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

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.

Roles of excess minority carrier recombination and chemisorbed O2 species at SiO2/Si interfaces in Si dry oxidation: Comparison between p-Si(001) and n-Si(001) surfaces.

This study provides experimental evidence for the following: (1) Excess minority carrier recombination at SiO2/Si interfaces is associated with O2 dissociative adsorption; (2) the x-ray induced enhancement of SiO2 growth is not caused by the band flattening resulting from the surface photovoltaic effect but by the electron-hole pair creation resulting from core level photoexcitation for the spillover of bulk Si electronic states toward the SiO2 layer; and (3) a metastable chemisorbed O2 species plays a decisive role in combining two types of the single- and double-step oxidation reaction loops. Based on experimental results, the unified Si oxidation reaction model mediated by point defect generation [S. Ogawa et al., Jpn. J. Appl. Phys., Part 1 59, SM0801 (2020)] is extended from the viewpoints of (a) the excess minority carrier recombination at the oxidation-induced vacancy site and (b) the trapping-mediated adsorption through the chemisorbed O2 species at the SiO2/Si interface.

High reactivity of H2O vapor on GaN surfaces.

Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H2O, O2, N2O, and NO gases. It was found that H2O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H2O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H2O molecules were present on the (0001) (+c) surface: non-dissociatively adsorbed H2O (physisorption), and dissociatively adsorbed H2O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H2O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (-c) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0) m-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al2O3 layer using H2O vapor, the reactions caused at the interface were more remarkable for p-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H2O and O2 should be used to avoid uncontrollable oxidation on GaN surfaces.

Oxidation of Anatase TiO2(001) Surface Using Supersonic Seeded Oxygen Molecular Beam.

We investigated the oxidation of oxygen vacancies at the surface of anatase TiO2(001) using a supersonic seeded molecular beam (SSMB) of oxygen. The oxygen vacancies at the top surface and subsurface could be eliminated by the supply of oxygen using an SSMB. Oxygen vacancies are present on the surface of anatase TiO2(001) when it is untreated before transfer to a vacuum chamber. These vacancies, which are stable in the as-grown condition, could also be effectively eliminated by using the oxygen SSMB.

The effects of alloying and segregation for the reactivity and diffusion of oxygen on Cu3Au(111).

We report results of the segregation induced by the adsorption of O2 and the barrier of the formation of Cu2O in Cu3Au(111) with an experimental and theoretical approach. Oxidation by a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident-energy dependence of the measured O-uptake curve, dissociative adsorption of O2 is less effective on Cu3Au(111) than on Cu(111). The dissociative adsorption is accompanied by the Cu segregation on Cu3Au(111) as well as on Cu3Au(100) and Cu3Au(110). The obvious growth of Cu2O for a 2.3 eV HOMB cannot be observed and it suggests that the Au-rich protective layers prevent the diffusion of O atoms into the bulk. The theoretical approach based on density functional theory indicates that O adsorption shows the same behavior on Cu3Au(111) and on Cu(111). For the diffusion case, the barrier to diffuse into the subsurface in segregated Cu3Au(111) is higher than in Cu(111). This indicates that the segregated Au-rich layer in Cu3Au(111) works as a protective layer.

Initial stages of Cu3Au(111) oxidation: oxygen induced Cu segregation and the protective Au layer profile.

We report results of our experimental and theoretical studies on the Au concentration profile of Cu3Au(111) during oxidation by a hyperthermal O2 molecular beam at room temperature, using X-ray photoemission spectroscopy (XPS), in conjunction with synchrotron radiation (SR), and density functional theory (DFT). Before O2 exposure, we observe strong Au segregation to the top layer, i.e., Au surface enrichment of the clean surface. We also observe a gradual Cu surface enrichment, and Au enrichment of the second and third (subsurface) layers, with increasing O coverage. Complete Cu segregation to the surface occurs at 0.5 ML O surface coverage. The Au-rich second and third layers of the oxidized surface demonstrate the protective layer formation against oxidation deeper into the bulk.

In situ synchrotron radiation photoelectron spectroscopy study of the oxidation of the Ge(100)-2 × 1 surface by supersonic molecular oxygen beams.

In situ synchrotron radiation photoelectron spectroscopy was performed during the oxidation of the Ge(100)-2 × 1 surface induced by a molecular oxygen beam with various incident energies up to 2.2 eV from the initial to saturation coverage of surface oxides. The saturation coverage of oxygen on the clean Ge(100) surface was much lower than one monolayer and the oxidation state of Ge was +2 at most. This indicates that the Ge(100) surface is so inert toward oxidation that complete oxidation cannot be achieved with only pure oxygen (O2) gas, which is in strong contrast to Si surfaces. Two types of dissociative adsorption, trapping-mediated and direct dissociation, were confirmed by oxygen uptake measurements depending on the incident energy of O2. The direct adsorption process can be activated by increasing the translational energy, resulting in an increased population of Ge(2+) and a higher final oxygen coverage. We demonstrated that hyperthermal O2 beams remarkably promote the room-temperature oxidation with novel atomic configurations of oxides at the Ge(100) surface. Our findings will contribute to the fundamental understanding of oxygen adsorption processes at 300 K from the initial stages to saturated oxidation.

