Effect Of Hydrogen Impurities Research Articles

Control of hydrogen concentration on InGaZnO thin film using cryopumping system

Amorphous InGaZnO thin film transistors (a-IGZO TFTs) have been extensively studied as driving transistors for large organic light emitting diode (OLED) displays due to its’ exceptional uniformity. However, the positive bias stability (PBS) of a-IGZO TFT is not sufficiently acceptable to endure the long on-stress time. Here, the effect of hydrogen impurities in a-IGZO channels on the stability and performance of TFTs used in OLED displays are investigated. The hydrogen content in the a-IGZO film is reduced to limit the formation of trapping sites for charge carriers and improve TFT stability. This study uses a cryogenic pumping system to reduce the hydrogen impurity density during the deposition process and examines the electrical characteristics of bottom-gate a-IGZO TFTs. The threshold voltage shift (ΔVth) of the a-IGZO TFT fabricated using the cryogenic pumping system was found to be reduced by 3.4–6.9 V, compared to the TFT fabricated using the turbo molecular pump system. The hydrogen and oxygen concentrations in the oxide thin films are quantitatively analyzed, and the mechanism of OH bonding acting as an electron trap during positive bias stress is explained. Overall, this study provides insights into the impact of hydrogen impurities in a-IGZO TFT stability and performance, as well as proposes a method using cryopump for reducing hydrogen impurities to improve OLED display reliability.

Effect of hydrogen impurities on hydrogen oxidation activity of Pt/C catalyst in proton exchange membrane fuel cells

AbstractHigh-purity of hydrogen is vital to the guarantee of end usage in proton exchange membrane fuel cell (PEMFC) electric vehicles (EVs) with superior durability and low expense. However, the currently employed hydrogen, primarily from fossil fuel, still contains some poisoning impurities that significantly affect the durability of PEMFCs. Here, we investigate the poisoning effect of several typical hydrogen impurities (S2–, Cl–, HCOO– and CO32–) on the hydrogen oxidation reaction (HOR) of the state-of-the-art carbon-supported platinum (Pt/C) catalyst used in the PEMFC anode. Electrochemical results indicate that the electrochemically active surface area of Pt/C is hampered by these hydrogen impurities with reduced effective Pt reactive sites due to the competitive adsorption against hydrogen at Pt sites showing the extent of the poisoning on Pt sites in the order: S2– > Cl– > HCOO– > CO32–. Density functional theory calculations reveal that the adsorption energy of S2– on Pt (111) is greater than that of Cl–, HCOO– and CO2, and the electronic structure of Pt is found to be changed due to the adsorption of impurities showing the downshift of the d-band centre of Pt that weakens the adsorption of hydrogen on the Pt sites. This work provides valuable guidance for future optimization of hydrogen quality and also emphasizes the importance of anti-poisoning anode catalyst development, especially towards H2S impurities that seriously affect the durability of PEMFCs.

Combustion characteristics of mixtures of certain hydrocarbon fuels in air

A theoretical study of the combustion of hydrocarbon fuel mixtures in water heating unit was carried out. The effect of hydrogen impurities and propane-butane fuel on the optimal (α = 1) combustion of natural gas in air is established.

The effect of hydrogen on the physical properties of natural gas and the metrological characteristics of its metering systems

Given the promise of the concept of using a mixture of hydrogen and natural gas as an energy source, studies were conducted in the following areas:
 
 determination of the effect of hydrogen impurities on the physical properties of natural gas;
 study of the effect of adding hydrogen to natural gas on the metrological characteristics of its consumption metering systems.
 
 To solve these problems, the following was carried out:
 
 determination of the dependence of the physical parameters of natural gas on the percentage of hydrogen in its composition:
 determination of the permissible fraction (permissible concentration) of hydrogen in natural gas in modern gas transmission and gas consuming systems.
 study of the effect of hydrogen additions on the metrological characteristics of measuring instruments and gas commercial metering systems.
 
