The production of non linear damage under molecular ion implantation

Germanium atomic (Ge₁) and molecular ions (Ge₂) of equivalent energy are implanted in silicon at an elevated temperature. The ion induced damage has been characterized by RBS channeling (RBS/C) and positron annihilation spectroscopy. The RBS/C studies indicate that the molecular ion implantation has produced more defects in the near surface regions compared to the atomic ion implantation. This paper reports a first time observation of an enhanced production of vacancy related defects in silicon implanted with molecular ions.     

Reaction kinetics of radiation-induced defects in vitreous silica under ion beam irradiation

The production behavior of radiation-induced defects in vitreous silica was studied by an in-situ luminescence measurement technique under ion beam irradiation of He⁺. The luminescence intensity of oxygen deficiency centers (ODCs) at 460 nm was observed to vary with irradiation time reflecting the accumulation behavior of the ODCs. The luminescence intensity increased after the start of irradiation and then decreased at room temperature, while it increased rapidly to a constant value at higher temperatures. Some differences were observed due to different OH contents in silica. The observations were analyzed by considering the production mechanisms and kinetics of the radiation-induced defects.     

Performance of the SIRAD ion electron emission microscope

An axial ion electron emission microscope (IEEM) is now working at the SIRAD irradiation facility of the INFN Laboratories of Legnaro (Italy). The IEEM is used to precisely reconstruct the impact points of single ions, information that may be used to determine the areas of a microelectronic device under test that are sensitive to single event effects (SEE). After describing the setup briefly reviewing its working principles, we show our first time resolved ion induced electron emission images of standard calibration targets. We also discuss a preliminary measurement of ion impact detection efficiency of the IEEM system and the available trigger signals for SEE studies. We finally make an assessment of ion electron emission microscopy at SIRAD and indicate future developments.     

Multi-step mechanism of damage accumulation in irradiated crystals

A new model of damage accumulation is presented, which is based on the assumption that the damage build-up process is composed of several steps. Each stage is triggered by the destabilization of the current structural organization of the solid. All transformations are described by a single impact mechanism. The analysis of the damage accumulation may thus be regarded as an identification of: (i) the structural properties of a material at each stage of the damage accumulation and (ii) the mechanisms of structural transformations from stage i to stage i + 1.     

Dislocation mobility study of heavy ion induced track damage in LiF crystals

The track damage created in LiF crystals by swift U, Xe and Kr ions with a specific energy of 11.1 MeV/u was studied using dislocation mobility measurements, track etching, SEM, AFM and optical microscopy. The results demonstrate high sensitivity of dislocation mobility to track core damage. The relationship between the energy loss of ions, dislocation mobility and track structure is discussed.     

Neutron induced damage in reactor pressure vessel steel: An X-ray absorption fine structure study

The radiation damage produced in reactor pressure vessel (RPV) steels during neutron irradiation is a long-standing problem of considerable practical interest. In this study, an extended X-ray absorption fine structure (EXAFS) spectroscopy has been applied at Cu, Ni and Mn K-edges to systematically investigate neutron induced radiation damage to the metal-site bcc structure of RPV steels, irradiated with neutrons in the fluence range from 0.85 to 5.0 × 10¹⁹ cm⁻². An overall similarity of Cu, Ni and Mn atomic environment in the iron matrix is observed. The radial distribution functions (RDFs), derived from EXAFS data have been found to evolve continuously as a function of neutron fluence describing the atomic-scale structural modifications in RPVs by neutron irradiations. From the pristine data, long range order beyond the first- and second-shell is apparent in the RDF spectra. In the irradiated specimens, all near-neighbour peaks are greatly reduced in magnitude, typical of damaged material. Prolonged annealing leads annihilation of point defects to give rise to an increase in the coordination numbers of near-neighbour atomic shells approaching values close to that of non-irradiated material, but does not suppress the formation of nano-sized Cu and/or Ni-rich-precipitates. Total amount of radiation damage under a given irradiation condition has been determined. The average structural parameters estimated from the EXAFS data are presented and discussed.     

