Ge nano-islands on the {111} planes

In Fig. A.3, the {111} facets fence the left and right sides of the (100) mesas appearing as the bright and dark bands in the image because of the derivative imaging processing. Three dimensional images Fig. A.4 and derivative images Fig. Fig. A.5 displays typical zoom-in images over these {111} side walls. The {111} facets have width (W111) of around 100, 180, 530, and 1190 nm for Fig. A.4(a)-Fig. A.4(d) and Fig.

A.5(a)-Fig. A.5(d), respectively. These islands ripen during growth, broadening the volume distribution. Presumably these islands consist of the SiGe alloy [69,70]. Similar to the (100) surface areas, nano-islands appear on the {111} facets once Ge coverage exceeds ~3–5 ML [70,71,72]. The average Ge thickness of about 15 ML is much larger than those of the wetting layers; therefore, the slight difference in the wetting layer thickness does not affect the nucleation behaviour observed herein.

On the large {111} facets such as in Fig. A.5 (c) and Fig. A.5(d), the nano-islands have a smaller average size and a higher number density (N111) compared to those (N100) on their neighbouring (100) facets. The equivalent thickness of the Ge growth obtained by integrating island volume per unit area is roughly the same for both the large {111} and (100) areas. Neglecting the difference in the critical thickness of their wetting layers, the Ge concentration of the wetting layers and islands, the effect of the finite radius of the AFM tip on the island sizemeasurement, and the scanning geometry difference on the two surfaces, this study estimated the deposition rates of Ge on the two facets to be roughly equal, while a similar study showed that the growth rate in {111} is about half of that in (100) [69]. Within the limitations of the AFM resolution, the nano-islands do not show facets as those observed in the molecular beam epitaxy at low rate [73]. This

morphology under similar growth conditions. The island number density N is roughly proportional to D^−1/3 at the same deposition rate, where D denotes the diffusion coefficient [74]. The higher island density indicates a smaller diffusion coefficient, that is, D111 ~D100/60 on the wetted layers of the two facets.

Fig. A.4 Zoom-in three-dimensional images over the {111} facet for the same samples,

Fig. A.5 Zoom-in derivative images over the {111} facet for the same samples, namely (a) Fig. A.3(a), (b) Fig. A.3(b), (c)Fig. A.3(c), and (d) Fig. A.3(d).

Compared to the central area in Fig. A.5(c) and Fig. A.5(d), N111 near the border of the (100) facets is noticeably smaller. In fact, nearly nuclei-free bands (known as the denuded zone) are clearly observable on both edges of the {111} facets in Fig. A.5(c);

their width (Wdz) is of the order of a few tens of nanometres. When W111 is compatible with Wdz, both the island density N111 and the average sizes of the nano-islands on the {111} facets decrease significantly, as shown in Fig. A.5(a) and Fig. A.5(b). In contrast, N100 displays little variation between the situations where it is near to and distant from the edges. As shown in Fig. A.6, similar island depletion zones on the {111} planes are also evident in the negative pyramid structure created on a square silicon oxide window on the same substrate as that in Fig. A.3(c).

The reduction in the island density and size on the {111} facets near their border with the (100) facets implies the loss of Ge adatoms on the {111} facets either to a good sink of adatoms at the boundary of the two facets or to the neighbouring (100) facet. Fig.

A.3 and their zoom-in images show that the (100) facet near the convex boundary contains nano-islands; however, few nucleated islands are visible above the convex boundary. Restated, preferential nucleation of nano-islands on the (100) facet near the {111} boundaries is not as evident as near the (100), (110) and a curved surface [75,60].

The convex edges between the {111} and (100) facets are not good sinks for Ge adatoms. Nevertheless, near the concave boundary (or the L-shape groove), however, preferential nucleation is slightly enhanced on the (100) plane, as shown in Fig. A.5(c).

The base areas of these nano-islands are on the (100) plane, indicating that they nucleate on the (100) plane and grow to contact with the V-groove. In comparison, Ge nano-islands preferentially grow on top of the V-grooves between two {111} family

can act as a local nucleation centre. The preferred nucleation can account in part for the depletion of Ge adatoms around the nearby {111} facets, assuming that the V-groove does not impose significant diffusion barrier. Additionally, the existence of the adatom sink on the V-groove and in the pits can lead to island depletion on the (100) surface [63]. However, a separate driving force is required for depleting Ge adatoms on the {111} facet near its convex edge, a location that lacks a good sink nearby. With no other driving forces, mass transport is driven by chemical-potential gradients associated with the wettinglayer thickness [76], that is, F = −

x Δ

μ

Δ . In addition, the diffusivity on the

{111} facet is smaller than that on (100), as discussed earlier. It can be concluded that a net flux of Ge adatoms from the {111} regions toward the (100) facets indicates that the effective chemical potential μ100 is smaller than μ111.

