We report on the origin of ferromagnetism in ZnO and Co-doped ZnO magnetic semiconductor films prepared by sol-gel technique on Si(100) and SiO2 substrates. X-ray diffraction measurements have shown that ZnO and Co-doped ZnO have the expected wurtzite structure and a preferential orientation along the c axis. Optical transmittance measurements revealed that Co is incorporated into the lattice, as Co2+ substituting Zn2+ ions. Magnetization measurements have shown room-temperature ferromagnetism for both ZnO and ZnO:Co films. After annealing, the magnetization curves recorded on free substrates show the same ferromagnetic behavior as the ZnO:Co film indicating that the ferromagnetism is extrinsic and due to external pollution.

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Al-doped ZnCoO films were grown by sol-gel process onto SiO2 and Si(100) substrates. Structural analysis shows that all samples have the hexagonal würtzite structure with a slight preferential orientation along the c-axis. The insertion of Al and Co into the ZnO matrix has been experimentally evidenced by UV-visible spectroscopy and transmission electron microscopy. This is further supported by x-ray photoelectron spectroscopy which indicated that all Co is under ionic form and that the samples contain no metallic clusters. The creation of free carriers by Al doping was confirmed by Hall effect and resistance measurements. Although a weak ferromagnetism is observed in all films, no clear influence of Al doping on the magnetic properties is evidenced which is in agreement with electronic structure calculations. The calculations show clearly that there is almost no overlap between the Al and the Co states, suggesting no change in the magnetic properties of ZnCoO with the Al doping. The only major role of Al is to change the conduction type of ZnCoO from p to n type.