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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Radiation defect dynamics in Si at room temperature studied by pulsed ion beams

The evolution of radiation defects after the thermalization of collision cascades often plays the dominant role in the formation of stable radiation disorder in crystalline solids of interest to electronics and nuclear materials applications. Here, we explore a pulsed-ion-beam method to study defect interaction dynamics in Si crystals bombarded at room temperature with 500 keV Ne, Ar, Kr, and Xe ions. The effective time constant of defect interaction is measured directly by studying the dependence of lattice disorder, monitored by ion channeling, on the passive part of the beam duty cycle. The effective defect diffusion length is revealed by the dependence of damage on the active part of the beam duty cycle. Results show that the defect relaxation behavior obeys a second order kinetic process for all the cases studied, with a time constant in the range of ∼4–13 ms and a diffusion length of ∼15–50 nm. Both radiation dynamics parameters (the time constant and diffusion length) are essentially independent of the maximum instantaneous dose rate, total ion dose, and dopant concentration within the ranges studied. However, both the time constant and diffusion length increase with increasing ion mass. This demonstrates that the density of collision cascades influences not only defect production and annealing efficiencies but also the defect interaction dynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Effect of Confinement of SIA Cluster Diffusion by Impurities on Radiation Defect Accumulation due to 14MeV neutrons in Tungsten

Impurities acting as traps, reduce the self-interstitial atom (SIA) cluster mobility, thereby increasing recombination and in turn enhancing radiation-resistance of polycrystalline materials. However, at elevated temperatures, depending on SIA-trap (impurity) binding energy, SIA clusters may trap and detrap (detach) numerous times. In tungsten, SIA clusters of all sizes glide one-dimensionally (1D) along their Burgers vector direction. Hence, detrapped SIAs would retrace their path, confining their 1D-glide between traps. However, small SIA clusters can change the direction of their 1D-glide by overcoming a rotation barrier. In addition, impurities or solutes are known to reduce this barrier. A lower rotation barrier can result in detrapped SIA clusters that diffuse in a random 1D direction, effectively leading to 3D (net-3D) diffusion. Here we investigate the effect of two cases of SIA diffusion, namely confined-1D diffusion and net-3D diffusion, on the damage accumulation in polycrystalline tungsten under 14 MeV neutron irradiation using the object kinetic Monte Carlo method. Simulations were performed using cascade debris obtained from molecular dynamics simulations with primary knock-on atom (PKA) energies corresponding to 14 MeV neutrons. In the simulations with net-3D diffusion, SIA clusters of all sizes diffuse in a random 1D direction after the clusters detrap. While for the confined 1D diffusion case, SIA clusters larger than size 5 retain their original direction of 1D-glide. In both types of simulations, trapped vacancies are assumed to be permanently immobilized. Unexpectedly, the damage accumulation was lower with confined 1D diffusion than that with net-3D. We present a systematic comparison of the influence of the two cases of SIA diffusion on the radiation damage accumulation as a function of dose rate, detrapping barrier, and impurity concentration at 1025 K.

Accumulation and recovery of lattice damage, tungs↗
DOE OSTI2019

Impact of pre-existing disorder on radiation defect dynamics in Si

The effect of pre-existing lattice defects on radiation defect dynamics in solids remains unexplored. Here, we use a pulsed beam method to measure the time constant of defect relaxation for 500 keV Ar ion bombardment of Si at 100 °C with the following two representative types of pre- existing lattice disorder: (i) point defect clusters and (ii) so-called “clamshell” defects consisting of a high density of dislocations. Results show that point defect clusters slow down defect relaxation processes, while regions with dislocations exhibit faster defect interaction dynamics. These experimental observations demonstrate that the dynamic aspects of damage buildup, attributed to defect trapping-detrapping processes, can be controlled by defect engineering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Radiation defect dynamics in SiC with pre-existing defects

