Multiple layers of self-asssembled Ge/Si islands: Photoluminescence, strain fields, material interdiffusion, and island formation

Strain fields in stacked layers of vertically aligned self-assembled Ge islands on Si(100) can cause a reduction of the wetting layer thickness in all but the initial layer and hence induce an energy separation $\Delta E_{\mathrm{wl}}$ between the energy transitions of the different wetting layers. Our systematic photoluminescence (PL) study on twofold stacked Ge/Si layers shows that the quantity $\Delta E_{\mathrm{wl}}$ is a sensitive function of the Si spacer thickness and reflects the degree of strain field interaction between the island layers. Pronounced PL blueshifts are also observed for the island related energy transition in twofold and multifold island layers. We suggest that with increasing number of stacked island layers strain field superposition of buried islands causes enhanced SiGe material intermixing during Si overgrowth of the islands. This effect naturally explains the strong PL blueshift of the island related energy transition. Recently observed shape transformations in stacked Ge islands are well explained by our model of superimposed strain fields. We also discuss the initial stages of island formation in the second Ge layer of twofold island stacks. Many of the effects observed in this paper on the Ge/Si system are probably also important for self-assembling III/V islands but due to extremely small sizes are much harder to evaluate.