Thermal Boundary Resistance Research Articles

Three-sensor 3ω-2ω method for the simultaneous measurement of thermal conductivity and thermal boundary resistance in film-on-substrate heterostructures

Solid heterostructures composed of substrates and epitaxial films are extensively used in advanced technologies, and their thermophysical properties fundamentally determine the performance, efficiency, and reliability of the corresponding devices. However, an experimental method that is truly appropriate for the thermophysical property measurement of solid heterostructures is still lacking. To this end, a three-sensor 3ω-2ω method is proposed, which can simultaneously measure the thermal conductivities of the film and the substrate, along with the film-substrate thermal boundary resistance (TBR) in a single solid heterostructure without any reference samples, showing broad applicability for miscellaneous heterostructures with film thickness ranging from 100 nm to 10 μm. In this method, three parallel metal sensors with unequal widths and distances conforming to guidelines for the three-sensor layout design are fabricated on the sample surface, in which the two outer sensors serve as heaters and the middle sensor as a detector. The respective 3ω signals of the two heaters and the 2ω signal of the detector are measured, and then the thermophysical properties of the sample are fitted within 3D finite element simulations. To verify this method, two typical wide bandgap semiconductor heterojunctions, i.e., GaN on SiC (#SiC) and GaN on Si (#Si) with ∼2.3 μm GaN epilayers, are measured. The thermal conductivity of the GaN film, the thermal conductivities of the SiC and Si substrates, and the GaN/substrate TBRs are derived, exhibiting good agreement with the literature. The proposed method will provide a comprehensive solution for the thermophysical property measurements of various solid heterostructures.

Water filling in carbon nanotubes with different wettability and implications on nanotube/water heat transfer via atomistic simulations

The peculiar heat and mass transfer properties of Carbon Nanotubes (CNTs) envision promising applications in nanoengineering and nanofluidic devices, such as heat sinks and desalination membranes. However, a comprehensive understanding of the intertwined effects of mass transfer (entrance and exit of liquid molecules inside CNTs) and heat transfer mechanisms (thermal exchange at the CNT/solvent interface) as a function of the properties of CNT surface is currently incomplete. In this work, we use molecular dynamics simulations to study heat and mass transfer in single wall CNTs with (5,5) and (10,10) chirality immersed in water. We present a sensitivity analysis where, starting from different choices of interaction potentials between CNTs and water molecules, we deduce the corresponding CNT/water wetting parameters, we model fill-in and fill-out water dynamics and arrangement of water molecules at the equilibrium. Spontaneous water entrance into CNTs is examined and a single energy parameter to model water filling is introduced. Secondly, we compute the CNT/water thermal boundary resistance for the different wetting properties. In perspective, this work supports a more rational design of CNT-based devices operating in nanothermal and nanobiological environments.

Wide-Temperature Tunable Phonon Thermal Switch Based on Ferroelectric Domain Walls of Tetragonal KTN Single Crystal.

Ferroelectric domain walls (DWs) of perovskite oxide materials, which can be written and erased by an external electric field, offer the possibility to dynamically manipulate phonon scattering and thermal flux behavior. Different from previous ferroelectric materials, such as BaTiO3, PbTiO3, etc., with an immutable and low Curie temperature. The Curie temperature of perovskite oxide KTa1-xNbxO3 (KTN) crystal can be tuned by altering the Ta/Nb ratio. In this work, the ferroelectric KTa0.6Nb0.4O3 (KTN) single crystal is obtained by the Czochralski method. To understand the role of ferroelectric domains in thermal transport behavior, we perform a nonequilibrium molecular dynamics (NEMD) calculation on monodomain and 90° DWs of KTN at room temperature. The calculated thermal conductivity of monodomain KTN is 9.84 W/(m·k), consistent with experimental results of 8.96 W/(m·k), and distinctly decreased with the number of DWs indicating the outstanding performance of the thermal switch. We further evaluate the thermal boundary resistance (TBR) of KTN DWs. An interfacial thermal resistance value of 2.29 × 10-9 K·m2/W and a large thermal switch ratio of 4.76 was obtained for a single DW of KTN. Our study shows that the ferroelectric KTN can provide great potential for the application of thermal switch at room temperature and over a broad temperature range.

