The interfacial thermal conductance of epitaxial metal-semiconductor interfaces
Date
2017
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Understanding heat transport at nanometer and sub-nanometer lengthscales is critical to solving a wide range of technological challenges related to thermal management and energy conversion. In particular, finite Interfacial Thermal Conductance (ITC) often dominates transport whenever multiple interfaces are closely spaced together or when heat originates from sources that are highly confined by interfaces. Examples of the former include superlattices, thin films, quantum cascade lasers, and high density nanocomposites. Examples of the latter include FinFET transistors, phase-change memory, and the plasmonic transducer of a heat-assisted magnetic recording head. An understanding of the physics of such interfaces is still lacking, in part because experimental investigations to-date have not bothered to carefully control the structure of interfaces studied, and also because the most advanced theories have not been compared to the most robust experimental data. ☐ This thesis aims to resolve this by investigating ITC between a range of clean and structurally well-characterized metal-semiconductor interfaces using the Time-Domain Thermoreflectance (TDTR) experimental technique, and by providing theoretical/computational comparisons to the experimental data where possible. By studying the interfaces between a variety of materials systems, each with unique aspects to their tunability, I have been able to answer a number of outstanding questions regarding the importance of interfacial quality (epitaxial/non-epitaxial interfaces), semiconductor doping, matching of acoustic and optical phonon band structure, and the role of phonon transport mechanisms apart from direct elastic transmission on ITC. In particular, we are able to comment on the suitability of the diffuse mismatch model (DMM) to describe the transport across epitaxial interfaces. ☐ To accomplish this goal, I studied interfacial thermal transport across CoSi2, TiSi2, NiSi and PtSi - Si(100) and Si(111), (silicides-silicon), interfaces with varying levels of disorder (epitaxial and non-epitaxial). The ITC values of silicides-silicon interfaces observed in this study are higher than those of other metallic interfaces to Si found in literature. Most surprisingly, it is experimentally found that ITC values are independent of interfacial quality and substrate orientation. Computationally, it is found that the non-equilibrium atomistic Green's Function technique (NEGF), which is specically designed to simulate coherent elastic phonon transport across interfaces, significantly underpredicts ITC values for CoSi2-Si interfaces, suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. In contrast, the Diffuse Mismatch Model closely mimics the experimentally observed ITC values for CoSi2-Si, NiSi-Si and TiSi2-Si interfaces, and only slightly overestimating the same for PtSi-Si interfaces. Furthermore, the results also show that ITC is independent of degenerate doping up to doping levels of 1*10^19 cm-3, indicating there is no significant direct electronic transport or transport effects which depend on long-range metal-semiconductor band alignment. ☐ Then, I study the effect of phonon band structure on ITC through measurements of epitaxial NiAl1-xGax-GaAs interfaces for varying levels of alloy composition, which independently tunes the mass of the metal's heavy atom without much affect on the lattice structure or interatomic force constants. The ITC values are found to linearly increase with increasing Ga content, consistent with the disappearance of a phonon band gap in NiAl1-xGax films with increasing Ga content, which enhances the phonon transmission coefficients due to a better density of states overlap between the two (NiAl1-xGax, GaAs) materials. ☐ Finally, I study a unique subset of epitaxial rocksalt interfaces between the Group IV metal nitrides (TiN, ZrN, and HfN) to MgO substrates as well as ScN layers. Prior to the currrent study, TiN-MgO was the only measured interface of this type, and maintained the record for the highest reported ITC for a metal-semiconductor interface. By varying the Group IV metal, the mass of the metal's light atom was independently tuned, allowing the ability to tune the acoustic phonon frequencies in the metal without significant effect to optical phonon band structure. We find that the ITC of all the studied interfaces are quite high, significantly exceeding the DMM predictions, and in the case of XN-ScN interfaces even exceed the radiative limit for elastic phonon transport. The results imply that mechanisms such as anharmonic phonon transmission, strong cross-interfacial electron phonon coupling, or direct electric transmission are required to explain the transport. The TiN-ScN interface conductance is the highest room temperature metal-dielectric conductance ever reported.
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Keywords
Applied sciences, Epitaxial interface, Interfacial thermal conductance, Metal-semiconductor, Phonon