Enhancement of interfacial thermal transport through evanescent electric field mediated acoustic phonon transmission for efficient cooling of high power Gallium Nitride devices

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Increasing power dissipation in electronic devices such as laptops, mobile phones and high-power Gallium Nitride (GaN) devices has led to the need for improved cooling, and to maintain device temperatures below permissible levels. A recent cooling strategy involves using a diamond substrate to cool electronic devices, due to the ultra-high thermal conductivity of diamond exceeding 2000 W/mK at 300 K. However, the interface between the diamond and the GaN electronic component has both poor interfacial bonding and structure-defects, which greatly diminish heat transfer across the interface, exacerbating the thermal management problem. The goal of this research is to explore the role of electric fields across the interface to reduce the interface thermal resistance and thus enable large enhancement of heat transfer across the electronic device-diamond substrate interface. The project will engage graduate and undergraduate students directly in the proposed research. High school and underrepresented students will be introduced to research activities through a summer-camp program and through outreach to tribal colleges in Oklahoma. It is well known that at nanometer gaps between polar dielectrics, evanescent electric fields lead to several orders of magnitude enhancement in heat transfer above the black body limit. Such enhancement in heat transfer is adequately described by continuum fluctuation-dissipation theorem, based on phonon polaritons (coupling of electric fields with optical phonons). At gaps of around 2 to 4 Angstroms, similar to those encountered across interfacial defects, a recent work demonstrated (through an atomistic formalism) that electric fields can also enable transmission of acoustic phonons, enhancing heat transfer. The project will explore such Coulomb interaction assisted acoustic phonon transmission, for enhancement of interfacial thermal conductance, using a combination of atomistic Green’s function method and classical and ab initio molecular dynamics. Simultaneously, the project will further explore materials with superior thermal conductivity relative to diamond, at nanometer to micron range length scales, through a first-principles approach based on three and four phonon scattering and an exact solution of the Boltzmann transport equation. Materials with superior thermal conductivity and improved interfacial thermal conductance will lead to next generation high power GaN devices with improved reliability and performance.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.