 To conduct objective research and modeling, natural gas samples having different component composition were prepared. An analysis of the physical properties of these gases was carried out — their physical parameters were calculated: adiabatic index and sound velocity, density, compressibility factor, higher calorific value and Wobbe number. Based on these samples, modeling was performed — the physical parameters of the gases were calculated by adding hydrogen at different concentrations (from 2 to 23 %).
 Based on the research results, the following conclusions are made:
 
 When hydrogen is added to natural gas in an amount of from 2 to 10 %, the physical parameters of the resulting mixture change slightly (within acceptable limits), therefore, the addition of hydrogen to natural gas in an amount of up to 10 % allows the use of existing gas transmission and gas-consuming systems without any reconstructions, improvements, changes in algorithms for calculating the physical parameters of the gas mixture and calculating (volume) volumetric flow for commercial accounting. The impact on gas meters of consumers will be within the acceptable ranges of accuracy and repeatability and, therefore, will not require the cost of updating the meters.
 The addition of up to 25 % hydrogen by volume does not require a radical new technology of burners and gas transmission systems. Safety will not be compromised by adding 25 % hydrogen by volume to the natural gas network. Changes in the physical properties in the gas mixture have a number of disadvantages, but, in the aggregate, they do not pose any additional safety risk.
 The addition of hydrogen in a volume of more than 25 %, as well as the use of the mixture at high pressures, requires additional research and the development of new algorithms.

Adsorption behavior of gas molecules on hydrogenated Fe(1 1 1)

The adsorption properties of gas molecules are studied on γ-Fe(1 1 1) surface with pre-adsorbed hydrogen, using density functional theory methods. Our results indicate that γ-Fe(1 1 1) is active for the adsorption of typical gas molecules, although it is the most stable facet of γ-Fe. The effect of hydrogen impurity on binding strength between gas molecules and γ-Fe(1 1 1) depends on the adsorption sites and the coverage of hydrogen. In general, when hydrogen atoms migrate from surface to inside of the iron, the effect of hydrogen on the adsorption of gas molecules on the iron surface becomes weaker. At the low hydrogen adsorption coverage of 1/16 ML, the main role of hydrogen atoms is to enhance the adsorption strength. As the adsorption coverage of hydrogen atoms increases to 1/4 ML, the binding energy of gas molecules will be weakened. These findings offer important clues for tuning the reactivity of γ-Fe(1 1 1) surface.

Effects of hydrogen impurities on performances and electrical reliabilities of indium-gallium-zinc oxide thin film transistors

The influences of hydrogen impurities on the performances of indium-gallium-zinc oxide (IGZO) thin film transistors (TFT) are summarized in this article. Firstly, the sources of hydrogen impurities in the IGZO channels of the TFTs are proposed, which could originate from the residual gas in the deposition chamber, the molecules absorbed on the sputtering target surface, the neighbor films that contain abundant hydrogen elements, doping during annealing processes, etc. The hydrogen impurities in the IGZO films can exist in the forms of hydroxyl groups and metal hydride bonds, respectively. The former originates from the reaction between H atoms and the O2- ions. This reaction releases free electrons, leading to a rise of the Fermi level of IGZO, and thus enhancing the mobilities of IGZO TFTs. The latter incurs negative charges on H atoms, and thus changing the distribution of the subgap density of states, hence improving the negative bias (or illumination) stabilities of IGZO TFTs. Subsequently, various methods are also proposed to characterize hydrogen elements in IGZO, such as secondary ion mass spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Finally, the effects of hydrogen impurities on the electrical characteristics of the IGZO TFTs, such as the field effect mobilities, subthreshold swings, threshold voltages, on/off current ratios as well as the positive and negative bias stress stabilities, are discussed. The results indicate that hydrogen element concentration and process temperature are two key factors for the device performances. With the increase of hydrogen element concentration in the IGZO channels, the TFTs exhibit higher electron mobilities, lower subthreshold swings and better reliabilities. However, annealing at too high or low temperatures cannot improve the device performance, and the most effective annealing temperature is 200-300℃. It is anticipated that this review could be helpful to the IGZO TFT researchers in improving the device performances and understanding the underlying mechanism.

