This presentation will examine two emerging strategies to develop high-performance materials with simultaneous high radiation resistance, high strength, good toughness and corrosion resistance, and moderate fabrication cost. A face-centered cubic 27%Fe-27%Mn-28%Ni-18%Cr high entropy alloy (HEA) was utilized in exploratory studies to compare HEA irradiation behavior with conventional Fe-Cr-Ni and Fe-Cr-Mn alloys. Significant differences in irradiation behavior compared to conventional alloys (in particular solute segregation to grain boundaries and defect microstructures) were observed following 5 MeV Ni ion irradiation (1-10 dpa at room temperature and 500°C). We will also describe results from an investigation of the effects of 5 MeV Ni ion irradiation at room temperature and elevated temperature (200°C) to 0.1 and 1 dpa on the BAM-11 bulk metallic glass (BMG) with a composition of 52.5%Zr-17.9%Cu-14.6%Ni-10%Al-5%Ti (Tg~390°C). Characterization of the HEA and BMG samples utilized nanoindentation mechanical properties testing, x-ray diffraction, and electron microscopy. Although the two model alloys examined in this scoping study did not possess complete radiation resistance, several aspects of their observed response to irradiation suggest the potential for developing alternative HEA and BMG materials that could exhibit favorable stability to irradiation.

Near-term initiation of new commercial deployment of advanced nuclear power reactors requires further development, testing and qualification on new structural materials with improved properties, specifically with respect to radiation, corrosion, thermal and other degradation processes. The International Atomic Energy Agency (IAEA) launched several coordinated research activities in this very important topic. The projects are focused primarily on the stability or instability of nano-dispersoids under irradiation, as observed in several studies. An alternate approach to explore the radiation performance of oxide dispersoid-strengthened (ODS) alloys to high dpa levels is to use charged particle irradiation at greatly accelerated dpa rates to simulate neutron damage. Such approach has been used successfully to study many aspects of void swelling, phase stability, and irradiation creep in other alloy systems [1]. However, some aspects of accelerated charged particle irradiation that produce behaviour atypical of neutrons at lower dpa rates. Because ODS alloys are rather complex in microstructure and very little data at high exposure are available, it is necessary to first demonstrate that modelling can bridge the gap between neutron and charged particle irradiation for a much simpler system before moving to more complex alloys. The research activities contributes to knowledge-base on the radiation-induced response of the ODS material in the following areas: (a) Radiation-induced swelling of ODS materials, (b) Radiation stability of oxide nanoparticles and nanoclusters (or nanofeatures) against swift-ion irradiation, (c) Re-crystallisation effects on the radiation resistance of ODS materials - by testing the radiation tolerance of ODS materials after extrusion and after re-crystallisation to gain insights into the effect of grain size on the radiation tolerance of ODS materials, (d) The effect of temperature on radiation damage development, (e) The effect of He on radiation damage development, (f) Synergistic effects of ballistic damage and simultaneous He and H accumulation on damage formation. The ion irradiations are carried out at different facilities in order to study the phenomena listed above, including: CEA/Saclay Triple-beam ion irradiation Jannus facility (France), ANSTO ANTARES accelerator, and STAR accelerator; Plasma Inert Ion Implantation (Australia); JINR IC-100 cyclotron for applied research (Russia); ITEP heavy ion RFQ HIP-1 (Russia) and Kyoto University DuET Accelerator (Japan). The in-situ TEM and post-irradiation experiments (PIE), static and dynamic nano-hardness measurements, micro-cantilever tests, compression test on micro-pillars, and APT analysis are scheduled in period 2013/2014.
References:
[1] P. Hähner, A.Zeman (ed.), Proceedings of the 2nd IAEA-EC Topical Meeting on Development of New Structural Materials for Advanced Fission and Fusion Reactor Systems, Ispra, Italy, 16-20 April 2012, J.Nucl.Mat.442 (2013) 408-538.

We present in-situ experiment with swift heavy ions at the GANIL facility. One of the applications of these ions is the simulation of materials under real-world conditions, such as radiative environments found in nuclear or space applications. Indeed, swift heavy ion irradiation leads to the deposition of high energy density with electronic excitations, known to entail modifications depending on material characteristics (compositions, electrical properties, crystallographic structures, etc).
Since few years, many efforts are under way to allow in-situ experiments directly at large scale infrastructures to provide time-saving and avoid reproducibility problems. For the study of induced structural phase transitions and kinetics, a X-ray diffractometer (“ALIX”) has been set up at the low-energy IRRSUD beamline of the GANIL facility [1]. This equipment allows to perform in-situ X-ray diffraction measurements simultaneous to irradiation. A special configuration, grazing incidence X-ray diffraction, is required to probe the upper irradiated part of sample due to the energy range of IRRSUD, i.e. 0.3-1 MeV/A, which implies a mean projected range in solid matter around only a few micrometers.
In this communication, key parameters will be presented about the GANIL facility and the capability of the ALIX setup to perform simultaneous irradiation - diffraction by using energy discrimination between X-rays from diffraction and from ion-target interaction. To illustrate its potential, results of sequential or simultaneous irradiation - diffraction will be detailed to show radiation effects on the structural properties of ceramics. Different standard oxide materials are chosen to highlight and to interpret damage build-up induced by ion irradiation, which are MgO, SrTiO3, ZrO2 and Al2O3, which were studied for their potential application as inert matrices for the immobilization of radioactive wastes. During experiments using ions with a relatively high electronic energy loss Se around 20 keV/nm, different sensitivities to electronic excitations are observed. By interpreting the transition characteristics, various cases are described for these materials, ranging from no transition, crystal-to-crystal transition to amorphization through different overlapping-track processes. Evolutions of structural parameters and damage accumulation models will be discussed to interpret the observed transition kinetics. To support the XRD results, transmission electron microscopy studies will be presented.
[1]: C. Grygiel et al., Rev. Sci. Instrum. Volume 83, Issue 1, 013902 (2012)

The interaction of ions with solids results in energy loss to both atomic nuclei and electrons. At low energies, nuclear energy loss dominates, leading to damage production via ballistic processes. At high energies typical of fission products and swift heavy ions, electronic energy loss dominates, leading to intense local ionization. At intermediate ion energies, nuclear and electronic energy losses are of similar magnitude and can lead to additive, competitive or even synergistic processes that affect damage production, defect recovery and microstructure evolution. This energy regime includes energies of primary knock-on atoms created by fission and fusion neutrons, energies of ions used to investigate neutron damage in materials, and ion energies used to modify or generate novel defects and structures in materials to tailor properties or create unique functionalities. We have integrated experimental and computational approaches to investigate the separate and combined effects of nuclear and electronic energy loss on the response of ceramics to ion irradiation over a range of energies. Experimentally, ion mass and energy are used to control the ratio of electronic to nuclear energy loss; whereas, large scale molecular dynamics simulations that include both ballistic collision processes and local heating, or inelastic thermal spike, from ionization via electron-phonon coupling are used to model these processes. Using these approaches, an additive effect of nuclear and electronic energy loss on damage production in amorphous silica and crystalline MgO is demonstrated over a wide range of energies. Similarly, the competitive effects of nuclear and electronic energy loss are confirmed for in situ transmission electron microscopy studies and in studies on ionization-induced recovery of pre-damaged states in SiC using ions and energies (MeV to tens of MeV) with high ratios of electronic to nuclear energy loss. A threshold in electronic energy loss for defect recovery in SiC has been determined. Finally, a synergistic effect of nuclear and electronic energy loss is observed in LiNbO₃. These results have significant implications for modeling the response of materials to extreme irradiation environments.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Tungsten is a prime candidate for building divertor (“exhaust”) components in fusion reactors. During its service life, the component would be subject to fast neutron displacement damage at high temperatures (plasma facing parts: 1200°C; structural parts: 500°C). Using self-ion implantation as an analogue of the primary knock-on atoms of fusion neutrons, the damage behaviour and microstructural evolution in tungsten (> 99.9999 wt%) were investigated as a function of temperature (R.T. → 800°C) and dose (le; 10¹⁸ W⁺m^-2; ~ 1.0 dpa) with in-situ 150 keV W⁺ ion irradiations on the IVEM-Tandem facility at Argonne National Laboratory. Dislocation loops with Burgers vectors of ½<111> and <100> coexisted, among which loops of interstitial nature dominated. In addition, spherical voids of size less than 2 nm were observed.
This work focuses on the thermal stability of the radiation damage induced by the 150 keV W⁺ ions. Thin foils of tungsten irradiated at R.T. up to 1.0 dpa were then annealed in-situ for up to 120 min at temperatures between 300°C and 800°C. Three major conclusions could be drawn: (1) radiation-induced SIA clusters were most stable as ½<111> dislocation loops and tended to configure into strings/rafts through elastic interactions, whilst vacancies resided in both spherical voids and ½<111> loops; (2) b = <100> is not a stable defect configuration in tungsten and the fraction of such loops decreased with increasing annealing temperature and/or time; (3) microstructural transformation during annealing was very sensitive to temperature, but less sensitive to annealing time. The majority of changes were completed within 15 min of annealing, and were associated with the annihilation of small (1-2 nm) dislocation loops. The origin of these trends are discussed by considering defect mobility with respect to the annealing stages in tungsten as well as the enthalpy of formation of defect configurations predicted by previous DFT calculations.

In situ measurements of irradiation effects are enormously valuable for systematic investigation of the effects of dose, dose rate and temperature, since variations in microstructure from one sample to another are avoided. This is particularly true for irradiation induced creep (IIC), yet such measurements are scarce owing to their difficulty. On the one hand, in situ neutron have become prohibitively expensive and time consuming, while on the other, ion irradiations are plagued by short penetration depth of ions in solids and the need for homogeneous damage. Typically high energy light ions in the MeV energy range or GeV heavy ions are employed to overcome this problems but they suffer from high electronic stopping which limits doses to less than 1 dpa /and/or simply do not reflect neutron damage. We have overcome some of these difficulties of in situ creep measurements by using bulge tests thin-film specimens or compression tests on micro-pillars in conjunction with MeV heavy ion on irradiations. We have employed the first method to study IIC in nanocrystalline Cu alloys, and the second to study IIC in amorphous Cu-Ti alloys. We have also modeled IIC in these materials using molecular dynamics simulations. We will first describe our methods in this presentation and then show that the creep response in nanocrystalline materials is directly related to that in amorphous alloys. In the limit that the grain size goes to zero, the creep response in the nanocrystal in fact approaches that of the glass. The model and preliminary results suggest that the magnitude of the creep response in these materials is nearly the same for all alloys and glasses
This work was supported by the U.S. DOE Office of Basic Energy Sciences under grant DEFG02-05ER46217.

In future generation nuclear systems cooled by Heavy Liquid Metals (HLMs), fuel cladding will be exposed to an extremely harsh environment, in which radiation dose will approach 150 displacements per atom (dpa) at a temperature of up to 800°C. In addition, corrosion of structural steels by HLMs stands as a major bottleneck. In this framework, Al2O3 coatings are being investigated for protecting steels [1].
Here, fully dense and compact, nanocrystalline/amorphous Al2O3 coatings are grown by Pulsed Laser Deposition. The mechanical properties of the coating are assessed with high accuracy and precision trough a novel opto-mechanical method, based on the combination of ellipsometry, Brillouin spectroscopy and nanoindentation [2], whereas the adhesive strength is evaluated by nanoscratch tests. The deposition process is tailored so as to obtain an advanced material with metal-like mechanical properties (E=195±9 GPa and nu;=0,29±0,02), strong interfacial bonding and outstanding wear resistance (ratio of hardness to elastic modulus H/E=0,049-0,091). Corrosion aspects are examined by short- (500 hours) and mid-term (2000 hours) exposure of samples to stagnant HLMs at 600°C. Post-test analysis reveals no signs of corrosion [1].
Concerning high dose radiation damage, the performance of the alloy substrate-ceramic coating system is studied by irradiation with 12 MeV Fe ions up to over 100 dpa at 600°C at the JANNUS platform of the CEA center of Saclay. The irradiation parameters (600°C, Fe self-ions, implantation depth beyond coating with an ENSP ratio around 100) are chosen in order to reach a compromise between the expected operating conditions in HLM-cooled fast reactors (ENSP around 4 in Al2O3) [3], and requirements for nanoindentation measurements. Post-test analysis is conducted by profilometry, SEM, TEM, nanoindentation and nanoscratch measurements. Results are compared to previous work on ion [3-5] and neutron [6,7] irradiation of crystalline Al2O3, standing to which the coating is expected to exhibit formation of dislocation loops of around 10-15 nm with densities similar to 1023 m-3 , along with macroscopic swelling of up to asymp;10% or more.
Finally, other coating materials and structures of interest, including cermets and graded barriers with an engineered composition, are proposed and discussed.
[1] F. García Ferré, M. Ormellese, F. Di Fonzo, M.G. Beghi. Corr Sci 77 (2013) 375
[2] F. García Ferré et al. Acta Mater 61 (2013) 2662
[3] S.J. Zinkle. J Nucl Mater 219 (1995) 113
[4] C.J. McHargue. Mater Sci & Eng A253 (1998) 94
[5] S.J. Zinkle, C. Kinoshita. J Nucl Mater 251 (1999) 200
[6] T. Yano, K. Ichikawa, M. Akiyoshi, Y. Tachi. J Nucl Mater 283 (2000) 947
[7] F.W. Clinard Jr. et al. J Nucl Mater 122 (1984) 1386

