Ultimate Confinement of Phonon Propagation in Silicon Nanocrystalline Structure

Machine-Learning-Based Interatomic Potentials for Group IIB to VIA Semiconductors: Toward a Universal Model

Rapid advancements in machine-learning methods have led to the emergence of machine-learning-based interatomic potentials as a new cutting-edge tool for simulating large systems with ab initio accuracy. Still, the community awaits universal interatomic models that can be applied to a wide range of materials without tuning neural network parameters. We develop a unified deep-learning interatomic potential (the DPA-Semi model) for 19 semiconductors ranging from group IIB to VIA, including Si, Ge, SiC, BAs, BN, AlN, AlP, AlAs, InP, InAs, InSb, GaN, GaP, GaAs, CdTe, InTe, CdSe, ZnS, and CdS. In addition, independent deep potential models for each semiconductor are prepared for detailed comparison. The training data are obtained by performing density functional theory calculations with numerical atomic orbitals basis sets to reduce the computational costs. We systematically compare various properties of the solid and liquid phases of semiconductors between different machine-learning models. We conclude that the DPA-Semi model achieves GGA exchange-correlation functional quality accuracy and can be regarded as a pretrained model toward a universal model to study group IIB to VIA semiconductors.

Interface thermal resistance induced by geometric shape mismatch: A multiparticle Lorentz gas model

Poor heat dissipation caused by interface thermal resistance (ITR, or Kapitza resistance) has long been the bottleneck that limits the further miniaturization of integrated circuit. In this paper, different from previous studies on ITR induced by conjunction of two different materials, the ITR of a homogeneous stepped system is studied through the multiparticle Lorentz gas model. It is found that ITR can be triggered by pure geometric shape mismatch, and decreases when the degree of mismatch decreases. The ITRs for forward and backward transport are asymmetrical; thus, thermal rectification effect is also obtained in this system. Moreover, the effects of absolute width, width ratio, mean temperature, and temperature difference on ITR and thermal rectification effect are discussed. The ITR induced by geometric shape mismatch provides physics for interfacial thermal transport.

Anomalous hybridization complementation effect on phonon transport in heterogeneous nanowire cross junction

Controlling phonon transport via its wave nature in nanostructures can achieve unique properties for various applications. In this paper, thermal conductivity of heterogeneous nano cross junction (hetero-NCJ) is studied through molecular dynamics simulation. It is found that decreasing or increasing the atomic mass of four side wires (SWs) severed as resonators, thermal conductivity of hetero-NCJ is enhanced, which is larger than that of homogeneous NCJ (homo-NCJ). Interestingly, by setting two SWs with larger atomic mass and other two SWs with smaller atomic mass, thermal conductivity of hetero-NCJ is abnormally decreased, which is even smaller than that of homo-NCJ. After further non-equilibrium Green's function calculations, it is demonstrated that origin of increase is attributed to the hybridization broken induced by unidirectional shift of resonant modes. However, the decrease in thermal conductivity originates from hybridization complementation induced by bidirectional shift of resonant modes, which synergistically blocks phonon transport. This work provides a mechanism for further strengthening resonant hybridization effect and manipulating thermal transport.

Weaker bonding can give larger thermal conductance at highly mismatched interfaces

Thermal boundary conductance is typically positively correlated with interfacial adhesion at the interface. Here, we demonstrate a counterintuitive experimental result in which a weak van der Waals interface can give a higher thermal boundary conductance than a strong covalently bonded interface. This occurs in a system with highly mismatched vibrational frequencies (copper/diamond) modified by a self-assembled monolayer. Using finely controlled fabrication and detailed characterization, complemented by molecular simulation, the effects of bridging the vibrational spectrum mismatch and bonding at the interface are systematically varied and understood from a molecular dynamics viewpoint. The results reveal that the bridging and binding effects have a trade-off relationship and, consequently, that the bridging can overwhelm the binding effect at a highly mismatched interface. This study provides a comprehensive understanding of phonon transport at interfaces, unifying physical and chemical understandings, and allowing interfacial tailoring of the thermal transport in various material systems.

Phonon transport in the nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity

Phonon transport in the nano-system has been studied using well-designed nanostructured materials to observe and control the interesting phonon behaviors like ballistic phonon transport. Recently, we observed drastic thermal conductivity reduction in the films containing well-controlled nanodots. Here, we investigate whether this comes from the interference effect in ballistic phonon transport by comparing the thermal properties of the Si or Si0.75Ge0.25 films containing Ge nanodots. The experimentally-obtained thermal resistance of the nanodot layer shows peculiar nanodot size dependence in the Si films and a constant value in the SiGe films. From the phonon simulation results, interestingly, it is clearly found that in the nanostructured Si film, phonons travel in a non-diffusive way (ballistic phonon transport). On the other hand, in the nanostructured SiGe film, although simple diffusive phonon transport occurs, extremely-low thermal conductivity (∼0.81 W m-1 K-1) close to that of amorphous Si0.7Ge0.3 (∼0.7 W m-1 K-1) is achieved due to the combination of the alloy phonon scattering and Ge nanodot scattering.