Molecular dynamics study of thermal transport across Ga2O3–diamond interfaces

Integration of β-Ga2O3 with high thermal conductivity materials such as diamond has been considered due to β-Ga2O3's low and anisotropic thermal conductivity, reaching only 27 W m–1 K–1. However, the effect of crystallographic orientation on thermal interface resistance has not been studied extensively, which is relevant for potential device architectures. In this work, we use molecular dynamics simulations to investigate the crystal orientation-dependent thermal boundary resistance (TBR) across van der Waals bonded diamond–β-Ga2O3 and ionicly bonded amorphous Al2O3–β-Ga2O3 interfaces. Al2O3 is often used as interlayer to grow diamond onto Ga2O3. We find that TBR values across the van der Waals interface may vary by up to 70% depending on the orientation of the β-Ga2O3, while the Al2O3–β-Ga2O3 TBR values remain around 0.9 ± 0.3 m2 KGW−1. We, thus, conclude on the optimal direction of β-Ga2O3 to use for reducing the TBR in these heterostructures.

AlN/diamond interface nanoengineering for reducing thermal boundary resistance by molecular dynamics simulations

Interfacial thermal transport has become a significant bottleneck in thermal management, particularly for the electronic high-power devices represented by III-V semiconductor devices. Diamond has great potential to be integrated with devices to dissipate heat efficiently due to its ultra-high thermal conductivity. In this paper, the Non-equilibrium Molecular Dynamics method, taking into consideration of the parameters such as the type of the interleaved nanopillars, the size and the height of the nanopillars, was used to study the influence of nanopillars on the thermal boundary resistance (TBR) at AlN/diamond interfaces. The TBR of the optimal AlN/diamond interface of nanopillar structures could be reduced by 28% compared to the planar interface. The vibrational density of states (VDOS) analysis of both AlN and diamond on each side of the interface can reveal that the enhancement of AlN intermediate frequency phonons and the shift of diamond VDOS towards the lower frequency can contribute to the optimization of the interfacial thermal transport. Hence, this work can provide a deeper understanding of the impact of nanostructures on the interfacial thermal transport and can also be a guideline for efficient thermal management through the introduction of nanostructures at the heterogeneous interfaces.

A data driven approach to model thermal boundary resistance from molecular dynamics simulations.

A new method is proposed to model the thermal boundary resistance (TBR) at the nanoscale, solid-liquid interface from macroscopic observables that characterize a nanoscale interface. We correlated the TBR with thermodynamic state variables, material properties, and geometric parameters to derive a generalized relationship with the help of data-driven heuristic algorithms. The results show that TBR can be expressed in terms of physical observables of the systems and material-specific parameters. We investigated the mutual independence of descriptor variables and quantified the weightage for each observable parameter in the TBR models. The interfacial liquid layering has a robust correlation with TBR. However, for systems with phonon size effects and under extreme thermodynamic conditions, the work of adhesion and system geometry also affects the variation in TBR. The data-driven approach followed in this study helps us gain better insight into the mechanism of TBR at nanoscale solid-liquid interfaces and shows significant improvement in our knowledge about interfacial thermal transport.

Tuning photoacoustics with nanotransducers via thermal boundary resistance and laser pulse duration

The photoacoustic effect in liquids, generated by metal nanoparticles excited with short laser pulses, offers high contrast imaging and promising medical treatment techniques. Understanding the role of the thermal boundary resistance (TBR) and the laser pulse duration in the generation mechanism of acoustic waves is essential to implement efficient photoacoustic nanotransducers. This work theoretically investigates, for the paradigmatic case of water-immersed gold nanocylinders, the role of the TBR and laser pulse duration in the competition between the launching mechanisms: the thermophone and the mechanophone. In the thermophone, the nanoparticle acts as a nanoheater and the wave is launched by water thermal expansion. In the mechanophone, the nanoparticle directly acts as a nanopiston. Specifically, for a gold–water interface, the thermophone prevails under ns light pulse irradiation, while the mechanophone dominates shortening the pulse to the 10 ps regime. For a graphene-functionalized gold–water interface, instead, the mechanophone dominates over the entire range of explored laser pulse durations. The results point to high-TBR, liquid-immersed nanoparticles as potentially efficient photoacoustic nanogenerators, with the advantage of keeping the liquid environment temperature unaltered.

