A mechanism of interface blocking was proposed to reduce the threading dislocations in the SiGe and Ge layers on Si(100) substrates. In this work, epitaxial Si1-xGex/Si1-(x-y)Gex-y and Ge/SiyGe1-y layers were grown by UHV/CVD. It was surprisingly found that if the variation of the Ge composition, y, across the interface of Si1-xGex/Si1-(x-y)Gex-y or Ge/SiyGe1-y is higher than a certain value, most of the threading dislocations appear to be blocked and confined in the underlying Si1-(x-y)Gex-y or SiGe1-y layer by the interface. It implies that this finding can provide a simple way to grow high-quality relaxed SiGe and Ge layers on the Si substrates. (C) 2004 American Vacuum Society.