Semiconductor nanostructures are classically grown by the Stranski-Krastanow mechanism for which, beyond a critical thickness, islands are formed on a two-dimensional wetting layer. By increasing the Ge coverage, the shape of the islands changes from square pyramids having {105} facets to dome-shaped islands with {105}, {113}, {15 3 23} facets, and the top (001) facet [1].The growth of Ge islands on Si(001) substrates has been extensively investigated [2]. However, the kinetics of the island growth and of the shape transitions is still not completely understood. This calls for in situ investigation as a function of temperature.
The evolution of the size of crystallographic facets of Ge domes has thus been analysed during their growth on Si(001) by using in situ grazing-incidence small-angle X-ray scattering (GISAXS) [3]. From the evolution of the widths of intensity streaks in reciprocal space, the kinetics of the growth of {113} and {15 3 23} facets has been determined as a function of the deposited Ge thickness, θ. For different growth temperatures from 500°C to 650°C, the facet size is found to obey a θ^(1/3) dependence [4]. This θ^(1/3) reveals the rare coalescence of domes and the in average self-similarly growth of their {113} and {15 3 23} facets in the investigated temperature and Ge deposit ranges. The critical thickness for the pyramid-to-dome transition is determined. Increasing the temperature, the critical thickness is found to decrease as observed by Cimalla [5]. The pyramid-to-dome transition is thus thermally activated. At 650°C, {111} and {20 4 23} facets have been observed. This confirms the formation of barn shaped islands for temperatures higher than 600°C [6].
[1] Medeiros-Ribeiro et al, Science 279, 353 (1998).
[2] D. J. Smith et al, J. Cryst. Growth 259, 232 (2003).
[3] J. Stangl et al, Rev. Mod. Phys. 76, 725 (2004).
[4] G. Renaud et al, Science 300, 1416 (2003).
[5] M.-I. Richard et al, submitted to Phys. Rev. B (2007).
[6] V. Cimalla et al, Appl. Phys. Lett. 77, 10 (2000).
[7] M. Stoffel et al, Phys. Rev. B. 74, 155326 (2006).
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