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.

Yb-doped ZnO thin films were prepared on glass substrates by spray pyrolysis technique in order to investigate the insertion of Yb ions in the ZnO matrix and the related optical properties of the films. The molar ratio of Yb in the spray solution was varied in the range of 0-5 at. %. X-ray diffraction patterns showed that the undoped and Yb-doped ZnO films exhibit the hexagonal wurtzite crystal structure with a preferential orientation along ͓002͔ direction. No secondary phase is observed in Yb-doped ZnO films. All films exhibit a transmittance between 75 and 90% in the visible range with a sharp absorption onset about 375 nm corresponding to the fundamental absorption edge at 3.3 eV. The photoluminescence measurements show a clear luminescence band at 980 nm that is characteristic of Yb 3+ transition between the electronic levels 2 F 5/2 and 2 F 7/2. This is an experimental evidence for an efficient energy transfer from ZnO matrix to Yb 3+. Hall effect measurements showed low resistivities and high carrier mobilities which makes these films of interest to photovoltaic devices.

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