Revealing Local Phonon Modes at Stacking faults and Interfaces*

Imperfections such as stacking faults and interfaces are recognized as controlling factors in modifying thermal properties and heat transport by scattering phonon and changing vibrational structure [1]. However, the effect of crystal defects on thermal conductivity is being theoretically treated by perturbation methods without considering the local change of phonon dispersion relation. Direct detection of local defect-induced phonon modes has not been realized until very recently [2,3]. Here we demonstrate that space- and angle-resolved vibrational spectroscopy in a transmission electron microscope enables the study of the vibrational structure of individual crystal defects. At a single stacking fault in a cubic silicon carbide, the acoustic vibration modes at X point undergo a red shift of several millielectronvolts, become enhanced, and are confined to within a few nanometers of the stacking fault [3]. The interfacial phonon modes localized at interfaces were also revealed in both Si-Ge epitaxial heterojunctions [4] and monolayer MoS₂-WSe₂ heterostructure [5]. Our work paves an avenue to investigating phonon propagation around crystal defects and provides guidance to the engineering of desired thermal performance for semiconductor and power electronic devices.