Formation Of Extended Defects Research Articles

Multiscale characterization of damage behavior of glass-ceramic bonded SiC joint under He ion irradiation

The synergies of glass-ceramic bonded SiC joint by He ion irradiation at 400 °C to the fluences of 2.5 × 1016, 1 × 1017 and 3 × 1017 ions/cm2 were investigated using a multiscale combination of atomic force microscopy (AFM), micro-Raman, grazing incidence X-ray diffraction, and transmission electron microscopy. The relative swelling difference between SiC and glass-ceramic reached 5.3 ± 0.6% irradiated to the fluence of 3 × 1017 ions/cm2. The main reason of swelling difference was the difference in irradiation-induced defects and He bubbles. Si-Si and C-C bonds formed in SiC substrate after irradiation were confirmed. Up to 3 × 1017 ions/cm2, crystalline phases in glass-ceramic were highly irradiation tolerant as they remained crystalline, but the irradiation-induced defects were significantly increased. Additionally, the size of He bubbles was larger in glass-ceramic compared to SiC. TEM visible extended defect formation such as dislocation loops were only detected in the larger phase grains, suggesting a potential influence of grain size on defect accumulation in glass-ceramic.

Effect of ion-induced secondary radiations on the formation of extended defects and radiolysis beyond the range of swift 12C, 50Ti, and 52Cr ions in LiF

The objective of this research is to study the role of ion-induced secondary radiations on structural damage in LiF crystals irradiated with ∼ 10 MeV/u energy 12C, 50Ti, and 52Cr ions using experimental methods of nanoindentation, Atomic Force Microscopy, optical absorption, depth-resolved photoluminescence and chemical etching. In LiF crystals irradiated at room temperature with 50Ti and 52Cr ions, the chemical etching method revealed formation of nano-sized colloids in a wide area (up to one mm) beyond the ion penetration depth. Annealing experiments on irradiated samples at temperatures above 620 K, show enhanced manifestations of radiolysis in ion-irradiated and beyond-range areas accompanied by the emission of radiolysis residues. The multicomponent secondary radiation following the ion-induced inelastic nuclear reactions is discussed as the plausible mechanism of the observed long-range damage.

In situ measurements of non-equilibrium positron state defects during He irradiation in Si

Radiation-induced property changes in materials originate from the energy transfer from an incoming particle to the existing lattice, displacing atoms. The displaced atoms can cause the formation of extended defects including dislocation loops, voids, or precipitates. The non-equilibrium defects created during damage events determine the extent of these larger defects and are a function of dose rate, material, and temperature. However, these defects are transient and can only be probed indirectly. This work presents direct experimental measurements and evidence of irradiated non-equilibrium vacancy formation, where in situ positron annihilation spectroscopy was used to prove the generation of non-equilibrium defects in silicon.

Effect of ion-induced nuclear reactions on structure modification and radiolysis in LiF irradiated by 410 MeV 36S ions

The objective of this research is to study the structural damage in LiF crystals irradiated with 410 MeV 36S ions. We report on the appearance of a remarkable structural damage far (hundreds of μm) beyond the penetration depth (136 μm) of sulfur ions. Selective chemical etching on samples irradiated at room temperature revealed the formation of extended defects within the ion-irradiated zone and also beyond the range of sulfur ions. Based on AFM image analysis, the defects were identified as nano-size colloids (10 nm). In the irradiated zone, nanoindentation tests indicated a significant hardening effect. Annealing experiments showed manifestations of intense radiolysis both in the ion-irradiated and beyond the range areas with a maximum of around 620 K. Comparison of experimental results with those of performed model simulation using Monte-Carlo particle transport code FLUKA support the assumption that the cause of the observed long-range damage is irradiation by secondary particles created in inelastic nuclear collisions between incident sulfur ions and lithium and fluorine nuclei of the target atoms. Neutrons, γ-rays and protons are rated as the dominating components of the secondary particle flux. The results can be helpful for assessing the risks for damage delocalization and for reasonable selection of ions in the applications of ion technologies to optical materials like LiF.

Impact of Nanosecond Laser Annealing on the Electrical Properties of Highly Boron-Doped Ultrathin Strained Si0.7Ge0.3 Layers

The CMOS scaling beyond 10 nm technology node requires high active dopant concentrations in source/drain modules to minimize contact resistance. Pulsed laser annealing has been targeted by chip manufacturers as a future option to enhance the activation level inside highly doped SiGe:B and Si:P regions, mostly used in PMOS and NMOS transistor fabrication, respectively. Indeed, this annealing process allows reaching high temperatures (above melt threshold), locally (~100nm below the surface) and with extremely fast temperature ramp rates (>109 °C/s), so that high doping levels have been demonstrated both in pure Si and Ge [1]. In a recent study, structural investigations allowed identifying the best conditions to obtain fully strained and defect-free undoped SiGe layers by liquid phase epitaxial regrowth (LPER) [2]. In this work, we report an analysis on the activation of boron dopants in similar layers. In-situ boron-doped 30 nm thick pseudomorphic Si0.7Ge0.3 layers were grown on p-type bulk Si (100) by CVD. Three different boron concentrations were incorporated inside these layers: 7.8x1019 (A), 1.4x1020 (B) and 2.3x1020 cm-3 (C). By combining the boron chemical profiles with the corresponding Hall effect measurements (Hall scattering factor: 0.35), it was possible to estimate the activation rate inside the as-grown strained SiGe layers and the impact of the possible inactive dopants on the transport properties. From the lowest to the highest boron chemical concentration, we found activation rates of ~100%, ~80% and ~60%, with no significant carrier mobility degradation, even in the sample with the highest fraction of inactive dopants.The SiGe layers were subsequently laser annealed in a SCREEN-LT3100 platform operating at 308 nm (XeCl laser) with a pulse duration around 160 ns. The laser energy densities (ED) ranged from 1.20 to 2.40 J/cm2 in order to investigate all the various annealing regimes. Results obtained using several characterization techniques were combined to determine the laser annealing regimes, quantify surface roughness and assess the layers’ composition, strain and crystalline quality. These results were compared to electrical measurements performed to analyse the evolution of the electrical parameters as a function of the laser anneal conditions, particularly the activation rate.For the lowly-doped and fully activated layer (A), the sheet resistance increases rapidly at the melt threshold (1.5 J/cm2, cf. Fig. 1, red curve), concomitantly with the appearance of a partial relaxation inside the layer (Fig. 2) and the formation of extended defects. The defects may induce a local dopant deactivation, while strain relaxation can result in a modification of the transport properties of the material (modified Hall scattering factor). Both phenomena can be therefore responsible for the observed increase of the sheet resistance. In contrast, for laser EDs allowing complete melt of the layer (i.e. beyond 2.0 J/cm2), the sheet resistance decreases with increasing ED and full activation is achieved (Fig. 3, red curve) together with strain recovery (Fig. 2) and no observable defects.For the highly-doped and partially activated layer (C), partial relaxation also occurs at the melt threshold (Fig. 2). However, thanks to the strain compensation effect of the small boron atoms, the relaxation level is lower compared to the lowly-doped sample and more quickly recovered when increasing the ED (no defects observed at 1.95 J/cm2, Fig. 4). In addition, the sheet resistance is found to continuously decrease as a function of the ED (Fig. 1, blue curve) independently of the strain state of the structure. This suggests that, in addition to the previously described phenomena, the initially inactive dopants are progressively incorporated into substitutional positions by LPER. Indeed, when full melt and strain recovery is achieved, a 100% dopant activation is also observed (Fig. 3, blue curve). Characterizations made on sample B suggest a similar behaviour to that of sample C.Finally, further results will be reported from additional experiments aiming at (i) better understanding the impact of strain relaxation on dopant activation and (ii) optimizing the laser annealing process to avoid relaxation. These experiments include the use of a shorter laser pulse for the annealing of the strained SiGe layers, as well as the comparison with results obtained from fully-relaxed boron-doped SiGe annealed under similar laser conditions.Acknowledgements:This work was supported by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 871813 MUNDFAB.

Особенности процессов структурообразования в соединениях сплава Д16, полученных сваркой трением с перемешиванием с инструментом типа «bobbin tool»

Introduction. One of friction stir welding types is the bobbin friction stir welding (BFSW) process, which allows to obtain welded joints in various configurations without using a substrate and axial embedding force, as well as to reduce heat loss and temperature gradient across the welded material thickness. This makes the BFSW process effective for welding aluminum alloys, which properties are determined by their structural-phase state. According to research data, the temperature and strain rate of the welded material have some value intervals in which strong defect-free joints are formed. At the same time, much less attention has been paid to the mechanisms of structure formation in the BFSW process. Therefore, to solve the problem of obtaining defect-free and strong welded joints by BFSW, an extended understanding of the basic mechanisms of structure formation in the welding process is required. The aim of this work is to research the mechanisms of structure formation in welded joint of AA2024 alloy obtained by bobbin tool friction stir welding with variation of the welding speed. Results and discussion. Weld formation conditions during BFSW process are determined by heat input into a welded material, its fragmentation and plastic flow around the welding tool, which depend on the ratio of tool rotation speed and tool travel speed. Mechanisms of joint formation are based on a combination of equally important processes of adhesive interaction in “tool-material” system and extrusion of metal into the region behind the welding tool. Combined with heat dissipation conditions and the configuration of the “tool-material” system, this leads to material extrusion from a welded joint and its decompaction. This results in formation of extended defects. Increasing in tool travel speed reduce the specific heat input, but in case of extended joints welding an amount of heat released in joint increases because of specific heat removal conditions. As a result, the conditions of adhesion interaction and extrusion processes change, which leads either to the growth of existing defects or to the formation of new ones. Taking into account the complexity of mechanisms of structure formation in joint obtained by BFSW, an obtaining of defect-free joints implies a necessary usage of various nondestructive testing methods in combination with an adaptive control of technological parameters directly in course of a welding process.

Ion Implantation Induced Structural Modifications and Functionalization of Ti3C2Tx MXenes

MXenes are single or few-layered two-dimensional (2D) transition metals carbides and/or nitrides successfully synthesized in 2011, with chemical composition Mn+1XnTx, where M stands for an early transition metal, X is C and/or N, Tx are a surface terminations and n = 1, 2, or 3 [1]. These materials are considered as promising candidates in many applications such as energy storage, sensing, catalysis, and optoelectronics. For instance, MXenes combine high conductivity and hydrophilicity making them interesting candidates for applications involving aqueous media and requiring good electronic properties or easy processing. Moreover, they exhibit a wide chemical variability thanks to their parent MAX phases precursors, a 150-plus member family of layered ternary carbides or nitrides with chemical composition Mn+1AXn (where the A-element mainly belong to the columns 13 and 14 of the periodic table). This offers the unique opportunity to design the MXenes on demand, with considerably different physicochemical properties for target applications [2-4]. Tuning the MXene properties is also possible through the manipulation of their surface termination groups T (i.e., T = −F, −OH, −O, or −Cl). The crucial role of the surface groups has recently triggered many studies aiming at controlling and characterizing the MXene surface structure/chemistry mainly by using different etching protocols [5]. However, the nature of the surface groups, inherited from the A element exfoliation step of the MAX phases by chemical reactions, is hardly tunable by this mean except by changing the etching protocole which might have significant impact on the quality of the synthetized MXenes.Here, we demonstrate that medium energy ion implantation can be used to modify the MXene surface functionalization, through the controlled formation of structural defects and the incorporation of foreign species in Ti3C2Tx thin films fabricated by spin coating (Fig.1 a-b). Defects created by Mn implantation in a wide fluence range (1x1014-5x1016 cm-2) are characterized at different depths by XPS on Ti2p core-level spectra, ToF-SIMS and EELS analyses. For the latest, we use an original approach that we have developed [6], allowing an estimation of the level of damage in the material by studying the C-K edge. Our results show that the 60 keV Mn implantation leads to successful incorporation of Mn in the Ti3C2 thin film. This is accompanied by the structural distortion of the Ti-C layers as evidenced by EELS experiments (Fig. 1c), preferential Ti sputtering as compared to C without any significant amorphization or phase transformation up to a fluence of 1x1016 cm-2. However, for the highest fluence studied (5x1016 cm-2), complete amorphization was observed. For the subthreshold amorphization fluence, the study of the fine structure of the O K-edge and of the electron diffraction pattern suggest the formation of extended defects in the Ti layers including the occupation of the defective Ti sites by oxygen, leading to local environment for the oxygen atoms like graphene oxide. Finally, our results show that the number of defects can be directly and in a reproductible way tunned by the implantation fluence.[1] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M.W. Barsoum, Adv. Mater. 2011, 23, 4248−4253[2] Y. Gogotsi, B. Anasori, ACS Nano 2019, 13, 8491−8494[3] Q. Tao, M. Dahlqvist, J. Lu, S. Kota, R. Meshkian, J. Halim, J. Palisaitis, L. Hultman, M. W. Barsoum, P. O. Persson, J. Rosen, Nat. Commun. 2017, 8, No. 14949.[4] B. Anasori, C. Shi, E.J. Moon, Y. Xie, C.A. Voigt, P. R. Kent, S.J. May, S.J. Billinge, M.W. Barsoum, Y. Gogotsi, Nanoscale Horiz. 2016, 1, 227−234[5] M. Benchakar, L. Loupias, C. Garnero, T. Bilyk, C. Morais, C. Canaff, N. Guignard, S. Morisset, H. Pazniak, S. Hurand, et al. Appl. Surf. Sci. 2020, 530, No. 147209[6] T. Bilyk, M. Benchakar, M. Bugnet, L. Loupias, P. Chartier, H. Pazniak, M.-L. David, A. Habrioux, S. Celerier, J. Pacaud, and V. Mauchamp, J. Phys. Chem. C (to be published, https://dx.doi.org/10.1021/acs.jpcc.0c06798) Figure 1

Synergy of electronic and nuclear energy depositions on the kinetics of extended defects formation in UO2, based on in situ TEM observations of ion-irradiation-induced microstructure evolution

During in reactor operations, the slowing down of fission fragments generates most of the radiation damage in uranium dioxide. Fission fragments deposit their energy to both atomic and electronic subsystems through nuclear and electronic interactions. To study the possible synergistic effects of nuclear and electronic energy deposition on the microstructure, UO2 thin foils have been irradiated with 0.39 MeV Xe and/or 6 MeV Si ions at 298 K using single or dual beam ion irradiations. The evolution of extended defects was characterized by in situ Transmission Electron Microscopy. Results show a similar evolution with fluence increase, irrespective of the ion beam: a nucleation of small dislocation loop that increase in density up to a saturation value, followed by an increase of the average loop size. However, the kinetics differs according to the irradiation conditions. In the case of the dual beam irradiation, all the phenomena occur at a lower value of damage level. The local increase of the temperature along the 6 MeV Si ion path seems to lead to an enhanced defect mobility, as in the case of irradiations performed at higher temperatures.

Influence of Sr deficiency on structural and electrical properties of SrTiO3 thin films grown by metal\u2013organic vapor phase epitaxy

Homoepitaxial growth of SrTiO3 thin films on 0.5 wt% niobium doped SrTiO3 (100) substrates with high structural perfection was developed using liquid-delivery spin metal–organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛr = 202 and a larger vertical lattice parameter. Current–voltage characteristics (IVCs) of metal–oxide–semiconductor (Pt/SrTiO3/SrTiO3:Nb) structures reveal that Sr deficient SrTiO3 films show an intrinsic resistive switching with on–off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.

Defect evolution in Mg ions implanted GaN upon high temperature and ultrahigh N2 partial pressure annealing: Transmission electron microscopy analysis

Defects in Mg ion-implanted GaN epitaxial layers formed after annealing at 1573 K and at 1753 K were analyzed by transmission electron microscopy. Degradation of the GaN surface, which occurs at temperatures higher than about 1573 K, was avoided by ultra-high-pressure annealing under a N2 atmosphere at 1 GPa. Annealing for damage recovery in ion-implanted compound semiconductors generally requires temperatures at about two-thirds of their melting point, which is reportedly 2518 K or higher for GaN. Thus, defect analysis in ion-implanted GaN annealed at temperatures higher than 1573 K is necessary to understand the defect recovery. Atomic-resolution transmission electron microscopy analysis showed that interstitial-type extended defects and inversion domains with Mg segregation were formed during the annealing at 1573 K. These defects were not observed in a sample annealed at 1753 K; instead, vacancy-type extended defects were formed. Such evolution of defects can be explained by previous theoretical studies that indicated that the migration energy of vacancy-type defects is higher than that of interstitial-type defects. Moreover, the formation of vacancy-type extended defects implies a reduction in the concentrations of vacancies and their complexes. Since the vacancies and their complexes can compensate for Mg acceptors, their reduced concentration should increase the acceptor activation efficiency. Also, the disappearance of Mg segregation during high-temperature annealing should increase the activation efficiency since the segregated Mg atoms are electrically inactive. It is thus concluded that the evolution of defects caused by high-temperature annealing above 1573 K increases the activation efficiency of acceptors in Mg ion-implanted GaN.

Real-time in situ observation of extended defect evolution near a crack tip in GaSb crystal under thermal loading

When an AlAsSb/InAs superlattice semiconductor material is grown by molecular beam epitaxy, extended defects in the GaSb buffer layer are inevitable. Therefore, the study the formation and evolution of extended defects is considerably important. In this study, a crack tip located in the buffer layer was considered. The evolution process of nucleation as well as development and extinction of two extended defects on the (1 1 −1) and (−1 1 −1) faces were observed in real time in situ by scanning transmission electron microscopy during heating. Analysis of the stress field results showed that thermal stress is the driving force for the propagation of extended defects, which promotes the nucleation and evolution of the (1 1 −1) and (−1 1 −1) surface extended defects. The variation in crack width before and after heating was measured according to the intensity of the atomic and crack regions in the high-resolution STEM-BF image. The results showed that the crack closes after heating at a high temperature compared to room temperature. It is considered that the amorphous GaSb at the crack tip and the edge portion at the nanometer scale during the heating process is transformed into a single crystal, thus causing the crack to shrink.

Defect and Threading Dislocations in Single Crystal Diamond: A Focus on Boron and Nitrogen Codoping

For the fabrication of vertical power electronic components, the use of a freestanding highly boron‐doped single crystal diamond substrate is mandatory, on top of which a thin low‐doped active layer is overgrown. The crystalline quality of this substrate is of prime importance as the high doping level required is also likely to lead to the generation of extended defects in the active layer. Herein, the improvement of the quality of thick highly boron‐doped diamond films grown by chemical vapor deposition (CVD) is investigated. Codoping with a low amount of nitrogen is explored as a way to limit the formation and propagation of extended defects. It is shown that the growth rate and boron concentration are not impacted by the addition of nitrogen with the parameters used in this study. Surface morphology and defect density estimated for each amount of nitrogen are presented; and Raman spectroscopy confirms that nitrogen addition compensates the strain induced by boron incorporation in single crystal diamond. Codoping with nitrogen is thus demonstrated as a useful strategy to obtain high quality boron‐doped diamond substrates potentially suitable for the development of power devices.

Role of Nitrogen in Defect Evolution in Zinc Oxide: STEM-EELS Nanoscale Investigations.

Direct evidence of the formation of nitrogen molecules (N2) after ion implantion of ZnO has been revealed by an atomically resolved scanning transmission electron microscopy (STEM)-electron energy-loss spectroscopy (EELS) investigation. Taking advantage of the possibility of using multiple detectors simultaneously in aberration-corrected STEM, we utilize the detailed correlation between the atomic structure and chemical identification to develop a model explaining the formation and evolution of different defect types and their interaction with N. In particular, the formation of zinc vacancy (VZn) clusters filled with N2 after heat treatment at 650 °C was observed, clearly indicating that N has not been stabilized in the O substitution site, thus limiting p-type doping. Previous results showing an exceptional thermal stability of vacancy clusters only for the case of N-doped ZnO are supported. Furthermore, VZn-N2 stabilization leads to suppression of VZn-Zni recombination; hence, the highly mobile Zn interstitials preferentially condense on the basal planes promoting formation of extended defects (basal stacking faults and stacking mismatched boundaries). The terminations of these defects provide energetically favorable sites for further N2 trapping as a way to reduce local strain fields.

Structure and crystal field analysis using ionoluminescence of Al2O3: Tm3+ phosphor

Structurally stable rare earth ion doped materials are attracting scientific interest for white light phosphors and laser system. α-Al2O3:Tm3+ (1 mol%) samples are synthesized by solution combustion method. Crystalline phase and structure are analyzed by X-ray diffraction (XRD) Rietveld refinement technique and it confirms multi-phases such as α-Al2O3 and TmAlO3. The average crystallite size is found to be ~125 nm. Ionoluminescence (IL) spectra of 100 MeV swift Si8+ ions irradiated Al2O3 show a sharp emission at 695 nm and a set of small humps at 678, 683 nm (N′ lines) and 708, 716, 729 nm (N lines) corresponding to Cr3+ impurity transition (2Eg→4A2g) of R lines. Emission peak at 672 nm is corresponding to Mn4+ (2E→4A2) characteristic emission. IL spectra of Tm3+ (1 mol%) doped Al2O3 exhibits Tm3+ characteristic emissions at 359.82, 397.79, 467.46, 528.01, 658.20, 757.00, 764.12 and 808.02 nm (1D2→3H6, 3P2→3H4, 1G4→3H6, 1D2→3H5, 1D2→3H4, 1D2→3F3, 1D2→3F2 and 3H4→3H6 transitions respectively) along with Cr3+ and Mn4+ impurity emissions. Further, 4f12 atomic energy configuration of Tm3+ in Al2O3 and TmAlO3 has been calculated by the crystal field theory method. Microstate energy levels of free Tm3+ ion, Tm3+ ions in C3v site of Al2O3 and Tm3+ ion in Cs site of TmAlO3 are fitted by varying the crystal field parameters. Root mean square (rms) deviation is found to be 0.30 16.41 and 18.18 cm−1 respectively. IL intensity decays with increasing ion fluence due to the formation of extended defects and local heating in host along by Si8+ ion trajectory. Irradiation-induced damage cross section of the cylindrical ion path is found to be 4.81 ± 0.20 and 5.10 ± 0.20 nm2 for undoped (695 nm) and Tm3+ doped Al2O3 (559 nm) respectively.

Plastic behaviour and deformation mechanisms in silicon nano-objects

Physical properties of nano-objects differ from what they are in bulk materials when the size decreases down to the nanometre scale. This behavioural change, named size effect, also applies to mechanical properties and has been evidenced in various materials. For instance, at low temperature, bulk silicon is known to be a brittle material while silicon nano-objects exhibit a ductile behavior. Although mechanical properties of silicon have been intensively studied over the last decades, the origin of this remarkable brittle-to-ductile transition at small scales remains, however, undetermined. In this article, a study of the plastic behaviour of nano-pillars is reported. The main results obtained from the combination of numerical calculations and experimental compression tests followed by atomically-resolved transmission electron microscopy imaging are described. We discuss the possibility for perfect dislocations to dissociate at low temperature and the underrated role of shuffle partial dislocations in plastic deformation of silicon. The formation of unexpected extended defects in the {115} planes with increasing plastic strain, also appears as a key-factor leading to the transition between ductile and brittle regimes at small scales.

Tailoring the switching performance of resistive switching SrTiO3 devices by SrO interface engineering

Redox-based resistive switching is one of the most-promising concepts in the focus of research to meet the ever-growing demand for faster and smaller non-volatile memory devices. In this work we present detailed studies of the impact of cation stoichiometry and surface segregation effects on the performance of the valence change memory model material SrTiO3. In order to clarify if the enhanced switching performance of Sr-rich SrTiO3 devices can be attributed to SrO segregation or to the formation of Sr-rich extended defects, we artificially engineered the formation of SrO islands by depositing additional SrO on top of stoichiometric SrTiO3. We thereby unravel that the enhanced switching performance is solely accounted for by the formation of SrO islands and not influenced by extended defects. Consequently following our findings, we design devices with a further improved retention by tailoring the amount of SrO on the surface.

Multiscale characterization of irradiation behaviour of ion-irradiated SiC/SiC composites

The irradiation tolerance of SiC/SiC composites was studied using 10 MeV Au ion irradiations at 350 °C, for surface doses between 1 and 50 displacements per atom (dpa). Atomic force microscopy and optical profilometry revealed irradiation-induced axial and radial shrinkage of SiC-fibers. At 50 dpa, net fiber shrinkage reached 2.8 ± 0.3%. We conclude that the primary cause of SiC-fiber shrinkage in SiC/SiC composites is the irradiation-induced loss of pre-existing carbon packets, which had occupied 2–3% fiber volume in unirradiated state. A compelling evidence of the carbon packet loss was revealed using a combination of state-of-art conventional transmission electron microscopy (TEM), high resolution TEM, energy-filtered TEM and electron energy loss spectroscopy. The carbon packet volume fraction decreased with increasing dose, reaching near-complete loss after 50 dpa. Carbon packet loss was further confirmed using Raman spectroscopy where the carbon D and G peaks disappeared after irradiation. In contrast, irradiation-induced swelling of 1 ± 0.5% was observed in the matrix after 50 dpa. The study also shows that up to 50 dpa, the multilayer pyrolytic-carbon (PyC) interface in the composite is highly irradiation tolerant as it maintained its morphology, graphitic nature and showed no signs of amorphization. Additionally, Raman spectroscopy revealed a saturation of TEM invisible disorder at 1 dpa for both ultra-fine grains of the fiber and the larger SiC-matrix grains. However, TEM visible extended defect formation such as dislocation loops were only detected in the larger matrix grains, thereby revealing a potential role of grain size on defect accumulation in SiC.

Diffusion and Defect Phenomena in III-V Semiconductors and their Investigation by Transmission Electron Microscopy

This article reviews the studies of diffusion and defect phenomena induced by high-concentration zinc diffusion in the single-crystal III-V compound semiconductors GaAs, GaP, GaSb and InP by methods of transmission electron microscopy and their consequences for numerical modelling of Zn (and Cd) diffusion concentration profiles. Zinc diffusion from the vapour phase into single-crystal wafers has been chosen as a model case for interstitial-substitutional dopant diffusion in these studies. The characteristics of the formation of diffusion-induced extended defects and of the temporal evolution of the defect microstructure correlate with the experimentally determined Zn profiles whose shapes depend on the chosen diffusion conditions. General phenomena observed for all semiconductors are the formation of dislocation loops, precipitates, voids, and dislocations and of Zn-rich precipitates in the diffusion regions. The formation of extended defects near the diffusion front can be explained as result of point defect supersaturations generated by interstitial-substitutional zinc exchange via the kick-out mechanism. The defects may act as sinks for dopants and as sources and sinks for point defects during the continuing diffusion process, thereby providing a path to establishing defect-mediated local point defect equilibria. The investigations established a consistent picture of the formation and temporal evolution of defects and the mechanisms of zinc diffusion in these semiconductors for diffusion conditions leading to high-concentration Zn concentrations. Based on these results, numerical modelling of anomalously shaped dopant concentration profiles leads to satisfactory quantitative results and yields information on type and charge states of the point defect species involved, also for near-surface Zn concentration profiles and the absence of extended defects.

Role of ZnS Segment on Charge Carrier Dynamics and Photoluminescence Property of CdSe@CdS/ZnS Quantum Rods

Growing a wide band gap shell on bare core and/or core@shell materials is a fascinating idea for improving the photoluminescence (PL) efficiency and stability. An epitaxially grown shell adds another degree of complexity to the system and modulates the excited-state relaxation dynamics, which remain poorly understood. Employing time-resolved PL and femtosecond transient absorption (TA) spectroscopy, we present a thorough study on charge carrier dynamics of CdSe@CdS and CdSe@CdS/ZnS quantum rods (QRs). Various excitation wavelengths were used to identify the contribution of individual segment toward the optical properties of the QRs. Our femtosecond TA measurements provide a clear evidence of excitation migration from CdS as well as ZnS to CdSe core within few picoseconds of photoexcitation. The excitons recombine faster in the CdSe moiety of the CdSe@CdS/ZnS than that of the CdSe@CdS QRs via an extra decay path. The interband trap states that are created via the formation of extended defects because of la...

Irradiation-induced defect formation and damage accumulation in single crystal CeO2

The accumulation of irradiation-induced disorder in single crystal CeO2 has been investigated over a wide range of ion fluences. Room temperature irradiations of epitaxial CeO2 thin films using 2 MeV Au2+ ions were carried out up to a total fluence of 1.3×1016 cm−2 Post-irradiation disorder was characterized using ion channeling Rutherford backscattering spectrometry (RBS/C) and confocal Raman spectroscopy. The Raman measurements were interpreted by means of a phonon confinement model, which employed rigid ion calculations to determine the phonon correlation length in the irradiated material. Comparison between the dose dependent changes in correlation length of the Raman measurements and the Ce disorder fraction from RBS/C provides complementary quantitative details on the rate of point and extended defect formation on the Ce and O sub-lattices over a broad range of ion fluences. Raman measurements, which are significantly more sensitive than RBS/C at low doses, reveal that the nucleation rate of defects is highest below 0.1 displacements per atom (dpa). Comparison between Raman and RBS/C measurements suggests that between 0.1 and 10 dpa the damage evolution is characterized by modest growth of point defects and/or small clusters, while above 10 dpa the preexisting defects rapidly grow into extended clusters and/or loops.