Swift Heavy Ion Beam Research Articles

MeV heavy ion beam induced epitaxial crystallization of buried Si3N4 layer

We report on swift heavy ion beam induced epitaxial crystallization of a buried Si3N4 layer. High-resolution transmission electron microscopy and selected area diffraction patterns are used to study the 70MeV Si5+ ion beam induced epitaxial growth of the Si3N4 layer. We observe good epitaxial crystallization at 150°C, which is a much lower temperature as compared to the conventional solid phase epitaxial growth. A possible mechanism of recrystallization is discussed on the basis of formation of vacancies along the track of the swift heavy ion and their migration at elevated temperatures during irradiation.

Swift heavy ion induced modification on the dielectric behaviours of pure and doped TGS crystals

Swift heavy ion beam irradiation has been used for creating the non-equilibrium condition in the ferroelectric crystals of TGS doped with alanine impurities. Dielectric measurements reveal that irradiated crystals retain their ferroelectric phase and Tc remains invariant. Dielectric peak, however, gets broaden and decreases with increasing fluence. Tangent loss also decreases. Dielectric dispersion in different crystals show interesting changes with fluence and is analyzed in terms of thermally generated electric field in the crystals that stabilizes the bi-domain structure.

Swift heavy ion beam induced recrystallization of amorphous Si layers

This paper focuses on the role of electronic energy loss in ion beam induced epitaxial crystallization for which swift heavy ions (100MeV Ag7+) have been used. We observed good epitaxial crystallization at 473–623K, which is a much lower temperature regime as compared to the one needed for conventional solid phase epitaxial growth. A systematic planar recrystallization has been observed as a function of temperature giving rise to an activation energy of 0.25±0.02eV. A possible mechanism of recrystallization is discussed on the basis of the production of vacancies along the track of the swift heavy ion and their migration at elevated temperatures.

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag + and Au + ions of 100 MeV at two different fluencies such as 1 × 10 12 ions/cm 2 and 1 × 10 13 ions/cm 2 at a pressure of 10 − 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi 2. The structural parameters such as crystallite size ( D), strain ( ε) and dislocation density ( δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi 2 have been discussed in this paper.

重イオンビームによる “ナノ構造制御” 高分子電解質膜の作製 (2)

重イオンビームによる “ナノ構造制御” 高分子電解質膜の作製 (2)

Decoration of swift heavy ion tracks in nickel-oxide with gold atoms

We have investigated the swift heavy ion beam induced relocation of Au marker layers in NiO. It was found that, above a certain threshold value, the electronic energy deposition results in gettering of the nearby Au atoms into the latest excited ion track. In that way a continuous Au redistribution from a recent track into a new one occurs. For Kr-ions only short and partly segmented Au-containing cylinders of about 5 nm in diameter are found, which are symmetrically located around the initial marker plane. In beam direction the laterally averaged Au distribution shows a slight broadening, which saturates at relatively low fluences. In case of Xe- and Au-ions coherent cylindrical Au-decorated zones are formed which reach up to the surface, and which obviously allow for transversal Au transport towards the surface and for segregation of the Au in spherical nano-particles on top of the surface. The transversal transport mechanism is not of diffusion-like nature, but most probably by pressure induced viscous flow in the molten ion track.

Atomic transport in ceramics induced by electronic energy deposition (invited)

In the present paper an overview over the results of a systematic study on swift heavy ion beam mixing of ceramic thin layer packages will be presented. It will be shown that interface mixing due to electronic energy deposition in these systems is a threshold process and can be regarded as transient interdiffusion in the molten ion track. The mixing threshold is given by the track formation threshold in the less sensitive of the interface forming materials with respect to beam induced damage. The mixing rate shows a square scaling with the electronic stopping power, in agreement with the global thermal spike model. The mixing efficiencies (except for NiO samples) do not vary much for different materials, and no influence of the solid state properties on the mixing effect can deduced. NiO seems to take an exceptional position within the investigated materials since it shows an extraordinary high mixing effect, which furthermore exhibits a spatial anisotropy. The latter is not yet fully understood.

Characterization of interfaces in nanoscale semiconductor devices by optimization of depth resolution in SIMS depth profiling

Optimization of depth resolution in secondary ion mass spectrometry (SIMS) depth profiling has been carried out prior to characterization of different nanoscale semiconductor devices based on Si, GaAs and SiGe. The optimization of depth resolution was carried out by adjusting different experimental parameters like primary ion beam energy, ion mass, angle of incidence, sputter rate, etc. Such adjustments control the basic processes like recoil mixing, cascade mixing or penetration depth which in turn control the depth resolution Δ z. Certified reference material Ta 2O 5/Ta was used as standard to optimize the parameters and the actual experiments were performed keeping them in view. Various nanoscale semiconductor devices were characterized for their interfaces, e.g.: 1. InP/InGaAs/InP… quantum well (QW) structures. 2. Si x N y / n-GaAs structures with NH 3 plasma pre-treatment of GaAs for MIS devices. 3. Ga 2O 3(Gd 2O 3)/Si 0.74Ge 0.26/Si structures for MIS devices. 4. Ga 2O 3(Gd 2O 3)/GaAs structures for MOS devices. 5. Au/GaN/Si schottky diodes. 6. Ge/Si bilayer films for swift heavy ion beam induced interface-mixing experiment. It has been observed that Cs + primary ion beam with 10–11 keV energy gives Δ z ∼2–3 nm for most of the depth profiles while O 2 + with 7 keV energy can also give Δ z of similar order (∼2 nm) in some of the systems depending on ion–matrix interaction and secondary ion yield response.

