We investigated the structure and formation of a surface oxide and bulk β-Ga2O3 on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 K 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 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 ≈ 1.6 eV above the stability limit for bulk β-Ga2O3. 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.
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