Correlation of Superlattice Cross-Plane Thermal Conductivity with Emission Wavelength in InAlAs/InGaAs Quantum Cascade Lasers.

The low cross-plane thermal conductivity of quantum cascade lasers (QCLs) is a significant limitation in their Continuous-Wave (CW) performance. Structural parameters such as individual layer thicknesses and interface density vary for QCLs with different target emission wavelengths, and these design parameters are expected to influence the cross-plane thermal conductivity. Though previous works have used theoretical models and experimental data to quantify thermal conductivity, the correlation between target wavelength and thermal conductivity has yet to be reported for QCLs. In this work, we observe a general trend across a group of QCLs emitting from 3.7 to 8.7 µm: as the QCL design changes to reduce wavelength, the thermal conductivity decreases as well. Numerically, we measured an approximate 70% reduction in thermal conductivity, from 1.5 W/(m·K) for the 8.7 µm device, to 0.9 W/(m·K) for the 3.7 µm device. Analysis of these structures with the Diffuse Mismatch Model (DMM) for thermal boundary resistance (TBR) shows that the largest contribution of this effect is the impact of superlattice interface density on the thermal conductivity. The observed changes in conductivity result in significant changes in projected CW optical power and should be considered in laser design.

Integrating Diamond for Cooling Electronics

III-Nitrides promise to offer high-frequency power amplifiers at higher power densities. This is attractive for realizing device platforms for 5G and beyond. It also enables integrating Si-based electronics to complement functionalities. However, to obtain the full potential of III-Nitrides one must operate these transistors at higher power densities, for which thermal management is crucial.Due to the continuous need for higher performance in various applications ranging from DC to W-band frequencies, semiconductor devices are pushed to higher power densities. This results in a higher junction temperature owing to the Joule heating in the channel resulting in device performance degradation and premature failure. Using devices based on wide-bandgap (WBG) semiconductors such as Gallium Nitride (GaN) and Gallium Oxide (GaOx) with a larger critical electric field and high-temperature tolerance, one could increase the power density. However, in order to increase the output power to the levels that is promised by these wide bandgap materials, junction temperature reduction through device-level cooling is critical. We are actively researching thermal management at device, circuit and packaging level using synthetic polycrystalline (PC) diamond. PC-diamond, at an appropriate grain size offers a TC is higher than any competing thermal vias with metals without interfering the electrical performance. Over the past 5 years we have demonstrated the potential of integrating diamond with GaN and recently achieving a record low diamond/GaN thermal boundary resistance (TBR) along with a relatively high diamond thermal conductivity (TC). This remarkable thermal performance was be achieved by maintaining the electrical performance, as demonstrated by the unharmed channel charge and mobility results.There are two key requirements for thermal management using heat spreading from the channel: first, a low thermal boundary resistance (TBR) between the hot spot in the channel and the heat spreader is required (in this study between GaN channel and diamond). Second, a low thermal resistance path to the heat-sink is crucial from the diamond layer which is directly proportional to diamond’s thermal conductivity (TC). In collaboration with Univ of Bristol, we have reported a TBR of ~3 m2K/GW, to date, known to be the lowest reported TBR for GaN HEMT technology. Diamond-GaN TBR depends strongly on the interface smoothness and the thickness of interfacial Si3N4 layer. A thinner Si3N4 resulted in lower TBR and allowed superior phonon coupling from the channel to the spreader. We have also measured a remarkably high TC for a 2 μm-thick diamond layer yielding over 650 W/m.K. The diamond grains are near isotropic in shape that allows excellent in-plane and cross-plane thermal conductivity. Achieving high TC within a thin film of diamond, grown heterogeneously, underscores the importance of our technique, since it reduces the residual stress when integrating with different materials that involves many thin layers like GaN/AlGaN HEMT epi-layers.β-Ga2O3 is an emerging ultra-wide bandgap material showing promises for both power and RF devices. However, the material’s low thermal conductivity poses to be a challenge for efficient power delivery (at any frequencies). PC diamond was successfully grown on (‾¯201)β-Ga2O3 using proper nucleation technique and thermal characterizations were conducted. A thermal conductivity (diamond + nucleation) and thermal boundary resistance at the diamond/β-Ga2O3 interface of 110 ± 33 W/mK and 30.2 ± 1.8 m2K/GW, respectively were measured.Our current results demonstrates a very promising roadmap for wide bandgap semiconductors via diamond integration.