Pre-amorphization of germanium by self-ion implantation is an advantageous step in forming shallow junctions. The germanidation process for forming low resistance contacts to the doped regions is investigated here. The germanide investigated is nickel germanide. Results for materials analysis using a high resolution transmission electron microscope for cross-sectional imaging, stoichometry using energy dispersive x-ray spectroscopy for individual grains of NiGe and recrystallized germanium, element mapping using electron energy loss spectroscopy and electron diffraction are presented. The amorphous region of germanium is 1.1 micron thick as expected from ion implantation modeling of the combined effect of the three energies used ( 550keV, 1MeV and 2MeV) which had the same does each at 1 x1015 cm-2. The HRTEM images confirmed this depth of amorphous region. To form the germanide, 50nm of Ni was deposited and the heat treatment to form the germanide, NiGe was 400 C in nitrogen for 15 minutes. The resulting NiGe analysis by TEM shows it to have a smooth surface but a rough interface with the amorphous Ge. The NiGe thickness is approximately 115nm. Recrystallization of the ion damaged Ge occurs during the NiGe formation. Electron diffraction images demonstrate the effects of this process. Element mapping shows the lack of uniformity of Ni and Ge and the formation of Ni islands below the NiGe-Ge interface. This is likely to be a more significant disadvantage than the rough interface. Electron diffraction shows that the degree of crystallinity of NiGe grains formed on amorphous Ge is high but not as high as that formed on crystalline Ge for the same heat treatments. Further heat treatment improves the crystallinity.

The JANNuS-Orsay facility [1], located at the CSNSM lab, is built around a transmission electron microscope (TEM) that is linked to two ion accelerators in the energy range 10-500 keV and 0.5-15 MeV. Since 2009 it has been an open facility for the scientific community in the field of ion-beam-induced effects in materials, allowing in-situ observation of the material microstructure modifications induced by single/dual ion irradiation/implantation.
A description of the facility will first be shown in this presentation. To illustrate the capability of the facility, and to help future potential users to prepare their experiment, we will expose feedbacks of our 4-years experiences of local contacts through chosen examples of in-situ experiments related to ageing of steels [2], nucleation-growth in semiconductor material [3], and ion-induced thin film structural modification [4].
This versatile tool is a quite efficient way to study materials submitted to radiation environments such as in space or in a nuclear reactor. Moreover it is much less time and money consuming than tests in real conditions. Immediate observation and characterization are allowed and the material is not activated.
Due to its large range of ions, energies and temperatures available, the JANNuS-Orsay facility is also a powerful tool to study ion-beam synthesis of new materials of interest in nanosciences.
[1] Web sites: http://www.csnsm.in2p3.fr/-MET- and http://jannus.in2p3.fr
[2] X. Li, S. Jublot-Leclerc, F. Fortuna, A. Gentils, M.-L. Lescoat and C. Pokor acknowledge financial support from the French National Research Agency through project ANR-11-BS09-006 CoIrrHeSim.
[3] F. Fortuna et al., Physical Review B 84, 144118 (2011).
[4] M. Seita et al., accepted in Acta Materialia (2013).

We present an experimental/numerical study on the variation of structural/mechanical properties of ion-implanted single crystal diamond at various annealing temperatures, up to full graphitization. In particular, optical interferometry is used to determine the variation of surface swelling in correspondence with the implanted areas at various annealing stages, and TEM and EELS measurements are also acquired and analysed. The experimental data are compared to the predictions of a phenomenological model and finite element simulations to describe the depth variation of mass density and strain of the considered samples. Damage saturation and graphitization thresholds are discussed.

Strain is often induced by ion implantation in crystals, where implantation defects usually lead to the lattice expansion along the surface normal [1]. Due to the predominant growth along the [0001] direction in wurtzite (hexagonal) crystals, this affects mainly the c lattice parameter for these systems (e.g., GaN or ZnO). The lattice expansion is frequently determined from the extra peak measured in X-ray diffraction (XRD) curves, but the depth resolution of this method is limited. Rutherford backscattering spectrometry under channeling conditions (RBS/C) has proven to be a powerful technique to perform strain profiling [2.3]. In this work we analyze the accuracy of RBS/C for the quantification of strain in challenging GaN-based heterostructures with different strain states (from -1% to 1%) and different thicknesses (from 50 to 250 nm). The angular scans were recorded with conventional solid state detectors but also with novel position sensitive Timepix pixel detectors [4,5]. The obtained values present deviations from the XRD data, confirming the importance of steering phenomena in the strain determination by RBS/C [6]. These effects, however, can be corrected with Monte Carlo simulations and controlled by the experimental parameters (e.g., the beam energy), allowing an accurate determination in such critical cases [2,5]. The applicability of RBS/C for the determination of strain in irradiated samples was evaluated in implanted GaN templates.
[1] B. Lacroix et al., EPL 96, 46002 (2011)
[2] A. Redondo-Cubero et al., Appl. Phys. Lett. 95, 051921 (2009)
[3] A. Redondo-Cubero et al., J. Phys. D 43, 055406 (2010)
[4] J. Jakubek et al., Nucl. Instr. Meth. A 591, 155 (2008)
[5] E David-Bosne, "Timepix and FitPix detection system for RBS/C materials analysis". M.Sc. Thesis, Universidade de Aveiro (2013)
[6] K. Lorenz et al., Phys. Rev. Lett. 97, 85501 (2006)

We describe the damage and recovery mechanisms for focused ion beam (FIB) implantation of Si using a wider range of primary ion species than available using conventional instruments. This is achieved by employing a mass-selecting FIB (MS FIB) that uses a Wien filter to separate ion species from an alloy source. In FIB systems the typical ion fluence is 0.1-10 A cm-2, which is at least three orders of magnitude greater than that in broad beam ion implanters applied to commercial doping of Si. In the present work we study the damage created by FIB (60 keV Si++ and Ge++, and 30 keV Ga+) and broad beam ion (60 keV Si+) implantations (doses: 1e12 - 5e15 ions cm-2) in Si, both in the as-implanted state and after annealing in a nitrogen ambient. For measuring implantation damage in these specimens we use Raman spectroscopy with laser probes of wavelengths 514 nm and 405 nm. The different absorption depths associated with these different wavelengths allows us to probe different volumes in the specimens. Measurements are made of Raman peak height and peak position. While the former was used for quantifying the degree of structural damage, the latter was used for characterizing stress state. We report on the effect of Si ion fluence on damage in as-implanted and annealed samples, comparing results from Si implants from the FIB and broad beam implants, covering three orders of magnitude of ion fluence. We also study the effects of FIB ion species on implant damage and recovery, comparing Ge, Ga and Si species.
Our measurements using 405 nm wavelength show that the structural damage increases with Si dose, as expected. Also, the structural damage is consistently higher for the higher fluence FIB implants than for the broad beam implants. However, the remnant structural damage after annealing is consistently substantially lower for the FIB implants. The relative variation of stress (inferred from peak shifts) with dose and fluence is more complex. Measurements using 514 nm and 405 nm wavelengths, consistently indicate higher stresses for the FIB implants for the 514 nm measurements, but lower stresses for the 405 nm measurements. In comparing the effects of FIB ion species, we observe that while Ge implants show greater sample damage than Ga—presumably due to the higher energy of Ge ions—in the as-implanted state, the measured damage after annealing (e.g. at 730 C, 10 mins) is substantially lower for the Ge-implanted samples. A model for damage development and recovery in FIB implanted Si will be presented.
We thank Hamed Parvaneh at Rensselaer Polytechnic Institute for providing Si specimens implanted by MS-FIB.

In recent years graphene has been studied as a novel material for the next generation electronic and photonic due to its unique properties.
Therefore many elaboration process have been studied based for example on chemical synthesis or CVD.
In this work, we present a new way of preparation of graphene or multilayer of graphene. This process is based on carbon implantation and diffusion in a metallic matrix displaying many advantages such as accurate control of carbon dose and depth localization.
We present the mechanism of Thin Layers Graphite (TLG) synthesis on a nickel film (mono or poly-crystalline) investigated by ¹³C implantation of four equivalent graphene layers and annealing at moderate temperatures (<600°C). Due to their isotopic separation, Nuclear Reaction Analyses (NRA) were used to determine the ¹²C and ¹³C concentrations at each step. During this process the implanted ¹³C segregates to the surface. However as evidenced by NRA involving ¹²C and ¹³C ions, a part of the carbon atoms also comes either from some previous impurities or from carbon absorbed into the metallic matrix. Raman spectroscopy and imagery were used to determine the main location of each carbon isotope into the TLG fragments. The Raman mappings especially emphasize the role of ¹²C previously present at the surface that first diffuses along grain boundaries. They play the role of nucleation sites. Around them the implanted ¹³C further aggregates and precipitate into graphene-like fragments.

Low dimensional wide bandgap semiconductors (WBS), such as III-nitride and II-oxide nanostructures and thin films are definitely established as active niches of research and applications. In this work we concentrate on the aspects concerning the use of ion implantation, a well-established tool in silicon-based device processing which provides universal doping and profile tuning. However, this method is not yet extensively studied in WBS and nanostructures in general. In particular, preoccupations regarding implantation damage formation, thermal annealing and lattice site location of dopants need to be addressed, especially when studying nanomaterials. When the mass of the dopant or the morphology of the sample hinders the use of traditional characterization techniques like RBS/C and emission channeling, this task can be accomplished with nuclear hyperfine techniques.
In this work, we used the Perturbed Angular Correlation (γ-γ PAC) nuclear technique, that provides, using only a small number of probes (<10^12 at/cm^2), information about the charge density distribution and polarization around the probe nuclei, thus allowing the characterization of the dopant and of its environment at the atomic scale, without external “tip” interference. The experimental results are then interpreted with the help of DFT simulations obtained for different atomic configurations and charge density models.
An extension of PAC, the electron-gamma PAC (e-γ PAC), further allows inducing an electronic excitation upon the conversion electron ejection from an atomic orbital. Then the measurement monitors the electronic recombination dynamics as a function of temperature and time, from the nano- to the microsecond time scale. The response depends on the availability of charge carriers and their mobility and on the intrinsic dopant-host electronic relationship.
Combining both techniques (γ-γ and e-γ PAC) on 181Hf and 111Cd implanted Ga2O3, GaN and AlN we will provide lattice site information and a dynamic picture of locally induced electronic excitations and subsequent recombination phenomena in the probe&’s neighborhood.

Colloidal silica particles are being intensively studied due to their potential applications in catalysis, intelligent materials, optoelectronic devices, photonic bandgap crystals, masks for lithographic nanopatterning, etc. Moreover, in nanoscale electronic, photonic and plasmonic devices, feature dimensions shrink towards a critical limit, and new experimental approaches have to be explored in lithographic patterning. For this work, spherical submicrometer-sized silica particles were prepared by the sol-gel technique and deposited as a self-assembled monolayer onto high-purity silica glass plates by means of a spin coater system. This monolayer is then used as a mask to create regular arrays of nanoscale features in the sample by MeV Ag ion implantation. The surface plasmon resonance (SPR) of the ordered arrays of Ag nanostructures was characterized by optical absorption measurements. On the other hand, some of the samples were irradiated at room temperature with 8 MeV Si ions in order to modify the shape of the Ag nanostructures, allowing the tuning of the SPR. The size, size distribution and shape of both the silica particles and the array of metallic deposits were determined by scanning and transmission electron microscopy, respectively, as a function of the ion irradiation parameters, whereas the long range order of the nanoparticle assembly was characterized by means of a Fast Fourier Transform study. Finally, the Ag distribution depth profile was studied by Rutherford Backscattering Spectrometry.

