Figure 4.

a and b AFM micrographs of islands grown on planar Si(001) (5.5 ML at T_Ge = 700°C). The color scale represents the local surface slope with respect to (001). Every color corresponds to a certain facet of the SiGe domes (blue: {105}, yellow: {113}, red: {15 3 23}). c–f: Calculated energy level scheme for islands grown on pit-patterned Si(001) substrates without regions of Ge enrichment (d, f) and islands grown on planar Si(001) substrates containing Ge-rich regions (c, e) as described in the text. The stronger Ge composition gradient of the islands grown on planar substrates compared to the ones grown on pit-patterned substrates [7-9] leads to a confining potential at the top of the island. The strong shift of the island signal on planar substrates at moderate P_exc as observed in Figs. 1c, 2 and 3 is explained by the decreased density of states (indicated by the horizontal lines) in this confining potential