Ballistic Thermal Transport at Sub-10 nm Laser-Induced Hot Spots in GaN Crystal

TL;DR

This study combines tip-enhanced Raman thermometry and simulations to investigate ballistic thermal transport at sub-10 nm hotspots in GaN, revealing the phonon mean free path and advancing thermal management in GaN electronics.

Contribution

It introduces a novel approach integrating experimental Raman thermometry with simulations to measure ballistic thermal transport at nanometer scales in GaN.

Findings

Temperature increase up to 40 K at hotspots

Localized optical penetration within 10 nm

Phonon mean free path estimated from combined methods

Abstract

Gallium nitride (GaN) is a typical wide-bandgap semiconductor with a critical role in a wide range of electronic applications. Ballistic thermal transport at nanoscale hotspots will greatly reduce the performance of a device when its characteristic length reaches the nanometer scale, due to heat dissipation. In this work, we developed a tip-enhanced Raman thermometry approach to study ballistic thermal transport within the range of 10 nm in GaN, simultaneously achieving laser heating and measuring the local temperature. The Raman results showed that the temperature increase from an Au-coated tip-focused hotspot was up to two times higher (40 K) than that in a bare tip-focused region (20 K). To further investigate the possible mechanisms behind this temperature difference, we performed electromagnetic simulations to generate a highly focused heating field, and observed a highly localized optical penetration, within a range of 10 nm. The phonon mean free path (MFP) of the GaN substrate could thus be determined by comparing the numerical simulation results with the experimentally measured temperature increase which was in good agreement with the average MFP weighted by the mode-specific thermal conductivity, as calculated from first-principles simulations. Our results demonstrate that the phonon MFP of a material can be rapidly predicted through a combination of experiments and simulations, which can find wide application in the thermal management of GaN-based electronics.