In this paper we present an optimization study of the quality of unintentionally doped GaN layers grown on Si(111) substrates. The GaN layers which were investigated here were grown as semi-insulating/high resistivity buffer layers in "standard" high electron mobility transistor (HEMT) GaN/AlGaN structures. High-resolution X-ray diffraction (HR-XRD) studies revealed that the full-width at half-maximum (FWHM) of the asymmetric ((1) over bar 102) peak decreases strongly with increasing growth temperature and saturates at a value of around 830 arcsec at a growth temperature of 1130 degrees C. This means that at these high growth temperatures the density of notably the edge-type dislocations is strongly reduced. Optical characterization was carried out by means of low-temperature (6 K) photoluminescence spectroscopy. The spectra showed that both blue- (BL) and yellow luminescence (YL) bands were present in samples grown at temperatures below 1100 degrees C. Secondary ion mass spectroscopy (SIMS), showed that the unintentionally incorporated carbon concentration in the layers decreased with increasing growth temperature. We attribute the BL to recombination involving donor-acceptor pairs C-Ga-C-N. The origin of the YL band can be attributed as follows: because the YL intensity strongly decreases with decreasing (edge) dislocation density this transition is based on the V-Ga-O-N complex captured at edge-type dislocations. Alternatively, the involvement of carbon cannot be completely excluded. (C) 2008 Elsevier B.V. All rights reserved.