Epitaxial relationships between GaN and Al2O3(0001) substrates

GaN thin layers (200 Angstrom) were grown by gas-source molecular beam epitaxy on c-plane Al2O3 substrates, Transmission electron microscopy reveals that two different epitaxial relationships may occur, The well-known GaN orientation with the c axis perpendicular to the Al2O3 surface and parallel to is observed when the substrate is nitridated prior to the growth. On the other hand, GaN layers deposited on bare Al2O3 surfaces exhibit a different crystallographic orientation: parallel to and parallel to. This corresponds to a tilt of about 19 degrees of the c axis with respect to the substrate surface. (C) 1997 American Institute of Physics.