New developments in the modeling and analysis of radiation damage in materials

Resume : The properties of iron alloys and steels significant to the use of these materials in magnetic fusion technology and engineering are their high temperature structural stability, allowing for the high thermal efficiency of operation of a fusion power plant, and magnetic properties that are expected to be compatible with the requirements of magnetic plasma confinement. Magnetism of iron alloys influences their phase stability, for example the position of the gamma-loop in the FeCr phase diagram [1], the solubility of chromium, and the relative stability of high temperature alpha and gamma phases [1,2]. Of particular interest is the effect of microstructure on magnetic properties, where simulations, confirmed by experimental observations, show that magnetism of iron is enhanced by vacancies produced by irradiation. The Curie temperature of FeCr alloys varies over a broad temperature range as a function of microstructure, increasing rapidly because of Cr precipitation [3]. We investigate the dynamics of magnetism in FeCr and FeNi alloys, relating it to the alloy microstructure, using dynamic magnetic cluster expansion models, parameterized using databases of DFT-generated atomic configurations. Dynamic magnetic cluster expansion simulations provide valuable information about the response of alloys to fluctuating magnetic field and temperature on a million atom scale, necessary for exploring structural and magnetic properties of steels under realistic plasma confinement conditions. [1] M.Y. Lavrentiev et al., Magnetic cluster expansion model for bcc-fcc transitions in Fe and Fe-Cr alloys, Phys. Rev. B 81, 184202 (2010); [2] P.-W. Ma et al., Dynamic simulation of structural phase transitions in magnetic iron, Phys. Rev. B 96, 094418 (2017); [3] M.Y. Lavrentiev et al., Magnetic cluster expansion model for high-temperature magnetic properties of iron and iron–chromium alloys, Journ. Appl. Phys. 109, 07E123 (2011)

Resume : For the design of future fusion devices radiation damage caused by high energy neutrons in structural materials of the first wall is a critical issue. In this context we investigate irradiation induced damage evolution in the European reduced activation ferritic martensitic steel EUROFER97 by means of a rate model for helium bubble nucleation and growth and Fe³⁺/He⁺ dual ion beam irradiation. The irradiation was carried out at the Jannus laboratory, CEA Saclay, introducing a damage of 15 dpa and a helium content up to 260 appm. The samples were irradiated at temperatures of 330 °C, 400 °C and 500 °C. In the post-irradiation analysis TEM bright-field and weak-beam-dark-field imaging modes are applied. Homogeneously distributed bubbles are observed in the matrix at all temperatures. At the highest irradiation temperature of 500 °C, heterogeneous formation of bubbles at grain boundaries and line dislocations is found in addition. With rising irradiation temperature the density of bubbles decreases and the bubble diameter increases. Dislocation loops with mean diameters of 4.2 nm and 6.1 nm were found at 330 °C and 400 °C, but not at 500 °C. The characterization of the Burgers vectors showed a domination of ½ a0 <111> orientation. A kinetic rate model is applied for nucleation and growth of helium bubbles determining the size and density of bubbles. The temperature dependence of these quantities is in good qualitative agreement with the experimental observation.

Resume : In the perspective of the development of the new generations of nuclear reactors based on fission or fusion one of the main challenges is to find new materials with more and more specific properties. Oxide-dispersion strengthened (ODS) steels are the main candidate for the structural materials of blanket/first-wall in fusion reactors as well as fuel cladding in advanced fast reactors. So they will be submitted to hard conditions : high temperatures (500°C-1000°C) and also high level of damage (200dpa)[1,2]. Based on a Fe-Cr ferritic or ferritic-martensitic matrix (Cr content from 9 to 18%) ODS steels demonstrate excellent properties (good high temperature creep resistance, irradiation resistance to swelling) due to their bcc structure and to the fine dispersion of oxides nanoparticles (Y,Ti,O). These ODS alloys are obtained by mechanical alloying of steel and oxide powders[3]. Their properties depend on the composition, size and dispersion into the matrix of nanometric oxides particles. The formation mechanism of these nanoparticles is not yet cleared and vacancies have been considered to be of most importance in the nanoparticles formation and nucleation. Strong interactions between vacancies and Yttrium and Oxygen have been already detected by Positron Annihilation Spectroscopy experiments and also by DFT calculations [4, 5]. By combining the techniques of positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry (SIMS), we aim to obtain information about the interaction between vacancy defects and solutes such as Y, Ti, and O in Fe. For these studies, solutes (Yttrium and/or Oxygen) have been introduced within vacancies by using implantation performed at room temperature (300K) into high purity Fe samples (99,99%). Single or co-implantations with Yttrium at 1.2MeV and Oxygen at 285keV have been realized in order to introduce both elements at the same depth. . Doppler broadening spectroscopy performed in a slow positron accelerator allows to probing the possible vacancy type defects including pure or complexed vacancy clusters in the implanted layers. Their profiles are compared with solutes (Yttrium, Oxygen) depth profiles measured by SIMS. The effect of annealing temperature on these profiles is studied up to 550°C. Some extra experiments have been made with EXAFS (Extended X-Ray Absorption Fine Structure ), to check the formation of precipitates. [1] S.J. Zinkle and J.T. Busby, Materials Today 12 (2009) 12. [2] J. Knaster, A. Moeslang, T. Muroga Materials research for fusion Nat. Phys., 5 (2016), pp. 424–434 [3]. D.T. Hoelzer, J. Bentley, M.A. Sokolov, et al., J. Nucl. Mater. 367–370 (2007) 166–172. [4] Thesis C He ,2014 ,CEMHTI ,University of Orleans [5] Thesis C Barouh ,2015,CEA-CEMHTI,Unversity of Orleans

Resume : The combination of radiation damage and incorporation of elemental species in a material during ion implantation is a recognized technique of nanostructure production. Ion implantation allows de-correlating the contributions of individual experimental parameters and, when accompanied by appropriate modeling, gives better understanding of the mechanisms involved in nano-oxide particle nucleation and growth, including, in particular, role of defects. This report presents the results of ion-beam synthesis of Ti oxide nano-particles by the implantation of low energy (keV) Ti+ and O+ ions into a high purity Fe-10wt%Cr alloy at room temperature, followed by high-temperature thermal annealing. The micro-structural changes induced by ion implantation have been characterized by conventional, energy-filtered and high resolution Transmission Electron Microscopy (TEM), as well as Time-of-Flight Secondary Ion Mass Spectroscopy. The nucleation of Ti oxide nano-particles via the ion beam synthesis was clearly demonstrated. Both in situ TEM annealing and bulk specimens annealing results will be presented in terms of the size, chemical composition and crystallographic structure of the nano-oxide particles.

Resume : The chemical complexity in concentrated solid solution alloys (SP-CSAs) can be modified by varying the number, type, and concentration of the constituent elements. Studying the effects of chemical complexity has advanced understanding and revealed some incredible physical properties, including excellent mechanical properties, good corrosion resistance, and enhanced radiation performance. Disentangling the role of chemical complexity on defect dynamics and microstructure evolution may provide new paradigms for designing structural materials for next-generation nuclear power systems. A specially selected set of SP-CSAs with different type and number of alloying elements (Ni80Fe20, Ni80Cr20 and Ni40Fe40Cr20) are investigated under Mn ion irradiation at room temperature. Considerable improvement in radiation resistance is observed in the ternary alloy, as compared to the binary alloys and elemental Ni in the low fluence regime. The results from ion channeling measurements and MD simulations suggest that an effective way to improve radiation performance is to suppress defect production and enhance recombination at early stages. Besides irradiation studies at room temperature, experimental and computational approaches are also used to investigate the damage accumulation in elemental Ni, binary NiFe, and quaternary NiFeCoCr irradiated at 16 K. Our results show that the defect evolution is delayed with increasing compositional complexity from elemental Ni to binary NiFe, and to the quaternary NiFeCoCr alloy, which is attributed to sluggish diffusion and dynamic annealing of simple defects at early stages during irradiation. The 16 K results demonstrate that the enhanced radiation resistance in complex SP-CSAs is an intrinsic property that is not a thermally activated process. Moreover, we show that the radiation response is not simply dependent on the number of alloying elements, but can be also tuned by altering the concentrations of specific elements. This work was supported by the Energy Dissipation to Defect Evolution, an EFRC funded by the U.S. DOE, BES.

