Nanoscale Imaging of Local Vibrational States and Phonon Dynamics at Interfaces

As the minimization of microelectronic devices continues, nanoscale thermal interface conductance becomes increasingly important in dissipating the heat flux generated in integrated circuits and helps prolong their lifetime. Phonon transport can be greatly modulated by interfaces between two differing materials. Thus, uncovering nanoscale phonon transport mechanisms at interfaces is crucial for heat management in operating electronics. However, it is challenging to detect local phonon properties due to the lack of effective experimental tools to investigate nanoscale vibrational spectra. Recent developments in monochromated electron energy-loss spectroscopy (EELS) have enabled the acquisition of vibrational spectra with few-meV energy resolution and sub-nm spatial resolution [1]. We developed a series of novel space- and angle-resolved vibrational EELS methods [2] to probe local vibrational states and phonon dynamics at diverse interfaces. Interfacial phonon modes at around 48 meV are observed in Si-Ge heterojunctions and confined to within a few nanometers of the interface [3]. Using our differential phonon momentum mapping method, we revealed a strong specular reflection of Si optical phonons at compositionally abrupt interfaces between Si and SiGe QDs [4]. Our work charts a definitive course for investigating local phonon spectra and phonon propagation around interfaces and provides guidance for the thermal nanoengineering of electronic devices.