Thermal Spike Process Research Articles

Raman spectroscopy study of damage induced in cerium dioxide by swift heavy ion irradiations

The damage induced in cerium dioxide by swift heavy ion irradiation was studied by micro-Raman spectroscopy. For this purpose, polycrystalline sintered pellets were irradiated by 100-MeV Kr, 200-MeV Xe, 10-MeV, and 36-MeV W ions in a wide range of fluence and stopping power (up to ∼28 MeV μm−1). No amorphization of ceria was found whatsoever, as shown by the presence of the peak of Raman-active T2g mode (centered at 467 cm−1) of the cubic fluorite structure for all irradiation conditions. However, a clear decrease of the T2g mode peak intensity was observed as a function of ion fluence to an asymptotic relative value of about 45%. Similar decays were also observed for satellite peaks and second-order peaks. Track radii deduced from the decay kinetics for the 36-MeV W ion data are in good agreement with previous determinations by X-ray diffraction and reproduced by the inelastic thermal spike model for low ion velocities. However, interaction between the nuclear and electronic stopping powers is needed to describe the decay kinetics of 10-MeV W ion data by the thermal spike process. Moreover, the asymmetrical broadening of the main T2g peak after irradiation was analyzed with different theoretical models.

Quantitative analysis of the epitaxial recrystallization effect induced by swift heavy ions in silicon carbide

This paper discusses recent results on the recrystallization effect induced by swift heavy ions (SHI) in pre-damaged silicon carbide. The recrystallization kinetics was followed by using increasing SHI fluences and by starting from different levels of initial damage within the SiC samples. The quantitative analysis of the data shows that the recrystallization rate depends drastically on the local amount of crystalline material: it is nil in fully amorphous regions and becomes more significant with increasing amount of crystalline material. For instance, in samples initially nearly half-disordered, the recrystallization rate per incident ion is found to be 3 orders of magnitude higher than what it is observed with the well-known IBIEC process using low energy ions. This high rate can therefore not be accounted for by the existing IBIEC models. Moreover, decreasing the electronic energy loss leads to a drastic reduction of the recrystallization rate. A comprehensive quantitative analysis of all the experimental results shows that the SHI induced high recrystallization rate can only be explained by a mechanism based on the melting of the amorphous zones through a thermal spike process followed by an epitaxial recrystallization initiated from the neighboring crystalline regions if the size of the latter exceeds a certain critical value. This quantitative analysis also reveals that recent molecular dynamics calculations supposed to reproduce this phenomenon are wrong since they overestimated the recrystallization rate by a factor ∼40.

An analysis of radiation effects on NdFeB permanent magnets

Radiation-induced demagnetization of permanent magnets may present a serious problem in a number of applications including robots conducting rescue and sampling missions in radiation-intense environments, NASA applications, and particle accelerators. Therefore, developing a good understanding of the underlying mechanisms of this phenomenon is crucial. In this study, hysteresis loops pre- and post-irradiation are examined via the Jiles–Atherton (JA) model, Molecular Dynamics (MD) simulations are conducted to establish length and time scales of the thermal spike process, and Density Functional Theory (DFT) calculations are performed to better characterize the effects of microstructure damage on the magnetization. It is found that the interaction between the domains of the magnet increases and that the pinning energy is also increased in the irradiated sample. Furthermore, the MD simulation allowed us to determine that the thermal spike process occurs on time scales of tens to hundreds of femtoseconds and on length scales of a few nanometers. Finally, the DFT simulations clearly depicted the effects of lattice structure defects on the magnetization. These observations showed qualitative agreement with previous studies. The results of this study will be used in a future Monte Carlo simulation that will attempt to take all these effects into account to model the process of radiation-induced demagnetization.

Molecular dynamics simulations of damage production by thermal spikes in Ge

Molecular dynamics simulation techniques are used to analyze damage production in Ge by the thermal spike process and to compare the results to those obtained for Si. As simulation results are sensitive to the choice of the inter-atomic potential, several potentials are compared in terms of material properties relevant for damage generation, and the most suitable potentials for this kind of analysis are identified. A simplified simulation scheme is used to characterize, in a controlled way, the damage generation through the local melting of regions in which energy is deposited. Our results show the outstanding role of thermal spikes in Ge, since the lower melting temperature and thermal conductivity of Ge make this process much more efficient in terms of damage generation than in Si. The study is extended to the modeling of full implant cascades, in which both collision events and thermal spikes coexist. Our simulations reveal the existence of bigger damaged or amorphous regions in Ge than in Si, which may be formed by the melting and successive quenching induced by thermal spikes. In the particular case of heavy ion implantation, defect structures in Ge are not only bigger, but they also present a larger net content in vacancies than in Si, which may act as precursors for the growth of voids and the subsequent formation of honeycomb-like structures.