Resume : GaN has attracted considerable interest due to the wide range of its applications in high power electronics, microwave and optoelectronic devices. Ion bombardment is an attractive choice in GaN-based device fabrication. This contribution will describe the results of investigation of mechanisms of irradiation?induced damage formation in GaN. We used keV monatomic and molecular ions. Ion-implantation-induced effects in wurtzite GaN bombarded with F, P, PF2, PF4, and Ag ions at room temperature are studied experimentally and by cumulative MD simulation in the correct irradiation conditions. In the low dose regime, damage formation is correlated with a reduction in photoluminescence decay time. In the high dose regime, it is associated with the thickness of the amorphous/disordered layer formed at the sample surface. In all the cases studied, an increase in the collision cascade density remarkably enhances the damage accumulation rate. The cumulative MD simulations do not reveal any significant difference in the total amount of both point defects and small defect clusters produced by light monatomic and molecular ions. On the other hand, increased production of large defect clusters by molecular PF4 ions is clearly seen in the vicinity of the surface. Ag ions produce almost the same number of small, but more large defect clusters compare to the others. This findings show that the higher probability of formation of a large defect clusters is important mechanism of the enhancement of stable damage formation in GaN under molecular, as well as under heavy monatomic ion irradiation. Work was supported by RFBR grant 18-08-01213.

Resume : In-pile irradiation of UO2 fuel material produces a large amount of fission gas atoms (mostly xenon) that may either precipitate or be released outside the material, thus increasing the fuel pin breakdown probability. Since irradiation defects are known to be very abundant and to strongly interact with the gas atoms, a Cluster Dynamics (CD) modelling approach was developed (among other techniques such as phase field related models) because it naturally allows one to account for both gas and defects using relatively rapid calculations. Nevertheless, because of the coupled behavior between xenon and defect, it is quite difficult to extract the relevant diffusion coefficients from gas release (i.e. annealing of samples implanted at a very low xenon dose). For this reason, a multi-scale approach based on atomistic calculations was developed to evaluate the various parameters of the CD model, i.e. defect or xenon production rates and formation or migration energies. A complete CD model in UO2 would require 3 dimensions (one for each component U, O, Xe) which would yield intractable calculations. We will present our model emphasizing the way 1) we reduced the model dimension and 2) we took into account the sensitivity of the mobile defect parameters to the local oxygen potential in UO2 as determined by the irradiation damage. We will also show the assessment of the model through a comparison with release experiments.

Resume : Polyethylene (PE), one of the simplest and most used aliphatic polymers, is generally provided with a number of additives, in particular antioxidants, because of its tendency to get oxidized. Carbonyl defects, a product of the oxidation of PE, are occurring in various forms, in particular saturated ones, known as ketones, where a C=O double bond substitutes a CH$_2$ group, and various unsaturated ones, i.e., with further missing hydrogens. Many experimental investigations of the optical properties in the visible/UV range mainly attribute the photoluminescence of PE to one specific kind of unsaturated carbonyls, following analogies to the emission spectra of similar small molecules. However, the reason why saturated carbonyls should not be optically detected is not clear. We investigated the optical properties of PE with and without carbonyl defects using perturbative GW and the Bethe-Salpeter equation in order to take into account excitonic effects. We discuss the calculated excitonic states in comparison with experimental absorption/emission energies and the stability of both saturated and unsaturated carbonyl defects. We conclude that the unsaturated defects are indeed the best candidate for the luminescence of oxidized PE, and the reason is mainly due to oscillator strengths.

Resume : Previous studies have shown that in layered crystals such as SrTiO3 or Mica, SHI irradiation under grazing incidence can produce chains of hillocks/grooves on the surface of the material. These structures have length scales of tens of nanometers and were associated with the local electron density along the ion track [1, 2, 3]. We develop a new approach to simulate SHI grazing incidence irradiation in order to reveal the underlying mechanism of hillock and groove formation. The new method calcualtes the energy locally deposited by the ion based on the electronic density along the trajectory and simulates the electronic energy redistribution and transfer to the lattice according to the two-temperature model. This energy is added to the atoms and simulated with Molecular Dynamics (MD), producing structural changes in the material such as hillocks or grooves. The simulated hillocks show similar length, interdistance and height as in the experiments, supporting the hypothesis that the hillocks appear due to the variation of the local electron density along the ion track. In the simulation, the hillocks and grooves observed on the surface are associated to molten and sublimated material respectively. Finally we study the dependence of the hillock separation on the irradiation angle and crystal orientation and search for characteristic features which could be observable in future experiments. References: -[1] Akcöltekin, Ender, et al. "Creation of multiple nanodots by single ions." Nature Nanotechnology 2.5 (2007): 290-294. -[2] Gruber, Elisabeth, et al. "Swift heavy ion irradiation of CaF2?from grooves to hillocks in a single ion track." Journal of Physics: Condensed Matter 28.40 (2016): 405001. -[3] Aumayr, Friedrich, et al. "Single ion induced surface nanostructures: a comparison between slow highly charged and swift heavy ions." Journal of Physics: Condensed Matter 23.39 (2011): 393001.

Resume : Diamond is a well-known electrical insulator (with a bandgap of 5.45 eV) with a high industrial and technological value because of its exceptional hardness, very high thermal conductivity and high charge-carrier mobility. Moreover, its electrical and optical properties can be significantly modified by introducing charge-carriers by ion implantation [1?3]. Nevertheless, the lattice is frequently affected due to the damage inherent to the ion implantation [4?6]. This damage can be later healed with thermal treatments, but in diamond it can lead to graphitization, which is the most stable phase at room temperature, above a certain graphitization threshold [7]. However, graphitization can be avoided for buried layers above the amorphization threshold [8,9]. In this work, we have used micro Raman and photoluminescence spectroscopy in order to study the damaged lattice of irradiated diamond and its recovery after high-temperature annealing. In particular, (100) diamond crystals were irradiated with highly-focused 9 MeV B and C ions with fluences in the range 1e15 ? 1e17 ions/cm2. In order to heal the irradiation damage, the samples were annealed at 1000 ºC during 1h. The lattice damage was studied by micro Raman and photoluminescence before and after annealing as a function of fluence and layer-depth. Our results show in which particular conditions amorphized diamond, produced by deeply-buried ion implantations, can be returned to crystalline diamond, avoiding graphitization. [1] J.F. Prins, Diam. Relat. Mater. 10 (2001) 1756?1764. [2] T. Vogel, J. Meijer, A. Zaitsev, Diam. Relat. Mater. 13 (2004) 1822?1825. [3] A. Lohrmann, S. Pezzagna, I. Dobrinets, P. Spinicelli, V. Jacques, J.-F. Roch, J. Meijer, A.M. Zaitsev, Appl. Phys. Lett. 99 (2011) 251106. [4] M.D. Ynsa, F. Agulló-Rueda, N. Gordillo, A. Maira, D. Moreno-Cerrada, M.A. Ramos, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. (2017). [5] F. Agulló-Rueda, M.D. Ynsa, N. Gordillo, A. Maira, D. Moreno-Cerrada, M.A. Ramos, Diam. Relat. Mater. 72 (2017) 94?98. [6] F. Agulló-Rueda, N. Gordillo, M.D. Ynsa, A. Maira, J. Cañas, M.A. Ramos, Carbon 123 (2017) 334?343. [7] C. Uzan-Saguy, C. Cytermann, R. Brener, V. Richter, M. Shaanan, R. Kalish, Appl. Phys. Lett. 67 (1995) 1194?1196. [8] J.O. Orwa, K.W. Nugent, D.N. Jamieson, S. Prawer, Phys. Rev. B 62 (2000) 5461. [9] P. Olivero, S. Rubanov, P. Reichart, B.C. Gibson, S.T. Huntington, J.R. Rabeau, A.D. Greentree, J. Salzman, D. Moore, D.N. Jamieson, S. Prawer, Diam. Relat. Mater. 15 (2006) 1614?1621.

