  We report a detailed study of the strong near-field Raman scattering
enhancement which takes place in tip-enhanced Raman scattering (TERS) in indium
nitride. In addition to the well-known first-order optical phonons of indium
nitride, near-field Raman modes, not detectable in the far-field, appear when
approaching the plasmonic probe. The frequencies of these modes coincide with
calculated energies of second order combinational modes consisting of optical
zone center phonons and acoustic phonons at the edge of the Brillouin zone. The
appearance of strong combinational modes suggests that TERS in indium nitride
represents a special case of Raman scattering in which a resonance condition on
the nanometer scale is achieved between the localized surface plasmons (LSPs)
and surface plasmon polaritons (SPPs) of the probe with the surface charge
oscillation of the material. We suggest that the surface charge accumulation
(SCA) in InN, which can render the surface a degenerate semiconductor, is the
dominating reason for the unusually large enhancement of the TERS signal as
compared to other inorganic semiconductors. Thus, the plasmon-resonant TERS
(PR-TERS) process in InN makes this technique an excellent tool for defect
characterization of indium-rich semiconductor heterostructures and
nanostructures with high carrier concentrations.
