phonon relaxation process, resulting in lower electron–electron energy transmission from Au and electron–phonon coupling due to the difference in the Fermi energy of Au relative to the conduction band minimum of TiOx when x >2.

The elastic moduli of amorphous and crystalline atomic layer-deposited Hf1-xZrxO2 (HZO, x = 0, 0.31, 0.46, 0.79, 1) films prepared with TaN electrodes on silicon substrates were investigated using picosecond acoustic measurements. The moduli of the amorphous films were observed to increase between 211 ± 6 GPa for pure HfO2 and 302 ± 9 GPa for pure ZrO2. In the crystalline films, it was found that the moduli increased upon increasing the zirconium composition from 248 ± 6 GPa for monoclinic HfO2 to 267 ± 9 GPa for tetragonal ZrO2. Positive deviations from this increase were observed for the Hf0.69Zr0.31O2 and Hf0.54Zr0.46O2 compositions, which were measured to have moduli of 264 ± 8 GPa and 274 ± 8 GPa, respectively. These two compositions contained the largest fractions of the ferroelectric orthorhombic phase, as assessed from polarization and diffraction data. The biaxial stress states of the crystalline films were characterized through sin2( ψ) x-ray diffraction analysis. The in-plane stresses were all found to be tensile and observed to increase with the increasing zirconium composition, between 2.54 ± 0.6 GPa for pure HfO2 and 5.22 ± 0.5 GPa for pure ZrO2. The stresses are consistent with large thermal expansion mismatches between the HZO films and silicon substrates. These results demonstrate a device-scale means to quantify biaxial stress for investigation on its effect on the ferroelectric properties of hafnia-based materials.

In this study, we report on the thermal conductivity of amorphous carbon generated in diamond via nitrogen ion implantation (N 3 + at 16.5 MeV). Transmission electron microscopy techniques demonstrate amorphous band formation about the longitudinal projected range, localized approximately 7 μm beneath the sample surface. While high-frequency time-domain thermoreflectance measurements provide insight into the thermal properties of the near-surface preceding the longitudinal projected range depth, a complimentary technique, steady-state thermoreflectance, is used to probe the thermal conductivity at depths which could not otherwise be resolved. Through measurements with a series of focusing objective lenses for the laser spot size, we find the thermal conductivity of the amorphous region to be approximately 1.4 W m − 1 K − 1, which is comparable to that measured for amorphous carbon films fabricated through other techniques.

Nanomultilayers are complex architectures of materials stacked in sequence with layer thicknesses in the nanometer range. Their application in microelectronics is challenged by their thermal stability, conductivity, and interface reactivity, which can compromise their performance and usability. By using different materials as thermal barriers and by changing their thickness, it is possible to manipulate interfacial effects on thermal transport. In this work, we report on the thermal conductivity of Cu/W, Cu/Ta, and Cu/TaN sputter deposited nanomultilayers with different thicknesses. The resistive interfacial effects are rationalized and discussed also in relation to the structural transformation into a nano-composite upon high-temperature annealing.

Germanium–antimony–telluride has emerged as a nonvolatile phase change memory material due to the large resistivity contrast between amorphous and crystalline states, rapid crystallization, and cyclic endurance. Improving thermal phase stability, however, has necessitated further alloying with optional addition of a quaternary species (e.g., C). Here, the thermal transport implications of this additional species are investigated using frequency-domain thermoreflectance in combination with structural characterization derived from x-ray diffraction and Raman spectroscopy. Specifically, the room temperature thermal conductivity and heat capacity of ( Ge 2 Sb 2 Te 5 ) 1 − x C x are reported as a function of carbon concentration ( x ≤ 0.12) and anneal temperature ( T ≤ 350 ° C) with results assessed in reference to the measured phase, structure, and electronic resistivity. Phase stability imparted by the carbon comes with comparatively low thermal penalty as materials exhibiting similar levels of crystallinity have comparable thermal conductivity despite the addition of carbon. The additional thermal stability provided by the carbon does, however, necessitate higher anneal temperatures to achieve similar levels of structural order.

Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelectric applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-crystalline metal/semiconductor superlattices for thermionic energy conversion. Solid-solution alloys of ScN with traditional III-nitrides such as AlScN have demonstrated piezoelectric and ferroelectric properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and physical and chemical vapor depositions for thermoelectric applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concentrations, and achieve high-mobility in epitaxial films. Here, we report the high-mobility and high-thermoelectric power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted molecular beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Electrical measurements demonstrated a high room-temperature mobility of 127 cm/V s and temperature-dependent mobility in the temperature range of 50-400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coefficients (-175 μV/K at 950 K) and, along with a moderately high electrical conductivity, a large thermoelectric power factor (2.3 × 10 W/m-K at 500 K) was achieved, which makes ScN a promising candidate for thermoelectric applications. The thermal conductivity of the films, however, was found to be a bit large, which resulted in a maximum figure-of-merit of 0.17 at 500 K., D.R., B.B., and B.S. acknowledge the International Center for Materials Science (ICMS) and Sheik Saqr Laboratory (SSL) in JNCASR for support. B.S. acknowledges the Science and Engineering Research Board (SERB) of the Government of India, Start-Up Research Grant No. SRG/2019/000613 for financial support. M.S.M.G. wants to acknowledge financial support from No. MAT2017-86450- C4-3-R and intramural CSIC Project No. 2D-MESES. M.G., V.B., and A.I.K.P acknowledge the facilities of Sydney Microscopy and Microanalysis at the University of Sydney. Y.R.K. and P.E.H. appreciate support from a MURI program through the Office of Naval Research, Grant No. N00014-18-1-2429.

In phase change memory cells, the majority of heat is lost through the electrodes during the programming process, which leads to significant drops in the performance of the memory device. In this Letter, we report on the thermal properties of thin film carbon nitride with a modest electrical resistivity of 5–10 mΩ cm, a low thermal conductivity of 1.47 ± 0.09 W m−1 K–1, and a low interfacial thermal conductance between carbon nitride and phase change material for length scales below 40 nm. The thermally insulating property of carbon nitride makes it a suitable thermal barrier, allowing for less heat loss during Joule heating within the memory unit. We compare the thermal properties of carbon nitride against the commonly used electrodes and insulators such as tungsten and silicon nitride, respectively, to demonstrate the promise of carbon nitride as a potential material candidate for electrode applications in phase change memory devices.

The thermal properties of plasma-generated aluminum oxyfluoride passivation layers at the surface of aluminum thin films are measured. The oxyfluoride layers are generated using plasmas produced in mixtures of NH3 and SF6 to simultaneously remove oxygen and add fluorine to the aluminum surface, an alternative approach to the more conventional two-step methods that utilize HF treatments to remove the native oxide followed by metal-fluoride (e.g., MgF2, LiF, and AlF3) thin film deposition that serves to protect the aluminum surface from further oxidation. Here, the change in thermal properties of the layers as a function of plasma processing time is determined. A significant reduction in thermal boundary conductance is measured with the increasing treatment time, which can be related to the increasing fluorine content in the layers. Acoustic reflection measurements indicate this reduced thermal boundary conductance is associated with lower bonding strength to aluminum with increasing fluorine.

Thermoreflectance techniques, namely, time- and frequency-domain thermoreflectance (TDTR and FDTR, respectively), are ubiquitously used for the thermophysical characterization of thin films and bulk materials. In this perspective, we discuss several recent advancements in thermoreflectance techniques to measure the thermal conductivity of solids, with emphasis on the governing length scales and future directions in expanding these advances to different length scales and material structures. Specifically, the lateral resolution of these techniques, typically on the order of several micrometers, allows for an understanding of the spatially varying properties for various materials. Similarly, limitations of TDTR and FDTR with respect to their volumetric probing regions are discussed. With a recently developed steady-state thermoreflectance technique, these limitations are overcome as probing volumes approach spot sizes. Finally, recent pushes toward the implementation of these techniques without the use of a thin metal transducer are presented, with guidelines for future avenues in the implementation under these specimen configurations.