Resume : Secondary ion mass spectrometry (SIMS) is the technique of reference for measuring depth distribution of ion implants. SIMS offers high sensitivity and high depth resolution but quantification is usually complex and requires matrix matched reference samples. In addition SIMS instrumentation is a metrology tool only accessible to a limited number of operators. The plasma sputtering based technique, Plasma Profiling Time of Flight Mass Spectrometry (PP-TOFMS), provides similar composition versus depth information as SIMS but much faster (typically a factor 10). This is due to not only the fast erosion rate of the high density and low energy plasma source but also the fact that sample does not need to be transferred to a high vacuum chamber and thanks to the ultra-fast and quasi-simultaneous TOFMS detection of all mass-to-charge ratios of the ions from the sample. Furthermore the separation between sputtering and ionisation processes makes this technique much less matrix dependent than SIMS. A calibration free semi-quantification can be performed by simply rationing the amount of ions detected from a given layer. PP-TOFMS data of boron implanted Si and SiGe will be shown. Results will be compared to TOF-SIMS measurements and aspects of analytical performance with regards to sensitivity, quantification, depth resolution, repeatability and sample throughput will be discussed. Other examples in microelectronics and nanotechnology will also be presented. [1]A. Tempez, S. Legendre, J.-P. Barnes, E. Nolot, Combining plasma profiling TOFMS with TOF-SIMS depth profiling for microelectronic applications, J. Vac. Sci. Technol. B 34, 03H120 (2016). http://dx.doi.org/10.1116/1.4943513

Resume : We present neutron total scattering with pair distribution function (PDF) analysis as a new strategy for the characterization of radiation effects in materials. Key to this approach is the use of swift heavy ions with a very high penetration depth to produce sufficiently large irradiated sample mass (~150 mg). Irradiation experiments were performed at the UNILAC accelerator of the GSI Helmholtz Center with ions of specific energy of 11.4 MeV/u. The irradiated samples were characterized at the Nanoscale Ordered Materials Diffractometer (NOMAD) beamline at the Spallation Neutron Source (Oak Ridge National Laboratory). We investigated various ion-induced structural modifications, such as defect formation (CeO2 and ThO2), disordering (Er2Sn2O7), and amorphization (Dy2TiO5). Neutrons scatter strongly from low-Z elements, permitting a detailed analysis of both cation and anion defect behavior. PDF analysis elucidates the local defect structure, including changes in site occupation, coordination, and bond distance. This is particularly important for characterizing radiation effects in oxides, as structural modifications induced by ion-beam irradiation are seldom ordered over long length scales and in extreme cases result in complete loss of long-range order (i.e., amorphization). Initial results demonstrate that irradiation-induced structural modifications are much more complex than previously thought with distinct processes occurring over different length scales.

Resume : Sapphire is a common substrate for a broad variety of materials due to its optical transparency, insulating property and its hexagonal structure allowing often an easy epitaxial growth. During the use in real world conditions, such materials can be exposed to extreme environments and their intrinsic properties can thereafter be modified. Thus it is necessary to study under ion irradiation the behaviour of sapphire substrate and its potential influence on the features of the epitaxial top layer. In this work, defect formation and structural modifications of a-Al2O3 induced by swift heavy ion irradiation are investigated. (0001)-Al2O3 single crystals have been irradiated along the c-direction by 92 MeV Xe at room temperature. The irradiation experiments have been performed at GANIL facility (Caen). High resolution X-ray diffraction and in-plane diffraction are mainly used to characterize samples. Using different fluences, the mechanisms of structural modifications are investigated. In the communication, the evolution of the X-Ray patterns recorded in different configurations for several reflections will be discussed. A careful study of the reflection characteristics will be detailed to highlight the material damaging through strain and amorphization. The kinetics of these evolutions with the ion fluence will be discussed. We will show how much the c-parameter (parallel to ion beam) and the a-parameter (perpendicular to ion beam) are affected by irradiation in the MeV range. A depth profile will be suggested as an explanation for the structural behaviour of Al2O3 under 92 MeV Xe irradiation. Complementary RBS/c, Raman spectroscopy and TEM results will be shown and discussed for a better understanding of these modifications under irradiation.

Resume : The nitride semiconductors, (Al,Ga,In)N, display optical and electronic properties which can provide useful applications such as light-emitting diode devices. Swift heavy ions are well known to induce structural and chemical changes in the materials which can modify these properties [1]. InN and GaN specimens were irradiated at GANIL facility with 950 MeV Pb while InGaN quantum well samples were irradiated at the Australian National University with 185 MeV Au. Transmission electron microscopy (TEM) instrument was used to describe the disorder induced in the InN, GaN and InGaN Quantum Wells irradiated samples [2]. High resolution TEM investigations were performed to identify the structural order along the ion tracks and the strain induced in the lattice neighboring the ion tracks. Chemical investigations were carried out by STEM - Energy Dispersive X-ray Spectroscopy and Electron Energy Loss Spectroscopy techniques to analyze the fluctuation rate of each element across an ion track and to describe their chemical environment within the track. STEM HAADF analysis reveals the presence of discontinuous tracks in GaN samples and a density fluctuation around the track. Chemical profiles plotted across ion tracks show a decrease of gallium rate within the ion path while higher density of gallium is clearly observed outside the track. Furthermore, the nitrogen K near-edge fine structure investigation reveals the encapsulation of nitrogen bubbles inside the ion tracks. [1] Ackermann J., Angert N., Neumann R., et al. Nucl Instrum Methods Phys ResB, 1996, vol. 107, no 1-4, p. 181-184. [2] Sall, M., Monnet I., Moisy, F. et al. Journal of Materials Science, 2015, vol. 50, no 15, p. 5214-5227.

Resume : Ion implantation is the technique of election for the local doping of substrates in out-of-equilibrium conditions. In the last years, this capability was efficiently exploited to incorporate optical rare-earth emitters in silica, as Er3+ ions emitting at 1.54 microns at room temperature, and to couple them with specific plasmonic nanostructures, to control and enhance their excitation and emission efficiency. To this regard, ion implantation is of paramount importance for controlling the early stages of nucleation in the synthesis of noble metal quantum clusters, which proved to be very efficient broadband sensitizers for the Er3+ excitation, and to get the incorporation of both optical emitters and sensitizing species on predefined patterns. In this presentation, we will report on the results of systematic photoluminescence (PL) characterizations aimed at investigating the mechanisms of energy-transfer between metallic quantum clusters and rare-earth ions incorporated in silica by ion implantation and subsequent thermal treatments. Moreover, the role of irradiation-induced defects on the luminescent emission properties and the cluster growth was investigated in order to decouple any detrimental effect due to implantation damage from the efficient sensitization process.

Resume : Solar cells based on complex lead halides with the perovskite structure have attracted particular attention of material scientists and engineers all over the world. Rapid improvement in terms of power conversion efficiencies, which exceeded 22% in combination with potentially low fabrication costs of perovskite solar cells might accomplish a revolution on the PV market. Perovskite solar cells can also be fabricated on ultrathin and lightweight substrates, which makes them particularly attractive for both terrestrial and space applications. Conventional solar cells based on silicon, gallium arsenide or A3B5 type absorbers are usually installed on the satellites. However, these materials rapidly degrade under severe radiation exposure in space. While high-energy particles can be blocked efficiently by the solar cell encapsulation layers, the degradation induced by gamma irradiation from random Sun flares can hardly be avoided. Therefore, development of novel PV materials and solar cell architectures capable of sustaining significant gamma irradiation represents a highly relevant task. In this paper, we will report an experimental evaluation of the gamma radiation tolerance of perovskite solar cells. The effects of the light absorber (MAPbI3 vs. triple cation MA(x)FA(y)Cs(z)PbBrnI3-n) formulation and the electron transport materials on the device stability will be discussed in detail. The proposed pathways of the radiation-induced degradation will be presented with a particular focus on the self-healing effects revealed in these experiments. Finally, we will conclude on the potential of the perovskite photovoltaics with respect to the space applications.

