Structure determination of the 1x1 GaN(0001) surface by quantitative low energy electron diffraction

A quantitative structural determination of the Ga-polar 1 x 1 (0001) surface of GaN is performed by quantitative low energy electron diffraction (LEED). The global best-fit structure is obtained by a new frozen LEED approach connected to a simulated annealing algorithm. The global minimization frozen (GMF) LEED search finds that the ordered structure consists of 1 ML of Ga adatoms at atop sites above Ga-terminated bilayers. The Ga adatoms are bonded with a Ga-Ga bond length of 2.51 Angstrom. The spacings within surface bilayers show a weak oscillatory trend, with the outmost bilayer thickness expanding to 0.72 Angstrom and the next bilayer thickness contracting to 0.64 Angstrom, compared to the bulk thickness of 0.65 Angstrom. The interlayer spacing between the first and second bilayers is 1.89 Angstrom, while the next interlayer spacing is 1.94 Angstrom, compared to the bulk value of 1.95 Angstrom. These results are compared with data from other theoretical and experimental studies.