Properties of GaN epitaxial layers grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy

The structural, electrical, and optical properties of GaN grown on 6H-SiC(0 001) substrates by molecular beam epitaxy are studied. Suitable substrate p reparation and growth conditions are found to greatly improve the structura l 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 <mu>m thickness. Mechanisms of dislocation generation and annihilation are discussed. Increasing the Ga to N flux rat io used during growth is found to improve the surface morphology, An uninte ntional electron concentration in the films of about 5 x 10(17) cm(-3) is o bserved, and is attributed to excess Si in the films due to a Si-cleaning s tep used in the substrate preparation, Results from optical characterizatio n are correlated with the structural and electronic studies.