Vanadium valency and hybridization in V-doped hafnia investigated by electron energy loss spectroscopy

The valency of vanadium, and thus indirectly the oxygen stoichiometry, of V-doped hafnia synthesized under different atmospheres have been investigated on a nanometer scale by means of electron energy loss spectroscopy (EELS). The EELS V L_2,3 spectra are compared with the results of crystal field multiplet calculations and experiments on reference vanadium oxides. The EELS spectra indicate that V-doped hafnia prepared under reducing (H₂) and neutral (Ar) atmosphere are unambiguously substituted with trivalent vanadium atoms leading to the creation of oxygen vacancies in the structure. On the contrary, stoichiometric (Hf, V)O₂ compound (i.e. V⁴⁺) is more likely to be stabilized under oxidative (air) atmospheres. We also show that the amount of hybridization alters for the different compounds studied but may in part be analyzed by high spatially resolved EELS. The crystal field multiplet calculations particularly indicate that a simple reduction of the Slater integrals gives a good account of the spectral modification induced by hybridization for the case of tetravalent vanadium atoms. Received 17 November 2000 and Received in final form 17 April 2001     

First-principles study of manganese-stabilized hafnia

We present a first-principles study of the electronic and magnetic properties of cubic hafnium dioxide stabilized by Mn. We find this material to be ferromagnetic and half-metallic, with the Mn-impurity electronic states lying in the band gap of hafnia for a wide range of manganese concentration. Our ab initio calculations, within the local spin-density approximation, demonstrate that Mn-doped hafnia may be ferromagnetic at 700 K while its high-T_C ferromagnetism is robust to the oxygen vacancy defects and to how the Mn impurities are distributed over the cation sublattice.     

Effects of humidified atmosphere on oxygen transport properties in La2Mo2O9

The effects of humidified atmosphere on oxygen surface exchange and diffusion in La₂Mo₂O₉ have been investigated. After annealing samples in D₂O vapour, the depth and line scan profiles of the OD^- species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La₂Mo₂O₉.     

Theoretical evaluation of zirconia and hafnia as gate oxides for si microelectronics

Parameters determining the performance of the crystalline oxides zirconia (ZrO2) and hafnia (HfO2) as gate insulators in nanometric Si electronics are estimated via ab initio calculations of the energetics, dielectric properties, and band alignment of bulk and thin-film oxides on Si (001). With their large dielectric constants, stable and low-formation-energy interfaces, large valence offsets, and reasonable (though not optimal) conduction offsets (electron injection barriers), zirconia and hafnia appear to have considerable potential as gate oxides for Si electronics.     

Oxygen reduction reactions in the SOFC cathode of Ag/CeO2

The interactions between oxygen molecules and a silver surface or a CeO₂(111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO₂(111), the atomic layer of silver, and O₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O₂ dissociation process on a monolayer of silver supported by CeO₂(111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with − 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.     

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10^–19 cm²/s with the precise value depending on the film morphology.     

Poisoning and non-poisoning oxygen on Cu(410)

We have investigated ethene and oxygen co-adsorption on Cu(410) by high resolution electron energy loss spectroscopy. We find that these two species compete for the adsorption sites and that pre-exposure to oxygen affects ethene adsorption more or less strongly depending on oxygen coverage and the kind of occupied sites. The c(2 × 2) O overlayer is inert with respect to ethene adsorption, while when some oxygen is removed by thermally induced subsurface incorporation, ethene chemisorption is restored. The latter species also adsorbs on the disordered oxygen phase formed when O(2) is dosed at low crystal temperature. Contrary to the bare surface case, most of the ethene ends up in a π-bonded configuration. Dehydrogenation occurs, too, albeit as a minority channel. The so-produced carbon reacts already at low temperature with adsorbed oxygen to yield carbon monoxide, which desorbs around 190 K.     

Quasiparticle excitations and charge transition levels of oxygen vacancies in hafnia

We calculate the quasiparticle defect states and charge transition levels (CTLs) of oxygen vacancies in monoclinic hafnia using density functional theory (DFT) and the GW method. We introduce the criterion that the quality and reliability of CTLs may be evaluated by calculating the same CTL via two physical paths and show that it is necessary to include important electrostatic corrections previously neglected within the supercell DFT + GW approach. Contrary to previous reports, the oxygen vacancies in hafnia are large positive U centers, where U is the defect charging energy.     

O2 diffusion in SiO2: triplet versus singlet

We address the diffusion of the oxygen molecule in SiO2, using first-principles spin-polarized total-energy calculations. We find that the potential energy surfaces for the singlet and triplet states are very different in certain regions, and that the O2 molecule preserves its spin-triplet ground state not only at its most stable interstitial position inside the solid but also throughout its diffusion pathway. Therefore, the singlet state is not a good approximation to describe the behavior of O2 inside SiO2, and spin-polarization effects are fundamental to understand the properties of this system.     

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.     

Superconductivity and properties of FeTeOx films

Films of the parent compound FeTe can be made superconducting via the addition of interstitial oxygen. The process is reversible. We have characterized the new superconductors with a variety of experiments. X-ray diffraction shows that the superconductor has the same overall structure but a small lattice constant change compared to pure FeTe. X-ray absorption shows that superconducting FeTeO_x has a nominal valence of 3+. DFT calculations show the most likely position for interstitial oxygen and confirm that such oxygen incorporation does not produce a large change in structure.     

