A number of superstructures of the GaN (0001) surface have been investigated systematically by reflection high-energy electron diffraction, scanning tunneling microscopy, and first-principles theoretical calculations. The GaN-thin films are grown on the Si-terminated 6H-SiC (0001) surface by an N plasma-assisted molecular-beam epitaxy under the Ga-rich condition. While the as-grown GaN surface is revealed to be a featureless 1×1 structure, post-growth deposition of Ga at lower temperatures results in the formation of a series of ordered structures, such as 2×2, 4×4, 5×5, $5\sqrt{3}\times 2\sqrt{13}$, $\sqrt{7}\times \sqrt{7}$, and 10×10 in the order of the increasing Ga coverage. An 1×1-Ga-fluid structure is obtained with the highest Ga coverage. Neither ordered 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 5×5 and $5\sqrt{3}\times 2\sqrt{13}$ phases are two configurations that exhibit a unique one-dimensional characteristic in the adatom arrangement. Their structures can be understood by Peierls distortion against Fermi-surface instability under the Ga-adatom scheme.

• Received 17 December 1998

DOI:https://doi.org/10.1103/PhysRevB.59.12604