Helium Implantation Research Articles

Non-uniform He bubble formation in W/W2C composite: Experimental and ab-initio study

Tungsten-tungsten carbide (W/W2C) composites are considered as possible structural materials for future nuclear fusion reactors. Here, we report on the effect of helium (He) implantation on microstructure evolution of polycrystalline W/W2C composite consolidated by field-assisted sintering technique (FAST), homogenously implanted at room temperature with 1 MeV 4He+ ions at the fluence of 8 × 1016 ions cm−2 and annealed at 1873 K for 20 minutes. Samples were analysed by scanning and transmission electron microscopy to study the presence and size of He bubbles. Monomodal He bubbles in W (30-80 nm) are limited to point defects and grain boundaries, with a considerable void denuded zone (150 nm). Bubbles do not form in W2C, but at the W|W2C interface and are considerably larger (200-400 nm). The experimental observations on He behaviour and migration in W and W2C were assessed by density functional theory (DFT) calculations, suggesting He migration and accumulation in the composite are determined by the effective He-He binding in clusters, which will give rise to decohesion. In the presence of He clusters, the decohesion of bulk W into free surfaces is energetically highly favourable but not sufficient in the W2C; hence bubbles are only observed in W grains and interfaces and not within bulk W2C.

Особенности повреждаемости ниобия импульсным лазерным излучением в сравнении с пучково-плазменным воздействием

The features of damageability of niobium by pulsed fluxes of laser radiation (LR) in the free-running modes (power density qFR = 105 – 106 W/cm2 with pulse duration τFR = 700 μs) and Q-switch mode (q = 108 – 109 W/cm2, τMD = 80 ns) in comparison with the pulsed effects of helium ion (HI) and helium plasma (HP) fluxes in the Plasma Focus (PF) setup at a flux power density qi ~ 108 W/cm2 and qp ~ 107 W/cm2, respectively and pulse durations τi ≈ 30 – 50 ns and τp ≈ 100 ns. LR fluxes were exposed to Nb in air; the working gas in the PF chamber was helium. It is shown that, in contrast to the effect of IG and HP fluxes on niobium in the PF installation, which contribute to the erosion of the material, the irradiation of niobium by pulsed LR in air fluxes under the conditions implemented does not cause noticeable surface erosion. When Nb is exposed to pulsed LR in the FR mode, the melt interacts with the air and forms on the irradiated surface of a thin film of elements of the liquid and gas phases. A similar nature of damage to Nb under the conditions of laser and beam-plasma treatment was found: a wavy relief of the irradiated surface with the presence of drop-like fragments on it, extended wave crests and microcracks. Irradiation of Nb by pulsed LR fluxes in the FR mode leads to the formation in the surface layer (SL) of sections with block and cellular structures, which are also formed after experiments in the PF chamber. It was found that after laser treatment in the FR and Q-switch modes, bubbles (blisters) are not formed in the SL of niobium, which are always present on the irradiated surface when exposed to pulsed fluxes of HI and HP in the PF chamber due to the implantation of helium ions in Nb. It is noted that in laser experiments there is no possibility of implanting working gas ions into the material, which is characteristic of beam-plasma impacts in PF devices, which affects the parameters of damageability and modification of the structure of the irradiated SL.

LiNbO3 channel and ridge waveguides based on helium ion implantation combined with lithography and precise diamond dicing

LiNbO3 channel and ridge waveguides based on helium ion implantation combined with lithography and precise diamond dicing

Fabrication and properties of the He+-implanted K9 glass waveguide

This work reports on the fabrication and characterization of a K9 glass planar waveguide structure. The helium ion implantation was employed to form the waveguide on the K9 glass. The choices of the energy of 0.4 MeV and the dose of [Formula: see text] ions/cm2 were conducted by the SRIM 2013. The [Formula: see text]-line spectroscopy and the modal profile of the He[Formula: see text]-irradiated K9 glass waveguide were performed by the prism-coupling system and the end-face coupling technique at 0.6328 [Formula: see text]m, which suggests the capability of light propagation. The He[Formula: see text]-implanted K9 glass waveguides can serve as potential candidates for advanced integrated optoelectronic devices.

