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 (Et) ≈ 0.03 eV] with those obtained following supersonic O2 molecular beam (SOMB) irradiation (Et ≈ 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.