Hydrogen Diffusion Barrier Research Articles

Molecular Dynamics-based Approach to Study the Anisotropic Self-Diffusion of Molecules in Porous Materials with Multiple Cage Types: Application to H2 in Losod

The anisotropic self-diffusion of molecular hydrogen in the multiple cage clathrasil losod (LOS) is modeled by means of molecular dynamics (MD) simulations of up to 1 micros for the temperature range 900-1200 K while treating the framework as fully flexible. The LOS diffusion tensor is calculated employing an analytical method based on hopping rates. The diffusion in the c-direction of the unit cell is found to be approximately two times more rapid than in the a- and the b-directions, a characteristic of importance for the application of LOS as a membrane. The overall diffusion is based on five different hop types for which the individual hopping rates and diffusion barriers are calculated separately. We show explicitly that the shape and volume of the cages have a significant effect on the hopping rates and further that even small deformations of the circular Si6O6 apertures have a large influence on the energetic barrier for hydrogen diffusion. Compared to the single cage clathrasils dodecasil 3C (MTN) and sodalite (SOD), LOS has a lower diffusion rate. However, from a technical point of view this rate (at 573 K) is still fast enough for LOS to be interesting as a size-selective membrane or as a hydrogen-adsorption medium.

Surface and Subsurface Hydrogen: Adsorption Properties on Transition Metals and Near-Surface Alloys

Periodic, self-consistent DFT-GGA calculations are used to study the thermochemical properties of both surface and subsurface atomic hydrogen on a variety of pure metals and near-surface alloys (NSAs). For surface hydrogen on pure metals, calculated site preferences, adsorption geometries, vibrational frequencies, and binding energies are reported and are found to be in good agreement with available experimental data. On NSAs, defined as alloys wherein a solute is present near the surface of a host metal in a composition different from the bulk composition, surface hydrogen generally binds more weakly than it binds to the pure-metal components composing the alloys. Some of the NSAs even possess the unusual property of binding hydrogen as weakly as the noble metals while, at the same time, dissociating H(2) much more easily. On both NSAs and pure metals, formation of surface hydrogen is generally exothermic with respect to H(2)(g). In contrast, formation of subsurface hydrogen is typically endothermic with respect to gas-phase H(2) (the only exception to this general statement is found for pure Pd). As with surface H, subsurface H typically binds more weakly to NSAs than to the corresponding pure-metal components of the alloys. The diffusion barrier for hydrogen from surface to subsurface sites, however, is usually lower on NSAs compared to the pure-metal components, suggesting that population of subsurface sites may occur more rapidly on NSAs.

Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite.

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.610(-10) to 1.810(-9) m(2)/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.

高強度低合金チタンの昇温脱離特性

We have investigated the thermal desorption properties and the temperature dependence of surface characteristic of a titanium alloy material using thermal desorption spectroscopy (TDS) and X-ray photoelectric spectroscopy (XPS). The results show drastic changes on the titanium oxide core XPS levels (TiO2 : 2P1/2, 2P3/2) of the surface, i.e. the TiO2 decomposed into the suboxides at 200°C, the suboxides vanish and metal Ti appears above 450°C. This sensitive change of the surface oxide layer affects to the hydrogen gas desorption of a titanium alloy, e.g. amount of the hydrogen gas desorption drastically increases with temperature above 450°C. This implies that the surface oxide layer acts as a barrier for the bulk hydrogen diffusion.

Development of Low Temperature Al2O3 MOCVD for Ferroelectric Film Passivation on 8″ Wafers

A long-term consideration in the application of ferroelectrics in device production is hydrogen-induced failures. Subsequent to ferroelectric formation, post-capacitor processes of inter-level dielectric layers contain hydrogen and are performed at elevated temperatures. Free hydrogen may react with the ferroelectric oxides, reducing portions of the dielectric layer and creating leakage paths. For optimum ferroelectric film performance, protection against hydrogen infusion is needed. In part, a design solution to this problem is to employ an Al2O3 hydrogen diffusion barrier in the device structure. We have focused on MOCVD of the barrier layer using an array of precursors—including trimethylaluminum (TMAl) and aluminum iso-propoxide (Al i-Pr) among others. We have successfully lowered the Al2O3 deposition temperature from 600°C to 350°C without sacrificing film quality or deposition rate. We have produced Al2O3 MOCVD films in a SpinCVD™ cluster tool rotating disk low-pressure reactor. The Al2O3 films are uniform and reproducible. The MOCVD process provides uniform precursor and temperature distributions necessary for coverage over topology in stacked architectures. This paper describes the MOCVD process and resultant film properties.

