Oxygen Point Defects Research Articles

Synthesis of High Density Boron Nitride Nanotube Film

Antiwetting BNNT films have been achieved by milling-ink method. Superhydrophobic (CA <5°) are demonstrated on films with stainless steel as substrate. The high density and purity are confirmed by EDX and NEXAFS. There are only a few oxygen point defects in the form of nitrogen vacancies due to ink and annealing process in air.

Ab initio study of oxygen point defects on tungsten trioxide surface

The gas response of tungsten trioxide (WO3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (− 1.65 eV in LDA and − 1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n-doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.

Correlation of residual impurity concentration and acceptor electron paramagnetic resonance linewidth in isotopically engineered Si

Electron paramagnetic resonance (EPR) experiments on boron acceptors in isotopically engineered 28Si samples with different degrees of chemical and isotopic purity are reported. The strong suppression of isotope-induced broadening effects in this material allows a direct observation of the linear correlation between the width of the inter-subband Δm = 1 EPR line and the concentrations of carbon, oxygen, and boron point defects down to a total concentration of ≈2 × 1015 cm−3. When the impurity level is decreased further, the linewidth does not fall below 2.3 ± 0.2 mT, for which we discuss possible origins.

Isotope effect on electron paramagnetic resonance of boron acceptors in silicon

The fourfold degeneracy of the boron acceptor ground state in silicon, which is easily lifted by any symmetry breaking perturbation, allows for a strong inhomogeneous broadening of the boron-related electron paramagnetic resonance (EPR) lines, e.g. by a random distribution of local strains. However, since EPR of boron acceptors in externally unstrained silicon was reported for the first time, neither the line shape nor the magnitude of the residual broadening observed in samples with high crystalline purity were compatible with the low concentrations of carbon and oxygen point defects, being the predominant source of random local strain. Adapting a theoretical model which has been applied to understand the acceptor ground state splitting in the absence of a magnetic field as an effect due to the presence of different silicon isotopes, we show that local fluctuations of the valence band edge due to different isotopic configurations in the vicinity of the boron acceptors can quantitatively account for all inhomogeneous broadening effects in high purity Si with a natural isotope composition. Our calculations show that such an isotopic perturbation also leads to a shift in the g-value of different boron-related resonances, which we could verify in our experiments. Further, our results provide an independent test and verification of the valence band offsets between the different Si isotopes determined in previous works.

Synthesis and thermoelectric properties of RuO2 nanorods

We have explored the effect of the O/Ru ratio on the morphology and the Seebeck coefficient of RuO2 nanorods (space group P42/mnm) synthesized by reactive sputtering. At an O/Ru ratio of 1.69, a faceted surface is observed, while nanorod formation occurs at O/Ru ratios of 2.03 and 2.24. Using classical molecular dynamics with the potential parameters derived in this work, we show that volatile species enable nanorod formation. Based on ab initio calculations, two effects of the nanorod formation on the Seebeck coefficient are observed: (i) increase due to additional states in the vicinity of the Fermi level and (ii) decrease due to oxygen point defects (volatile species). These two competing effects give rise to a moderate increase in the Seebeck coefficient upon nanorod formation.

Effects of proton irradiation on indium zinc oxide-based thin-film transistors

In this research, we performed a high-dose proton-beam irradiation process to investigate the effect of irradiation on the performance of indium-doped zinc oxide-based thin-film transistors (IZO-TFTs) by controlling their electrical and structural properties. The resistivity of IZO thin films dramatically increased with increasing proton-irradiation doses up to ∼ 10 14 cm − 2, after which the resistivity recovered to a level similar to as-deposited IZO thin film. After proton irradiation, the crystallinity of IZO thin films was reduced due to defect-isolation and/or chemical-isolation. The field effect mobility of IZO-TFTs decreased from 2.20 cm 2 V − 1 s − 1 to 1.22 cm 2 V − 1 s − 1, turn-on voltage shifted −7 V to −21 V and subthreshold swing value increased after proton irradiation with a 10 12 cm − 2 dose. With proton-irradiation doses over 10 14 cm − 2, IZO-TFTs device performance recovered to a level similar to that of a pristine device. Moreover, irradiated thin films and devices have low sensitivities to post-thermal annealing due to the creation of antisite oxygen point defects.

