In an epi-layer of GaAs less than 300nm to 100nm thick grown on silicon substrate by two-step MOCVD lethod, broad Raman scattering centered at 700cm-1(L+) and TO-like phonon at 268cm-1(L-1) were observed as well as LO phonon spectrum (292cm -1) in a back-scattering optical geometry. Emission peak of photoluminescence spectra in these epilayer of GaAs were found to shift by 33meV at 300K towards high energy side. In a thin buffer layer of GaAs less than 20nm thick, however, Raman spectra showed only LO phonon peak. These experimental facts shows n-carrier generation of 1018cm-3 only within interfacial epi-layer of GaAs less than 300nm thick, except for the very thin buffer layer of GaAs, because of incorporation of silicon atoms during growth of epilayer at 700°C. In addition, in both layers of GaAs less than 300nm, compressive stress of 109dyn·cm-2 was observed, which were found to relax abruptly with thickness.