The stress state and its influence on structural and optical properties of GaN films grown on sapphire and 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD) were investigated.The E2 (high) phonon shift shows that relatively large tensile stress exists in GaN epilayer grown on 6H-SiC while a small compressive stress appears in the film grown on sapphire,which also indicates that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films.Narrower full width at half maximum (FWHM) of E2 (high) phonon and double crystal X-ray diffraction(DCXRD) as well as higher E2 (high)phonon Raman intensity are visible for GaN film grown on sapphire,which illustrates a decreased dislocation density and better crystalline quality.The presence of the A free exciton (3.495 eV) and reduction of the neutral donor bound exciton (D0X) FWHM to 1 meV in the spectrum of the GaN layer grown on sapphire reaffirm the same result.Finally,a numerical relationship between the luminescent bandgap and the biaxial stress of the Gabfilms is obtained at 10 K.