GeO 2 /Ge/In x Ga 1Àx As heterostructures grown on (100) GaAs substrates were studied using Raman spectroscopy and photoluminescence (PL) spectroscopy. Both nearly pseudomorphic tensile-strained and nearly completely relaxed Ge films were grown and studied. The maximum tensile strain for Ge films with a thickness of %7 nm reaches 2.25%. PL data confirm the conclusions that the band gap offset of Ge/In x Ga 1Àx As is sensitive to the polarity of the bonds at the interface, and also to a parameter of x and the relaxation of strain. Depending on these parameters, the Ge/In x Ga 1Àx As may be type-I or type-II heterostructures. V