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Using the atomic cluster expansion (ACE) framework, we develop a machine learning interatomic potential for fast and accurately modelling the phonon transport properties of wurtzite aluminum nitride. The predictive power of the ACE potential against density functional theory (DFT) is demonstrated across a broad range of properties of w-AlN, including ground-state lattice parameters, specific heat capacity, coefficients of thermal expansion, bulk modulus, and harmonic phonon dispersions. Validation of lattice thermal conductivity is further carried out by comparing the ACE-predicted values to the DFT calculations and experiments, exhibiting the overall capability of our ACE potential in sufficiently describing anharmonic phonon interactions. As a practical application, we perform a lattice dynamics analysis using the potential to unravel the effects of biaxial strains on thermal conductivity and phonon properties of w-AlN, which is identified as a significant tuning factor for near-junction thermal design of w-AlN-based electronics.

Anisotropic thermal transport plays a key role in both theoretical study and engineering practice of heat transfer, but accurately measuring anisotropic thermal conductivity remains a significant challenge. To address this issue, we propose the three-sensor 2{\omega} method in this study, which is capable of accurately measuring the isotropic or anisotropic thermal conductivity of solid materials. In this method, several three-sensor groups following the design guidelines are fabricated upon the sample along different characteristic directions, and each group consists of three parallel metal sensors with unequal widths and distances optimally designed based on sensitivity analysis. Among the three sensors, the outer two serve as AC heaters and the middle one as a DC detector. The 2{\omega} voltage signals across the detector in each three-sensor group are measured, and then the data are processed by the proposed Intersection Method to derive the thermal conductivities along directions of interest. The application of the detector's 2{\omega} instead of the heater's 3{\omega} voltage signals eliminates the errors introduced by the uncertainties of thermal resistance in superficial structures (metal layer, insulation layer, interface, etc.). Meanwhile, by replacing the fitting algorithm with the Intersection Method, the local optimum trap of multivariate fitting is avoided. To verify the accuracy and reliability, four typical monocrystalline semiconductors, i.e., Si, GaN, AlN, and {\beta -Ga _2 O _3}, are measured, and the results are consistent with the literature. This method will provide a comprehensive and versatile solution for the thermal conductivity measurements of solid materials., Comment: 25 pages, 6 figures

We present a comprehensive investigation of self-heating in gallium nitride (GaN) high-electron-mobility transistors (HEMTs) through technology computer-aided design (TCAD) simulations and phonon Monte Carlo (MC) simulations. With microscopic phonon-based electrothermal simulations, we scrutinize both the temperature profiles and electrothermal coupling effect within GaN HEMTs. Two metrics, maximum channel temperature ($T_\text{max}$) and equivalent channel temperature ($T_\text{eq}$), are introduced to measure the reliability and electrical performance degradation of the device, respectively. The influence of bias-dependent heat generation and phonon ballistic transport on the two indicators is thoroughly examined.

To develop effective thermal management strategies for GaN transistors, it is essential to accurately predict the device junction temperature. Since the width of the heat generation in the devices is comparable to phonon mean free paths of GaN, phonon ballistic transport exists and can significantly affect the heat transport process, which necessitates a thorough understanding of the influence of the phonon ballistic effects in GaN transistors. In this paper, the ballistic-diffusive phonon transport in GaN-on-SiC devices is examined by measuring the hotspot temperature using the thermoreflectance thermal imaging TTI combined with the hybrid phonon Monte Carlo-diffusion simulations. A series of Au heaters are fabricated on the top of the GaN layer to quantitatively mimic the different heat source distributions during device operation. The experimental and simulation results show a good consistency and both indicate that the phonon ballistic effects can significantly increase the hotspot temperature. With the size of the heat source decreasing, the errors of Fourier's law-based predictions increase, which emphasizes the necessity to carefully consider the phonon ballistic transport in device thermal simulations.