Magnesium aluminate spinel is one of the oxides envisaged to be used in manufacturing of inert matrix fuel. Although the material is recognized for its radiation resistance, most of the experiments were performed on single crystals mainly by the use of Rutherford Backscattering/channeling (RBS/c) method and not much is known about the effects of irradiation of this material in polycrystalline form. The very recent concept is to use luminescence techniques as an experimental method able to measure the level of disorder in polycrystals, as it may be applied to both single and polycrystalline solids, is non-destructive, fast and can be easily implemented in-situ.
The ion beam-induced luminescence (IBIL or IL, ionoluminescence) technique has been chosen to analyze the level of damage formation in ion-irradiated single and polycrystals of magnesium aluminate spinel. The main aim of the study is to better understand the potential of the luminescence technique to analyze the damage accumulation process in irradiated materials with the special emphasis to polycrystalline solids. Samples were irradiated with 320 keV Ar+ ions at fluences ranging from 1x1012 to 2x1016 cm-2 in order to create various levels of radiation damage. The ionoluminescence (IL) was measured using a homemade system based on the use of Hamamatsu spectrometer collecting the light from a sample installed inside a target chamber of an ion implanter (Balzers MPB 202RP). A H2+ ion beam with an energy of 50 keV and low fluence (selected so to correspond to the projected range of 320 keV Ar ions) was used to excite the luminescence.
The IL spectra of single and polycrystalline samples were analyzed in terms of the variation of peaks intensities with the irradiation fluence. Additionally, for single crystals, the damage-build up as a function of accumulated ion fluence was established through RBS/c. The results of IL and RBS/c analysis were then processed using Multi-Step Damage Accumulation (MSDA) model. That allowed for the determination of damage build-up kinetics, and finally cross-section for radiation damage build-up. Complementary information has been extracted from RBS/C and IL measurements in the case of single crystals. The analysis presented confirms the huge potential of luminescence techniques for damage analysis in single- and polycrystalline samples.
This work was partially sponsored by the National Science Centre (Poland) under the contract number DEC-2011/03/D/ST8/04490.

Because of their high-aspect ratio, excellent electrical and mechanical property, CNT is a potential candidate as the next generation field electron emitters. And in the passed twenty years, field emission investigation has become one of the hot tops in CNTs world. However, the relatively high work function of CNTs limits its ability to emit electrons. So there is a requirement to enhance the field emission properties of CNTs further by reducing their work function. Doping CNTs with metal elements could get better field emission performance, because the metal element could enhance the electronic density of states near the Fermi level of CNTs and make field emission easier. The ion implantation technique is an effective tool for doping. In this paper, we summarized the progress of electron field emission enhancement from carbon nanotubes implanted by energetic ions in our group. We investigated the field emission performance and structure of the vertically aligned multi-walled carbon nanotube arrays implanted by energetic Zn, Ag,Si, Ti and C ions with average energy of 40 keV respectively. After energetic ion implantation, the turn-on electric field and the threshold electric field of samples decreased. The lowest turn-on electric field and the threshold electric field (0.67 and 0.98 V/m respectively) was obtained on the sample implanted by C ions. Structural analysis of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy indicates that the enhancement of electron field emission is due to the formation of DLC nanowires at the tip of carbon nanotube arrays, which is an electron emitting material with low work function.

Determining the composition and structure of the near-surface of solids is essential for understanding their physical and chemical properties. As a consequence, many experimental techniques have been developed to characterize such layers, each having their specific advantages and limitations. Here we introduce a new technique for probing the composition and electronic structure of the near-surface region of solids, namely: high-energy electron backscattering spectrometry (or electron Rutherford backscattering spectrometry (e-RBS)).
Measurements are performed by irradiating samples with mono-energetic (5-40 keV) electrons and detecting the backscattered electrons with high-energy resolution (~0.3 eV FWHM). This energy resolution is sufficient to distinguish electrons scattered elastically from different mass atoms, as well as those that undergo additional inelastic scattering events (e.g. plasmon excitation, or band-gap transitions). The near-surface composition can be determined directly from the elastically scattered electrons in a manner analogous to Rutherford backscattering spectrometry. However, in this case depth profiling is achieved by performing measurements at different energies and/or geometries, taking account of the inelastic mean free path of the electrons. This allows the stoichiometry of the near-surface layer to be determined with an accuracy of 5%-10%. Electrons that have undergone additional inelastic scattering provide information about electronic excitations in the probed layer and can be used to monitor the presence of specific phases, including the presence of bandgaps.
The utility of the e-RBS technique is illustrated for the case of ion-beam synthesised transition metal oxides that are of interest for non-volatile memory applications. Two systems are considered: the synthesis of Ta2O5 by oxygen implantation of Ta films, and the synthesis of HfO2 by oxygen implantation of Hf films. The e-RBS technique is shown to provide quantitative compositional analysis of the implanted films and to provide insight into the formation of near-stoichiometric layers of Ta2O5 an HfO2 through the presence of characteristic bandgaps. Results are compared with x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy-dispersive x-ray analysis (EDX) and dielectric breakdown measurements.

Dynamic investigation of defects induced by short, high current pulses of high energy lithium ions
Hua Guo1,2,3, Arun Persaud1, Steve Lidia1, Andrew M. Minor2,3, and Thomas Schenkel1
1Accelerator and Fusion Research Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, 50A2177, Berkeley, CA 94720, USA
2Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
3National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
Intense, short pulses of energetic ions are highly desirable for studies of warm dense matter, high energy density physics experiments with volumetrically heated targets, and for studies of defect dynamics in solids [1]. To investigate radiation induced defects and their dynamic behavior on time scales from nanoseconds up to milliseconds, a unique induction type linear accelerator, the Neutralized Drift compression eXperiment (NDCX-II) [2], was employed. NDCX-II was built to produce Lithium ions with an energy of up to 3MeV with a pulse length of <1ns. For the experiments we will discuss here, Lithium ions at energies of 170 keV and 320 keV were used to examine a micron thick Si membrane. High current pulses (30nC) with pulse lengths of ~20 ns (compressed beam) and ~600 ns (un-compressed beam) were used to probe defect dynamics in ion transmission experiments with a fast Faraday cup. The ion energy and membrane thickness was selected so that the defect creation in the membrane and ion transmission are maximized simultaneously. Therefore, ions arriving later during a pulse can be used as a probe to investigate the damage created by ions that arrive earlier. This allows accessing the defects dynamics at time scales shorter than the beam pulse length and we will discuss the observed trends in defect recombination times for a series of irradiation dose rates. Further pump-probe experiments using other diagnostics, such as back scattering ions, are currently under development.
To examine other materials, electrical properties were measured in situ during beam exposure. Electrical measurements provide access to dynamic effects on different timescale, from the length of the beam pulse to milliseconds and longer. This is important to give a broader understanding of the multi-scale behavior of defects in solids under irradiation.
This work is supported by the Director, Office of Science, Office of Fusion Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
References:
[1] A. Friedman, et al., Phys. Plasmas17, 056704 (2010).
[2] T. Schenkel, et al., Nucl. Instr. Meth. B, in press, http://dx.doi.org/10.1016/j.nimb.2013.05.074

Raman scattering spectroscopy displays peculiar interests for characterizing damages induced by ion beam irradiation in matter. The method can probe different types of damage, in both electronic and ballistic regimes, through either a specific vibrational spectrum of the damaged zone, or through change of the Raman selection rules induced by damage, leading to appearance of symmetry-forbidden modes. The recent developments in Raman imaging offer a very powerful tool for probing inhomogeneities in the 1-50µm length scale range, i.e., the same order of magnitude as the depth attained by incident ions below surface, depending of their nature and energy. The way to extract the relevant information from numerous data given by imaging approach will be particularly emphasized, as for instance source separation methods and multivariate analysis. The potentialities of modern Raman methods will be reviewed through the following examples.
(i) 6H-SiC, irradiated by 20 MeV Au ions, where the main effect is the release of the Raman selection rules. Two different types of amorphization were evidenced, depending of the depth below the surface [1, 2]. One is only due to the loss of translation symmetry, the second one is a much more disordered structure.
(ii) Gd₂(Zr_xTi_1-x)₂O₇ pyrochlores, irradiated by 870 MeV Xe ions. The large penetration depth of Xe ions (30 µm) allows here an accurate probing of the damage, from crystalline structure with increasing linewidths, up to amorphization close to the surface [3, 4].
(iii) UO₂ ceramics, irradiated by 25 MeV He²⁺ ions, where irradiation reveals so-called defect Raman lines, some of them being characteristic of local changes of the oxygen stoichiometry, and other due to release of selection rules[5].
In a second part, we will present Raman in-situ measurements recorded during irradiation of UO₂ ceramics with He beams on the CEMHTI cyclotron accelerator. A detailed presentation of the experimental set-up, originally developed for radiolysis studies, will be given [6]. Recent results obtained of ceramic uranium oxide [7] allow to record the kinetics of damage under irradiation, via the growing of the defect lines cited above. Besides, the frequency shift and broadening of the UO₂ Raman line give access to temperature increase induced by the ion beam.
[1] A. Gentils et al., Journal of Materials Science. 46, 6390 (2011).
[2] F. Linez et al., Journal of Raman Spectroscopy 43, 939 (2012).
[3] S. Moll et al., Physical Review B 84, 064115 (2011).
[4] G. Sattonnay et al., Journal of Applied Physics 108, 103512 (2010).
[5] G. Guimbretière et al., Applied Physics Letters 100, 251914 (2012).
[6] A. Canizarès et al., Journal of Raman Spectroscopy 43, 1492 (2012).
[7] G. Guimbretière et al, Applied Physics Letters 103, 041904 (2013).

Here we introduce a new technique to better understand material deformation and defect formation at small scales. A straightforward approach allowing three-dimensional (3D) visualization of subsurface deformation beneath nanofeatures (i.e. nanoindents, ion beam patterns) using reconstructed cross-sectional transmission electron microscopy (TEM) data is demonstrated. It is applicable to any array of nominally identical features that can be patterned with regular spacing. This opens the approach for emerging areas of research to study photon/ion damage from 3D printing to new ion implant devices for electronics on inorganic and organic materials.
Cross-sectional transmission electron microscopy (TEM) provides essential atomic scale imaging and probing for unmatched characterization of nanoscale structures, interfaces, alloy phase evolution, subsurface defect formation, and more. However, it is a very challenging and tedious task to prepare the TEM lamella of the small volume in a specific site. The most common way to prepare site-specific TEM cross sections is via focused-ion-beam (FIB) lift outs.
When lifting out site-specific features < 100 nm, the primary challenge lies in centering a feature within the TEM cross section. Even with the aid of fiducial marks, centering a feature within the final cross section is nontrivial, and it is reasonable to assume an error of ~ 50 nm or more. Due to this error it is common to find the final thinned sample no longer contains the feature of interest because it has been milled away. Even with success in extracting a cross section, important structural information (e.g. crystal anisotropy, interfaces, etc.) in three dimensions is still missing. This is because the cross section represents a single, isolated, 2D projection of a fraction of the 3D nanofeature. This limited information results in gaps of our understanding of experimental nanoscale mechanics, impeding our ability to accurately model and predict small-scale mechanical behavior.
Here we address the above problems, by introducing an alternative approach to FIB preparation that allows for 3D visualization of small volumes via TEM. The approach involves generating nominally identical indents in an m x n pattern, where m and n are integers, which is offset from the FIB lamella direction by an optimal angle, theta;. This rotation of the pattern with respect to the FIB lamella results in a different section of each indent to be lifted out. When these sections are compiled, they collectively represent the subsurface deformation volume of a single indent feature. This approach allows all sections of one representative feature to be contained in a single ion-milled cross section. The cross section is extracted using the same procedure as the conventional in situ FIB lift-out technique. The TEM images of each section of the indent are captured and 'reconstructed' using a software routine to create a 3D image of the nanoindent/feature.

In semiconductor or insulator materials, charged particle beam creates electron-hole (e-h) pairs, which can be used to study defects and transport properties. Moreover the secondary electrons produced can stimulate the emission of light in a similar way as in electroluminescence (EL) excited by means of an electron beam. We present results about He-ion induced current and EL experiments carried out by means of a commercial He-Ion microscope (Orion, Zeiss). The microscope can in principle operate with a beam spot size of about 0.5 nm at 15-35kV, and the operating beam current could be chosen from 0.1 pA (high resolution mode) to about 100 pA. Here we investigate on the limits and advantages of the He-ion techniques in comparison with the standard electron beam induced current and cathodoluminescence.