Numerical study of silicon vacancy color centers in silicon carbide by helium ion implantation and subsequent annealing

Molecular dynamics simulation is adopted to discover the formation mechanism of silicon vacancy color center and to study the damage evolution in 4H-SiC during helium ion implantation with different annealing temperatures. The number and distribution of silicon vacancy color centers during He ion implantation can be more accurately simulated by introducing the ionization energy loss during implantation. A new method for numerical statistic of silicon vacancy color centers is proposed, which takes into account the structure around the color centers and makes statistical results more accurate than the Wigner–Seitz defect analysis method. Meanwhile, the photoluminescence spectra of silicon vacancy color centers at different helium ion doses are characterized to verify the correctness of the numerical analysis. The new silicon vacancy color center identification method can help predicting the optimal annealing temperature for silicon vacancy color centers, and provide guidance for subsequent color center annealing experiments.

Microstructural effects of high dose helium implantation in ErD2

Metal hydrides can store hydrogen isotopes with high volumetric density. In metal tritides, tritium beta decay can result in accumulation of helium within the solid, in some cases exceeding 10 at.% helium after only 4 years of aging. Helium is insoluble in most materials, but often does not readily escape, and instead coalesces to form nanoscale bubbles when helium concentrations are near 1 at.%. Blistering or spallation often occurs at higher concentrations. Radioactive particles shed during this process present a potential safety hazard. This study investigates the effects of high helium concentrations on erbium deuteride (ErD2), a non-radioactive surrogate material for erbium tritide (ErT2). To simulate tritium decay in the surrogate, high doses of 120 keV helium ions were implanted into ErD2 films at room temperature. Scanning and transmission electron microscopy indicated spherical helium bubble formation at a critical concentration of 1.5 at.% and bubble linkage leading to nanoscale crack formation at a concentration of 7.5 at.%. Crack propagation occurred through the nanocrack region, resulting in spallation extending from the implantation peak to the surface. Electron energy loss spectroscopy was utilized to confirm the presence of high-pressure helium in the nanocracks, suggesting that helium gas plays a predominant role in deformation. This work improves the overall understanding of helium behavior in ErD2 by using modern characterization techniques to determine: the critical helium concentration required for bubble formation, the material failure mechanism at high concentration, and the nanoscale mechanisms responsible for material failure in helium implanted ErD2.

Radiation induced hardening of beryllium during low temperature He implantation

• Helium implantation resulted in significant hardness increase of beryllium. • Dislocation loops and helium bubbles are responsible for hardening. • Low purity grade has higher irradiation induced hardening after 200°C implantation. • Crystallography leads to significant scatter in indentation hardness. • Softer grains exhibited higher irradiation induced hardening. The effect of ion irradiation on evolution of microstructure and hardening of beryllium with different impurity levels was investigated using TEM and nanoindentation. High purity S-65 grade and less-pure S-200-F grade were implanted by helium ions at temperatures of 50°C and 200°C. 11 different energies were used, so as to create a quasi-homogeneous 3 µm irradiated layer with average radiation damage of 0.1 dpa and average He content of 2000 appm. Nanoindentation experiments demonstrated that before irradiation, the S-200-F and S-65 grades have an average hardness of 3.7±0.8 GPa and 3.4±0.8 GPa correspondently. After implantation the hardness of both grades increased by about 60% for the 200°C irradiation and 100% for the 50°C irradiation. The crystallographic analysis of indented grains demonstrated that in the as-received materials the hardness is about 2.5 times higher when the indentation direction is close to the [0001] c-axis of beryllium compared to indentation perpendicular to [0001]. Hardness anisotropy significantly decreased after irradiation: the “soft orientation” was most sensitive to irradiation-induced hardening, with hardness increasing by about 140% after irradiation at 50°C and 100% after irradiation at 200°C, compared to about 15 - 20% for the “hard” orientation at both irradiation temperatures. The higher purity grade had smaller increase of the “soft orientation” hardness: 2.5±0.3 GPa for the S-65 and 2.9±0.2 GPa for the S-200-F. At both temperatures in both grades, under TEM investigation the radiation damage appears as “black dots” which are likely to be small dislocation loops with the number density of ~ 10 22 m −3 . No bubbles were observed by TEM inside grains and at grain boundaries. Analysis of the possible hardening contribution demonstrated that the observed “black dots” could be responsible for up to half of the measured hardening, while the rest of the hardening should originate from helium bubbles with the size below the TEM resolution (at or below 1.5 nm).

