Space- and Angle-Resolved Vibrational Spectroscopy to Probe the Local Phonon Modes at Planar Defects

Abstract

Combined with aberration-corrected scanning transmission electron microscopy (STEM) imaging, the state-of-the-art monochromated electron energy-loss spectroscopy (EELS) is capable of detecting vibrational spectra for a variety of materials with an attainable energy resolution less than 10 meV and sub-nanometer resolution [1, 2]. Additionally, by varying the convergence semi-angle (α) and EELS collection geometry, momentum resolution is achieved, enabling angle-resolved vibrational spectroscopy that maps out the phonon dispersion relations [3]. Balancing these three aspects: spatial, energy, and momentum resolutions, we can obtain a powerful spaceand angle-resolved vibrational spectroscopy to investigate local phonon modes near the crystalline defects in semiconductors and their heterostructures [4]. Planar defects such as stacking faults and interfaces are believed to impede phonon propagation and modify the vibrational structure [5]. However, the experimental evidence of phonon-defect interactions is fundamentally elusive from either thermal conductivity measurements or optical spectroscopies due to their insufficient spatial resolution. Here we show two cases to demonstrate how to utilize the high spatial, energy, and momentum resolution capabilities of monochromated EELS to reveal the local defect phonon modes at (1) a stacking fault in cubic silicon carbide and (2) an interface in a Si–Ge heterojunction.