Nonequilibrium electron-phonon coupling across the interfaces between Al nanofilm and GaN

The metal Al is commonly attached to external circuits as the source and drain in GaN-based field effect transistors, so profound comprehension of the energy transfer between electrons and phonons in Al/GaN is crucial for nanofabrication and thermal management of electronic devices. Time-domain thermoreflectance (TDTR) is an effective technique for measuring the strength of non-equilibrium electron-phonon (e-ph) coupling. The two-temperature model (TTM) is widely employed in conjunction with TDTR methods to determine e-ph coupling factors. However, TTM is a gray method and cannot take into account interactions between electrons and different phonon modes. Therefore, in this work, we use the TDTR technique to analyze the non-equilibrium transport properties of pure Al and the thickness dependence of the e-ph coupling with Al nanofilms, and the coupling strengths of high-energy electrons excited by femtosecond lasers with different modes of phonons are obtained in conjunction with MTM. The results show that the e-ph coupling coefficients of Al nanofilms on GaN substrates are larger than those of pure Al. In conjunction with the TTM, we determined the coupling strength between high-energy electrons excited by femtosecond laser pulses and various phonon modes. Compared to the transverse acoustic branch-1 (TA1) and transverse acoustic branch-2 (TA2) modes, the longitudinal acoustic (LA) phonon mode of Al exhibits a higher e-ph coupling factor. This suggests that the LA mode predominates in the electron relaxation process after ultrafast femtosecond laser excitation. This study provides experimental and theoretical guidance for laser processing and electronic device design. Nano-metallic Al films are usually attached as source and drain in the external circuits of gallium nitride-based field effect transistors, and thus unbalanced carrier transport in Al/GaN is critical for thermal management.