Molecular Dynamics Study of Ballistic Rearrangement of Surface Atoms During Ion Bombardment on Pd(001) Surface Sang-Pil Kim and Kwang-Ryeol Lee Computational Science Center Korea Institute of Science and Technology, Seoul, Korea (S.-P. Kim), (K.-R. Lee)

Ion Bombardment (Sputtering) Deposition  Film coating Ion bombardment  Surface treatment Science, 285, 1551 (1999). -Most works utilized sputtering as deposition tool are focused on the reaction phenomena between recoiled ions from the target and substrate atoms to deposit. -Since a possibility to manufacture the nano patterns on the surface was introduced, it has taken an enormous attention to researchers. -Such a peculiar process resulted in enhancing the possibility for designing nano sized patterning by cheap and simple method.

Applications Chaudhari et al., NATURE 411, 56 (2001) Ordered adsorption of large molecules Facsko et al., SCIENCE, 285, 1551 (1999) Optoelectronic devices Azzaroni et al., APL 82, 457 (2003) Molding templates Deposition Sputtering Moroni et al., PRL 91, (2003) Manipulating magnetism Catalytically active surface Manipulating film texture

Theoretical Approach Sigmund theory Agreement: ripple formation/ orientation Disagreement: in-plane ordering, wavelength coarsening Toward improvement  Nonlinear terms considered  New terms included to the equation (ex. shadowing effect, surface anisotropy…) * P. Sigmund, Phys. Rev. 184, 383 (1969). Incident energy spreading: Gaussian Local correction to the uniform flux due to non-flat geometry Normal erosion velocity at O Considering the surface diffusion to reduce surface area  Bradley-Harper (BH) theory Fundamentally based on the “Negative Deposition” concept

10keV Ar on Au(001) *Total simulation time: 41 ps 32.64×32.64×20.4 nm 3 Au substrate (1.28 mil. atoms) Initial2.5 ps9.5 ps 15 ps22 psFinal Relative Height Simulation movie 

Sputtering Process  Erosion + Rearrangement Rearrangement Effect 10 keV Ar ion impacts on Au(001) crater with rim recurving atoms recurving cluster Rearrangement atoms

Research Strategy To understand formation mechanism of surface patterning during ion bombardments  Direct observation of atomic scale behavior  Quantitative analysis Molecular Dynamics (MD) Simulation - Massive MD - EAM+ZBL interatomic potential - MD statistics (1,000 individual calculations) - Auto-correlation function

Computational Procedure Pd(001) 0.5, 1.0, 2.0 keV 0, 30, 45, 60, 75 ° Materials Polar Angle ( θ ) Incident Energy 45° ([110] direction) Ar Ion Azimuthal Angle (Φ) ZBL Ar LJ potential Pd EAM + ZBL Inter-atomic potential -Substrate temperature: 300K -LAMMPS code ( Simulation geometry on Pd(001) surface Simulation Conditions

Sputtering vs. Rearrangement Yield  Y rearrangement >> Y sputtering  Various for the incident angle  Ratio = 2.8±0.5 YieldsRatio

Rearrangement Distribution Pd X: impact point 0.5keV Ar [100] [110] Beam dir. -Symmetric but anisotropic distribution could be obtained at normal incidence. -In the case of 30° and 45°, rearrangement atoms were accumulated in front of the impact point along the beam direction. -In the case of more than 60°, the atoms moved beside the beam direction.

Surface Structure Evolution 15.56×15.56 nm 2, 4,200 (17.34 ions/nm 2 ) Ar bombardments Atomic Configuration 2D Auto-correlation function -To confirm the rearrangement effect on the formation of surface patterns, many Ar atoms were bombarded on the Pd(001). -The correlation image is a little distorted diamond shape along and direction. Initial surface height

Comparison 0.5 keV Ar on Pd(001) with normal direction Experimental Result*4,200 Bombarding Result Rearrangement Distribution 2D auto-correlation function *T.C. Kim et al., PRL 92, (2004). -2D auto-correlation function after 4,200 bombarding is in consistent with an experimental result. -4 fold anisotropic surface pattern results from the accumulation of the rearranged atoms of anisotropic lateral distribution.

MD simulation shows that ballistic rearrangement of the surface atom by ion bombardment plays an important role in the surface structure evolution. Present simulation of Ar ion bombardment on Pd(001) surface demonstrates the formation of 4 fold symmetric patterns which is in good agreement with the previous experimental observation. Existing kinetic models which are based on the negative deposition concept should be revised to consider this effects. Conclusion