One-dimensional Ostwald Ripening on Island Growth

1
One-dimensional Ostwald Ripeningon Island
Growth

• An-Li Chin (???)
• Department of Physics
• National Chung Cheng University
• Chia-Yi 621
• Taiwan, ROC
• Prof. Fu-Kwo Men (???)
• Prof. Chin-Rong Lee(???)

2
Outline

• Introduction
• Growth modes
• Experimental setup
• Our works
• Nucleation and growth of islands
• Selective growth
• Coalescence of islands
• 1-D island ripening
• Conclusion

3
RT-Scanning Tunneling Microscopy
4
Substrate structure
1010nm2
(52)
5
Growth modes
?F
f
?S
?F/S
The force equilibrium can be written as ?S
?F/S ?F cosf
f the island wetting layer ?S the surface
tension of the substrate ?F/S the inter-surface
tension of the film/substrate ?F the surface
tension of the film substrate
?S ? ?F/S ?F (layer-by-layer) ?S lt ?F/S
?F (island growth))
6
Growth of Cobalt on clean Si(111)
620?
0.1ML
0.3ML

• (v7 v7)
• structure.
• Steps of double bi-
• layer height
• transformed to
• single bi-layer
• height.
• CoSi2 islands
• emerging
• at Co coverages
• above 0.3 ML.

500 Å 500 Å
1000 Å 1000 Å
(Å)
0
7
Cobalt on Si(111)-5 2/Au
0.1ML
0.3ML

• Islands are formed
• on surface with only
• 0.1ML Co deposition.

6000 Å 6000 Å
600?
600?
0.5ML
0.5ML
2000 Å 2000 Å
500?
700?
8
Surface structure vs. Au coverage
v3 v3
( 4 )
9
Controlled structural change via Au deposition
700?
630?
500 Å 4000 Å
(7 7)
(5 2)
(5 2)
(7 7)
2000 Å 2000 Å
240 Å 240 Å
240 Å 240 Å
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The selective island growth

• Islands grow
• only on (5 2)
• terraces.
• No islands grows
• on (v7 v7)
• terraces up to 0.3
• ML of Co.
• The island is
• consisted of Si
• and Co atoms.

v7 v7
500 Å 500 Å
5 2
4000 Å 4000 Å
12000 Å 4000 Å
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Growth Scheme

• I. Depositing Au onto a nominally flat
  Si(111)-(7?7) surface to induce a (5?2)
  reconstruction. (Au coverage 0.443 ML)
• II. Depositing Co onto the Si(111)-(5?2)/Au
  surface at
• room temperature (A disordered surface
  results.)
• III. Observing surface morphological change as a
  function
• of sample heating time.

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Coalescence of islands
30 sec
210 sec
90 sec
900 sec
510 sec
330 sec
200?200nm2
With islands on terrace decreasing gradually in
size, atoms diffuse away from edges of terrace
islands and feed the growth of islands at step
edges.
0.5 ML Co on Si(111)-(5?2)/Au at RT followed by
620?C heating
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Islands on step edge and terrace
terrace
step edge
Heating for 30sec
Heating for 90sec
Heating for 210sec
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Relative populations of two types of islands
Most islands appear at step edges at late stage
of ripening process. (note that the number
density of the islands at step edges decreases as
well.)
15
Conservation of sum of island volume
Total island volume is conserved during the
ripening process.
16
Average island size vs. growth time
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Ripening growth
Gibbs-Thomson effect
low
high
diffusion length
2D-adatom gas
18
Overview of clustering
aggregation
nucleation
late stage growth
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Model for island ripening 1/2
I.M. Lifshitz and V.V. Slyozov (1958)
Consider the adatom diffusion among neighboring
islands resulting from the chemical potential
differences in islands of different sizes, the
change in island radius, r, can be expressed as
r gt rcr, island grows r lt rcr, island shrinks
where rcr is some critical grain radius. A grain
in the solution grows (shrinks) if its radius is
larger (smaller) than rcr. D is the diffusion
coefficient and the S size of the region involved
in the adatom exchange process, the concentration
of the solution, the grain surface energy per
unit area, and the molar volume of the dissolved
material.
(i)
(1)
(ii)
where W has the width of a step if the diffusing
atoms are confined to move along step edges.
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Model for island ripening 2/2
Let f(r, t) be the number distribution function
of island with radius r at time t, from the
equation of continuity we have
(2)
With the constraint that the number of adatoms on
the surface is conserved, we solve equations (1)
and (2). The results are
rcr(t) ? t 01/4 N(t) ? t -3/4
rcr(t) ? t 1/5 N(t) ? t -3/5
(i)
(ii)
rcr(t) ? t 0.201 N(t) ? t -0.55
(Experimental results )
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Island distributions vs. time
Average island density
Average island height
Slope 0.2
Slope -0.55
? Island density decreases as time to the -0.55
power. ? Island height increases as time to the
0.2 power. (Island shape independent of island
size.)
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Diffusing species diffusion pathway
Diffusing species must
1. escape from islands on terraces 2. diffuse
toward step edges, which act as sinks 3. diffuse
along step edges (rate-limiting) 4. attach to
islands at step edges followed by edge diffusion.
Single bi-layer-height step (3.1 Å)
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Conclusion

• We have demonstrated the self-selective growth
  of CoSi2 islands with narrow size distribution on
  only one of the two domains by depositing up to
  0.3 ML of Co.
• We have observed a unique 1D diffusion process
  leading to the growth of step-edge islands at the
  expense of terrace islands.

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Island distribution