The characteristics of GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy have been investigated. The A1N thickness was found to decrease the stress sufficiently to avoid crack formation in a subsequent thick (2.6 mu m) GaN layer. X-ray diffraction and photoluminescence measurements were used to determine the effect of A1N thickness on the strain in the subsequent GaN layers. Strong band edge photoluminescence of GaN on Si (111) was observed with a full width at half maximum of the bound exciton line as low as 17 meV at 13 K. The narrow (437 '') linewidth on the (002) X-ray rocking curve also attests to the high crystalline quality of GaN on Si (111). (c) 2007 Elsevier Ltd. All rights reserved.