Metastable surface oxide on CoGa(100): Structure and stability

We investigated the structure and formation of a surface oxide and bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a $p2mm$ symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential $\ensuremath{\sim}1.6\text{ }\text{eV}$ above the stability limit for bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.