Structural measurement of electron-phonon coupling and electronic thermal transport across a metal-semiconductor interface

Scattering of energetic charge carriers and their coupling to lattice vibrations (phonons) in dielectric materials and semiconductors are crucial processes that determine the functional limits of optoelectronics, photovoltaics, and photocatalysts. The strength of these energy exchanges is often described by the electron-phonon coupling coefficient, which is difficult to measure due to the microscopic time- and length-scales involved. In the present study, we propose an alternate means to quantify the coupling parameter along with thermal boundary resistance and electron conductivity by performing a high angular-resolution time-resolved X-ray diffraction measurement of propagating lattice deformation following laser excitation of a nanoscale, polycrystalline metal film on a semiconductor substrate. Our data present direct experimental evidence for identifying the ballistic and diffusive transport components occurring at the interface, where only the latter participates in thermal diffusion. This approach provides a robust measurement that can be applied to investigate microscopic energy transport in various solid-state materials.

Thermal and Dielectric Properties of 3D Printed Parts Based on Polylactic Acid Filled with Carbon Nanostructures

AbstractPolylactic acid (PLA) containing 12 wt% of nanofillers, such as multi‐walled carbon nanotubes (MWCNTs), graphene nanoplates (GNPs), is considered to obtain a non‐conventional filament suitable for additive manufacturing (AM) process with enhanced thermal and electrical properties. In details, three‐dimensional (3D) printed specimens obtained by means of fused deposition modeling (FDM) are experimentally characterized in terms of thermal conductivity and relative permittivity. The results are interpreted on the basis of the different features of the fillers and their interaction with the PLA, as evidenced by a morphological analysis. It is found better thermal transport by using two‐dimensional (2D) shape nanoparticles (GNPs) with an improvement of about 260% respect to the unfilled polymer most likely due to its favorable arrangement and the lower thermal boundary resistance between the two phases, matrix, and fillers. Differently, mono‐dimensional filler like MWCNTs is to be preferred in order to obtain higher value for the relative permittivity, due to the enhancement of the interfacial polarization and the presence of functionalized groups. It is found a value of 5.35 × 103 much greater than that of 3.7 measured for pure PLA. Given the good combined thermal and dielectric properties, such materials are promising candidates for packaging applications with electromagnetic (EM) shielding capability.

Self-heating temperature measurement in AlInN/GaN HEMTs by using CeO2 and TiO2 micro-Raman thermometers

Thermal characterization of GaN-based components is an important and challenging issue that requires the measurement of the self-heating temperature of the channel, the metal contact surface and the substrate with high accuracy. This paper points out that all these thermal parameters can be measured by combining confocal Raman spectroscopy and micro-Raman thermometers. To prove these assertions, thermal resistance and Thermal Boundary Resistance (TBR) were accurately estimated experimentally. In addition, temperature measurements with TiO2 and CeO2 micro-Raman thermometers were carried out to determine which is more accurate and reliable in measuring the surface self-heating temperature of these components.

