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A. Vlad, A. Stierle, M. Marsman, G. Kresse, I. Costina, H. Dosch, M. Schmid, P. Varga:
"Metastable surface oxide on CoGa(100): Structure and stability";
Physical Review B, 81 (2010), 1154021 - 11540210.

We investigated the structure and formation of a surface oxide and bulk 􏰈-Ga2O3 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 􏰂1.6 eV above the stability limit for bulk 􏰈-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.