Cracks and blisters formed in nanocrystalline tungsten films by helium implantation

Tungsten is regarded as a potential plasma facing material (PFM) in nuclear fusion reactors owing to its high melting point, good thermal conductivity, and low sputtering yield. However, various types of defects such as interstitial atoms, vacancies, dislocation rings, cavities, helium bubbles, and “fuzz” occur in W under helium irradiation. In this study, we obtained α- and β-phase W films with nanochannels and used a helium ion microscope to conduct helium ion implantation experiments on the films. The implantation dose and energy were 1.5 × 1022 ions/m2, and 30 keV, respectively. There was no helium bubble formation in the β-W film until the completion of the implantation. The uniformly distributed columnar crystals growing perpendicular to the substrate constitute remarkable nanochannels. These effectively inhibit the aggregation of helium atoms. For α-W films prepared at high temperatures, the escape efficiency of the implanted helium atoms was reduced owing to the chaotic and reduced number of nanochannels, and helium bubbles are eventually formed. Cross-sectional images revealed that helium atoms tended to gather near the grain boundaries and gradually followed the nanochannels formed by the grain boundaries. The expansion eventually formed numerous longitudinal fissures. The results show that grain boundaries play a decisive role in the radiation resistance of nanostructured materials. Furthermore, the preparation of regular nanochannels that reach the sample surface is an effective method to improve the helium escape capability.

Helium Bubbles and Blistering in a Nanolayered Metal/Hydride Composite.

Helium is insoluble in most metals and precipitates out to form nanoscale bubbles when the concentration is greater than 1 at.%, which can alter the material properties. Introducing controlled defects such as multilayer interfaces may offer some level of helium bubble management. This study investigates the effects of multilayered composites on helium behavior in ion-implanted, multilayered ErD2/Mo thin film composites. Following in-situ and ex-situ helium implantation, scanning and transmission electron microscopy showed the development of spherical helium bubbles within the matrix, but primarily at the layer interfaces. Bubble linkage and surface blistering is observed after high fluence ex-situ helium implantation. These results show the ability of metallic multilayers to alter helium bubble distributions even in the presence of a hydride layer, increasing the lifetime of materials in helium environments.

Metal–Insulator Transition via Ion Irradiation in Epitaxial La0.7Sr0.3MnO3−δ Thin Films

Complex oxides provide rich physics related to ionic defects. For the proper tuning of functionalities in oxide heterostructures, it is highly desired to develop fast, effective, and low‐temperature routes for the dynamic modification of defect concentration and distribution. Herein, the use of helium implantation to efficiently control the vacancy profiles in epitaxial La0.7Sr0.3MnO3−δ thin films is reported. The viability of this approach is supported by lattice expansion in the out‐of‐plane lattice direction and dramatic change in physical properties, i.e., a transition from ferromagnetic metallic to antiferromagnetic insulating. In particular, a significant increase of resistivity up to four orders of magnitude is evidenced at room temperature, upon implantation of highly energetic He ions. The result offers an attractive means for tuning the emergent physical properties of oxide thin films via strong coupling between strain, defects, and valence.

