Hydrogen Effusion Research Articles

Computer — Aided Determination of Diffusible Hydrogen in Deposited Weld Metal

The vacuum method is described and compared with other methods for the determination of diffusible hydrogen in deposited and fused weld metal. It is shown that the vacuum and mercury methods are practically equivalent. A calculation method to reduce the time of determination of diffusible hydrogen in deposited weld metal is proposed. It uses measurements at the beginning of the hydrogen effusion. The functional-analytical solution to the direct 3D hydrogen diffusion problem for a parallelepiped-shaped solid is found. The inverse problem for unknown initial concentration of hydrogen introduced through a deposited metal is stated and solved numerically. It is demonstrated that as much as a quarter of total diffusible hydrogen can effuse prior to its measurement according to the standard procedure. The proposed method allows the time of determination of diffusible hydrogen to be reduced by several factors.

The use of SIMS in quality control and failure analysis of electrodeposited items inspected for hydrogen effects

In high-strength steels it is often difficult to distinguish between hydrogen embrittlement and various other brittle failure mechanisms. The objective of this work was to develop a sensitive analytical procedure based on secondary ion mass spectrometry (SIMS) that would allow in-service identification of local hydrogen accumulation, either during quality control or during failure analysis of electroplated items. Dynamic SIMS was found useful in identifying when baking of Cd-plated AISI 4340 steel was not carried out, thus potentially leading to hydrogen embrittlement. In all non-baked samples, an increase in the hydrogen signal was found at the Cd/steel interface. In baked samples, either a peak was not observed at the interface, or it was insignificant based on determination of the ratios between the hydrogen signals in the coating, interface and substrate. This reproducible effect was monitored even after 16 months storage in a desiccator. These observations make the procedure practical in suggesting more accurate, reliable and cost effective recommendations for prevention of failures. The main effect of baking was found to be effusion of hydrogen from the interface and the substrate steel into the atmosphere. A mechanism for delayed failure is suggested.

Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04eV. The second process, slow tritium outgassing with a time constant of 200–300h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60years.

Metal induced hydrogen effusion from amorphous silicon

Hydrogen effusion from hydrogenated amorphous silicon (a-Si:H) films, induced by various metal layers (Al, Ni, Cu, Ge, Cr, Au, Ag, Fe, and Sn), has been studied by temperature programed desorption spectroscopy. Significant reduction of the effusion temperature is observed in the presence of Cr, Ni, or Al on the a-Si:H film. Al has the strongest catalytic effect on hydrogen effusion from a-Si:H; H2 effusion is intensive, even at a temperature of 150°C, when the Al layer thickness is larger than 40nm. The strong catalytic effect of Al is considered to be related to the large diffusion constant of Al in a-Si:H.

The influence of Fermi energy on structural and electrical properties of laser crystallized P-doped amorphous silicon

A series of phosphorous-doped hydrogenated amorphous silicon films (a-Si:H) were crystallized using step-by-step laser crystallization process. The structural changes during the sequential crystallization process were detected by Raman measurements. The dehydrogenation was monitored by measuring the Si–H local vibrational modes using Raman spectroscopy and hydrogen effusion measurements. Interestingly, hydrogen bonding is affected by doping of the amorphous material. The influence of doping concentrations, thus the Fermi energy on electronic properties has been investigated employing secondary ion mass spectroscopy (SIMS), dark-conductivity- and Hall-effect measurements. The results from hydrogen effusion are consistent with the results obtained from Raman spectroscopy, Hall-effect- and dark-conductivity measurements.

Low-temperature formation of SiO2 layers using a two-step atmospheric pressure plasma-enhanced deposition-oxidation process

Silicon oxide (SiO2) layers were fabricated at low temperatures (⩽400°C) by combining the deposition of hydrogenated amorphous Si (a-Si:H) with its oxidation using atmospheric pressure plasmas excited by a 150MHz very high-frequency (VHF) power. The surface excitation by the atmospheric pressure VHF plasma was capable of reducing the temperature for the hydrogen effusion from a-Si:H. As a result, a porous a-Si:H film containing a large amount of hydrogen could be transformed into a stoichiometric SiO2 with an approximately 24% increase in oxidation rate compared with the oxidation of Si(001) at a temperature of 400°C.

Hydrogen isotopic substitution experiments in nanostructured porous silicon

Nanostructured porous silicon is usually prepared by electrochemical anodization of monocrystalline silicon using a fluorine-rich electrolyte. As a result of this process, the silicon atoms conserve their original crystalline location, and many of the dangling bonds appearing on the surface of the nanostructure are saturated by hydrogen coming from the electrolyte. This work presents an IR study of the effects produced by partial substitution of water in the electrolytic solution by deuterium oxide. The isotopic effects on the IR spectra are analyzed for the as-prepared samples and for the samples subjected to partial thermal effusion of hydrogen and deuterium. We demonstrate that, although deuterium is chemically indistinguishable from hydrogen, it presents a singular behaviour when used in porous silicon preparation. We found that deuterium preferentially bonds forming Si–DH groups. A possible explanation of the phenomenon is presented, based on the different diffusivities of hydrogen and deuterium.

