Computer Simulation of Sputtering & Collision Cascades in Ionic Materials

1. Computer Simulation of Sputtering & Collision Cascadesin Ionic Materials D Ramasawmy*, S D Kenny and Roger Smith Department of Mathematical Sciences * Email: madr@lboro.ac.uk

2. Sputtering: Definition, History & Applications. Computer Simulation of Sputtering of NaCl by impact with a Na+ ion. Use of DPMTA code‡ Order (N) to evaluate Coulombic interactions. Collision Cascades in NaCl. Outline ‡ DPMTA : http://www.ee.duke.edu/Research/SciComp/Docs/Dpmta/dpmta.html

3. Sputtering Definition • Sputtering is the removal of surface atoms due to energetic particle bombardment. • This is caused by collisions between the incoming particles and the atoms in the near surface layers of a solid. History • The first recorded observation of sputtering was made by W R Grove‡ 150 years ago. Applications • Sputtering is not just an unwanted effect which destroys cathodes and contaminates plasmas. • It is used in many modern industrial processes including surface cleaning and etching, thin film deposition, surface and surface layer analysis. ‡ W R Grove, Philosophical Transactions, vol 142, page 87, 1852.

4. MD Methodology • Target : a NaCl lattice • System size : 1944 particles • Impact particle : a Na+ ion at normal incidence with energy of 1 KeV • Fixed boundary conditions were taken along the sides while the top and bottom surfaces were free. • Several hundreds of trajectories with run-time of 2.0 ps were carried out for different impact positions to yield a good statistics. • A particle is considered sputtered if it is moving away from the surface and it has sufficient KE to overcome the electrostatic attraction.

5. MD Methodology • The electrostatic interactions were evaluated using a “brute force” method • The potential used was that of the Buckingham form as given by Catlow et al. ‡ • This potential as given was not suitable for modelling collisional phenomena. • The Na+ - Cl- potential was hardened using a screened coulomb potential ‡‡ to overcome the over attractive forces for small separation. ‡ C.R.A. Catlow, K.M. Diller and M.J.Norgett, J. Phys. C: Solid State Phys., 10, 1394 (1977). ‡‡ D. Ramasawmy, S.D. Kenny, Roger Smith, NIMB (2002).

6. Simulation • Due to the symmetry of the (100) surface of the NaCl lattice, we have considered only one quarter of the area of the surface unit cell. Cl- ion From the results, sputtering was observed to occur only for impacts concentrated around the Na+ ion and the Cl- ion. Furthermore, for the majority of impacts outside these regions, channelling was observed. Reduced Impact Zone Na+ ion

7. % 1st layer 2nd layer 3rd layer 5th layer Reflected Results • The overall sputtering yield was determined to be 0.36 with a variance of 0.01. • The total no of sputtered particles was almost evenly distributed between the 2 species [ 51% Na+ & 49% Cl-]. • A lower yield of sputtered particles was observed compared to similar impacts on metals. • The sputtered particles were classified into groups according to their kinetic energies and atomic types. • More low energy particles were ejected compared from metals and semi-conductors. • The origin of the ejected particles is summarised in the table. It shows a substantial contribution from subsurface layers. 76.5 6.5 3.3 3.5 4th layer 4.1 6.0

8. Further Results • The angular distributions show less structure representative than is typical for sputtering from metals and semi-conductors. • The majority of trajectories lead to only a small number of sputtered particles. • The ions are often seen to come off as NaCl dimers.

9. Movie of Sputtering • Example of a Computer Simulation of the Sputtering of NaCl by impact with a Na+ ion with 1 KeV at normal incidence.

10. We have observed that ionic materials show a number of characteristic differences from metals and semi-conductors. They are as follows: a) Lower ejection yields b) Larger contribution from subsurface layers c) Less well-defined angular distributions d) Large number of low energy ejected particles. There is a number of features that warrant further investigation, namely the effect of bombarding species, the crystal size and cluster formation. Discussion

11. DPMTA • DPMTA‡ (a Distributed Implementation of the Parallel Multipole Tree Algorithm) code developed at Duke University was implemented within our MD code. • DPMTA is based on the FMM (the Fast Multipole Method) and was originally developed by Greengard and Rokhlin‡‡. • This method is O(N) meaning that it is faster compared with the “crude” method which is O(N2) and which we used in our initial study. • We are now simulating bigger system sizes and this will enable us to study sputtering, collision cascades and other effects in more detail. ‡ DPMTA : http://www.ee.duke.edu/Research/SciComp/Docs/Dpmta/dpmta.html ‡‡ L. Greengard, V. Rokhlin, J. Comp. Phys. 82 (1997) 135

12. Collision Cascades We are currently doing some test simulations on Collision Cascades. Below is one example in which a Cl- ion about the centre of a NaCl lattice is given 250 eV along a certain direction. Temperature: 0 K; System Size : 5832 particles. Legend: Colours of Spheres Blue / Purple Interstitial (Cl- ion) Red Interstitial (Na+ ion) Brown Vacancy (Cl- ion) Grey Vacancy (Na+ ion) Acknowledgements: H Hurchand ( Collision Cascades )