Post-Irradiation Annealing of Bi Ion Tracks in Si3N4: In-Situ and Ex-Situ Transmission Electron Microscopy Study

Recrystallization effects of swift heavy 209Bi ions irradiation on electrical degradation in 4H-SiC Schottky barrier diode

The basic physical mechanisms of damage formation in semiconductors due to swift heavy ion (SHI) irradiation are not yet fully understood. In the present paper damage evolution and the formation of ion tracks during SHI irradiation in InP, InSb, GaAs, GaP, and Ge are investigated for irradiation with Xe or Au ions having specific energies ranging from about 0.8 to 3 MeV/u. Based on these experimental results and those obtained by other authors for cluster-ion irradiation of InP, GaAs, Ge, and Si, extensive calculations were performed in the framework of the thermal spike model. As we published previously, the model was extended to correctly treat processes being specific to semiconductors. Additionally, the computer code was modified to perform calculations for cluster ions too. The calculated track radii are compared with those measured for the various irradiation conditions. Thereby, one unknown parameter in the calculations was determined by fitting one data point. With this procedure a very good agreement between calculated and measured track radii is obtained for InP, Ge, and Si irradiated under various conditions. This implies that the extended thermal spike model is well capable to explain track formation in SHI irradiated semiconductors. Furthermore, special experiments were performed, themore » results of which also support the thermal spike model of ion track formation and contradict competing mechanisms such as Coulomb explosion, shock waves, or lattice relaxation. Thus, visible (amorphous/heavily damaged) ion tracks occur if the electronic energy deposition per ion and unit length clearly exceeds the threshold value necessary for melting. This is possible for elemental ion irradiation of InP and InSb. In Ge and Si (and probably also in GaAs and GaP) the energy deposition necessary for melting is that high that it cannot be reached by elemental ion irradiation. Moreover, at least in InP and GaAs ion tracks can be formed also in a subthreshold irradiation regime if the material is predamaged. This suggests that the existence of point defects and clusters of point defects in the crystal lattice noticeably increases the electron-phonon coupling efficiency, resulting in a more efficient energy transfer to the lattice. Within the thermal spike model, this means that for a given electronic energy deposition in the predamaged crystal a higher temperature is reached than in the perfect one.« less

Swift heavy ion irradiation causes several effects on the target material. This doctoral thesis is about two effects, namely track formation and ion beam induced annealing.Tetrahedral amorphous carbon, a material consisting of 80% sp3 bound carbon, is known to form sp2 rich filaments after swift heavy ion irradiation along the ions path (i.e. 1 GeV U-ions). This phase transformation causes a change in conductivity, the ion tracks with a diameter of 8 nm form conductive nanowires embedded in an insulating matrix. The aim of this thesis was increasing the conductivity of the tracks and retaining the insulating properties of the surrounding matrix. Two approaches were made to achieve this. First, the ta-C samples were doped with nitrogen, boron, iron or copper during deposition. The concentration was kept below 2 at% to avoid sp2 bond formation. It is expected that these dopants enhance conductivity by increasing the amount of localized states at the Fermi level. Second, as irradiation with a higher electronic energy loss forms more conductive tracks, the electronic energy loss during irradiation was increased. The electronic energy loss of 1 GeV U ions in ta-C is about 40 keV/nm. The ta-C samples were also irradiated with molecules, here 30 MeV C60 molecules. The electronic energy loss in ta-C, which can be calculated as the sum of the electronic loss of each atom, equals 72 keV/nm. The conductivity of the ion tracks in ta-C was analyzed using two methods, atomic force microscopy (AFM) with a conductive cantilever and macroscopic transport measurements at low temperatures.Doping of ta-C increases track conductivity. The best results were obtained after copper doping. All other dopants also increase the matrix conductivity, thus, the conductivity contrast decreases. Irradiation with C60 molecules also results in very conductive tracks. It was also found that tracks created by ion irradiation show a broad distribution in conductivity, visible in the AFM-images. After cluster irradiation, all tracks show the same conductivity.The other effect of swift heavy ion irradiation analyzed in this work is ion beam induced annealing. In principle, ion beam induced annealing has some advantages compared to furnace annealing, like only locally heated for a short time span (1 ps), sample decomposition and dopant diffusion can be suppressed. A possible ion beam induced annealing effect was analyzed on three different semiconductors, gallium nitride, diamond and silicon carbide. These materials were irradiated with a variety of ions and energies. For GaN, the irradiation was performed at room temperature; the other materials were irradiated at elevated temperatures. The samples were either implanted with Mg-ions (GaN) or Ar-ions (diamond, SiC) for damage production. The crystal quality before and after irradiation was monitored by luminescence analysis.An annealing effect was observed for low fluence (3 x 1013 Mg-ions/cm2) implanted GaN after irradiation with 578 MeV Cr-ions (8 keV/nm) with a low fluence (5 x 1011 ions/cm2). The intensity ratio of the near band-edge to the intensity of the pristine blue band was found to be an empirical figure of merit for crystal quality. However, Mg-related luminescence could not be recorded. An annealing effect comparable to the effect found for GaN could not be found for diamond. Nonetheless, the luminescence spectra show that unimplanted diamond is unaffected by swift heavy ion irradiation. SiC is damaged further during swift heavy ion irradiation. These results can support the feasibility of ion beam induced annealing under certain preconditions.

