We have applied micro‐Raman spectroscopy to the analysis of structural quality of GaAs‐on‐Si where the GaAs growth was performed through openings in an amorphous mask. The presence of the symmetry forbidden transverse optic (TO) phonon band and line shape of the longitudinal optic (LO) phonon band have been used to extract information concerning the structural quality of microscopic regions of GaAs from the Raman spectra. The utility of TO to LO phonon intensity ratios as a measure of crystal quality has been corroborated by correlation to x‐ray rocking curve full width. The structural quality of selectively grown GaAs as determined from first‐order Raman ratios is found to degrade in the vicinity of the transition between single crystal and polycrystalline regions. This work also shows that post‐growth annealing significantly improves the quality of structures with minimum feature size as small as 2 &mgr;m.