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.
The objective of this project was to use computational tools to study the nature of heat transfer near and across interfaces in semiconducting materials (e.g., silicon) at the atomic level. Such interfaces are prevalent in technologies including computer chips, solid-state lighting, and lasers. The interfaces help to control electricity in these technologies. They also lead to increased thermal resistance, however, which causes higher operating temperatures and earlier-than-desired failure. The increased thermal resistance is desirable in thermoelectric energy conversion, where waste heat (e.g., from a car exhaust) can be converted into electricity. The research work had three main components. First, we studied heat transfer across an Angstrom-sized gap between two pieces of silicon, as shown in Fig. 1. This system was representative of what might be found in an atomic-force microscope. We found that the rate of heat transfer due to conduction showed a non-monotonic behavior with gap thickness (Fig. 2) and was four times greater than that by radiation. Second, we considered heat transfer through a superlattice (Fig. 3), which is a material built from very thin films (~nanometers) of two different materials. We investigated how defects at the interfaces, which will be found in experimental samples, changed the thermal conductivity of the full structure. Defects were found to play an important role when the superlattice layers are thin, but that this effect decreases as the film thickness increases. Third, we examined how heat flows through a finite number of closely-spaced interfaces (i.e., a multi-thin film system, see Fig. 4). We were particularly interested in how heat transfer through such a system is different/similar to that through a single interface, as has been studied previously. As shown in Fig. 5, we found that due to the close-spacing of the interfaces, the behavior of the multi-film system cannot be predicted from the properties of the constituent materials, as can be done for larger-scale systems. The educational and outreach activities included (i) the mentoring of undergraduate and graduate students, (ii) the development and presentation of lectures aimed at teaching undergraduate students about nanoscience and nanotechnology, and (iii) the organization of a yearly meeting in Pittsburgh that brings together researchers in the simulation field.
