Surface optical phonon and A1(LO) in ZnO submicron crystals probed by Raman scattering: Effects of morphology and dielectric coating

The authors studied the surface optical phonon and A(1)(LO) in ZnO submicron crystals as well as ZnO single crystals with submicron surface structures by employing ultraviolet Raman scattering. The small variation in Raman frequency of A(1)(LO), when the growth condition is changed, is mainly caused by the crystal imperfection via two mechanisms: one is the frequency lowering by native defects and the other is the frequency increasing due to the tilt of c-axis of ZnO crystals away from the surface normal direction that leads to the mixing of A(1)(LO) and E-1(LO). The evolution of Raman features, before and after dielectric coatings, in conjunction with the photoluminescence, provides clear-cut evidence for the observation of a surface optical phonon. The frequency of the observed surface mode is lowered by the existence of surface states, and the surface states can be effectively reduced by the dielectric coatings.