Swift heavy ion (SHI) irradiation of damaged germanium (d-Ge) layer results in porous structure with voids aligned along ion trajectory due to local melting and resolidification during high electronic energy deposition. The present study focuses on the irradiation temperature- and incident angle-dependent growth dynamics and shape evolution of these voids due to 100 MeV Ag ions irradiation. The d-Ge layers were prepared by multiple low-energy Ar ion implantations in single crystalline Ge with damage formation of ~7 displacements per atom. Further, these d-Ge layers were irradiated using 100 MeV Ag ions at two different temperatures (77 and 300 K) and three different angles (7°, 30° and 45°). After SHI irradiation, substantial volume expansion of d-Ge layer is detected which is due to formation of nanoscale voids. The volume expansion is observed to be more in the samples irradiated at 77 K as compared to 300 K at a given irradiation fluence. It is observed that the voids are of spherical shape at low ion irradiation fluence. The voids grow in size and change their shape from spherical to prolate spheroid with increasing ion fluence. The major axis of spheroid is observed to be aligned approximately along the ion beam direction which has been confirmed by irradiation at three different angles. The change in shape is a consequence of combination of compressive strain and plastic flow developed due to thermal spike generated along ion track. Post-SHI irradiation annealing shows increase in size of voids and reversal of shape from prolate spheroid towards spherical through strain relaxation. The stability of voids was studied with the effect of post-growth annealing.

Swift heavy ion irradiation induced phase transformation in undoped and niobium doped titanium dioxide composite thin films

Atomistic strain and structural analysis of 120 MeV Ni ions irradiated CdSe nanocrystals through molecular dynamics simulation method

The influence of swift heavy ion (SHI) irradiation on the photoluminescence (PL) of silicon nanoparticles (SiNPs) and defects in SiO2-film is investigated. SiNPs were formed by implantation of 70 keV Si+ and subsequent thermal annealing to produce optically active SiNPs and to remove implantation-induced defects. Seven different ion species with energy between 3–36 MeV and fluence from 1011–1014 cm−2 were employed for irradiation of the implanted samples prior to the thermal annealing. Induced changes in defect and SiNP PL were characterized and correlated with the specific energy loss of the employed SHIs. We find that SHI irradiation, performed before the thermal annealing process, affects both defect and SiNP PL. The change of defect and SiNP PL due to SHI irradiation is found to show a threshold-like behaviour with respect to the electronic stopping power, where a decrease in defect PL and an anticorrelated increase in SiNP PL after the subsequent thermal annealing are observed for electronic stopping exceeding 3–5 keV nm−1. PL intensities are also compared as a function of total energy deposition and nuclear energy loss. The observed effects can be explained by ion track formation as well as a different type of annealing mechanisms active for SHI irradiation compared to the thermal annealing.