Resume : The recent advent of nanofocused hard X-ray beams and of their high power densities has raised the point of synchrotron-induced material damage. In the case of the high temperature superconductors Bi2Sr2CaCu2O8+? (Bi-2212) and YBa2Cu3O7-? (Y-123), whose properties highly depend on the non-stoichiometric oxygen content ?, photon damage has been studied and controlled for the purpose of X-ray nanopatterning (XNP). Indeed, Josephson devices have been fabricated out of Bi-2212 by means of 50 nm beams at 17.5 keV, also allowing device on-line monitoring during fabrication [1]. Although the microscopic mechanism for defect formation is not fully understood, Monte Carlo simulations have proved that photogenerated electrons can knock on the Bi-2212 interstitial oxygen atoms and significantly modify their amount ? [2]. Moreover, nano-XRD maps have revealed the formation of mosaic domains showing a misalignment that increases with increasing the irradiation dose. XNP has been recently demonstrated also in a more conventional oxide like TiO2 by means of directly writing conducting channels via nanobeam irradiation. These findings represent a potential breakthrough for the whole class of oxide materials, paving the way to the fabrication of novel devices via XNP, which could be advantageous in terms of device heat dissipation and of its native 3D structure. 1. L. Mino, et al., Scientific Reports, 2017, 7(1), 9066 2. D. Torsello, et al., Physical Review Materials, 2018, 2(1), 014801

Resume : Organic electronic devices fabricated on flexible substrates are promising candidates for applications in environments where flexible, light-weight and radiation hard materials are required1. In this work, we have recently reported on how flexible devices based on 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) can be used as real-time, direct ionizing radiation detectors2, opening unprecedent perspectives for space applications of organic electronics. We monitored the device parameters as threshold voltage, charge mobility and trap density of TIPS-pentacene based OTFTs, before and after irradiation by high-energy protons3. The observed reduction of charge carrier mobility following irradiation can be only partially ascribed to the increased trap density. Indeed, Atomic Force Microscopy reveals morphological defects occurring in the organic dielectric layer induced by the impinging protons, which, in turn, induce a strain on the TIPS-pentacene crystallites lying above. The effects of this strain are investigated by Density?Functional?Theory simulations of two model structures, which describe the TIPS-pentacene crystalline films at equilibrium and under strain. The two different density of states distributions in the valence band have been correlated with the Photo-Current spectra acquired before and after proton irradiation. We conclude that the degradation of the dielectric layer and the organic semiconductor sensitivity to strain are the two main phenomena responsible for the reduction of OTFT mobility after proton irradiation.

Resume : Radiation-induced hardening and dynamic mechanical properties changes of isotropic graphite are analyzed by nano- and micro-indentation. The samples were exposed to C, Ca, Xe, Sm, Au and U ion beams in the GeV energy regime. The irradiations were performed at the M-branch facility at the UNILAC accelerator at GSI achieving fluences between 1e11 ions/cm² and 5e13 ions/cm². For graphite, a general increase of Young?s modulus and surface hardness with increasing ion fluence is observed. For fluences larger than 1e13 ions/cm2 and energy-loss values larger than 20 keV/nm, the hardness and Young?s modulus increased up to 500%. Investigations by Raman spectroscopy indicate that the steep increase in hardness is associated with an irradiation-induced allotropic transformation of graphite to a glassy carbon structure. In contrast to amorphization by keV-MeV ion energies, this type of transformation is related to specific structural defects created by ions with electronic energy-loss above a certain threshold. In addition, we investigated beam-induced embrittlement by repetitive nano-impact measurements that show an earlier failure of the material in high strain rate fatigue tests. The damping coefficient is decreased in irradiated samples indicating a reduction in crystallite size and an evolution towards a stiffer structure by cross-linking of the basal planes in graphite.

Resume : We report on the effect of gamma (γ) radiation on epitaxially grown AlGaN/GaN heterostructure using metal organic chemical vapor deposition (MOCVD) technique and the high electron mobility transistor (HEMT) devices fabricated on same structure separately. The layer structure has been irradiated cumulatively with γ-ray dose of the order of 10kGy. HR-XRD analysis of irradiated sample shows the reduction in defect density due to lowering in FWHM in comparison to that of pristine sample. While edge dislocations remain unaffected, a decrease in screw dislocation density along (0002) plane has been observed. Hall measurement shows an improvement in the mobility post irradiation which can be attributed to the reduction in defect density. The reduction in defect density is also supported by CL imaging. Electrical characterization of HEMT devices post γ irradiation shows a negative shift in the threshold voltage with a trend of improvement in the drain current. Simultaneously, transfer length measurement (TLM) characterization shows reduction in the contact resistance due to the local thermal heating effect after irradiation. Further, SEM imaging corroborates TLM analysis where the increase in local thermal heating is due to reduced perimeter to area ratio of metal-semiconductor contact. Hence this study infers that initial dose of γ irradiation enhances the quality of GaN material and improves the metal-semiconductor interface in the devices.

Resume : Since decades Molecular Dynamics (MD) and the Binary Collision Approximations (BCA) are used to simulate collision cascades in solids under irradiation. MD algorithm uses interatomic potentials and integrates the equations of motion of all atoms in the system. It is thus well-suited to study complex phenomena at the atomistic level and many-body effects. However, simulation boxes become very large as the initial energy of the projectile increases, resulting in a large number of atoms in the system and hence, a prohibitive computing time to solve all the Newton's equations. Using MD when the energy of the projectile is high (~ MeV) is thus not realistic. On the other hand, Binary Collision Approximation (BCA) is much more efficient than Molecular Dynamics (MD) to simulate collision cascades in solids as it only follows displaced atoms. The BCA is thus preferred over MD when simulating high-energy collision cascades in solids . However, though the BCA is accurate in the high-energy regime, it is not able to accurately predict last stages of cascades, i.e. when atoms have a low kinetic energy, since it does not take into account the long-range potential between atoms and neither solves for the equations of motion of each atom as MD does. As a consequence, the BCA cannot account for mechanisms such as the recombination of defects or the formation of clusters during the cooling phase of the cascade. In this work we combine the BCA and the MD, which results in a significant speedup of MD calculations. In order to demonstrate the accuracy of the BCA-MD method, we shall focus on the case of Fe under irradiation, a material of high importance for future fusion reactors. We shall show that the results obtained with the BCA-MD are in excellent agreement with those obtained with full MD calculations. We shall demonstrate that collision cascades for initial projectile energies in the order of MeV and that would be prohibitive for full MD calculations, can be simulated in a reasonable computational time with the BCA-MD.

