We have grown and characterized GaN/Si(1 1 1) epitaxial layers having different total thicknesses and periods of Al0.3Ga0.7N/GaN superlattices using metalorganic chemical vapor depositon. When the total thickness of Al0.3Ga0.7N/ GaN superlattice is 60 nm, GaN/Si(1 1 1) epitaxy shows the best surface and crystal quality. Under the same total thickness of Al0.3Ga0.7N/GaN superlattice of 60 run, as the number of periods of Al0.3Ga0.7N/GaN superlattices increases, the quality of GaN/Si(1 1 1) epitaxy is also improved. GaN/Si(1 1 1) epitaxy grown With optimized Al0.3Ga0.7N/GaN superlattice of total thickness of 60 nm and 20 periods, shows crack distance of about 120 nm between cracks, FWHM of double crystal X-ray diffractometry rocking curve for GaN(0 0 0 2) of 690 arcsec and FWHM of photoluminescence at RT of 34.49 MeV, which is compared with the best results reported for GaN/Si(1 1 1), so far. Therefore, we obtained a high-quality and nearly crack-free GaN/Si(1 1 1) epitaxy by optimizing Al0.3Ga0.7N/ GaN. superlattice, possibly for the applications of -LED or other photon devices, and, we found that the optimized total thickness and periods of Al0.3Ga0.7N/GaN superlattice play a very important role in the improvement of quality and reducing cracks in the growth of GaN/Si(1 1 1) epitaxy. (C) 2003 Elsevier Science B.V. All rights reserved.