Characterizations of ferromagnetic Zn1−xCoxO thin films grown on Al2O3 (0001) by reactive radio-frequency magnetron sputtering coupled with post-growth annealing

High-quality ferromagnetic Zn1−xCoxO thin films were deposited on a sapphire (0001) substrate at 600 °C by using reactive radio-frequency magnetron sputtering coupled with post-annealing treatment for 1 h at 580 °C under an Ar atmosphere. High resolution X-ray diffraction patterns show that hexagonal wurzite crystal structures of undoped ZnO film were maintained even after Co doping up to 4.5 at.% without forming Co clusters or oxides. X-ray photoelectron spectroscopy spectra represent the energy difference of 15.42 eV between Co2p3/2 and Co2p1/2, which is different from 15.05 eV of Co clusters. The characteristic absorption bands near 658, 616, and 568 nm wavelengths out of UV–VIS–IR spectroscopy spectra are correlated with the d–d transitions of tetrahedrally coordinated Co2+ ions. The low temperature photoluminescence spectrum for undoped ZnO shows a strong near-band edge (NBE) emission peak of 3.42 eV without deep level emission peaks. But, Co content increases in Zn1−xCoxO film, the NBE emission peak intensity decreases and another emission peak at 3.37 eV as well as a broad green emission peak at around 2.5 eV starts to appear with larger intensity due to the more actively creating oxygen vacancies. The emission peak at 3.37 eV proves the interaction between Co ions and the hydrogenic electrons in the impurity band and also supports the typical ferromagnetic hysteresis curves obtained by superconducting quantum interface device magnetometry at 300 K for Zn1−xCoxO films. High insulator characteristics are observed for as-grown Zn1−xCoxO films whereas it exhibits n-type characteristics with the increased carrier concentration, mobility, and resistivity after post-growth annealing. The spintronic devices could be fabricated with the utilization of Zn1−xCoxO films grown by the economically feasible reactive radio-frequency magnetron sputtering coupled with the post annealing treatment.