Resume : Future nuclear power plants will have to cope with severe harsh radiation environments generating defects in their structural materials that will undergo changes in their mechanical properties. Both experimental and simulation investigations are being carried out to observe which defects are being formed in such materials as well as to understand their formation mechanisms. Ferritic iron alloys are proposed, among others, as structural materials for such nuclear power plants, so this work is focused in such materials. Ion beam irradiation experiments using Fe ions are a prolific source of current data that is aimed to be reproduced and explained using simulation codes. In order to validate our simulation results we are using two experiments in which samples of UHP Fe at different temperatures are being irradiated with Fe ions of two different energies: 1) 0.5 MeV Fe ions toward UHP Fe samples at room temperature in the experiment of Prokhodtseva and co-workers [1], and 2) 2.0 MeV Fe ions toward UHP Fe samples at 500ºC from the experiment of Brimbal and co-workers [2]. We determine the cascades of vacancies and self-interstitials using a combination of codes. First we use SRIM [3] as well as MDRANGE [4] to calculate the deposited energy as a function of depth. With this information, we generate the initial distribution of defects (vacancies and self-interstitials) using a database of collision cascades generated by molecular dynamics simulations. Then, this defect distribution is used as the input data to study the evolution of microstructures with the OKMC code MMonCa [5]. Two different models for the formation of immobile self-interstitial (SIA) dislocation loops <100> are introduced (and separately evaluated) in this code. One of the mechanisms allows the formation of such loops as the interaction result of two 1/2<111> SIA loops under certain conditions [6,7], and the other one establishes that it can be rarely produced during the cascade collisions [8]. One important component on the microstructure evolution in Fe, carbon, is also included in these calculations. References [1] A. Prokhodtseva, B. Décamps, A. Ramar, R. Schäublin, Acta. Mat. 61, 6647-7034 (2013) [2] D. Brimbal, B. Décamps, J. Henry, E. Meslin, A. Barbu, Acta. Mat. 64, 391 (2014) [3] SRIM, J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instrum. Meth. B 268, 1818-1823 (2010) [4] K. Nordlund, F. Djurabekova, G. Hobler, Phys. Rev. B 94, 214109 (2016) [5] I. Martin-Bragado, A. Rivera, G. Valles, J.L. Gomez-Selles, M.J. Caturla, Computer Physics Communications 184, 2703-2710 (2013) [6] J. Marian et al. PRL 88, 25 (2002) [7] H. Xu, R. Stoller, Y. Osetsky, D. Terentyev, PRL 110, 265503 (2013) [8] M.C. Marinica, F. Williaime, J.P. Crocombette, PRL 108, 025501 (2012)

Resume : We present theoretical studies of single-shot damage of ruthenium thin films induced by femtosecond extreme ultraviolet (EUV) free-electron laser (FEL). Previous experimental analysis of the damaged spots with different types of microscopy (SEM, TEM and AFM) revealed the nature of the induced damage to be melting of the Ru surface with subsequent spallation of the top Ru layer. A combined model is used to simulate the interaction of the EUV fs pulse with the Ru film: (i) Photoabsorption and nonequilibrium electron kinetics are described with the Monte Carlo code XCascade-3D [1], which enables to calculate the time-space evolution of electron cascades induced by the EUV pulse. We show that since the electron cascading practically ends within a femtosecond, the EUV-induced electronic distribution in Ru may be considered as thermalized at all times. (ii) Diffusion of thermalized electrons into the depth of the material and heating of the lattice via electron-phonon interaction are described with the two-temperature model [2]. (iii) Finally, the lattice dynamical effects such as melting and spallation are modeled with the two-temperature hydrodynamics code [3] and the classical molecular dynamics approach. The simulations show that heating of Ru by the FEL EUV pulse occurs in the stress confinement regime. Large stresses develop inside the material during the heating process, leading to spallation of the top Ru layer, which is in agreement with the mentioned experimental observations. [1] V. Lipp, N. Medvedev, and B. Ziaja, SPIE Optics+ Optoelectronics, International Society for Optics and Photonics, 102360H, 2017. [2] Anisimov, S. I., B. L. Kapeliovich, and T. L. Perelman. Zh. Eksp. Teor. Fiz 66.2 (1974): 375-377. [3] Inogamov, N. A., et al. Applied Surface Science 255.24 (2009): 9712-9716.

Resume : Nano- and micromechanical testing is important for assessing damage induced by neutron irradiation (minimized volume of activated material) and ion irradiation (limited penetration depth) the latter being of interest for the emulation of neutron irradiation damage. Besides validated experimental methods a robust analytical framework is required. A modification to the Nix–Gao framework is introduced to describe the breakdown of the scaling regime of the indentation size effect (ISE) at small indentation depths. The modification consists in accounting for the spatial profile of the density of geometrically necessary dislocations (GNDs). The model sheds new light on the role of GNDs and statistically stored dislocations, thereby rationalizing a material's propensity to developing pile-up or sink-in behaviour, respectively, and elucidating the effect of residual stresses. The model is applied to a quantitative analysis of the ISE of the ferritic-martensitic steels T91 and Eurofer97, here investigated in the non-irradiated reference state, but envisaged as structural materials for nuclear fission and fusion applications, respectively. Depth-controlled single cycle measurements at various final indentation depths, force-controlled single cycle, force-controlled progressive multi-cycle measurements, and continuous stiffness measurements (CSM) all using a Berkovich tip at room temperature have been combined to assess the ISE. The present model captures the extremely high dislocation density associated with martensitic lath boundary misorientation. While the quasi-static methods show good agreement, CSM gives somewhat different results; hence breakdown of the scaling regime is sensitive to the test mode.

Resume : The origin of the microstructural changes under irradiation is the super-saturation of point defects that can agglomerate in the form of loops or voids and can enhance or modify solute atom diffusion, resulting in enhanced or induced precipitation or segregation. In ultrafine grain (UFG) alloys, the volume fraction of grain boundaries is dramatically higher than in coarse grain (CG) materials. Since grain boundaries act as point defect sinks, a large part of irradiation induced defects can be annihilated. Therefore, a limitation of radiation damage is expected. The objective of the work presented here is to investigate the stability of a nanostructured FeCrW alloy under irradiation. A Fe-14%Cr-1%W model alloy was nanostructured by high pressure torsion in order to get grains of several tens of nanometers. Both CG and UFG alloys were ion irradiated at 400°C in JANNUS facilities. Their microstructures were characterized by transmission electron microscopy and atom probe tomography. Nanoindentation tests were performed to estimate irradiation hardening in both CG and UFG alloys. This talk will describe experimental results. The effect of nanostructuration on the stability of the microstructure and the hardening will be discussed.

Resume : Nowadays, one of the most important challenges in the development of fusion energy is to predict and mitigate the effects of large levels of transmuted gas atoms (He and H) along with high displacement damage produced by high energy neutrons that affect the mechanical properties of the structural materials and gives rise to phenomena like swelling. In fact, many issues related to the irradiation defects are still unknown. The understanding of the macroscopic effects of loops due to irradiation on the structural materials requires further research at different levels: experimental observations by TEM in basic and simple metals (model alloys) as a backup to modelling. Computational modelling of radiation damage in fusion materials may help to predict the damage effects and how could mitigate it and contribute also to design of DEMO (DEMOnstration fusion power plant) in the medium term. Modelling would help to understand the underlying physical mechanisms of the damage and to predict its evolution along the reactor's lifetime. In order to achieve a sturdy computational approach, the models and computational codes must be validated with experiments. In the present research, we have performed some validation experiments in order to evaluate the formation of dislocation loops in pure Fe when the loops have been created far away from the surface since the Fe ions are highly energetic (20 MeV) in bulk. This would prevent the generated dislocation loops from migration and disappearance at the surface of the well-known surface effect acting as defect sink. Understanding dislocation loop formation and interaction mechanisms are the main objectives of this research, given that this interaction is suspicious of being responsible for embrittlement of structural steels in fusion reactors. On the other hand, we carried out as well the same irradiations on thin films (electropolished TEM discs), in which the ions get through them, and the effect of free surfaces should be enhanced. The irradiations were as follows: 5 and 10 dpa (in peak, being lower on thin films) at different irradiation temperatures 350 and 450 ºC. After irradiations, the damaged areas were observed under transmission electron microscopy (TEM). Bulk specimens were prepared by means of lift-out technique and their surface will be polished with low Ar kV. The lamellae contained all the irradiation depth, so a deep characterization correlating depth and dpa level was achieved. However, thin films were studied directly by TEM after irradiation maintaining a constant dpa level within the thickness. The defects that were taken into consideration are only dislocation loops of the types ½ a0 <111> and a0 <100> since those are the only ones that form in ferritic materials, in agreement with the literature are considered. A swelling phenomenon produced by the loops (depending on their nature, vacancy or interstitials) was calculated comparing between thin films and bulk irradiation. The microstructure was completely different comparing the thin film thickness with the first 150 nm of the bulk irradiation, even having the same PKA profile calculated by MARLOWE. Large <100> loops of hundreds of nanometers were detected on thin films, and a few of them belong to <111> too. In contrast, in bulk irradiation very few embryos-like loops were discovered at the beginning of the lamella, the maximum loop density was found in the bulk, but even though the size was much smaller than the found ones on thin films. Complex modelling studies corroborate these experimental observations, which mean the validation was successful.

