Native Gallium Adatoms Discovered on Atomically-Smooth Gallium Nitride Surfaces at Low Temperature

In
advanced compound semiconductor devices, such as in quantum dot and
quantum well systems, detailed atomic configurations at the growth
surfaces are vital in determining the structural and electronic properties.
Therefore, it is important to investigate the surface reconstructions
in order to make further technological advancements. Usually, conventional
semiconductor surfaces (e.g., arsenides, phosphides, and antimonides)
are highly reactive due to the existence of a high density of group
V (anion) surface dangling bonds. However, in the case of nitrides,
group III rich growth conditions in molecular beam epitaxy are usually
preferred leading to group III (Ga)-rich surfaces. Here, we use low-temperature
scanning tunneling microscopy to reveal a uniform distribution of
native gallium adatoms with a density of 0.3%–0.5% of a monolayer
on the clean, as-grown surface of nitrogen polar GaN(0001̅)
having the centered 6 × 12 reconstruction. Unseen at room temperature,
these Ga adatoms are strongly bound to the surface but move with an
extremely low surface diffusion barrier and a high density saturation
coverage in thermodynamic equilibrium with Ga droplets. Furthermore,
the Ga adatoms reveal an intrinsic surface chirality and an asymmetric
site occupation. These observations can have important impacts in
the understanding of gallium nitride surfaces.