Coverage Dependence upon Early Oxidation Stages of Hafnium-Adsorbed Si(111)-7 × 7

Coverage dependence on the early oxidation stages of hafnium–adsorbed silicon (111) [Hf–Si(111)] was investigated by synchrotron radiation X-ray photoelectron spectroscopy. The Hf–Si(111) surface with a 0.5 monolayer (ML) comprised Hf tetrasilicide (HfSi4) and Hf monosilicide (HfSi). HfSi4 is inferred to be single Hf adsorption around the Si rest-atoms/corner-holes on the 7 × 7 reconstructed surface, while HfSi is inferred to be a unit of six Hf atoms adsorbed on the top of the Si center adatoms/rest-atoms on the same surface. HfSi was oxidized by exposure to thermal oxygen molecules (O2) with a translational energy (Et) of 0.03 eV; subsequently, a surface passivation behavior was observed. HfSi4 was oxidized by supersonic O2 beam (SOMB) irradiation at Et = 0.39 eV. In Hf–Si(111) with six MLs, Si migration into the Hf overlayers and segregation on the surface were observed by depth profiling. The segregated Si atoms were oxidized by SOMB irradiation with Et = 2.2 eV. The interfacial compounds of metallic Hf, HfSi, and HfSi4 remained regardless of the Et of the impinging O2 because no oxygen reached near the Si substrate. The oxide composition and distribution near the surface were completely different depending on the Hf coverage.