Topological semimetal states which are constrained by symmetries and give birth to innovative excitations are the frontiers of topological quantum matter. Nodal chains in which two nodal rings connect at one point were first discovered in non-symmorphic electronic systems and then generalized to symmorphic phononic systems. In this work, we identify a new class of planar nodal chains in non-symmorphic phononic systems, where the connecting rings lie in the same plane. The constituting nodal rings are protected by mirror symmetry, their intersection is guaranteed by the combination of time-reversal and non-symmorphic two-fold screw symmetry. In addition, the connecting points are four-fold degenerate while those in previous works are two-fold degenerate. We searched all 230 space groups and found 8 space groups that can host the proposed planar nodal chain phonons. Taking wurtzite GaN (space group No.186) as an example, the planar nodal chain is confirmed by first-principles calculations. The planar nodal chains result in two distinct classes of drumhead surface. The first category lies on the [10(-1)0] surface Brillouin zone and the second lies on the [0001] surface Brillouin zone. Our finding reveals a class of planar nodal chains in non-symmorphic phononic systems, expands the catalog of topological nodal chains, and enriches the family of topological surface states.

Solid heterostructures composed of substrates and epitaxial films are extensively used in advanced technologies, and their thermophysical properties fundamentally determine the performance, efficiency, reliability, and lifetime 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 3{\omega}-2{\omega} 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 {\mu}m. In this method, three parallel metal sensors with unequal width and spacing are fabricated on the sample surface, in which the two outer sensors are used as heaters, and the middle sensor is used as a detector. The respective 3{\omega} signals of the two heaters and 2{\omega} 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 {\mu}m 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 reported values in the literature. The proposed method will provide a comprehensive solution for the thermophysical property measurements of various solid heterostructures.

Exact assessment of thermal spreading resistance is of great importance to the thermal management of electronic devices, especially when completely considering the heat conduction process from the nanoscale heat source to the macroscopic scale heat sink. The existing simulation methods are either based on convectional Fourier's law or limited to small system sizes, making it difficult to accurately and efficiently study the cross-scale heat transfer. In this paper, a hybrid phonon Monte Carlo-diffusion method that couples phonon Monte Carlo (MC) method with Fourier's law by dividing the computational domain is adopted to analyze thermal spreading resistance in ballistic-diffusive regime. Compared with phonon MC simulation, the junction temperature of the hybrid method has the same precision, while the time costs could be reduced up to 2 orders of magnitude at most. Furthermore, the simulation results indicate that the heating scheme has a remarkable impact on phonon transport. The thermal resistance of the heat source (HS) scheme can be larger than that of the heat flux (HF) scheme, which is opposite from the prediction of Fourier's law. In the HS scheme, the enhanced phonon-boundary scattering counteracts the broadening of the heat source, leading to a stronger ballistic effect as the heat source thickness decreases. The conclusion is verified by a one-dimensional thermal resistance model. This work has opened up an opportunity for the fast and extensive thermal modeling of cross-scale heat transfer in electronic devices and highlighted the influence of heating schemes.

To develop efficient thermal management strategies for wide bandgap (WBG) semiconductor devices, it is essential to have a clear understanding of the heat transport process within the device and accurately predict the junction temperature. In this paper, we used the phonon Monte Carlo (MC) method with the phonon dispersion of various typical WBG semiconductors, including GaN, SiC, AlN, and \ce{\beta-Ga_2O_3}, to investigate the thermal spreading resistance in a ballistic-diffusive regime. It was found that when compared with Fourier's law-based predictions, the increase in the thermal resistance caused by ballistic effects was strongly related to different phonon dispersions. Based on the model deduced under the gray-medium approximation and the results of dispersion MC, we obtained a thermal resistance model that can well address the issues of thermal spreading and ballistic effects, and the influences of phonon dispersion. The model can be easily coupled with FEM based thermal analysis and applied to different materials. This paper can provide a clearer understanding of the influences of phonon dispersion on the thermal transport process, and it can be useful for the prediction of junction temperatures and the development of thermal management strategies for WBG semiconductor devices.