Hydrogen bonding in laser-crystallized poly-Si

Hydrogen bonding and diffusion during laser dehydrogenation and crystallization of hydrogenated amorphous silicon (a-Si:H) are investigated. The samples were crystallized at room temperature. After each crystallization step the specimens were characterized by hydrogen effusion measurements and Raman spectroscopy. The as-deposited a-Si:H films contain a hydrogen concentration of approximately 13 at.%. Initially, step-by-step laser crystallization produces a stratified structure. The poly-Si surface layer acts as a diffusion barrier for hydrogen. Completely crystallized poly-Si films contain a residual H concentration of approximately 1×10 21 cm −3 .

Improvement of Hydrogen Degradation in Pt/SrBi2Ta2-xNbxO9/Pt Capacitor

AbstractFerroelectric SrBi2(Ta1-xNbx)2O9 (SBTN) thin films are under evaluation for IC memory applications. It has been reported that SrBi2(Ta1-xNbx)2O9 has higher fatigue endurance and imprint resistance than other common ferroelectric materials such as lead zirconate titanate (PZT). One of the most important technical issues for integrating ferroelectric oxide materials into IC memory applications is the degradation of ferroelectric capacitor properties from the oxide reduction from hydrogen. Hydrogen annealing processes performed to terminate dangling bonds in silicon, or, elevated temperature processes, which dissociate hydrogen, are expected to reduce the ferroelectric oxide thereby degrading the dielectric properties. Therefore, in this work, we have investigated reducing degradation of Pt/SBTN/Pt capacitors due to hydrogen exposure by evaluating the compositional dependence of the Ta/Nb ratio and incorporating TiN as a hydrogen diffusion barrier. To investigate compositional dependence, the Pt/SBTN/Pt capacitors with the desired composition were fabricated from enhanced metal organic decomposition solutions that were spin deposited and delineated with a conventional patterning and etching techniques. Post annealing the samples in H2 (1%)-N2 ambient at 200°C for 10–60min, hysteresis and current-voltage measurements were conducted to evaluate any degradation to pre-anneal measurements. This experimental condition revealed that the best electrical characteristics were obtained with the Ta/Nb ratio of 1.5/0.5. Therefore, Pt/SBTN/Pt capacitors with a Ta/Nb ratio of 1.5/0.5 were evaluated using a TiN barrier and its effect characterized. After depositing a TiN film on the Pt/SBTN/Pt capacitors, the samples were annealed in H2 (5%)-N2 at 400°C for 10–60 min. The TiN was removed with NH4OH: H2O2: H2O = 1: 3: 1, respectively, at 60 - 70°C and the hysteresis and current-voltage characteristics of the capacitors measured. It was found that the degradation was extremely reduced by incorporating a TiN barrier. Moreover, the degradation effects decreased with increased density of the deposited TiN film. TiN densities greater than 3.8g/cm3 in these experiments kept the leakage current degradation to less than 10−6A/cm2 while the degradation in remanent polarization is less than 20%.

Hydrogen adsorption on palladium: a comparative theoretical study of different surfaces

The interaction of atomic hydrogen with the Pd(111), Pd(100) and Pd(110) surfaces is studied by ab-initio density functional calculations within the generalized gradient approximation (GGA). For the three surfaces, we have determined the preferred adsorption sites, the adsorption structures, the work function changes and the surface diffusion barrier, including relaxation effects. This comparative study allows some common features to be seen, in particular in the adsorption energies and geometries for both surface and subsurface H-atoms, and some significant differences such as the surface diffusion and the dispersion of the H-induced surface state. The origin of these differences is explained by a detailed analysis of the electronic structures of both clean and hydrogen-covered surfaces. Our study leads to an interesting correlation between the hydrogen diffusion barrier and the surface roughness since it plays an important part in the catalytic activity of the respective surfaces.

