Kinetic Evolution and Equilibrium Morphology of Strained Islands

Self-assembled SiGe islands grown on Si(001) leave behind characteristic “footprints” that reveal that small islands shrink, losing material to nearby larger islands. The critical size, dividing shrinking from growing islands, corresponds to the pyramid-to-dome shape transition, consistent with “anomalous coarsening.” While shrinking, $\{105\}$-faceted pyramids transform into truncated pyramids and ultimately into unfaceted mounds. The similarity to behavior during island growth indicates that island shape and facet formation are thermodynamically determined.

Figure 1

AFM images of SiGe islands obtained by deposition of Ge on Si(001) at (a),(c) 740 and (b),(d) 840 °C. Note color scale reflects surface slope in (a) and (b) to highlight shape, and shallow and steep facets can be distinguished. Color reflects height in (c) and (d), with scale chosen to highlight trenches. Islands at different stages of evolution are observed: truncated pyramids (TP), pyramids (P), transitional domes (TD), domes (D). In (c) and (d), empty trenches (ET) are also seen.

Figure 2

Statistical analysis of islands contained in the sample grown at 840 °C: (a) Shape (aspect ratio $r$) vs volume and (b) histogram of volumes. The area of (c) steep, (d),(f) $\{105\}$, and (e) shallow facets vs volume were computed from the “facet plot” [22] (with logarithmic intensity) shown in the inset of (c). They show that prepyramids (PP), truncated pyramids (TP), and pyramids Ps are shrinking, while TDs and domes (D) are growing, and that the critical size distinguishing shrinking and growing islands corresponds to the P-TD shape transition.

Figure 3

Sequence of AFM magnifications illustrating shrinking pyramids and the pyramid-to-prepyramid transition occurring with decreasing volume during the ripening process. Images have color scale according to local surface slope with respect to the (001) plane.

Figure 4

Statistical analysis of the correlation between the position of shrinking islands with respect to the surrounding trenches and the environment. The inset of (b) shows the definition of the angles $\alpha$ and $\beta$ on top of an AFM image of an island, which shrinks because of material transfer to the neighboring large island.