In-situ Irradiation, Helium Implantation and Heating to Elucidate Mechanisms in Tungsten Alloys

In-situ Irradiation, Helium Implantation and Heating to Elucidate Mechanisms in Tungsten Alloys

Optical planar and ridge waveguides in terbium scandium aluminum garnet crystal fabricated by ion implantation and precise diamond blade dicing

Terbium scandium aluminum garnet crystal is a promising material which has excellent characteristics for magneto-optical devices. We report on the fabrication of optical planar and ridge waveguides in the terbium scandium aluminum garnet by using combination of helium ion implantation and precise diamond blade dicing. The conditions for the ion implantation are an energy of 400 keV and a fluence of 6 × 1016 ions/cm2. The rotating velocity and cutting speed of the diamond blade were set to be 20,000 rpm and 0.1 mm/s, respectively. The implantation depth and distributions of energy loss were calculated by the stopping and range of ions in matter 2013. The refractive index distribution and the dark-mode spectrum for the implanted planar waveguide were obtained by the reflectivity calculation method and the prism coupling system, respectively. The near-field intensity profiles of both the planar and ridge waveguides were measured by the end-face coupling technique. The optical guiding properties of the terbium scandium aluminum garnet waveguides suggest that they can serve as candidates for optical waveguide isolators.

Investigation of Substrate Swell-Induced Defect Formation in Suspended Graphene upon Helium Ion Implantation

Investigation of Substrate Swell-Induced Defect Formation in Suspended Graphene upon Helium Ion Implantation

Molecular dynamics simulation of color centers in silicon carbide by helium and dual ion implantation and subsequent annealing

Atomic and close-to-atomic scale fabrication with high yield for the color centers in silicon carbide is critical in developing its applications. Combined with Wigner-Seitz method and identify diamond structure method to consider the structure around the silicon vacancy (VSi), it is found that He ion implantation is more likely to fabricate a small number of silicon vacancies with complete structure but locating deep, while Si ion implantation is more likely to introduce more silicon vacancies with incomplete structure but a large number and closer to the near surface. Therefore, a method of dual ion implantation is proposed in this paper. By adjusting the ratio of He to Si ion concentration for dual ion implantation, the fabrication yield of color centers with depth below 5 nm can be increased compared with that of He implantation after high temperature annealing. Molecular dynamics (MD) simulation is employed to discover the underlying mechanism of VSi color center and damage evolution by helium ion and dual ion implantation into four-hexagonal silicon carbide (4H–SiC) with subsequent annealing. Density-functional theory (DFT) calculation proves that magnetic-spin polarization enhances the stability of carbon anti-vacancy pair (CSiVC) defect, which indicates CSiVC defects are more stable than VSi defects. The evolution of the VSi color centers of different defect models are also calculated at various temperatures by MD, and the dynamic process of VSi defects to CSiVC defects is demonstrated. It is revealed that the decrease of the VSi color center at high temperature annealing is partly due to the transformation of some silicon vacancies to CSiVC defects.