Nanocrystalline oxides, compared to their microcrystalline counterparts, are of high interests for a wide range of applications due to their exceptional size-dependent materials properties. Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, ions deposit their energy to both atomic nuclei and electrons, and both energy loss pathways contribute to grain growth. The response of nanocrystalline CeO2 and ZrO2 to MeV ion irradiation has been investigated. Energy loss to atomic collisions and electrons both have significant impact on microstructure evolution and lead to effective grain growth. Atomic level MD simulations provide insights on a disorder-driven grain growth mechanism as a result of active interaction between irradiation-induced disorder and GBs. By varying the amount of energy deposition into the electronic and atomic structures, an additive effect from both displacement and ionization as the overall integrated effect is observed that grain growth can be described well as a function of total energy deposition in the nanocrystalline films. This study provides important evidence for understanding the effects of ionization on the kinetics of atomic processes, as well as data for validation of computational results. Moreover, we demonstrate an effective use of energetic ions to tailor the grain size in nanocrystalline ceria and zirconia ranging from a few nm up to ~ a few tens nm, which is the critical region for controlling size-dependent material property. The observed of ionization effect and unraveling of disorder-driven grain growth mechanism may open up new possibilities to better control grain sizes, improve crystallinity and tailor the functionality of nanocrystalline materials.
This work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Heavy gold and bismuth ions of energies of a few tens of keV deposit a high energy density into the collision cascade due to their relatively short projected range. At higher ion energies the energy density becomes diluted as the collision cascade volume grows faster than the ion energy. However, dimer, trimer, etc. ions deposit a multiple of energy at the same time in a volume of a few nm size. In this case the energy density is sufficient to form a very local but almost classical melt in Ge and Si for several tens up to a few hundreds of ps, at least at enhanced substrate temperature.
The local melting and resolidification by individual impacts of polyatomic ions is demonstrated by molecular dynamics simulations. The self-organization of very pronounced surface patterns of very regular, hexagonally ordered dots under high-fluence irradiation will be explained using features of single ion impacts. The melt pool kinetics leads to the same generic partial differential equation of the surface evolution as that proposed for the first time by Bradley und Harper, but which is based here on completely different mechanisms of the ion-surface interaction.
The local melting and quenching process is so fast that particularities of the phase diagram, e.g. that of Bi-Si, are frozen into the nanostructure of the surface layer which has been studied by transmission electron microscopy. This opens the possibility to study extremely fast solid-liquid phase transitions.
[1] Bischoff, Heinig, Schmidt et al., Nucl. Instr.Meth. B 272, 198 (2012)
[2] Böttger, Bischoff, Heinig et al., J. Vac. Sci. Technol. B 30, 06FF12 (2012)
[3] Böttger, Heinig, Bischoff et al., Appl Phys A (2013) 113, 53 (2013)
[4] Anders, Heinig, Urbassek, Phys. Rev. B 87, 245434 (2013)
[5] Böttger, Heinig, Bischoff et al., Phys. Status Solidi RRL 7, 501 (2013)
The financial support by the German Research Foundation via the Research Unit 845 “Self-organized nanostructures induced by low-energy ion beam erosion.” is acknowledged.

Controlling the size, distribution and composition of nano-particles in a material is a fundamental problem for a broad variety of applications from electronics and photonics to nuclear industry. Ion implantation is a powerful technique to synthesize nano-structures under well-controlled conditions. In this presentation we demonstrate that ion implantation can be successfully applied to create nano-size oxide precipitates in Fe-Cr steel.
We report the results of an experiment on the metal-oxide particle formation in a high purity Fe-10wt.%Cr steel by ion implantation (using JANNuS-Orsay facility, where two accelerators are linked to a Transmission Electron Microscope), together with a structural characterization of these nano-oxide clusters. The behaviour of these particles under in situ thermal annealing will be also addressed.
A dispersion of nano-size precipitates of metallic oxides is known to strongly improve mechanical properties of ferritic-martensitic steels. The Oxide Dispersion Strengthened (ODS) steels are promising candidates for structural components of future nuclear reactors. The broad application of these steels is however hindered by the relatively complicated and expensive production technology (powder co-grinding and high-pressure thermomechanical treatment) of ODS steels. Our experimental results indicate the feasibility of unconventional ways for nano-size oxide ensemble creation with high potential for control over the steel property amelioration by tailoring the parameters of oxide ensembles.
Mode of presentation: an oral contribution is highly preferred.

At room temperature, it is very difficult to immobilize single atoms, in particular the least reactive noble gases. Ion implantation into a crystal lattice possesses this capability, but the randomness of the involved processes does not permit much control over their distribution within the solid. However, with the assistance of single layers of hexagonal boron nitride or graphene, site-selective immobilization of atoms at surfaces becomes feasible. If ions with energies near the penetration threshold are applied [1].
In the present study, we demonstrate that the hexagonal boron nitride (h-BN) nanomesh on rhodium [2] can trap argon atoms at distinct subsurface sites and form so-called "nanotents" at room temperature. The h-BN nanomesh is a corrugated single layer with a 3.2 nm honeycomb superstructure on a Rh(111) surface [3]. Due to the mismatch between h-BN and Rh, and the preference of nitrogen to bond on top of Rh, the superstructure appears as a “mesh” with two bonding areas, the &’pores&’ with 2 nm diameter, and the surrounding &’wire&’ regions where the h-BN is weakly bonding to the substrate. The Ar implantation process exhibits a pronounced site selectivity, which can be controlled by the ion exposure and the sample temperature. Scanning tunneling microscopy and photoemission data show the Ar atoms at two distinct wire crossing sites within the nanomesh super cell, which are confirmed by density functional theory calculations. Remarkably, these “nanotents” that are predicted to house up to six Ar atoms are stable in air. In-situ variable temperature measurements reveal that Ar nanotents decrease in number while growing in size upon annealing up to 450 K. Further annealing of the ion irradiated structures to 900 K induces the formation of highly regular holes of 2 nm diameter in the h-BN layer, with flakes of the same size found near the holes on top of the surface. We propose that this "can-opener" effect is due to vacancy defects, generated during the penetration of the Ar through the h-BN lattice, and the propagation along the rim of a nanomesh pore where the h-BN lattice is strongly bent. These observations provide ways to functionalize ultimately thin sp2 membranes on surfaces at room temperature. The reported effects are robust and quite general: they are also observed in graphene on ruthenium and for neon atoms.
References:
[1] H. Y. Cun et al., Nano Lett. 2013, 13, 2098-2103.
[2] M. Corso et al., Science 2004, 303, 217-220.
[3] S. Berner et al., Angew. Chem. Int. Ed. 2007, 46, 5115-5119.

Oxides with fluorite-related structure such rare earth oxide with bixbyite structure (Ia3) Re2O3 have been recently an active area of researches because of their high radiation damage tolerance. The physical properties of these materials deposited as thin films strongly depend on their elaboration conditions. Y2O3 thin films were grown by Ion Beam Sputtering (IBS) on different substrates with different and controlled nonstoichiometry. By combining RBS, XRD and HRTEM it is shown that oxygen stoichiometry of the as-deposited thin films can be accommodated either by a local disorder of the pre-existing oxygen vacancy network of the cubic-C Re2O3 structure (anti-Frenkel defects leading to a disordered fluorite-like structure) or, in a strong nonstoichiometric case (the unique valence state of yttrium cation do not allow a charge compensation), by extended defects (aggregation of oxygen vacancies collapsing to create prismatic dislocation loops).
Both types of defects, oxygen vacancy network disorder and dislocations, induce an important triaxial strain field and leads to a very high compressive stress in the films which can be modified by post irradiation process. Irradiations were performed at cryogenic temperature using xenon ions with different energies (70-380 keV), and in a fluence range of 1.10E14 - 1.10E16 ions/cm2.
Two original dissimilar structural transitions are evidenced, depending on the irradiation energy conditions. For energies below 180 keV, a partial polygonization leading to a complete amorphization occur. Above 180 keV, a transition from cubic-to-amorphous-to-monoclinic structure is observed with a high degree of coherency with the initial phase.
These two structural transformations and their energy dependence can be explained by taking into account the stress dependence in the Gibbs free energy formulation for the nucleation of the monoclinic phase. The mechanism of cubic/monoclinic phase transformation is well described by a Multi-Step-Damage-Accumulation (MSDA).
This calculation clearly evidence the strong correlation between the size of the oxygen vacancy loops created by the irradiation process in the cubic {111} planes (assimilated to the first germ of the monoclinic phase by atomistic considerations) and the residual triaxial strain field induced by the created point defect (isolated vacancies and anti-Frenkel defects). This result supports that vacancy clustering during irradiation leads to prismatic loop. These loops can be stable at extremely small sizes by a reducing of the critical radius by the strain field allowing the monoclinic transformation with non equivalent coherency probability between the cubic {111} planes and the (40-2) monoclinic planes.
This approach leads to a full description of the amorphous/monoclinic phase stability as a function of the irradiation energy and gives a full explanation of the coherence relationship observed between the cubic and the monoclinic structures.

Highly energetic ions interacting with solids are slowed down by inelastic collisions with the electrons of matrix atoms that result in localized deposition of extremely high energy. Particularly visible in insulating materials, including natural minerals, glasses, and plastics, high electronic excitations lead to the formation of cylindrical damage regions referred to as ion tracks. In reality, tracks correspond to permanent structural modifications at a nanometer scale along ion trajectory. The most common method of revealing the ion tracks has been a chemical etching process. However, this technique has serious drawbacks, the major one being the elimination of the information concerning the track morphology. Another commonly used technique is the direct track imaging by transmission electron microscopy; nevertheless, up to now, the examination of the track over the entire ion path was not feasible.
The morphology of tracks created by swift heavy ions in a Gd2Ti2O7 crystal is characterized over the entire ion trajectories (from the surface of the sample down to the end of the ion path, i.e. ~10 mu;m) by using advanced focused ion beam milling and high-resolution transmission electron microscopy techniques. Several peculiarities were observed: a layered nanostructure of ion tracks that are composed of an amorphous core surrounded by a strained crystalline envelope, a variation of the track diameter and orientation as a function of the ion energy loss, a progressive transformation from continuous to discontinuous tracks. These observations are compared to electronic and nuclear energy loss depth distributions and can be interpreted with a model based on the concept of effective local energy deposition defined as the sum of the inelastic energy deposited by the swift track-forming ion and the elastic energy deposited by a primary knock-on atom inside the ion track.
This work was supported by the research grant from the Polish Ministry of Science and Higher Education number 714/N-EMSL/2010/0, the French-Polish cooperation program 09-133, the Nuclear Energy Research & Development, the U.S. Department of Energy under Contract DE-AC05-76RL01830 and the EMSL Open Access project 34930. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

The very large surface area to volume ratio in nanostructures dramatically modifies many of their properties compared to bulk systems. While this is well established with respect to for instance the electronic and optical properties, the effect of the nanoscale on sputtering yields has not been studied systematically. Using experiments and computer simulations, we find that 80 keV Xe ion irradiation of Au nanorods can produce sputtering yields exceeding 1000, which to our knowledge are the highest yields reported for sputtering by single ions in the nuclear collision regime. This value is enhanced by more than an order of magnitude compared to the same irradiation of flat Au surfaces. Using MD simulations, we show that the very high yield can be understood as a combination of enhanced yields due to low incoming angles at the sides of the nanowire, as well as the high surface-to-volume ratio causing enhanced explosive sputtering from heat spikes.

The investigation into radiation-solid interactions of advanced reactor materials must provide quantitative measurements of parameters like defect accumulation rate, defect annihilation rate, and defect mobility while also providing identification of metastable phases that are only stable in extreme environments. Here we present findings of a study where refractory nuclear reactor materials are prepared in the form of thin film capacitors. This geometry allows for rapid and uniform irradiation while providing post-dose and in-situ monitoring by conventional dielectric measurements to detect damage at defect levels that are transparent to most ex-situ techniques.
The material of choice for this study is CeO₂, which can be prepared using thin film techniques with dense and tunable microstructures featuring large columnar grains or nanoscaled equiaxed grains. Films spanning these characteristics were prepared using magnetron sputtering and were characterized using x-ray diffraction and AFM image analysis resulting in films with grain sizes approximately 50nm in diameter. Thin film capacitors utilizing CeO₂ as a dielectric were prepared as metal-insulator-metal stacks built on Si(001) substrates with a dielectric thickness of 500nm. As built dielectric analysis revealed highly insulating ceria with a permittivity of approximately 24 and negligible dispersion in the range from 1kHz to 1000kHz.
In-situ measurements were performed utilizing dielectric analysis in conjunction with ion beam irradiation. We demonstrate the ability to detect significant events such as beam on and beam off in addition to measuring increases in capacitance and loss tangent corresponding to the accumulation of point defects created during ion dosing. We will provide the results from investigations of dielectric response to changes in beam current ranging from 5nA to 100nA and thus the defect generation rate as well as response to the application of an electric field. With this technique we can directly compare the results of the in-situ measurements with the ex-situ results from the same capacitor structure as well as comparisons to capacitors that were not measured in-situ. Ex-situ results show increases in dispersion after ion irradiation that compare favorably to previous experiments on thin film capacitors with a ceria dielectric. In general we find that in-situ measurements reveal extremely subtle trends with irradiation that are nearly impossible to see with ex-situ analysis. Specifically measuring the slope of capacitance and loss allows one to characterize how the defects created by irradiation interact with the as-prepared defect chemistry thus giving insights on how one can engineer this equilibrium to create a desired property response.

