  The apertureless SNOM mapping of the slightly-doped 4H-SiC epitaxial layer
grown on a heavily-doped 4H-SiC substrate was performed with a cleaved edge
geometry. ASNOM images taken at the light frequencies of a $C^{13}O_{2}^{16}$
laser show a clear contrast between the substrate and the epitaxial layer. The
contrast vanishes at the laser frequency of $884cm^{-1}$, and gets clearer at
higher frequencies $(923cm^{-1})$. This can be explained by changes in the
local polarizability of SiC caused by the carrier concentration, which are more
pronounced at higher frequencies. Since the light frequency is tuned up further
($935cm^{-1}$), a transversal mode structure appears in the ASNOM map,
indicating a waveguide-like confinement of a surface phonon polariton wave
inside the strip of an epi-layer outcrop.
