In-Situ X-Ray Diffraction Study of the Evolution of NiO Microstructure Under 120 MeV Au Ion Irradiation

Modification of the microstructure of 300 nm thick NiO thin film deposited by pulsed laser deposition method under 120 MeV Au ion irradiation was studied by in-situ X-ray diffraction(XRD) at 300 K. Irradiation led to different effects on the crystallinity of the films at different ion fluences. In the low fluence regime, crystalline quality of the films improved with consequent increase in the XRD peak intensity and decrease of the full width at half maximum (FWHM) of the XRD peaks with increasing ion fluence up to 3 x 10(11) ions cm(-2). At higher ion fluences, the crystalline quality decreased. Thermal spike model with a transient temperature profile exceeding the melting temperature inside the track explains the second effect and a temperature radially decreasing away from the track region, but still high enough to anneal out the pre-existing defects in the region surrounding the ion tracks explains the first effect. Radius of the ion tracks (similar to 1.9 nm) calculated by fitting the fluence dependence of XRD peak intensity agrees with the value (2 nm) predicted by the inelastic thermal spike model. Low density of the disordered material in the track region created due to the electronic energy loss of the energetic ions exceeding the threshold value for track formation was found to produce radial compressive strain on the crystalline matrix around the ion tracks leading to shifting of XRD peaks towards higher angles with increasing ion fluence. Fitting the variation of the strain with ion fluence to Poisson relation yielded radius of the strained region as 6.9 nm. Our study thus suggests three modes of materials modification under the attendant ion beam: (i) annealing of defects in regions surrounding the ion tracks, (ii) creation of columnar defects along the ion path and (iii) a strained matrix around each ion track.