We report on the high-temperature ferromagnetism in Co-doped ZnO films fabricated by the sol-gel method above 350 K. The lattice constant of c axis of wurtzite Zn 1Ϫx Co x O follows Vergard's law for 0ϽxϽ0.25. For Zn 1Ϫx Co x O with xу0.25, a secondary phase is detected. The Zn 1Ϫx Co x O exhibits ferromagnetic behavior with a Curie temperature higher than 350 K. By the results of x-ray photoelectron spectroscopy measurement, we assume that Co occupied the Zn site without changing the wurtzite structure. In the case of xϭ0.2, the coercive field measured by a magnetizationmagnetic field hysteresis curve at 350 K was nearly 80 Oe. Additionally, we investigated the electric structure through first-principles pseudopotential plane-wave calculation.
Based on first-principles calculations, a model for large-size-mismatched group-V dopants in ZnO is proposed. The dopants do not occupy the O sites as is widely perceived, but rather the Zn sites: each forms a complex with two spontaneously induced Zn vacancies in a process that involves fivefold As coordination. Moreover, an As(Zn)-2V(Zn) complex may have lower formation energy than any of the parent defects. Our model agrees with the recent observations that both As and Sb have low acceptor-ionization energies and that to obtain p-type ZnO requires O-rich growth or annealing conditions.
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