Ion Irradiation Conditions Research Articles

Etch-stop mechanisms in plasma-enhanced atomic layer etching of silicon nitride: A molecular dynamics study

Possible mechanisms of etch-stops in plasma-enhanced atomic layer etching (PE-ALE) for silicon nitride (SiN) were examined with molecular dynamics (MD) simulations. Recent experiments [Hirata et al., J. Vac. Sci. Technol. A 38, 062601 (2020)] have shown that the PE-ALE process of SiN consisting of hydro-fluorocarbon (HFC) adsorption and argon ion (Ar+) irradiation can lead to an etch-stop. The MD simulations have revealed that carbon (C) remnants at the end of a PE-ALE cycle can enhance further accumulation of C in the subsequent cycle. Under typical Ar+ ion irradiation conditions, nitrogen (N) atoms are preferentially removed from the surface over silicon (Si) atoms, and therefore, the SiN surface becomes more Si rich, which also promotes C accumulation by the formation of Si–C bonds. It is also seen that fluorine atoms contribute to the removal of Si, whereas hydrogen and C atoms contribute to the removal of N from the SiN surface.

Investigating the crystal structure of ZrB2 under varied conditions of temperature, pressure, and swift heavy ion irradiation

The influence of temperature, pressure, and swift heavy ions on the structural characteristics of a nano-sized zirconium diboride crystal (43 nm) was investigated using X-ray diffraction and Small-Angle X-ray Scattering/Wide-Angle X-ray Scattering (SAXS/WAXS) powder diffraction methods. Within the temperature range of 300–773 K, alterations in lattice parameters, lattice volume, coefficient of thermal expansion, and the kinetics of water molecule absorption on the active surface of the zirconium diboride crystal were comprehensively analyzed. Furthermore, the mechanism responsible for the structural degradation of absorbed water molecules was elucidated. The degree of amorphization in the ZrB2 nano crystal reached its maximum when subjected to irradiation by fast 132Xe26+ ions with an energy of 167 MeV, at fluences of up to 3.0 × 1014 ions/cm2. Additionally, the expansion mechanism of lattice parameters and the kinetics of the phase transformation process under the influence of swift heavy ions were studied. Notably, pressure variations did not induce a phase transition within the lattice structure. However, when exposed to an 8.0 GPa pressure, the lattice underwent substantial maximum contraction, reducing by up to 20.5 Å3.

Identifying Luminescent Boron Vacancies in h-BN Generated Using Controlled He+ Ion Irradiation.

The defect emission from h-BN at 1.55 eV is interesting as it enables optical readout of spins. It is necessary to identify the nature of the relevant point defects for its controlled introduction. However, it is challenging to engineer point defects in h-BN without changing the local atomic structure. Here, we controllably introduce boron vacancies in h-BN using an ultrahigh spatial resolution and low-energy He+ ion beam. By optimizing the He+ ion irradiation conditions, we control the quantity and location of defects spatially and along the depth of h-BN to achieve a robust photoluminescence emission at 1.55 eV from 10 K to room temperature. We show that as-generated defects activate an additional Raman mode at 1295 cm-1. Electron energy loss spectroscopy confirms introduction of boron vacancies without modification of the local h-BN crystal structure. Our results provide a deterministic strategy to create scalable boron vacancy emitters in h-BN for quantum photonics.

Composite effect of helium ion irradiation and transient thermal shock on recrystallized pure W and W-Y2O3 alloys

Tungsten (W) is the material of choice for the thermal nuclear fusion reactors of the future. In this research, pure W (PW) and W-Y2O3 alloys made by the SWIP (Southwestern Institute of Physics) wet chemical process are chosen. Simulation of long-term extreme operating conditions of fusion reactor materials by annealing and recrystallization of forged tungsten materials. The thermal shock resistance of recrystallized PW and W-Y2O3 alloys to radiation by helium (He) ions at a flux of 1.4 × 1022 ions/(m2s) was investigated. The surface microscopic pictures of recrystallized W and W-Y2O3 alloys were analyzed using scanning electron microscopy (SEM) and a laser surface profiler under the conditions of helium ion irradiation and electron beam transient thermal shock. After helium ion irradiation, the surface of PW and W-Y2O3 alloys exhibits a fuzzy structure. Under the same 0.33 GW/m2 thermal shock condition, the average width of the main cracks of PW and W-Y2O3 alloys significantly increases after helium ion irradiation, indicating that helium ion irradiation can reduce the thermal shock resistance of the surface of recrystallized W materials.