Structural characterization of buried nitride layers formed by nitrogen ion implantation in silicon

The synthesis of buried silicon nitride insulating layers was carried out by SIMNI (separation by implanted nitrogen) process using implantation of 140 keV nitrogen (¹⁴N⁺) ions at fluence of 1.0 × 10¹⁷, 2.5 × 10¹⁷ and 5.0 × 10¹⁷ cm⁻² into 〈1 1 1〉 single crystal silicon substrates held at elevated temperature (410 °C). The structures of ion-beam synthesized buried silicon nitride layers were studied by X-ray diffraction (XRD) technique. The XRD studies reveal the formation of hexagonal silicon nitride (Si₃N₄) structure at all fluences. The concentration of the silicon nitride phase was found to be dependent on the ion fluence. The intensity and full width at half maximum (FWHM) of XRD peak were found to increase with increase in ion fluence. The Raman spectra for samples implanted with different ion fluences show crystalline silicon (c-Si) substrate peak at wavenumber 520 cm⁻¹. The intensity of the silicon peak was found to decrease with increase in ion fluence.     

Critical issues in the formation of quantum computer test structures by ion implantation

The formation of quantum computer test structures in silicon by ion implantation enables the characterization of spin readout mechanisms with ensembles of dopant atoms and the development of single atom devices. We briefly review recent results in the characterization of spin dependent transport and single ion doping and then discuss the diffusion and segregation behaviour of phosphorus, antimony and bismuth ions from low fluence, low energy implantations as characterized through depth profiling by secondary ion mass spectrometry (SIMS). Both phosphorus and bismuth are found to segregate to the SiO₂/Si interface during activation anneals, while antimony diffusion is found to be minimal. An effect of the ion charge state on the range of antimony ions, ¹²¹Sb²⁵⁺, in SiO₂/Si is also discussed.     

Structural changes in helium implanted Zr0.8Y0.2O1.9 single crystals characterized by atomic force microscopy and EXAFS spectroscopy

The present work is devoted to investigate the local atomic environment (of Zr, Y and O) as well as surface modifications associated with excess helium in the cubic phase of (1 0 0)-oriented Zr_0.8Y_0.2O_1.9 single crystal substrates. Commercially available oxide crystals have been implanted at various fluences in the range 0.15-2.0 × 10¹⁶ He-atoms/cm² using a 2.74 MeV He⁺ ion beam passing through a 8.0 μm Al foil. The microstructure and surface morphology of the irradiated surface are examined using atomic force microscopy (AFM). The local atomic environments of Zr, Y and O in the implanted layer are studied using synchrotron radiation and by extended X-ray absorption fine structure (EXAFS) measured at glancing angles to probe the implanted layer. From AFM studies it was observed that the surface roughness increases as fluence increases and above a critical fluence stage, small blister-like structures originating from helium bubbles are scattered on the irradiated surface. The radial distribution functions (RDFs), derived from EXAFS data at the Zr K-edge, have been found to evolve continuously as a function of ion fluence describing the atomic scale structural modifications in YSZ by helium implantation. From the pristine data, long range order (beyond the first- and second-shell) is apparent in the RDF spectrum. It shows several nearest neighbour peaks at about 2.1, 3.6, 4.3 and 5.4 Å. In the implanted specimens, all these peaks are greatly reduced in magnitude and their average positions are changed, typical of damaged material. A simple model taking into account only the existence of lattice vacancies has been used for the interpretation of measured EXAFS spectra.     

Radiation damage in ZnO ion implanted at 15 K

Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10¹¹ to 7 × 10¹⁶ cm⁻². The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms.Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10¹³ cm⁻² the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E_d(Zn) = 65 eV and E_d(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10¹⁵ cm⁻² a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10¹⁶ cm⁻² the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.     

Irradiation induced dislocation loop and its influence on the hardening behavior of Fe-Cr alloys by an Fe ion irradiation

Nano indentation analysis and transmission electron microscopy observation were performed to investigate a microstructural evolution and its influence on the hardening behavior in Fe-Cr alloys after an irradiation with 8 MeV Fe⁴⁺ ions at room temperature. Nano indentation analysis shows that an irradiation induced hardening is generated more considerably in the Fe-15Cr alloy than in the Fe-5Cr alloy by the ion irradiation. TEM observation reveals a significant population of the a₀<1 0 0> dislocation loops in the Fe-15Cr alloy and an agglomeration of the 1/2a₀<1 1 1> dislocation loops in the Fe-5Cr alloy. The results indicate that the a₀<1 0 0> dislocation loops will act as stronger obstacles to a dislocation motion than 1/2a₀<1 1 1> dislocation loops.     

