He⁺ Ion Irradiation Induced Sn Extrusion out of SnO Covered Tin Spheres – a Combined Computer Simulation of Defect Generation and Defect Kinetics

Here we report on a combined computer simulation of defect generation and defect kinetics for 30 keV He⁺ ion irradiation of sub-μm-scale tin spheres. In the process to be simulated, the irradiation was performed in a Helium Ion Microscope which allows the in-situ monitoring of morphological changes of the nanospheres during He⁺ ion irradiation. Above a He⁺ fluence of ∼10¹⁷ /cm², Sn extrusions appear on the surface of the spheres. Initially, small, pyramidally facetted extrusions evolve at the equator of the tin spheres (north pole pointing to the ion source), later on each sphere becomes completely covered with tin, then turning into facetted single crystals. No Sn extrusions were observed for tin spheres with diameters smaller than ∼100nm. Transmission electron microscopy and Auger electron spectroscopy investigations show that the tin spheres are covered with a few-nm-thick SnO skin and that the extrusions are single crystals.
For the computer simulations a model was developed which assumes that He⁺ ions generate interstitials ISn and vacancies VSn in the body-centered tetragonal lattice of tin. Due to the SnO skin, the ISn and VSn are confined to the tin sphere. A coherent Sn-SnO interface with a strong Sn-SnO interaction prevents ISn and VSn annihilation and void formation here. The projected range of 30 keV He⁺ ions is smaller than the diameter of the sub-μm spheres, the He accumulates and partly fills the VSn. Thus, the “pressure” of ISn increases steadily. Simultaneously, He⁺ ion erosion creates openings in the SnO skin. The sputter coefficient increases with the angle of incidence, thus openings in the SnO skin form at the equator regions first. Once the tin interstitials find such an opening in the SnO skin, they can escape from the interior of the Sn sphere and form an epitaxial, regular Sn lattice outside. Due to the high Sn-SnO bond strength the extruded tin wets the outer SnO surface.
The defect generation at He⁺ ion irradiation was simulated with TRI3DYN [1], a 3D program calculating atomic displacements in the binary collision approximation. The reaction-diffusion behavior of the ISn and VSn as well as their clustering into voids and growth to extrusions were simulated with a 3D kinetic lattice Monte Carlo program [2] using an RGL potential for tin. The simulated reaction pathway of the morphology agrees very well with the sequence of HIM images taken during He⁺ ion irradiation. A quantitative comparison of the extruded material with simulations provides conclusions on the defect kinetics under ion irradiation.

[1] Möller, Nucl. Instr. Meth. B 322 (2014) 23
[2] Strobel et al., Phys. Rev. B 64 (2001) 245422