Ion-induced Amorphization Research Articles

Rethinking radiation effects in materials science using the plasma-focused ion beam

A new methodology for fundamental studies of radiation effects in solids is herein introduced by using a plasma Focused Ion Beam (PFIB). The classical example of ion-induced amorphization of single-crystalline pure Si is used as a proof-of-concept experiment that delineates the advantages and limitations of this new technique. We demonstrate both the feasibility and invention of a new ion irradiation mode consisting of irradiating a single-specimen in multiple areas, at multiple doses, in specific sites. This present methodology suggests a very precise control of the ion beam over the specimen, with an error in the flux on the order of only 1%. In addition, the proposed methodology allows the irradiation of specimens with higher dose rates when compared with conventional ion accelerators and implanters. This methodology is expected to open new research frontiers beyond the scope of materials at extremes such as in nanopatterning and nanodevices fabrication.

Morphology modification of Si nanopillars under ion irradiation at elevated temperatures: plastic deformation and controlled thinning to 10 nm

Si nanopillars of less than 50 nm diameter have been irradiated in a helium ion microscope with a focused Ne+ beam. The morphological changes due to ion beam irradiation at room temperature and elevated temperatures have been studied with the transmission electron microscope. We found that the shape changes of the nanopillars depend on irradiation-induced amorphization and thermally driven dynamic annealing. While at room temperature, the nanopillars evolve to a conical shape due to ion-induced plastic deformation and viscous flow of amorphized Si, simultaneous dynamic annealing during the irradiation at elevated temperatures prevents amorphization which is necessary for the viscous flow. Above the critical temperature of ion-induced amorphization, a steady decrease of the diameter was observed as a result of the dominating forward sputtering process through the nanopillar sidewalls. Under these conditions the nanopillars can be thinned down to a diameter of ∼10 nm in a well-controlled manner. A deeper understanding of the pillar thinning process has been achieved by a comparison of experimental results with 3D computer simulations based on the binary collision approximation.

Selective etching of focused gallium ion beam implanted regions from silicon as a nanofabrication method

A focused ion beam (FIB) is otherwise an efficient tool for nanofabrication of silicon structures but it suffers from the poor thermal stability of the milled surfaces caused by segregation of implanted gallium leading to severe surface roughening upon already slight annealing. In this paper we show that selective etching with KOH:H2O2 solutions removes the surface layer with high gallium concentration while blocking etching of the surrounding silicon and silicon below the implanted region. This remedies many of the issues associated with gallium FIB nanofabrication of silicon. After the gallium removal sub-nm surface roughness is retained even during annealing. As the etching step is self-limited to a depth of 25–30 nm for 30 keV ions, it is well suited for defining nanoscale features. In what is essentially a reversal of gallium resistless lithography, local implanted areas can be prepared and then subsequently etched away. Nanopore arrays and sub-100 nm trenches can be prepared this way. When protective oxide masks such as Al2O3 grown with atomic layer deposition are used together with FIB milling and KOH:H2O2 etching, ion-induced amorphization can be confined to sidewalls of milled trenches.

Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing

Swift heavy ion (2GeV 181Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd2Ti2O7 (Fd3¯m) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La2Ti2O7 (P21). The averaged electronic energy loss, dE/dx, was 37keV/nm and 35keV/nm in Gd2Ti2O7 and La2Ti2O7, respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with the growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: ∼10nm) for the perovskite-type material; whereas a concentric, core–shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: ∼9nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A2B2O7 materials decreases as a function of the cation radius ratio rA/rB, the current study correlates this behavior with the stability field of monoclinic structures, where rLa/rTi>rGd/rTi. Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La2Ti2O7 at 775°C and of Gd2Ti2O7 at 850°C. The annealing behavior is discussed in terms of enhanced damage recovery in La2Ti2O7, compared to the pyrochlore compounds Gd2Ti2O7. The difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.

