Phonon Transport Near and Across Seminductor Interfaces

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Technical Summary This award supports theoretical and computational research and educational activities related to the transport of thermal energy by phonons across interfaces in nanostructured materials. Atomistic modeling tools including lattice dynamics calculations, the Boltzmann transport equation, molecular dynamics simulations, and density functional theory calculations, as well as theoretical development will be applied to address fundamental questions regarding phonon propagation and scattering under conditions very different from what exists in the bulk phase. Specifically, the PIs aim to: (1) Derive an expression for the phonon-interface scattering rate. (2) Resolve the discrepancies between different thermal boundary resistance models by predicting the non-bulk-like phonon distributions that exist near an interface. (3) Demonstrate that density functional theory calculations can be used to provide the input for lattice-dynamics based thermal boundary resistance models. The predictions will then be used to assess the role of electrons in thermal transport across metal-semiconductor interfaces. (4) Identify how Bloch phonon modes develop in the transition from an isolated interface to multiple interfaces to a periodic superlattice, including the effect of interfacial species mixing. This research is immediately relevant to the wealth of technologically important systems that contain multi-layer components, such as the silica layer in a field-effect transistor, silicon-germanium and tellurium-based superlattices for thermoelectric energy conversion applications, and quantum cascade lasers and light emitting diodes built from layers of GaAs, AlGaAs, and GaN. Interactions with Professor Jon Malen (Carnegie Mellon University) will allow for direct comparison of the theoretical and computational predictions to experimental measurements. This project will promote education in the emerging field of heat transfer physics: the study of thermal transport at the carrier-level, i.e. via phonons, photons, electrons, and fluid particles. An undergraduate elective course will be developed. NanoHUB and thermalHUB, two online resources, will be used to disseminate general information and research findings. Discovery-based lectures will be developed and presented in undergraduate classes and through Pittsburgh-based outreach programs. Nontechnical Summary This award supports theoretical and computational research and educational activities related to heat transfer across solid-solid interfaces. When such interfaces are separated by distances of the order of one thousandth to one millionth the size of the human hair, as they are in computer chips and light-emitting diodes, they can dominate thermal resistance. High thermal resistance makes it difficult to remove heat, leading to undesirably high operating temperatures. Furthermore, closely spaced interfaces behave differently than interfaces that are far apart, the topic of most previous studies. The PI will perform computer simulations and theoretical calculations to consider interfaces at the atomic level. The motions of individual atoms will be studied so as to determine how energy flows across an interface. Calculations will be performed at different levels of accuracy, with some based on quantum mechanics, allowing for comparison to experimental results. The work is relevant to the wealth of technologically important systems that contain multi-layer components, such as field-effect transistors, materials made of periodically alternating regions of different compositions for thermoelectric energy conversion applications, lasers, and light emitting diodes. This project will promote education in the study of heat transfer at the atomic-level. An undergraduate elective course will be developed. NanoHUB and thermalHUB, two online resources, will be used to disseminate general information and research findings. Discovery-based lectures will be developed and presented in undergraduate classes and through Pittsburgh-based outreach programs.