We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼93% as compared to the thermal conductivity of pristine graphene) via the introduction of covalently bonded fullerenes on the surface of the graphene sheets. We demonstrate large tunability in the thermal conductivity by the inclusion of covalently bonded fullerene molecules at different periodic inclusions, and we attribute the large reduction in thermal conductivities to a combination of resonant phonon localization effects, leading to band anticrossings and vibrational scattering at the sp3 bonded carbon atoms. The torsional force exerted by the fullerene molecules on the graphene sheets and the number of covalent bonds formed between the two carbon allotropes is shown to significantly affect the heat flow across the hybrid structures, while the size of the fullerene molecules is shown to have a negligible effect on their thermal properties. Moreover, we show that even for a large surface coverage, the mechanical properties of these novel materials are uncompromised. Taken together, our work reveals a unique way to manipulate vibrational thermal transport without the introduction of lattice defects, which could potentially lead to high thermoelectric efficiencies in these materials.

It is shown using classical molecular dynamics simulations that phonon scattering from disorder in the interatomic forces introduced by charge transfer and not from mass disorder is needed to explain the thermal conductivity reduction experimentally measured that accompanies the addition of a sixth cation to the entropy stabilized oxide J14 [(Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5]. The simulations were performed on five entropy-stabilized oxides, J14, and J14 plus Sc, Sn, Cr, or Ge in equi-molar cation proportions. Comparing the simulation results to predictions from the Bridgman equation using properties from the simulations suggests that despite phonon scattering from disorder in both atomic forces and mass, the thermal conductivity for these systems is still above an analytical limit for an amorphous structure.

We demonstrate a steady-state thermoreflectance-based optical pump-probe technique to measure the thermal conductivity of materials using a continuous wave laser heat source. The technique works in principle by inducing a steady-state temperature rise in a material via long enough exposure to heating from a pump laser. A probe beam is then used to detect the resulting change in reflectance, which is proportional to the change in temperature at the sample surface. Increasing the power of the pump beam to induce larger temperature rises, Fourier’s law is used to determine the thermal conductivity. We show that this technique is capable of measuring the thermal conductivity of a wide array of materials having thermal conductivities ranging from 1 to >2000 W m−1 K−1, in excellent agreement with literature values.

Spectral linewidths are used to assess a variety of physical properties, even as spectral overlap makes quantitative extraction difficult owing to uncertainty. Uncertainty, in turn, can be minimized with the choice of appropriate experimental conditions used in spectral collection. In response, we assess the experimental factors dictating uncertainty in the quantification of linewidth from a Raman experiment highlighting the comparative influence of (1) spectral resolution, (2) signal to noise, and (3) relative peak intensity (RPI) of the overlapping peaks. Practically, Raman spectra of SiGe thin films were obtained experimentally and simulated virtually under a variety of conditions. RPI is found to be the most impactful parameter in specifying linewidth followed by the spectral resolution and signal to noise. While developed for Raman experiments, the results are generally applicable to spectroscopic linewidth studies illuminating the experimental trade-offs inherent in quantification.

We report on the thermal resistances of thin films (20 nm) of hafnium zirconium oxide (Hf1–xZrxO2) with compositions ranging from 0 ≤ x ≤ 1. Measurements were made via time-domain thermoreflectance and analyzed to determine the effective thermal resistance of the films in addition to their associated thermal boundary resistances. We find effective thermal resistances ranging from 28.79 to 24.72 m2 K GW−1 for amorphous films, which decreased to 15.81 m2 K GW−1 upon crystallization. Furthermore, we analyze the heat capacity for two compositions, x = 0.5 and x = 0.7, of Hf1–xZrxO2 and find them to be 2.18 ± 0.56 and 2.64 ± 0.53 MJ m−3 K−1, respectively.

The energy flux to a surface during plasma exposure and the associated surface heating are of long standing interest as they contribute to the physico-chemical changes that occur during plasma-based materials synthesis and processing. Indeed, the energy delivered to the surface, via a flux of particles and photons, in concert with a flux of reactive species serves to chemically modify, etch, and/or deposit materials, with an efficacy that depends on the plasma processing environment. A unique feature of plasma synthesis and processing is that most of the delivered energy is absorbed at or very near the surface over short (picosecond) time scales. The dissipation of thermal energy proceeds through electron-electron and/or electron-phonon interactions as they propagate through the material, with relaxation time scales that can be orders of magnitude slower. Typically then, the surface is not in thermal equilibrium with the bulk material. Fast, surface-sensitive techniques are thus required to fully appreciate the dynamics of the plasma-surface interaction. In this work, we employ pump-probe Time-Domain Thermoreflectance, a surface sensitive technique typically used to measure thermal properties of thin films, to determine electron heating of thin metal films during exposure to an atmospheric pressure plasma jet. The results, in conjunction with current measurements, are used to develop a first order understanding of plasma jet-surface interactions. The results show that the energy delivered by the plasma jet causes a localized increase in electron energy within the thin film over an area commensurate with the plasma jet radius.

