arXiv subjects

Tianli Feng

Publications and source records attributed to Tianli Feng.

At least 19 recordsLinked to original sources

arXiv2026

Significant impact of Al1-xGaxN interlayer on GaN/AlN thermal boundary conductance

AlN-GaN heterostructures are central to high-power and high-frequency electronics, including RF devices, power converters, and AI accelerators. An intermediate Al1-xGaxN (AlGaN) layer is often present, either unintentionally during growth or intentionally to induce a 2D electron gas, yet its impact on the interfacial thermal boundary conductance (TBC) remains unknown due to the lack of reliable measurement or modeling methods. Here, we report a first principles-based evaluation of the TBCs of AlN-AlGaN, AlGaN-GaN, and AlN-AlGaN-GaN interfaces over the full alloy range. This is realized by the development of accurate deep learning interatomic potentials based on first-principles simulations. Contrary to other material systems where mixed interlayers enhance thermal coupling, we find that an AlGaN interlayer markedly degrades TBC between GaN and AlN, explaining the observation in experiments. Finally, we show that if the Al composition is sigmoidally transitioned from 0 to 1 across the AlN-GaN interface, it can remarkably increase the TBC, compared to an abrupt or a linear transition. This work is expected to shed light on an accurate thermal analysis and electro-thermal co-design of future AlGaN-based devices.

cond-mat.mtrl-sci↗

arXiv2025

Bridging Finite Element and Molecular Dynamics for Non-Fourier Thermal Transport Near Nanoscale Hot Spot

Nanoscale hot spots forming tens of nanometers beneath the gate in advanced FinFET and HEMT devices drive heat transport into a non-Fourier regime, challenging conventional (Fourier-based) finite-element (FEM) analyses and complicating future thermal-aware chip design. Molecular dynamics (MD) naturally captures ballistic transport and phonon nonequilibrium, but has not been applied to hot-spot problems due to computational cost. Here, we perform the first MD simulations of hot-spot heat transfer across ballistic-diffusive regimes and benchmark them against FEM. We find that FEM using bulk thermal conductivity $\kappa_0$ significantly underestimates hot-spot temperature, even when the channel thickness is ~10 times the phonon mean free path, indicating persistent non-Fourier effects. We introduce a size-dependent "best" conductivity, $\kappa_{\mathrm{best}}$, using which FEM can reproduce MD hot-spot temperatures with high fidelity. We further decompose the MD-extracted thermal resistance into: (i) diffusive spreading, (ii) cross-plane ballistic, (iii) heat-carrier selective heating, and (iv) residual 3D ballistic-spreading resistances, and quantify each contribution. The resulting framework offers a practical route to embed non-Fourier physics into FEM for hot-spot prediction, reliability assessment, and thermally aware design of next-generation transistors.

cond-mat.mes-hall↗

arXiv2025

Beyond surfaces: quantifying internal radiative heat transport in dense materials

While phonons and electrons are well-established heat carriers in solids, photons are typically associated only with radiative transfer between surfaces. Yet for over 70 years, theorists have speculated that thermal photons could also conduct heat within dense, opaque materials -- an idea that has remained unproven and unquantified. Here, we resolve this longstanding question by developing a first-principles framework that reveals and quantifies the internal radiative contribution to thermal conductivity in solids. By analyzing 15 crystalline materials, we uncover photon mean free paths (MFPs) ranging from $\sim$100$\mu$m to over 1cm, with some materials exhibiting surprisingly large radiative thermal conductivity ($\kappa_{\text{rad}}$). Contrary to common assumptions, we show that $\kappa_{\text{rad}}$ can scale steeply with temperature (from $T^{1}$ to $T^{4}$), even as MFPs decrease (from $T^{-0.3}$ to $T^{-3}$). We also discover a robust link between photon MFP and phonon linewidths, revealing an unexpected interplay between radiative and phononic heat transport. Crucially, we establish a general formalism to calculate $\kappa_{\text{rad}}$ across arbitrary sample thicknesses and surface emissivities -- bridging ballistic and diffusive regimes. Our findings overturn long-held assumptions, uncover a missing channel of heat conduction, and provide a powerful new tool for thermal management in extreme environments.