In contrast to previous reports, where the modifications of the elastic constants of semiconductors irradiated with heavy ions are associated to crystalline to amorphous transition, here we show that H-implantation cause a dramatic effect on the shear modulus of Si at relatively low levels of damage. To study the system, we developed an alternative and rather general method to determine the shear modulus of a buried damaged layer. For this we associate two simple measurements, which are performed at different length scale: i) at the mesoscale we measure the wafer curvature, allowing to extract the in-plane stresses developed in the implanted region, and; ii) a local nanoscale measurement to determine strains in the implanted layer is performed using x-ray diffraction. The results are combined under elasticity theory. Considering H-implantation at the energy ranges around 30 keV, we demonstrate and quantify that the shear modulus of the irradiated material decreases for fluences > 1 x 1016 H+/cm2. For 6 x 1016 H+/cm2, the effective shear modulus of the implanted layer falls by approximately 50 %. The method/strategy used here to study the elastic properties of H-implanted Si can be applied to other ion implantation systems.

Due to the strong polarization of wurtzite-type semiconductors (GaN and ZnO), ion-beam mixing has been suggested to increase the internal quantum efficiency through the formation of graded superlattices (SLs) [1]; however, the actual mechanism controlling the damage are greatly unknown. This work explores ion-induced intermixing and damage build up in GaN/AlN SLs with different dimensionality: quantum wells (QWs) and quantum dots (QDs) [3]. These systems show reduced intermixing by thermal treatments [2], but this work addresses the successful intermixing by ion implantation at very low temperatures. Sequential 100 keV Ar+ implantations were performed at 15 K and in situ analyzed by Rutherford backscattering spectrometry in channelling mode. Further characterization was done by means of X-ray diffraction and transmission electron microscopy (TEM). Results agree with a 3-step damage model with an amorphization threshold of 40 displacements per atom (higher than for bulk GaN). However, the SLs show significant differences in the saturation level of defects at high fluences (>1E15 cm-2), this being higher for QDs than for QWs. Compositional depth profiles obtained by TEM were fitted with an interdiffusion model, demonstrating that the higher damage in 0D structures is correlated with a larger diffusion length. Furthermore, the high radiation resistance of SLs as compared to bulk GaN is demonstrated.
1 K.P. O&’Donnell et al., Phys. Stat. Sol. RRL 1 (2011)
2 C. Leclere, et al., J. Appl. Phys. 113, 034311 (2013).
3 A. Redondo-Cubero et al., Nanotechnology, in press (2013).

Positron annihilation spectroscopy has been widely applied to identify vacancy defects in semiconductors [1]. In a semiconductor material, positrons can get trapped at negative and neutral vacancy defects, and at negatively charged non-open volume defects given the temperature is low enough. The trapping of positrons at these defects is observed as well-defined changes in the positron-electron annihilation radiation. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities (sublattice, decoration by impurities) and the concentrations of the vacancies. Performing measurements as a function of temperature gives information on the charge states of the detected defects.
We have constructed a facility allowing generation of point defects with proton irradiation employing the 5MV tandem accelerator of the Helsinki University Accelerator Laboratory and on-line studies of both vacancies with positron annihilation spectroscopy and current-voltage (IV) measurements of irradiated electronic devices [2]. The particle accelerator is equipped with four different types of ion sources, providing a wide selection of ions from protons to heavy ions. In particular, uur system allows proton irradiation at low temperatures (down to 10 K) with in situ positron lifetime measurements. The possibility to perform on-line measurements during the irradiation as well as the available temperature range (10-300K) enables a detailed examination of different (vacancy-impurity-interstitial) reactions under irradiation and subsequent thermal treatments, as well as their macroscopic effects on the functionality of different semiconductor devices.
We present results obtained in bulk Si, Ge and AlN semiconductors [2-4]. We analyze the interplay between the irradiation-induced point defects and the in-grown defects and impurities in these materials. As an example, these experiments were used to demonstrate that divacancies in Ge can exist at room temperature. We will discuss the potential extensions of our approach to studying thin films and the possibilities of employing complementary in situ defect characterization techniques.
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys., in press.
[2] S. Väyrynen et al., Nucl. Instr. Meth. Phys. Res. A 572, 978 (2007).
[3] J. Slotte et al., Phys. Rev. B 83, 235212 (2011).
[4] J.-M. Mäki et al., Phys. Rev. B 84, 081204(R) (2011).

Swift heavy ion beams in the MeV to GeV energy range are provided by the UNILAC linear accelerator of the GSI Helmholtz Centre. At the three M-branch experimental sites several in-situ and on-line monitoring techniques are being currently employed for the investigation of ion-beam material modification and degradation studies. In-situ high resolution scanning electron microscopy (HRSEM) and ultra-high vacuum (UHV) atomic force and scanning tunneling microscopy (AFM/STM) have been integrated into the M1-beamline and are devoted to in-situ characterization of ion-irradiation induced effects on surfaces. In addition, a four-circle X-ray diffractometer is installed at the M2-beamline. This allows the study of crystallographic and structural changes as function of irradiation parameters and material. Finally, at the M3-beamline multi-purpose chambers are available for sample irradiation between 10 and 1200 K at two target positions under various gas atmospheres. Optical spectroscopy such as FTIR, UV/Vis, thermal imaging by a fast IR camera, and a residual gas analyzer are installed and operated during irradiation.
In this contribution we will present recent results obtained at the UNILAC M1-branch: In-situ HRSEM investigations at the nanometer scale of ion-beam induced shaping processes of pre-structured thin metal oxide samples as well as AFM/STM studies of ion tracks created under small impact angles.

Ion beam modification of materials has become an essential tool in the nanoscale modification of materials for many advanced applications. Despite this, the structural dynamics that result from the interaction of either high energy irradiation or low energy implantation with materials in various environments are often unknown. In order to visualize in real time and at the nanoscale the interaction of ion species and mater, Sandia National Laboratories&’ Ion Beam Lab has developed a unique facility that combines a 6 MV Tandem accelerator, a 200 kV transmission electron microscope (TEM), and a 10 kV Colutron. A complex design was developed and large number of beam line components was needed to both control and characterize the two ion beams, as the ions are concurrently transported into the objective lens of the TEM. In addition to the electron beam and two ion beams, this facility also has introduced two light optical pathways that are also aligned to the same three millimeter location as the three charged particle beams. The addition of light optical pathways permits the capabilities for laser heating, ion beam induced luminescence, cathodoluminescence, photoluminescence, as well as direct imaging of the TEM sample during ion beam irradiation or implantation. The capabilities provided by these various forms of radiation are complimented by a suite of TEM stages that introduce gas or liquid environments inside of the microscope, as well as stages for quantitative thermal or mechanical loading of the sample. This combination permits the ability to study at the nanoscale the radiation interaction with mater under a wide range of conditions and environments. A limited number of initial studies will be presented that highlight some of the unique capabilities of this in-situ ion irradiation TEM facility. Finally, this presentation will conclude with a vision for both this facility and the overall in-situ irradiation TEM field.
This work is supported by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

In contrast to their bulk form, nanosized metals surrounded by a dielectric medium exhibit a remarkable optical phenomenon known as localized surface plasmon resonance (LSPR) when they interact with light. Excitation of surface plasmons in metal nanoparticles at the resonance wavelength gives rise to specific effects (i.e., an intense optical absorption and a strong enhancement of the local field around the nanoparticles), which are the bases of recent developments with various applications ranging from photonic and photovoltaic devices to biochemical sensors and markers. Despite the progress in nanofabrication techniques, the realization of nanostructured materials with optimized plasmonic properties remains a significant challenge, which demands a full control of the size, shape, organization, and environment of the nanoparticles. In this context, it has recently been shown that periodic nanoripple patterns produced by ion-beam sputtering of insulating surfaces can be used as templates to fabricate self-aligned noble metal nanoparticles and nanowires by glancing-angle deposition (GLAD) [1-3]. It has been proved that these systems possess original dichroic properties, reflected in a polarization-dependent excitation of their LSPR and strong coupling between particles for a longitudinal polarization, which might be exploited, e.g., for surface enhanced Raman scattering applications [4].
In this presentation, we will focus on the formation of self-organized arrays of Au and Ag nanoparticles prepared by GLAD on periodic nanoripples produced by xenon sputtering of alumina thin films. We will show that reciprocal space mapping by grazing incidence small-angle X-ray scattering (GISAXS) associated with quantitative analysis in the distorted wave-Born approximation provide accurate information, averaged over macroscopic dimensions, on the morphology and organization of both the nanorippled alumina surfaces and the nanoparticle arrays deposited thereon. Moreover, we will show that the combined use of real-time and in situ GISAXS and surface differential reflectance spectroscopy (SDRS) [5] experiments can deliver unique insights into the growth mechanisms and LSPR characteristics of anisotropic plasmonic nanostructures produced by GLAD, from the early stages of nucleation to the formation of continuous films [6].
References
[1] S. Camelio, D. Babonneau, D. Lantiat, L. Simonot, and F. Pailloux, Phys. Rev. B 80, 155434 (2009).
[2] D. Babonneau, S. Camelio, L. Simonot, F. Pailloux, P. Guérin, B. Lamongie, and O. Lyon, Europhys. Lett. 93, 26005 (2011).
[3] D. Babonneau, S. Camelio, E. Vandenhecke, S. Rousselet, M. Garel, F. Pailloux, and P. Boesecke, Phys. Rev. B 85, 235415 (2012).
[4] M. Ranjan and S. Facsko, Nanotechnology 23, 485307 (2012).
[5] L. Simonot, L. Simonot, D. Babonneau, S. Camelio, D. Lantiat, P. Guérin, B. Lamongie, and V. Antad, Thin Solid Films 518, 2637 (2010).
[6] R. Dohrmann et al., Rev. Sci. Instrum. 84, 043901 (2013).

The AlGaN compounds present many advantages over conventional semiconductors. These include chemical stability, carrier generation and physical stability over a wide temperature range [1]. Despite the well-known high density of defects these semiconductors with a wide direct bandgap have already proven to be efficient high temperature emitters. The large forbidden energy band in the AlGaN compounds is suitable for the incorporation of several electronic energy levels of dopants, namely the ones produced by the rare earth (RE) ions. When optically active in the trivalent charge state, these ions exhibit sharp optical emission lines due to the intra-4fⁿ shell transitions making the tuning of light emission from the ultraviolet to the infrared in the AlGaN host realized by an appropriated choice of the dopant ion [2].
In this work, Al_xGa_1-xN films grown by halide vapor phase epitaxy with different AlN molar fractions were intentionally implanted with lanthanides. The ion implantation was carried out with three different RE ions (Pr³⁺ ,Tm³⁺ and Tb³⁺), for fluences between 1x10¹⁴ ion/cm² and 1x10¹⁵ ion/cm² and with the beam tilted (10°) or aligned with the c-axis. The implantations were studied for two energies, 150 and 300 keV. The structural information was accessed by High Resolution X-ray Diffraction (HRXRD), Rutherford Backscattering / Channeling Spectrometry (RBS/C), and Raman Spectroscopy. HRXRD shows an expansion of the c-lattice parameter in all the studied layers. Rapid thermal annealing at 1200 °C under N₂ ambient promotes the lattice recovery. The RBS/C results reveal a higher resistance of the lattice to irradiation damage with the increase of AlN content. The simulation of experimental angular RBS/C scans along different axes using the Monte Carlo code FLUX allows the determination of RE lattice sites. Results show that RE ions occupy two preferential sites, the high symmetry substitutional Ga/Al site and a site displaced along the c-axis from this regular site. The relation between AlN molar fraction and RE ion displacement is investigated. Photoluminescence (PL) measurements reveal optical activation of RE ions after annealing treatment. The influence of the AlN molar fraction on the emission intensity is discussed. Temperature dependent PL measurements were also performed in order to clarify the thermal luminescence quenching mechanisms present in these samples.
[1] H.X. Jiang and J. Y. Lin, Opto-Electron. Rev. 10(4), 271-286 (2002)
[2] A.Wakahara, Opt. Mater. 28, 731-737 (2006)

We examine crystalline-texture evolution during ion-beam assisted deposition (IBAD) of MgO thin films. This process is also known as ion texturing at film nucleation because the crystalline alignment occurs at the initial stages of film growth. These artificially textured films are used as templates for epitaxial growth of energy functional layers such as superconductors and are today manufactured in industry in kilometer long tapes. However, a fundamental understanding of the process is still lacking.
To perform our analytical experiments we developed a unique experimental methodology based on linear combinatorics in a reel-to-reel sample tape transport system. This technique allows us to fabricate film-thickness wedges that maximize data collection and allow us to systematically study film evolution as function of thickness. MgO texture evolution can be separated into three different regions. During initial ion beam assisted deposition an amorphous MgO layer is formed which is crucial for obtaining <100> out-of-plane grain alignment. Onset of texture appears in the first 1-2 nm of film deposit when MgO crystallizes. We separate out-of-plane <100> fiber texture that appears first, followed by in-plane grain alignment along the ion-beam assist direction.
The IBAD film evolution is studied by analyzing wedge samples with an in situ, but post deposition, time-of-flight ion scattering system. Using this diagnostic tool we can analyze Ion Scattering Spectroscopy (ISS) as well as Direct Recoil Spectroscopy (DRS) data to get the most surface sensitive analysis. Additionally we utilize Mass Spectroscopy of Recoiled Ions (MSRI) for accurate chemical determination and complementary data to the ISS. The full suite of ion scattering spectroscopy data as a function of film evolution provides us with a better understanding of the ion texturing process.
This work was supported by the Department of Energy Office of Electricity Delivery & Energy Reliability.

