Up to 10 monolayers of Al were deposited on Si (1 1 1) surfaces at low (450 degrees C) and high (640 degrees C) temperatures before the molecular beam epitaxy growth of GaN. The influence of the Al monolayers on the overall GaN epitaxial layers was investigated by reflection high-energy electron diffraction, atomic force microscopy, high-resolution X-ray diffraction and transmission electron microscopy. At high-temperature deposition, similar to 1.3 monolayer Al gave the smoothest GaN surface and best crystalline quality. At the low temperature, only similar to 0.8 ML provided the same GaN quality. (c) 2006 Elsevier B.V. All rights reserved.