The thermal conductivities of β-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80–495 K using the time domain thermoreflectance method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0 ± 2.0 W/mK, while that along the [100] direction has the lowest value of 10.9 ± 1.0 W/mK. At high temperatures, the thermal conductivity follows a ∼1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the β-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations, which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

Anisotropic thermal conductivity in single crystal β-gallium oxide

Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.

Charge-Induced Disorder Controls the Thermal Conductivity of Entropy-Stabilized Oxides

Heat transfer can differ distinctly at the nanoscale from that at the macroscale. Recent advancement in computational and experimental techniques has enabled a large number of interesting observations and understanding of heat transfer processes at the nanoscale. In this review, we will first discuss recent advances in computational and experimental methods used in nanoscale thermal transport studies, followed by reviews of novel thermal transport phenomena at the nanoscale observed in both computational and experimental studies, and discussion on current understanding of these novel phenomena. Our perspectives on challenges and opportunities on computational and experimental methods are also presented.

pdf/nanoscale-heat-transfer-from-computation-to-experiment-xduvmb8rlr.pdf

Nanoscale heat transfer – from computation to experiment

Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.

/pdf/measuring-phonon-mean-free-path-distributions-by-probing-4zc5821v09.pdf

Measuring Phonon Mean Free Path Distributions by Probing Quasiballistic Phonon Transport in Grating Nanostructures

The development of GaN-on-diamond devices holds much promise for the creation of high-power density electronics. Inherent to the growth of these devices, a dielectric layer is placed between the GaN and diamond, which can contribute significantly to the overall thermal resistance of the structure. In this work, we explore the role of different interfaces in contributing to the thermal resistance of the interface of GaN/diamond layers, specifically using 5 nm layers of AlN, SiN, or no interlayer at all. Using time-domain thermoreflectance along with electron energy loss spectroscopy, we were able to determine that a SiN interfacial layer provided the lowest thermal boundary resistance (<10 m2K/GW) because of the formation of an Si–C–N layer at the interface. The AlN and no interlayer samples were observed to have TBRs greater than 20 m2K/GW as a result of a harsh growth environment that roughened the interface (enhancing phonon scattering) when the GaN was not properly protected.

Low Thermal Boundary Resistance Interfaces for GaN-on-Diamond Devices

Semiconductors with high thermal conductivity and electron-hole mobility are of great importance for electronic and photonic devices as well as for fundamental studies. Among the ultrahigh–thermal conductivity materials, cubic boron arsenide (c-BAs) is predicted to exhibit simultaneously high electron and hole mobilities of >1000 centimeters squared per volt per second. Using the optical transient grating technique, we experimentally measured thermal conductivity of 1200 watts per meter per kelvin and ambipolar mobility of 1600 centimeters squared per volt per second at the same locations on c-BAs samples at room temperature despite spatial variations. Ab initio calculations show that lowering ionized and neutral impurity concentrations is key to achieving high mobility and high thermal conductivity, respectively. The high ambipolar mobilities combined with the ultrahigh thermal conductivity make c-BAs a promising candidate for next-generation electronics. Description Swift carriers Boron arsenide is a semiconductor with several interesting properties, including a high thermal conductivity. Theoretical calculations also suggest that it has high ambipolar mobility, a measure of the mobility of electrons and holes. Yue et al. and Shin et al. used different types of measurements to observe a high ambipolar mobility in very pure cubic boron arsenide. Shin et al. were able to simultaneously measure the high thermal and electrical transport properties in the same place in their samples. Yue et al. found even higher ambipolar mobility than the theoretical estimates at a few locations. Boron arsenide’s combination of transport properties could make it an attractive semiconductor for various applications. —BG Boron arsenide is a semiconductor with high thermal conductivity and electron-hole mobility.

High ambipolar mobility in cubic boron arsenide

The thermal properties of thin films and material interfaces play an important role in many technologies such as microelectronics and solid-state energy conversion. This thesis examines the characterization of thermal transport over length scales from nanometers upward in solids and liquids using pump-probe optical techniques. The design and implementation of a system that is uniquely suited to thermal measurements is described in detail, and the theory for interpreting the measurement results is developed using a linear systems approach, accounting for pulse accumulation effects and anisotropic thermal properties. Cross-plane thin film and bulk thermal conductivity measurements spanning more than two orders of magnitude are presented, along with a discussion on the importance of sample preparation on thermal boundary conductance and thermal conductivity measurements. An approach for measurement of in-plane thermal properties of anisotropic thin films is presented, and as a demonstration the method is used to extract both the cross-plane and in-plane thermal conductivity of highly ordered pyrolytic graphite. In addition, the measurement techniques are extended to liquids, providing a way to measure liquid volumes less than 500 nm thick as well as solid-liquid thermal interface conductances. Finally, optical techniques for the study of nanofluids are discussed. Transmission pump-probe measurements on gold nanorods in suspension are presented. The heat transfer dynamics of the rods are measured, and the critical role played by surfactants in the thermal transport between the rod and the surrounding fluid is studied systematically. An optical transient grating technique is used to explore the bulk thermal and viscous properties of nanofluids. Thermal conductivity measurements show that the observed thermal conductivity enhancement of nanofluids is repeatable and is not a function of the measurement technique, while acoustic attenuation measurements indicate that the nanoparticles do not form clusters in solution. Thesis Supervisor: Gang Chen Title: Warren and Towneley Rohsenow Professor

Optical characterization of thermal transport from the nanoscale to the macroscale

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High ambipolar mobility in cubic boron arsenide

Optical characterization of thermal transport from the nanoscale to the macroscale