Synchrotron x-ray diffraction analysis of gadolinium and lanthanum titanate oxides irradiated by xenon and tantalum swift heavy ions

ABSTRACTA synthetic cubic pyrochlore, Gd2Ti2O7 (Fd3̅m) irradiated with swift heavy ions is compared with a compositionally-related composition La2Ti2O7 (P21), which has a monoclinic, layered, perovskite-type structure. Irradiation experiments were performed at the GSI Helmholtz Center with 181Ta ions and 129Xe ions at specific energies of 11MeV/amu. At these energies the ions pass entirely through the sample thickness of ∼ 40 μm. Angle-dispersive synchrotron powder x-ray diffraction (XRD) measurements were completed and an increasing ion-induced amorphization with increasing ion fluence was for both phases. The ion track cross-sections for the radiation-induced crystalline-to-amorphous transformation, as determined from the evolution of the integrated peak intensities as a function of fluence, reveal that La2Ti2O7 (track diameter, d ∼ 7.2 nm with 181Ta and 5.1 nm with 129Xe) is more susceptible to amorphization than Gd2Ti2O7 (d ∼ 6.2 nm with 181Ta and 4.6 nm with 129Xe). The radiation response of the two titanate compounds can be understood in the context of their different structures and cation ionic radius ratios rA/rB, where the susceptibility of radiation of titanate pyrochlores is proportionate with this radius ratio. The higher electronic linear energy loss of the 181Ta ions as compared with 129Xe ions leads to a consistent increase of volume amorphized per ion in both materials, which manifests as a larger track diameter.

Improved zircon fission-track annealing model based on reevaluation of annealing data

The thermal recovery (annealing) of mineral structure modified by the passage of fission fragments has long been studied by the etching technique. In minerals like apatite and zircon, the annealing kinetics are fairly well constrained from the hour to the million-year timescale and have been described by empirical and semi-empirical equations. On the other hand, laboratory experiments, in which ion beams interact with minerals and synthetic ceramics, have shown that there is a threshold temperature beyond which thermal recovery impedes ion-induced amorphization. In this work, it is assumed that this behavior can be extended to the annealing of fission tracks in minerals. It is proposed that there is a threshold temperature, T0, beyond which fission tracks are erased within a time t0, which is independent of the current state of lattice deformation. This implies that iso-annealing curves should converge to a fanning point in the Arrhenius pseudo-space (ln t vs. 1/T). Based on the proposed hypothesis, and laboratory and geological data, annealing equations are reevaluated. The geological timescale estimations of a model arising from this study are discussed through the calculation of partial annealing zone and closure temperature, and comparison with geological sample constraints found in literature. It is shown that the predictions given by this model are closer to field data on closure temperature and partial annealing zone than predictions given by previous models.

Swift heavy ion-induced amorphization of CaZrO3 perovskite

Perovskite, ABO3, structures are an important class of ceramics with a large variety of derivative structure-types (cubic, tetragonal, hexagonal, orthorhombic, and rhombohedral). Radiation damage in perovskites is of interest due to their potential as actinide waste forms and to understand radiation effects in uranium- and thorium-bearing phases. Powder CaZrO3 perovskite was irradiated with 940-MeV Au ions up to 1.5×1013ions/cm2. Changes in the crystal structure were followed in situ as function of fluence by means of a sequence of X-ray diffraction (XRD) measurements. Ion-induced amorphization is evidenced by a decrease in diffraction intensity and an increase in diffuse scattering. Based on XRD measurements, as well as transmission electron microscopy (TEM), CaZrO3 is completely amorphized at a fluence of 1.5×1013ions/cm2. From the evolution of the integrated XRD-maxima intensities with fluence, the diameter of the amorphous tracks is estimated to be 6.0±0.6nm, which is independently confirmed by bright-field TEM images: 6.7±0.4nm. Changes in the positions of diffraction maxima may be caused by at least two processes. Broadening of the diffraction maxima is analyzed using a Williamson–Hall plot. Strain-induced broadening is the dominant process.

