Atomic-Scale Pathway of the Pyramid-to-Dome Transition during Ge Growth on Si(001)

By high resolution scanning tunneling microscopy, we investigate the morphological transition from pyramid to dome islands during the growth of Ge on Si(001). We show that pyramids grow from top to bottom and that, from a critical size on, incomplete facets are formed. We demonstrate that the bunching of the steps delimiting these facets evolves into the steeper dome facets. Based on first principles and Tersoff-potential calculations, we develop a microscopic model for the onset of the morphological transition, able to reproduce closely the experimentally observed behavior.

Figure 1

Plot of the aspect ratio vs volume for Ge islands on Si(001) obtained in experiment ${\mathrm{E}\mathrm{x}\mathrm{p}}_B$. Different symbols refer to islands at various stages of the pyramid-to-dome transition, as illustrated by representative STM images. Similar results are obtained for ${\mathrm{E}\mathrm{x}\mathrm{p}}_A$.

Figure 2

STM topographs illustrating the pathway of the pyramid-to-dome evolution. Images in panels (a)–(c) correspond to ${\mathrm{E}\mathrm{x}\mathrm{p}}_A$, while the three bottom panels are taken from ${\mathrm{E}\mathrm{x}\mathrm{p}}_B$. The image gray scale corresponds to local surface slope with respect to the (001) plane (dark areas are steeper). Arrows in (c) point at two of the eight {15 3 23} facets composing the dome surface.

Figure 3

(a) Energy per atom vs $h/L$ at different depths, as computed by Tersoff-potential calculations for $L=27\mathrm{n}\mathrm{m}$ (full curves with small, solid circles). For comparison, the energy per atom is also plotted for $d=5\AA$ and $L=16\mathrm{n}\mathrm{m}$ (full curve with large, solid squares). The energy per atom typical of bulk Ge and of Ge tetragonally strained at the Si lattice parameter are indicated. (b) In-plane average lattice parameter $a_{\parallel }$ vs $h/L$ for different depths, and in the presence or not of Si ($35\%$) intermixed into the Ge pyramid.

Figure 4

(a) Geometry of the configuration $S_N$, characterized by $N$ steps bunched together. (b) Energy difference between the flat-facet state ($S_0$) and the stepped state ($S_N$) plotted as a function of the pyramid base $L$ and of the number of steps $N$ for $\Gamma =12\mathrm{m}\mathrm{e}\mathrm{V}/\AA$.