Raman scattering is measured along a bevel etched GaAs epitaxial film grown on Si by molecular beam epitaxial (MBE). From the correlation length profile of Raman scatteringmost dislocation lines are confined in the 2000Å regions close to the interface. The strain profile calculated from the Raman peak shift shows that about 0.6% compressive strain exists near the interface because of lattice mismatch. However, as one moves away from the interface, the compressive strain is gradually counterbalanced by thermal expansion. Transmission electron microscope (TEM) studies of the local dislocation image and properties show that an ultra clean Si surface is essential for dislocation confinement. From high resolution TEM, we find that the distance between dislocations at the interface is nonuniform, varying from 50Å to 125Å with an average distance at 81Å. Finally, a GaAs p-i-n photodetector on Si substrate is fabricated. Even though a normal photoresponse curve is obtained, the high dark current (50nA) and relatively low responsivity (0.01A/W) show that the material quality needs to be further improved to make a minority carrier vertical transition device.