Structural and optical properties of lithium borate glasses under extreme conditions of ion irradiation

Lithium borate (LBO) glasses of the composition 10LiO2.90B2O3 were prepared by melt quenching technique and characterized for morphological, structural, and optical properties before and after the exposure to the extreme conditions of 100 MeV Ni ion irradiation. Both the morphological images and structural properties confirm the transformation of LBO glasses from amorphous to polycrystalline nature. This Ni irradiation provides thermal energy that causes the ceramization of glass. The beam interacts with the material where the top layer of the glass remelts, and ultrafast re-solidification occurs after the termination of the ion beams. The Rietveld refinement using the x-ray diffraction data shows that the irradiated LBO possesses a monoclinic crystal structure of lithium pentaborate pentahydrate (B5H10LiO13). The FTIR spectra of Ni ion irradiated glasses show the creation of non-bridging oxygens (NBO) by the formation of BO4 units at the expense of BO3 units. The direct band gap and the Urbach energy of the glasses are affected by the nickel ion irradiation and are in the range 2.73 to 2.55 eV and 0.59 to 0.42 eV. The refractive index also reveals a minor change (1.66 to 1.63). The molar electronic polarizability, metallization criteria, and transmission coefficient were estimated. The coexistence of crystalline and amorphous phases leads to a multiscale phase structure that has multilevel relaxation processes. This is a suitable condition for the improvement of the rheological properties of glass and glass ceramics. Future work involves optimization of the coexistence of these two phases during irradiation.

Structure and Chemical Composition of Ion-Synthesized Gallium Oxide Nanocrystals in Dielectric Matrices.

The ion-beam synthesis of Ga2O3 nanocrystals in dielectric matrices on silicon is a novel and promising way for creating nanomaterials based on gallium oxide. This research studies the regularities of changes, depending on the synthesis regimes used, in the chemical composition of ion-implanted SiO2/Si and Al2O3/Si samples. It has been shown that the formation of Ga-O chemical bonds occurs even in the absence of thermal annealing. We also found the conditions of ion irradiation and annealing at which the content of oxidized gallium in the stochiometric state of Ga2O3 exceeds 90%. For this structure, the formation of Ga2O3 nanocrystalline inclusions was confirmed by transmission electron microscopy.

Quantifying defect production in solids at finite temperatures: Thermally-activated correlated defect recombination corrections to DPA (CRC-DPA)

It has long been recognized from low-temperature defect production experiments and molecular dynamics (MD) simulations of displacement events in metals that the surviving defect fraction relative to the calculated Norgett-Robinson Torrens (NRT) displacements per atom (dpa) ranges from ∼1 for near-threshold displacements (representative of electron irradiation) to ∼0.3 for energetic displacement cascades (relevant for fast neutrons or energetic heavy ion irradiations). This correlated point defect recombination that occurs within a few picoseconds following the creation of a primary knock-on atom (PKA) has recently been formalized in the athermal recombination corrected dpa (ARC-DPA) parameter. However, at elevated temperatures additional thermally-stimulated correlated defect recombination processes need to be considered. For example, at irradiation temperatures above defect recovery Stage I (typically ∼40–100 K for most metals), mobile self-interstitial atoms induce additional thermally-activated correlated defect recombination that can amount to 70–80% reduction in the surviving defect fraction for low PKA energies and ∼20 to 30% reduction for high PKA energies. Since most nuclear material applications involve operation above room temperature, it is generally important to include this additional correlated defect recombination in radiation effects models. Published experimental defect recovery and defect production studies on 26 metals are analyzed to extract the surviving defect fraction following thermally-activated correlated recombination as a function of PKA energy. These experimental estimates of the athermal- plus thermal-activated correlated recombination-corrected DPA (CRC-DPA) are compared with new long-term MD simulation results on Cu and Fe. The analysis indicates the CRC-DPA fraction of surviving defects in most elements for irradiation at temperatures where interstitials are mobile is ∼25% of the NRT DPA over a wide range of PKA energies relevant for electron, ion and fast neutron irradiation conditions. Limited experimental and kinetic Monte Carlo studies suggest a further reduction to ∼10% of the NRT DPA over a wide range of PKA energies when correlated recombination due to thermally activated vacancy migration is considered. This temperature-dependent CRC-DPA should be used as the defect production parameter (rather than NRT DPA or ARC-DPA) for materials irradiated at all temperatures where defects are mobile.

