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GaAs hereroepitaxial films on Si contain a high number of misfit dislocations and large internal stresses due to the lattice and thermal mismatch of the two materials. These greatly affect the structural, optical, and electrical properties of the films. We report a study of these effects in films grown by metalorganic chemical vapor deposition using x‐ray diffraction, sample curvature, photoluminescence, and carrier concentration measurements. The x‐ray data indicate that the lattice misfit strain is almost entirely relieved by the generation of dislocations, but that the difference in thermal expansion between the film and substrate causes significant tetragonal distortion of the GaAs lattice which results in wafer bowing and, for thicker GaAs layers, film cracking. Wafer bowing was successfully eliminated by growth of GaAs films on both sides of the Si substrate. Photoluminescence spectra of crack‐free GaAs layers indicated that …