Hydrogen passivation of silicon surfaces: A classical molecular-dynamics study

We present a computationally efficient classical many-body potential that is capable of predicting the energetics of bulk silicon, silicon surfaces, and the interaction of hydrogen with silicon. The potential includes well established models for one-component Si and H systems and incorporates a newly developed Si-H interaction. It is shown that the present model yields hydrogen diffusion barriers, hydrogen abstraction, and ${\mathrm{H}}_{2}$ desorption reactions on silicon surfaces in excellent agreement with experiment and/or previous ab initio results. Detailed molecular-dynamics simulations are performed that elucidate the complex balance between adsorption and abstraction reactions during hydrogen passivation on Si(100) surfaces. We find a very high sticking coefficient of 0.6 for atomic hydrogen on clean Si(100)2$\ifmmode\times\else\texttimes\fi{}$1 surfaces and provide a detailed qualitative and quantitative explanation for this prediction. Furthermore, we find that there are two efficient competing surface reactions of atomic hydrogen with monohydride Si surfaces. One is the Eley-Rideal abstraction of ${\mathrm{H}}_{2}$ molecules, and the other one is adsorption. Additionally, adsorbed hydrogen on hydrogenated Si surfaces acts as a reservoir that can lead to complete passivation of Si surfaces despite the efficient creation of voids in the hydrogen layer by the abstraction.

Interaction of hydrogen with transition metal fcc(111) surfaces

The interaction of atomic hydrogen with the fcc(111) surfaces of Pd and Rh was investigated theoretically with an ab initio method, to find out the differences and similiarities between these neighboring metals. At the Rh surface the hcp site of the threefold-coordinated adsorption sites is preferred, while at Pd almost no difference between the hcp and fcc sites was found. For Pd, the occupation of subsurface positions was calculated to be more stable than bulklike positions. The energy gain caused by hydrogen absorption in subsurface positions is only about 100 meV lower than for hydrogen adsorption at the surface. In contrast, for Rh, significant differences between adsorption and absorption were calculated. The diffusion barrier for hydrogen diffusion from surface to subsurface positions was calculated and compared to the diffusion barrier in bulk. The hydrogen-induced work-function changes for the considered 4d transition-metal surfaces were positive for coverage \ensuremath{\theta}=1.

Effect of surface oxide layers on deuterium permeation through stainless steels with reference to outgassing reduction in ultra- to extremely high vacuum

Hydrogen is a dominant outgassing species from stainless steel vacuum chambers in ultra- to extremely high vacuum. Oxidation of stainless steel surfaces has been known to reduce the outgassing. The oxide layer formed on the stainless steel surface is expected to serve as a diffusion barrier for hydrogen diffusing from the bulk. In the present study the effects of oxidation on the outgassing rate, on hydrogen thermal desorption spectra, on surface contamination and on deuterium permeation and diffusion characteristics through the stainless steels are summarized. Discussion is given to the role of the surface oxide layer in outgassing reduction as well as to the effect of thickness and chemical compositions on reduction in hydrogen permeability and diffusion. The surface oxide formed in the present study appears to serve as the diffusion barrier for hydrogen and is found to decrease the deuterium permeability to 1 2 – 1 3 of the unoxidized one by lowering the diffusion coefficient. The most effective oxide is proposed to be Cr-rich oxide with 100–200Å thickness.

Hydrogen absorption and diffusion into and in palladium: ac-impedance analysis under impermeable boundary conditions

The hydrogen absorption reaction ( har) and diffusion into and in palladium electrode has been investigated in 0.1 M NaOH solution under impermeable boundary conditions by using ac-impedance, open-circuit potential transient and current transient techniques. The ac-impedance measurements were carried out in the overpotential range of − 0.10 to 0.25 V rhe . Measured impedance spectra were analyzed by using complex non-linear least squares (CNLS) fitting method on the basis of Faradaic admittance equations for hydrogen absorption under impermeable boundary conditions. From the occurrence of plateau region of the open-circuit potential transients and hydrogen content below 0.03 determined from the current transients it is suggested that a thin β-phase palladium hydride layer is formed beneath the electrode surface. The indirect to direct har transition in mode occurs at the overpotential of 0.08 V rhe below which the direct har is predominant. From the hydrogen diffusivity reduced with decreasing overpotential, it is indicated that the thin β-phase palladium hydride layer acts as a barrier for hydrogen diffusion in the electrode. The formation of the thin β-phase palladium hydride layer between the electrode surface and subsurface accounts for the predominant direct har mode and hydrogen diffusion impeded by the phase boundary between α-and β-phase palladium hydride below 0.08 V rhe .