Synthesis of embedded Si nano-particles using swift heavy ions and its optical properties

ABSTRACTEmbedded Si nano-particles of average size around 5nm were synthesized in an amorphous Si matrix by two stage ion implantation processes. It has been observed that amorphous Si (a-Si) layers were recrystallized using 50 MeV Au ions with enhanced regrowth rate with activation energy in the range of 0.29 eV. During the crystallization process Si nanocrystals were formed in the a-Si layers due to sudden quenching of the molten tracks created by MeV Au ions. The recrystalizations were confirmed by Rutherford backscattering spectrometry-Channeling (RBSC) technique. The structural modification and nanocluster creation that emerged during recrystallization process was observed in high-resolution transmission electron microscopy and photoluminescence (PL) spectroscopy. The PL emission was observed over a broad band of 2.8 – 3.4 eV and centered at 3.25 eV. The Si nano-crystal formation can be explain by a mechanism combining the melting within the ion tracks by thermal spike process and the subsequent recrystallization nucleated from the crystalline sides at the interface.

Mechanism of the swift heavy ion induced epitaxial recrystallization in predamaged silicon carbide

Although silicon carbide has attracted extensive investigations of ion irradiation effects at low energy owing to its potential use in harsh environments, very few works were carried out in the field of ion irradiation at high energy. A recent preliminary study exploring the combination of low and high energy ion irradiation effects in silicon carbide revealed that the damaged layer formed by low energy ion irradiation can undergo an epitaxial recrystallization under subsequent swift heavy ion irradiation. The present paper is devoted to the investigation of the mechanisms at the origin of this phenomenon by performing additional experiments. A detailed analysis of the kinetics of this recrystallization effect demonstrates that the latter cannot be explained by the models proposed for the well-known ion-beam-induced epitaxial crystallization process. Furthermore, it is found that this effect can be accounted for by a mechanism combining the melting within the ion tracks of the amorphous zones through a thermal spike process and their subsequent epitaxial recrystallization initiated from the neighboring crystalline regions wherever the size of the latter surpasses a certain critical value.

Measurement of energy and angular distributions of secondary ions in the sputtering of gold by swiftAunclusters: Study of emission mechanisms

Energy and angular distributions of negative ions (Au–, Au 2 - , Au 3 - , and Au 5 - ) emitted from gold target bombarded by Au, Au4, and Au9 projectiles at 200 keV/atom were measured with a multipixel position sensitive detector. The angular distributions are symmetrical with respect to the normal to the target surface and forward peaked. They depend on the type of emitted ions, on the emission energy, and on the projectile size. More forward directed emission is observed with Au9 projectiles. The secondary ion energy distributions obtained with Au and Au4 projectiles are well reproduced by a sum of linear collision cascades and thermal spike processes. However, in the case of Au9 projectiles the energy distributions are better described by using a simple spike model with two different average temperature regimes: the first one corresponds to high emission energy occurring in the early stage of the whole process, and the second to the low energy component.

Radial variation of track damage in polycarbonate

Several authors reported data showing a differentiated structure of ion tracks in some polymers, when analysed at submicroscopic level. Four zones can be distinguished: core, halo, cross-linking and bulk, at least in tracks produced by ions with specific energies higher than 0.2 MeV/n. Nevertheless there is a lack of experimental data for tracks produced by low specific energy ions, where thermal spike mechanisms prevails. In the present work, data of the track diameter evolution as a function of etching time are reported for ions with 2.6, 0.09 and 0.02 MeV/n specific energies. The four typical track zones can be identified in tracks produced by 2.6 MeV/n ions. Conversely, for 0.09 and 0.02 MeV/n ion tracks, the variations are smooth and the transitions between zones are not clear. This effect could be related to the thermal spike process.

Observation of local heating in an ion track by measuring the ion-induced luminescence spectrum

The rise of the local temperature in an ion track was observed by utilizing fast exciton radiative recombination in the layered perovskite-type materials (C 3H 7NH 3) 2PbBr 4 and (C 4H 9NH 3) 2PbBr 4. In (C 3H 7NH 3) 2PbBr 4, the local temperature was an increasing function of the excitation density or linear energy transfer (LET). However, the same local temperature was obtained for different LETs in (C 4H 9NH 3) 2PbBr 4, which was attributed to local melting of the organic layers. Furthermore, the obtained value of the local “melting point” was much higher than that in the bulk analog of the organic and inorganic layers, which indicates that nonequilibrium phonon dynamics plays a crucial role in the thermal spike process.

