Thermal transport across metal silicide-silicon interfaces: First-principles calculations and Green's function transport simulations

Abstract

Heat transfer across metal-semiconductor interfaces involves multiple fundamental transport mechanisms such as elastic and inelastic phonon scattering, and electron-phonon coupling within the metal and across the interface. The relative contributions of these different transport mechanisms to interface conductance remains unclear in the current literature. In this work, we use a combination of first-principles calculations under the density functional theory framework and heat transport simulations using the atomistic Green's function (AGF) method to quantitatively predict the contribution of the different scattering mechanisms to the thermal interface conductance of epitaxial CoSi 2 -Si interfaces. An important development in the present work is the direct computation of interfacial bonding from density functional perturbation theory (DFPT) and hence the avoidance of commonly used 'mixing rules' to obtain the cross-interface force constants from bulk material force constants. Another important algorithmic development is the integration of the recursive Green's function (RGF) method with Büttiker probe scattering that enables computationally efficient simulations of inelastic phonon scattering and its contribution to the thermal interface conductance. First-principles calculations of electron-phonon coupling reveal that crossinterface energy transfer between metal electrons and atomic vibrations in the semiconductor is mediated by delocalized acoustic phonon modes that extend on both sides of the interface, and phonon modes that are localized inside the semiconductor region of the interface exhibit negligible coupling with electrons in the metal. We also provide a direct comparison between simulation predictions and experimental measurements of thermal interface conductance of epitaxial CoSi 2 -Si interfaces using the time-domain thermoreflectance technique. Importantly, the experimental results, performed across a wide temperature range, only agree well with predictions that include all transport processes: elastic and inelastic phonon scattering, electron-phonon coupling in the metal, and electron-phonon coupling across the interface.2

Phonon transmission at crystalline-amorphous interfaces studied using mode-resolved atomistic Green's functions

“…Compared to wave packet and lattice dynamics methods, the atomistic Green's function (AGF) method is more efficient and easier to implement, and it has been used extensively to compute the frequency dependent transmission through a device connected to reservoirs [21][22][23][24][25][26]. Such quantum transport problems have been studied by the quantum transmitting boundary method (QTBM) [27] and the nonequilibrium Green's function (NEGF) method [28], which provide general framework for electron transport and the modeling of physics of nanoscale devices [29].…”

Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces

“…Above 100K a coherent AGF approach significantly underpredicts interface conductance in the case of CoSi 2 suggesting that energy trans- port does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. A full-dispersion diffuse mismatch model closely predicts the experimentally observed interface conductances for CoSi 2 , NiSi, and TiSi 2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electronphonon coupling, or other mechanisms could also account for the high temperature behavior 54 . The effect of degenerate semiconductor dopant concentration on metalsemiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (ntype or p-type) ≈ 1 × 10 19 cm −3 , indicating that there is no significant direct electronic transport and no transport effects which depend on long-range metal-semiconductor band alignment.…”

“…Inelastic phonon scattering has been identified as an important transport mechanism for material combinations with a large acoustic mismatch such as Pb and diamond 56,57 . In the case of CoSi 2 of Si(111), we show in a forthcoming publication that the high temperature behavior of interface conductance can be matched quite well by invoking these mechanisms 54 . It remains unclear, however, whether these mechanisms are insensitive to interfacial structure.…”

“…Figure 5(a) shows a comparison between temperaturedependent CoSi 2 experiments and our DMM and AGF calculations. 54 Since the experimental Si (111)-CoSi 2 (111) interfaces considered here are epitaxial with submonolayer interfacial roughness it would be reasonable to expect the AGF method to be applicable. We observe, however, that the experimental value of thermal interface conductance (≈500 MW/m 2 -K) exceeds the AGF prediction by more than 50%.…”

Atomistic origin of the reduced lattice thermal conductivity of silicon nanotubes

“…8,17 Recent experimental work 17 has shown that the thermal conductivity of SiNTs is determined on the thickness, indicating that the ultralow thermal conductivity of SiNTs could be achieved. In theoretical studies, some methods have been developed to deal with the thermal transport properties of nanostructures, including thermodynamic model, 18 Boltzmann transport equation, 19 first-principles calculations, 20,21 and molecular dynamics simulations, 22 etc.…”