A versatile method to transfer a thin silicon film on a host substrate is based on the Smart Cuttrade; procedure comprising H+ implantation, substrate bonding and annealing. Standard Smart Cuttrade; process suffers from a high fluence of hydrogen indispensable to achieve a fracture. It was demonstrated that helium/hydrogen co-implantation greatly reduces the total implanted fluence required to achieve a splitting. However, little is known about the effect of the order of co-implantation on the fracture.
In this work, we have addressed this problem studying the effect of He⁺ and H⁺ co-implantation order on fracture. Hydrogen and helium implanted fluences were fixed to 1x10¹⁶ cm^-2 and 1.5x10¹⁶ cm^-2 respectively. Hydrogen ions were implanted at 32 keV. The implantation energies of helium ions were adjusted in a way to keep a He depth distribution either above or below the depth distribution of H. Concentration depth distributions of both He and H ions were measured by secondary ion mass spectroscopy. Strain depth profiles were extracted from the simulation of the (004) X-ray diffraction spectra. Besides, Raman spectroscopy was applied to detect the repartition of Si vacancies-hydrogen atoms complexes. Transmission electron microscopy was used for a visualization of platelets being the co-precipitates of H and Si-vacancies, formed after annealing well prior to a fracture. Optical microscopy was used for the visualization of the population of microcracks being at the base of the fracture.
The statistical analysis of the evolution of platelets and microcracks as function of annealing time allowed us to conclude that splitting can be achieved if He atoms diffused towards the H depth distribution serving to overpressurize H platelets. This phenomenon, independently of the order of co-implantation, was observed for the only case when He⁺ ions were initially implanted below the depth distribution of H. However, the quickest splitting is attained if He⁺ ions were implanted first. In case of the inversed order of the co-implantation, i.e. when He⁺ ions are implanted through the layer containing implanted H, He⁺ ions destroy the precursors of H platelets that drastically retard the formation of platelets and, subsequently, the growth kinetics of microcracks. The role of the damage and the generated strain created by the co-implantation in a different order will be further discussed.

The conductive bridge non-volatile memory technology is an emerging way to replace the traditional charge based memory devices for future neural networks and configurable logic applications. An array of the memory devices that fulfills logic operation must be developed for implementing such architectures. A scheme to fabricate these arrays, using ion bombardment through a mask, has been suggested and advanced by us. In this work, the formation of vias, on which the memory devices performance are based, and damage accumulation due to the interactions of Ar⁺ ions with Ge_xSe_1-x (x=0.2, 0.3 and 0.4) chalcogenide glasses as a function of the ion energy and dose dependence are studied. Blanket films and devices were created to study the structural changes, surface roughness, and device performance. Raman Spectroscopy, Atomic Force Microscopy (AFM), Energy Dispersive X-Ray Spectroscopy (EDS) and electrical measurements expound the Ar⁺ ions behavior on thin films of Ge_xSe_1-x system. Raman studies show that there is a decrease in area ratio between edge-shared to corner-shared structural units, revealing an occurrence of structural reorganization within the system as a result of ion/film interaction. AFM results demonstrate a tendency in surface roughness improvement with increasing Ge concentration. EDS results revealed a compositional change in the vias, the Ge atoms are more susceptible to the ion sputtering causing the films to become more Se rich. Combination of these structural and compositional alterations by ion interaction influences the device performance. The electrical testing on the devices was achieved through a measurement of current-voltage (I-V) curves, Read/Write voltages and the two resistive states.
The advantage of this method for array formation is that it provides a unique alternative to conventional photo-lithography, for prototyping redox conductive bridge memristor array without involving any wet chemistry.

When an ion traverses matter, it can lose its energy mainly by two processes. In the keV to low MeV region, the ion-matter interaction is dominated by elastic collisions with the target atoms leading to displacements and collision cascades (nuclear stopping). In contrast, ions with a few hundreds of MeV predominantly lose their energy via excitation and ionization of the electronic system (electronic stopping). The energy deposition process is thus fundamentally different in these two regimes leading to a distinctly different damage formation behavior of various III-V compound semiconductors. Although InP is readily amorphized in both regimes, the question remains whether the amorphous phase structure varies due to the different interaction mechanisms.
This question can be successfully answered using X-ray absorption spectroscopy (XAS) which is a powerful and versatile technique for structural analysis. It is widely used in a large number of scientific fields including solid state physics and material science due to its following characteristic features. (i) XAS measures the structural environment of a particular element of the material and is thus element-specific. (ii) It probes the atomic neighborhood up to about 10 Å and is thus applicable to crystalline, disordered and amorphous materials. (iii) XAS is sensitive to the correlated motion of neighboring atoms and thus provides information about vibrational properties of the material.
The talk presents examples for all three features demonstrating the power of XAS analysis. Studies of the atomic-scale structure of semiconductor alloys such as (In,Ga)P are based on the element-specific nature of the technique while bond stretching force constants are deduced from temperature-dependent XAS measurements on crystalline and amorphous InP. The main focus is placed on the study of the atomic-scale structure of InP amorphized by ion irradiation with either dominant nuclear or dominant electronic energy loss. Despite the fundamentally different interaction mechanisms, the amorphous phase structure is found to be very similar for both regimes. This suggests a common amorphization mechanism which can be identified as a “melt and quench” process based on molecular dynamics simulations and thermal spike calculations for the nuclear and electronic stopping regime, respectively. XAS thus provides unique information about ion irradiation effects in compound semiconductors and contributes significantly to a more comprehensive understanding of ion-solid interactions.

Ion beams are commonly used in the framework of nuclear materials studies to reproduce, in a controlled way, the different sources of irradiation to which these materials are submitted to. The interaction of energetic ions with matter induces the formation of crystalline damage (i.e. defects) along the path of the ions, associated with high strains in the implanted/irradiated region.
In this work we make use of advanced X-ray diffraction techniques in order to retrieve the strain and damage profiles in the irradiated region of single crystals. Although XRD is highly sensitive to atomic displacements, the determination of displacement profiles is hindered by the phase problem, i.e. the displacement field u(r) associated with a given defect affects the phase of the amplitude E, whereas the quantity measured in practice is the intensity I = EE*. Thereby, only relative positions (i.e. as function of a correlation distance r - r') can be obtained by a direct inversion of the X-ray diffraction data.
We show that this issue can be circumvented by an original fitting approach where the strain and damage profiles are modeled with cubic B-spline functions and the diffracted intensity is computed using the dynamical theory of diffraction. XRD data have been simulated using the procedure detailed in [1]. The fitting is performed using the generalized simulated annealing algorithm that allows to find the global minimum of the fitting problem. Here we present the results obtained for cubic yttria-stabilized zirconia single crystals, irradiated with 4 MeV Au2+ ions at different temperatures (80K, 300K, 573K, 773K and 1073K). For each temperature, samples have been irradiated at various fluences ranging from 1012 to 1016/cm2.
In this talk, we will show that upon increasing fluence, the width of the damaged region increases and both the level of strain and damage inside this region increases. The damage build-up occurs according to a two-step mechanism: in the first step, the damage increases slowly up to a critical fluence, above which the second step takes place and is characterized by dramatic increase of the damage. The transition fluence is shifted towards lower values at higher temperatures. For the highest fluences, the damage level is so high that the strain cannot be measured anymore in the most defective regions but can still, using the presented methodology, be obtained at the damaged/pristine interfaces. Increasing the temperature induces an enhanced defect-clustering that both lowers the strain and damage in the first step of the damage build-up and shifts the threshold fluence towards lower values.
[1]A.Boulle, A.Debelle,J. Appl. Cryst. 43 (2010) 665

Adopted from the small sample volumes for high-pressure experimental procedures, we have developed an approach to characterize structural and chemical modifications induced by high energy heavy ions, which lose their energy predominantly by electronic excitations, using synchrotron-based x-ray diffraction (XRD) and x-ray absorption spectroscopy measurements (XAS). Key to the experimental procedure is that the GeV ions completely pass through the sample thickness and deposit an almost constant energy per unit length along their path through electronic energy loss. High energy x-rays (30 keV) are used in transmission geometry to analyze ion-induced structural and chemical modifications throughout the entire sample thickness. Such experiments required a new holder system to facilitate the preparation of microgram samples with precision control of the sample thickness. Sample chambers were prepared in a 50 mu;m thin stainless steel foil by drilling several holes with a diameter of 200 mu;m. Powder was pressed into the sample chambers between two steel die-pieces in a hydraulic laboratory press using a pressure of ~20 MPa. This resulted in sample pellets with well-defined dimensions of 200 mu;m in diameter and 50 mu;m in thickness. Several set of samples were irradiated at the GSI Helmholtz Center in Darmstadt, Germany at room temperature with swift heavy ions (e.g., Au ions, 2.2 GeV) to fluences typically between 1×10¹⁰ and 5×10¹³ ions/cm². Angle-dispersive synchrotron powder XRD and XAS measurements were performed as function of increasing fluence at the beam line HPCAT 16-BMD of the Advanced Photon Source at Argonne National Laboratory. This provides high-quality diffraction patterns in terms of resolution and signal-to-background ratio, yielding superior structural data as a function of irradiation fluence. After x-ray characterization, samples can be additionally analyzed by Raman spectroscopy and transmission electron microscopy (TEM). For the latter, suitable specimens are produced without further elaborate preparation by simply emptying the sample chamber over a TEM holder.
As an application for such XRD experiments, ion-induced defect formation, amorphization, and crystalline-to-crystalline phase transformations are presented for a number of oxide samples (e.g., CeO₂, Gd₂Ti₂O₇, and Gd₂O₃). Changes of the electronic structure in CeO₂ as a consequence of swift heavy ion irradiation are presented as example of XAS measurements. Finally, this experimental approach is particularly useful for ion-beam experiments with actinide materials, as it minimizes the level of radioactivity at the ion-beam and synchrotron user facility.

Insulators and semiconductors that are exposed to swift heavy ions can form ion tracks as a result of their interaction with the material&’s electrons. These tracks are narrow, cylindrical-shaped regions of high defect concentration, only a few nanometres in diameter and up to tens of micrometers in length. In minerals, such as apatite, or natural quartz, ion tracks can occur naturally from the fission of incorporated uranium impurities, setting the foundation for fission track dating and thermochronology. In synthetic quartz, they are utilized to modify the (opto-)electronic properties. We have previously shown that synchrotron-based small angle x-ray scattering (SAXS) is a powerful and non-destructive technique, well suited for studying ion tracks with unprecedented precision [1-3].
Here, we present our recent results on the temperature-induced recovery of ion track damage. Quartz of different crystallographic orientations was irradiated with 2.2 GeV Au ions. Using SAXS, we monitored the recrystallization of the ion tracks during isothermal in situ annealing. We observed an anisotropy in the track annealing process with tracks generated along the [110]-direction showing a significantly lower transformation temperature when compared to tracks along the [100] and [001] directions. The activation energies of this process were determined for various track directions and temperature regimes. These results are complemented with those on the temperature-dependence of the track formation mechanism in quartz and apatite [4].
[1] P. Kluth et al., Phys. Rev. Lett. 101 (2011) 175503.
[2] B. Afra et al., Phys. Rev. B 83 (2011) 064116.
[3] M. Ridgway et al., Phys. Rev. Lett. 110 (2013) 24.
[4] D. Schauries et al., J. Appl. Cryst 46 (2013), in press.

