We have investigated the initial growth of GaAs films on Si(111) subst
rates by molecular beam epitaxy. GaAs grew while accomplishing high-qu
ality grain-boundaries at a size of about 100 nm with no dislocations
during the initial stage (t less than or equal to 200 nm). The high-qu
ality was confirmed based on the sharpness (165 s at an as-grown thick
ness of 100-200 nm) of the full width at half maximum (FWHM) of the X-
ray rocking curves of (111) GaAs diffraction, and on cross-sectional t
ransmission electron microscope observations. Up to a thickness of 100
nm, the FWHM coincides well with the values of a perfect crystal, cal
culated based on dynamic diffraction theory. This is greatly related t
o bond matching near to the interface between GaAs and Si in the (111)
-orientation. However, in addition to the growth of high-quality grain
boundaries, the growth accompanying areas having bad-quality regions
surrounding them was dominant as the growth proceeded to thicknesses g
reater than 300 nm.