Tin and silicon incorporate on gallium sites in GaO and act as shallow donors. The monoclinic structure of GaO has two inequivalent Ga sites; density functional theory calculations for bulk show that Sn prefers the octahedral site, while Si prefers the tetrahedral site. Experiments have indicated that Si and Sn can also incorporate on the thermodynamically less preferred site. We use density functional theory to study the adsorption of Si and Sn and also the co-adsorption of these impurities with Ga and O adatoms on the GaO(010) surface. We identify a number of surface reconstructions in which Si adatoms prefer octahedral sites and Sn adatoms prefer tetrahedral sites. By applying the electron counting rule, we also study the mechanisms of the preferred adsorption sites for Si and Sn. We conclude that Si and Sn can also occupy the thermodynamically unfavored site due to surface reconstructions during the growth, which potentially leads to Si and Sn occupying both octahedral and tetrahedral sites in GaO.
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