Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al2O3 (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm−1 due to the substitution of Co2+ in wurtzite ZnO lattice. The LVM at 723 cm−1 is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm−1 can be attributed to enrichment of Co2+ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn1−xCoxO films associated with ferromagnetism. The intensity of LVM at 699 cm−1, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.