Tip-enhanced Raman spectroscopy of 6H-SiC with graphene adlayers: selective suppression of E1 modes

We have characterized 6H-SiC substrates with tip-enhanced Raman spectroscopy in a broad spectral range between 100 and 3000 cm(-1). A few adlayers of graphene previously grown on the semiconductor's C surface enabled us to approach the Au tip of a scanning tunneling microscope (STM) into tunnelling contact. In the recorded STM images of the substrate, we discern the height of the carbon adlayers in different sample regions. We compare tip-enhanced Raman (TER) spectra of the sample to normal Raman spectra recorded with the field vector of the linearly polarized light oriented at 30 degrees or 90 degrees with respect to the surface normal. All spectra show the phonon fingerprint of SIC and of graphene. We find the E1 modes of 6H-SiC selectively suppressed in the TER spectra whereas they are present in the normal Raman spectra excited at 30 degrees. This effect is explained by a tip-induced symmetry change. Copyright (C) 2009 John Wiley & Sons, Ltd.