  Cubic GaN layers were grown by plasma-assisted molecular beam epitaxy on
3C-SiC (001)substrates. In situ reflection high energy electron diffraction was
used to quantitatively determine the Ga coverage of the GaN surface during
growth. Using the intensity of the electron beam as a probe,optimum growth
conditions of c-GaN were found when a 1 ML Ga coverage is formed at the
surface. 1 micrometer thick c-GaN layers had a minimum surface roughness of 2.5
nm when a Ga coverage of 1 ML was established during growth. These samples
revealed also a minimum full width at half maximum of the (002)rocking curve.
