Structural model for the 5x5 reconstruction. Ga-adatoms in T4 sites and N-adatoms in H3 sites are shown by large black and large grey circles respectively. The small open circles in the diagram represent the Ga rest atoms in the 2nd layer. In the locations where the small open circles are missing, Ga vacancies occur. The N atoms in the 3rd layer are not shown. The light grey circle labelled DB (dangling bond) is a particular Ga rest atom site. In an alternative model, this site could conceivably contain another adatom in a nearby T4 or H3 site. (From [36]).
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