Photoluminescence of F-passivated ZnO nanocrystalline films made from thermally oxidized ZnF₂films

F-passivated ZnO nanocrystalline films were prepared by thermal oxidation of ZnF 2 films. ZnF 2 films were deposited on a Si wafer by the electron beam evaporation technique. X-ray diffraction and x-ray photoelectron spectroscopy were used to study the structural changes of ZnF 2 as a function of oxidation temperature. When the ZnF 2 film was oxidized at 400 °C for 30 min, a polycrystalline hexagonal wurtzite structure of ZnO:F was obtained. The room temperature photoluminescence spectrum of the ZnO:F film showed a strong near band edge ultraviolet emission located at 379 nm with a narrow linewidth of 70 meV and a very weak visible emission associated with deep level defects. The results demonstrated that the presence of residual F ions in ZnO nanocrystalline film can dramatically decrease the visible emission and increase the ultraviolet emission of ZnO. On the basis of the experimental findings, two possible mechanisms are proposed: (1) the residual F ions in the film occupy the lattice sites of V o * centres (the oxygen vacancies with one electron) inside the ZnO nanocrystals, which results in an appreciable decrease in visible emission and (2) some of the F ions also passivate ZnO nanocrystal surface states, which prevents the holes in the valence band from being trapped in surface states and then tunnelling back into nanocrystals to combine with V o * to form V o ** centres ( ) which are another source of visible emission.