Evolution of resonant Raman scattering spectra of ZnO crystallites upon post-growth thermal annealing

The authors report on observations regarding thermal annealing of ZnO submicron crystals grown by radio-frequency magnetron sputtering. The surface optical (SO) and A1(LO) phonons in the resonant Raman scattering spectra, upon sample annealing, exhibit blue- and red-shift, respectively. The blue-shift of the SO mode is strongly correlated with the intensity reduction of the A1(LO) mode, which is an indication of the weakening of the surface electric field that was built by surface and near-surface defects via forming surface states. Photoluminescence measurements reveal defects annihilation in the ZnO crystallites that led to the surface states passivation, while the off-axis X-ray diffraction mappings provide evidence that the red-shift in the A1(LO) mode originated from the anneal-induced crystal reorientations.