First-principles electron transport and electron-phonon coupling in thin films and interfaces

Kumar, Sushant
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Gall, Daniel
Meunier, Vincent
Shi, Jian
Rhone, Trevor
Sundararaman, Ravishankar
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Materials engineering
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This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
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Electron-phonon interactions have implications in some of the most important current and future technologies like integrated circuits, solar cells, superconductors, spintronics, and quantum information. Though the study of electron-phonon scattering is a decades-old problem, up until recently it has mostly evaded a rigorous and fully first-principles treatment. In this work, we use parameter-free ab initio techniques for modeling electron transport and electron-phonon coupling in metallic films and nanostructures. We focus our efforts on two specific applications—plasmonics and electrical interconnects. In the field of plasmonics, we model the optical response of metallic nanoparticles in pump-probe spectroscopy. Generation of hot carriers followed by thermalization through electron-electron and electron-phonon scattering is a complex phenomenon. However, a multiscale approach which is able to capture the coupling between individual electronic states and phonon modes is able to predict the optical response with quantitative accuracy. We look at a long-standing puzzle in the ultrafast dynamics field which has evaded explanation for many years. The ultrafast pump-probe measurements of aluminum show a slow rise time and no decay in stark contrast to gold, even though carriers relax faster in aluminum by both electron-electron and electron-phonon scattering. We identify strong electron-phonon coupling and insensitivity of probe response to electron temperature as the solution to this long-standing puzzle. In our second research thrust, we study electron-phonon coupling in thin metallic films. A major hurdle in scaling down the size of integrated circuits is the increased resistivity of metallic Back-End-Of-Line (BEOL) interconnects at smaller dimensions. Enhanced surface and grain boundary scattering in the narrow limit leads to dramatic increase in the resistivity of the metal nanowires. Research in the last decade has focused on the search for highly conductive elemental metals like Co, Ru, Ir and Rh which could potentially replace the ubiquitously used Cu. Here, we explore the use of new classes of materials including intermetallics, metallic carbides, oxides, nitrides and topological metals and semimetals as next-generation interconnect materials. We show that metals with suitably anisotropic Fermi velocity distributions can strongly suppress electron scattering by surfaces and outperform isotropic conductors such as copper in nanoscale wires. We derive a corresponding descriptor for the resistivity scaling of anisotropic conductors, screen thousands of metals using first-principles calculations of this descriptor and identify the most promising materials for nanoscale interconnects. Previously-proposed layered conductors such as MAX phases and delafossites show promise in thin films, but not in narrow wires due to increased scattering from side walls. We find that certain intermetallics (notably CoSn) and borides (such as YCo3B2 ) with one-dimensionally anisotropic Fermi velocities are most promising for narrow wires. Combined with first-principles electron-phonon scattering predictions, we show that the proposed materials exhibit 2-3× lower resistivity than copper at 5 nm wire dimensions. For its potential as low-resistance interconnects, we pursue a fundamental understanding of electron transport properties of thin films of topological metals/semimetals. Studies have shown that in the case of Cu, the resistance-area RA product remains constant for pristine films and increases for films with defects with decreasing thickness. Our study show that the RA product decreases with film thickness for a Weyl semimetal—NbAs. This is attributed to the disproportionately large number of surface conduction states which dominate the ballistic conductance by up to 70%. The results presented here underscore the promise of topological semimetals as a future BEOL interconnect metal. Lastly, we find that some of the candidates we shortlisted for interconnects have significant advantages for efficient hot carrier harvesting. We show that the optical response of film-like conductors, PtCoO2 and Cr2AlC, resemble that of 2D metals, while that of wire-like conductors, CoSn and YCo3B2 , resemble that of 1D metals, which can lead to high mode confinement and efficient light collection in small dimensions, while still working with 3D materials with high carrier densities. Carrier lifetimes and transport distances in these materials, especially in PtCoO2 and CoSn, are competitive with noble metals. Most importantly, we predict that carrier injection efficiency from all of these materials into semiconductors can exceed 10% due to the small component of carrier momentum parallel to the metal surface, substantially improving upon the typical < 0.1% injection efficiency from noble metals into semiconductors.
December 2022
School of Engineering
Dept. of Materials Science and Engineering
Rensselaer Polytechnic Institute, Troy, NY
Rensselaer Theses and Dissertations Online Collection
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