The influence of pre-existing lattice disorder on radiation defect dynamics in SiC remains unexplored. Here, we use a pulsed ion beam method to study dynamic annealing in Ar-ion-bombarded 3C-SiC at 200°C with different levels of pre-existing lattice disorder. Results reveal a nonmonotonic dependence of the defect relaxation time constant on the level of pre-existing disorder, exhibiting a maximum of ~4 ms at a level of relative initial disorder of ~0.4, while crystals without pre-existing damage are characterized by a time constant of ~1.4 ms. These observations demonstrate that radiation defect dynamics in SiC can be controlled by defect engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2013

MD simulations of phase stability of PuGa alloys: Effects of primary radiation defects and helium bubbles

We present classical molecular dynamics (MD) with Modified Embedded Atom Model (MEAM) simulations to investigate the role of primary radiation defects and radiogenic helium as factors affecting the phase stability of PuGa alloys in cooling–heating cycles at ambient pressure. The models of PuGa alloys equilibrated at ambient conditions were subjected to cooling–heating cycles in which they were initially cooled down to 100 K and then heated up to 500 K at ambient pressure. The rate of temperature change in the cycles was 10 K/ns. The simulations showed that the initial FCC phase of PuGa alloys undergo polymorphous transition in cooling to a lower symmetry α'-phase. All the alloys undergo direct and reverse polymorphous transitions in the cooling–heating cycles. The alloys containing vacancies shift in both transitions to lower temperatures relative to the defect-free alloys. The radiogenic helium has much less effect on the phase stability compared to that of primary radiation defects (in spite of the fact that helium concentration is twice of that for the primary radiation defects). Lastly, this computational result agrees with experimental data on unconventional stabilization mechanism of PuGa alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Non-monotonic temperature dependence of radiation defect dynamics in silicon carbide

Understanding response of solids to particle irradiation remains a major materials physics challenge. This applies even to SiC, which is a prototypical nuclear ceramic and wide-band-gap semiconductor material. The lack of predictability is largely related to the complex, dynamic nature of radiation defect formation. Here, we use a novel pulsed-ion-beam method to study dynamic annealing in 4H-SiC ion-bombarded in the temperature range of 25–250 °C. We find that, while the defect recombination efficiency shows an expected monotonic increase with increasing temperature, the defect lifetime exhibits a non-monotonic temperature dependence with a maximum at ~100 °C. This finding indicates a change in the dominant defect interaction mechanism at ~100 °C. As a result, the understanding of radiation defect dynamics may suggest new paths to designing radiation-resistant materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Simulation of radiation-defect formation processes in heterostructures with self-assembled Ge(Si)/Si(001) nanoislands under neutron irradiation

TRISQD software is developed for the computer simulation of processes in which radiation defects are formed under the corpuscular irradiation of semiconductor heterostructures with lenticular nanoinclusions of various shapes. The computer program is used to study defect-formation processes in p-i-n diodes with the i region having a built-in 20-period lattice of self-assembled Ge(Si) nanoislands formed under irradiation with high-energy neutrons. It is found that the fraction of Ge(Si) nanoislands in which point radiation defects are formed under the impact of atomic-displacement cascades is ≤3% of their total number in the lattice. More than 94% of the defects are localized in the bulk of the p, n, and i regions of the diode and in silicon layers that separate sheets of Ge(Si) nanoislands.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Specific features of formation of radiation defects in the silicon layer in 'silicon-on-insulator' structures

Specific features of formation of radiation defects in thin silicon layer of silicon-on-insulator (SOI) structures have been studied. It is shown that there are differences between variations in the structural and electrical properties of the thin silicon layer and those in bulk silicon crystals (with similar electrical characteristics) subjected to the same radiation effect. It is established that the embedded insulator in the SOI structure represents a barrier for motion of radiation-induced intrinsic interstitial silicon atoms, which brings about an increase in the dose of bombarding ions, which leads to the loss of single-crystallinity of the silicon layer in a SOI structure. It is shown that {gamma}-ray irradiation with doses unaffecting the electrical conductivity of bulk silicon crystals appreciably affects the conductivity of the silicon layer in the SOI structures. In addition, variation in the conductivity of silicon layer is related to variation in the density of surface states at the interface between the silicon layer and the built-in insulator, rather than to generation of conventional radiation-induced structural defects in silicon.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A Finite Difference informed Random Walk solver for simulating radiation defect evolution in polycrystalline structures with strongly inhomogeneous diffusivity

