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 (E_t) of 0.03 eV; subsequently, a surface passivation behavior was observed. HfSi₄ was oxidized by supersonic O₂ beam (SOMB) irradiation at E_t = 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 E_t = 2.2 eV. The interfacial compounds of metallic Hf, HfSi, and HfSi₄ remained regardless of the E_t 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.