1. One-dimensional Ostwald Ripening on Island Growth An-Li Chin ( 秦安立 ) Department of Physics National Chung Cheng University Chia-Yi 621 Taiwan, ROC Prof. Fu-Kwo Men ( 門福國 ) Prof. Chin-Rong Lee ( 李進榮 )

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 (7×7) 24×24nm 2 (5×2) 10×10nm 2

5. The force equilibrium can be written as γ S = γ F/S + γ F cos φ φ : the island wetting layer γ S : the surface tension of the substrate γ F/S : the inter-surface tension of the film/substrate γ F : the surface tension of the film substrate γ S ≧ γ F/S + γ F (layer-by-layer) γ S ＜ γ F/S + γ F (island growth)) Growth modes γSγS γ F/S γFγF φ

6. Growth of Cobalt on clean Si(111) 0.1ML 0.3ML (√7 × √7) structure. Steps of double bi- layer height transformed to single bi-layer height. CoSi 2 islands emerging at Co coverages above 0.3 ML. 500 Å × 500 Å 1000 Å × 1000 Å 0 (Å) 620 ℃

7. Cobalt on Si(111)-5 × 2/Au 0.1ML0.3ML 0.5ML Islands are formed on surface with only 0.1ML Co deposition. 6000 Å × 6000 Å 500 ℃ 700 ℃ 600 ℃ 2000 Å × 2000 Å

8. (7×7) 24×24nm 2 (5×2) 36×36nm 2 Surface structure vs. Au coverage √3× √3 ( 4° )

9. Controlled structural change via Au deposition 2000 Å × 2000 Å (5 × 2)(7 × 7) 240 Å × 240 Å 500 Å × 4000 Å (7 × 7) (5 × 2) 700 ℃ 630 ℃

10. 12000 Å × 4000 Å Islands grow only on (5 × 2) terraces. No islands grows on (√7 × √7) terraces up to 0.3 ML of Co. The island is consisted of Si and Co atoms. The selective island growth 4000 Å × 4000 Å 500 Å × 500 Å √7 × √7 5 × 2

11. 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.

12. Coalescence of islands 0.5 ML Co on Si(111)-(5  2)/Au at RT followed by 620  C heating 30 sec210 sec90 sec 900 sec510 sec 200  200nm 2 330 sec 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.

13. Islands on step edge and terrace Heating for 30sec Heating for 90sec Heating for 210sec terrace step edge

14. 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

17. 2D-adatom gas diffusion length low high Ripening growth Gibbs-Thomson effect:

18. Overview of clustering nucleation aggregation late stage growth

19. Model for island ripening 1/2 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 I.M. Lifshitz and V.V. Slyozov (1958) where r cr is some critical grain radius. A grain in the solution grows (shrinks) if its radius is larger (smaller) than r cr. 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. (1) where W has the width of a step if the diffusing atoms are confined to move along step edges. r > r cr, island grows r < r cr, island shrinks (i) (ii)

20. Model for island ripening 2/2 With the constraint that the number of adatoms on the surface is conserved, we solve equations (1) and (2). The results are r cr (t)  t 1/5 N(t)  t -3/5 (Experimental results: ) 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) r cr (t)  t 0.201 N(t)  t -0.55 r cr (t)  t 01/4 N(t)  t -3/4 (i) (ii)

21. Island distributions vs. time  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.) Slope = -0.55 Slope = 0.2 Average island density Average island height

22. Diffusing species diffusion pathway Single bi-layer-height step (3.1 Å) 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. Diffusing species must

23. Conclusion We have demonstrated the self-selective growth of CoSi 2 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.

24. Island distribution