The initial stage of amorphous Si and Si0.7Ge0.3 deposition on SiO2 by low-pressure chemical vapor deposition was investigated in the temperature range from 375 to 450degreesC. Both amorphous Si and Si0.7Ge0.3 layers with a nominal thickness of about 5 nm did not form a continuous layer. Rather, the layers grew by island formation. The results also show that the surface roughness of amorphous Si and Si0.7Ge0.3 films increases with decreasing temperature at a similar nominal film thickness. The formation of islands and the dependence of the surface roughness of amorphous Si and Si0.7Ge0.3 films on deposition temperature are explained on the basis of the nucleation and growth behavior of thin films with respect to the thermodynamic driving force of nucleation and the mechanism of adatom attachment.