Defect characterization and magnetic properties in un-doped ZnO thin film annealed in a strong magnetic field

Highly c-axis oriented un-doped zinc oxide (ZnO) thin films, each with a thickness of ～ 100 nm, are deposited on Si (001) substrates by pulsed electron beam deposition at a temperature of ～ 320 ℃, followed by annealing at 650 ℃ in argon in a strong magnetic field. X-ray photoelectron spectroscopy (XPS), positron annihilation analysis (PAS), and electron paramagnetic resonance (EPR) characterizations suggest that the major defects generated in these ZnO films are oxygen vacancies. Photoluminescence (PL) and magnetic property measurements indicate that the room-temperature ferromagnetism in the un-doped ZnO film originates from the singly ionized oxygen vacancies whose number depends on the strength of the magnetic field applied in the thermal annealing process. The effects of the magnetic field on the defect generation in the ZnO films are also discussed.