Influence of thermal spikes on preferred grain orientation in ion-assisted deposition

Theoretical models are presented to explain how thermal spike processes can induce the development of preferred orientation of polycrystalline grains during ion-assisted atomic deposition and thin film growth. Two ion energy regimes are investigated. In the first, higher-energy regime, ions penetrate growing grains and generate defects at rates dependent upon the probability of ion channeling in individual grains. The volume free-energy density of different grains is, therefore, different and the thermal spike arising from each ion impact results in preferential growth of grains with the lowest volume free-energy density. In the second, lower-energy regime, ions do not penetrate nor create defects in grains but the ion-induced thermal spikes centered on the surface can enhance surface atomic migration and lead to preferential growth of grains with the lowest surface free-energy density. The results in these two regimes are compared with the predictions of a model for preferred grain orientation evolution in the absence of ion assistance and where minimization of surface free-energy density is the driving process. Ion energy, ion flux density, depositing atom flux density, and system temperature conditions are established where ion-assisted and non-ion-assisted rates of preferred grain orientation become comparable. It is shown that, in general, the tendency toward preferred orientation is not a simple function of the energy per deposited atom.

Modeling of damage creation in metallic materials under swift heavy ion irradiations

A variety of recent experiments in metallic targets unambiguously prove that a dense electron excitation induces atomic displacements leading to bulk damage. However, till now there is no convincing theoretical model that explains these. In the present work, a model based on the thermal spike mechanism has been proposed to account for the experimental evidence and a microscopic energy transfer mechanism between excited electrons and target atoms. This model follows four stages in the time scale of events: (1) ejection of target electrons and atoms, (2) slowing down of ejected electrons and thermalization of target valence electrons, (3) conversion of energy from thermalized electrons to target atoms, and (4) structural modification of the target. The characteristics of energy transfers are indirect and coherent in space and time. Based on mathematical developments of this model, electronic energy loss-induced thermal spike process and accompanying defect evolution are numerically simulated. In a collective process involving many neighboring target atoms, this thermal spike process is sufficient to result in microstructure modification in the bulk of metallic materials. Therefore, in the framework of this model, experimental results available in metallic materials can be explained.

Modification of defects induced by nuclear collisions in Fe and Ni in electronic stopping power regime

Electronic energy loss (Se) induced modification of collisional defects in Fe and Ni was theoretically studied under the framework of thermal spike model. The initial spatial density of defects created by nuclear collisions was calculated by use of binary collision theory with charge screen. Taking into account the characteristics of defect annealing substages, temperature dependent atomistic jumps and migrations, the modification of collisional defects in Fe and Ni caused by Se was simulated in a cylindrical geometry. For iron, the theoretical damages were deduced from calculations and compared with experiments. It was found that the number z of quasi-free electrons in iron participating Se induced thermal spike process is about 1.5 per atom and, by use of z=1.5, the deduced defect recombination cross section values in iron are in agreement with experiments. Furthermore, three regions characterized by the concentrations of interstitials and vacancies are proposed. Similar variation tendency of damage efficiency in nickel was observed from numerical calculations. Thus, the Se induced defect modification in pure metals may be due to a transient thermal process.

On the mechanisms of target modification by ion beams and laser pulses

A comparison is made between the importance of the following categories of processes in ion-surface and laser-surface interactions: ballistic, thermal-spike, residual-defect-induced, and electronic. It is shown that ballistic processes are important for ion sputtering, ion mixing, and ion composition change. Also, they exist with laser-pulse sputtering but are not important. The picture with thermal-spike processes is the inverse, such processes being unimportant in most ion-surface interactions, but very important indeed with laser-pulse sputtering. In the latter case one must distinguish between normal vaporization and phase explosion, both of which are fundamental. On the other hand, normal boiling may possibly be unimportant owing to the negligible density of heterogeneous nuclei formed in the bulk, ∼ 10 6 kg −1. However, it is essential to note that, if nuclei can be formed at the surface, the density will be much higher. Sputtering due to subsurface heating probably does not exist. Residual defects are basic to all ion-surface interactions, but most especially to ion mixing. Here most (∼90%) of the observed mixing is due to residual defects and only a lesser amount is due to ballistic events. But the role of thermal spikes in ion mixing appears to have been exaggerated. Finally, electronic processes are somewhat important to ion sputtering but have not yet been quantified for ion mixing and composition change. On the other hand, although they are very important indeed with laser-surface interactions, a basic qualification must be made: the primary laser-surface interactions are dominantly electronic, but most of the deposited energy is rapidly converted from excitation to heat, and it is for this reason that thermal-spike sputtering is so prominent. Finally, we consider the fact that laser sputtering has two aspects. These are the primary processes, which lead to particle expulsion, and the secondary processes. The latter arise due to collisions amongst the emitted particles, with the result that the particles, effectively, “lose memory” of the primary process.