Influence of hydrogen impurity on the resistivity of low temperature grown p-AlxGa1-xN layer (0.08 ≤ x ≤ 0.104)

Low temperature grown p-AlGaN layer with a small resistivity is crucial to improve the performance of the GaN-based laser diodes (LDs). In this study, growth temperature of the p-AlxGa1-xN (0.08 ≤ x ≤ 0.104) layers are controlled to be relatively low, and the influence of hydrogen impurity on the resistivity is investigated in detail. According to the dependence of hole concentration and resistivity on hydrogen impurity concentration, it is found that when Mg doping concentration is unchanged, reducing hydrogen impurity concentration in p-AlxGa1-xN (0.08 ≤ x ≤ 0.104) layers could reduce the resistivity effectively, which is attributed to weaken the compensation or passivation effect of hydrogen impurities on Mg acceptors. A p-Al0.09Ga0.91N layer with a low resistivity of 4.6 Ω cm has been achieved.

Small polarons and point defects in barium cerate

Barium cerate (${\mathrm{BaCeO}}_{3}$) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of doping and native defects are not fully understood. Computational studies have been stymied by the nature of the conduction band, which is made up of cerium $4f$ states. These states present a challenge to ab initio techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of ${\mathrm{BaCeO}}_{3}$. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties.

Effect of hydrogenic impurity on linear and nonlinear optical absorption coefficients and refractive index changes in a quantum dot

The analytical expressions of linear and nonlinear optical absorption coefficients and refractive index changes in a quantum dot with a hydrogenic impurity are obtained by using the compact-density-matrix approach and iterative method. The wave functions and the energy levels are obtained by using the variational method. Numerical results show that the optical absorption coefficients and refractive index changes are strongly affected by the hydrogenic impurity.

Influence of hydrogen impurities on p-type resistivity in Mg-doped GaN films

The effects of hydrogen impurities on p-type resistivity in Mg-doped GaN films were investigated. It was found that hydrogen impurities may have the dual role of passivating MgGa acceptors and passivating donor defects. A decrease in p-type resistivity when O2 is introduced during the postannealing process is attributed to the fact that annealing in an O2-containing environment can enhance the dissociation of MgGa-H complexes as well as the outdiffusion of H atoms from p-GaN films. However, low H concentrations are not necessarily beneficial in Mg-doped GaN films, as H atoms may also be bound at donor species and passivate them, leading to the positive effect of reduced compensation.

An experimental overview of the effects of hydrogen impurities on polymer electrolyte membrane fuel cell performance

Hydrogen quality is critical for increasing the reliability, stability, and durability of polymer electrolyte (PEM) fuel cells. In this work, several hydrogen impurities have been studied to understand their effects on PEM fuel cell performance at various operating concentrations. Our studies have shown that the following impurities suggested by industry stakeholders do not result in substantial fuel cell degradation when they are the sole impurity in hydrogen: 5 ppm formaldehyde, 2 ppm formic acid, 19 ppm chloromethane, 30 ppm acetaldehyde, 5% ethylene, 20 ppm toluene, and 10 ppm benzene. In addition, a specific mixture of impurities called the “specification concentration level cocktail” consisting of 0.2 ppm carbon monoxide, 4 ppb hydrogen sulphide, 0.2 ppm formic acid, 2 ppm benzene, and 0.1 ppm ammonia in hydrogen, also does not show significant effects on cell performance. In comparison, when a cocktail having five times the specification concentration is introduced into the cell, significant performance loss is evident.

Effect of hydrogenic impurity on the third-harmonic generation in a quantum well

The third-harmonic generation (THG) coefficients in a quantum well with hydrogenic impurity are theoretically investigated with the compact-density-matrix approach and iterative method. The wave functions and the energy levels can be obtained by using variational method and numerical method. Numerical results show that the THG coefficients are strongly affected by the hydrogenic impurity.