We use aluminum nano-inclusions in silicon to demonstrate the dominance of elastic modulus mismatch induced scattering in phonon transport. We use time domain thermoreflectance to measure the thermal conductivity of thin films of silicon co-deposited with aluminum via molecular beam epitaxy resulting in a Si film with 10% clustered Al inclusions with nanoscale dimensions and a reduction in thermal conductivity of over an order of magnitude. We compare these results with well-known models in order to demonstrate that the reduction in the thermal transport is driven by elastic mismatch effects induced by aluminum in the system.We use aluminum nano-inclusions in silicon to demonstrate the dominance of elastic modulus mismatch induced scattering in phonon transport. We use time domain thermoreflectance to measure the thermal conductivity of thin films of silicon co-deposited with aluminum via molecular beam epitaxy resulting in a Si film with 10% clustered Al inclusions with nanoscale dimensions and a reduction in thermal conductivity of over an order of magnitude. We compare these results with well-known models in order to demonstrate that the reduction in the thermal transport is driven by elastic mismatch effects induced by aluminum in the system.

We experimentally demonstrate the role of oxygen stoichiometry on the thermal boundary conductance across Au/TiOx/substrate interfaces. By evaporating two different sets of Au/TiOx/substrate samples under both high vacuum and ultrahigh vacuum conditions, we vary the oxygen composition in the TiOx layer from 0 ≤ x ≤ 2.85. We measure the thermal boundary conductance across the Au/TiOx/substrate interfaces with time-domain thermoreflectance and characterize the interfacial chemistry with x-ray photoemission spectroscopy. Under high vacuum conditions, we speculate that the environment provides a sufficient flux of oxidizing species to the sample surface such that one essentially co-deposits Ti and these oxidizing species. We show that slower deposition rates correspond to a higher oxygen content in the TiOx layer, which results in a lower thermal boundary conductance across the Au/TiOx/substrate interfacial region. Under the ultrahigh vacuum evaporation conditions, pure metallic Ti is deposited on the substrate surface. In the case of quartz substrates, the metallic Ti reacts with the substrate and getters oxygen, leading to a TiOx layer. Our results suggest that Ti layers with relatively low oxygen compositions are best suited to maximize the thermal boundary conductance.

In controlling the thermal properties of the surrounding environment, we provide insight into the underlying mechanisms driving the widely used laser direct write method for additive manufacturing. We find that the onset of silver nitrate reduction for the formation of direct write structures directly corresponds to the calculated steady-state temperature rises associated with both continuous wave and high-repetition rate, ultrafast pulsed laser systems. Furthermore, varying the geometry of the heat affected zone, which is controllable based on in-plane thermal diffusion in the substrate, and laser power, allows for control of the written geometries without any prior substrate preparation. These findings allow for the advance of rapid manufacturing of micro- and nanoscale structures with minimal material constraints through consideration of the laser-controllable thermal transport in ionic liquid/substrate media.

We investigate the effect of mass disorder, temperature, and pressure on the spectral thermal conductivity of multicomponent crystalline solid solutions via molecular dynamics simulations. The thermal conductivities of Lennard-Jones based solid solutions with one to five different atomic components in the crystalline lattice are simulated at a range of uniaxial strain levels and temperatures. Our results show that for multicomponent alloys, increasing only the mass impurity scattering by adding atoms with different masses in the solid solution does not lead to significant changes in the spectral contributions to thermal conductivity. However, increasing the impurity concentration or changing the local force-field of the impurity atoms in the solid solution has a relatively significant impact on the spectral contributions to thermal conductivity. The effect of chemical order in these alloys is shown to drastically alter the temperature dependence due to the different scattering mechanisms dictating thermal...

Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4 × 10−4 K−1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers.