Ion beam induced atomic transport in Pd/Ge system

Bilayer system of Pd/Ge is irradiated at room temperature using a 400 keV Ar + beam. The evolution of the intermixed layer at the interface in this system is studied by Rutherford backscattering spectrometry (RBS), and by measurement of the variation in resistance. The experimental results are compared with the existing theoretical models. Both RBS and resistance measurements indicate that Pd and Ge atoms in this system are found to follow collisional cascade processes and to a less degree, thermal spike processes.

The effect of dense and dilute collision cascades on helium bubbles in metals

The evolution of small helium bubbles in thin gold and aluminum foils has been followed during irradiation with high energy heavy ions (400 keV Ar + and 200 keV Xe +) using in-situ transmission electron microscopy. Substrate materials were chosen in order that the heavy ion irradiation would produce dilute (aluminum) and dense (gold) collision cascades. Significant differences in bubble behaviour are observed in the two cases, the major effect of dilute cascades being bubble shrinkage due to helium resolution by direct displacement of the gas out of the bubbles. Effects observed for dense cascades, however, include the disappearance and Brownian motion of bubbles under irradiation. The paper will present recent experimental results as well as simple numerical modelling of the observed behaviour. In particular, our interpretation of the dense cascade effects is consistent with the type of displacement and thermal spike processes that recent molecular dynamics simulations have indicated may be important in radiation damage in metals.

The Effect of Displacement Cascades on Small Helium Bubbles in Aluminum and Gold

AbstractThe evolution of individual helium bubbles in thin foils of gold and aluminum irradiated with 400 keV Ar+ and 200 keV Xe+ has been followed with in-situ transmission electron microscopy for a comparison between the effects of dilute (Al) and dense (Au) collision cascades. Bubble shrinkage in Al has been attributed to direct displacement of the gas out of the bubbles. Effects in Au, include the disappearance and Brownian motion of bubbles under irradiation, and are consistent with thermal spike processes seen in molecular dynamics simulations.

Track recrystallization in semiconductors observed for xenon irradiations at 0.9 MeV amu-1

Abstract High radiation resistance of monocrystalline diamond, Si, GaP and Ge is observed in exposures ti Xe ions. This fact indicates that no extended destructions induced by (d E /d x e are formed in these materials. The analysis of experimental data leads to the conclusion that the excited spike relaxation in semiconductors may be accompanied by a lattice recrystallization. Coulomb field and thermal spike processes are discussed as sequential stages of the dissipation of the deposited energy. Recrystallization suggests that the process can develop in a way that is opposite to that of latent track formation.

Modification of the optical and structural properties of dielectric ZrO2 films by ion-assisted deposition

Low-energy bombardment by argon and oxygen ions has been used in the deposition of thin dielectric films of ZrO2. The film packing density has been improved from 0.83 to unity with a corresponding increase in the refractive index from 1.84 to 2.19. The highest stable refractive index measured was 2.23 for oxygen ion-assisted deposition of ZrO2 on a substrate heated to 300 °C. Ion bombardment during condensation of evaporated ZrO2 on a room temperature substrate results in crystallization into the cubic phase which is consistent with previous studies of ion impact crystallization by thermal-spike processes. At elevated substrate temperatures the monoclinic phase is also present.

Ion bombardment-induced emission of gas from glass

Krypton ions were injected into Pyrex glass at temperatures of 145, 210 and 270°C. The glass was then bombarded with argon ions at various temperatures below the primary ion injection temperature and the rate of release of the primary species observed. The variation of the bombardment-induced gas release rate with temperature shows a rapid increase as the secondary bombardment temperature approaches the primary injection temperature. These measurements, and observations of the preferential removal of gas trapped with low activation energies, suggest the operation of a thermal spike process (i.e. an effective violent local heating of the glass near the point of ion impact) which causes gas release.

The mechanism of ion bombardment induced release of gas from tungsten

The release of entrapped inert gas ions from polycrystalline tungsten during bombardment with a second ion species has been studied as a function of target temperature during bombardment. The form of the variation of release rate with temperature, and the preferential removal of ions trapped with low activation energy, suggests operation of a thermal spike process rather than Target sputtering as the mechanism responsible for bombardment induced desorption.