Overview of Polyethylene Terephthalate Foils Patterned Using 10 MeV Carbon Ions for Realization of Micromembranes

Polymer membranes are conventionally prepared using high-energy particles from radioactive decay or by the bombardment of hundreds of MeVs energy ions. In both circumstances, tracks of damage are produced by particles/ions passing through the polymer, and successively, the damaged material is removed by chemical etching to create narrow pores. This process ensures nanosized pore diameter but with random placement, leading to non-uniform local pore density and low membrane porosity, which is necessary to reduce the risk of their overlapping. The present study is focused on the use of polyethylene terephthalate (PET) foils irradiated by 10.0 MeV carbon ions, easily achievable with ordinary ion accelerators. The ion irradiation conditions and the chemical etching conditions were monitored to obtain customized pore locations without pore overlapping in PET. The quality, shape, and size of the pores generated in the micromembranes can have a large impact on their applicability. In this view, the Scanning Transmission Ion Microscopy coupled with a computer code created in our laboratory was implemented to acquire new visual and quantitative insights on fabricated membranes.

Field-free spin-orbit torque switching of GdCo ferrimagnet with broken lateral symmetry by He ion irradiation

Current-induced magnetization switching by spin-orbit torque (SOT) is of great importance for the energy-efficient operation of spin-based memory and logic devices. However, the requirement of an external in-plane magnetic field to deterministically switch the perpendicular magnetization of a device is a bottleneck for device application. There have been many efforts to realize field-free SOT switching using interlayer/exchange coupling, the spin valve structure, or materials with lateral symmetry breaking. However, limitations of material selection or layer structure modification hinder the application of these methods in practice. Here, we demonstrate the field-free SOT switching of a GdCo ferrimagnet with lateral symmetry breaking by He ion irradiation. Local control of the magnetic property with different He ion irradiation conditions induces a lateral magnetic gradient orthogonal to the current flow direction in the ferrimagnet. We also observe out-of-plane-SOT generation due to lateral symmetry breaking, which is essential for field-free switching. Since the He ion irradiation technique is utilized for the fabrication of complementary−metal−oxide−semiconductors, and resolution can reach the nanometer level, our findings have the potential to serve as the basis for new developments in the fabrication of wafer-scale spintronic memory and logic devices with high energy efficiency and high density.

Theoretical study on the correlation of swelling peaks between neutron and heavy ion irradiated 15-15Ti stainless steel

Titanium-modified austenitic stainless steels (15-15TiSS) are currently used as the fuel cladding material of fast reactors, which are subject to higher radiation damage during their application. In this work, the radiation-induced swelling of 15-15TiSS under a fast reactor neutron and heavy ion irradiation conditions was studied by the rate theory (RT). The simulated swelling properties of 15-15TiSS under fast neutron conditions were calculated initially. The swelling peak, swelling rates, and swelling-related microstructural properties are consistent with the neutron irradiation results, indicating that the selected RT model and material parameters are reasonable. Then, the swelling properties of 15-15TiSS under various damage rates were predicted by changing the radiation damage rates from 1 × 10−6 to 1 × 10−3 dpa/s. It shows that swelling peaks are strongly dependent on temperature and the swelling peaks shift ∼50 °C toward the higher temperature with each order of magnitude increase of defects generation rate. The swelling rates and swelling-related defect evolution at 1 × 10−3 dpa/s (with a swelling peak temperature of 590 °C) are consistent with that under neutron irradiation with 1 × 10−6 dpa/s (with a swelling peak temperature of 460 °C). At length, the RT-predicted heavy ions irradiation results were verified by the previous positron annihilation lifetime spectroscopy results of ions irradiated 15-15TiSS. It indicates that heavy ion irradiation can be used to study the radiation effect of materials under neutron irradiation and should be a feasible technique used in the further screening of radiation-resistant materials.

Optimizing ion implantation to create shallow NV centre ensembles in high-quality CVD diamond

The negatively charged nitrogen-vacancy centre (so-called NV-centre) in diamond is one of the most promising systems for applications in quantum technologies because of the possibility to optically manipulate and read out the spin state of this defect, even at room temperature. Nevertheless, obtaining high NV densities (>500 ppb) close to the surface (5–20 nm) while maintaining good spin properties remain challenging. In this work we rely on a versatile ion implantation system allowing both implanting nitrogen using N2 + and creating vacancies with He+ ion bombardment at variable energies and fluence to create shallow NV ensembles. By optimizing the ion irradiation conditions as well as the surface preparation prior to treatment we successfully increase the amount of created colour centres while demonstrating narrow magnetic resonance linewidths.