Low Thermal Budget Growth of Near‐Isotropic Diamond Grains for Heat Spreading in Semiconductor Devices

AbstractThe ever‐increasing power density is a major trend for electronics applications from dense computing to 5G/6G networks. Joule heating and resulting high temperature in the device channel due to the increased power density results in performance degradation and premature failure. Diamond integration near the hot spot can spread the heat by increasing the heat transfer coefficient. Diamond is mostly grown at high temperatures (700–1000 °C), which limits its integration with many semiconductor technologies. Here, a high‐quality 400 °C‐diamond by modifying the gas chemistry at different nucleation stages, with a sharp sp3 Raman peak (FWHM≈6.5 cm−1) and high phase purity (97.1%), similar to 700 °C‐diamond (>98%) is demonstrated. An average grain size of 650 nm with a thickness of 790 nm corresponding to an anisotropy ratio of 1.21 at 400 °C close to the best‐reported of 1.12 at 700 °C is achieved. This near‐isotropic diamond exhibits a relatively high thermal conductivity of ≈300 W m−1 K−1 and a thermal boundary resistance as small as only 5 m2K G−1 W−1 (on SiO2 and Si3N4). Achieving such a high‐quality diamond at 400 °C demonstrates the possibility to grow the diamond on a wide range of semiconductors including Si, InP, Ga2O3, SiC, and GaN where SiO2 or its variations, and Si3N4 are commonly used dielectrics.

Integrating thin wall into block: A new scanning strategy for laser powder bed fusion of dense tungsten

• A novel scanning strategy to manufacture pure tungsten with less defects was developed in this study using laser powder bed fusion (LPBF). Dense pure tungsten samples from the novel strategy present a nice brick-stacking structure. • The report on thermal properties of pure tungsten processed by LPBF is rather limited. In this study, their thermal contributions, electrical properties and thermoelectric figure of merit (zT) were firstly reported. • Although the temperature dependence of samples fabricated by different strategies was similar, phonon contribution to the total thermal conductivity varied significantly, which could be interpreted by the higher grain boundary thermal resistance in the brick-stacking tungsten builds. By integrating thin-wall into block (ITWB), a novel scanning strategy to manufacture dense tungsten with few cracks was developed in this study using laser powder bed fusion (LPBF). Different from the S-shaped grains observed from the other LPBFed tungsten, samples fabricated by the ITWB strategy present a pore-free microstructure composed of stacked columnar grains. The thermal conductivities of tungsten parts manufactured by the different strategies were compared. Although their temperature dependence was similar, phonon contribution to the total thermal conductivity varied significantly, which could be primarily interpreted by the differences on the grain boundary thermal resistance.

Low Energy Switching of Phase Change Materials Using a 2D Thermal Boundary Layer.

The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon substrate and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ∼100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interface. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide; thus, this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices.

Theoretical and experimental study on the anisotropic thermal conductivity of composite phase change materials prepared by hot-pressing method

A convenient and efficient anisotropic thermal conductivity prediction model of composite phase change materials (CPCMs) prepared by hot-pressing method was proposed. This model quantitatively calculates the angular distribution of thermal conductivity particles in CPCMs. The predicted value of anisotropic thermal conductivity is in good agreement with the experimental value. The distribution of ellipsoidal thermal conductivity particles in the CPCMs tends to be horizontal with increasing compression ratio, which makes the anisotropy of the thermal conductivity of CPCMs obvious. By decreasing the particle thickness, increasing the particle length and reducing the boundary thermal resistance of the particles, the radial thermal conductivity of the CPCMs can be improved.