Hydrogen sorption kinetics in partly oxidized Mg films

The influence of oxygen on the hydrogen uptake by Pd-coated Mg films was investigated, by exposing the Mg films to various amounts of oxygen, after deposition of the Mg film but before deposition of the Pd layer. The hydrogen uptake rate of these PdMgO x Mg samples shows a peculiar dependence on the thickness of the MgO x layer. For thin oxide layers (small added amounts of oxygen) the hydrogen rate uptake increases, compared with pure PdMg samples. Thicker oxide films (larger amounts of added oxygen) cause a monotonic decline of the hydrogen uptake rate. However, even very large oxygen exposures/uptakes cannot prevent hydrogen uptake completely. MgO x is thus only a partial but not a perfect diffusion barrier for hydrogen. Repeated hydriding-dehydriding cycles increase the hydrogen uptake rate by PdMgO x Mg samples, compared with the rate during the first cycle. The results are discussed in terms of a combined effect of the MgO x , layer, affecting both hydrogen transport and nucleation of the hydride phase.

Hydrogen permeation properties of molybdenum coatings from absorption-desorption experiments

The permeation properties of molybdenum coatings deposited as hydrogen diffusion barriers are studied. Absorption-desorption experiments with molybdenum-coated nickel substrates (previously published) are quantitatively interpreted using a non-steady-state analytical solution, developed in detail in a companion paper. Given the known solubility and diffusion coefficient of hydrogen in nickel, the effective permeation constant of the molybdenum coating is measured to be (8.4 ± 2.5) × 10 −16 and (37 ± 13) × 10 −16 m 2 s −1 at at −1 at 350 and 450 °C, respectively. Surface and interface effects are evident. While performing rather well as hydrogen diffusion barriers, the coatings exhibit about 9 (at 450 °C) or 17 (at 350 °C) times less protection than expected from literature diffusion data for bulk molybdenum. The relatively low efficacy of the molybdenum coating is attributed to material microdamage resulting from high internal stress.

Self-discharge behaviour of sealed Ni-MH batteries using MmNi3.3+xCo0.7Al1.0−x anodes

The thermodynamic and kinetic factors controlling self-discharge behaviour of the Ni-MmNi3.3+xCo0.7Al1.0−x (x = 0.0, 0.2) sealed cells have been investigated.A modified sealed cell is employed to measure the charge retention of the anode and cathode separately in the Ni-MmNi3.3+xCo0.7Al1.0−x (x = 0.0, 0.2) sealed cells. It is found that the charge retention of the MmNi3.3+xCo0.7Al1.0−x electrode is the same as the total charge retention and is much smaller than the Ni cathode in air at 1 atm and 30°C.From the results of hydrogen thermal desorption spectra with respect to open-circuit time, it is confirmed that the cause of the self-discharge for the MmNi3.3+xCo0.7Al1.0−x anode is due to the desorption of hydrogen from the metal hydride electrode owing to the pressure difference between the hydrogen partial pressure in the sealed cell and the equilibrium hydrogen pressure of the metal hydride alloy. Also, it is found that the self-discharge behaviour of Ni-MH cells is not correlated with the type or structure of the MH anode.As a diffusion barrier for hydrogen, MmNi3.3Co0.7Al1.0 alloy is encapsulated with Cu (which has lower hydrogen diffusivity than MH alloy); however the charge retention of the Ni-MmNi3.3Co0.7Al1.0 sealed cell is not improved because hydrogen is desorbed through cracks rather than the Cu-coated layer.The charge retention of the Ni-MmNi3.3Co0.7Al1.0 sealed cell is greatly improved by applying 1 atm of hydrogen gas. This pressure is higher than the equilibrium hydrogen pressure of the metal hydride and decreases the driving force for hydrogen desorption from the MH electrode.