Thermonuclear Fusion Reactor Plasma-Facing Materials under Conditions of Ion Irradiation and Plasma Flux

A review of experimental studies carried out at the NRC “Kurchatov Institute” on plasma-facing thermonuclear fusion reactor materials is presented in the paper. An experimental method was developed to produce high-level radiation damage in materials simulating the neutron effect by surrogate irradiation with high-energy ions. Plasma-surface interaction is investigated on materials irradiated to high levels of radiation damage in high-flux deuterium plasma. The total fluence of accelerated ions (3–30 MeV, 4He2+, 12C3+, 14N3+, protons) on the samples was 1021–1023 m−2. Experiments were carried out on graphite materials, tungsten, and silicon carbide. Samples have been obtained with a primary defect concentration from 0.1 to 100 displacements per atom, which covers the predicted damage for the ITER and DEMO projects. Erosion dynamics of the irradiated materials in steady-state deuterium plasma, changes of the surface microstructure, and deuterium retention were studied using SEM, TEM, ERDA, TDS, and nuclear backscattering techniques. The surface layer of the materials (3 to hundreds µm) was investigated, and it was shown that the changes in the crystal structure, the loss of their symmetry, and diffusion of defects to grain boundaries play an important role. The most significant results are presented in the paper as an overview of our previous work for many years (carbon and tungsten materials) as well as the relatively recent results (silicon carbide).

Temperature dependence of irradiation-induced nanocrystallization in amorphous silicon carbide

Ion irradiation induced nucleation and growth of nanoparticles in amorphous SiC at elevated temperatures are investigated in this study. Both as-deposited amorphous and Kr ion irradiation-amorphized SiC films are used for the investigation. Similar behavior of particle precipitation in the two types of the films is observed. It is found that the threshold temperature for nucleation is between 550 and 700 K, which is considerably higher than the critical temperature for full amorphization in SiC irradiated with Kr ions. There is a temperature regime up to at least 900 K, where the nucleation and growth rates are only weakly dependent of the irradiation temperature. This temperature regime is potentially useful to tailor the size and density of nanocrystalline SiC precipitates by optimizing the ion irradiation conditions.

Photoluminescence of ion-synthesized 9R-Si inclusions in SiO2/Si structure: Effect of irradiation dose and oxide film thickness

The study of hexagonal silicon polytypes attracts special attention due to their unique physical properties compared to the traditional cubic phase of Si. Thus, for some hexagonal phases, a significant improvement in the emission properties has been demonstrated. In this work, the luminescent properties of SiO2/Si structures irradiated with Kr+ ions at different doses and annealed at 800 °C have been systematically investigated. For such structures, a photoluminescence line at ∼ 1240 nm is observed and associated with the formation of hexagonal 9R-Si phase inclusions. It is found that the variation in the thickness of oxide film and the relative position of ion distribution profile and film/substrate interface leads to a regular change in the luminescence intensity. The nature of the observed dependencies is discussed as related mainly to the interplay between the factors contributing to the formation of 9R-Si inclusions and the generation of radiation defects in the Si substrate—centers of nonradiative recombination. The revealed regularities suggest optimal ion irradiation conditions for synthesis of optically active 9R-Si phase in diamond-like silicon.

Filament-assisted reactive magnetron sputter deposition of VSiN films

Many vapor deposition techniques utilize ion irradiation during growth of transition metal nitride-based films to obtain low porosity and to control nanostructural evolution, phase content, and crystallographic texture. In this study, V1-xSixN films (0.18 ≤ x ≤ 0.30) were deposited by a reactive magnetron sputtering process in which a heated filament was used to enhance growth temperature and ion bombardment. Filament discharge current (0.0 A ≤ ID ≤ 2.6 A) and sputter target powers (PV = 500 W,250 W, PSi = 180 W,90 W) were varied among depositions. With filament-assistance, substrate current densities doubled relative to the current densities achieved without the filament. Atomic force microscopy, scanning electron microscopy, and electrical resistivity measurements indicated that the use of the hot filament reduced the porosity present in the deposited films. X-ray diffraction revealed that all VSiN films contained a single crystalline phase with a crystal structure and lattice spacing similar to NaCl-type VN. Texture coefficient calculations showed that increases in filament discharge current initially enhanced (111) texturing until, at the maximum filament discharge current, a mixed (111)/(200) texture was obtained. Film hardness and elastic modulus were measured using nanoindentation and found to scale with filament discharge current to maximum values of 16 GPa and 218 GPa, respectively. Rockwell indentation tests revealed that film adhesion decreased with increases in filament discharge current and film deposition rate. Telephone cord blisters were observed in films deposited at the highest filament discharge currents confirming that the ion irradiation conditions generated compressive residual stresses.

