GaN layers were grown by molecular beam epitaxy using an RF activated nitrogen plasma source on Si substrates with (1 0 0)- and (1 1 1) orientations. Previous to the GaN growth a SiC layer was formed on the substrates by UHV-annealing under a C2H2 over-pressure. The structural characterization of the samples by scanning- and transmission electron microscopy, atomic force microscopy, and X-ray diffraction, showed that alpha -GaN acid beta -GaN epitaxial layers were obtained on the (1 1 1)- and (1 0 0) SiC coated Si substrates, respectively. The optical properties of the epilayers were studied by room temperature photoreflectance, and 12K photoluminescence spectroscopy. The results show that the cubic GaN on (1 0 0) SiC coated Si substrates presented the higher crystal quality.;(C) 2001 Elsevier Science B.V. All rights reserved.