LAUSR

Spatially resolved high resolution electron energy loss in a high energy resolution monochromated EELS system (HERMES) system has attracted a lot of attention after its inception while opening new avenues of research and many unanswered questions. The recent advancements in EELS have made it possible for < 1 nm probe sizes while still managing high probe current of around 10 pA [1]. With this, spatially resolved transmission EELS is possible, paving the way for accurately quantifying bulk vibrational mode energies and distinguishing them from surface modes. [2] It is instructive to begin with a relatively simple system whose vibrational modes can be more precisely studied than other complex systems. We have chosen SiC as the system of choice due to its simple cubic structure in its 3C polytype and hexagonal structure in its 4H and 6H polytypes. Here we discuss only the SiC 6H polytype. SiC vibrational modes have been meticulously studies previously by means of Fourier transform infra-red spectroscopy and Raman spectroscopy. With a well-documented database for the SiC system, we hope to correlate our findings in spatially resolved high resolution EELS. Typically, Raman spectroscopy studies the surface vibrational modes of a material due to the low penetration depth of the laser. Although transmission Raman Spectroscopy [3] is capable of probing bulk modes in a material, it lacks the spatial resolution to map local bulk modes and study thickness and size dependent effects. With our HERMES system, we can show the distinction between surface and bulk modes in a thin high quality thin film single crystal sample as well as increase in scattering probability of bulk modes as a function of thickness.