We present a theoretical study of zinc blende and wurtzite GaN surfaces employing density-functional theory calculations. For clean GaN surfaces our results predict novel surface structures that are very different from those found on traditional III-V semiconductors. Adding impurity atoms of the type commonly present in significant concentrations during growth strongly modifies surface reconstructions and significantly reduces surface energies. For example, we find that arsenic has a low solubility and strongly stabilizes the cubic GaN (001) surface, thus making it interesting as a potential surfactant. We have also found that GaN surfaces are very reactive with impurities such as oxygen. Finally, the diffusion of Ga and N adatoms on both the equilibrium and non-equilibrium surfaces is discussed. These results give insight into the fundamental growth mechanisms and allow conclusions concerning optimum growth conditions.
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