Diffusivity of species and defects on grain boundaries is usually several orders of magnitude larger than that inside grains. Such strongly inhomogeneous diffusivity requires prohibitively high computational demands for modeling microstructural evolution. Here, this paper presents a highly-efficient numerical solver, combining the Finite Difference method and Random Walk model, designed for accurately modeling strongly inhomogeneous diffusion within polycrystalline structures. The proposed solver, termed Finite Difference informed Random Walk (FDiRW), integrates a customized Finite Difference (cFD) scheme tailored for fast diffusion along thin grain boundaries represented by a single-layer of nodes. Numerical experiments demonstrate that the FDiRW solver achieves an impressive efficiency gain of 1560x compared to traditional Finite Difference methods while maintaining accuracy, making it feasible for personal computer machines to handle diffusional systems with strongly inhomogeneous diffusivity across static polycrystalline microstructures. The model has been successfully applied to simulate radiation defect evolution, showcasing its scalability to engineering scales in both length and time dimensions.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Formation and annealing of radiation defects in tin-doped p-type germanium crystals

The effect of tin on the formation and annealing of radiation defects in p-type germanium crystals irradiated with 6-MeV electrons at a temperature of 80 K is studied. It is shown that acceptor complexes SnV with a hole ionization enthalpy of 0.16 eV are dominant in irradiated Ge:(Sn, Ga) crystals after their heating to a temperature of 300 K. These complexes disappeared as a result of the annealing of irradiated crystals in the temperature range 30-75 Degree-Sign C. Annealing of irradiated crystals at temperatures in the range 110-150 Degree-Sign C brings about the formation of deep-level centers with a donor level at E{sub v} + 0.29 eV; this center is presumably related to a complex consisting of a tin atom and an interstitial gallium atom.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Deterministic Role of Collision Cascade Density in Radiation Defect Dynamics in Si

The formation of stable radiation damage in solids often proceeds via complex dynamic annealing (DA) processes, involving point defect migration and interaction. The dependence of DA on irradiation conditions remains poorly understood even for Si. Here, we use a pulsed ion beam method to study defect interaction dynamics in Si bombarded in the temperature range from ~ -30 °C to 210 °C with ions in a wide range of masses, from Ne to Xe, creating collision cascades with different densities. We demonstrate that the complexity of the influence of irradiation conditions on defect dynamics can be reduced to a deterministic effect of a single parameter, the average cascade density, calculated by taking into account the fractal nature of collision cascades. For each ion species, the DA rate exhibits two well-defined Arrhenius regions where different DA mechanisms dominate. These two regions intersect at a critical temperature, which depends linearly on the cascade density. The low-temperature DA regime is characterized by an activation energy of ~ 0.1 eV , independent of the cascade density. The high-temperature regime, however, exhibits a change in the dominant DA process for cascade densities above ~ 0.04 at.%, evidenced by an increase in the activation energy. These results clearly demonstrate a crucial role of the collision cascade density and can be used to predict radiation defect dynamics in Si.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Radiation defect dynamics in GaAs studied by pulsed ion beams

Gallium arsenide under ion bombardment at room temperature and above exhibits pronounced dynamic annealing that remains poorly understood. Here, we use a pulsed beam method to study radiation defect dynamics in GaAs in the temperature range of 20–100 °C irradiated with 500 keV Xe ions. Results show that, with increasing temperature, the defect relaxation time constant monotonically decreases from ~5.2 to ~0.4 ms. A change in the dominant dynamic annealing process occurs at a critical temperature of ~60 °C, as evidenced by a change in the activation energy. A comparison with the other semiconductors studied by the pulsed beam method (Si, Ge, and 4H-SiC) reveals that both the high-temperature activation energy and the temperature below which dynamic annealing becomes negligible scale with the melting point.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effects of collision cascade density on radiation defect dynamics in 3C-SiC

