Modified Stranski–Krastanov growth in stacked layers of self-assembled islands

In a stack of vertically aligned Stranski–Krastanov grown islands, the critical thickness for planar growth for all but the initial dot layer is reduced, if the thickness of the spacer layer $t_s$ is smaller than a certain value $t_0.$ We present structural and photoluminescence results on the basis of the extensively studied lattice-mismatched material system Si/Ge.