1. Multi-Scale modelling of atomic layer deposition Presented by: Mahdi Shirazi Supervised by: Dr. Simon Elliott

2. Outline • Atomic layer deposition (ALD) • Description of ALD • Goals • Obstacles • Findings from atomic-scale modelling • Strategy for Kinetic Monte Carlo

3. What is ALD? • ALD is based on self-limiting surface reactions of two chemicals. For an oxide, a metal precursor & an oxygen precursor. • Process is cyclic : • Pulse of metal precursor - a monolayer of metal precursor molecules chemisorbs onto surface . • Purge - to remove unreacted precursor and by-products from chamber. • Pulse of oxygen precursor – to create a monolayer of chemisorbed oxygen precursor on surface • Purge – to remove unreacted precursor and by-products from chamber. www.isr.umd.edu/~hennlec/images/ALD/ALD_reaction_475.jpg The desired film thickness is reached by repeating the cycle. Typical growth per cycle is about 0.1 nm/cycle and cycle time is typically 1-4 s/cycle. 

4. Goals • Model interaction of precursors with surface and growth by ALD. • Go beyond the atomic length scale and the time scale of individual reactions. • Explain why amorphous or crystalline layers are deposited?

5. Obstacles • Reaction mechanisms consist of rare events. • Need to evaluate system beyond picosecond timescale. • Using density functional theory (DFT) is time consuming.(e.g. Cl-NEB calculation takes 3 hours for 300 atoms by 120 Intel-Xeon CPU). • To find reaction events: how efficient are Nudged Elastic Band (NEB), Conjugate Gradient (CG), quasi-Newton algorithms and ab initio Molecular Dynamics (MD)?  We should take advantage of Kinetic Monte Carlo (KMC) to describe ALD.

6. Outline • Atomic layer deposition • Findings from atomic-scale modelling • ALD reactions • Non-ALD reaction • Evaluating barriers by NEB • Importance of coordination number • Strategy for Kinetic Monte Carlo

7. Slab modelling • Growth of HfO2 from Hf(N(CH3)2)4 and H2O • Monoclinic structure is stable phase in low temperature. • Direction of growth (111) • Four layers have been regarded as slab • Extended surface 22 • We used hydroxylated surface • VASP code. Slab=yellow, oxygen=red, hydrogen=white VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html

8. Adsorption and dissociation of H2O at HfO2 surface1 • H2O dissociates at active Lewis acid and base sites at surface • Cover the surface with hydroxyl groups and water molecules • Rate of proton diffusion depends on coverage of OH. Hafnium =grey, oxygen=red, hydrogen=white 1-Charles B. Musgrave et al., Chem. Mater. 2006, 18, 3397-3403.

9. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

10. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

11. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

12. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

13. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

14. Sequence of ALD reactions Barriers were calculated by Cl-NEB1 Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey HfX4+2H2O 4HX+HfO2 X=N(CH3)2 • We used DFT2,3 to calculate activation energies to implement them into the  KMC • Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000 • VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html • Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K

15. Barrier to H+ diffusion to amide group Cl-NEB • Our calculations showed that H+ diffusion barrier varies between 0 to 2eV.

16. 3.71eV Barrier to H+ diffusion to amide group Cl-NEB • Test whether amide ligand can desorb without combining with H+? • No: barrier increases from 1.6 eV to 3.7 eV in absence of H+.

17. Discovery of reaction events(MD superior to optimisation) • Ligand transfer Densification1 show role of coordination number • A. Este`ve, M. Djafari Rouhani et al, J. Chem. Theory Comput. 2008, 4, 1915–1927

18. Non-ALD reaction(MD superior to optimization) • Ligand decomposition We find that activation energies are tuned by coordination number

19. Rate catalogue

20. Outline Atomic layer deposition Findings from atomic-scale modelling Strategy for Kinetic Monte Carlo1 Call reaction catalogue Stick to on-site KMC Tie rates to coordination number of atoms at surface Implementation of new application into the SPPARKS2 code in progress • Arthur F. Voter, Introduction to the Kinetic Monte Carlo Method • SPPARKS http://www.sandia.gov/~sjplimp/spparks.html

21. BKL algorithm1 • A. Bortz, M. Kalos, and J. Lebowitz, J. Comput. Phys. 17, 10 1975

22. Conclusions • New mechanisms of ALD reactions were found and quantified. • Ab initio MD superior to optimisation methods in identifying global basins. • Role of coordination number is important in growth of complex material. • Introduce new application for KMC.

23. Acknowledgement We are grateful for funding by Science Foundation Ireland under the FORME project, http://www.tyndall.ie/forme/ and acknowledge a generous grant of computing time from the SFI and HEA-funded Irish Centre for High End Computing (ICHEC). We also thank A. Esteve & M. D. Rouhani in LAAS and Steve Plimpton & Corbett Battaile at Sandia National Laboratory for their collaboration. Thank you for your attention