We investigate the effect of crystalline configuration on the thermal conductivity of binary Lennard-Jones based solid solutions via classical molecular dynamics simulations and harmonic lattice dynamics calculations. We show that the pronounced effect of Umklapp scattering causes the cross-plane thermal conductivity of the chemically ordered alloy (1 × 1 monolayer period superlattice) to approach the thermal conductivity of the disordered counterpart (alloy limit) at elevated temperatures. However, we find that for superlattices with thicker periods and larger acoustic mismatch between the layers, the thermal conductivity can approach a minimum that is well below the alloy limit and can even approach the theoretical minimum limit of the corresponding amorphous phase. Our simulations over a wide range of mass ratios between the species suggest two contrasting effects of increasing mass ratio: (i) flattening of modes that leads to lower group velocities and lower overall thermal conductivity and (ii) reduc...

This work represents a thorough investigation of the thermal conductivity (κ) in both thin film and bulk PbZr1–xTixO3 (PZT) across the compositional phase diagram. Given the technological importance of PZT as a superb piezoelectric and ferroelectric material in devices and systems impacting a wide array of industries, this research serves to fill the gap in knowledge regarding the thermal properties. The thermal conductivities of both thin film and bulk PZT are found to vary by a considerable margin as a function of composition x. Additionally, we observe a discontinuity in κ in the vicinity of the morphotropic phase boundary (MPB, x = 0.48) where there is a 20%–25% decrease in κ in our thin film data, similar to that found in literature data for bulk PZT. The comparison between bulk and thin film materials highlights the sensitivity of κ to size effects such as film thickness and grain size even in disordered alloy/solid-solution materials. A model for the thermal conductivity of PZT as a function of com...

In this study, we present a method to calculate the temperature and heat flux profiles as a function of depth and radius for bulk, homogeneous materials and samples with layered thin-film structures, including geometries supporting bidirectional heat fluxes, during pulsed and continuous wave (CW) laser heating. We calculate the temperature profiles for both modulated and unmodulated heating events to reveal that the thermal penetration depth (defined as the depth at which temperature decays to 1/e of the surface temperature) for a pulsed laser is highly dependent on time and repetition rate. In the high repetition rate limit, the temperature profile relaxes to that of a CW source profile, while in the opposite extreme, a single pulse response is observed such that the concept of the thermal penetration depth loses any practical meaning. For modulated heating events such as those used in time- and frequency-domain thermoreflectance, we show that there is a limit to the thermal penetration depth obtainable ...

We demonstrate a method to reduce the thermal conductivity of fully dense (above the rigidity percolation threshold) amorphous thin films below the minimum limit by systematically changing the coordination number through hydrogenation. Studying a-SiO:H, a-SiC:H, and a-Si:H thin films, we measure the thermal properties using time-domain thermoreflectance to show that thermal conductivity can be reduced below the amorphous limit by a factor of up to two. By experimentally investigating the thermophysical parameters that determine thermal conductivity, we show that sound speed, atomic density, and heat capacity cannot explain the measured reduction in thermal conductivity, revealing that the coordination number can significantly alter the scattering length scale of heat carriers. Reformulating the minimum limit to consider the propensity for energy to transfer through the non-hydrogen network of atoms, we observe greatly improved agreement with experimental data.

The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3–4 μm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015–1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends—and their overall reduction relative to bulk—are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.

We report on the thermal boundary resistances across crystalline and amorphous confined thin films and the thermal conductivities of amorphous/crystalline superlattices for Si/Ge systems as determined via non-equilibrium molecular dynamics simulations. Thermal resistances across disordered Si or Ge thin films increase with increasing length of the interfacial thin films and in general demonstrate higher thermal boundary resistances in comparison to ordered films. However, for films ≲3 nm, the resistances are highly dependent on the spectral overlap of the density of states between the film and leads. Furthermore, the resistances at a single amorphous/crystalline interface in these structures are much lower than those at interfaces between the corresponding crystalline materials, suggesting that diffusive scattering at an interface could result in higher energy transmissions in these systems. We use these findings, together with the fact that high mass ratios between amorphous and crystalline materials can...

Understanding the impact and complex interaction of thermal carrier scattering centers in functional oxide systems is critical to their progress and application. In this work, we study the interplay among electron and phonon thermal transport, mass-impurity scattering, and phonon-vacancy interactions on the thermal conductivity of cadmium oxide. We use time domain thermoreflectance to measure the thermal conductivity of a set of CdO thin films doped with Dy up to the saturation limit. Using measurements at room temperature and 80 K, our results suggest that the enhancement in thermal conductivity at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation-vacancy scattering. With these results, we gain insight into the comple...

