Ge surface segregation on Si(100) and Si(111) substrates during silicon molecular beam epitaxy is studied as a function of growth temperature. During growth, the Ge atoms are partly incorporated into the growing layer and the residuals segregate on the grown surface. The incorporation coefficient cannot be expressed in terms of a simple thermal activation process. That is, the Ge segregation phenomena increase to maxima at around 450°C for the Si(100) substrates and at around 650°C for the Si(111) substrates. Segregation decreases above these temperatures. These reverse temperature dependences enable not only good crystallinity of the heteroepitaxial layer, but also abrupt heterointerfaces. The incorporation coefficients for Si(111) are much larger than those for Si(100), which is explained by a model based on surface migration.