Study on self heating effect of enhancement-mode Ga<sub>2</sub>O<sub>3</sub> vertical MOSFET

<p indent="0mm">In this paper, a vertical enhanced Ga₂O₃ MOSFET is designed. Due to the low thermal conductivity of gallium oxide material, the output characteristics of the device under different crystal orientation materials, different thermal resistance and different temperature are studied. The thermal characteristics of Ga₂O₃ MOSFET are analyzed based on the temperature distribution characteristics of the device. The thermal conductivity model studies the effect of different orientation of gallium oxide on the output characteristics and temperature distribution of the device, and it is found that the thermal conductivity of [010] orientation is the best. When <italic>T</italic>=<sc>300 K,</sc> the thermal conductivity is about <sc>0.1 W/cm K,</sc> which is higher than that of other crystal directions, and the corresponding output saturation current is the largest under the same voltage (<italic>V</italic>_gs=<sc>3 V, </sc><italic>I</italic>_dsat&gt;<sc>400 A/cm²).</sc> Furthermore, the output characteristic curves of different crystal directions under different temperature and different thermal resistance are studied. With the decrease of thermal resistance, the heat dissipation capacity at the device boundary is improved. Thus, the influence of self-heating effect is suppressed and the drain saturation current increases. The normal operation of the device can be ensured when the boundary thermal resistance is <sc>0.01–0.005 cm² K/W.</sc> All of these provide reliable methods and reference value for optimizing device performance in the future.

The Impact of Interface Defects on Thermal Boundary Resistance at a Si|C Interface

Due to the increasing complexity and miniaturization of microelectronics, material interfaces are now the dominant source of thermal resistance in many integrated circuits and electronic devices. While the literature on thermal boundary resistance is extensive, the theory remains poorly understood. This is particularly true for the case of defects at the interface. Data and results on three defects at a Si|C interface are presented—an interstitial N pair and two different substitutional Ge's. It is found that the interstitial N pair, as well as one of the Ge defects, reduces thermal boundary conductivity considerably whereas the substitutional Ge placed directly at the interface is largely transparent.

The Thermal Conductance of Sapphire Ball Based Detector Clamps

In order to provide secure clamping with a low thermal conductance, some low temperature detectors are held with point contact sapphire ball clamps. While this method is increasingly common, the thermal conductance across this interface has not been well studied. We present a direct measurement of the thermal conductance of such clamps between 200 and 600~mK, with a clamping force of approximately 2.7~$\pm0.3$~N/clamp. The thermal conductance of a single sapphire-on-copper clamp was found to be 660$^{+360}_{-210}$~$(T/K)^{3.1}$~[nW/K]. For a sapphire-on-silicon clamp the conductance was found to be 380$^{+190}_{-120}$~$(T/K)^{2.8}$~[nW/K]. The conductance measured is consistent with thermal boundary resistance.

(Invited, Digital Presentation) Thermal Conductance across Heterogeneously Integrated Interfaces for Thermal Management of Wide and Ultra-Wide Bandgap Electronics

Thermal management is important for wide and ultra-wide bandgap power electronics because overheating degrades device reliability and performance. High thermal conductivity substrates such as SiC and diamond facilitate heat dissipation of these devices while the thermal boundary resistance between devices and substrates accounts for a large portion of the total thermal resistance, which prevents devices from taking the full advantage of the high thermal conductivity of the substrates. Recently, heterogeneously integrated wide and ultra-wide bandgap semiconductor interfaces are found to have low thermal boundary resistances, which provides a new degree of freedom to design and fabricate thermally conductive interfaces for thermal management of related power devices. For example, GaN can be bonded with single crystal SiC or single crystal diamond directly at room temperature. β-Ga2O3 can be bonded with SiC with or without Al2O3 interfacial layers. Compared with growth, the bonded interfaces are not limited by lattice mismatch. Moreover, the room-temperature bonding process discussed in this talk can possibly eliminate the effects of thermal stress existed in high temperature growth or bonding techniques. Recent progresses in this sub-area will be discussed in this talk, especially thermal conductance across surface-activated bonded GaN and β-Ga2O3 interfaces measured by time-domain thermoreflectance (TDTR) and the effects of thermal boundary resistance values on device temperatures. Finally, the potential challenges will also be pointed out, for instance, high-throughput thermal measurements of buried interfaces, thermal property-structure relations of interfaces bonded under different conditions, theoretical understanding of interfacial thermal transport, and device demonstrations.