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arXiv2025

Thermal boundary conductance in standalone and non-standalone GaN/AlN heterostructures predicted using machine learning interatomic potentials

GaN/AlN interfaces are essential in advanced high-power and high-frequency electronic devices, where effective thermal management is crucial for optimal performance and reliability. This work investigates the thermal boundary conductance (TBC) of standalone and non-standalone GaN/AlN heterostructures using non-equilibrium molecular dynamics (NEMD) driven by accurate machine learning interatomic potentials trained from density-functional theory calculations. For the standalone interface, the TBC is found to be ~600 MW m-2 K-1 at room temperature after quantum correction, with possible ~60% elastic and ~40% inelastic contributions. The result revises previous NEMD predictions (400 - 2000 MW m-2 K-1) using empirical interatomic potentials. When a second GaN/AlN interface is brought close to the original interface, the TBC of the original interface can increase to 1000 MW m-2 K-1, and this value gradually decreases with increasing distance between the two interfaces. When multiple interfaces are introduced in proximity, the first interface's TBC can be further enhanced to above 1150 MW m-2 K-1. After comparing double interfaces, superlattices, and random multilayers, it is concluded that such enhancement of TBC is not caused by the emergence of coherent superlattice modes but rather by the ballistic transport of the original bulk phonon modes of each material. Additionally, a "critical separation distance" (l_cs) is defined as the threshold beyond which the two interfaces no longer influence each other and behave independently. l_cs is determined by the mean free path of the phonons filtered by the interfaces that transport in the middle layer between two interfaces. Our findings provide insights into the thermal transport mechanisms that may aid the design of future electronic devices.

cond-mat.mes-hall↗

arXiv2024

Thermal boundary conductance of metal diamond interfaces predicted by machine learning interatomic potentials

Thermal boundary conductance (TBC) across metal diamond interfaces plays a critical role in the thermal management of future diamond based ultrawide bandgap semiconductor devices. Molecular dynamics is a sophisticated method to predict TBC but is limited by the lack of reliable potential describing metal diamond interfaces. In this work, we report the development of machine learning interatomic potentials and the prediction of TBCs of several technologically promising metal diamond interfaces using nonequilibrium molecular dynamics. The predicted TBCs of Al, Zr, Mo, and Au-diamond interfaces are approximately 316, 88, 52, and 55 MW/m2K, respectively, after quantum corrections. The corresponding thermal boundary resistances are equivalent to 0.8 μm thick of Al, 1.4 μm Mo, 0.3 μm Zr, and 5.3 μm Au, respectively. We also find that the conventional simple models, such as the acoustic mismatch model and diffuse mismatch model, even including the full-band phonon dispersion from first principles, largely misestimate the TBC values because of their inability to include inelastic transmission as well as interfacial structural and bonding details. The quantum-corrected TBC values for the metal diamond interfaces correlate well with the quantum corrected phonon specific heat of metals, instead of diamond. Additionally, our comparative analysis of Debye temperature and elastic modulus in these systems reveals that the former parameter correlates more strongly with the TBC than the latter. These low TBC values need to be considered in future diamond based semiconductor devices.

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arXiv2024

Thermal boundary conductance and thermal conductivity strongly depend on nearby environment

