GaN films were grown on Si(100) substrates by molecular beam epitaxy employing an RF activated N-plasma source. The substrates were coated with a thin SiC layer to reduce the reaction of N with Si. The substrate temperature was set at 750 degreesC, and the flux of Ga atoms was varied by changing the Ga-Knudsen cell temperature (T-Ga) from 950 to 1100 degreesC. The effects of the different growth conditions on the optical and structural characteristics of the films were studied by X-ray diffraction, atomic force microscopy, photoluminescence and photoreflectance spectroscopy. The results show that for T-Ga = 950 degreesC, the films presented a very poor crystal quality with a mixture of hexagonal (alpha) and cubic (beta) GaN phases. By increasing T-Ga the crystal quality improved. The films presented predominately the beta-GaN phase for an optimal temperature T-Ga of 1050 degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.