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.