At the nanoscale, the thermal boundary conductance (TBC) and thermal conductivity are not intrinsic properties of interfaces or materials but depend on the nearby environment. However, most studies focused on single interfaces or superlattices, and the thermal transport across heterostructures formed by multiple different materials is still mysterious. In this study, we demonstrate how much the TBC of an interface is affected by the existence of a second interface, as well as how much the thermal conductivity of a material is affected by the nearby materials. Using Si and Ge modeled by classical molecular dynamics simulations, the following phenomena are demonstrated. (1) The existence of a nearby interface can significantly change the TBC of the original interface. For example, by adding an interface after Si/Ge, the TBC can be increased from 400 to 700 MW/m2K. This is because the nearby interface serves as a filter of phonon modes, which selectively allows particular modes to pass through and affect the TBC of the original interfaces. This impact will disappear at the diffusive limit when the distance between interfaces is much longer than the phonon mean free path so that phonon modes recover equilibrium statistics before arriving at the second interface. (2) The thermal conductivity of a material can be significantly changed by the existence of neighboring materials. For example, a standalone 30-nm-thick Si's thermal conductivity can be increased from 50 to 280 W/mK, a more than 4-fold increase, beating the bulk thermal conductivity of Si, after being sandwiched between two Ge slabs. This is because the Ge slabs on the two sides serve as filters that only allow low-frequency phonons to transport heat in Si, which carry more heat than optical phonons. This work opens a new area of successive interface thermal transport and is expected to be important for nanoscale thermal characterization and thermal management of semiconductor devices.

cond-mat.mes-hall↗

arXiv2024

Understanding the flat thermal conductivity of La2Zr2O7 at ultrahigh temperatures

Many crystals, such as lanthanum zirconate (La2Zr2O7), exhibit a flat temperature dependence of thermal conductivity at elevated temperatures. This phenomenon has recently been attributed to the inter-band phonon tunneling (or diffuson) contribution using different formalisms. However, the contributions of finite-temperature corrections (e.g., higher-order phonon-scattering, phonon renormalization, and phonon scattering cross-section softening effects) remain unclear. In this work, we predict and compare the thermal conductivity of La2Zr2O7 using three distinct first-principles methods. The first method is Green-Kubo molecular dynamics (MD) based on temperature-dependent machine learning interatomic potentials (MLIPs) trained from ab initio MD simulations, which successfully predict the flat trend at ultra-high temperatures. The second method is the Peierls Boltzmann transport equation (BTE), within the phonon particle framework, using phonon lifetime that includes all the finite-temperature corrections. Four-phonon scattering is found large but is cancelled by the phonon scattering cross-section softening effect. As a result, BTE with temperature corrections does not reproduce the flat thermal conductivity. The third method is Wigner formalism, which includes both phonon particle and wave contributions, which successfully reproduce the flat thermal conductivity. Diffuson and phonon contribute about 67% and 27% of thermal conductivity at 1800 K, respectively. The radiation contribution to thermal conductivity is around 6%. The scaling laws of the phonon, diffuson, radiation, and total thermal conductivity are found to be ~T-0.97, ~T0.43, ~T2.01, and ~T-0.40, respectively. This work clarifies the thermal transport mechanisms in La2Zr2O7 at ultra-high temperatures from different aspects.

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arXiv2024

Impacts of Point Defects on Shallow Doping in Cubic Boron Arsenide: A First Principles Study

Cubic boron arsenide (BAs) stands out as a promising material for advanced electronics, thanks to its exceptional thermal conductivity and ambipolar mobility. However, effective control of p- and n-type doping in BAs poses a significant challenge, mostly as a result of the influence of defects. In the present study, we employed density functional theory (DFT) to explore the impacts of the common point defects and impurities on p-type doping of Be$_\text{B}$ and Si$_\text{As}$, and on n-type doping of Si$_\text{B}$ and Se$_\text{As}$. We found that the most favorable point defects formed by C, O, and Si are C$_\text{As}$, O$_\text{B}$O$_\text{As}$, Si$_\text{As}$, C$_\text{As}$Si$_\text{B}$, and O$_\text{B}$Si$_\text{As}$, which have formation energies of less than $1.5$ eV. While the O impurity detrimentally affects both n- and p-type dopings, C and Si impurities are harmful for n-type dopings. Interestingly, the antisite defect pair As$_\text{B}$B$_\text{As}$ benefits both p- and n-type doping. The doping limitation analysis presented in this study can potentially pave the way for strategic development in the area of BAs-based electronics.

