Giant Enhancement of Hole Mobility for 4H-Silicon Carbide through Suppressing Interband Electron-Phonon Scattering.

4H-silicon carbide (4H-SiC) possesses a high Baliga figure of merit, making it a promising material for power electronics. However, its applications are limited by low hole mobility. Herein, we found that the hole mobility of 4H-SiC is mainly limited by the strong interband electron-phonon scattering using mode-level first-principles calculations. Our research indicates that applying compressive strain can reverse the sign of crystal-field splitting and change the ordering of electron bands close to the valence band maximum. Therefore, the interband electron-phonon scattering is severely suppressed and the electron group velocity is significantly increased. The out-of-plane hole mobility of 4H-SiC can be greatly enhanced by ∼200% with 2% uniaxial compressive strain applied. This work provides new insights into the electron transport mechanisms in semiconductors and suggests a strategy to improve hole mobility that could be applied to other semiconductors with hexagonal crystalline geometries.

One Stone Two Birds: Anomalously Enhancing the Cross-Plane and In-Plane Heat Transfer in 2D/3D Heterostructures by Defects Engineering.

This study addresses a crucial challenge in two-dimensional (2D) material-based electronic devices-inefficient heat dissipation across the van der Waals (vdW) interface connecting the 2D material to its three-dimensional (3D) substrate. The objective is to enhance the interfacial thermal conductance (ITC) of 2D/3D heterostructures without compromising the intrinsic thermal conductivities (κ) of 2D materials. Using 2D-MoS2/3D-GaN as an example, a novel strategy to enhance both the ITC across 2D/3D interface and κ of 2D material is proposed by introducing a controlled concentration (ρ) of vacancy defects to substrate's bottom surface. Molecular dynamics simulations demonstrate a notable 2.1-fold higher ITC of MoS2/GaN at ρ = 4% compared to the no-defective counterpart, along with an impressive 56% enhancement in κ of MoS2 compared to the conventional upper surface modification approaches. Phonon dynamics analysis attributes the ITC enhancement to increased phonon coupling between MoS2 and GaN, resulting from polarization conversion and hybridization of phonons at the defective surface. Spectral energy density analysis affirms that the improved κ of MoS2 directly results from the proposed strategy, effectively reducing phonon scattering at the interface. This work provides an effective approach for enhancing heat transfer in 2D/3D vdW heterostructures, promisingly advancing electronics' heat dissipation.

Phonon dynamic behaviors induced by amorphous layers at heterointerfaces.

An amorphous layer is commonly found at the interfaces of heterostructures due to lattice and thermal mismatch between dissimilar materials. While existing research has explored the impact of these layers on interfacial thermal transport, a comprehensive understanding of the underlying microscopic mechanisms remains essential for advancing thermal nanodevice development. Through phonon wave packet simulations, we investigated the dynamic behaviors of phonons crossing the amorphous interlayer at the GaN/AlN interface from the mode level. Our results highlight the amorphous layer's capability to notably adjust the polarization properties of incoming phonons, culminating in phonon localization. By examining transmission outcomes on a per-mode basis, we demonstrate the amorphous layer's impediment on phonon transport. Notably, this resistance escalates with an increase in the amorphous layer thickness (L), with certain high-frequency TA phonons showing unexpectedly high transmissivity due to polarization conversion and inelastic scattering at the amorphous interface. In addition, we observe that the amorphous layer prompts multiple reflections of incident phonons, instigating discernible from the two-beam interference equation. Finally, in pursuit of enhanced phonon transport, we employ annealing techniques to optimize the interface morphology, leading to the recrystallization of the amorphous layer. This optimization yields a substantial enhancement of interfacial thermal conductance by up to 38% for L = 3 nm.

Atomic-Scale Surface Engineering for Giant Thermal Transport Enhancement Across 2D/3D van der Waals Interfaces.

Heat dissipation in two-dimensional (2D) material-based electronic devices is a critical issue for their applications. The bottleneck for this thermal issue is inefficient for heat removal across the van der Waals (vdW) interface between the 2D material and its supporting three-dimensional (3D) substrate. In this work, we demonstrate that an atomic-scale thin amorphous layer atop the substrate surface can remarkably enhance the interfacial thermal conductance (ITC) of the 2D-MoS2/3D-GaN vdW interface by a factor of 4 compared to that of the untreated crystalline substrate surface. Meanwhile, the ITC can be broadly manipulated through adjusting substrate surface roughness. Phonon dynamic and heat flux spectrum analyses show that this giant enhancement is attributed to the increased phonon densities and channels at the interfaces and enhanced phonon coupling. The slight surface fluctuation in MoS2 and the increased diffuse interfacial scattering facilitate energy transfer from MoS2's in-plane phonons to its out-of-plane phonons and then to the substrate. In addition, it is further found that the substrate and its surface topology can dramatically influence the thermal conductivity of MoS2 due to the reduction of phonon relaxation time, especially for low-frequency acoustic phonons. This study elucidates the effects of the amorphous surface of the substrate on thermal transport across 2D/3D vdW interfaces and provides a new dimension to aid in the heat dissipation of 2D-based electronic devices via atomic-scale surface engineering.

Anomalously Isotropic Electron Transport and Weak Electron-Phonon Interactions in Hexagonal Noble Metals.

The electrical transport properties of typical hexagonal metals are anisotropic because of their anisotropic lattice structures. Unexpectedly, we show that the electron transport properties in hexagonal close-packed (hcp) noble metals are almost isotropic. Although the electron transport properties of an individual electronic band are highly anisotropic, the total contributions are almost equal in different crystalline orientations because of the complementary contributions of different bands. The electron transport is severely limited by phonons for metals with multisheet Fermi surfaces and optical phonon polarizations. However, it is found the electron-phonon interactions are weak in hcp noble metals, although their Fermi surfaces and phonon spectra are complicated. The electronic structure acts as a phonon filter, resulting in small electron-phonon scattering rates. The weak electron-phonon interactions are beneficial to electron and thermal transport, indicating hcp noble metals have great potential to be used in electronics and solar cells.

Thermal transport by electrons and phonons in PdTe2: an ab initio study.

Palladium ditelluride (PdTe2) is expected to have great promise in electronics and quantum computing due to its exotic type-II Dirac fermions. Although the electronic structure and electrical transport properties of PdTe2 have been comprehensively investigated, its thermal transport properties have not been well understood yet. In this work, we study the lattice and electronic thermal conductivity of PdTe2 using mode-level ab initio calculations. We find its thermal conductivity is ∼35 W m-1 K-1 on the a-axis at room temperature, mainly attributed to the strong lattice anharmonicity and electron-phonon interactions. The lattice thermal conductivity is smaller than 2 W m-1 K-1 and it only contributes a small ratio of ∼5% to the total thermal conductivity. The electronic thermal conductivity is relatively small compared to common metals mainly due to the strong electron-phonon scattering. The Lorenz ratio has a large deviation from the Sommerfeld value below 200 K. In addition, the mean free path of the phonons is about five times larger than that of the electrons. Our results provide a thorough understanding of the thermal transport in PdTe2 and can be helpful in the design of PdTe2-based devices.