Effects of the collision cascade density on radiation damage in SiC remain poorly understood. We study damage buildup and defect interaction dynamics in 3C-SiC bombarded at 100 °C with either continuous or pulsed beams of 500 keV Ne, Ar, Kr, or Xe ions. Here, we find that bombardment with heavier ions, which create denser collision cascades, results in a decrease in the dynamic annealing efficiency and an increase in both the amorphization cross-section constant and the time constant of dynamic annealing. The cascade density behavior of these parameters is non-linear and appears to be uncorrelated. Our results demonstrate clearly (and quantitatively) an important role of the collision cascade density in dynamic radiation defect processes in 3C-SiC.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Radiation defect dynamics in solids studied by pulsed ion beams

The buildup of radiation damage is often a dynamic phenomenon proceeding via migration and interaction of point defects ballistically generated in collision cascades. In this work, we describe how the dynamic aspects of damage formation via inter-cascade defect interaction can be studied by pulsed ion beam bombardment. We illustrate the pulsed beam method by an example for Si bombarded at 100°C with 1.35 MeV Xe ions and compare with previously reported data for 0.5 MeV Xe ion irradiation. Results reveal that the time constant of defect relaxation strongly depends on both ion energy and the depth from the sample surface. This observation cannot be entirely explained as an effect of the collision cascade density on defect interaction dynamics, based on depth dependencies of cascade densities calculated with a recently developed model that accounts for the fractal nature of cascades. There results call for future systematic studies to better understand ion energy and cascade density effects on defect relaxation dynamics in Si.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Radiation defect dynamics in 3 C -, 4 H -, and 6 H -SiC studied by pulsed ion beams

Radiation damage behavior of SiC depends on its lattice structure. Here, we use a pulsed-ion-beam method to study defect interaction dynamics in 6H-SiC and damage buildup in 4H- and 6H-SiC irradiated at 100 °C with 500 keV Ar ions. These results are compared with previously reported data for Ar-ion-irradiated 3C- and 4H-SiC. We find that, for these irradiation conditions, damage buildup in 3C- and 6H-SiC is statistically indistinguishable and is significantly more efficient than in 4H-SiC. Within a stimulated amorphization model, 4H-SiC is described by a reduced amorphization cross-section constant, while the point defect cluster production cross-section is the same (within experimental errors) for the three polytypes studied. Moreover, 4H-SiC exhibits slower defect relaxation dynamics than 3C- and 6H-SiC. These results clearly demonstrate the importance of the lattice structure in damage buildup and defect interaction dynamics in SiC.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Radiative Defects in Chloride-Activated CdSe Thin Films

Defect recombination limits wide-gap Se-based chalcogenide devices, yet how chloride activation reshapes radiative pathways remains unclear. Here we show that a 40 min CdCl 2 anneal converts evaporated CdSe from porous nanograins into dense micrometer-scale polycrystals and sharpens the optical band edge, reducing the Urbach energy from 85 to 17 meV at 300 K. Combining temperature- and fluence-dependent photoluminescence (PL), time-resolved PL, hyperspectral mapping, and hybrid-DFT, we resolve three emissive channels and identify their mechanisms. The near-edge band is excitonic at low temperature and evolves into free-carrier emission at elevated temperature. A sub-gap band at E g −0.45 eV requires above-gap carriers and thermally quenches with a 0.16 eV activation energy. A broad ∼1.05 eV infrared band is excited by above- and below-gap photons and retains microsecond lifetimes at room temperature; patial mapping links it to edge-rich microstructure. Calculations suggest selenium-vacancy and cadmium-vacancy−chlorine complexes, pointing to routes to suppress defect-related losses in wide-gap chalcogenide devices.

Abudulimu, Abasi [Univ. of Toledo, OH (United Stat↗