In a stack of vertically aligned Stranski-Krastanov grown islands, the crit
ical thickness for planar growth for all but the initial dot layer is reduc
ed, if the thickness of the spacer layer t(s) is smaller than a certain val
ue t(0). We present structural and photoluminescence results on the basis o
f the extensively studied lattice-mismatched material system Si/Ge. (C) 199
9 American Institute of Physics. [S0003-6951(99)04409-5].