Facetting Morphology of Crystallites under Ion Irradiation – a 3D Kinetic Lattice Monte Carlo Study

The orientation-dependent surface energies of crystal facets determine the shape of crystallites via Wulff’s theorem. Thus, in the framework of a nearest neighbor Ising model, the equilibrium shape of a fcc crystallite is a truncated octahedron. This crystallite shape is found frequently, e.g. for A-type CoSi2 crystallites embedded in silicon.
In this contribution we show that crystallites change their shape under intense ion irradiation. A first indication for a crystallite shape change under ion irradiation was reported several years ago [1]. In a Monte Carlo simulation the continuing vacancy production transformed the equilibrium fcc crystallite shape of a truncated octahedron into the steady-state shape of a cube. Here we present a systematic study of ion-irradiation-induced crystallite shape changes for different crystal structures and different irradiation intensities and temperatures. For the first time, an explanation of the shape changes based on the surface defect kinetics will be given: Whereas in thermodynamics the equilibrium concentration of surface defects is determined by energetics (“detailed balance”), under ion irradiation its steady-state value is controlled by the geometry of atomic displacements. Consequently, facets with a high atomic area density become unstable. It will be shown that this facet instability might be the reason for temperature dependent pattern formation on metal surfaces under ion erosion [2].
[1] P. Bellon, Phys. Rev. Letters 81, 4176 (1998).
[2] U. Valbusa et al., J. Phys.: Condens. Matter 14, 8153 (2002).