Direct Observation of Substantial Phonon Nonequilibrium Near Nanoscale Hotspots in Gallium Nitride

Phonon modal nonequilibrium is believed to widely exist around nanoscale hotspots, which can significantly affect the performance of nano-electronic and optoelectronic devices. However, such a phenomenon has not been explicitly observed in 3D device semiconductors at the nanoscale. Here, by employing a tip-enhanced Raman thermal measurement approach, substantial phonon nonequilibrium in gallium nitride near sub-10 nm laser-excited hotspots is directly revealed for the first time. As further evidence, quantitative agreements between measurements and accurate first-principles-based phonon Boltzmann transport equation calculations are obtained. The large nonequilibrium is attributed to the strong Fröhlich coupling of electrons with longitudinal optical phonons and the large acoustic-optical phonon frequency gap in gallium nitride, which is further demonstrated in other common III-V semiconductors. This work establishes a viable approach for understanding nanoscale nonequilibrium phonon transport and can potentially benefit the future modulation of hot carrier dynamics.