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Motivations

• In adatoms as a tracer of vacancy selfdiffusion
• (R. van Gastel et al. Nature (London) 408
  (2000))
• (R. van Gastel et al. Phys. Rev. Lett. 86
  (2001))
• In adatoms foster the layers by layer selfgrowth
  on Cu(100)
• (H.A. van der Vegt et al., Phys. Rev. B 51,
  14806 (1995))

No ab initio calculations available
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Simulation details

• Ab initio - density functional theory (DFT)
• Electron-ions interaction US-PP
• Code VASP using PAW, supercells
• NEB for diffusion barriers
• Brillouin zone summation
• Monkhorst Pack 8x8x1
• Methfessel-Paxton, ?0.2 eV
• Slabs 3x3x545 atoms, 5x5x5125 atoms (100)
• 5x4x3 for 110-step and 6x6x3 for 100-step
• all layers (exc. bottom) relaxed, vacuum region
  10 Å

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In adatom adsorbtion on Cu(100)

• Indium adsorbtion Hollow vs Substitutional

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In segregation
As any good surfactant, In floats (dE1.1 eV)...
Also, low Insub-Cuad exchange activation of 0.46
eV (220 K)
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Vacancy formation energy and diffusion

• Eform of a vacancy beside an embedded In atom is
  0.37 eV,
• i.e 0.12 eV less than in a clean terrace !!
• gtIndium-Vacancy complex
• is formed

• Extremely small barrier for
• In Vac exchange just 0.07 eV (0.24 eV via EAM,
  Van Gastel et al. 2001)

• Cu Vac. barriers in proximity of an embedded
  In are lower, between 0.28 eV and 0.41 eV

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In-Vac correlated motion
vac
Ediff 0.41 eV
Both formation and diffusion activation energies
are consistent with experiment (notice
improvement due to entropy)
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Vacancy generation at surface kinks

• Barrier for generation via direct expulsion of
  surface Cu 1.72 eV ! ... but ....

Generation at 110-kink step
Generation at 100-step kink
Eatt0.69 eV
Eatt0.7 eV
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Indium incorporation at steps
(STM picture by R. van Gastel et al. Nature
(London) 408 (2000))
No barrier against In incorporation at kink
vacancy ! according to our ab initio calculations
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Insubst-Cuadat Interactions
Indium fosters layer by layer Cu(100) selfgrowth
(surfactant).
Why?
First step gtcheck Insubst Cuadat interactions
Indium-Adatoms interaction is slightly repulsive
0.1eV
Qualitatively similar to the interaction of Sb
(surfactant) with Ag adatoms in Sb/Ag(111)
Is the surfactance understandable similarly in
the two systems ?
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Summary

• Surface-substituted In favored over In at surface
  hollow or bulk typical behaviour of a
  surfactant
• Cu-Insost exchange barrier is not high
  (0.46 eV) gt In
• segregation relatively inexpensive
• first-principle calculations support the
• a) vacancy-assisted In diffusion in the form of
  In-V complex
• b) vacancy-assisted In incorporation at steps
• c) Cu nucleation at Inads
• Insub Cuad interactions remind Sbsub-Agad on
  Sb/Ag(111)
• but smaller and shorter-range.

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Acknowledgements
Project MIUR-FIRB 01 Microscopic bases of thin
film morphology SLACS-HPC Cluster at CASPUR
Roma INFM Supercomputing at CINECA Bologna
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Indium as a surfactant
(R. van Gastel PhD thesis Leiden Univerisity
2001)
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Indium as a vacancy tracer ?
(R. van Gastel et al. Nature (London) 408 (2000))
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Cu adatom diffusion on Cu(100) by NEB
2 Diffusion processes are considered
Jump
Ediff 0.55 eV
Cu-Cu exchange
Ediff 0.95 eV via NEB
Experimental Barrier 0.40 eV !!!
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Incorporation of In atoms

• Nucleation
• Eleg-0.27 eV gt

lt Eleg-0.60 eV

• Exchange
• Eatt 0.86 eV
• Tatt gt400 K

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Insubst-Insubst interaction (eV)
-0.05
-0.025
-0.02
0.13
-0.03
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Mass transport due to vacancy diffusion in Cu(100)