Atomic scale characterization of ion-induced amorphization of GaAs and InAs using perturbed angular correlation spectroscopy

The perturbed angular correlation technique has been utilized to understand the production and nature of the implantation induced crystalline to amorphous transformation in GaAs and InAs. This technique, which is based upon the nuclear hyperfine interaction of the electric-quadrupole moment of the probe nucleus with the electric field gradient from extra nuclear charges, requires introduction of radioactive probe nuclei in host material. The radioactive probes 111In/111Cd were produced with the 14UD heavy-ion accelerator via nuclear reaction that recoil implants the 111In nuclei deep into single crystals of GaAs (100) and InAs (100). After removal of radiation damage, caused by recoil implantation, single crystals of GaAs (100) and InAs (100) were implanted with stable 74Ge ions (MeV) over a wide dose range at liquid nitrogen temperature. The irradiated samples were investigated with respect to the damage production. The crystalline, disordered and amorphous probe environments were identified from the measurement. The evolution of damage is described within the framework of different amorphization models. In GaAs, amorphization is obtained by direct-impact amorphization and by the growth of amorphous zones due to defect-stimulation at crystalline/amorphous interface. In InAs, the amorphization is first initiated by accumulation of simple point defects and then direct-impact/defect-stimulated mechanism contributes to further stimulate the transformation.

Ion irradiation effects on amorphization and thermal crystallization in Zr–Al–Ni–Cu alloys

Structural modification and primary precipitates in the Zr55Al10Ni5Cu30 alloy caused by rradiation with 300–500keV H, Ag, Cu and Au ions has been studied. A metastable primary phase was formed in the ion irradiated alloys followed by the heat treatment, while no difference was observed just after the ion irradiation at room temperature. The deposited energy dependence of the precipitation behavior indicated an increase of the nucleation sites by the implanted metal atoms, simultaneously with a decrease of the growth rate by higher energy deposition density. Ion-induced amorphization was found in the alloys containing crystalline phases by Au ion irradiation at room temperature. Thermal precipitation of the Zr2Ni type crystalline phase in the alloy was effectively suppressed with higher incident Au ion fluence.

Ion-induced amorphization in ceramic materials

Amorphization induced by swift heavy ions is discussed, the main features are reviewed and explained by the author’s model. In Al2O3 and MgAl2O4 the recrystallization reduces considerably the track diameters. The threshold electronic stopping power for amorphization Set can be estimated reliably for these solids from the position of the amorphous–crystalline boundary formed at high ion fluences. The results are in good agreement with the predictions of the model. Experiments on CeO2 and UO2 are discussed. Estimates of Set are made for β-Si3N4, CeO2, pure ZrO2, ZrSiO4, and UO2, and 10.1>Set>6.2keV/nm is obtained at room temperature for fission fragment energies. At about 1000°C operation temperature, Set is reduced by about 35–50%. No amorphization is expected by ion bombardment in AlN, SiC (semiconductors) and MgO (ionic crystal).

Low current MeV Au 2+ ion-induced amorphization in silicon: Rutherford backscattering spectrometry and transmission electron microscopy study

The amorphization due to MeV Au 2+ ion implantation in Si(1 1 1) has been studied using Rutherford backscattering spectrometry/channeling (RBS/C) and transmission electron microscopy (TEM) methods. 1.5 MeV Au 2+ ions were implanted into Si(1 1 1) substrates at various fluences at low currents (0.02–0.04 μA cm −2) while the samples were kept at room temperature. The RBS/C results for as-implanted specimen shows the onset fluence for amorphization to be ≈5×10 13 ions cm −2 which is much lower than the fluence reported earlier. Selected area diffraction (TEM) for a sample implanted at a of 1×10 14 ions cm −2 confirms the occurrence of the amorphization. Earlier, amorphization studies by Alford and Theodore, using 2.4 MeV gold ions in silicon (1 0 0) reported a threshold fluence of 1.8×10 15 ions cm −2 for amorphization when the implantation was carried out at higher currents (0.2–5 μA cm −2) [J. Appl. Phys. 76 (1994) 7265]. The nuclear energy loss ( S n ) for 1.5 MeV gold ions in silicon is ≈13% greater than the value for 2.4 MeV and cannot be the sole reason for lower threshold fluence for the amorphization. The amorphization at a relatively lower fluence for the low current implantations could be possible due to reduction in the dynamical annealing effects.