Hydrogen transport properties in zinc oxide

Abstract Hydrogen diffusion in single crystal and polycrystalline zinc oxide (ZnO) was investigated by deuterium diffusion and hydrogen effusion measurements. The diffusion coefficient exhibits thermally activated behavior and varies between E A = 0.17 and 0.37 eV. Interestingly, the change in E A is accompanied by a variation of the diffusion prefactor by about eight orders of magnitude. This indicates that E A does not represent the diffusion barrier. On the other hand, the H density of states derived from effusion data is consistent with originally reported values of the diffusion activation energy of E A ≈ 1.0 eV .

Role of hydrogen in the photoinduced evolution of porous silicon luminescence

AbstractPhoto‐induced post preparation evolution effects in porous silicon were studied by IR, EPR, photoluminescence, and hydrogen effusion spectroscopies. The results show that two independent mechanisms are present during the photo‐induced evolution. We also show that hydrogen photo‐effusion takes place, in agreement with our previously proposed model. Photo‐effusion experiments performed in vacuum, combined with IR and photoluminescence spectroscopies allow to discriminate the competing mechanisms present in the evolution of the photoluminescence. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Annealing effect on structures and luminescence of amorphous SiN films

The microstructures and chemical bonding configurations of amorphous silicon nitride films with various compositions are investigated. Room temperature photoluminescence is observed which depended on the film concentrations. The post-annealing treatment at moderate temperature region of 700–900 °C is performed and the annealing effect on the structures and luminescence is studied. It is found that the structural rearrangements occurred after thermal annealing due to the effusion of hydrogen from the films. The luminescence is also changed after annealing and the possible originations are briefly discussed.

AFM and FTIR characterization of microcrystalline Si obtained from isothermal annealing of Al/a‐Si:H

AbstractAtomic force microscopy and Fourier transform infrared spectroscopy were used to investigate the morphology of the microcrystalline surface, and also the amorphous‐crystalline structural transformation of a‐Si:H films, isothermally annealed during several hours. Crystallization process was strongly influenced by the deposition of an Al layer on the surface of a‐Si:H samples. Representative AFM images show the presence of grains, which increase in diameter with the annealing time. Relative crystallized fraction as a function of the annealing time can be described adequately by using the Avrami equation. The kinetic of this crystallization process suggest a two‐dimensional growth of the Si nuclei. Fourier transform infrared measurements show the presence of an intense band near 512 cm–1 associated to Si–Si bonding. We observed the relative diminishing of the intensity of the Si–H wagging mode at 694 cm–1 with annealing time, suggesting effusion of hydrogen to the surface of microcrystalline films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallization of thin-film Si monitored in real time by in-situ spectroscopic techniques

We have developed optical techniques for real-time, in-situ monitoring of crystallization and epitaxial growth of silicon. Real-time spectroscopic ellipsometry is used for evaluating epitaxial growth during hot-wire CVD (HWCVD) reveals the thickness at which epitaxy fails and amorphous cones start to nucleate. To distinguish the various processes related to solid phase crystallization of amorphous silicon, we employ, instead, an in-situ real-time reflectance spectroscopy technique, which is simpler and less expensive. Here, we demonstrate the sensitivity of reflectivity spectroscopy to key changes in materials properties. By analyzing reflectance in strongly absorbing UV and transparent IR spectral regions, we distinguish and study the: (1) crystallization mode (random crystallization or solid-phase epitaxy); (2) nucleation location (uniformly in bulk, near film-substrate interface, or film surface); and (3) hydrogen effusion prior to crystallization.

Hydrogen equilibration in polycrystalline silicon

Hydrogen equilibration in polycrystalline silicon was investigated as a function of annealing time and temperature using electron spin resonance and hydrogen effusion measurements. During a vacuum anneal at least 1.5×1021cm−3 H atoms are mobile in the lattice, however, only about 3.7×1018cm−3 H atoms passivate Si dangling bonds. The results show that the annealing treatment can cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defect passivation preferentially occurs at grain boundaries and platelet nucleation and growth are confined to the interior of single-crystal grains, H equilibration is governed by two spatially separated processes. Moreover, the data demonstrate that the hydrogen density-of-states distribution is dynamic and changes in response to experimental parameters.

Influence of pH solution on photoluminescence of porous silicon

The radiative lifetime of as-prepared and modified layers of porous silicon (por-Si) were studied in liquid solutions with different pH. It was observed that por-Si photoluminescence (PL) intensity and decay lifetime strongly depend on pH value. This phenomenon is explained by competition of the following processes: UV-induced hydrogen effusion, hydrogen adsorption from the buffer solution, and por-Si oxidation. Regarding the phenomenon a pH change sensor can be proposed. Por-Si layer degradation can be decreased somehow by protective PEDOT layer.