Several dynamic thermal and nonthermal scattering processes affect ultrafast heat transfer in metals after short-pulsed laser heating. Even with decades of measurements of electron-phonon relaxation, the role of thermal vs. nonthermal electron and phonon scattering on overall electron energy transfer to the phonons remains unclear. In this work, we derive an analytical expression for the electron-phonon coupling factor in a metal that includes contributions from equilibrium and nonequilibrium distributions of electrons. While the contribution from the nonthermal electrons to electron-phonon coupling is non-negligible, the increase in the electron relaxation rates with increasing laser fluence measured by thermoreflectance techniques cannot be accounted for by only considering electron-phonon relaxations. We conclude that electron-electron scattering along with electron-phonon scattering have to be considered simultaneously to correctly predict the transient nature of electron relaxation during and after short-pulsed heating of metals at elevated electron temperatures. Furthermore, for high electron temperature perturbations achieved at high absorbed laser fluences, we show good agreement between our model, which accounts for d-band excitations, and previous experimental data. Our model can be extended to other free electron metals with the knowledge of the density of states of electrons in the metals and considering electronic excitations from non-Fermi surface states.

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-Webber 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 study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an ∼3 nm Ti adhesion layer between the Au film and the non-metal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate with temperature, which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electron-interface scattering is shown to have negligi...

We study the interplay between the contributions of electron thermal flux and interface scattering to the Kapitza conductance across metal-metal interfaces through measurements of thermal conductivity of copper-niobium multilayers. Thermal conductivities of copper-niobium multilayer films of period thicknesses ranging from 5.4 to 96.2 nm and sample thicknesses ranging from 962 to 2677 nm are measured by time-domain thermoreflectance over a range of temperatures from 78 to 500 K. The Kapitza conductances between the Cu and Nb interfaces in multilayer films are determined from the thermal conductivities using a series resistor model and are in good agreement with the electron diffuse mismatch model. Our results for the thermal boundary conductance between Cu and Nb are compared to literature values for the thermal boundary conductance across Al-Cu and Pd-Ir interfaces, and demonstrate that the interface conductance in metallic systems is dictated by the temperature derivative of the electron energy flux in the metallic layers, rather than electron mean free path or scattering processes at the interface.

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devices due 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...

The electronic transport properties of metals with weak electron-phonon coupling can be influenced by non-thermal electrons. Relaxation processes involving non-thermal electrons competing with the thermalized electron system have led to inconsistencies in the understanding of how electrons scatter and relax with the less energetic lattice. Recent theoretical and computational works have shown that the rate of energy relaxation with the metallic lattice will change depending on the thermalization state of the electrons. Even though 20 years of experimental works have focused on understanding and isolating these electronic relaxation mechanisms with short pulsed irradiation, discrepancies between these existing works have not clearly answered the fundamental question of the competing effects between non-thermal and thermal electrons losing energy to the lattice. In this work, we demonstrate the ability to measure the electron relaxation for varying degrees of both electron-electron and electron-phonon thermalization. This series of measurements of electronic relaxation over a predicted effective electron temperature range up to ∼3500 K and minimum lattice temperatures of 77 K validate recent computational and theoretical works that theorize how a nonequilibrium distribution of electrons transfers energy to the lattice. Utilizing this wide temperature range during pump-probe measurements of electron-phonon relaxation, we explain discrepancies in the past two decades of literature of electronic relaxation rates. We experimentally demonstrate that the electron-phonon coupling factor in gold increases with increasing lattice temperature and laser fluences. Specifically, we show that at low laser fluences corresponding to small electron perturbations, energy relaxation between electrons and phonons is mainly governed by non-thermal electrons, while at higher laser fluences, non-thermal electron scattering with the lattice is less influential on the energy relaxation mechanisms.

Thermal boundary conductance is of critical importance to gallium nitride (GaN)-based device performance. While the GaN-substrate interface has been well studied, insufficient attention has been paid to the metal contacts in the device. In this work, we measure the thermal boundary conductance across interfaces of Au, Al, and Au-Ti contact layers and GaN. We show that in these basic systems, 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.

