Phase formation and separation in NiO/SiO2 and NiO/Si layer systems during swift heavy ion irradiation


Phase formation and separation in NiO/SiO2 and NiO/Si layer systems during swift heavy ion irradiation

Ferhati, R.; Guilliard, N.; Fritzsche, M.; Bolse, W.

We have recently investigated restructuring effects in thin oxide films (NiO, Fe2O3 and ZnO on pure Si- or oxidized Si-substrates) during swift heavy ion (SHI) irradiation. On the one hand we have observed that the films rupture upon SHI bomdardment under normal ion incidence and exhibit dewetting patterns similar to those observed for liquid polymer films on Si. On the other hand, grazing incidence irradiation of NiO-films leads to an instability against cracking perpendicular to the beam direction projected onto the surface and subsequent growth of an almost periodic lamellae pattern with lamellae thicknesses of the order of 100 nm and heights of the order of 2 µm. Subsequent irradiation under perpendicular directions at sufficiently low ion fluences results in a more or less rectangular crack pattern, i.e. the generation of spacially separated rectangular NiO-plates on top of the substrate. By further irradiation under grazing incidence and continuous azimuthal rotation of the sample, the oxide plates shrink in their lateral dimensions and grow in height. At the same time the egdes become round and finally a ”forest” of cylindrical NiO-pillars with radii of the order of 100 nm forms. However, because of the irregular cracking, the pillar are not arranged in a regular manner and exhibit a wide distribution of heigth and radius. These limitations can be overcome by replacing the initial cracking due to the ”Grinfeld instability” by artificial production of regularely ordered rectangular ”oxide plates” utilizing the focused ion beam (FIB) technique. SHI irradiation under grazing angle incidence and continuous azimuthal rotation than indeed results in the formation of regularely ordered objects of various (not only pillar-like) shapes, depending on the size of the initial plate. These phenomena can be understood in terms of the ”ion hammering effect” and ion-induced visco-elastic flow due to yield stresses and surface tensions as well as interface energies (capillary forces). However, until now it is not clear whether or not the above described phenomena are influenced also by phase formation and separation in the film and at its interfaces. Related studies require the utilization of phase- or at least element-sensitive analysis techniques with good lateral spacial or/and depth resolution. In the present report we’ll present our first EDX- (Energy Dispersive X-Ray Spectroscopy) and XPS- (Foto-Electron Spectroscopy) results on SHI irradiated coherent as well as pre-structured NiO-layers on Si and SiO2, respectively.

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