Pyrochlore-type oxides (A2B2O6X - Fd m space group) are ordered superstructures of the ideal fluorite structure with twice the lattice constant where A and B are two cation sites. Pyrochlores provide a large array of structural and physical properties related to their various chemical compositions which make them suitable for potential applications in different fields. Recently, due to the high radiation resistance of some compositions, pyrochlores were considered as potential matrices for the transmutation or immobilization of actinides produced in nuclear power plants1. In operating conditions or during long-term storage, nuclear matrices are submitted to severe radiations which induce atomic rearrangements. These structural changes lead to the alteration of the physico-chemical properties of materials.
Gd2Ti2O7, Y2Ti2O7, Gd2Zr2O7 and Nd2Zr2O7 pyrochlores were elaborated by a standard solid state process, then irradiated with 93-MeV Xe ions at GANIL in Caen. The phase transformation build-ups were investigated by in situ X-ray diffraction (XRD) experiments using the ALIX setup on the IRRSUD beam line. Moreover, ex-situ XRD experiments and Raman spectroscopy were also performed in order to investigate the structural modifications induced by irradiation. Both the nature of phases induced by irradiation and the phase transformation kinetics depend on the composition. The high electronic excitation along the ion trajectory results in the amorphization of ion tracks for Gd2Ti2O72 and Y2Ti2O7. For Gd2Zr2O7, an anion-deficient fluorite phase is formed. A more complex behavior is observed for Nd2Zr2O7: pyrochlore-fluorite phase transformation and amorphization occur simultaneously3. Finally, a specific study concerning the thermal recovery of irradiated pyrochlores was performed by using in situ high temperature X-ray diffraction in order to determine the thermal stability of phases produced by irradiation. It was observed that the recrystallization temperature for amorphized pyrochlores increases with increasing the cation radius ratio rA/rB. However, for the zirconates, the temperature of the anion-deficient fluorite to pyrochlore transition is independent of the rA/rB ratio3.
References
1 R.C. Ewing, W.J. Weber, J. Lian, J. Appl. Phys. 95 (2004) 5949.
2 G. Sattonnay, C. Grygiel, I. Monnet, C. Legros, M.Herbest-Ghysel, L. Thomé, Acta Materialia 60 (2012) 22-34.
3 G. Sattonnay, N. Sellami, L. Thomé, C. Legros, C. Grygiel, I. Monnet, J. Jagielski, I. Jozwik-Biala, P. Simon, Acta Materialia 61 (2013) 6492-6505

Radiation damage is usually believed to be produced either by displacement cascades or elec-
tronic energy deposition acting separately in space.
However, ion irradiation in transition energy regime might lead to a synergy between two channels of energy transfer, interactively producing the damage, which cannot be found if both mechanisms act independently. Here, by synergy we understand the possible collaborative or competitive effects, which may occur if the energy deposited via both channels in the same place and at the same time. The collaborative effect between these two mechanisms will most likely result in enhanced damage production, but the competition between them might lead to the enhanced recombination or partial annealing during damage formation. Thus, contrary to the conventional theories, we expect both nuclear and electronic energy deposition to produce damage in synergy.
We use molecular dynamics simulations to analyze the interplay of both channels of energy deposition with respect to the damage produced by the Au ions of transient energies between 0.6 MeV and 76.5 MeV in pure silica and the silica structure which contains Au or Ge nanoparticles. We also consider two models of introducing the electronic energy deposition via a single event of inelastic thermal spike along the ion path or as two-temperature exchange model, naturally dissipating through both channels in lattice and electronic subsystems. The results are compared to the available experiments.

Hydrogen implantation is used to slice and finally transfer thin Si layers from a donor substrate onto a host material, providing a versatile method for the fabrication of SOI wafers. The separation of the top layer from the donor substrate is achieved during annealing by a controlled fracture, an outcome of a complex microstructure evolution in which stress and strain play crucial roles already after implantation. Whether Si self-interstitials, vacancies and/or H-related defects all contribute (and how) to the build in strain is still controversial. Without this knowledge, optimization of the technique and transfer to other materials remains elusive.
In this work, we have built up a semi-analytic model that predicts absolute concentrations of different complexes of H atoms, Si interstitials and vacancies formed at room temperature after implantation of H+ in Si. For this, we have introduced a concept of a local threshold concentration of point defects necessary to form a VnHm or InHm complex dependent on the diffusion coefficients of H atoms, Si interstitials and vacancies at room temperature and on the time of precipitation. Then, an average concentration of a VnHm or InHm complex is determined through the integration over all possible local concentrations of point defects exceeding respective threshold concentration times a probability to have given local concentration and, finally, weighted by the relative formation energy of the complex. The formation energies and the associated strain fields of the different point defects and complexes have been calculated by DFT. The probability functions of the distribution of local point defect concentrations have been deduced analytically. Finally, we have linked the absolute concentrations of defects to the strain generated by H implantation for the large set of H concentrations investigated here and measured by X-ray diffraction and dark-field electron holography. We deduce the role and contribution of every defect in the mechanism of strain generation.

Our understanding of implantation damage evolution has been crippled for many decades by the disconnect between experimental and atomistic simulation times for these systems. This limitation is overcome here by combining nanocalorimetry measurements with a newly proposed atomistic simulation method that runs over time scales reaching seconds. [1] 10 or 80 keV Si ions are implanted at low-fluence (0.02-0.1 Si/nm²) in monocrystalline Si at 110 K or 300 K. The heat release during annealing is measured in a nanocalorimetry scan from the implantation temperature up to 1200 K. For all implants, damage annealing generates a broad heat release, incompatible with a model that considers the implantation damage to consist of interstitial-vacancy pairs (or bond defects) clusters. To develop a better model, we turn to the kinetic Activation-Relaxation Technique (k-ART), an off-lattice kinetic Monte-Carlo method with on-the-fly catalogue construction. This method takes fully into account all elastic effects both for energy minima and barriers. Here, k-ART is applied to a 27 000-atom cell of Stillinger-Weber silicon self-implanted with a single ion at a 3-keV. The simulation results reveal a logarithmic time dependence of defect annealing and closely reproduce the heat-release experiments. Exploring the microscopic origin of the relaxation, we find that this dependence results from a two-step relaxation process. First, the logarithmic potential energy decrease is associated with the need to unlock metastable states, a process that requires crossing ever-higher barriers with time. These unlocking steps do not generally decrease the potential energy, they only allow other processes, the relaxation steps, to take place. These are taken from a uniform distribution of energy barriers that is replenished after each unlocking step. The picture that emerges is that self-implantation or keV-recoil-induced damage in c-Si consists of a collection of relatively simple structures that, rather than only relaxing by interacting with the crystal surrounding them, overcome reconfiguration barriers in order to interact with each other and undergo relaxation, resulting in a logarithmic time-dependent evolution. Given that long-range elastic effects are a general feature found in most materials, we estimate that these conclusions apply at least to many covalently bond crystals, but probably also to many other materials.
[1] L.K. Béland, Y. Anahory, D. Smeets, M. Guihard, P. Brommer, J.-F. Joly, J.-C. Pothier, L.J. Lewis, N. Mousseau, F. Schiettekatte, Phys. Rev. Lett. 111 (2013) 105502

Under irradiation conditions, the displacement defects produced in a metal can migrate and aggregate to form larger clusters of defects and consequently alter the microstructure of the material and lead to the degradation of mechanical properties. Therefore, a fundamental understanding of defects formation and migration is important for the investigation of the changes in microstructure and mechanical properties of structural materials. In this presentation, we show an ab initio approach to the force field calculation for structural materials in a cascade process. Specifically, we apply full-potential locally self-consistent multiple scattering (LSMS) method to the study of the Fe-alloy material with radiation damage, and determine from first principles the classical force acting on each atom. Because the LSMS method is a linear scaling ab initio method, it allows us to investigate the underlying mechanism that drives defect migration and vacancy formation in materials at a nanometer scale.

The MIAMI* facility at the University of Huddersfield, UK has been designed to permit the ion irradiation of thin foils in-situ in a transmission electron microscope (TEM). A particular focus of MIAMI is to enable the implantation of light ions such as helium and hydrogen into TEM foils, and this has necessitated a design that yields good ion fluxes for energies in the range 1 - 10 keV. In addition, however, the Facility can provide ions of a variety of species at energies of up to 100 keV.
The ability to observe the changes taking place in specimens under ion irradiation, as they occur and at temperatures from 100 - 1300 K, can provide powerful insights into mechanisms responsible for the defect structures, phase changes and other morphological changes that occur under ion irradiation and may also permit observation of effects due to individual ion impacts. The paper will present recent work on helium bubble formation in a number of materials including W and SiC - both of interest as nuclear materials - and Cu, where the interest is in revisiting the formation (and specifically formation mechanisms) of He bubble superlattices. Recent observations of mechanical deformation of thin graphite films (the formation of so-called “kink-bands”) will be presented and the implications for our understanding of radiation damage processes in this material will be discussed. Finally, observations of single ion impact effects and the measurement of enhanced sputtering yields for heavy-ion impacts on Au nanorods will also be presented and discussed.
*Microscope and Ion Accelerator for Materials Investigations

Raman spectroscopy is a powerful technique for studying the evolution of the microstructure of materials under irradiation. For that purpose, a confocal Raman spectrometer was recently installed on the multi-irradiation platform JANNUS-Saclay, that allows characterizing a variety of materials of nuclear interest, for example advanced ceramics (SiC, B4C, TiC, ZrC) and strengthened alloys (ODS). This facility aims at performing point acquisition but also 2D maps and depth scans, before and after single- and dual-beam ion irradiations.
We used Raman spectroscopy to determine the phases present before and formed after irradiation, to highlight the presence of gases such as molecular hydrogen implanted in ODS, to outline the stress induced by irradiation and to monitor the damage build-up. The damage was investigated at different irradiation doses and temperatures and by varying the nuclear to electronic energy loss ratio. We deduced the critical amorphization thresholds in materials where amorphization occurred. The comparison of these thresholds to those obtained by RBS-C revealed a very good agreement in the case of SiC for example. Raman spectroscopy is particularly efficient to compare the damage induced in materials having different microstructures, such as SiC single-crystals, SiC fibers and SiCf/SiC composites. The damage has also been measured as a function of depth with confocal mode or by observing transverse cross sections made after irradiation. Finally, Raman was used to study recrystallization phenomena after thermal annealing.
In addition, a system dedicated to perform in-situ measurements, linked to the actual Raman spectrometer, has been installed in the triple beam chamber of the JANNUS-Saclay platform. Our purpose is to monitor in real time the evolution of the damage induced by single-, dual- or triple beam irradiations in different materials. The first results obtained with this in-situ facility will be presented.

High-level nuclear wastes resulting from the reprocessing of the spent nuclear fuel from PWR reactors are immobilized in a borosilicate matrix called R7T7 by vitrification. As the glass canisters will be placed in a geological repository, research is carried out to predict the long-term behavior of the glass under disposal conditions. During the disposal time, helium will be generated from the minor actinides (Np, Am, Cm...) α-decays. At the same time, the glass will be exposed to nuclear damages induced by the recoil nuclei which are the main source of radiation damage inside the glass. In order to predict whether helium will be released or not outside the glass under geological disposal, it is mandatory to investigate the mechanisms involved in helium diffusion.
For that purpose R7T7 type glasses irradiated in the OSIRIS nuclear reactor (CEA Saclay, France) have been studied. The nuclear reaction 10B(n,α)7Li has induced either ballistic and electronic damages corresponding to around 100000 years of storage of nuclear glass.. To study the diffusion mechanisms, we have used a specific setup developed on the analysis chamber of the nuclear microprobe dedicated to radioactive samples in Saclay [1]. 3He ions have been implanted at energy of 700 keV corresponding to a mean range Rp of 2.2 µm according to SRIM calculation at a fluence of 2 x 1016 He.cm-2 and at a low temperature (<-80°C). Then several thermal annealing have been carried out in a temperature ranging from 25°C to 50°C. The 3He distribution profiles have been measured by using in situ mu;NRA technique through the 3He(d,p)4He nuclear reaction. In order to characterize the effect of radiation damage on helium behavior, non-irradiated R7T7 type glasses, free of defects, have also been studied in the same conditions of implantation and annealing. This paper presents the first results obtained for OSIRIS irradiated glasses on helium diffusion that will be compared with the ones obtained on un-irradiated glass.
[1] Nuclear Instruments and Methodology in Physics Research B 266 (2008) 2424-2428

Cu-Nb nanolaminates have been investigated for decades due to their high thermal stability, high yield strength and excellent radiation damage tolerance properties. Yield strengths as high as 2.5GPa have been reported for nanolaminates with 5nm layer thickness. However, both atomic simulations and experiments find that the interfacial shear strength is far less than the theoretical shear strengths of perfect Cu or Nb crystals. This interfacial shear strength could ultimately limit certain engineering applications. In this work, we applied in-situ TEM nanopillar compression to study the shear strength and shear mechanism of Cu-Nb nanolaminates. The shear strength of Cu-Nb at different irradiation doses was compared in order to investigate the effects of interfacial roughness and mixing on interfacial shear strength. The investigation also extends to nanolaminates of differing chemistry.

Understanding the behavior of complex oxides submitted to ion irradiation is crucial in order to predict their radiation tolerance, to establish criteria controlling their structural stability and to identify relevant factors for the occurrence of phase transformations. Among complex oxides, pyrochlores were thoroughly investigated during the last decades, due to the large array of structural and physical properties related to their chemical composition. For instance, pyrochlores were considered as potential immobilization matrices for the actinides produced in nuclear power plants. In this work, we used a combined experimental and computational approach to try to elucidate the mechanisms governing the structural stability of pyrochlores under irradiation.
Gd2Ti2O7 and Gd2Zr2O7 pyrochlores were irradiated with swift heavy ions (in electronic excitation regime) and with low energy ions (in ballistic regime) in order to investigate the effect of the chemical composition on the structural changes induced by irradiation. The first part of this study deals with experimental results on the amorphization resistance of pyrochlores under irradiation characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and also by X-ray Absorption Fine Spectroscopy performed at SOLEIL synchrotron facility. The second part is devoted to the calculation of defect properties with a variable-charge model (called SMTB-Q), particularly concerning the oxygen-Frenkel pairs, the cation-Frenkel-pair and the cation antisite defects in both Gd2Ti2O7 and Gd2Zr2O7. New results explaining the origin of the difference in the radiation tolerance of Gd2Ti2O7 (high susceptibility to amorphization) and Gd2Zr2O7 (no amorphization) will be shown. For instance, the key role played by the titanium defect in the amorphization mechanism of pyrotitanates will be underlined.

Irradiation effects on SiC are well-known in the literature. In this presentation, we aim at comparing the properties under irradiation of other types of carbides, namely TiC and ZrC. For this purpose, we performed simultaneous irradiations of the three above-mentioned crystals at room temperature with 1.2 MeV Au ions at several fluences. Irradiation effects were investigated using several complementary characterization techniques, namely Rutherford backscattering Spectrometry in channeling geometry (RBS/C), X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). The use of this set of techniques allowed obtaining damage and strain depth profiles, disordering kinetics and visualization of radiation defects. Hence, a relatively comprehensive understanding of the behaviour of the three materials has been acquired. The most important finding is that the whole set of results clearly reveal a drastic difference between TiC/ZrC versus SiC regarding the radiation resistance, with TiC and ZrC being much more tolerant than SiC. Hints for explaining this different behaviour will be proposed.

Understanding of radiation damage by fission fragments is one of the essentials to clarify the microstructure stability of the nuclear fuel/target materials. Fission fragments with 70-100 MeV hevay ions are known to induce high-density electronic excitation along the ion-path, resulting in the formation of ion tracks. Previous investigations of ion tracks in fluorite-type oxides, such as yttria stabilized-cubic zirconi, ceria and urania, have found that the ion tracks in those oxides are not amorphous but retains the crystallinity [1]. We have investigated the structure of ion tracks in ceria irradiated with 200 MeV Xe by using atomic resolution transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) [2]. Overlapping effects of radiation damage of ion tracks are also investigated to gain insights into the microstructure evolution of fuel/target materials exposed to high burn-up conditions [3].
TEM observations of ion tracks showed continuous images with diffraction and structure factor contrast, and the analysis on the thickness-dependent contrast of ion tracks has evaluated the decrease in the atomic density inside the ion tracks to be about 10%. High resolution STEM using high-angle annular dark-field (HAADF) technique showed that the fluorite structure of the Ce cation column is retained at the core region of ion tracks, although the signal intensity of the Ce cation lattice is reduced over a region 4-5 nm in size. Annular bright-field (ABF) STEM observations have detected that the O anion column is preferentially distorted at the core region of ion tracks within a diameter of 4 nm. These STEM observations imply that the core region of ion tracks in ceria contains a high concentration of vacancies or small vacancy clusters and generate interstitials in the surrounding regions [4].
Overlap of high-density electronic excitation damage at high fluence (ge;10¹³ ions/cm²) was found to induce dislocation networks in ceria. Further, sub-divided small grains with approximately 100 nm in diameter or less were formed at high fluence (ge;10¹⁶ ions/cm²) . Those microstructure changes were discussed in terms of the accumulation of interstitials due to the significant overlap of high-density electronic excitation damage.
[1] T. Sonoda, et al., Nucl. Instr. Meth. B250 (2006) 254. [2] S. Takaki et al. Nucl. Instr. Meth. B (2013) in press. [3] K. Yasuda et al., Nucl. Instr. Meth. B314 (2013) 185. [4] K. Yasuda et al. Int. J. Mater. Res. (2011)1082.

One of the requirements for the effective manufacturing of electronic devices in the mesoscopic range is the fabrication of controlled, reliable and robust tunnel junctions. These fundamental circuital elements are the core for many nanodevices in quantum metrology, quantum computation and ultra-high sensitivity sensing. Typical configurations consist of tunnel junctions combined forming Single Electron Transistors (SET), Josephson's Junctions and magnetic flux interferometers for Superconducting Quantum Interference Devices (SQUIDs).
Many different lithographic techniques have been proposed during the last decades for the fabrication of thin-film metallic or semiconductor-based tunnel junctions with sub-micrometric effective areas; the majority of these techniques lays on dedicated thin film deposition systems or on the uncontrolled random assembly of nanoparticles among mesoscopic electrical leads. These constrains heavily limit the choice in materials and circuit complexity binding the exploration of the quantum world to few fundamental effects. Indeed the continuous demand for new material properties explorations coupled to the need for more and more complex nanostructures has opened the thin film technology to many different three dimensional structuring techniques, one of them being the 3D Direct Focused Ion Beam Lithography (3D DFIBL).
In the present contribution, an overview of the previously mentioned 3D structuring technique will be given, stressing the versatility of this mask-less lithographies process to create rather complex circuits based on tunnel junctions and suspended nanostructures.
In particular the following topics will be addressed:
- The fabrication process of metallic tunnel junction based devices, as SETs and nanoSQUIDs, by means of a commercial available dual-beam system equipped with a Focused Ion (Ga+) Beam column will be described in detail. This kind of structuring is commonly performed on a multilayered thin film incorporating the dielectric barrier which is deposited or grown by means of different techniques as sputtering, dry or wet oxidation. A detailed comparison on the influence of the FIB milling process on different multilayered films will be presented.
- A detailed investigation on the effect of Ga+ milling on the electrical and structural properties of our mesoscopic tunnel junctions will be presented together with some particular expedients for the reduction of the well known detrimental effects, typical during high energy ion milling, like amorphization and implantation: common barrier-type anodization process or low energy cleaning can in fact inactivate the uncontrolled conduction channel introduced at the boundaries of the dielectric barrier. For the purpose Transmission Electron Microscopy analysis and in-situ electrical measurements will be presented, providing a solid basis to the new powerful 3D lithography technique for mesoscopic devices fabrication.

The capability of ion beam sputtering (IBS) to induce surface nanopatterns on different materials (metals, semiconductors or insulators) is well-known since the early 60&’s [1]. The first description of such phenomenology was proposed by Bradley and Harper (BH) [2] invoking the interplay between surface relaxation mechanisms and the sputtering yield dependence on the local surface curvature. BH model and posterior generalizations [1] predict pattern formation for any ion incidence angle and successfully account for the observation of nanoripple or nanodot structures depending on the irradiation geometry (anisotropic or isotropic, respectively). However, the universality of the BH approach has been recently questioned by the disparity of (sometimes conflicting) results. This ambiguous scenario has been partially clarified after the awareness of the role played by compositional modifications during IBS for both monoelemental [3] and binary [4] semiconductors and, specially, for nanodot pattern formation. In these studies, silicon has become a sort of model system due to its technological relevance but also from the mono-elemental nature as well as extreme flatness. Thus, IBS of silicon surfaces yields (ripple) pattern formation only above an incidence angle threshold (~45° for low-energy Ar⁺ [5]) unless (metal) impurities prone to react with the target (forming silicides) are inadvertently or intentionally added during the irradiation [6,7]. Despite the a-priori undesirable presence of impurities, IBS with simultaneous co-deposition has emerged as a novel method to tune and modify the pattern morphology and characteristics [6-10]. In addition, the morphological pattern is correlated with a compositional one, offering new potential applications. However, assessing such compositional variations at the nanoscale is not straightforward and demands of advanced characterization tools. In this talk, the present status of metal co-deposition during low- (< 10 keV) and medium-energy (10-200 keV) IBS of silicon surfaces will be presented, making special emphasis on the efforts to elucidate correlated morphological and compositional issues.
[1] J. Muñoz-García, L. Vázquez, R. Cuerno, M. Castro, R. Gago, “Toward Functional Nanomaterials: Self-organized surface nanopatterning by IBS” pp.323-398 (Springer, 2009); [2] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988); [3] G. Ozaydin et al. Appl. Phys. Lett. 87,163104 (2005); [4] S. Le Roy et al. J. Appl. Phys. 106, 094308 (2009); [5] M. Castro, R. Gago et al. Phys. Rev. B 86, 214107 (2012) ; [6] J.A. Sánchez-García, R. Gago et al. J. Phys.: Condens. Matter 21, 224009 (2009); [7] H. Hofsäss et al. Appl. Phys. A 111, 653 (2013); [8] J.A. Sánchez-García, L. Vázquez, R. Gago et al. Nanotechnology 19, 355306 (2008); [9] K. Zhang et al. New J. Phys. 13, 013033 (2011); S. Macko et al. New J. Phys. 13, 073017 (2011); [10] A. Redondo-Cubero, R. Gago et al. Phys. Rev. B 86, 085436 (2012).

Tantalum exists under two phases: the equilibrium α-Ta phase (bcc structure) which has a low resistivity, is ductile, and used as diffusion barrier in microelectronic devices and also the metastable β-Ta phase (tetragonal) which have a high resistivity, is brittle and commonly observed in tantalum sputtered thin films. Despite its use in a variety of applications (resistor, capacitor, X-ray optic), and the numerous studies reported in the literature, the origin of the β-phase stabilization is not fully understood.
In this study, our approach is to understand the fundamental phenomena driving the nucleation and growth of the β-phase by combining highly sensitive in-situ stress measurements by the substrate curvature technique, ex-situ structural investigations (XRD, AFM and HRTEM) and Monte Carlo simulations of growth energetics. We also address the issue of stress development and defect incorporation in these nanoscale layers.
β- and α-Ta films were deposited by magnetron sputtering in Ar atmosphere on neutral (a-Si) and crystalline bcc Mo (110) template layers, respectively. Stress development during growth was evaluated in real-time using a multiple beam optical stress sensor (MOSS). Ion irradiation with 360 keV Kr ions, with low ion dose varying from 0.16 to 2 dpa, was used as an effective tool to study stress relaxation and possible phase change. The elastic strain field in as-deposited and irradiated states was determined from XRD using the sin^2(psi) method, adapted for the case of textured layers, while the micro-strain was obtained from a Williamson-Hall analysis of diffraction lines integral breadth. The influence of deposited energy either using substrate bias voltage or post-growth irradiation on the intrinsic stress and related lattice-expansion was investigated for both phases.
We show that the nucleation of β-Ta films on a-Si is the result of minimization of the surface/interface energy. No phase transition towards the equilibrium α-Ta is observed upon film thickening at room temperature. However, the α-Ta phase is stabilized on Mo(110) due to template (epitaxial) effects.
Also, β-Ta films were found to be more sensitive to the applied bias voltage, resulting in larger compressive stress (up to -2.4 GPa at -190 V) compared to α-Ta films (typically around -1.5 GPa). This different stress behavior is explained by a higher propensity to point-defect creation during growth, and their subsequent instability, under ion irradiation, in the metastable β-Ta lattice compared to equilibrium α-Ta.

Ion-solid interactions in the swift heavy-ion irradiation (SHII) regime are dominated by inelastic processes that result in the excitation and ionisation of substrate atoms. The fraction of the energy subsequently transferred from the electronic subsystem to the lattice is governed by the material- and phase-specific electron-phonon coupling constant. Should the lattice temperature increase beyond that required for melting, a narrow cylinder of molten material is formed along the ion path. The ensuing rapid resolidification can produce remnant structural modifications within the substrate in the form of an ion track.
For this report, we have combined calculation, simulation and experiment to identify and characterise ion track formation in amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) and thereafter examined the influence of ion track formation on materials properties. For both materials, the ion track is comprised of an under-dense core and over-dense shell with respect to unirradiated material. We attribute this structure to a quenched-in pressure wave subsequent to the rapid heating and thermal expansion about the incident ion path.
When spherical metal nanoparticles (NPs) are embedded within an a-SiO2 layer, SHII not only yields an ion track in the substrate but a spherical-to-rod-like shape transformation in the NPs. The ion track confines the shape transformation and a melt-and-flow process within the ion track has been identified. The minimum width sustainable by the elongated NPs is governed by the thermodynamic properties of the metal, specifically the energy required for vapourisation.
In a-Ge, SHII again yields an ion track and, at sufficient fluence, porosity. Given the density of molten Ge exceeds that of the amorphous solid, a volume contraction ensues within the ion track upon melting. A fraction of the resulting open volume remains after resolidification in the form of voids with a characteristic bow-tie shape. Radially-inward cooling yields this unique structure and remarkable agreement between molecular dynamics simulations and transmission electron microscopy observations have been achieved. We suggest these voids are the embryonic precursors to the porosity apparent at higher fluences.
In summary, SHII under the appropriate conditions can readily produce ion tracks and the cumulative effect of multiple ion track overlaps can yield dramatic changes in materials properties.