Interface modification by swift heavy ions

The modification of thin layer interfaces by ion irradiation (ion beam mixing) is the most elegant way to learn about the atomic transport processes induced by the energy deposited along the ions path. Ion beam mixing in the nuclear stopping regime has been intensively studied for about 30 years, while ion beam mixing by electronic energy deposition became a matter of interest only after about 1993. In this paper an attempt will be made to briefly summarise the present knowledge on atomic transport processes induced by the electronic energy loss of swift heavy ions on the basis of our recent ion beam mixing experiments with layered systems. Swift heavy ion beam mixing is strongly related to nuclear track formation and therefore a short introduction into the present ideas of the ion's energy deposition and the track formation processes will be given. Afterwards the few experiments on high-energy ion beam mixing performed until recently will be shortly reviewed. Finally we will discuss in detail our mixing data in ceramic systems and show that swift heavy ion beam mixing is a threshold process and results from transient interdiffusion in the molten ion track.

Atomic transport in hot ion tracks

In the present paper we will summarize our recent results on swift heavy ion beam mixing (IBM) of oxide ceramics and compare them with the IBM results which where obtained in the nuclear stopping regime. We will show that the strong mixing observed after high energy irradiation is most likely due to the interdiffusion in the molten ion track. The arguments for such a conclusion are based on the fact, that the track formation threshold in both interface forming materials must be exceeded for effective interdiffusion and, even more, that the estimated diffusion constants compare with liquid rather than solid state diffusion. We will show that the “global thermal spike” model, which was developed but failed for mixing in the nuclear stopping regime can be applied for mixing by electronic stopping.

Swift heavy ions in dense plasmas: The interaction process as a probe of the plasma properties

In this article, we analyze the sensitivity of the charge and energy distribution of a swift heavy ion beam interacting with a dense plasma on the thermodynamic and dynamic properties of the target. We study more particularly partially ionized carbon targets in which both the bound and the free electrons yield a significant contribution to the stopping. The emphasis is put on direct and indirect correlation between charge transfer and stopping. We show that nonlinearities, appearing in the interaction process for heavy ions, increase the dependency of the stopping on the plasma properties, indicating that diagnostics based on the analysis of the beam ions can provide valuable information on the target properties.

Electronic excitation effects on secondary ion emission from a foil of conducting material bombarded by high energy heavy ions

It is recognized that high electronic excitations play an important role in the damage process of solid targets by high-energy heavy ions. However, the conversion mechanism of the energy of the excited electrons into kinetic energy for target atom displacement is not well understood. In order to investigate the electronic excitation effects of high energy heavy ions in solids, we have studied the secondary ions mass spectra from thin conductive solid targets irradiated by swift heavy ion beams from the JAERI tandem accelerator in the energy region where the electronic stopping power is dominant. The results show the importance of the electronic excitation effect even in simple metallic (Cu) target. It is suggested that Coulomb explosion is the major mechanism of producing columnar defects along the incident ion path.

Modification of icosahedral quasicrystal Al 65Cu 20Fe 15 by 170-MeV 58Ni +12 beam

Thin ribbons of stable icosahedral quasicrystal Al 65Cu 20Fe 15 were irradiated with swift heavy ion (SHI) beam of 170 million electron volt (MeV) 58Ni +12 in high and clean vacuum environment at a temperature of 80 K to study the modification of its properties. Dynamic variation of resistance was studied on-line during and after irradiation for the first time, to understand the effect of SHI. The resistance decreased sharply at the time of incidence of ion beam due to creation of a large number of charge carriers by intense ionization by SHI and did oscillate around a mean value during irradiation due to defect creation and annihilation. The resistance increased rapidly just after the beam was stopped from its mean value during irradiation. The quasicrystalline peaks of the XRD data on these samples irradiated by SHI, were prominent till the ion beam fluence of 5 × 10 12 particles/cm 2, but the peak height consistently decreased with increasing fluence after a fluence of 10 12 particles/cm 2.

Development of an in situ ultra-high-vacuum scanning tunneling microscope in the beamline of the 15 MV tandem accelerator for studies of surface modification by a swift heavy ion beam

We report the installation and testing of an ultra-high-vacuum (UHV) scanning tunneling microscope (STM) in the beamline for materials science studies of the 15 MV Pelletron accelerator at the Nuclear Science Center (NSC), New Delhi. This is a new facility for in situ irradiation-induced surface modification studies, available in an accelerator laboratory. The article describes its vibration isolation, in-vacuum sample transfer and other salient features for integrating the STM to the beamline. The UHV STM is tested by obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG). In situ topographic and spectroscopic studies of defect structures produced by impact of 200 MeV Au ions on HOPG, p-type Si, and 200 MeV Ag-irradiated Y1Ba2Cu3O7+δ are studied.

