Surface-enhanced Raman scattering of wurtzite-type GaN(0001) and ZnO(0001):

We first-time report surface-enhanced Raman scattering (SERS) of Ag-deposited wurtzite-type GaN(0001) epitaxial film and ZnO(0001) substrate. On non-deposited region, two Raman-active modes, A$_{1}$(LO) and E$_{2}$(high), were observed in backscattering geometry, which is consistent with the Raman selection rule on wurtzite structure. In contrast, on the Ag-deposited region of both samples, only A$_{1}$(LO) mode (734 cm$^{-1}$ for GaN and 572 cm$^{-1}$ for ZnO) exhibits clear Raman enhancement. We propose that the macroscopic polarization field accompanied by LO-phonons is responsible for this anomalous Raman enhancement. The study of SERS effect on ionic crystals thus provides a simple test to investigate the mechanism beside electromagnetic effect in enhanced Raman scattering.