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arXiv2023

Accurate prediction of thermal conductivity of Al2O3 at ultrahigh temperatures

Many complex crystals show a flattening or even increasing lattice thermal conductivity at high temperatures, which deviates from the traditional 1/T decay trend given by conventional phonon theory. In this work, we predict the thermal conductivity of Al2O3 that matches with experimental data from room temperature to near melting point (2200 K). The lattice thermal conductivity is found to be composed of contributions of phonon, diffuson, and radiation. Phonon particle thermal conductivity decays approximately as ~T^-1.14 after considering four-phonon scattering as well as finite temperature corrections to lattice constant, harmonic, and anharmonic force constants. Diffuson (inter-band tunneling) thermal conductivity increases roughly as ~T^0.43. Radiation thermal conductivity increases as ~T2.51, being slightly smaller than ~T^3 due to the increase of phonon linewidth with temperature, which increases photon extinction coefficient and reduces photon mean free path. At room temperature, phonon, diffuson, and radiation contribute about 99%, 1%, and 0, respectively. At 2200 K, the contributions change to 61%, 20%, and 19%, respectively. Four-phonon scattering is important at ultra-high temperature, decreasing the phonon thermal conductivity by a maximum of 24%. The finite-temperature softening effects of harmonic and anharmonic force constants increase the phonon thermal conductivity by a maximum of 36% at ultra-high temperatures. We also verify that Green-Kubo MD can capture phonons' both particle and wave natures, similar to the Wigner formalism.

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arXiv2023

Ruddlesden-Popper chalcogenides push the limit of mechanical stiffness and glass-like thermal conductivity in single crystals

Insulating materials featuring ultralow thermal conductivity for diverse applications also require robust mechanical properties. Conventional thinking, however, which correlates strong bonding with high atomic-vibration-mediated heat conduction, led to diverse weakly bonded materials that feature ultralow thermal conductivity and low elastic moduli. One must, therefore, search for strongly-bonded single crystals in which heat transport is impeded by other means. Here, we report intrinsic, glass-like, ultralow thermal conductivity and ultrahigh elastic-modulus/thermal-conductivity ratio in single-crystalline Ruddlesden-Popper Ban+1ZrnS3n+1, n = 2,3, which are derivatives of BaZrS3. Their key features are strong anharmonicity and intra-unit-cell rock-salt blocks. The latter produce strongly bonded intrinsic superlattices, impeding heat conduction by broadband reduction of phonon velocities and mean free paths and concomitant strong phonon localization. The present study initiates a paradigm of mechanically stiff phonon glasses.

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arXiv2023

Intrinsic thermal conductivity of ZrC from low to ultra-high temperatures: A critical revisit

Current phonon transport theory based on ground-state calculations has been successful in predicting thermal conductivity at room and medium temperatures but may misrepresent behavior at high temperatures. In this work, we predict the thermal conductivity ($κ$) of ZrC including electronic and phonon contributions from 300 K to 3500 K, by including high-order phonon scattering, lattice expansion, temperature-dependent (TD) harmonic and anharmonic force constants, and inter-band phonon conduction by using first principles. For the phonon transport, we find that four-phonon scattering significantly reduces the phonon thermal conductivity ($κ_{ph}$), e.g., by $\sim$60% and $\sim$75% at 2500 K and 3500 K, respectively. After including four-phonon scattering and all other factors, $κ_{ph}$ shows a $\sim$T$^{-1.5}$ rather than $\sim$T$^{-1}$ dependence. The contribution from inter-band (Wigner) phonon conduction is small, even at ultra-high temperatures. The temperature dependence of anharmonic force constants decreases the phonon scattering cross-section at elevated temperatures and increases the $κ_{ph}$ significantly (by 52% at 3500 K). For the electronic thermal transport, we find that it is sensitive to and can be changed by 20% by the TD lattice constants. The Lorenz number varies from 1.6 to 3.3$\times$10$^{-8}$ W$\cdotΩ\cdot$K$^{-2}$ at different temperatures. The theoretical prediction in the literature overpredicts $κ_{ph}$ (e.g., $\sim$28%) and underpredicts the $κ_{el}$ (e.g., $\sim$38%), resulting in an overall underprediction of $κ$ ($\sim$26% at 1500 K). The impacts of grain size and defects are found strong, and no reported experimental data has reached the intrinsic theoretical thermal conductivity of ZrC yet.

