Molecular Dynamics Study of Ballistic Molecular Dynamics Study of Ballistic Rearrangement of Surface Atoms During Rearrangement of Surface Atoms During

Ion Bombardment on Pd(001) SurfaceIon Bombardment on Pd(001) Surface

Sang-Pil Kim and Kwang-Ryeol Lee

Computational Science CenterKorea Institute of Science and Technology, Seoul, Korea

spkim@kist.re.kr (S.-P. Kim), krlee@kist.re.kr (K.-R. Lee)

Ion Bombardment (Sputtering)Ion Bombardment (Sputtering)

Deposition Film coating

Ion bombardmentIon bombardment Surface treatmentSurface 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.

ApplicationsApplicationsChaudhari 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

De

po

sit

ion

Sp

utt

eri

ng

Moroni et al., PRL 91, 167207 (2003)

Manipulating magnetism

Catalytically active surface

Manipulating film texture

Theoretical ApproachTheoretical ApproachSigmund theorySigmund 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)10keV Ar on Au(001)

*Total simulation time: 41 ps

32.64×32.64×20.4 nm3 Au substrate (1.28 mil. atoms)

Initial 2.5 ps 9.5 ps

15 ps 22 ps Final

RelativeHeight

Simulation movie http://diamond.kist.re.kr/CSC

Sputtering Process Erosion + Rearrangement

Rearrangement EffectRearrangement Effect

10 keV Ar ion impacts on Au(001)

crater with rimrecurving atoms

recurving cluster

Rearrangement atoms

Research StrategyResearch StrategyTo understand formation mechanism of surface patterning during ion bombardments

Direct observation of atomic scale behaviorDirect 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 ProcedureComputational Procedure

Pd(001)

0.5, 1.0, 2.0 keV

0, 30, 45, 60, 75°

Materials

Polar Angle (θ)

Incident Energy

45° ([110] direction)

ArIon

Azimuthal Angle (Φ)

ZBLAr

LJ potential

Pd

EAM + ZBL

Inter-atomic potential

-Substrate temperature: 300K-LAMMPS code (http://lammps.sandia.gov)

Simulation geometry on Pd(001) surface

Simulation Conditions

Sputtering vs. Rearrangement YieldSputtering vs. Rearrangement Yield

Yrearrangement >> Ysputtering

Various for the incident angle

Ratio = 2.8±0.5

Yields Ratio

Rearrangement DistributionRearrangement 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 EvolutionSurface Structure Evolution15.56×15.56 nm2, 4,200 (17.34 ions/nm2) 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 <100> and <010> direction.

Initial surface height

ComparisonComparison0.5 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, 246104 (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.

ConclusionConclusion