Effects of AlxGa1-xN/GaN superlattice (SL) insertion on the structural homogeneity, photoluminescence (PL) lifetime, and defect densities were studied in cubic (c-) GaN epilayers on (001) GaAs substrates grown by low-pressure metalorganic vapor-phase epitaxy. Values of the full-width at half-maximum (FWHM) of both the (002) X-ray diffraction peak and near-band-edge excitonic PL peak were decreased by a factor of 2, and the room-temperature PL intensity was improved by a factor greater than 10 through the use of double 15-period 3.8-nm-thick Al0.35Ga0.65N/2.5-nm-thick GaN SL layers between the c-GaN epilayer and the c-GaN template layer prepared on a substrate-decomposition-shielding GaN layer deposited at a low temperature. The density or size of Ga-vacancy (V-Ga)-related defects in the c-GaN epilayer was also significantly reduced, and simultaneous increase in the excitonic PL lifetime at 293 K from approximately 20 ps to 230 ps indicated a tremendous reduction of the nonradiative defect density. (C) 2004 Elsevier B.V. All rights reserved.