(Invited) Helium in Silicon: From the Atom to the Bubble

Helium is one of the most inert elements in nature and its solubility in most materials is very low. When introduced by implantation, the energetic helium atoms induce the displacement of a large number of atoms from their lattice position creating excess defects (self-interstitials and vacancies) that may agglomerate to form larger clusters and/or may interact with the implanted atoms as well. In particular, the interaction of vacancies with helium leads to the formation of bubbles that may modify the mechanical and physical properties of materials. This leads to a very active research field in the nuclear domain, where bubbles are responsible for the embrittlement of materials. In covalent materials, helium bubbles can be formed following high fluence implantation as well and the idea to transform bubbles from a liability into an asset has emerged twenty years ago. The most known application is probably the formation of helium and hydrogen disc-like bubbles, the platelets, used to produce silicon-on-insulator wafers. But other potential applications have been demonstrated as well such as proximity gettering of metallic impurities for instance. For all these applications, the control of the bubble formation and evolution is essential. This requires an in-depth understanding of the underlying physical mechanisms.In this work, a multiscale picture of the formation mechanisms of helium bubbles in silicon and their evolution under annealing is derived from the combination of numerical simulations and Electron Energy Loss Spectroscopy (EELS) in the Transmission Electron Microscope (TEM).Using molecular dynamics (MD) and rate equation dynamics calculations, we have identified the atomic scale mechanisms involved in the nucleation and early growth steps of the bubbles and followed their dynamics during experimental timescale. For the smallest length scale, the optimal helium filling in small vacancy clusters was determined (Fig. 1a) [1]. Regarding bubble growth, MD simulations (Fig. 1b) suggest that both Ostwald ripening and migration-coalescence mechanisms are jointly activated during bubble growth. We also discover that an original mechanism, based on the splitting of bubbles, could have a significant contribution. Overall, helium atoms are found to delay growth, proportionally to their concentration. This can be clearly observed at the nanosecond timescale. However, for longer timescales, cluster dynamics calculations also reveal periods of accelerated growth for specific helium concentrations [2].At larger length scales, the physical properties of the bubbles (helium density, pressure, morphology and size) were investigated experimentally using an original approach based on spatially resolved EELS that we have developed [3]. These experiments allow for an accurate determination of size, aspect ratio and helium density for a large number of single bubbles. These bubbles, 6 to 20 nm in diameter, were synthesized by high fluence helium implantation in silicon, followed by annealing. Very high helium densities, from 60 to 180 He/nm3, were measured in the bubbles depending on the conditions, in stark contrast with previous investigations of helium bubbles in metal with similar sizes. These results were confirmed by atomistic calculations performed for helium bubbles in the diameter range 1 to 13 nm [4].The structural modifications and, simultaneously, the helium emission from individual bubbles were investigated by spectrum imaging during in situ annealing in the transmission electron microscope (Fig. 1c). We show that helium emission surprisingly takes place at temperatures where bubble migration had hardly started. At higher temperatures, the migration (and coalescence) of voids is clearly revealed. For helium density lower than 150 He.nm-3, the Cerofolini's model taking into account the thermodynamical properties of an ultradense fluid reproduces well the helium emission from the bubbles, leading to an activation energy of 1.8 eV. When bubbles exhibit a higher initial helium density, the Cerofolini's model fails to reproduce the helium emission kinetics. We ascribe this to the fact that helium may be in the solid phase and we propose a model to take into account the properties of the solid [5].[1] L. Pizzagalli, M.-L. David, J. Dérès, Phys. Stat. Solidi A, 1700263 (2017)[2] L. Pizzagalli, J. Dérès, M.-L. David, T. Jourdan, J. Phys. D : Appl. Phys. 52, P. 455106 (2019)[3] K. Alix, M.-L. David, G. Lucas, D.T.L. Alexander, F. Pailloux, C. Hébert, L. Pizzagalli, Micron 77, p.57 (2015)[4] J. Dérès, M.-L. David, K. Alix, D.T.L. Alexander, C. Hébert, L. Pizzagalli, Phys. Rev. B 96, p. 014110 (2017)[5] K. Alix, M.-L. David, J. Dérès, C. Hébert, L. Pizzagalli, Phys. Rev. B 97, p. 104012 (2018) Figure 1

Effects of helium implantation fluence on the crystal-ion-slicing fabrication of Y-cut lithium niobate film

In the present work, the contribution of Helium implantation fluence on the Crystal-Ion-Slicing fabrication (CIS) of Y-cut LiNbO3 film were systematically investigated. Y-cut Lithium niobate (LN) thin films with a submicron thickness on the Y-cut LN substrates were successfully fabricated by CIS technique by employing He implantation. By varying the implantation fluence, the structure evolution of the LN exfoliation layer was systematically investigated by Rutherford backscattering/channeling (RBS/C), X-ray Diffraction (XRD), Raman spectroscopy, as well as Atomic Force Microscopy (AFM). Upon gradually increasing the implantation fluence from 1.0 x 1016 to 4.0 x 1016 cm-2, the lattice damage together with the tensile strain in the as-implanted LN becomes intensive according to the Raman spectroscopy investigations. In addition to the crystalline quality, the root mean square roughness becomes 1.4 nm upon increasing fluence till 4.0 x 1016 cm-2. The blistering threshold fluence for Y-cut LN is found to be between 1.0 x 1016 and 2.0 x 1016 cm-2, and the Arrhenius plot of the blistering time as a function of reciprocal temperature shows that the activation of blistering is 2.14 eV, 1.77 eV and 1.44 eV for samples under fluence of 2.0 x 1016, 3.0x 1016 and 4.0 x 1016 cm-2, respectively. Our experimental results unambiguously contribute to exploring the avenue of achieving high-quality layer, particularly highly anisotropic ones, by CIS techniques.

