Raman spectroscopic studies of oxygen defects in Co-doped ZnO films exhibiting room-temperature ferromagnetism

The optical properties, Raman spectra, and magnetization of Zn1-xCoxO (0 < =x < 0.1) thin films are reported. Optical transmission spectra confirm the substitution of Co2+ cations for Zn2+ ions at the tetrahedral sites of ZnO. Raman spectra of films annealed in air show an additional broad vibrational mode, attributed to disordered -Zn-O-Co- local vibrations. These modes disappear on vacuum annealing, suggesting the formation of oxygen vacancies close to the dopant sites (-Zn-Co-). Magnetization measurements show that air-annealed films lack significant moment, whereas vacuum-annealed samples with intermediate Co concentrations (0.012 < x < 0.05) exhibit room-temperature ferromagnetism. Using the framework of the bound magnetic polaron model, the polaron size is estimated to be roughly 7.8 A for this system.