Disordering of ultra thin WO3 films by high-energy ions

We have studied disordering or atomic structure modification of ultra thin WO3 films under impact of high-energy ions with non-equilibrium and equilibrium charge incidence, by means of X-ray diffraction (XRD). WO3 films were prepared by thermal oxidation of W deposited on MgO substrate. Film thickness obtained by Rutherford backscattering spectrometry (RBS) is as low as 2nm. Smoothness of film surface was observed by atomic force microscopy. It is found that the ratio of XRD intensity degradation per 90MeV Ni+10 ion (the incident charge is lower than the equilibrium charge) to that per 90MeV Ni ion with the equilibrium charge depends on the film thickness. Also, film thickness dependence is observed for 100MeV Xe+14. By comparison of the experimental result with a simple model calculation based on the assumption that the mean charge of ions along the depth follows a saturation curve with power-law approximation to the charge dependent electronic stopping power, the characteristic length attaining the equilibrium charge is obtained to be ∼7nm for 90MeV Ni+10 ion incidence or the electron loss cross section of ∼1016cm2, demonstrating that disordering of ultra WO3 films has been observed and a fundamental quantity can be derived through material modification.