Mechanical and Corrosion Properties of VT6 Titanium Alloy after Irradiation with Helium and Aluminum Lons

The effect of high-dose aluminum implantation on the structural-phase state of the surface layer of titanium alloy VT6 with a fine structure (average grain size 2.3 μm) on the mechanical and corrosion properties has been investigated. It is shown that, as a result of ion irradiation, polyphase implanted layers based on α-titanium grains are formed, containing an intermetallic Ti3Al phase along the grain boundaries of α-titanium. The modified surface layers are characterized by improved mechanical properties and corrosion resistance. The noted effect is enhanced by the use of preliminary helium implantation with a dose of 1.3 × 1017ion / cm2.

Crystallographic orientation dependent of blisters formation on tungsten surface exposed to helium ions

ABSTRACT The mechanism of crystallographic orientation dependent on helium-induced blister formation in plasma-facing tungsten was studied by helium implantation, scanning electron microscopy (SEM) observation, and molecular dynamics (MD) simulation. In the present work, the electropolished high-purity tungsten was exposed to 18 keV helium ions to a fluence of 1.1 × 1018 He·cm-2at room temperature. Combined with the electron backscatter diffraction (EBSD) techniques, scanning electron microscopy observations indicate that crystallographic orientation of the underlying grain controls the behavior of blister formation. It is obvious that different grains developed different morphologies after helium implantation, and the severe blisters with larger size and higher exfoliation ratio were observed on grains oriented near <100> direction. Based on the strain–stress relation obtained from MD simulation, it was found that the blisters on grains with orientation near <100> direction grows quickly and exfoliate firstly, and the severe blistering will be achieved for grains oriented near <111> direction. It could also be foreseen that much higher fluence was expected to introduce more severe blisters on grains near <111> direction.

Effect of Sequential Helium and Nickel Ion Implantation on the Nano-Indentation Hardness of X750 Alloy

Abstract Sequential He+ and Ni+ implantations were performed to investigate their combined effect on the indentation hardness of heat-treated X750 alloy. The microstructure of the ion-implanted region was also characterized with transmission electron microscope (TEM). The X750 alloy displayed a pronounced softening with very low Ni+ implantation levels, ψ = 0.01–1.0 dpa, however it showed a clear increase in hardness when implanted with He+ up to CHe = 5000 appm. Samples subjected to sequential He+ and Ni+ implantations displayed hardness values between those presented by sole He+ or Ni+ implantation suggesting that the effects of ion-induced microstructural damage and helium accumulation on the hardness of this alloy can be considered as independent and additive over the range of conditions studied. This observation is in contradiction to previously reported TEM-based studies, which suggest that accumulated helium slows the dissolution/disordering of the γ′ hardening phase in this alloy. In our study, established theories were applied to assess the contribution of ion-induced defect clustering, γ′ precipitate disordering, and helium bubble accumulation to the hardness of the X750 alloy. It was observed that generation of ion-induced defect clusters and the formation of helium bubbles increased the indentation hardness slightly while the disordering of γ′ precipitates resulted in a dramatic decrease in the total hardness. Ni+ and He+ implantation also had different effects on the depth dependence of the indentation hardness indentation size effect (ISE). The ISE was pronounced in the samples subjected to only Ni+ implantation while it was almost absent in samples subjected to only He+ implantation.

Blistering of potassium-tantalate single crystals induced by helium implantation

Potassium-tantalate crystals were implanted with helium ions at 190 keV and then thermally treated at different temperatures to study their blistering. The surface blistering, hydrogen depth distribution, and lattice damage induced by He+ implantation were characterized using secondary ion mass spectroscopy, optical and transmission electron microscopy, and Rutherford backscattering spectrometry. The blistering of potassium-tantalate crystals was achieved at certain ion fluences followed by thermal treatment.