Ion-Induced Amorphization of Murataite

ABSTRACTMurataite A4B2C7O22-x, where A = Na+, Ca2+, REE3+, An3+/4+; B = Mn2+/3+, Zn2+; C = Ti4+, Fe3+, Al3+; 0≤x≤1, is an isometric, derivative of the fluorite-structure. Murataite is potentially suitable as a phase for the immobilization of rare earth (REE) and actinide elements (An). Murataite structures with three-(3C), five-(5C), and eight-fold (8C) multiples of the fluorite unit cell parameters have been identified. Radiation-induced amorphization of murataite has been investigated by 1 MeV Kr+ ion irradiation of three ceramic samples produced by melting in a resistive furnace and a cold crucible at 1400-1600 °C. The 1 MeV Kr+ ion irradiations were performed at room temperature using IVEM-Tandem Facility at Argonne National Laboratory. Radiation damage was observed by in-situ TEM. Initially, the irradiation caused disordering of the murataite structure. Murataite was rendered fully amorphous at a dose of (1.7∼1.9)Á1018 ion/m2. The pyrochlore structure phase (2C) is more radiation resistant to ion irradiation-induced amorphization than the murataite structure. Combining results on murataite with those pyrochlore and fluorite, a generally increasing trend in the susceptibility to ion beam damage is found in the fluorite-related structures as a function of the increasing multiples of the fluorite unit cells.

Sequential disordering during ion-induced amorphization in the Mo–Fe multilayered films

Amorphous alloys were formed within an extended broad composition range from 26 to 67 at.% of Mo by room temperature 200 keV xenon ion mixing of multilayered films in the Mo–Fe system with a small negative heat of formation of −3 kJ/mol, provided the multilayered films were properly designed to include sufficient fraction of interfacial atoms, thus possessing enough excess interfacial free energy. Moreover, a sequential disordering of Mo and Fe lattices was observed, for the first time, in the process of ion-induced amorphization in the Fe–Mo multilayered films. The sequential behavior can be attributed to the fact that Fe acted as a fast diffuser into Mo lattice, leading to an earlier disordering of the Mo crystalline structure and might be also associated with a difference in Debye temperature of the two metals.

Study of argon-irradiation-induced defects and amorphization in silicon using a positron beam, Raman spectroscopy and ion channelling

Studies of irradiation-induced defects have been carried out on silicon single-crystal samples, irradiated with 140 keV argon ions to doses in the range 5 × 1014 to 5 × 1016 cm-2, using a variable-low-energy positron beam, Raman spectroscopy and ion channelling. The Doppler broadening lineshape S-parameter has been found to exhibit a peak as a function of the positron beam energy Ep and a subsequent saturation behaviour for all irradiated samples. The peak damage occurs at a depth of 100 nm, consistent with TRIM code calculations. The measured S-Ep curves are analysed using a positron diffusion model to obtain the depth profile of positron-trapping defects. The behaviour of the S-W correlation plots and the variation of the R-parameter indicate that the nature of the open-volume defects is independent of the dose and the dominant defects are vacancy clusters larger than divacancies. Raman spectroscopic studies indicate that all of the irradiated samples are amorphized, and the degree of residual crystallinity across the irradiated zone of the sample is obtained. Ion-channelling studies carried out on the samples have yielded thicknesses of the amorphized layer which are consistent with positron beam results. In contrast to the TRIM code calculations, the present results show the presence of defects at depths far beyond the Ar-ion range. The results of the positron beam, Raman and ion-channelling studies are discussed in the context of ion-induced defects and amorphization.

Stress-induced amorphization at moving crack tips in NiTi

In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glass, i.e., the glassy state that has the same entropy as that of the defect-free crystal. As the glassy state with the lowest free energy, its formation can be understood as the most energetically-favored, kinetically-constrained response of crystalline alloys driven far from equilibrium.