Nanostructure evolution in hydrogenated amorphous silicon during hydrogen effusion and crystallization

The authors report a study by small-angle x-ray scattering (SAXS) and tilting SAXS of nanovoids in amorphous silicon films undergoing solid phase crystallization (SPC) by annealing. SPC causes nanovoids in hot-wire chemical vapor deposited (HWCVD) amorphous Si films to decrease in total void fraction but increase in individual void volume. During annealing, the voids also change shape from prolate spheroids to more spherical geometries. Preexisting, large H clusters in as-deposited HWCVD films may provide favorable sites for void expansion driven by H2 pressure, while energy minimization and strain relaxation drive geometric changes.

Post-deposition thermal annealing studies of hydrogenated microcrystalline silicon deposited at 40 °C

Post-deposition thermal annealing studies, including gas effusion measurements, measurements of infrared absorption versus annealing state, cross-sectional transmission electron microscopy (X-TEM) and atomic force microscopy (AFM), are used for structural characterization of hydrogenated amorphous and microcrystalline silicon films, prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at low substrate temperature ( T S). Such films are of interest for application in thin semiconductor devices deposited on cheap plastics. For T S ∼ 40 °C, H-evolution shows rather complicated spectra for (near-) microcrystalline material, with hydrogen effusion maxima seen at ∼ 200–250 °C, 380 °C and ∼ 450–500 °C, while for the amorphous material typical spectra for good-quality dense material are found. Effusion experiments of implanted He demonstrate for the microcrystalline material the presence of a rather open (void-rich) structure. A similar tendency can be concluded from Ne effusion experiments. Fourier Transform infrared (FTIR) spectra of stepwise annealed samples show Si–H bond rupture already at annealing temperatures of 150 °C. Combined AFM/X-TEM studies reveal a columnar microstructure for all of these (near-) microcrystalline materials, of which the open structure is the most probable explanation of the shift of the H-effusion maximum in (near-) microcrystalline material to lower temperature.

ESR Study of Crystallization of Hydrogenated Amorphous Silicon Thin Films

AbstractSolid-phase crystallization and the subsequent re-hydrogenation of the amorphous silicon thin films provides a low cost approach for thin-film crystalline Si:H-based photovoltaic devices. During the hydrogen effusion, significant lattice reconstruction occurs, as hydrogen is driven out of the film, accompanied by creation and migration of a large number of dangling bonds. We used electron-spin-resonance (ESR) to study evolution of the local order surrounding these dangling bonds during crystallization. When samples made by both plasma enhanced chemical vapor deposition (PECVD) and the and hot wire CVD (HWCVD) are heated to 560°C, hydrogen effuses within 30 min, giving rise to H-effused defect densities of about 5x1018 cm-3. Further heating at 560°C results in crystallizati°n in the HWCVD sample after about 200 min. On the other hand, PECVD samples crystallize only when heated up to 580°C, and then only after much longer times (Dt ~ 1300 min) [1,2]. ESR defects in both samples persist at the 5x1018 cm-3 level as long as the sample remains amorphous during the grain nucleation period. As the crystallites appear, the defect densities gradually decrease and saturate at about 3x1017 cm-3 as the crystallization is completed, both in HWCVD and PECVD samples.In the H-effused states before crystallization, the ESR signals for both the HWCVD and PECVD samples show significant exchange-narrowing, suggesting that the defects are probably clustered. As the sample crystallizes, the defect clustering largely disappears, yet the line-widths in fully crystallized films are somewhat narrower than those in typical micro-crystalline silicon thin films as reported earlier [3]. This difference is probably due the specific structures of the grain boundaries in the present study. The effect of re-hydrogenation on both the H-effused amorphous and crystallized states will be discussed.

Effects of doping on structural change and hydrogen bonding in laser crystallized polycrystalline silicon films

Doped polycrystalline silicon films were produced by employing a step-by-step laser crystallization of doped hydrogenated amorphous silicon (a-Si:H). The influence of laser crystallization on structural properties and hydrogen bonding were investigated using Raman backscattering spectroscopy and hydrogen effusion measurements. Crystallization with low laser fluence, E L, results a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. In fully crystallized polycrystalline silicon the Raman lines in both P- and B-doped specimens are asymmetric, which is indicative of the Fano effect. From the hydrogen effusion spectra, the hydrogen density-of-states distribution is derived. Laser crystallization results in an increase of the hydrogen binding energy by about 0.2–0.3 eV compared to the amorphous starting material.

Effects of hydrogen plasma on passivation and generation of defects in multicrystalline silicon

Hydrogenation by plasma is a low cost and efficient method to improve the photovoltaic properties of multicrystalline silicon. The role of plasma parameters on the efficiency of hydrogenation was studied using secondary ion mass spectrometry (SIMS), hydrogen effusion, electrochemical impedance spectroscopy and electron beam induced current (EBIC). The experimental results showed a deuterium concentration of 10 20 atoms cm −3 could be reached in the sample after a 15-min treatment. Optimal treatment time depends on temperature and leads to maximum electrical conductivity and minority carrier diffusion length. The results confirm the reduction of defects densities and potential barriers associated with grain boundaries.

Photoluminescence properties of SiN x/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH 2Cl 2/H 2/NH 3 plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV–VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N–H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN x interfaces.