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arXiv2023

Wide-range continuous tuning of the thermal conductivity of $\rm La_{0.5}Sr_{0.5}CoO_{3-δ}$ films via room-temperature ion-gel gating

Solid-state control of the thermal conductivity of materials is of exceptional interest for novel devices such as thermal diodes and switches. Here, we demonstrate the ability to continuously tune the thermal conductivity of nanoscale films of $\rm La_{0.5}Sr_{0.5}CoO_{3-δ}$ (LSCO) by a factor of over 5, via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (with $δ\approx 0.1$) to an oxygen-vacancy-ordered brownmillerite phase (with $δ=0.5$), accompanied by a metal-insulator transition. Combining time-domain thermoreflectance and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport, and structural characterization by X-ray diffraction, we uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. The wide-range continuous tunability of LSCO thermal conductivity enabled by low-voltage (below 4 V) room-temperature electrolyte gating opens the door to non-volatile dynamic control of thermal transport in perovskite-based functional materials, for thermal regulation and management in device applications.

cond-mat.mtrl-sci↗

arXiv2021

Raman Linewidth Contributions from Four-Phonon and Electron-Phonon Interactions in Graphene

The Raman peak position and linewidth provide insight into phonon anharmonicity and electron-phonon interactions (EPI) in materials. For monolayer graphene, prior first-principles calculations have yielded decreasing linewidth with increasing temperature, which is opposite to measurement results. Here, we explicitly consider four-phonon anharmonicity, phonon renormalization, and electron-phonon coupling, and find all to be important to successfully explain both the $G$ peak frequency shift and linewidths in our suspended graphene sample at a wide temperature range. Four-phonon scattering contributes a prominent linewidth that increases with temperature, while temperature dependence from EPI is found to be reversed above a doping threshold ($\hbarω_G/2$, with $ω_G$ being the frequency of the $G$ phonon).

cond-mat.mtrl-sci↗

arXiv2021

FourPhonon: An extension module to ShengBTE for computing four-phonon scattering rates and thermal conductivity

FourPhonon is a computational package that can calculate four-phonon scattering rates in crystals. It is built within ShengBTE framework, which is a well-recognized lattice thermal conductivity solver based on Boltzmann transport equation. An adaptive energy broadening scheme is implemented for the calculation of four-phonon scattering rates. In analogy with $thirdorder.py$ in ShengBTE, we also provide a separate python script, $Fourthorder.py$, to calculate fourth-order interatomic force-constants. The extension module preserves all the nice features of the well-recognized lattice thermal conductivity solver ShengBTE, including good parallelism and straightforward workflow. In this paper, we discuss the general theory, program design, and example calculations on Si, BAs and $\mathrm{LiCoO_2}$.

cond-mat.mtrl-sci↗

arXiv2020

Quantum Prediction of Ultra-Low Thermal Conductivity in Lithium Intercalation Materials

Lithium-intercalated layered transition-metal oxides, LixTMO2, brought about a paradigm change in rechargeable batteries in recent decades and show promise for use in memristors, a type of device for future neural computing and on-chip storage. Thermal transport properties, although being a crucial element in limiting the charging/discharging rate, package density, energy efficiency, and safety of batteries as well as the controllability and energy consumption of memristors, are poorly managed or even understood yet. Here, for the first time, we employ quantum calculations including high-order lattice anharmonicity and find that the thermal conductivity k of LixTMO2 materials is significantly lower than hitherto believed. More specifically, the theoretical upper limit of k of LiCoO2 is 6 W/m-K, 2-6 times lower than the prior theoretical predictions. Delithiation further reduces k by 40-70% for LiCoO2 and LiNbO2. Grain boundaries, strains, and porosity are yet additional causes of thermal-conductivity reduction, while Li-ion diffusion and electrical transport are found to have only a minor effect on phonon thermal transport. The results elucidate several long-standing issues regarding the thermal transport in lithium-intercalated materials and provide guidance toward designing high-energy-density batteries and controllable memristors.

