The structural, electrical, and optical properties of GaN grown on 6H-SiC(0001) substrates by molecular beam epitaxy are studied. Suitable substrate preparation and growth conditions are found to greatly improve the structural quality of the films. Threading dislocation densities of about 3 x 10(9) cm(-2) for edge dislocations and < 1 x 10(6) cm(-2) for screw dislocations are achieved in GaN films of 0.8 m thickness. Mechanisms of dislocation generation and annihilation are discussed. Increasing the Ga to N flux ratio used during growth is found to improve the surface morphology, An unintentional electron concentration in the films of about 5 x 10(17) cm(-3) is observed, and is attributed to excess Si in the films due to a Si-cleaning step used in the substrate preparation, Results from optical characterization are correlated with the structural and electronic studies.