Revisit of XPS Studies of Supersonic O2 Molecular Adsorption on Cu(111): Copper Oxides.

We report the X-ray photoemission spectroscopy (XPS) characterization of the bulk Cu2O(111) surface and "8" and "29" oxide structures on Cu(111) prepared using a 0.5 eV O2 supersonic molecular beam. We propose a new structural model for the "8" oxide structure and also confirm the previously proposed model for the "29" oxide structure on Cu(111), based on the O 1s XPS spectra. The detection angle dependence of the O 1s spectra supports that the nanopyramidal model is more preferable for the (√3 × âˆš3)R30° Cu2O(111). We also report electronic excitations that O 1s electrons suffer.

Interface atom mobility and charge transfer effects on CuO and Cu2O formation on Cu3Pd(111) and Cu3Pt(111).

We bombarded [Formula: see text] and [Formula: see text] with a 2.3 eV hyperthermal oxygen molecular beam (HOMB) source, and characterized the corresponding (oxide) surfaces with synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS). At [Formula: see text], CuO forms on both [Formula: see text] and [Formula: see text]. When we increase the surface temperature to [Formula: see text], [Formula: see text] also forms on [Formula: see text], but not on [Formula: see text]. For comparison, [Formula: see text] forms even at [Formula: see text] on Cu(111). On [Formula: see text], [Formula: see text] forms only after [Formula: see text], and no oxides can be found at [Formula: see text]. We ascribe this difference in Cu oxide formation to the mobility of the interfacial species (Cu/Pd/Pt) and charge transfer between the surface Cu oxides and subsurface species (Cu/Pd/Pt).

Gas Barrier Properties of Chemical Vapor-Deposited Graphene to Oxygen Imparted with Sub-electronvolt Kinetic Energy.

Graphene gas-barrier performance holds great interest from both scientific and technological perspectives. Using in situ synchrotron X-ray photoelectron spectroscopy, we demonstrate that chemical vapor-deposited monolayer graphene loses its gas-barrier performance almost completely when oxygen molecules are imparted with sub-electronvolt kinetic energy but retains its gas-barrier performance when the molecules are not energized. The permeation process is nondestructive. Molecular dynamics-based simulation suggests kinetic energy-mediated chemical reactions catalyzed by common graphene defects as a responsible mechanism.

Kinetics of oxygen adsorption and initial oxidation on Cu(110) by hyperthermal oxygen molecular beams.

Oxygen adsorption and subsequent oxide formation on Cu(110) using a hyperthermal oxygen molecular beam (HOMB) were investigated using X-ray photoelectron spectrometry. The O-uptake curves, which were determined from the evolution of the O-1s peaks, indicate that simple Langmuir type kinetics can describe dissociative adsorption of O(2) with an incident energy (E(i)) below 0.5 eV under Theta < or = 0.5 ML. The reaction order dependence on E(i) implies two competing dissociation mechanisms, trapping-mediated and directly activated adsorption. Oxidation at Theta > or = 0.5 ML proceeds rather effectively using highly energetic HOMB at E(i) > or = 1.0 eV. The azimuthal dependence of the sticking probability during the effective oxidation using HOMB incidence suggests that the added rows, which consist of the Cu-O structure, shade the reactive hollow sites in the trough where oxygen penetrates into the subsurface. The surface Cu(2)O formed with highly energetic HOMB incidence decomposes with desorbing subsurface oxygen even at room temperature, demonstrating that HOMB can induce a metastable surface structure that cannot be produced in the thermal equilibrium process.

Experimental and Theoretical Studies on Oxidation of Cu-Au Alloy Surfaces: Effect of Bulk Au Concentration.

We report results of our experimental and theoretical studies on the oxidation of Cu-Au alloy surfaces, viz., Cu3Au(111), CuAu(111), and Au3Cu(111), using hyperthermal O2 molecular beam (HOMB). We observed strong Au segregation to the top layer of the corresponding clean (111) surfaces. This forms a protective layer that hinders further oxidation into the bulk. The higher the concentration of Au in the protective layer formed, the higher the protective efficacy. As a result, of the three Cu-Au surfaces studied, Au3Cu(111) is the most stable against dissociative adsorption of O2, even with HOMB. We also found that this protective property breaks down for oxidations occurring at temperatures above 300 K.

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si.

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni3C/NiCx)/graphene/Ni/Ni silicides (Ni2Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene.