When Si layers were grown on Ge(100) by alternatingSiH2Cl2gas exposure and irradiation with synchrotron radiation, bistable growth occurred at temperatures (i) below 400 °C (0.21 Å/cycle) and (ii) between 470 and 530 °C (0.42 Å/cycle). The growth rate saturated for bothSiH2Cl2exposures and synchrotron-radiation (SR) irradiation time. In regime (i) chemisorption and photon-stimulated desorption of H and Cl atoms on the Si overlayer resulted in Stranski–Krastanov-type growth. In regime (ii) chemisorption on a bare Ge surface, removal of the ligands by photothermal desorption, and enhanced migration of Si adatoms resulted in Volmer–Weber-type growth. Island size increased with increasing SR irradiation time in the exposure-irradiation cycle. ©1997 American Institute of Physics.