Resume : Among other effects, neutron irradiation hardens ferritic/martensitic (F/M) nuclear steels; this hardening is suspected to be due to the formation of dislocation loops, α’ phase and solute-rich clusters (SRCs). In neutron irradiated FeCr model alloys the SRCs—which are made of unavoidable impurities such as P—are likely to be the main contributors to the yield strength increase, together with the presence of α’ precipitates, if any [1,2]. Ion irradiation is an extended tool used to investigate the creation and evolution of radiation damage. Since the experiments are fast they offer the possibility to tune the parameters to design model-oriented experiments. Their use is also oriented to reproduce the effects of neutrons for nuclear applications. However, the characteristics of the nano-sized features (solute concentration, density, size and size distribution) are difficult to reproduce using ions with respect to neutrons, since the increase of the sink strength due to injected interstitials [3,4] and the high values of damage rate [5] influence defect production and evolution. Atom probe tomography (APT) is used in the present work to investigate the hardening nanofeatures in irradiated materials. First, an Fe14CrNiSiP model alloy has been irradiated with both Fe+ ions and neutrons up to a dose of 0.1 dpa at 300°C. At such low doses some features, such as SRCs or Cr-rich regions, are formed, thus a direct comparison can be made to highlight differences and commonalities between the two kinds of irradiation; these results can also be used for the development of models. Second, a similar investigation has been made on neutron irradiated EUROFER97 up to 15 dpa at 300°C. The presence of radiation induced segregation (RIS) and radiation enhanced features can be correlated to the model alloy providing some insights on the nature of the hardening in F/M steels. [1] F. Bergner, C. Pareige, M. Hernández-Mayoral, L. Malerba, C. Heintze, Application of a three-feature dispersed-barrier hardening model to neutron-irradiated Fe–Cr model alloys, J. Nucl. Mater. 448 (2014) 96–102. doi:10.1016/j.jnucmat.2014.01.024. [2] G. Monnet, Multiscale modeling of precipitation hardening: Application to the Fe–Cr alloys, Acta Mater. 95 (2015) 302–311. doi:10.1016/j.actamat.2015.05.043. [3] F. Soisson, T. Jourdan, Radiation-accelerated precipitation in Fe?Cr alloys, Acta Mater. 103 (2016) 870–881. doi:10.1016/j.actamat.2015.11.001. [4] O. Tissot, C. Pareige, E. Meslin, B. Décamps, J. Henry, Influence of injected interstitials on α′ precipitation in Fe–Cr alloys under self-ion irradiation, Mater. Res. Lett. 5 (2016) 117–123. doi:10.1080/21663831.2016.1230896. [5] E.R. Reese, N. Almirall, T. Yamamoto, S. Tumey, G. Robert Odette, E.A. Marquis, Dose rate dependence of Cr precipitation in an ion-irradiated Fe 18Cr alloy, Scr. Mater. 146 (2018) 213–217. doi:10.1016/j.scriptamat.2017.11.040.

Resume : Ion irradiation experiments give important insight about the mechanisms that govern the nucleation and growth of defects in materials exposed to a nuclear environment. In this contribution, we report on Au and Ag ion irradiation effects in a solubilized AISI 316L alloy to investigate the influence of ions of different masses on the development of irradiation induced precipitate reactions. Mechanically polished AISI 316L samples were implanted with Ar ions and then, along with control samples without Ar, were irradiated at 550 ºC with 5 MeV Au ions and with 3.5 MeV Ag ions reaching fluences of 20 and 40 dpa. The samples were analyzed via transmission electron microscopy. The Au irradiation caused the formation of small Ar bubbles (d≈2nm) and of precipitates identified as M23C6 and MC phases only in the samples implanted with Ar; the control samples are free from precipitates and it is observed the presence of typical radiation-induced defects. Differently, the Ag irradiation experiment triggered the formation of precipitates and small Ar bubbles/cavities in both samples, with and without Ar. The mechanisms of defect formation are discussed considering the density of collision cascades induced by each ion/matrix-atom interaction and the consequent number of vacancies produced by the cascades. The evolution of the precipitate system is also discussed considering the diffusion controlled supply of C atoms from the matrix and the efficiency of the Ar gas as a vacancy attractor.

Resume : Point defects as interstitial atoms and vacancies play an important role in controlling properties of materials and their kinetic evolution. As a consequence, a proper understanding and modelling of properties of materials require a precise knowledge of point defect properties, in particular their formation and migration energies. In alloys as Fe-Cr alloys, the investigation of point defects is complicated, because the properties of point defects can be dependent on the various parameters such as: Cr composition, short-range ordering of the alloy and the local environment of a defect. The DFT simulations performed within this study showed that the properties of interstitials in concentrated Fe-Cr alloys may fluctuate significantly as a function of Cr concentration, short-range ordering and the local environment of a defect. These fluctuations are observed to a lesser or greater extend for all investigated properties: lattice parameters, chemical potentials, bulk moduli, relaxation volumes, magnetic moments, and migration energies of dumbbells. Moreover, the most stable directions of Fe-Fe, Fe-Cr and Cr-Cr dumbbells were investigated. The formation energy of dumbbells depends on Cr content and on the number of Cr atoms in the local environment of a defect. The trends in the mean values of investigated properties are observed. The mean migration energies of Cr atoms are significantly smaller than those of Fe atoms. The relaxation volumes of dumbbells increase in the ordered and decrease in the disordered structure with the increase of number of Cr atoms in the local environment of a defect.

Resume : The Li doped-ZnSnO (ZTO) thin films were fabricated on a SiO2 (100 nm)/p+-Si substrate using a spin coating method. The Zn:Sn ratio and concentration of Li-ZTO solution were 1:2 and 0.5 M, respectively, and the Li content was 3 at.%. Particle irradiation can change the properties by forming defects. Neutron irradiation on Li-ZTO thin films was conducted using a cyclotron (MC-50 in KIRAMS). A fast neutron was generated through 30 MeV proton irradiation on a Be target. The neutron flux was about E8 n/cm2 s, and the dose was varied by changing irradiation time from 10 to 2000 s. To fabricate thin film transistor (TFT), a patterned Al electrode was deposited after the neutron irradiation. The saturation mobility of Li-ZTO TFT increased to 11.3 cm2/Vs at a neutron dose of 2×E11 n/cm2. The X-ray photoelectron spectroscopy (XPS) analysis showed that the chemical bonding states such as O 1s, Zn 2p and Sn 3d were not changed regardless of the neutron dose. From the XPS and spectroscopic ellipsometer (SE) results, it was shown that the Fermi level of the sample irradiated for 1000 s (1×E11 n/cm2) was located at the closest position to the conduction band minimum from the band gap alignment. The increase in saturation mobility and the variation in Fermi level for the samples irradiated above 1×E11 n/cm2 were due to the electrons exited easily toward the conduction band by increasing the D1 state (carrier generating state) at the conduction band edge.