Amorphization and recrystallization of yttrium iron garnet under swift heavy ion beams

The room-temperature dc conductivity is used to monitor the damage and structural modifications induced by swift heavy ion irradiations in yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers doped with calcium (CaYIG) or silicon (SiYIG), with a variable conductivity due to a variable degree of compensation, and amorphous YIG layers. Irradiations are performed with heavy ions in the 0.8–6 MeV amu−1 energy range, in the electronic slowing down regime, with an electronic stopping power ranging between 7 and 41 MeV μm−1 above the amorphous track formation threshold (4.5 MeV μm−1) in this low-ion velocity range. A conductivity decrease versus ion fluence is found in the case of the high-conductivity uncompensated epilayers whereas an increase occurs for the low-conductivity compensated ones, either p-type (CaYIG) or n-type (SiYIG). These results are discussed by considering the competing effects of disorder on the carrier density and mobility in the case of compensated and uncompensated semiconductors. In both cases, the low-fluence data display a plateau at around the same conductivity value corresponding to the amorphous YIG above an amorphous fraction around 50% regardless of the ions. All the high-fluence data exhibit a power-law behavior without saturation, above a threshold fluence decreasing with increasing amorphization cross section (A). These results are interpreted by the formation of amorphous tracks and of a more conducting nanophase after recrystallization of the tracks under ion impacts. All the data are rescaled versus the product of A times fluence (φ) where amorphization dominates for Aφ⩽1, whereas recrystallization dominates for Aφ>10. However, significantly larger A values than the ones previously determined from the RBS-channeling data are derived from a mean-field analysis of the low-fluence conductivity data with a 2D Bruggeman model. These deviations are ascribed to a contribution of the crystalline track halos where internal stresses are accumulated due to the atomic density difference between the crystal and amorphous phase. A simple phenomenological approach of the amorphization and recrystallization processes is proposed on the basis of two kinetic rate equations with a recrystallization cross section (S) at least one order of magnitude smaller than A. These S values are in agreement with a thermal spike model assuming vaporization of the amorphous YIG phase along the ion path. At such high temperatures in the ion tracks, the garnet phase may decompose into a more conducting nanocrystalline phase. Finally, an exp(−T)−1/4 law for the thermal dependence of conductivity at low temperature is found in the nanophase like in the amorphous one, most probably because of the strong contribution of the disordered grain boundary cores in the conduction process.

Carbon sputtering of polymer-like amorphous carbon by swift heavy ions

Thin films of “polymer-like” deuterated amorphous carbon have been irradiated at GANIL (Caen, France) by several swift heavy ion beams : S, Ar, Nb, Sn, Pb and U in the energy range between 2 and 10 MeV/amu, covering the electronic stopping power ( S e) range between 1 and 13 keV/nm. Using Rutherford backscattering spectrometry, the carbon content remaining in the film has been determined versus fluence for several values of S e. The deduced yield of carbon sputtering increases significantly above a S e threshold value of 2.5 ± 1.0 keV nm −1, which is larger than the S e threshold value of deuterium effusion (0.75 ± 0.35 keV nm −1). Using the thermal spike model, an effective temperature around 2800 K is defined for carbon sputtering in polymer-like amorphous carbon in the S e regime.

Improvement of the properties of cuprate superconductors by creating controlled columnar defects

Interaction of swift heavy ion beam with copper oxide based high temperature superconducting (HTS) materials is discussed. The thin films and crystals of HTS are irradiated by swift heavy ion beam in a vacuum chamber to produce the columnar defects as per our requirements. These controlled columnar defects serve as pinning centers for vortices. This phenomenon is of interest in increasing critical current density in moderate magnetic field and producing a shift in the irreversibility line towards higher temperatures. These make the materials better device-worthy. The results of the modifications of the properties in some of the HTS materials after introduction of these defects at NSC are presented.

Amorphization of rare earth - cobalt intermetallic alloys by swift heavy-ion irradiation

(R = Y, Ce and Tm) and ( R= Y and Ce) intermetallic compounds have been irradiated for the first time by swift heavy-ion beams (U, Pb) of energy 1 - 5 GeV at GANIL (Caen, France). The compounds , and are found to be partially amorphized after irradiation. The amorphized volume fraction has been determined as a function of the ion beam fluence by x-ray diffraction and magnetic measurements. Results are found to be in agreement. By means of a phenomenological model, associating the phenomena of amorphization and recrystallization, the diameters of the latent tracks are estimated to lie in the range 2 - 3 nm. In contrast, no amorphization is observed in and . The different behaviours observed are well understood, in the framework of the thermal spike model, in terms of the different values of the electron - phonon coupling constant of the irradiated compounds.