Effect of interfacial structures on phonon transport across atomically precise Si/Al heterojunctions

ZX Lu and NP Smith and MP Prange and RA Bunker and JL Orrell and AM Chaka, PHYSICAL REVIEW MATERIALS, 5, 086002 (2021).

DOI: 10.1103/PhysRevMaterials.5.086002

Phonons are important carriers of energy and information in many cryogenic devices used for quantum information science and in fundamental physics experiments such as dark matter detectors. In these systems phonon behaviors can be dominated by interfaces and their atomic structures; hence, there is increasing demand for a more detailed understanding of interfacial phonon transport in relevant material systems. Previous studies have focused on understanding thermal transport over the entire phonon spectrum at and above room temperature. At ultralow temperatures, however, knowledge is missing regarding athermal phonon behavior due to the challenge in modeling the extreme conditions in microscale, heterogeneous cryogenic systems, as well as extracting single-phonon information from a large ensemble. In this paper, we delineate the effects of interfacial atomic structures on phonon transport using a combination of classical molecular dynamics (MD) and phonon wave-packet simulations, to illustrate the consistency and differences between the ensemble- and single-phonon dynamics. We consider three single-crystal Si surface reconstructions-(1 x 1), (root 3 x root 3) and (7 x 7)-and model both experimentally observed Si(1 x 1)/Al interfaces and hypothesized Si(v 3 x v 3)/Al and Si(7 x 7)/Al interfaces. The overall interfacial thermal conductance calculated from non-equilibrium MD shows that for the Si(1 x 1)/Al system, the presence of Al twin boundaries can hinder phonon transport and reduce thermal conductance by 2-12% relative to single-crystal Al; whereas the Si (v 3 x v 3) and (7 x 7) reconstructions can enhance it by 6-19%. Normal mode decomposition reveals that both the increase and decrease in conductance are related to inelastic phonon scattering. Single-phonon wave-packet simulations predict phonon transport properties consistent with non- equilibrium MD, while further suggesting that phonon polarization conversion is significant even when elastic transmission dominates, and that the interfacial structures have anisotropic impacts on atomic vibrations along different lattice directions. Our findings suggest avenues for achieving selective phonon transport via controlling interfacial structures of materials using atomically precise fabrication techniques, and that the phonon wave-packet formalism is a potentially powerful method for developing a detailed understanding of non- equilibrium phenomena in the low-temperature limit.

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