Atomistic investigation of various GaN (0001) phases on the 6H-SiC (0001) surface

A number of superstructures of the GaN (0001) surface have been investigate d systematically by reflection high-energy electron diffraction, scanning t unneling microscopy, and first-principles theoretical calculations. The GaN -thin films are grown on the Si-terminated 6H-SiC (0001) surface by an N pl asma-assisted molecular-beam epitaxy under the Ga-rich condition. While the as-grown GaN surface is revealed to be a featureless 1x1 structure, post-g rowth deposition of Ga at lower temperatures results in the formation of a series of ordered structures, such as 2X2, 4X4, 5X5, 5 root 3X2 root 13, ro ot 7X root 7, and 10x10 in the order of the increasing Ga coverage. An 1x1- Ga-fluid structure is obtained with the highest Ga coverage. Neither ordere d structure nor smooth morphology has been observed under the N-rich regime . We conclude that the atomic structures of all these Ga-rich phases can be described best by a Ga-adatom scheme. We further show that the 5x5 and 5 r oot 3X2 root 13 phases are two configurations that exhibit a unique one-dim ensional characteristic in the adatom arrangement. Their structures can be understood by Peierls distortion against Fermi-surface instability under th e Ga-adatom scheme. [S0163-1829(99)10719-7].