FE-SEM observations of multiple nanohillocks on SrTiO₃ irradiated with swift heavy ions

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Single crystals of SrTiO₃ (100) and 0.5% Nb-doped SrTiO₃ (100) were irradiated with 80 MeV and 200 MeV ¹³⁶Xe¹⁴⁺ swift heavy ions at grazing incidence. The angle between the sample surface and the beam was less than 2˚. The irradiated surfaces were examined by field-emission scanning electron microscopy (FE-SEM). On the SrTiO₃ surface after osmium coating, chains of nanohillocks are observed along the ion path as reported in previous atomic force microscopy (AFM) study. In contrast, black lines accompanied by hillocks are observed on the Nb-doped SrTiO₃ surface. When the Nb-doped SrTiO₃ was coated with osmium, the chains of nanohillocks similar to the ones observed for SrTiO₃ were observed instead of black lines. The result suggests that the black lines are depressed regions (grooves). On this basis, we proposed a new model of the hillock chain formation in the framework of Rayleigh instability. The present model assumes that the outmost surface near ion trajectory is molten, the molten column is necked at nearly equal distance interval, and finally the separated segments form individual spheres. The spheres correspond to hillocks, and the traces of molten zones would be grooves.