Because of the considerable difference in atomic radii of group III (Ga, In, and Al) elements and N, and more importantly the large binding energy of the N2 molecule, nitride surfaces terminated with group-III adlayers are more energetically favorable. As such, group-III nitride molecular beam epitaxy (MBE) is typically carried out in group-III rich conditions. These metal-rich adlayers can favorably impact the epitaxial growth of nitrides, since the excess metal atoms on top of the growing surface can serve as a surfactant during growth, modifying the growth kinetics from diffusion-limited to that of exchange-limited.
In this work, we investigate the Ga-N site exchange, a critical process of the auto-surfactant effect during epitaxial growth of GaN. On the GaN(0001) pseudo (1×1) surface, we find that the presence of two Ga adlayers instigates two Ga-N site exchanges of the N adatoms before their incorporation and the formation of a new GaN bilayer. The first site-exchange results in N being incorporated at the subsurface T1 site between the top and second Ga layer, forming metastable “ghost” islands. The second exchange that converts these “ghost” islands to that of bilayer-height can be triggered by continued STM imaging, which is driven by electrostatic forces induced by electrons tunneling to or from localized states associated with the second layer of Ga. This sets off a chain reaction which frees these Ga atoms, allowing N to form covalent Ga-N-Ga bonds of a new GaN bilayer. These results provide the first atomistic view of the auto-surfactant effect of Ga during MBE growth of GaN.
Published in Physical Review Letters:
“An atomistic view of the autosurfactant effect during GaN epitaxy“, S. T. King, M. Weinert, and L. Li, Phys. Rev. Lett. 98, 206106 (2007).