Near surface latent track morphology of SHI irradiated TiO2

Abstract

The material response to SHI irradiation of simple oxides such as TiO2 is markedly different in the near surface volume than in the bulk. In this subsurface region, several tens of nanometers in thickness, the material exhibits a much higher sensitivity to damage creation. The enhanced sensitivity of the material in this region is related to the proximity of a free surface and not strongly dependent on electronic stopping power. At least for these materials, the assumption of cylindrical latent tracks is dubious and great care should be taken when inferring track radii from indirect techniques. Direct observation by TEM can also easily lead to grossly over estimated track radii when most standard specimen preparation techniques are employed. We suggest reduced internal pressure in the crystal in the vicinity of a free surface as the enabling factor for this enhanced sensitivity. Lower pressure leads to increased probability of a phase change due to elevated temperatures in the track core and volume expansion leads to the formation of hillock on the surface of the crystal.