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

Abstract

Coverage dependence on the early oxidation stages of hafnium−adsorbed silicon (111) ] was investigated by synchrotron radiation X-ray photoelectron spectroscopy. The Hf−Si(111) surface with a 0.5 monolayer (ML) comprised Hf tetrasilicide (HfSi 4 ) and Hf monosilicide (HfSi). HfSi 4 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 (O 2 ) with a translational energy (E t ) of 0.03 eV; subsequently, a surface passivation behavior was observed. HfSi 4 was oxidized by supersonic O 2 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 HfSi 4 remained regardless of the E t of the impinging O 2 because no oxygen reached near the Si substrate. The oxide composition and distribution near the surface were completely different depending on the Hf coverage.