Hydrogen-Mediated Spin-Spin Interaction in ZnCoO

The effect of hydrogen impurities on the electronic and magnetic properties of ZnO-based diluted magnetic semiconductors is examined through first-principles pseudopotential calculations. We suggest that interstitial H can mediate a strong short-ranged ferromagnetic spin-spin interaction between neighboring magnetic impurities through the formation of a bridge bond. Results based on first-principles total-energy calculations and Monte Carlo simulations indicate that such H-mediated spin-spin interactions can lead to high temperature ferromagnetism.

Energetics of hydrogen impurities in aluminum and their effect on mechanical properties

The effects of hydrogen impurities in the bulk and on the surface of aluminum are theoretically investigated. Within the framework of density functional theory, we have obtained the dependence on H concentration of the stacking fault energy, the cleavage energy, the Al/H surface energy and the Al/H/Al interface formation energy. The results indicate a strong dependence of the slip energy barrier in the $[\bar 211]$ direction the cleavage energy in the [111] direction and the Al/H/Al interface formation energy, on H concentration and on tension. The dependence of the Al/H surface energy on H coverage is less pronounced, while the optimal H coverage is $\leq 0.25$ monolayer. The calculated activation energy for diffusion between high symmetry sites in the bulk and on the surface is practically the same, 0.167 eV. From these results, we draw conclusions about the possible effect of H impurities on mechanical properties, and in particular on their role in embrittlement of Al.

Spectroscopic studies of hydrogen related defects in CVD diamond

Thin diamond films prepared by the hot filament chemical vapour deposition (HFCVD) method at various deposition pressures have been characterized using a variety of spectroscopic techniques. Interpretation of the spectral details have provided useful information about the nature of the films. Deposition pressure appears to affect the quality of the diamond films which is reflected in terms of the position and width of the characteristic Raman peak of diamond. Raman spectra of the films prepared at low deposition pressures showed the presence of a sharp peak at ∼1332 cm−1 characteristic of theT 2g mode of diamond. The study of the effect of deposition pressure on the diamond growth, shows that in the range between 20 torr and 60 torr, there is little effect on the width and the shift of the 1332 cm−1 Raman peak. However, at higher pressures the peak showed a blue shift and was considerably broadened. These studies indicate the development of strain in the lattice due to the introduction of unetched hydride layer, at higher deposition pressures, as well as distortions in the lattice leading to partial lifting of the degeneracy of theT 2g mode. A broad band corresponding to the non-diamond phase (which exists at the grain boundaries, interface or as inclusions inside the grain), which can be attributed to the effect of hydrogen impurity creeping into the lattice at higher deposition pressures is also observed. SEM and XRD patterns have confirmed the dominance of diamond phase in these films.

Effects of hydrogen impurities on the diffusion, nucleation, and growth of Si on Si(001).

Effects of hydrogen impurities on the diffusion, nucleation, and growth of Si on Si(001).

Charge-density-wave transport in hydrogen-doped NbSe3.

The effects of hydrogen impurities on charge-density waves (CDW's) in ${\mathrm{NbSe}}_{3}$ have been investigated. Hydrogen can be introduced interstitially into ${\mathrm{NbSe}}_{3}$ in concentrations of at least several atomic percent with no effects on the crystal structure and only weak effects on the Peierls transitions. The threshold electric field ${E}_{T}$ for charge-density-wave depinning and the inverse of the residual resistivity ratio ${r}_{R}^{\mathrm{\ensuremath{-}}1}$ increase monotonically with H concentration for small concentrations. For concentrations above 1.5 at. %, ${E}_{T}$ and ${r}_{R}^{\mathrm{\ensuremath{-}}1}$ exhibit a plateau, which we interpret as evidence for H clustering or ordering. Several different measurements indicate that CDW interaction with H is roughly 100 times smaller than with Ta, the most weakly perturbing substitutional impurity in ${\mathrm{NbSe}}_{3}$. Hydrogen doping should thus allow study of CDW's in a new, very weakly pinned regime. In nominally pure ${\mathrm{NbSe}}_{3}$ prepared by standard methods, H is present in larger quantities than any other impurity, and may play an important role in memory effects and long-time-scale fluctuation phenomena.