Resume : An approach to emulate neutron irradiation with ion irradiation with Ni and MgO as test-case materials is conducted. The neutron irradiation and ion irradiation properties are compared through weighted recoil spectra obtained by computational simulations using the DART code. The weighted recoil spectrum provides the relation between the Primary Knock-on Atom (PKA) energy and target atoms displacements. The 300 keV Bi-209 and 600 keV Ni-58 ion irradiations on Ni and the 1 MeV Cl-35 ion irradiation on MgO are determined to represent neutron irradiations reported in papers available in the literature. The X-Ray Diffraction (XRD) technique is used to examine if the strain induced in the post irradiation sample matches that in the neutron irradiation.

Resume : Additive manufacturing has presented several potentials in the engineering industries. However, the limited material range has slowed down the progress of this technology despite its rapid adoption and implementation in industries. For instance, the development of processes to additive manufacture ceramics is still an on-going challenge. Moreover, due to the nature of layerwise manufacturing, new standards are needed for part qualification and characterisation. This work focuses on the density measurement of a lithium aluminosilicate glass-ceramic. The most common method in the community for quantifying the relative density of additively manufactured parts is by the use of Archimedes method. In this method, a relative density of 100 % implies that the manufactured part is fully dense without porosity. However, the use of this method is not appropriate for characterising additively manufactured materials which undergo phase changes accompanied by a drastic change in material density. An example of such a material is spodumene. A transformation from the α-phase to β phase reduces the density of the material from 3.15 kg/m3 to 2.4 kg/m3 after heat-treatment.3 However, the physical dimensions of the printed part does not change. Therefore, the use of Archimedes method for this material results in an increase in relative density, providing false information on the internal porosity. For better qualification of such materials in future, this work proposes the use of computer tomography (CT) scan.4 The preference of this method over the others is the prevention of possible confusion between part density and material density. In addition, CT scan is able to provide visualisation of the internal structure of the printed parts, enabling defects such as porosity to be located.

Resume : Low energy charged particle damage requires attention in various areas like ion thrusters, fusion reactors, lithography optics etc. Models involving binary collision approximations break down at low energies due to a linear increase in collision cross-section below 100eV, and thus making predictability questionable and necessitating experimental verification. We report the physical and chemical modification of ruthenium under the influence of noble gas and nitrogen ions near the sputter threshold. Measuring frequency changes in an AT-cut quartz crystal microbalance coated with ruthenium, allowed us to measure thickness losses with nanometer level precision. Comparing the obtained yields to the semi-empirical model by Yamamura showed that the model overestimates yields for argon by 10% for 300eV while nitrogen shows larger discrepancies from model predictions which can be explained by the formation of a nitride.

Resume : Rare earth doped GaN is a promising material for light emitters and optical sensors. In particular, doping with Eu offers temperature stable red emission. In this study, GaN nanowires, grown by Molecular Beam Epitaxy, were implanted with Eu to different fluences. Strain introduced by implantation defects in nanowires remains considerably lower than in thin films. Surface degradation is observed for high fluences, consistent with increased sputtering in nanowires. Nevertheless, the nanowire core remains of high crystalline quality. Moreover, no formation of extended defects (e.g. stacking faults) is seen, in contrast to GaN thin films. After post-implant rapid thermal annealing, the typical Eu3+ intra-ionic transitions were observed in all samples. X-ray absorption spectroscopy (XAS) shows the coexistence of two different charge states (2+ and 3+) and gives information on the coordination of Eu in GaN nanowires and thin films. A similar local environment was found in both structures: for low fluences, Eu is mainly incorporated on substitutional Ga-sites while for high fluences XAS points at the formation of a EuN-like next neighbour structure.

Resume : Perovskite solar cells (PSCs) have emerged and been developed quickly over the past a few years as a promising low cost and highly efficient solution of renewable energy: some PSCs have reached an efficiency of higher than 20% [1]. However, the understanding of the lattice changes under illumination is still lacking. In this work, we propose a novel terahertz (THz) thin film total internal reflection (TF-TIR) spectroscopy to characterize the perovskite thin films. We show that TF-TIR spectroscopy enhances sensitivity compared to transmission geometry. The light induced spectral change and partial recovery process of two single halide perovskites (MAPbI3, MAPbBr3) and two mixed halide perovskites (MAPbI2.5Br0.5, MAPbI2Br1) were recorded with the proposed THz TF-TIR spectroscopy method. The effects of the moisture and oxygen were also included. The measured TF-TIR spectra were analyzed with Lorentz oscillator model (LOM) and the resonance frequency and oscillation strengths were examined physically. The X-ray diffraction (XRD) and Photoluminescence (PL) of the perovskite thin film samples under the same situation were also measured to verify the degradation and the phase segregation in the perovskites. From the THz TF-TIR spectral results, we presume there was two-step structural change in the perovskite thin films under illumination. Firstly, we showed that the intensity of the second THz phonon mode was enhanced after 5 minutes of illumination, which was attributed to the rotational vibration of the MA cations and the coupling between the cations and inorganic octahedra; Secondly, the long-term light soaking induced a decrease in the intensity of both THz phonon modes. In particular, the strength of the first phonon mode rapidly decreased to zero (within 30 minutes for MAPbI3) in the presence of moisture, which corresponds to the decreased bending strength of the halide-lead-halide bond. Our results match with the assumption proposed by Gottesman et al that inorganic octahedra would realign to adapt to the rotated MA cations induced by photo illumination [2]. Moreover, based on the experimental results, we are able to give the time scales of the two-step structural change. The rotation of the MA cations occurred within the first 5 minutes of illumination. In the presence of moisture, the halide-lead-halide bond bending strength remained stable for the first 10 minutes of illumination and then rapidly decreased to zero within the next 20 minutes. Without moisture, this bond bending strength was modified at a much slower rate. We also elaborate on the details of changes in the octahedra under illumination: the bending and stretching strength of the lead halide bond decreased; and in the presence of moisture and oxygen the vibrational resonance frequency of the halide-lead-halide bond bending had a blue shift. We demonstrate that our technique could give information about photo-induced structural changes in the perovskite crystal structure and our conclusions are reinforced by XRD and PL measurements. The proposed THz TF-TIR spectroscopy breaks the limitations of the traditional THz transmission TDS and ATR spectroscopy techniques, as it is able to capture features of weekly-absorbing materials without photoexcitation. Using this TF-TIR geometry we are potentially able to characterize materials that are too thin to be characterized in transmission geometry. This is very helpful for studying the dynamics of the carriers in materials. This method is also potentially able to examine the film quality of perovskite samples, which is an essential factor to determine the performance of the perovskite solar cell devices [3]. Reference [1] J.-P. Correa-Baena, M. Saliba, T. Buonassisi, M. Grätzel, A. Abate, W. Tress, et al., "Promises and challenges of perovskite solar cells," Science, vol. 358, pp. 739-744, 2017. [2] R. Gottesman, L. Gouda, B. S. Kalanoor, E. Haltzi, S. Tirosh, E. Rosh-Hodesh, et al., "Photoinduced reversible structural transformations in free-standing CH3NH3PbI3 perovskite films," The journal of physical chemistry letters, vol. 6, pp. 2332-2338, 2015. [3] U. Mehmood, A. Al-Ahmed, M. Afzaal, F. A. Al-Sulaiman, and M. Daud, "Recent progress and remaining challenges in organometallic halides based perovskite solar cells," Renewable and Sustainable Energy Reviews, vol. 78, pp. 1-14, 2017.

