On a clean Si(111)-7X7 surface, Ge grows in the Stranski-Krastanov mode. At 300-450 degrees C, relaxed islands begin to form when. the Ge growth is 6 monolayers (ML) thick. When Ge is grown on a Pb/Si(111)-root 3X root 3 surface, Pb segregates on the surface and agglomerates into two-dimensional islands. And the critical thickness for the formation of relaxed island becomes 4 ML. This change in critical thickness is explained by Pb reducing the surface energy or by an improved crystallinity of Ge layers (due to a simplified substrate surface reconstruction), or by both.