Enhanced Thermal Boundary Conductance across GaN/SiC Interfaces with AlN Transition Layers.

Heat dissipation plays a crucial role in the performance and reliability of high-power GaN-based electronics. While AlN transition layers are commonly employed in the heteroepitaxial growth of GaN-on-SiC substrates, concerns have been raised about their impact on thermal transport across GaN/SiC interfaces. In this study, we present experimental measurements of the thermal boundary conductance (TBC) across GaN/SiC interfaces with varying thicknesses of the AlN transition layer (ranging from 0 to 73 nm) at different temperatures. Our findings reveal that the addition of an AlN transition layer leads to a notable increase in the TBC of the GaN/SiC interface, particularly at elevated temperatures. Structural characterization techniques are employed to understand the influence of the AlN transition layer on the crystalline quality of the GaN layer and its potential effects on interfacial thermal transport. To gain further insights into the trend of TBC, we conduct molecular dynamics simulations using high-fidelity deep learning-based interatomic potentials, which reproduce the experimentally observed enhancement in TBC even for atomically perfect interfaces. These results suggest that the enhanced TBC facilitated by the AlN intermediate layer could result from a combination of improved crystalline quality at the interface and the "phonon bridge" effect provided by AlN that enhances the overlap between the vibrational spectra of GaN and SiC.

Direct Visualization of Localized Vibrations at Complex Grain Boundaries.

Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The functionality at a GB results from the unique atomic arrangements, different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic-scale GB structures to macroscopic electronic, infrared optical, and thermal properties. In this work, a SrTiO3 GB is examined using atomic-resolution aberration-corrected scanning transmission electron microscopy and ultrahigh-energy-resolution monochromated electron energy-loss spectroscopy, in conjunction with density functional theory. This combination enables the correlation of the GB structure, nonstoichiometry, and chemical bonding with a redistribution of vibrational states within the GB dislocation cores. The new experimental access to localized GB vibrations provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.

Direct Measurement of Ballistic and Diffusive Electron Transport in Gold.

We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Vacancy-Induced Temperature-Dependent Thermal and Magnetic Properties of Holmium-Substituted Bismuth Ferrite Nanoparticle Compacts.

Multiferroics have gained widespread acceptance for room-temperature applications such as in spintronics, ferroelectric random access memory, and transistors because of their intrinsic magnetic and ferroelectric coupling. However, a comprehensive study, establishing a correlation between the magnetic and thermal transport properties of multiferroics, is still missing from the literature. To fill the void, this work reports the temperature-dependent thermal and magnetic properties of holmium-substituted bismuth ferrite (BiFeO3) and their dependencies on oxygen vacancies and structural modifications. Two distinct magnetic transitions on temperature-dependent magnetic and heat capacity responses are identified. Experimental analysis suggests that the excess of oxygen vacancies shifts the magnetic transition temperature by ∼64 K. The holmium substitution-induced structural modification increases BiFeO3 heat capacity by 30% up to the antiferromagnetic phase transition temperature. Furthermore, an unsaturated heat capacity even at temperatures as high as 850 K is observed and is ascribed to anharmonicity and partial densification of the nanoparticles used during heat capacity measurements. The room-temperature thermal conductivity of BiFeO3 is ∼0.33 ± 0.11 W m-1 K-1 and remains unchanged at high temperatures due to defect scattering from porosities.

Observation of solid-state bidirectional thermal conductivity switching in antiferroelectric lead zirconate (PbZrO3).

Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO3 bidirectionally by -10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO3 that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO3 can serve as a fast (<1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m-1 K-1.

Suppressed electronic contribution in thermal conductivity of Ge2Sb2Se4Te.

Integrated nanophotonics is an emerging research direction that has attracted great interests for technologies ranging from classical to quantum computing. One of the key-components in the development of nanophotonic circuits is the phase-change unit that undergoes a solid-state phase transformation upon thermal excitation. The quaternary alloy, Ge2Sb2Se4Te, is one of the most promising material candidates for application in photonic circuits due to its broadband transparency and large optical contrast in the infrared spectrum. Here, we investigate the thermal properties of Ge2Sb2Se4Te and show that upon substituting tellurium with selenium, the thermal transport transitions from an electron dominated to a phonon dominated regime. By implementing an ultrafast mid-infrared pump-probe spectroscopy technique that allows for direct monitoring of electronic and vibrational energy carrier lifetimes in these materials, we find that this reduction in thermal conductivity is a result of a drastic change in electronic lifetimes of Ge2Sb2Se4Te, leading to a transition from an electron-dominated to a phonon-dominated thermal transport mechanism upon selenium substitution. In addition to thermal conductivity measurements, we provide an extensive study on the thermophysical properties of Ge2Sb2Se4Te thin films such as thermal boundary conductance, specific heat, and sound speed from room temperature to 400 °C across varying thicknesses.

Thermal conductivity measurements of sub-surface buried substrates by steady-state thermoreflectance.

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump-probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump-probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump-probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries.

High In-Plane Thermal Conductivity of Aluminum Nitride Thin Films.

High thermal conductivity materials show promise for thermal mitigation and heat removal in devices. However, shrinking the length scales of these materials often leads to significant reductions in thermal conductivities, thus invalidating their applicability to functional devices. In this work, we report on high in-plane thermal conductivities of 3.05, 3.75, and 6 µm thick aluminum nitride (AlN) films measured via steady-state thermoreflectance. At room temperature, the AlN films possess an in-plane thermal conductivity of ∼260 ± 40 W m-1 K-1, one of the highest reported to date for any thin film material of equivalent thickness. At low temperatures, the in-plane thermal conductivities of the AlN films surpass even those of diamond thin films. Phonon-phonon scattering drives the in-plane thermal transport of these AlN thin films, leading to an increase in thermal conductivity as temperature decreases. This is opposite of what is observed in traditional high thermal conductivity thin films, where boundaries and defects that arise from film growth cause a thermal conductivity reduction with decreasing temperature. This study provides insight into the interplay among boundary, defect, and phonon-phonon scattering that drives the high in-plane thermal conductivity of the AlN thin films and demonstrates that these AlN films are promising materials for heat spreaders in electronic devices.

Experimental Control and Statistical Analysis of Thermal Conductivity in ZnO-Benzene Multilayer Thin Films.

We have fabricated a model system of precisely layer-engineered inorganic-organic thin-film structures using atomic/molecular-layer deposition (ALD/MLD). The samples consist of nanoscale polycrystalline ZnO layers and intervening benzene layers, covering a broad range of layer sequences. The samples characterized in this study combined with previous publications provide an excellent sample set to examine thermal transport properties in inorganic-organic thin films. The cross-plane thermal conductivity is found to depend on multiple factors, with the inorganic-organic interface density being the dominating factor. Our work highlights the remarkable capability of interface engineering in suppressing the thermal conductivity of hybrid inorganic-organic materials, e.g., for thermoelectric applications.