Electric fields commonly exist in semiconductor structures of electronics, bringing to bear on phonon thermal transport. Also, it is a popular method to tune thermal transport in solids. In this work, phonon and thermal transport properties of GaN with wurtzite and zincblende structures at finite electric field are investigated using first principles calculations from perspectives of symmetry breaking and lattice deformation. Effects of electric field on phonon transport properties including phonon dispersion and thermal conductivity from response of electron density distribution only and response from lattice changes are studied in zincblende GaN. It is found that the former has a small but qualitative impact on phonon dispersion relations, i.e., splitting of phonon branches, since it breaks symmetry of zincblende lattice. While the latter affects both lattice symmetry and size, causing significant changes of phonon properties and increase of thermal conductivity. In wurtzite GaN, space-group-conserved lattice changes at finite electric field are studied with lattice deformation only, where thermal conductivity decreases at electric fields significantly with increase of anisotropy, much different from the changes in zincblende GaN. This work provides a comprehensive understanding on phonon thermal transport properties in GaN at finite electric field, which promises to benefit phonon transport tuning and provide reference for thermal management in GaN-based information and power electronics., Comment: 19 pages, 12 figures. International Journal of Heat and Mass Transfer (submitted 2021/01/24, accepted 2021/06/28)

Tuning thermal transport in semiconductor nanostructures is of great significance for thermal management in information and power electronics. With excellent transport properties, such as ballistic transport, immunity to point defects and disorders, and forbidden backscattering, topological phonon surface states show remarkable potential in addressing this issue. Herein, topological phonon analyses are performed on hexagonal wurtzite GaN to check the topological characteristics of phonons. And other nitrides of the same family, i.e., AlN and AlGaN alloy, are also calculated from a topological phonon phase transition perspective. With the aid of first-principle calculations and topological phonon theory, Weyl phonon states, which host surfaces states without backscattering, are investigated for all these materials. The results show that there is no nontrivial topological phonon state in GaN. However, by introducing Al atoms, i.e., in wurtzite type AlN and AlGaN, more than one Weyl phonon point is found, confirmed by obvious topological characteristics, including non-zero integer topological charges, source/sink in Berry curvature distributions, surface local density of states and surface arcs. As AlN and AlGaN are typical materials in AlGaN/GaN heterostructure based electronics, the existence of topological phonon states in them will benefit thermal management by facilitating the design of one-way interfacial phonon transport without backscattering.

Heat conduction is investigated on three levels: equilibrium, Fourier, and Cattaneo. The Fourier level is either the point of departure for investigating the approach to equilibrium or the final stage in the investigation of the approach from the Cattaneo level. Both investigations bring to the Fourier level an entropy and a thermodynamics. In the absence of external and internal influences preventing the approach to equilibrium the entropy that arises in the latter investigation is the production of the classical entropy that arises in the former investigation. If the approach to equilibrium is prevented, then the entropy that arises in the investigation of the approach from the Cattaneo level to the Fourier level still brings to the Fourier level the entropy and the thermodynamics even if the classical entropy and the classical thermodynamics is absent. We also note that vanishing total entropy production as a characterization of equilibrium state is insufficient., Comment: Submitted to the Journal of Non-equilibrium Thermodynamics

An extended social force model with a dynamic navigation field is proposed to study bidirectional pedestrian movement. The dynamic navigation field is introduced to describe the desired direction of pedestrian motion resulting from the decision-making processes of pedestrians. The macroscopic fundamental diagrams obtained using the extended model are validated against camera-based observations. Numerical results show that this extended model can reproduce collective phenomena in pedestrian traffic, such as dynamic multilane flow and stable separate-lane flow. Pedestrians' path choice behavior significantly affects the probability of congestion and the number of self-organized lanes.