Effect of hydrogen impurity on the electrical characteristics of neon-argon gas mixtures. I. Measurement of ionization coefficients

The effect of H2 on the breakdown potential and Townsend ionization coefficients for Ne-A mixtures were measured by the luminous-flux method. Both argon and hydrogen form Penning mixtures with neon. The breakdown potentials for Ne-A gas mixtures are determined by the Penning reaction, Nem+A→Ne+e+A+, between metastable neon (Nem) and ground state A atoms. At low H2 concentrations (<0.1%) the breakdown potentials and Townsend ionization coefficients are unaffected. At higher H2 levels (>0.1%), increase in breakdown potential caused by the lowering of the Townsend ionization coefficients depends upon the value of the product, pd, of the fill pressure ( p) and the electrode separation (d). This is due to (1) lowering of electron average energy which determines Nem density, and (2) nonionizing reaction, Nem+H2→H*(n=3)+H+Ne, competing with the Penning reaction between Nem and A atoms.

Effects of hydrogen impurities on shock structure and stability in ionizing monatomic gases. Part 1. Argon

At shock Mach numbers Ms ∼ 16 in pure argon with initial pressures p0 ∼ 5 torr and final electron number densities ne ∼ 1017 cm−3, the translational shock front in a 10 x 18 cm hypervelocity shock tube develops sinusoidal instabilities which affect the entire shock structure including the ionization relaxation region, the electron-cascade front and the final quasi-equilibrium state. By adding a small amount of hydrogen (∼ 0·5% of the initial pressure), the entire flow is stabilized. However, the relaxation length for ionization is drastically reduced to about one-third of its pure-gas value. Using the familiar two-step collisional model coupled with radiation-energy loss and the appropriate chemical reactions, it was possible to deduce from dual-wavelength interferometric measurements a precise value for the argon-argon collisional excitation cross-section SAr Ar* = 1·0 x 10−19 cm2/eV with or without the presence of a hydrogen impurity. The reason for the success of hydrogen, and not other gases, in bringing about stabilized shock waves is not clear. It was also found that the electron-cascade front approached the translational-shock front near the shock-tube wall. This effect appears to be independent of the wall material and is not affected by the evolution of adsorbed water vapour from the walls or by water vapour added deliberately to the test gas. The sinusoidal instabilities investigated here may offer some important clues to the abatement of instabilities that lead to detonation and explosions.

Effects of hydrogen impurities on shock structure and stability in ionizing monatomic gases: 2. Krypton

At shock Mach numbers [Formula: see text] in pure krypton, at initial pressures p0 ~ 5 Torr, and final electron number densities ne ~ 1017 cm−3, the translational shock front in a 10 cm × 18 cm hypervelocity shock tube develops sinusoidal instabilities which affect the entire shock structure including the ionization relaxation region, the electron-cascade front and the final quasi-equilibrium state. By adding a small amount of hydrogen (~0.5% of the initial pressure), the entire flow is stabilized. However, the relaxation length for ionization is drastically reduced to about one half of its pure-gas value. Unlike argon the stability appears to diminish with the addition of hydrogen beyond about 0.5%. Using the familiar two-step collisional model coupled with radiation-energy loss and the appropriate chemical reactions, it was possible from dual-wavelength interferometric measurements to deduce a more precise value for the krypton–krypton collision excitation cross-section, S*Kr–Kr = 1.2 × 10−19 cm2/eV, with or without the presence of hydrogen impurities. The reason for the success of hydrogen, and not other gases, in bringing about stabilized Shock waves in argon and krypton is not clear. It was also found that the electron-cascade front approached closely to the translational-shock front with increasing proximity to the shock-tube wall. This effect appears independent of the wall material and is not affected by the evolution of adsorbed water vapour from the walls or by water added deliberately to the test gas. The sinusoidal instabilities investigated here may offer some important clues to the abatement of instabilities that lead to detonations and explosions.