Strained Si device with first SiGe layer with higher Ge concentration being relaxed to have substantially same lattice constant as second SiGe layer with lower Ge concentration

A semiconductor device comprising

• a first Si1−αGeα film, a first cap film, a second Si1−βGeβ film (β<α≦) and a second cap film sequentially formed in this order on a substrate whose surface comprises silicon,
• wherein the first Si1−αGeα film is relaxed to have substantially the same lattice constant as that of the second Si1−βGeβ film in a horizontal plane, while the first Si1−αGeα film has a higher Ge concentration than does the second Si1−βGeβ film.