Sridhar Sadasivam

Electron-phonon coupling and thermal conductance at a metal-semiconductor interface: First-principles analysis

The mechanism of heat transfer and the contribution of electron-phonon coupling to thermal conductance of a metal-semiconductor interface remains unclear in the present literature. We report ab initio simulations of a technologically important titanium silicide (metal) - silicon (semiconductor) interface to estimate the Schottky barrier height (SBH), and the strength of electron-phonon and phonon-phonon heat transfer across the interface. The electron and phonon dispersion relations of TiSi

Combined Microstructure and Heat Transfer Modeling of Carbon Nanotube Thermal Interface Materials

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Phonon-eigenspectrum-based formulation of the atomistic Green's function method

with C49 structure and the TiSi

Phonon wave effects in the thermal transport of epitaxial TiN/(Al,Sc)N metal/semiconductor superlattices

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Microscopic Evaluation of Electrical and Thermal Conduction in Random Metal Wire Networks

-Si interface are obtained using first-principles calculations within the density functional theory (DFT) framework. These are used to estimate electron-phonon linewidths and the associated Eliashberg function that quantifies coupling. We show that the coupling strength of electrons with interfacial phonon modes is of the same order of magnitude as coupling of electrons to phonon modes in the bulk metal, and its contribution to electron-phonon interfacial conductance is comparable to the harmonic phonon-phonon conductance across the interface.

Microstructure-Dependent Heat Transfer Modeling of Carbon Nanotube Arrays for Thermal Interface Applications

A microstructure-sensitive thermomechanical simulation framework is developed to predict the mechanical and heat transfer properties of vertically aligned CNT (VACNT) arrays used as thermal interface materials (TIMs). The model addresses the gap between atomistic thermal transport simulations of individual CNTs (carbon nanotubes) and experimental measurements of thermal resistance of CNT arrays at mesoscopic length scales. Energy minimization is performed using a bead–spring coarse-grain model to obtain the microstructure of the CNT array as a function of the applied load. The microstructures obtained from the coarse-grain simulations are used as inputs to a finite volume solver that solves one-dimensional and three-dimensional Fourier heat conduction in the CNTs and filler matrix, respectively. Predictions from the finite volume solver are fitted to experimental data on the total thermal resistance of CNT arrays to obtain an individual CNT thermal conductivity of 12 W m−1 K−1 and CNT–substrate contact conductance of 7 × 107 W m−2 K−1. The results also indicate that the thermal resistance of the CNT array shows a weak dependence on the CNT–CNT contact resistance. Embedding the CNT array in wax is found to reduce the total thermal resistance of the array by almost 50%, and the pressure dependence of thermal resistance nearly vanishes when a matrix material is introduced. Detailed microstructural information such as the topology of CNT–substrate contacts and the pressure dependence of CNT–opposing substrate contact area are also reported.

Cross-plane thermal conductivity of (Ti,W)N/(Al,Sc)N metal/semiconductor superlattices

While the atomistic Greens function (AGF) method has the potential to compute spectrally resolved phonon transport across interfaces, most prior formulations of the AGF method provide only the total phonon transmission function that includes contributions from all phonon branches or channels. In this work, we present a formulation of the conventional AGF technique in terms of phonon eigenspectra that provides a natural decomposition of the total transmission function into contributions from various phonon modes. The method involves the use of Dyson and Lippmann-Schwinger equations to determine surface Greens functions from the phonon eigenspectrum of the bulk, and establishes a direct connection between the transmission function and the bulk phonon spectra of the materials forming the interface. We elucidate our formulation of the AGF technique through its application to a microscopic picture of phonon mode conversion at Si-Ge interfaces with atomic intermixing. Intermixing of atoms near the interface is shown to increase the phase space available for phonon mode conversion and to enhance thermal interface conductance at moderate levels of atomic mixing. The eigenspectrum-based AGF (EAGF) method should be useful in determination of microscopic mechanisms of phonon scattering and identification of the specific modes that dominate thermal transport across an interface.

THE ATOMISTIC GREEN'S FUNCTION METHOD FOR INTERFACIAL PHONON TRANSPORT

Epitaxial single crystalline TiN/(Al,Sc)N metal/semiconductor superlattice metamaterials have generated significant interest in recent years for their potential applications in high temperature thermoelectric devices, optical hyperbolic metamaterials in the visible and near infrared-spectral range, and as candidates for solar-thermophotovoltaics and high temperature electronic materials. While significant progress in their structural, mechanical, and optical properties has been made, in-depth analysis and detailed understanding of their thermal transport mechanism remain to be addressed. In this article, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/semiconductor superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thickness. Due to the increasing contribution of such phonon wave-modes, the cross-plane thermal conductivity at short-periods increases...

Combined Microstructure and Heat Conduction Modeling of Heterogeneous Interfaces and Materials

Ideally, transparent heaters exhibit uniform temperature, fast response time, high achievable temperatures, low operating voltage, stability across a range of temperatures, and high optical transmittance. For metal network heaters, unlike for uniform thin-film heaters, all of these parameters are directly or indirectly related to the network geometry. In the past, at equilibrium, the temperature distributions within metal networks have primarily been studied using either a physical temperature probe or direct infrared (IR) thermography, but there are limits to the spatial resolution of these cameras and probes, and thus, only average regional temperatures have typically been measured. However, knowledge of local temperatures within the network with a very high spatial resolution is required for ensuring a safe and stable operation. Here, we examine the thermal properties of random metal network thin-film heaters fabricated from crack templates using high-resolution IR microscopy. Importantly, the heaters achieve predominantly uniform temperatures throughout the substrate despite the random crack network structure (e.g., unequal sized polygons created by metal wires), but the temperatures of the wires in the network are observed to be significantly higher than the substrate because of the significant thermal contact resistance at the interface between the metal and the substrate. Last, the electrical breakdown mechanisms within the network are examined through transient IR imaging. In addition to experimental measurements of temperatures, an analytical model of the thermal properties of the network is developed in terms of geometrical parameters and material properties, providing insights into key design rules for such transparent heaters. Beyond this work, the methods and the understanding developed here extend to other network-based heaters and conducting films, including those that are not transparent.

PHONON EIGENSPECTRUM-BASED FORMULATION OF THE ATOMISTIC GREEN'S FUNCTION METHOD

A growing interest has developed in the use of carbon nanotube (CNT) arrays as thermal interface materials (TIMs). However, theoretical modeling of CNT TIMs has largely been limited to semi-empirical methods without detailed consideration of array microstructure, primarily due to the inherent randomness of the microstructure and the computational complexity involved in full atomistic modeling of CNTs. In this work, we report combined thermo-mechanical simulation of CNT arrays with a coarse-grain approach for the mechanics modeling and a thermal network approach for the heat transfer modeling. Parametric studies on the effects of CNT height on the Young’s modulus and buckling load of CNT arrays are reported. The thermal network model is used to estimate the pressure dependence of diffusive and tip contact resistances of CNT arrays; the predictions are compared with thermal resistance measurements using the photoacoustic method. The resulting simulation framework enables a particularly rich and broad thermo-mechanical data set. Selected parametric variations are computed to assess the stress-strain behavior, effective conductivity within the CNT array, and aspects of the contact topologies of the CNT-substrate interface.Copyright