Oxidation Mechanisms of Hafnium Overlayers Deposited on an Si(111) Substrate

Abstract

The oxidation mechanism of hafnium overlayers on an Si(111) substrate [Hf–Si(111), including the outermost metallic Hf overlayers and interfacial Hf silicides (HfSi and HfSi4)] was investigated via high-resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) of Hf 4f5/2,7/2, Si 2p1/2,3/2, and O 1s core levels. The atomic-scale interaction of O2 molecules with Hf–Si(111) is discussed by comparing the results obtained following thermal O2 exposures [translational energy (E t) ≈ 0.03 eV] with those obtained following supersonic O2 molecular beam (SOMB) irradiation (E t ≈ 2.2 eV). Metallic Hf and interfacial HfSi were immediately oxidized to HfO2 and Hf (sub)­silicates (Hf–O–Si configurations) via trapping-mediated dissociative adsorption. Upon excessive SOMB irradiation, the other interfacial HfSi4 was oxidized via direct dissociation. When oxidation proceeded at the Si(111) substrate via excess SOMB irradiation, volatile Si atoms were emitted from the interfacial SiO2/Si-strained layers. When the volatile Si atoms were trapped in the overlayers, the HfO2 overlayers were converted into completely oxidized Hf silicate layers. However, when the volatile Si atoms passed through the HfO2 overlayers, they reacted with the impinging O2, and the outermost SiO2 deposition layers were formed on HfO2 (or Hf silicate) layers.

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

“…According to this result as well as consideration of the difference electronegativity between the Si atom (χ Si = 1.9) and the Hf atom (χ Hf = 1.3), the peaks at 14.70 and 15.29 eV can be assigned to empirical HfSi and HfSi 4 compounds as second-and firstproducts, respectively. 8,11,28,29 These HfSi 4 and HfSi compounds are abbreviated as H1 and H2, respectively, in the figures of Hf 4f 5/2,7/2 CLSs. The notations of HfSi 4 and HfSi quote precedents.…”

“…To our knowledge, the adsorbed structures/configurations of empirical HfSi 4 and HfSi compounds on the Si(111)-7 × 7 surface have not yet been clarified. 8,11,28,29 In this study, the HfSi 4 and the HfSi structures on the 0.5-ML Hf−Si(111) surface become the important key to understand the early oxidation stages based on CLSs obtained from all elements on the surface. Therefore, the possible configurations of empirical HfSi 4 and HfSi are supposed here.…”

“…The lower- and higher-binding-energy (BE) peaks appeared at 14.7 and 15.29 eV, respectively. Their chemical shift values based on metallic Hf peak at 14.34 eV are +0.36 and +0.95 eV. , In previous papers, metal silicide formation on titanium (Ti) adsorbed Si(111) was investigated. , As a result of fitting analysis of Si 2 p 1/2,3/2 CLSs at different Ti coverages, the first component that arose at low coverage was assigned as an empirical Ti tetrasilicide (TiSi 4 ), then the second component that arose when the Ti coverage increased was assigned as an empirical Ti monosilicide (TiSi). According to this result as well as consideration of the difference electronegativity between the Si atom (χ Si = 1.9) and the Hf atom (χ Hf = 1.3), the peaks at 14.70 and 15.29 eV can be assigned to empirical HfSi and HfSi 4 compounds as second- and first-products, respectively. ,,, These HfSi 4 and HfSi compounds are abbreviated as H1 and H2, respectively, in the figures of Hf 4 f 5/2,7/2 CLSs.…”

“…A commercial n-type Si(111) wafer (thickness 0.5 mm; Nilaco Ltd.) was carved into a small substrate of 15 × 15 mm 2 . The organic/inorganic contaminants on the Si(111) surface were removed by chemical treatments, and natural SiO 2 layers were eliminated by flashing at 1370 K by radiation heating under ultrahigh vacuum (UHV) conditions at a base pressure of 2.0 × 10 −8 Pa. 8 The 7 × 7 spots were confirmed by the low-energy electron diffraction (LEED) pattern at room temperature (RT) of ∼300 K. Sample temperatures were measured with an infrared pyrometer in the range 923−1370 K and a W/Re thermocouple in the range of ∼300−923 K.…”

“…We have recently investigated the early oxidation of Hf-deposited Si (Hf–Si) surfaces using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). As a result, the oxide/suboxide products formed at the Hf–Si interface and surface were affected by the Si plane directions, thermal treatments, translation energies ( E t ) of the impinging O 2 , and several other factors. − The coverage of Hf overlayers is another significant factor in understanding the early oxidation of Hf–Si because the local configuration on the surface, migration of Si atoms toward Hf overlayers, and electronic state at the valence considerably affect the interaction of Hf–Si with the impinging O 2 . According to previous oxidation studies on metal-adsorbed Si substrates, coverage dependences were clearly observed. − Reactive site blocking and passivation behaviors toward the impinging O 2 molecules were reported at extremely low coverages of noble metals (less than 1 ML). , When the coverage exceeded 1 ML, the oxidation of the Si(111) substrate was enhanced by the effect of noble metal deposition, and the presence of SiO 2 was unexpectedly confirmed on the outermost surface.…”