Resume : Ion beam sputtering is a well-established technique for the production of nanopatterns in a large range of materials [1], but the formation of patterns in some binaries, such as GaN or ZnO, has been proven difficult to achieve [2,3] due to their high radiation hardness. In this work, we present the results on modeling and analysis of radiation damage in ZnO bulk substrates. We study the role of the energy (20 and 40 keV) and the fluence (from 1016 to 2·1018 cm-2) when samples are irradiated with Ar+ ions at oblique incidence (60º). We successfully create ripple patterns with an asymmetric shape and an uncorrelated direction with the beam. A detailed study of the damage-induced changes on the near-band-edge is carried out by photoluminesence and the evolution of both the damage and the roughness vs. the fluence are analyzed by spectroscopic ellipsometry. The results are contrasted with atomic force microscopy and Rutherford backscattering spectrometry in channeling mode and medium energy ion scattering measurements. With these methods we can confirm that the irradiation causes: i) a decrease of the refractive index, ii) a broadening of the excitonic transitions, and iii) an enhanced absorption below bandgap. [1] Muñoz-García et al. Mater. Sci. Eng. R-Rep. 86, 1 (2014). [2] Qian et al. J. Phys. D: Appl. Phys. 42, 105304 (2009). [3] Bhattacharjee et al. Appl. Surface Science 257, 6775 (2011).

Resume : Investigations of radiation effects in III–VI semiconductors are promising from practical point of view for creation of photoconverters for use in photo-, opto- and quantum electronics, radiation hardness of which substantially exceeds that of silicon one. Layered crystal structure of III–VI compounds enables both to obtain their plates with atomically smooth surface by cleaving the ingot in air and to apply simple techniques (thermal oxidation and direct optical contact) for the preparation of device structures. In this work the influence of 10 MeV electron irradiation within a wide fluence range 10^13–10^15 cm-3 on the parameters of intrinsic oxide–p-InSe:Cd heterojunctions and p-InSe:Cd–n-InSe homojunctions is established. Current-voltage (dark and light), capacitance-voltage and load characteristics of the junctions, charge transfer mechanisms through the potential barriers, photoresponse spectra, open-circuit voltage, short-circuit current, current and voltage monochromatic sensitivities as well as solar energy conversion coefficient are investigated before and after radiation. The detected insignificant changes in the electrical and photoelectric characteristics for the diode InSe-based structures even at the maximum irradiation fluence showed a high resistance of the investigated photoconverters to high-energy electron irradiation. Its influence leads to the appearance of simple radiation-induced point defects located preferably in the interlayer spaces.

Resume : Electron or ion irradiation of solids may result in irradiation-induced amorphization, recrystallization and annealing and may induce changes of electrical properties of solid-state materials and devices. Irradiation may also cause formation of amorphous nanosized domains and nanocrystals by decomposition of the target material. Therefore during the last few decades there was significant activity to explore such alterations in various types of semiconductor devices, in particular in those based on metal-oxide-silicon structures. Homogeneous films from SiOx (x=1.3) and composite a-Si-SiOx films, containing amorphous Si nanoparticles (a-Si) were prepared on crystalline Si substrates by evaporation of silicon monoxide. The nanoparticles were grown by post-deposition annealing at 700 °C. The samples were irradiated by 20-MeV electrons at fluencies of around 1015 el/cm2. The film thicknesses and optical constants were explored by spectroscopic ellipsometry. The development of the films’ structure caused by the electron-beam irradiation was studied by transmission electron microscopy. It has been shown that the SiOx films are optically homogeneous and the electron irradiation has led to small changes in the optical constants. The irradiation of the a-Si-SiOx composite films caused a decrease of the effective refractive index and, at the same time, an increase of the refractive index of the oxide matrix. Irradiation induced increase of the optical band gap and decrease of the absorption coefficient of the thermally grown amorphous Si nanoparticles has also been observed. The obtained results are discussed in terms of formation of small amorphous silicon nanoclusters in the homogeneous layers and electron irradiation induced nanoparticle size reduction in the composite films. The conclusion for the nanoparticle size reduction is supported by infrared transmittance results.

Resume : Thin films of WO3 were grown at 400 °C under 5 and 590 Torr oxygen pressure by thermal oxidation of tungsten substrates with different grain size. Two samples were exposed to helium plasma in order to mimic the conditions of ITER diverter W-plasma facing components and to explore the effects on their properties. Ellipsometry measurements and modelling on the obtained samples were performed in order to determine the thicknesses of the films, thus consolidating the thickness values estimated previously between 20 and 270 nm by the FIB cross-section method [1]. The film thicknesses are influenced by the substrate grain size. Specular reflectance of the WO3 thin films in the 190-2100 nm range measured by spectroscopic ellipsometry shows that the minimum of the reflectance is shifted to lower wavelengths when the thickness of the films decreases. The substrate grain size does not significantly influence the reflectance spectra of the films. The reflectance spectra of the virgin tungsten, regardless the substrate grain size, does not present a sharply defined minimum. The modeled n and k dependencies of wavelength were compared with their measured counterparts. For the He exposed samples n and k have higher values than the unexposed ones. [1] Y. Addab et. al, Formation of thin tungsten oxide layers: characterization and exposure to deuterium, Phys. Scr. T167 (2016) 014036.

Resume : Well known high radiation hardness makes GaN the material of interest for electronic applications where systems are exposed to fluxes of ionizing radiation like cosmic rays. Understanding of the processes affecting irradiation damage formation is extremely important to design technologically improved and radiation hard GaN devices. In this work investigation of radiation hardness of GaN epitaxial layers under irradiation with swift O, Ni, Ag and Au ions has been carried out by employing in-situ electrical resistivity measurements and study of microstructure of irradiated samples. Un-intentionally doped and Si-doped n-GaN epitaxial layers grown by MOCVD on sapphire were used for SHI irradiation. Irradiation with light O ions has very weak effect on the sample resistivity. On the other hand, the resistivity sharply increases for more than 6 orders of magnitude under bombardment with other ions. Interestingly, shape of the experimental curves is very similar to one obtained in the case of much lower (few MeV) energy ion irradiation. Formation of aligned chains of defect clusters was found by XTEM after Ag irradiation. Estimation of total amorphized area gives ~3% after target irradiation to highest experimental fluence of 2×10^11 ?m^-2. Such a low value will not significantly affect sample resistivity. On the other hand, nuclear energy loss can generate enough simple point defects to determine the effect observed. The mechanism of resistivity increase is formation of complexes consisting of a doping atom and a simple point defect generated by stopping ion. Corresponding model will be presented. Work was partly supported by the State Assignment for Higher Education Institutions (basic part), project ? 3.5469.2018

Resume : Insulation cabling is an essential engineering component in nuclear power plants. The insulation around these wires is made of ethylene-propylene rubber and crosslinked polyethylene that is subjected to environmental and radiation damage. For the purpose of maintenance and reactor recertification, it is necessary to develop a non-destructive approach to test the impact of damage on the insulation. Accelerated aging experiments are used to develop these methods, although it is unclear how to relate these experimental results to in situ aged specimen. Here we use a kinetic rate model to investigate the impact of radiation dose rate and total dose on the crosslinking and chain scission of polyethylene. Analytical expressions for the concentration of crosslinking and scission sites as a function of time and radiation rate both for during and post radiation and crossover point between these two are presented in terms of radiation rate and time. The scission reaction rate is significantly slower than crosslinking for lower total doses whereas the situation is changed for higher total doses causing the ambiguity between the accelerated aging and in-situ aging methods. Also, chain scission forming reactions can continue for months or years during storage after being radiated, whereas crosslink forming ones end almost immediately after the radiation is turned off.

Resume : In determining the effects of neutron irradiation on structural reactor materials, the author have utilised both light, He at 1-3 MeV, and heavy ion, Au at 12 MeV, irradiation to examine the hardening behaviour of SS 316 and Duplex steel. Utilising two nano-indentation techniques (top-down and inclined indentation methods) and TEM analysis of the dislocation distribution the authors were able to observe the depth dependent hardening of these materials as a function of ion and indenter penetration depth. The usefulness and veracity of each technique were contrasted with numerical models that evaluated the growth and distribution of the complex plastic zones below the Berkovich indenter. Through a combination of an Orowan based hardening model and SRIM outputs, the authors were able to relate the dpa dose with an increased hardening response at an FE scale. The simulation results were qualitatively and quantitatively in good agreement with the experimentally derived results and proved a robust and predictive capability in broadly examining layered structures, of which ion irritated materials form an interesting subset group.