MAT science Week

Silicon nanopillars down to a diameter of ~20 nm and up to an aspect ratio 3 have been exposed to high-fluence irradiation of 50 keV Si${}^{+}$ ions. When the pillars are kept at room temperature (RT) they change their shape drastically under 10${}^{16}$ Si${}^{+}$cm${}^{-2}$ irradiation: They become bell-shaped, i.e. their heights decreases and their diameters increases strongly. To understand this shaping we performed 3D simulations using the program TRI3DYN [1] which show clearly that this shaping cannot be explained by sputtering effects. The shape change originates probably from ion-induced viscous flow [2, 3]. During irradiation at RT the Si becomes amorphous which allows a plastic deformation. Surprisingly, under irradiation at 400°C the bell-like shaping disappears completely. The nanopillars become thinner without a substantial reduction of their height. This agrees nicely with predictions of our 3D TRI3DYN simulations, i.e. sputtering is at 400°C the dominating mechanism. At high-T irradiation viscous flow is blocked as the Si pillars remain crystalline. The authors acknowledge support from the H2020 project “IONS4SET”, contract number 688072. [1] W. Möller, W. Eckstein, Nucl. Instr. Meth. B322 (2014) 23. [2] H. Trinkaus, A.I. Ryazanov, Phys. Rev. Lett. 74 (1995) 5072. [3] T. van Dillen et al., Appl. Phys. Letters 83 (2003) 4315, ibid. 84 (2004) 3591.