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

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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\ifmmode\times\else\texttimes\fi{}1 structure, post-growth deposition of Ga at lower temperatures results in the formation of a series of ordered structures, such as 2\ifmmode\times\else\texttimes\fi{}2, 4\ifmmode\times\else\texttimes\fi{}4, 5\ifmmode\times\else\texttimes\fi{}5, $5\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{13}$, $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$, and 10\ifmmode\times\else\texttimes\fi{}10 in the order of the increasing Ga coverage. An 1\ifmmode\times\else\texttimes\fi{}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\ifmmode\times\else\texttimes\fi{}5 and $5\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{13}$ 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.

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