On the Assessment of Radiation Resistance of Materials in Imitation Experiments

The theoretical and experimental data determining the similarities and differences between the defect formation processes under the conditions of neutron and imitation ion irradiation are analyzed. The analysis involves such factors as the type and energy of cascade-forming particles, the composition and structure of irradiated materials, and the irradiation temperature. A number of proven procedures for calculating the concentration of radiation defects and the number of displacements per atom for cascade-forming types of irradiation are revised. The key factors ensuring the similarity of full-scale and simulation experiments are considered, with the following two of them singled out as the principal factors: the damaging dose accumulation rate (flux) and the temperature, which determine the course of relaxation processes. Simple methods are proposed for estimating the number of primary knocked-on atoms (PKA) under neutron irradiation and the fraction of elastic energy losses spent on defect formation. The substantiation of a unified fractal structure of atomic displacement cascades in a given target is provided, regardless of the nature and energy of the cascade-forming types of irradiation.

Temperature-Dependent Cavity Swelling in Dual-Ion Irradiated Fe and Fe-Cr Ferritic Alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550 °C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50 °C lower peak swelling temperature than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470 °C, but seems to have little effect or slightly enhances swelling above ~470 °C. Cavities were observed in all the irradiated samples between 400-550 °C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artefact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

Striking effect of solute elements (Mn, Ni) on radiation-induced segregation/precipitation in iron-based model alloys

The evolution of radiation damage in steels is a major issue for the safe operation of nuclear power plants. Mn and Ni contribution to the formation and evolution of microstructural features remains a controversial issue. The present study aims at investigating their effects on microstructure by irradiating undersaturated BCC Fe-3.3at.%Ni and Fe-2.8at.%Mn model alloys. Two different ion irradiation conditions have been applied to study the effect of irradiation dose rate in both of these Fe-based alloys at a temperature of 673 K. In all cases, irradiation promotes the formation of dislocation loops. In the FeNi alloy, radiation-induced segregation is observed on loops, leading to precipitation of a FCC phase in the low dose rate irradiation condition only. In the case of FeMn, even if no precipitation was revealed, highly enriched decorated dislocation loops are observed, with a higher Mn segregation at radiation defects than Ni.

Temperature-dependent cavity swelling in dual-ion irradiated Fe and Fe-Cr ferritic alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550°C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50°C lower peak swelling temperature difference than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470°C, but seems to have little effect or slightly enhances swelling above ~470°C. Cavities were observed in all the irradiated samples between 400-550°C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artifact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

Effects of Nitrogen Impurity Concentration on Nitrogen-Vacancy Center Formation in 4H-SiC

Spin defects of which states can be manipulated in Silicon Carbide (SiC) have drawn considerable attention because of their applications to quantum technologies. The single negatively-charged pairs of VSi and nitrogen atom (N) on an adjacent C site (NCVSi- center) in SiC is suitable for them. This paper reports the formation of NCVSi- centers on 4H-SiC epilayers with different nitrogen concentrations using light/heavy ion irradiation and subsequent thermal annealing. The formation of NCVSi- centers is characterized by the near infrared photoluminescence (PL) spectroscopy. It is shown that the PL intensity from NCVSi- centers depends on the N concentration and the ion irradiation conditions. The PL intensity increases monotonically with increasing the N concentration when the N concentration is above 2.6×1016 cm-3, whereas no linear correlation between them does not appear below that N concentration. Although the PL intensity increases with increasing defects induced by ion irradiation, the PL quenching due to neighboring residual defects appear at above the areal vacancy concentration of 1017 vac/cm2 and the broad Raman scattering spectra originated from vibration modes of amorphized regions hinder the PL from NCVSi- centers at above 1018 vac/cm2. The formation mechanism and the charge state stability of NCVSi- centers are discussed based on the obtained results.