Annealing-based manipulation of thermal phonon transport from light-emitting diodes to graphene

We demonstrate that the thermal boundary conductivity (TBC) between graphene and GaN-based light-emitting diodes (LEDs) can be manipulated through thermal annealing, which is verified by measuring the acoustic phonons after reflection at the interface. Thermal annealing affects the interfacial morphology as evaluated by both the Raman spectra and the spatial profile of the graphene wrinkles in atomic force microscopy. By tracing the phase of ultrafast acoustic oscillations on the basis of the pump-probe scheme, we extract the phonon reflection coefficient at the interface as a function of annealing temperatures up to 400 $°$C. Specifically, the phase shift of transient phononic oscillations at the graphene/LED interface conveys the photoelastic response during the phonon transfer process and can be used for extracting the interfacial coupling rate, which is strongly enhanced around $≈200°$C. By incorporating the heat capacity and the interfacial coupling constants into TBC, along with analytical modeling based on the phonon reflection coefficients, we show that the TBC increases with the minimized surface roughness of graphene side at 200 $°$C. This new comprehensive TBC extraction scheme could spark further discussion on improving the heat dissipation of LEDs.