High Thermoelectric Power Factor Realization in Si-Rich SiGe/Si Superlattices by Super-Controlled Interfaces

A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained, not to give strain to the above Si layers. Therein, a drastic thermal conductivity reduction occurs by larger phonon scattering at the interfaces with the large atomic size difference between Si layers and Ge segregation layers, while unstrained Si layers preserve a high conduction band degeneracy leading to a high Seebeck coefficient. As a result, the n-type Si_0.7Ge_0.3/Si SL with controlled interfaces achieves a higher power factor of ∼25 μW cm^-1 K^-2 in the in-plane direction at room temperature, which is superior to ever reported SiGe-based films: SiGe-based SLs and SiGe films. The Si_0.7Ge_0.3/Si SL with controlled interfaces also exhibits a low thermal conductivity of ∼2.5 W m^-1 K^-1 in the cross-plane direction, which is ∼5 times lower than the reported value in a conventional Si_0.7Ge_0.3/Si SL. These results demonstrate that strain and atomic differences controlled by ultrathin layers can bring a breakthrough for realizing high-performance light-element-based thermoelectric films.

Nanoscale heat transport through the hetero-interface of SrRuO3 thin films

A SrRuO₃ thin film has been widely used as a metal electrode in electronic devices based on transition metal oxides, and hence it is important to understand its thermal transport properties to minimize a thermal degradation problem during the device operation. Using the time-domain thermoreflectance measurement technique, we investigate the cross-plane thermal conductivity of the SrRuO₃ thin films with a thickness variation from 1 μm to 8 nm. We find that the thermal conductivity is reduced from about 6 W m^-1 K^-1 for the 1 μm thick film to about 1.2 W m^-1 K^-1 for the 8 nm thick film, and attribute this behavior to the boundary scattering of thermal carriers which originally have the mean free path of about 20 nm in a bulk state. Also, we observe a clear dip behavior of the thermal conductivity in the intermediate thickness around 30 nm which suggests an existence of a strong scattering source other than the film boundary. We explain this result by considering an additional interfacial scattering at the tetragonal-orthorhombic phase boundary which is formed during the strain relaxation with an increase of the film thickness.

Phonon-Grain-Boundary-Interaction-Mediated Thermal Transport in Two-Dimensional Polycrystalline MoS2

Although dislocations and grain boundaries (GBs) are ubiquitous in large-scale MoS₂ samples, their interaction with phonons, which plays an important role in determining the lattice thermal conductivity of polycrystalline MoS₂, remains elusive. Here, we perform a systematic study of the heat transport in two-dimensional polycrystalline MoS₂ by both molecular dynamics simulation and atomic Green's function method. Our results indicate that the thermal boundary conductance of GBs of MoS₂ is in the range from 6.4 × 10⁸ to 35.3 × 10⁸ W m^-2 K^-1, which is closely correlated with the overlap between the vibrational density of states of GBs and those of the pristine lattice, as well as the GB energy. It is found that the GBs strongly scatter the phonons with frequency larger than 2 THz, accompanied by a pronounced phonon localization effect and significantly reduced phonon group velocities. Furthermore, by comparing the results from realistic polycrystalline MoS₂ to those from different theoretical models, we observe that the Casimir model is broken down and detailed phonon dynamics at a specific GB should be taken into account to accurately describe the phonon transport in polycrystalline materials. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-GB interaction.

Semiconductor Nanostructure Design for Thermoelectric Property Control

We present the methodologies for developing high-performance thermoelectric materials using nanostructured interfaces by reviewing our three studies and giving the new aspect of nanostructuring results. (1) Connected Si nanocrystals exhibited ultrasmall thermal conductivity. The drastic thermal conductivity reduction was brought by phonon confinement and phonon scattering. Here, we present discussion about the new aspect for phonon transport: not only nanocrystal size but also shape can contribute to thermal conductivity reduction. (2) Si films including Ge nanocrystals demonstrated that phonon and carrier conductions were independently controlled in the films, where carriers were easily transported through the interfaces between Si and Ge, while phonons could be effectively scattered at the interfaces. (3) Embedded-ZnO nanowire structure demonstrated the simultaneous realization of power factor increase and thermal conductivity reduction. The [Formula: see text] increase was caused by the interface-dominated carrier transport. The nanowire interfaces also worked as phonon scatterers, resulting in the thermal conductivity reduction.

Tuning phonon transport spectrum for better thermoelectric materials

The figure of merit of thermoelectric materials can be increased by suppressing the lattice thermal conductivity without degrading electrical properties. Phonons are the carriers for lattice thermal conduction, and their transport can be impeded by nanostructuring, owing to the recent progress in nanotechnology. The key question for further improvement of thermoelectric materials is how to realize ultimate structure with minimum lattice thermal conductivity. From spectral viewpoint, this means to impede transport of phonons in the entire spectral domain with noticeable contribution to lattice thermal conductivity that ranges in general from subterahertz to tens of terahertz in frequency. To this end, it is essential to know how the phonon transport varies with the length scale, morphology, and composition of nanostructures, and how effects of different nanostructures can be mutually adopted in view of the spectral domain. Here we review recent advances in analyzing such spectral impedance of phonon transport on the basis of various effects including alloy scattering, boundary scattering, and particle resonance.