Variation of the oxygen content and point defects in tephroite, Mn 2SiO 4 + δ

Changes in the oxygen content of Mn 2SiO 4 + δ in equilibrium with MnSiO 3 were measured thermogravimetrically at 1000, 1100 and 1200 °C at a total pressure of about 0.5 atm as a function of the oxygen activity, a O 2 , and were used to determine absolute values for δ. The obtained values for δ vary from about 15 · 10 − 4 at 1200 °C and about 5 · 10 − 4 at 1000 °C at high oxygen activities, to below 1 · 10 − 4 at log 10 a O 2 ≈ − 8.5 at both temperatures. Thus Mn 2SiO 4 + δ is non-stoichiometric over the entire oxygen activity range investigated. The deviation from stoichiometry in manganese silicate is modeled based on the formation of localized holes, (Mn Mn 2+ 3+) , manganese vacancies, ( V Mn 2+ )″, and Mn 4+ ions on silicon sites, (Mn Si 4+ 4+) x . This defect model accounts for the observed oxygen activity dependence of δ. Furthermore, to better understand oxygen activity dependencies of point defect concentrations at experimentally relevant conditions, model calculations using a more general defect model were performed to derive Kröger–Vink diagrams for a generic orthosilicate Me 2SiO 4 + δ for constant Si/(Me + Si)-ratios and for being in equilibrium with a second oxide of the type MeSiO 3.

Temperature accelerated dynamics study of migration process of oxygen defects in UO 2

We studied the migration dynamics of oxygen point defects in UO 2 which is the primary ceramic fuel for light-water reactors. Temperature accelerated dynamics simulations are performed for several initial conditions. Though the migration of the single interstitial is much slower than that of the vacancy, clustered interstitial shows faster migration than those. This observation gives us important insight on the formation mechanism of high-burnup restructuring, including planar defects and grain sub-division (the rim structure), found in UO 2.

Field-induced tip–sample oxygen transfer in scanning tunneling microscopy on TiO 2(1 1 0) (1 × 1)

A study on the field-induced tip–surface oxygen transfer at room temperature and its influence on the tunneling conditions for stable STM imaging of the TiO 2(1 1 0) (1 × 1) surface is reported. A simple model of field-induced transfer is applied to tungsten and platinum–iridium tips. The oxygen transition rates from the sample to the tip or from the tip to the sample depend on the oxygen desorption barriers formed at tunneling distance. For stable imaging the applied bias voltage has to balance the oxygen transfer probabilities in both directions. In the case of Pt/Ir tips, the tunneling conditions for images with clear evidence of bridging oxygen point defects have been established.

Selective thermal reduction of single-layer MoO 3 nanostructures on Au(1 1 1)

MoO 3 is an interesting oxide prototype because its catalytic activity is sensitive to the presence and nature of defects. In this work, we demonstrate that we can control the number of defects in single-layer MoO 3 nanostructures grown on Au(1 1 1) by a thermal reduction treatment. X-ray photoelectron spectroscopy demonstrates the formation of Mo 5+ species and oxygen vacancies during annealing at 650 K. The percentage of Mo 5+ increases with the duration of annealing, until a stable composition containing 50% Mo 6+ and 50% Mo 5+ is obtained. Surprisingly, the formation of lower oxidation states such as Mo 4+ was not observed. The reduced MoO x islands remain one layer high, based on scanning tunneling microscope (STM) images. The two-dimensional nature of the reduced oxide nanocrystals may be due to a large barrier for structural reorganization and, thus, may account for the absence of Mo oxidation states lower than +5. Based on scanning tunneling microscopy images and density functional calculations, we propose that the formation of Mo 5+ ions during annealing is not associated with formation of oxygen point defects, but can be attributed to the formation of extended one-dimensional shear defects. These reduced structures are useful for studying the dependence of reactivity on defect type, and present exciting possibilities for chemical sensors and other applications.

Visualization of microscopic stress fields in silica glass in the scanning electron microscope

Quantitative measurements were made in silica glass of highly graded stress fields, as they developed: (i) in the K-dominated zone ahead of the tip of a median-type indentation micro-crack; and, (ii) at a silicon-silica interface of a metal-oxide semiconductor (MOS) device. Stress fields could be visualized on a microscopic scale using a field-emission scanning electron microscope (FE-SEM) equipped with a spectrally resolved cathodoluminescence (CL) device, according to piezo-spectroscopic (PS) assessments. The peculiarity of this newly proposed PS assessment resides in the fact that the performed CL/PS analysis exploited a peculiar luminescence emitted by optically active oxygen point defects in silica glass.

