POLARITY OF EPITAXIAL LAYERS AND ((1)OVER-BAR-2(1)OVER-BAR-0) PRISMATIC DEFECTS IN GAN AND ALN GROWN ON THE (0001)(SI) SURFACE OF 6H-SIC

The polarity of GaN films and their AIN buffer layer grown on the (000 1)(Si) surface of 6H-SiC by electron cyclotron resonance plasma-enhanc ed molecular beam epitaxy has been investigated by convergent-beam ele ctron diffraction (CBED) and high-resolution electron microscopy (HREM ). Good results were obtained in GaN layers by CBED. For the AlN buffe r layers, which contain a very high density of defects, the polarity w as determined by extensive HREM and image simulation investigations. I n both cases, the experimental results are in good agreement with the simulations and demonstrate that the free surfaces of the GaN and AlN layers are Ga and Al terminated respectively. Moreover, ((1)over-bar-2 (1)over-bar0) prismatic planar defects observed in the AlN layers have been identified as stacking faults, and analyses carried out in diffe rent areas of the specimens have confirmed that the layers are unipola r. From crystallographic and energy considerations, it is concluded th at the formation of inversion domains in nitride layers over SiC shoul d be difficult without the presence of impurities such as O.