We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of Barium Titanate (BaTiO3) decreases with decreasing grain size as a result of increased phonon scattering from grain boundaries. We analyze our results with a model for thermal conductivity that incorporates a spectrum of mean free paths in BaTiO3. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths.

We investigate the fundamental mechanisms driving thermal transport across solid/classical-liquid interfaces via non-equilibrium molecular dynamics simulations. We show that the increase in thermal boundary conductance across strongly bonded solid/liquid interfaces compared to weakly bonded interfaces is due to increased coupling of low-frequency modes when the solid is better wetted by the liquid. Local phonon density of states and spectral temperature calculations confirm this finding. Specifically, we show that highly wetted solids couple low frequency phonon energies more efficiently, where the interface of a poorly wetted solid acts like free surfaces. The spectral temperature calculations provide further evidence of low frequency phonon mode coupling under non equilibrium conditions. These results quantitatively explain the influence of wetting on thermal boundary conductance across solid/liquid interfaces.

We report on the thermal conductivity of atomic layer deposition-grown amorphous alumina thin films as a function of atomic density. Using time domain thermoreflectance, we measure the thermal conductivity of the thin alumina films at room temperature. The thermal conductivities vary ∼35% for a nearly 15% change in atomic density and are substrate independent. No density dependence of the longitudinal sound speeds is observed with picosecond acoustics. The density dependence of the thermal conductivity agrees well with a minimum limit to thermal conductivity model that is modified with a differential effective-medium approximation.

We develop an analytical model for the thermal boundary conductance between a solid and a liquid. By infusing recent developments in the phonon theory of liquid thermodynamics with diffuse mismatch theory, we derive a closed form model that can predict the effects of wetting on the thermal boundary conductance across an interface between a solid and a classical liquid. We account for the complete wetting (hydrophilicity), or lack thereof (hydrophobicity), of the liquid to the solid by considering varying contributions of transverse mode interactions between the solid and liquid interfacial layers; this transverse coupling relationship is determined with local density of states calculations from molecular dynamics simulations between Lennard-Jones solids and a liquids with different interfacial interaction energies. We present example calculations for the thermal boundary conductance between both hydrophobic and hydrophilic interfaces of Al/water and Au/water, which show excellent agreement with measured values reported by Ge et al. [Z. Ge, D. G. Cahill, and P. V. Braun, Phys. Rev. Lett. 96, 186101 (2006)]. Our model does not require any fitting parameters and is appropriate to model heat flow across any planar interface between a solid and a classical liquid.

We study the scattering mechanisms driving electron-phonon relaxation in thin gold films via pump-probe time-domain thermoreflectance. Electron-electron scattering can enhance the effective rate of electron-phonon relaxation when the electrons are out of equilibrium with the phonons. In order to correctly and consistently infer electron-phonon coupling factors in films on different substrates, we must account for the increase in steady-state lattice temperature due to laser heating. Our data provide evidence that a thermalized electron population will not directly exchange energy with the substrate during electron-phonon relaxation, whereas this pathway can exist between a non-equilibrium distribution of electrons and a non-metallic substrate.

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.

In nanosystems, the thermal resistance between materials typically dominates the overall resistance. While size effects on thermal conductivity are well documented, size effects on thermal boundary conductance have only been speculated. In response, we characterize the relationship between interfacial resistance and material dimension using molecular dynamics. We find that the interfacial resistance increases linearly with inverse system length but is insensitive to cross-sectional area. Also, from the temperature-dependence of interfacial resistance, we conclude that contributions of short-wavelength phonons dominate. Lastly, by coupling the molecular dynamics to a two-temperature model, we show that electron-mediated transport has little effect on thermal resistance.

Ferroelectric and ferroelastic domain structure has a profound effect on the piezoelectric, ferroelectric, and dielectric responses of ferroelectric materials. However, domain walls and strain field effects on thermal properties are unknown. We measured the thermal conductance from 100–400 K of epitaxially grown BiFeO3 thin films with different domain variants, each separated primarily by 71° domain walls. We determined the Kapitza conductance across the domain walls, which is driven by the strain field induced by the domain variants. This domain wall Kapitza conductance is lower than the Kapitza conductance associated with grain boundaries in all previously measured materials.