Early nitridation and oxynitridation stages of hafnium-adsorbed Si(111) surfaces using supersonic nitrogen and nitric oxide molecular beams

The use of a highly reactive hafnium–adsorbed Si(111) substrate, which is temporarily stabilized under ultrahigh vacuum conditions, is expected to the enable surface- and interface-selective incorporation of nitrogen atoms. However, no studies have yet elucidated the nitrogen-incorporation process or the resulting changes in the chemical states of the constituent elements. In this study, the early stages of nitridation and oxynitridation of hafnium–adsorbed Si(111) substrates induced by irradiation with supersonic nitrogen and nitric oxide molecular beams were investigated by synchrotron radiation X-ray photoelectron spectroscopy. N 1s core-level spectra obtained following irradiation with a 2.0 eV supersonic nitrogen molecular beam revealed that hafnium layers are nonreactive toward nitrogen molecules, whereas silicon atoms react to form N≡Si3 without charge transfer. Under irradiation with a 0.07 eV supersonic nitric oxide molecular beam, the oxynitridation of the hafnium-adsorbed silicon (111) substrate proceeds depending on the nitric oxide dose. Valence, N 1s, and O 1s core-level spectra obtained below 1.0 × 1017 molecules/cm2 show that, via charge transfer, dissociated oxygen atoms preferentially react with hafnium layers, whereas dissociated nitrogen atoms react only with silicon. During irradiation up to 6.0 × 1017 molecules/cm2, the silicon oxidation at the interface continues, while its nitridation is suppressed. There, the N 1s peak corresponding to the N—O bonds clearly appeared. Consequently, the incorporation ratio of nitrogen to oxygen at the interface rapidly decreased from ∼8.4 to ∼1.6. These results demonstrate that high-purity atomic-scale silicon nitride and oxynitride layers can be selectively formed at the interface between hafnium dioxide overlayers and the Si substrate.