Strong dependency of reaction rates on coordination number of
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Transcript Strong dependency of reaction rates on coordination number of
Multi-Scale modelling of
atomic layer deposition
Presented by: Mahdi Shirazi
Supervised by: Dr. Simon Elliott
Outline
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Atomic layer deposition (ALD)
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Description of ALD
Goals
Obstacles
Findings from atomic-scale modelling
Strategy for Kinetic Monte Carlo
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 :
1.
Pulse of metal precursor - a
monolayer of metal precursor
molecules chemisorbs onto surface .
2.
Purge - to remove unreacted
precursor and by-products from
chamber.
3.
Pulse of oxygen precursor – to
create a monolayer of chemisorbed
oxygen precursor on surface
4.
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.
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?
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.
Outline
• Atomic layer deposition
• Findings from atomic-scale modelling
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ALD reactions
Non-ALD reaction
Evaluating barriers by NEB
Importance of coordination number
• Strategy for Kinetic Monte Carlo
Slab modelling
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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 22
We used hydroxylated surface
VASP code.
Slab=yellow, oxygen=red, hydrogen=white
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VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
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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.
1-Charles B. Musgrave et al., Chem. Mater. 2006, 18, 3397-3403.
Hafnium =grey, oxygen=red, hydrogen=white
Adsorption and dissociation of H2O
at HfO2 surface1
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Hafnium =blue, oxygen=red, hydrogen=white, Nitrogen= darkblue, carbon=grey
Sequence of ALD reactions
Barriers were calculated by Cl-NEB1
HfX4+2H2O 4HX+HfO2 X=N(CH3)2
We used DFT2,3 to calculate activation energies to implement them into the KMC
1.
Graeme Henkelman, Hannes Jo´nsson et al J. Chem. Phys.113, 22, 9901 2000
2.
VASP: http://cms.mpi.univie.ac.at/vasp/vasp/vasp.html
3.
Entropy calculated by TURBOMOLE http://www.turbomole.com/ T=500K
Barrier to H+ diffusion to amide group
Cl-NEB
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Our calculations showed that H+ diffusion barrier varies
between 0 to 2eV.
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+.
Discovery of reaction events
(MD superior to optimisation)
Densification1
Ligand transfer
show role of coordination number
1.
A. Este`ve, M. Djafari Rouhani et al, J. Chem. Theory Comput. 2008, 4, 1915–1927
Non-ALD reaction
(MD superior to optimization)
• Ligand decomposition
We find that activation
energies are tuned by
coordination number
Rate catalogue
Reactions
Barrier (eV)
HfX4(g)HfX4(s)
-
HfX4(s)HfX3(s)
2.80
HfX3(s)HfX2(s)
2.96
Densification
0<0.5
HfX2(s)HfX1(s)
1.29
HfX1(s)HfX0(s)
1.64
HfX0(s)HfX1(s)
0.69
HfX1(s)HfX2(s)
2.24
HfX2(s)HfX3(s)
5.26
HfX3(s)HfX4(s)
4.92
Ligand decomposition
<0.5
Ligand transfer
<0.5
H2OOH-+H+
Depends on coordination number
of hafnium atoms at surface
H+ diffusion to amide group
0-2eV
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
1.
2.
Arthur F. Voter, Introduction to the Kinetic Monte Carlo Method
SPPARKS http://www.sandia.gov/~sjplimp/spparks.html
BKL algorithm1
1.
A. Bortz, M. Kalos, and J. Lebowitz, J. Comput. Phys. 17, 10 1975
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.
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