Static elasticity measurements at high pressures were carried out on oriented fluorapatite single crystals, some of which contained oriented amorphous ion tracks (ITs) implanted with relativistic Au ions (2.2 GeV) from the UNILAC linear accelerator at GSI, Darmstadt. High-pressure experiments on irradiated and non-irradiated crystal sections were carried out in diamond-anvil high-pressure cells under hydrostatic conditions. In situ single-crystal diffraction was performed to determine the high-precision lattice parameters, simultaneously monitoring the widths of X-ray diffraction Bragg peaks. High-pressure Raman spectra were analyzed with respect to the frequency shift and widths of bands, which correspond to the Raman-active vibrational modes of the phosphate tetrahedra. Swift heavy ion irradiation was found to induce anisotropic lattice expansion and tensile strain within the host lattice dependent on the ion-track orientation. The relatively low Gruneisen parameter for the ν 1b(A g) mode, which has been assigned to originate from the volume fraction of the amorphous tracks, and the γ(ν 1a)/γ(ν 1b) ratio reveals compressive strain on the amorphous ITs. The comparative compressibilities for the host lattice reveal approximately equivalent bulk moduli, but significantly different pressure derivatives (K T = 88.4 ± 0.7 GPa, ∂K/∂P = 6.3 ± 0.3 for non-irradiated, K T = 90.0 ± 1.7 GPa, ∂K/∂P = 3.8 ± 0.5 for irradiated samples). The axial compressibility moduli β −1 reveal significant differences, which correlate with the ion-track orientation [\( \beta_{a}^{ - 1} \) = 240 ± 5 GPa, \( \beta_{c}^{ - 1} \) = 361 ± 14 GPa, ∂\( \left( {\beta_{a}^{ - 1} } \right) \)/∂P = 11.3 ± 1.2, ∂\( \left( {\beta_{c}^{ - 1} } \right) \)/∂P = 11.6 ± 3.4 for irradiation ⊥(100); 246 ± 9 GPa, 364 ± 57 GPa, 9.5 ± 2.9, 14.7 ± 14.1 for irradiation ⊥(001), 230.7 ± 3.6 GPa, 373.5 ± 5.1 GPa, 19.2 ± 1.4, 20.1 ± 1.8 for no irradiation]. Line widths of XRD Bragg peaks in irradiated apatites confirm the strain of the host lattice, which appears to decrease with increasing pressure. By contrast, the bandwidths of Raman modes increase with pressure, and this is attributed to increasing strain gradients on the length scale of the short-range order. The investigations reveal considerable deviatoric stress on the [100]-oriented tracks due to the anisotropic elasticity, while the compression is uniform for the directions perpendicular to the tracks, which are aligned parallel to the c-axis. This difference might be considered to control the diffusion properties related to the annealing kinetics and its observed anisotropy, and hence to cause potential pressure effects on track-fading rates.

The healing effect of intense electronic energy deposition arising during swift heavy ion (SHI) irradiation is demonstrated in the case of 3C-SiC damaged by nuclear energy deposition. Experimental (ion channeling experiments) and computational (molecular dynamics simulations) studies provide consistent indications of disorder decrease after SHI irradiation. Furthermore, both methods establish that SHI-induced recrystallization takes place at amorphous-crystalline interfaces. The recovery process is unambiguously accounted for by the thermal spike phenomenon.

Molecular dynamics simulations of swift heavy ion induced defect recovery in SiC

We demonstrate the generation of a persistent conductivity increase in vanadium dioxide thin films grown on single crystal silicon by irradiation with 1 GeV 238U swift heavy ions at room temperature. VO2 undergoes a temperature driven metal-insulator-transition (MIT) at 67 °C. After room temperature ion irradiation with high electronic energy loss of 50 keV/nm the conductivity of the films below the transition temperature is strongly increased proportional to the ion fluence of 5·109 U/cm2 and 1·1010 U/cm2. At high temperatures the conductivity decreases slightly. The ion irradiation slightly reduces the MIT temperature. This observed conductivity change is persistent and remains after heating the samples above the transition temperature and subsequent cooling. Low temperature measurements down to 15 K show no further MIT below room temperature. Although the conductivity increase after irradiation at such low fluences is due to single ion track effects, atomic force microscopy (AFM) measurements do not show surface hillocks, which are characteristic for ion tracks in other materials. Conductive AFM gives no evidence for conducting ion tracks but rather suggests the existence of conducting regions around poorly conducting ion tracks, possible due to stress generation. Another explanation of the persistent conductivity change could be the ion-induced modification of a high resistivity interface layer formed during film growth between the vanadium dioxide film and the n-Silicon substrate. The swift heavy ions may generate conducting filaments through this layer, thus increasing the effective contact area. Swift heavy ion irradiation can thus be used to tune the conductivity of VO2 films on silicon substrates.