Hot-carrier-induced degradation of N/sub 2/O-oxynitrided gate oxide NMOSFETs

Measurement of long term electrical characteristics of N/sub 2/O-oxynitrided gate oxide NMOSFETs revealed that the role of the nitrogen-rich layer as a blocking barrier for molecular hydrogen diffusion is dominant in the reduced device degradation. A two-step model was proposed, in which the release of hydrogen species and their reaction with the trivalent silicon atoms are the main factors for hot-carrier-induced-degradation. Hot carrier immunity of the NMOSFETs was also found to originate partially from the small increase of both the effective barrier height for electron injection and the interface trap creation energy due to the negatively charged nitrogen-rich layer.

The formation of ω-Zr during the oxidation of Zircaloy-4

The stability of the oxide scales formed on zirconium alloys has added to the attractiveness of their use as fuel cladding and pressure tubes in nuclear reactors. Under some conditions, however, the oxides have been found to undergo changes in either their passive rate of growth or their ability to act as hydrogen diffusion barriers. The mechanisms of the oxidation processes thus need to be determined if these types of behaviour changes are to be avoided and the in-reactor use of the alloy optimised. We have therefore been examining the microstructural development of the scales formed under a variety of conditions and here describe our cross-sectional TEM observations of some of the changes which occur during the oxidation of Zircaloy-4 in pure oxygen at 500°C. A knowledge of the metal-oxide interface microstructure has been found to be particularly useful for following the historical development of the oxide scale and determining how changes in the oxidation process can be initiated but it requires an accurate determination of the lattice parameters of the different phases which can be formed.

Stability of hydrogen in silicon nitride films deposited by low-pressure and plasma enhanced chemical vapor deposition techniques

Hydrogen concentration depth profiles in silicon nitride films deposited by low-pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) techniques were studied. Quantitative hydrogen profiling was carried out using the resonant nuclear reaction 15N+1H→12C+4He+γ ray. Hydrogen concentration in as-deposited LPCVD silicon nitride films was ∼2.5×1021 atoms/cm3 and was stable even after a furnace anneal at 450 °C in 3% H2/Ar for 30 min or a rapid thermal anneal at 1000 °C in oxygen for 30 s. These nitride films thus appear to be good hydrogen diffusion barriers. In contrast, hydrogen concentration in as-deposited PECVD silicon nitride films was ∼1.75×1022 atoms/cm3 and dropped to ∼7.5×1021 atoms/cm3 after rapid thermal annealing at 1000 °C in oxygen for 30 s. During high-temperature anneals, the hydrogen diffused from PECVD silicon nitride film into the underlying SiO2 layer. A comparison of the hydrogen concentration in these deposited oxides under the nitrides with those previously reported for as-deposited, but uncovered, SiO2 films points out that the observed threshold voltage shifts in nitride covered metal–oxide semiconductor capacitors are related to the loss of hydrogen from the silicon oxide during vacuum processing for nitride deposition and to the subsequent diffusion in SiO2 from the PECVD nitride films.

Reaction of bulk hydrogen in Pd(111) with adsorbed CN: A possible controlled barrier for hydrogen diffusion

The reactivity of bulk hydrogen in Pd(111) with chemisorbed CN has been studied by temperature programmed desorption. It has been found that adsorbed CN effectively reacts above 420 K with surface H produced by diffusion from the bulk. This reaction completely suppresses C 2N 2(g) formation and H 2(g) formation, producing HCN(g). Thus, CN(a) is an effective barrier material for bulk H diffusion in Pd.

First Principles Study of Hydrogen Adsorption and Diffusion on Transition Metal Surfaces: Application to the Ru (0001) Surface

AbstractHydrogen adsorption and diffusion on the (0001) surface of ruthenium is investigated using ab initio pseudopotentials within the local density approximation. The adsorption energies at a number of different sites and bond lengths were investigated via total energy calculations. Using these calculated values a potential surface was constructed and an estimate for the activation barrier for hydrogen diffusion was obtained. The calculated value of 4.0 kcal is in good accord with the value of ≈ 4 kcal as determined from laser-induced thermal desorption experiments.