First-principles study of native point defects in hafnia and zirconia

A first-principles study of native point defects in hafnia $(\mathrm{Hf}{\mathrm{O}}_{2})$ and zirconia $(\mathrm{Zr}{\mathrm{O}}_{2})$ is carried out to identify dominant defects under different oxygen chemical potentials and Fermi levels. Oxygen vacancies and oxygen interstitials in both $\mathrm{Hf}{\mathrm{O}}_{2}$ and $\mathrm{Zr}{\mathrm{O}}_{2}$ show negative-$U$ behavior. It is shown that $\mathrm{Hf}{\mathrm{O}}_{2}$ is less prone to the formation of oxygen point defects than $\mathrm{Zr}{\mathrm{O}}_{2}$ under the same oxygen chemical potential. When the Fermi level is constrained to be within the band gap of silicon, the dominant defects are negatively charged hafnium or zirconium vacancies under intermediate to high oxygen chemical potential. We find no evidence for magnetic defects.

Oxygen behavior in aluminum nitride

The infrared lattice-vibration spectra of three polycrystalline samples of wurtzite AlN differing in their oxygen contamination have been studied by measuring the room-temperature reflectivity at near-normal incidence in the 400-3000cm−1 frequency range using unpolarized light. A type of highly-contaminated-material reflectivity spectrum has been observed. Two-mode behavior has been observed at low oxygen concentration, one-mode behavior tends to be dominant when the oxygen concentration increases and only one-mode behavior has been observed at high oxygen concentration. Otherwise, a careful analysis of the data using the Kramers-Kronig technique and classical dispersion theory gives, in addition to the transverse and longitudinal mode frequencies, two in-band resonance modes attributed to oxygen point defects in AlN. Changes in the frequencies of these modes with oxygen concentration are interpreted as a transition in the oxygen accommodation defect as the concentration of oxygen increases. A model for the behavior of oxygen in AlN is proposed.

Copper Compatible Barium Titanate Thin Films for Embedded Passives

Barium titanate thin films have been prepared by chemical solution deposition on 18 μ m thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO3 grains with diameters between 0.1 and 0.3 μ m, and an equiaxed morphology. The average film thickness is 0.6 μ m and the microstructure is free from secondary or interfacial phases. The BaTiO3 films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities of 2500 are observed at zero bias and room temperature, with permittivities greater than 3000 at the coercive field. Loss tangents under 1.5% are demonstrated at high fields. The BaTiO3 phase exhibits pronounced ferroelectric switching and coercive field values near 10 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100∘C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature—as compared to typical film processing—provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO3 films on copper foil represents an important step towards high capacitance density embedded passive components and elimination of economic constraints imparted by traditional noble metallization.

Atom-Resolved Noncontact Atomic Force Microscopic Observations of CeO2(111) Surfaces with Different Oxidation States: Surface Structure and Behavior of Surface Oxygen Atoms

Hexagonally arranged surface oxygen atoms, oxygen point defects, and multiple oxygen defects at oxygen-terminated CeO2(111) surfaces in different oxidation states were visualized by noncontact atomic force microscopy (NC-AFM). The multiple defects such as line defects and triangular defects were stabilized by a local reconstruction, where edge oxygen atoms surrounding the multiple defects were displaced and gave enhanced brightness due to a geometric reason. Successive NC-AFM measurements of the same area of a slightly reduced CeO2(111) surface revealed that hopping of surface oxygen atoms faced to metastable multiple defects was thermally activated even at room temperature. In contrast, no hopping was observed either at a point oxygen vacancy or a line defect that is stabilized by local reconstruction. It was also confirmed from atom-resolved NC-AFM observations that the surface oxygen defects were easily healed by exposure to O2 gas at room temperature.

Proximity effects on semiconducting mineral surfaces II:: Distance dependence of indirect interactions