Fig. A.6 Three-dimensional AFM images showing a negative pyramid following Ge growth. The depletion zone is discernible on the edge of {111} facets near the (100) planes, but no such zone emerges on the border between the {111} facets. The scales

A3.4 Conclusion

Nanosized surfaces with well-defined sidewall facets provide opportunities for both new methods of growth-control engineering, and also fundamental understanding of the size-dependent crystal growth phenomena during nano-island formation. Various nanosized Si{111} facets bordered by (100) planes were fabricated and Ge nano-islands were simultaneously grown on the two facets. Nano-island formation was suppressed on the {111} facets as the width of the {111} facets reduced below ~500 nm at growth temperature 650℃. By excluding Ge adatom sinks near the convex border of the two facets, we conclude that the effective chemical potential for Ge adatoms on the {111} facets is smaller than that on the (100) facet, resulting in an adatom flux from the {111} facets to the (100) plane. Our results provide the first direct comparison of the adatom chemical potential of two wetting layers and its influence on the growth behaviour on the nanosized surfaces.

References

[1] T. C. Shen, Surf. Sci. 390, 35 (1997).

[2] C. Thirstrup, Surf. Sci. 411, 203 (1998)

[3] T. C. Shen, J. A. Steckel, and K. D. Jordan, Surf. Sci. 446, 211 (2000).

[8] J. A. Kubby, and J. J. Boland, Surface Science Reports, 26, 61 (1996) [9] X. Tong and R. A. Wolkow, Surf. Sci. 600, L199 (2006).

[10] T. Hallam, T. C. G. Reusch, L. Oberbeck, N. J. Curson, and M. Y. Simmons, J.

Appl. Phys. 68, 2187 (1990)

[11] A. Gross, M. Bockstedte, and M. Scheffler, Phys. Rev. Lett. 79, 701 (1997); E.

Pehlke and M. Scheffler, ibid. 74, 952 (1995).

[12] Dynamics of Gas-Surface Interactions, edited by C. T. Rettner and M. N. R.

Ashfold (The Royal Society of Chemistry, Cambridge, 1991).

[13] S. Ciraci and I. P. Batra, Surf. Sci. 178, 80 (1986).

[14] S. F. Shane, K.W. Kolasinski, and R. N. Zare, J. Chem. Phys. 97, 1520 (1992) [15] J. J. Boland, Adv. Phys. 42, 129 (1993), and references therein.

[16] See, for example, H. N. Waltenburg and J. T. Yates, Jr., Chem. Rev. 95, 1589 (1995), and references therein.

[23] J. E. Northrup, Phys. Rev. B 44, R1419 (1991).

[24] Y. J. Chabal and K. Raghavachari, Phys. Rev. Lett. 54, 1055 (1985).

[25] P. Nachtigall, K. D. Jordan, and C. Sosa, J. Chem. Phys. 101, 8073 (1994).

[26] M. C. Flowers, N. B. H. Jonathan, Y. Liu, and A. Morris, J. Chem. Phys. 99, 7038 (1993).

[27] A. Vittadini and A. Selloni, Chem. Phys. Lett. 235, 334 (1995).

[28] M. R. Radeke and E. A. Carter, Phys. Rev. B 54, 11803 (1996).

[29] S. K. Jo, B. Gong, G. Hess, J. M. White, and J. G. Ekerdt, Surf. Sci. Lett. 394, 162 (1997).

[30] A. R. Laracuente and L. J. Whitman, Surf. Sci. 545, 70 (2003).

[31] S. M. Gates, R. R. Kunz, and C. M. Greenlief, Surf. Sci. 207, 364 (1989).

[32] J. J. Boland, Phys. Rev. Lett. 67, 1539 (1991).

[33] D.-S. Lin and R.-P. Chen, Phys. Rev. B 60, R8461 (1999); Surf. Sci. 454, 196 (2000).

