Intense ultraviolet photoluminescent emission from Yb doped ZnO thin films on Si after high temperature annealing

We report that the near band edge ultra-violet (UV) emission of ZnO sputtered films on Si can be enhanced greatly by incorporating about 5 at.% ytterbium in the films (ZnO + YbO). After annealing at 1100 °C in N₂, the UV peak intensity at around 380 nm was hundred times stronger from ZnO + YbO than from un-doped ZnO. The as-deposited ZnO + YbO film is amorphous and forms ZnO crystal grains by annealing above 700 °C. After 1100 °C annealing reactions with Si substrate have induced Zn silicates and Yb silicates and left embedded ZnO nanocrystals and Yb₂O₃ nano crystals on the surface as observed by X-ray diffraction, high resolution transmission electron microscopy and scanning electron microscopy together with energy dispersive electron spectroscopy. The photoluminescence (PL), PL excitation and PL decay spectra indicate that excitation energy is transferred between parts of the surrounding silicates, the Yb-oxides and the ZnO nanocrystals, while the energy transfer to Yb³⁺ is not efficient.