Double-crystal X-ray diffraction (DCXD), photoluminescence (PL), and Raman spectroscopy measurements on GaAs/Si heterostructures grown with various kinds of buffer layers by molecular beam epitaxy were performed in order to characterize the crystallinity of the samples. The strain and dislocation density of the GaAs epitaxial layer determined from the DCXD curve for the GaAs/Si/SiGe/Si heterostructure are 3.90 x 10(-3) and 6.99 x 10(7) cm(-3), respectively, and the full width at half maximum (FWHM) of the GaAs epitaxial layer for the heterostructures is 405 arcsec. The results of the PL spectrum for GaAs/Si with a Si/SiGe buffer showed that the FWHM of the band-to-band luminescence is only 5.4 meV. The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for GaAs/Si with a Si/SiGe/Ge buffer increased dramatically in comparison with the corresponding rates for the GaAs/Si with Ge and SiGe/Ge buffers. These results indicate that the crystallinity of the GaAs epilayer was remarkably improved using a Si/SiGe/Ge buffer, and a strong binding energy of the Si-Si bond in the top Si buffer layer reduced the formation of the dislocation in the GaAs active layer.