Probing phonon transport dynamics across an interface by electron microscopy

Understanding thermal transport mechanisms across material interfaces is crucial for advancing semiconductor technologies, particularly in miniaturized devices operating under extreme power densities1,2. Although the interface phonon-mediated processes are theoretically established3–6 as the dominant mechanism for interfacial thermal transport in semiconductors7, their nanoscale dynamics remain experimentally elusive due to challenges in measuring the temperature and non-equilibrium phonon distributions across the buried interface8–11. Here, we overcome these limitations by using in-situ vibrational electron energy-loss spectroscopy in an electron microscope to nanoscale profile temperature gradients across the AlN-SiC interface during thermal transport and map its non-equilibrium phonon occupations at sub-nanometer resolution. We observe a sharp temperature drop within ~2 nm across the interface, enabling direct extraction of relative interface thermal resistance. During thermal transport, the mismatch of phonon modes’ thermal conductivity at the interface causes substantial non-equilibrium phonons nearby, making the populations of interface modes different under forward and reverse heat flow, and also leading to significant changes in the modal temperature of AlN optical phonons within ~3 nm of the interface. These results reveal the phonon transport dynamics at the (sub-)nanoscale and establish the inelastic phonon scattering mechanism involved by interface modes, offering valuable insights into engineering of thermal interfaces.