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arXiv2019

A Unified Phonon Interpretation for the Non-Fourier Heat Conduction by Non-equilibrium Molecular Dynamics Simulations

Nanoconfinement induces many intriguing non-Fourier heat conduction phenomena that have been extensively studied in recent years, such as the nonlinear temperature profile inside the devices, the temperature jumps near the contacts, and the finite-size effects. The understanding of these phenomena, however, has been a matter of debate over the past two decades. In this work, we demonstrate a unified phonon interpretation of non-Fourier heat conduction which can help to understand these phenomena by a mode-to-mode correspondence between the non-equilibrium molecular dynamics (NEMD) simulations and the mode-resolved phonon Boltzmann transport equation (BTE). It is found that the nanoscale phonon transport characteristics including temperature profile, the heat flux value and the modal temperature depend on the applied thermal reservoirs on the two contacts. Our NEMD simulations demonstrate that Langevin thermostat behaves like an infinitely large thermal reservoir and provides thermally equilibrium mode-resolved phonon outlets, while biased reservoirs, e.g., Nose-Hoover chain thermostat and velocity rescaling method behave like non-equilibrium phonon outlets. Our interpretation clearly demonstrates that the non-Fourier heat transport phenomena are originated from a combination of non-diffusive phonon transport and phonon thermal nonequilibrium. This work provides a clear understanding of nanoscale heat transport and may guide the measurement and control of thermal transport in various applications.

cond-mat.mes-hall↗

arXiv2019

Four-phonon scattering diminishes the optical phonon contribution and isotope effect to thermal conductivity of III-V semiconductors

Recent studies reveal that four-phonon scattering is generally important in determining thermal conductivities of solids. However, these studies have been focused on materials where thermal conductivity $κ$ is dominated by acoustic phonons, and the impact of four phonon scattering, although significant, is still generally smaller than three-phonon scattering. In this work, taking AlSb as example, we demonstrated that four-phonon scattering is even more critical to three-phonon scattering as it diminishes optical phonon thermal transport, and therefore significantly reduces the thermal conductivities of materials in which optical branches have long three-phonon lifetimes. Also, our calculations show that four-phonon scattering can play an extremely important role in weakening the isotope effect on $κ$. Specifically, four-phonon scattering reduces the room-temperature $κ$ of the isotopically pure and natural-occurring AlSb by 70$\%$ and 50$\%$, respectively. The reduction for isotopically pure and natural-occurring c-GaN is about 34$\%$ and 27$\%$, respectively. For isotopically-pure w-GaN, the reduction is about 13$\%$ at room temperature and 25$\%$ at 400 K. These results provided important guidance for experimentalists for achieving high thermal conductivities in III-V compounds for applications in semiconductor industry.

cond-mat.mtrl-sci↗

arXiv2019

Role of higher-order phonon scattering in the zone-center optical phonon linewidth and the Lorenz oscillator model

Zone-center optical phonon linewidth is a key parameter for infrared and Raman spectra as well as the Lorenz oscillator model. While three-phonon scattering was often assumed to be the leading contribution, in this work we find, surprisingly, that higher-order phonon scattering universally plays a significant or even dominant role over three-phonon scattering at room temperature, and more so at elevated temperatures, for a wide range of materials including diamond, Si, Ge, boron arsenide (BAs), cubic silicon carbide (3C-SiC), and $α$-quartz. This is enabled by the large fourphonon scattering phase space of zone-center optical phonons, and distinct from heat conduction where at room temperature four-phonon scattering is still secondary to three-phonon scattering. Moreover, our results imply that five-phonon and even higher-order scattering may be significant for some large band-gap materials, e.g., BAs. Our predicted infrared optical properties through the Lorenz oscillator model, after including four-phonon scattering, show much better agreement with experimental measurements than those three-phonon based predictions. This work will raise broad interest of studying high-order scattering in various areas beyond heat conduction.

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