Zinc nanoparticles (NPs) of ∼10 nm in diameter were irradiated with 200 MeV Xe14+ ions with various fluences ranging from 2 × 1011 to 4 × 1014 ions/cm2. The shape elongation of the NPs and the mean inter-particle (IP) distance were evaluated by small angle X-ray scattering (SAXS) in the transmission configuration using synchrotron X-ray of 18 keV. The azimuth angle dependence of SAXS signal, which was isotropic for unirradiated state, changed to anisotropic exceeding the fluence of 1 × 1013 ions/cm2. It indicated that the NPs collectively deformed from spheres to spheroids. The signal due to the ion tracks was observed at all the fluences where the measurements were carried out, i.e., between 2 × 1011 and 4 × 1014 ions/cm2 except the unirradiated state. This observation is consistent with the erasing and overwriting mechanism for the core-shell tracks instead of the simple black-painting model. The IP distance perpendicular to the SHI beam increased with the fluence, indicating that dynamical dissolution and agglomeration of NPs proceed with the fluence in addition to the elongation. Swift heavy ion (SHI) irradiation induces not only the shape elongation of nanoparticles (NPs) but an increase of the inter-particle distance perpendicular to the SHI beam, i.e., dissolution of smaller NPs and agglomeration to larger ones.

Size characterization of ion tracks in PET and PTFE using SAXS

This chapter presents the basic concepts of conducting or π-conjugated polymers and their different nanostructures and physico-chemical properties, which ushered in a new era of functional organic materials with potential applications. Most importantly, they can replace the traditional metallic conductors owing to their excellent properties of high conductivity, thermal stability, light weight, low corrosion, high flexibility, ease of synthesis and low cost. The first studied conducting polymer was polyacetylene, and in the last two decades, the most extensively studied conducting polymers are polyaniline (PAni), polypyrrole (PPy) and polythiophine (PTh) and their derivatives owing to their interesting physico-chemical properties. Irradiation on polymers with energetic heavy ions is used to tailor their different physico-chemical properties. The energetic heavy ion irradiation-induced modifications on various properties of polymers depends on various parameters viz. type of energy transferred (i.e., nuclear or electronic) to the target, species of ion and ion fluences. The ion-matter interaction with low energy (eV to keV) range causes implantation of the ions, while ions with high energy (keV to MeV) interaction cause irreversible structural modification along the cylindrical ion track, which is of the order of few nanometers in diameter. The fundamental aspects of ion-solid interaction, different related parameters and models governing the ion-solid interaction have been described in details in this chapter. PPy nanotubes, potential candidate of highly conducting π-conjugated polymers, have been chosen for irradiation at different ion fluences to enhance their structural, morphological, electrical, optical and thermal properties. Room temperature swift heavy ion (SHI) irradiation on thin PPy films (thickness ~30–35 µm) was investigated under high vacuum (~10−5 Torr) condition by 160 MeV Ni12+ SHI using various irradiation fluences such as 1010, 5 × 1010, 1011, 5 × 1011 and 1012 ions/cm2. High-resolution transmission electron microscopy (HRTEM) was used to investigate the morphological changes of SHI-irradiated PPy nanotubes. The irradiated nanotubes exhibit denser structure, and density is highest at 5 × 1011 ions/cm2 irradiation fluence. However, on irradiation with the highest ion fluence of 1012 ions/cm2, the density of irradiated PPy nanotubes is decreased. Up to the ion fluence of 5 × 1011 ions/cm2, reduction in optical band gap energy (Eg) of irradiated PPy nanotubes is observed; however, at the investigated highest irradiation fluence of 1012 ions/cm2, value of Eg is found to be higher as compared to the unirradiated PPy nanotubes. Micro-Raman studies exhibit that upon SHI irradiation up to the ion fluence of 5 × 1011 ions/cm2, the π-conjugation length and crystallinity of PPy nanotubes are increased. Thermogravimetric analysis (TGA) shows enhanced thermal stability of irradiated PPy nanotubes with increasing ion fluence, while thermal stability of PPy nanotubes decreases at the highest irradiation fluence. The current-voltage (I-V) characteristics for the irradiated PPy nanotubes get enhanced with increasing ion fluence, while their I-V characteristics decrease at the highest irradiation fluence of 1012 ions/cm2. The scaling of modulus spectra of irradiated PPy nanotubes at different irradiation fluences depicts irradiation fluence-independent relaxation dynamics of charge carriers. At the end of the chapter, the challenges in the field of ion-matter interaction in pre-/post-irradiation as well as the processing, characterization and application of the target materials have been discussed.

We present experimental evidence for the formation of ion tracks in amorphous Si induced by swift heavy-ion irradiation. An underlying core-shell structure consistent with remnants of a high-density liquid structure was revealed by small-angle x-ray scattering and molecular dynamics simulations. Ion track dimensions differ for as-implanted and relaxed Si as attributed to different microstructures and melting temperatures. The identification and characterization of ion tracks in amorphous Si yields new insight into mechanisms of damage formation due to swift heavy-ion irradiation in amorphous semiconductors.