A systematic study of Sb sufactant-mediated relaxed SiGe graded layers has been performed. The results have been compared with those of relaxed SiGe layers grown at high temperatures, showing that the Sb-mediated SiGe graded layers have been significantly improved both in surface smoothness and in threading dislocation density. We have investigated the grading rate dependence on the resulting threading dislocation density and surface smoothness of as-grown Si0.5Ge0.5 buffer layer samples. With the use of Sb surfactant mediation, we have also fabricated high-quality Ge photo diodes, showing very low leaky current at the reverse bias at 1 V.