[34] Scientific Background on the Nobel Prize in Chemistry 2007, Chemical Processes on Solid Surfaces:

Laughlin, Z. Zhang, M. T. Schulberg, D. J. Gladstone, M. McGonigal, and S. T.

Ceyer, Phys. Rev. Lett. 74, 2603 (1995).

[38] J. A. Jensen, C. Yan, and A. C. Kummel, Phys. Rev. Lett. 76, 1388 (1996).

[39] R. S. Nord and J. W. Evans, J. Chem. Phys. 82, 2795 (1985).

[40] J. J. Boland, Science 262, 1703 (1993).

[41] D. Rioux, F. Stepniak, R. J. Pechman, and J. H. Weaver, Phys. Rev. B 51, 10981

[46] G. J. Xu, A. W. Signor, A. Agrawal, K. S. Nakayama, B. R. Trenhaile, and J. H.

Weaver, Surf. Sci. 577, 77 (2005).

[47] Gabor A. Somorjai, Introduction to Surface Chemistry and Catalysis (John Wiley

& Sons, New York, 1994).

[48] G. R. Darling and S. Holloway, Rep. Prog. Phys. 58, 1595 (1995).

[49] H. Doshita, K. Ohtani, and A. Namiki, J. Vac. Sci. Technol. A 16, 265 (1998).

[50] Y. Liu, D. P. Masson, and A. C. Kummel, Science 276, 1681 (1997).

[51] E. J. Buehler and J. J. Boland, Surf. Sci. 425 L363 (1999).

[52] T. C. Shen, C. Wang, G. C. Abeln, J. R. Tucker, J. W. Lyding, P. Avouris, and R.

E. Walkup, Science 268, 1590 (1995).

[53] X. Tong and R. A. Wolkow, Surf. Sci. 600, L199 (2006).

[54] M. McEllistrem, M. Allgeier, and J. J. Boland, Science 279, 545 (1998).

[55] M. Dürr, A. biedermann, Z. Hu, U. Höfer, and T. F. Heinz, Science 296, 1838 (2002).

[56] A. Bruce et al 2005 Quantum Dots: Fundamentals, Applications, and Frontiers (NATO Science Series vol 190) (Dordrecht: Springer), and reference therein [57] P. Michler, 2004 Single Quantum Dots: Fundamentals, Applications and New

Concepts (Berlin: Springer), and reference therein.

[58] E. S. Kim, N. Usami, and Y. Shiraki, Semicond. Sci. Technol. 14, 257 (1999).

[59] T. Kitajima, B. Liu, and S. R. Leone, Appl. Phys. Lett. 80, 497 (2002).

[63] Y. Suda, S. Kaechi, D. Kitayama, and T. Yoshizawa, Thin Solid Films 464/465, 190 (2004).

[64] A. Olzierski, A. G. Nassiopoulou, I. Raptis, and T. Stoica, Nanotechnology 15, 1695 (2004).

[65] Y. Y. Zhang, J. Zhang , G. Luo, X. Zhou, G. Y. Xie, T. Zhu, and Z. F. Liu,

324 (200) and references therein.

[68] H. J. Kim, Z. M. Zhao, J. Liu, V. Ozolins, J. Y. Chang, and Y. H. Xie, J. Appl.

Phys. 95, 6065 (2004).

[69] A. Hartmann, L. Vescan, C. Dieker, and H. Lüth, J. Appl. Phys. 77, 1959 (1994).

[70] F. Ratto et al, Appl. Phys. Lett. 84, 4526 (2004).

[71] P. D. Szkutnik, A. Sgarlata, N. Motta, and A. Balzarotti, Mater. Sci. Eng. C 23, 1053 (2003).

[72] F. Ratto, A. Locatelli, S. Fontana, S. Ashtaputre, S. K. Kulkarni, S. Heun, and F.

Rosei, Phys. Rev. Lett. 96, 96103 (2006).

[73] N. Motta, F. Rosei, A. Sgarlata, G. Capellini, S. Mobilio, and F. Boscherini, Mater. Sci. Eng. B 88, 264 (2002).

[74] Y. W. Mo, J. Kleiner, M. B. Webb, and M. G. Lagally, Phys. Rev. Lett. 66, 1998 (1991).

[75] T. I. Kamins, and R. S. Williams, Appl. Phys. Lett. 71, 1201 (1997).

[76] K. N. Tu, J. W. Mayer, and L. C. Feldman, 1992 Electronic Thin Film Science for Electronical Engineers and Materials Scientists (New York: Macmillan)