Temperature dependence of Kr ion-induced amorphization of mica minerals

Four different mica compositions, i.e., muscovite [KAl 2(AlSi 3O 10)(OH,F) 2], phlogopite [KMg 3(AlSi 3O 10)(OH,F) 2], biotite [K(Mg,Fe) 3(AlSi 3O 10)(OH,F) 2] and lepidolite [K(Li,Al) 3(Al,Si) 4O 10(OH,F) 2], were irradiated with 1.5 MeV Kr + ions in the temperature range between 20 and 1023 K using the HVEM-Tandem Facility at Argonne National Laboratory. The irradiation effects were characterized by in situ and high resolution transmission electron microscopy (TEM). All four micas are susceptible to irradiation-induced amorphization with relatively low critical amorphization doses ( D c) (1–2 × 10 14 ions/cm 2, fractions of a dpa) below or at room temperature. D c increased with irradiation temperature for all phases above 400 K. The predicted critical temperatures ( T c), above which amorphization will not occur, for the four micas are all at or above 1000 K. Lepidolite is the most easily amorphized among the four micas at all irradiated temperatures with a predicted T c of 1300 K. Large gas bubbles (1–2 μm in diameter) were observed in a 800 keV Kr 2+ irradiated biotite after 1 × 10 15 ions/cm 2.

Rigidity Constraints in Amorphization of Multiply-Polytopic Multiply-Connected Ceramic Structures

AbstractThe ease with which ceramic structures amorphize under displacive radiation is considered from the point of view of connectivity and the consequent topological constraints which impose rigidity on the original atomic arrangement. These constraints define a measure of topological freedom to rearrange which has been shown previously to correlate with measured energy deposition density during ion-induced amorphization. That earlier assessment of topological freedoms is extended here to include more complex connectivities and is shown to be representable in an analytical form which reflects the progressive destruction of topological constraints.

Radiation Damage Effects in Ferroelectric Litao3 Single Crystals

ABSTRACTZ-cut lithium tantalate (LiTaO3) ferroelectric single crystals were irradiated with 200 keV Art++ ions. LiTaO3 possesses a structure that is a derivative of the corundum (A12O3) crystal structure. A systematic study of the radiation damage accumulation rate as a function of ion dose was performed using ion-beam channeling experiments. An ion fluence of 2.5•1018 Ar2+ ions/m2 was sufficient to amorphize the irradiated volume of a LiTaO3 crystal at an irradiation temperature of ∼120K. This represents a rather exceptional susceptibility to ion-induced amorphization, which may be related to a highly disparate rate of knock-on of constituent lattice ions, due to the large mass difference between the Li and Ta cations. We also observed that the c− end of the ferroelectric polarization exhibits slightly higher ion dechanneling along with an apparent greater susceptibility to radiation damage, as compared to the c+ end of the polarization.

Ion-induced crystallization and amorphization at crystal/amorphous interfaces of silicon

New empirical equations describing the rate of ion-induced crystallization at a Si crystal/amorphous interface have been developed. In our model, crystallization/amorphization at the interface arises from formation of hot spots and of knock-ons in the collision cascades. It is presumed that the hot spots induce amorphous-to-crystal transformation which lowers the free energy, similarly to heating to high temperatures, and that the bond rearrangement by a series of displacements by knock-ons and recombination to the original lattice point in collision cascades can lead to both crystal-to-amorphous and amorphous-to-crystal transformations. In both hot-spot and knock-on effects, the presence of di-vacancies under irradiation with ion beams is assumed to prohibit crystallization. The model can explain the experimental observation that the crystallization/amorphization rate is scaled by X = ø 1 2 exp(C v /2kT) , the product of the root of the flux and the inverse of root of the Boltzmann factor for the motion of the di-vacancies. Crystallization rate in the hot-spots derived assuming that an incident ion induces spontaneous crystallization within a characteristic volume along the track reveals that the radius is 10 atomic distances and the thickness of is about 0.3 monolayer for 1.5 MeV Xe ions. The calculated crystallization/amorphization rate fits to experimental results over a wide temperature range.

Effect of concurrent irradiation with electrons on ion-induced amorphization in silicon

The effect of concurrent irradiation with electrons on ion-induced amorphization has been investigated under irradiation with high energy ions and fast electrons in the HVEM-TANDEM Facility at Argonne National Laboratory. The simultaneous irradiation with a focused electron beam prevents or retards the ion-induced amorphization, forming a distinct interface between crystalline and amorphous regions. The position of the interface has been converted to the critical electron flux, which increases with increasing energy deposition density and flux of ions and energy of electrons. It is concluded that amorphous embryos are essentially formed through overlap of subcascades, and that the prevention of ion-induced amorphization is mainly caused by athermal migration of point defects.