Cubic GaN films were grown on Si (0 0 1)coated with an ultra-thin Rat SiC buffer layer under both Ga-rich and N-rich conditions. The SIC buffer layer (thickness of about 2.5 nm) was grown by carbonization of Si (0 0 1) substrates in C2H2 (pressure of 5 x 10(-6) h Torr.) at 970 degrees C, The GaN films prepared under the Ga-rich condition had a local atomically smooth surface. High-resolution transmission electron microscopy (TEM) showed that the main defects in the GaN films are stacking faults along the [1 (1) over bar 1] and [(1) over bar 1 1] directions. Plan-view TEM showed that the dislocation density within grains was much lower than the overall dislocation density in the GaN films. The dislocation density within grains was about 1 x 10(5) cm(-2). X-ray diffraction (XRD) and TEM showed that the GaN films had a mosaic structure. The full-width at half-maximum of the (0 0 0 2) XRD peak of a 0.82-mu m-thick film was 19 min, one of the lowest values reported so far for cubic GaN films. And the GaN films prepared under both Ga-rich and N-rich conditions showed a strong near-bandedge photoluminescence. Yellow-band luminescence and donor-acceptor recombination peaks were also detected when the GaN films contained more extended defects and/or native-point defects.