In a previous study, we described proximity effects on surfaces of the semiconducting minerals galena and pyrite, whereby a chemical reaction at one surface site modifies the reactivity of a remote surface site several Ångstroms or even nanometers away (Becker et al., 2001). The modification of interest does not arise because of a direct “through space” interaction between the two sites, but rather an indirect interaction via the electronic structure of the substrate. Here we investigate the distance and direction dependence of proximity effects using quantum mechanical modeling. The direct and indirect interactions between co-adsorbed oxygen atoms and between adsorbed oxygen atoms and point defects on vacuum-terminated galena (100) surfaces were modeled. Density functional theory cluster and plane wave pseudopotential calculations were used to calculate the modifications to the adsorption energy as a function of separation. Energy-distance plots indicate that the proximity effect energy can become very strong at separations decreasing below about 5 to 6 Å, and persist at increasing separations up to 12 Å in a slowly decaying form. A strong attractive indirect interaction out-competes direct electrostatic repulsion for O-vacancy interactions. An oscillatory asymptotic behavior is found for co-adsorbed O-O indirect interactions, which indicates that the proximity effect energy can vary with surface crystallographic direction. It implies the presence of a strong organizing force on like adatoms that may explain the progressive oxidation of certain sulfide minerals by patchwork growth. These findings begin to pave the way for improved adsorption isotherms and extended surface complexation models that will include the specific influence of semiconductor-type proximity effects.

Cu-Compatible Ultra-High Permittivity Dielectrics for Embedded Passive Components

ABSTRACTBarium titanate thin films have been prepared by chemical solution deposition on 18 μm thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO3 grains with diameters between 0.1 and 0.2 μm, and an equiaxed morphology. The average film thickness is 0.6 μm and high resolution cross sectional microscopy shows no indication of interfacial phases. The BaTiO3 films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities greater than 3000 are observed, with loss tangents under 2.5%. The BaTiO3 phase exhibits pronounced ferroelectric switching and coercive field values near 20 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100 °C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature – as compared to typical film processing – provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO3 films on copper foil represents an important step towards high capacitance density embedded passive components.

Variation of the oxygen content and point defects in olivines, (Fe x Mg 1−x ) 2 SiO 4 , 0.2 ≤ x ≤ 1.0

The variation of the oxygen content in olivines, (Fe x Mg1− x )2SiO4, with 0.2 ≤ x ≤ 1.0, was investigated by thermogravimetric measurements. Mass changes occurring upon oxygen activity changes were measured as a function of oxygen activity and cationic composition at 1130 and 1200 °C. During the measurements the samples were in direct contact with gases containing CO, CO2 and N2 and, at a few spots at the bottom of the sample stack, also with SiO2. By fitting experimental data of mass changes to equations derived using point defect thermodynamics, it was shown for olivines with 0.2 ≤ x ≤ 1.0 at 1130 °C and 0.2 ≤ x ≤ 0.7 at 1200 °C within the oxygen activity ranges investigated that the observed variations in the oxygen contents are compatible with cation vacancies and Fe3+ ions on M sites and Fe3+ ions on silicon sites as majority defects if it is assumed that only three types of point defects occur as majority defects. Different cases were considered, closed systems, taking into account that ξ=[Si]/([Si]+[Fe]+[Mg]) is not necessarily equal to 1/3, and olivines in equilibrium with SiO2 or pyroxenes. The oxygen content variations observed in this study are significantly smaller than those reported previously in the literature. It is proposed that these differences are related to the dissolution of Fe into noble metal containers used as sample holders in earlier studies and/or to the presence of secondary phases.

Analysis of localized vibration of nitrogen complexes in CZ silicon

Nitrogen doping reduces secondary defects in CZ silicon. It is necessary to establish the infrared measurement method of nitrogen (N) and nitrogen–oxygen (N–O) point defect (N–V) complex concentrations. In this study, we examine the vibration of N–O and N–V complexes by the molecular orbital calculation. First, we analyze the NNO and NNOO in as-grown crystal. The structures used for calculation are Si 40N 2O(and O 2)H 54. Vibration mode and induced dipole moment by localized vibration are obtained by vibrational calculation. We have shown that the sum of absorption of N and N–O peaks is closely related to the total N concentration measured by SIMS. It is to be noted that oscillator strength is proportional to square of induced-dipole. Weighted sum of absorption coefficient inversely proportional to oscillator strength is used in determining the conversion coefficient to N concentration. Next, we calculate the frequency and dipole moment of normal vibration modes of NNV and NNVV which are thought to be formed and affect the defect formation at high temperature. The structures used to calculate are Si 34N 2H 36 and Si 42N 2H 42. H 2O-type and BF 3-type asymmetric stretch vibrations are obtained. The frequency of NNV is 940 cm −1 in H 2O-type and 867 cm −1 in BF 3-type. In NNVV 909 and 920 cm −1 are obtained corresponding to H 2O-type and the BF 3-type vibrations, respectively. We are trying to find these peaks experimentally.

Visible thermal emission from sub-band-gap laser excited cerium dioxide particles

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ∼1200–1400 °C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes.