Molecular Dynamics as a tool to interpret macroscopic amorphization-induced swelling in silicon carbide

We present here an investigation of the irradiation-induced swelling of SiC using Classical Molecular Dynamics simulations. Heavy ion irradiation has been assumed to affect the material in two steps: (a) creation of local atomic disorder, modeled by the introduction of extended amorphous areas with various sizes and shapes in a crystalline SiC sample at constant volume (b) induced swelling, determined through relaxation using Molecular Dynamics at constant pressure. This swelling has been computed as a function of the amorphous fraction introduced. Two different definitions of the amorphous fraction were introduced to enable meaningful comparisons of our calculations with experiments and elastic modeling. One definition based on the displacements relative to the ideal lattice positions was used to compare the Molecular Dynamics results with data from experiments combining ion implantations and channeled Rutherford Backscattering analyses. A second definition based on atomic coordination was used to compare the Molecular Dynamics results to those yielded by a simplified elastic model. The simulation results using the lattice-based definition of the amorphous fraction compare very well with the experimental results. This proves that the modeling in two steps chosen for the creation of the amorphous regions is reasonable. Moreover, the results show very clearly that SiC swelling does not scale linearly with the amorphous fraction introduced. Two swelling regimes are observed relatively to the size of the amorphous area. Comparison of the elastic model with the Molecular Dynamics results using the coordination-based definition of the amorphous fraction has also enabled us to shed light on the swelling mechanisms and has shown that amorphization-induced swelling exhibits an elastic behavior. Furthermore, scalings for the swelling as a function of the two amorphous fractions considered, which can be used by larger scale models, have been determined. Finally, our study shows that classical Molecular Dynamics calculations enable one to connect the results of the available experiments with the elastic calculations and to get further insight into the swelling mechanisms.     

Driving mechanism of neutron-irradiation-induced amorphization in silicon carbide

In this study, irradiation-induced amorphization in silicon carbide (SiC) by 1 MeV neutrons was investigated using molecular dynamics (MD) simulations. The crystalline-to-amorphous (c-a) transition occurred at 0.27 dpa with a structure relaxation of the whole lattice. Fast neutrons have produced many displacement spikes with unsaturated coordinated atoms at the center. Our results have shown that the two-coordinated Si atoms play a key role in defect accumulation and amorphization. There are two types of such defects: displaced-atom-induced (D-type) defect and vacancy-induced (V-type) defect. The D-type defect tends to form clusters and promotes the formation of C Frenkel pairs after 0.13 dpa. The V-type defect enhances the driving force of c-a transition and finally triggers amorphization at high concentration based on thermodynamics.     

Interface effects on Gd induced disordering of Co films on Pt(111)

Amorphization of epitaxial Co thin films grown on top of a Pt(111) surface has been studied by surface X-ray diffraction after deposition of Gd overlayers. The results indicate strong differences of the disordering process depending on the thickness of the Co film. First basic difference is that thick Co films (15 atomic layers) are only partially amorphized by 4 atomic layers of Gd on top of them, whereas thinner Co films (5 atomic layers) are completely disordered by just 2 atomic layers of Gd. Moreover, amorphization by Gd overlayers induces different stress relaxation processes in both cases. For 15 atomic layers thick Co films a preferential amorphization of the more strained Co grains is observed, leading to an effective relaxation of about ? 0.5% of the in-plane lattice parameter during amorphization, approaching its relaxed value. On the contrary, for 5 atomic layers thick Co films, the initial steps of disordering are accompanied by a stronger increase of the in-plane lattice constant, by about 1.4%, typical of Co–Pt interface alloy formation, followed by a complete amorphization. Furthermore, the magnetic characterization, carried out by magneto-optical Kerr effect and resonant magnetic surface X-ray diffraction, strongly supports that the amorphization of thin Co films is changing the nature of the Co/Pt interface. In particular, as Gd overlayers are deposited, and the amorphization proceeds, the structural disordering of the Co/Pt interface flips its characteristic perpendicular magnetic anisotropy toward in-plane orientation before the complete magnetic depolarization of the interface Pt atoms is reached. All these results confirm a marked dependence of amorphization processes on film thickness, which can be related to the enhanced influence of the nearby film/substrate interface.     

Two-stage amorphization of scandium molybdate at high pressure

Using Raman spectroscopy and X-ray diffraction we find that at high pressure amorphization of scandium molybdate occurs in two stages. Excessive broadening of molybdate internal modes arising from distortion and disordering of MoO₄ tetrahedra, together with rapid weakening of diffraction from large d-spacings and disappearance of diffraction from small d-spacings suggest gross molybdate ion disorder above 4 GPa. On the other hand, complete amorphization occurs at the significantly higher pressure of 12 GPa, where diffraction disappears completely. The amorphization is also found to be irreversible.     

Lu2Ti2O7和Lu2TiO5陶瓷材料的Kr离子辐照损伤研究

钛酸盐因其优异的物理化学性能,可作为高放射性核废物（HLW）和锕系元素（钚）的重要候选固化材料之一。采用传统的陶瓷烧结工艺制备了多晶的Lu₂Ti₂O₇和Lu₂TiO₅陶瓷材料。在室温下,用800 keVKr2+对两种材料进行辐照,辐照后的样品采用GIXRD进行表征,观察到两种样品都经历了先肿胀、然后再发生非晶相变的过程。不同的是Lu₂Ti₂O₇的晶格肿胀程度大于Lu₂TiO₅。另外,Lu₂TiO₅样品的辐照到2×10₁₄ ions/cm₂时非晶含量达95.54%,而Lu₂Ti₂O₇样品在此剂量下非晶含量只有74.66%。通过第一性原理计算了Lu₂Ti₂O₇晶体的晶格肿胀随反位浓度的变化关系,结果表明,Lu₂Ti₂O₇出现非晶前的晶格肿胀主要由阳离子反位导致,而Lu₂TiO₅是无序的萤石结构,其辐照所导致的晶格肿胀不含阳离子反位的贡献,晶格肿胀程度较低。     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

Effect of lattice strain on the oxygen vacancy formation and hydrogen adsorption at CeO2(111) surface

Using first-principles calculation, the effect of lattice strain on the oxygen vacancy formation at CeO₂(111) surface has been investigated. The tensile strain facilitates the oxygen vacancy formation at the surface and the compressive strain hinders the process. This is in part due to the strengthening or weakening of the surface Ce–O bond under the lattice strain. On the other hand, a more open surface with a larger lattice constant can better accommodate the larger Ce³⁺ and thus facilitate the structural relaxation of the reduced surface. The studies on the strain effect on the atomic hydrogen adsorption at the defect-free CeO₂(111) surface show that the adsorption strength monotonously increases with the increase of the lattice strain, further confirming the tunable surface chemical activity by lattice strain.     

Surface modifications by swift heavy-ion irradiation of indium phosphide

InP (001) samples were irradiated with 200 MeV Au ions at different fluences. The surface nanotopographical changes due to increasing fluence of swift heavy ions were observed by Atomic Force Microscopy (AFM), where the onset of a large increase in surface roughness for fluences sufficient to cause complete surface amorphization was observed. Transmission Electron Microscopy (TEM) was used to observe bulk-ion tracks that formed in InP, and high resolution TEM (HRTEM) revealed that single-ion tracks might not be amorphous in nature. Surface-ion tracks were observed by AFM in the form of ill-defined pits (hollows) of ~12 nm in diameter (width). In addition, Rutherford backscattering was utilized to follow the formation of disorder to amorphization in the irradiated material. The interpretation of the large increase in surface roughness with the onset of amorphization can be attributed to the plastic phenomena induced by the change of states from crystalline to amorphous by ion irradiation. The text was submitted by the authors in English.     

Anomalies of the baric and temperature dependences of the elastic characteristics of ice during solid-phase amorphization and the phase transition in the amorphous state

The elastic characteristics of ice up to pressures of 1.7 GPa are determined for the first time at a temperature of 77 K, along with features of their variation associated with the phase transformation of hexagonal ice Ih into high-density amorphous ice hda. The elastic instability of the ice lattice before solid-phase amorphization is experimentally confirmed. Elastic instability during a transition from one amorphous state to another amorphous state was also observed for the first time; this took place when hda ice was warmed at p=0.05 GPa from T=77 K. Zh. éksp. Teor. Fiz. 112, 200–208 (July 1997)     

Clustering effect in the crystallization process of a CuZr amorphous alloy

The structural relaxation and crystallization processes of the Cu₅₀Zr₅₀ amorphous alloy have been studied by field ion microscopy (FIM) on an atomic scale. An interesting phenomenon which we call the clustering effect was observed for the first time as far as we know. In the temperature range 673–723 K, clusters consisting of 3, 4, or 5 atoms formed and migrated towards certain crystalline centers. They then combined with one another and rearranged to produce an ordered atomic array. This clustering process including the formation, migration, combination and rearrangement of clusters is considered as a structural relaxation process.     

Photoluminescence characteristics from amorphous SiC thin films with various structures deposited at low temperature

Hydrogenated amorphous SiC thin films deposited at low substrate temperature (100 °C) show the different bonding configurations and microstructures which depend on the carbon concentrations in the films controlled by the gas ratio R of methane to silane during the deposition. Photoluminescence characteristics are investigated for these samples with different structures. A strong luminescence in red light region can be observed for samples deposited with low gas ratio R which is significantly reduced its intensity with increasing the carbon concentrations in the films. On the other hand, the luminescence bands located at blue-green light region are detected under UV light excitation for samples deposited with high gas ratio R, which can be associated with the existence of amorphous SiC clusters in the films.     

Randomisation of crystal structure within energetic collision cascades revealed by the sputtering of Au single crystals during ion bombardment

The ratio of random to preferential atomic ejection which occurs from the surface of a gold single crystal during ion bombardment has been found to be very sensitive to the ion mass. For instance, in the case of 50 keV Ne⁺ ion bombardment preferential ejection dominates, whereas in the case of 50 keV Au⁺ ion bombardment preferential ejection is only a small fraction of the total. These results are interpreted in terms of a dynamic randomisation of the crystal lattice which occurs during the creation of energetic atomic collision cascades. The disorder occurs so rapidly that a substantial fraction of atoms sputtered from the surface are ejected from an essentially random structure. It is thought that such gross disorder is only a transient phenomenon, and leaves no significant permanent effects, except perhaps for a few clustered defects.     

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

Raman spectroscopy was used to study the radiation damage of fluorapatite single crystals and sinters. Krypton and iodine ion irradiations were performed at high energies (∼1 MeV amu⁻¹) for fluences ranging between 1 × 10¹¹ and 5 × 10¹³ cm⁻². Evolution of the symmetric stretching mode of the PO₄³⁻ tetrahedral building blocks (strongest Raman mode observed at 965 cm⁻¹) versus ion fluence was investigated. After irradiation, this peak decreases in intensity and a second broader peak appears at lower wavenumber. The well‐resolved peak has been assigned to the crystalline phase, and the broader one to the amorphous phase. The integrated intensity ratios of these two peaks versus fluence are in good agreement with the damage fractions determined by X‐ray diffraction (XRD). Fits of the amorphous fraction versus fluence show that the amorphization mechanisms is dominated by a single‐impact process for iodine ions and by a double‐impact process for krypton ions in the case of single crystals and sinters. For both irradiations, complete amorphization could not be obtained. The amorphous fraction saturates at a maximum value of 88% for sinters and 72% for single crystals. This is attributed to a recrystallization effect which is more important in single crystals than in sinters. For both types of samples, the crystalline peak shifts slightly to a lower wavenumber with fluence, and then shifts back to its initial value for an amorphous fraction larger than 60%. This feature is attributed to a stress relaxation, as shown in the XRD data, which is accompanied by a decrease of the crystalline peak full‐width at half‐maximum. Copyright © 2011 John Wiley & Sons, Ltd.     

Double-well optical lattices with atomic vibrations and mesoscopic disorder

Double-well optical lattice in an insulating state is considered. The influence of atomic vibrations and mesoscopic disorder on the properties of the lattice are studied. Vibrations lead to the renormalization of atomic interactions. The occurrence of mesoscopic disorder results in the appearance of first-order phase transitions between the states with different levels of atomic imbalance. The existence of a nonuniform external potential, such as trapping potential, essentially changes the lattice properties, suppressing the disorder fraction and rising the transition temperature.     

高电荷态离子$lt;sup$gt;126$lt;/sup$gt;Xe$lt;sup$gt;$lt;i$gt;q$lt;/i$gt;+$lt;/sup$gt;引起GaN表面形貌变化研究

用不同电荷态的¹²⁶Xe^q+离子（9≤q≤30）在室温下轰击GaN晶体表面,经原子力显微镜分析表明,当q>18,辐照区域由隆起转为显著的刻蚀.被轰击后的GaN晶体表面形貌主要取决于入射离子的电荷态.同时,样品表面形貌还与入射离子的剂量和入射角有关;在实验参数范围,与入射离子的初动能没有明显关系（180 keV≤E_k≤600 keV）.当入射离子的电荷态q=18,与样品表面法线成60°角倾斜入射和垂直表面入射时,样品的表面几乎没有变化,只是倾斜入射后有很微小的隆起;当q＜18时,样品表面膨胀隆起,粗糙度增强,倾斜入射时表面隆起比垂直入射时更明显,而且都有清晰的峰状分界区;当q＞18时,样品表面被蚀刻呈凹陷状,有明显的齿状刻痕,且侵蚀深度与离子剂量近似呈线性关系,倾斜入射时的刻蚀深度大于垂直入射时的刻蚀深度. 关键词： 高电荷态离子 GaN晶体 原子力显微镜 表面形貌     

Sub-gap absorption study of defects in ion-implanted and annealed Si layers

Sub-gap absorption measurements are presented as a tool to characterize the amorphization and recrystallization processes in ion-implanted and annealed Si layers. The gap state density associated with the disorder introduced in the target crystalline lattice has been shown to saturate once the amorphization dose is exceeded. The doping effect due to implantation of impurity species is also reported. The absorption spectra have also been shown to be very sensitive to defects associated with precipitation of the implanted atoms.     

Pressure amorphization through displacive disorder

After classifying amorphous materials according to their topology, we review a recently proposed theory of pressure amorphization (PA) that arises from some degree of displacive disorder while retaining a crystalline topology. That theory is based on the notion that one or more branches of the phonon spectrum become soft and flat with increasing pressure and is illustrated by a simple model that possesses the range of features displayed by many of the materials which undergo PA with displacive disorder. We report the results of Langevin simulations of the simple model which show how the probability of amorphization increases with the number of unit cells in the system and support our theory. We comment on how to generalize the model for the study of real systems. Received 29 march 2002     

Radiation induced microstructural evolution and amorphization of intermetallic compounds

Abstract

A theoretical model is developed to study the influence of radiation-induced microstructural evolution on the amorphization kinetics of intermetallic compounds. The amorphization mechanism is assumed to be the buildup to a critical level of defect complexes. A complex consists of a coupled interstitial-vacancy pair. It is shown that the process of amorphization under particle bombardment is obstructed in alloy systems in which interstitials exhibit a tendency to cluster. In these systems, interstitial clustering delays the buildup of complexes. Under electron irradiation, the complex concentration attains a very low level after high doses, and the crystalline-to-amorphous transition is inhibited down to fairly low temperatures. During heavy ion bombardment, cascade damage produces an enhancement of complex formation and the transition takes place. It is shown that the kinetics of amorphization under ion bombardment depends on temperature at low temperatures, where amorphization is mostly due to complex accumulation. On the other hand, the present analyses indicate that direct in-cascade amorphization becomes more important as the bombardment temperature increases. Zr₃Al is used as a model system. The theoretical calculations yield good agreement with experimental results.     

Irradiation-induced amorphization processes in nanocrystalline solids

A theoretical model is suggested which describes irradiation-induced amorphization in nanocrystalline solids, using the rate theory approach. In the framework of the model, interfaces (grain boundaries) cause the two basic effects on irradiation-induced damage and amorphization processes in nanocrystalline solids where the volume fraction of the interfacial phase is extremely large. First, amorphization is enhanced in nanocrystalline solids, because high-density ensembles of interfaces essentially contribute to the total energy of the crystalline state and thereby provide a shift in the energetics of amorphization. Second, interfaces serve as effective sinks of irradiation-produced point defects and thereby hamper amorphization driven by defect accumulation. The competition between these effects is described by kinetic equations for densities of point defects in nanoscale grains in nanocrystalline solids under irradiation treatment. This competition is shown to be responsible for the specific behavior of irradiated nanocrystalline solids, which is different from that of their coarse-grained counterparts. The suggested model accounts for the experimental data reported in the literature.PACS 61.46.+w; 61.72.Cc; 61.80.Az     

Density of phonon states in amorphous germanium and silicon

The density of phonon states in amorphous germanium and silicon is calculated by statistically averaging the crystalline phonon density of states according to the radial distribution function. A simple rigid ion model is used to calculate the density of phonon states at various lattice spacings. The appropriate model parameters are obtained from the pressure dependent elastic constants and the Raman frequency. The calculated results compare favorably to experimental data obtained by infrared and Raman scattering and the results of other theoretical calculations.