UV assisted low temperature nitridation and post deposition oxidation technique for hafnium oxide gate dielectric

An evaluation of a low temperature method (∼400 °C) for synthesis of nitrogen incorporated hafnia gate dielectric has been reported. This method is based on metal film growth in ammonia ambient and subsequent oxidation under ultraviolet (UV) irradiation. X-ray photoelectronic spectroscopy confirmed the presence of nitrided interface layer with a thickness of ∼12 ?. Equivalent oxide thickness values of around 11.5 ? and leakage current densities lower than 1 × 10⁻⁴ A/cm² at an operation voltage (−1 V) were achieved. The post deposition ultraviolet oxidation process was performed to check the interface oxidation resistance. The interface growth rate showed that as the interface bonding characteristics changed from Si-N to Si-O predominant bonding system of nitrogen incorporated films, the activation energy for oxygen diffusion changed from 18.0 kJ/mol to 9.8 kJ/mol and the activation energy of undoped hafnia films was 2.3 kJ/mol in every growth region.     

Oxygen incorporation in the alkaline earth fluorides

We consider the thermodynamics and kinetics of oxygen incorporation in the alkaline earth fluor ides, basing our discussion on the results of calculated energies of formation of a range of defects and defect clusters. Our results strongly suggest that oxygen may be incorporated only by hydrolysis and not by oxidation, and that the reaction will only occur in doped crystals. We propose a mechanism for the reaction, involving proton exchange at the surface of the crystals followed by diffusion of hydroxyl ions into the crystal; there they react with dopant-interstitial complexes to produce dopant-oxide complexes of types that have been detected in spin resonance studies.     

Reactions and diffusion of water and oxygen molecules in amorphous SiO2

Water and oxygen molecules determine many of the properties of amorphous SiO2 used in several technologies, but the underlying atomic-scale processes remain unresolved. We report results of first-principles calculations showing that a wide range of behavior is possible in an amorphous environment, including diffusion of the molecule as a whole and various reactions with the network. Experimental data including oxygen exchange reaction and radiation sensitivity are accounted for. The possibility of H3O+ formation as a source of positive charge is discussed.     

Kinetics of laser-induced oxidation of silicon near room temperature

The growth of ultra-thin (<6 nm) silicon-dioxide films on Si(100):H, Si(111):H, and a-Si:H surfaces in a dry oxygen atmosphere (0.1–10 Pa) at low temperatures (35–200 °C) was investigated. Oxidation was induced by pulsed F₂-laserradiation at 157 nm. The thickness and composition of the growing films were monitored in real time by spectroscopic ellipsometry in the photon energy range of 1.15–4.75 eV. The kinetics of low-temperature oxidation was similar for the Si surfaces investigated and differs from that of high-temperature thermal oxidation (900–1200 °C) that can be described by the Deal–Grove model. To explain the faster growth at the initial stage, it is proposed that oxidation occurs by diffusion of oxygen atoms O and/or ions O^-rather than oxygen molecules. The recombination of diffusive species to oxygen molecules limits their penetration into the bulk. A diffusion model is developed for low-temperature oxidation which takes into account the recombination process of the diffusive species. Good agreement between theory and experiment is found. The activation energy of diffusion of the active species was found to be 0.15 eV, in agreement with previous results and recent calculations for O^- ions. PACS 82.65.+r; 07.60.Fs; 81.65.Mq; 82.50.Hp     

Computer simulation of defects and oxygen transport in yttria-stabilized zirconia

We have used molecular dynamics simulations and energy minimization calculations to examine defect energetics and oxygen diffusion in yttria-stabilized zirconia (YSZ). Oxygen vacancies prefer to be second nearest neighbors to yttrium dopants. The oxygen diffusion coefficient shows a peak at 8 mol% yttria consistent with experimental findings. The activation energy for oxygen diffusion varies from 0.6 to 1.0 eV depending on the yttria content. The Y_Zr′–V_O^··–Y_Zr′ complex with a binding energy of − 0.85 eV may play an important role in any conductivity degradation of YSZ.     

Understanding the Effects of Point Defects on the Electronic Properties of Titania Nanoparticles: An Ab Initio Study

First-principles calculations are used to study the electronic structure of the TiO₂(101) surface. The effect of oxygen vacancies and interstitial titanium ions on the electronic structure is investigated, and models for optical single-electron charge transfer transitions in the structure are proposed. We found that the addition of an uncharged oxygen vacancy leads to a shift of the total density of states toward lower energy, and the bandgap increases. Therefore, interstitial titanium ion incorporation induces donor states above the valence band and increases the bandgap. These results can be used to explain the observed blueshift in nanoscale TiO_2.     

Vacancy-assisted diffusion of alkoxy species on rutile TiO2(110)

Using scanning tunneling microscopy (STM) and density functional theory simulations, we have studied the diffusion of alkoxy species formed by the dissociation of alcohols on bridge-bonded oxygen (BBO) vacancies (BBO(V)'s) on TiO2(110). At elevated temperatures (>or=400 K) the sequential isothermal STM images show that mobile BBO(V)'s mediate the diffusion of alkoxy species by providing space for alkyl-group-bearing BBO atom to diffuse into. The experimental findings are further supported by simulations that find that BBO(V) diffusion is the rate limiting step in the overall diffusion mechanism.