Growth of less than 2 monolayers Ge on a submonolayer amount of predeposited C on Si results in the formation of very small Ge quantum islands. In a photoluminescence study, we compare these C-induced Ge (CGe) dots with carefully chosen reference structures incorporating the same total amount of C and Ge but with different deposition orders and with varied C distribution below the Ge islands. Our investigations imply that the special combination of pregrown low surface mobility C and post-grown high surface mobility Ge constitutes a distinct microstructure within the SiGeC material system, causing dot formation at a very early stage and showing particularly intense photoluminescence signal. Moreover, structures combining CGe dots withSi1−yCyquantum wells are well explained by the model of spatially indirect type-II recombination within the CGe islands. ©1998 American Institute of Physics.