A new growth approach has been investigated for GaN single crystal epiatxy on sapphire substrates using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). Compared to the conventional two-step growth mode (C2SG), the new procedure features an extra-step that consists of depositing a thin film of AIN on nitrided sapphire by atomic layer epitaxy and subsequent low-temperature surface treatment under thrimethylaluminum flow. This three-step technique was found to dramatically influence the quality of GaN epilayers. Interestingly, it revealed to be more efficient than the two-step technique, with or without pre-nitridation of the substrate, for the growth of samples exhibiting high crystallinity and a specular surface morphology as well. Si donors incorporation in GaN was also carried out, leading to free-electron concentrations as high as 1.3 x 10(19)cm(-3). The samples exhibited X-ray diffraction peaks widths of similar to 320 arcsec and in the range (435similar to504) arcsec along the [002] and [102] directions, respectively. The high-optical quality of these n-type samples was also evidenced using photoluminescence (PL) measurements. In addition, the PL spectra showed the presence of the near-band edge UV transition at similar to3.41 eV and the broad yellow luminescence (YL) centered at similar to2.2eV, With increasing the doping level, the UV emission line broadened and its intensity was enhanced, while the quantum efficiency ratio of the YL to the UV greatly decreased. These results are quite consistent with those reported for device-quality samples grown by C2SG in an atmospheric pressure MOCVD reactor. (C) 2002 Elsevier Science B.V. All rights reserved.