Mechanisms of damage formation in semiconductors

The damage accumulation in ion-implanted semiconductors is analysed using Rutherford backscattering spectrometry (RBS). When energetic ions are implanted in a material, they transfer their energy mainly into atomic collision processes (nuclear energy loss) and in electronic excitations (electronic energy loss). For a given material this primary energy deposition is determined by the mass and energy of the implanted ions and the ion fluence (number of ions per unit area). However, the damage concentration which is measured after implantation does not only depend on the primary energy deposition, but is strongly influenced by secondary effects like defect annealing and defect transformation. For the latter processes the target temperature and the ion flux (number of ions per unit area and time) play an important role. In this presentation the influence of the various parameters mentioned above on the damage accumulation is demonstrated for various materials. Simple empirical models are applied to get information about the processes occurring and to systematize the results for the various semiconductors.     

Ti-PS nanocomposites by plasma immersion ion implantation and deposition

We synthesize Ti-PS nanocomposites by plasma immersion ion implantation and deposition (PIII&D) technique. Ti nanoparticles at size of 5-15 nm are found in PS matrix. We propose the formation of Ti nanoparticles as a result of the combined effect of ion implantation and ion condensation in PIII&D process. X-ray photoelectron spectroscopy measurements reveal that Ti atoms have three different chemical states, metal, oxide and carbide. While surface Ti atoms are oxidized, embedded Ti atoms keep their metallic states by surrounding PS matrix. We characterize optical absorbance of Ti-PS nanocomposites by UV-VIS measurements. An adsorption peak due to the excitation of localized surface plasmon is found at wavelength 337.5 nm and the fractal nature of Ti-PS nanocomposites broaden absorption wavelength from UV to infrared. In addition, we use a protein assay to measure protein immobilization. It is found that the amount of protein immobilized on Ti-PS nanocomposites is almost twice than that on pristine PS. The enhancement mechanisms are attributed to the increased surface roughness as well as covalent linkages between protein molecules and functional groups on the surface of Ti-PS nanocomposites.     

Recovery of fused silica surface damage resistance by ion beam etching

An increase of photoluminescence induced by laser irradiation in vacuum was observed for the fused silica. The degradation of transmittance and damage resistance performance of fused silica surfaces may be due to substoichiometric silica and a sufficient defect population introduced in the near surface. When the laser-irradiated surface layers were removed by ion beam etching, the laser-induced damage threshold recovered to that of un-irradiated samples. The photoluminescence also decreased after ion beam etching. According to the calculated etching depth, the laser-induced defects formed in the surface layer of 10-20 nm when different parameters used during vacuum exposure. In addition, the evolution of surface root-mean-square microroughness as a function of ion beam etching time was studied by the optical interferometric technique and atomic force microscopy.     

Modeling of ion beam induced damage in Si during channeling Rutherford backscattering spectrometry analysis

We have modeled damage creation by an analyzing beam during channeling Rutherford backscattering spectrometry (RBS) analysis. Based on classic scattering theory and the assumption that only a dechanneled ion beam can cause displacements, a chi-square approach is used to fit the modeled spectra with experimental profiles, to extract the dechanneling cross section and the displacement creation efficiency. The study has shown that, for a 2.0 MeV He beam channeled along a Si(1 0 0) axis, the efficiency of defect creation by dechanneled beams is about 8% of the value predicted from the Kichin-Pease model. This suggests a significant dynamic annealing of point defects. The modeling procedure in this work can be used to predict the displacement creation during channeling RBS analysis.     

Analyzing the effect of displacement rate on radiation-induced segregation in 304 and 316 stainless steels by examining irradiated EBR-II components and samples irradiated with protons

Recent studies have indicated that, at temperatures relevant to fast reactors and light water reactors, void swelling in austenitic alloys progresses more rapidly when the radiation dose rate is lower. A similar dependency between radiation-induced segregation (RIS) and dose rate is theoretically predicted for pure materials and might also be true in complex engineering alloys. Radiation-induced segregation was measured on 304 and 316 stainless steel, irradiated in the EBR-II reactor at temperatures near 375 °C, to determine if the segregation is a strong function of damage rate. The data taken from samples irradiated in EBR-II is also compared to RIS data generated using proton radiation. Although the operational histories of the reactor irradiated samples are complex, making definitive conclusions difficult, the preponderance of the evidence indicates that radiation-induced segregation in 304 and 316 stainless steels is greater at lower displacement rate.     

Structural studies of silicon oxynitride layers formed by low energy ion implantation

Silicon oxynitride (Si_xO_yN_z) layers were synthesized by implanting ¹⁶O₂⁺ and ¹⁴N₂⁺ 30 keV ions in 1:1 ratio with fluences ranging from 5 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻² into single crystal silicon at room temperature. Rapid thermal annealing (RTA) of the samples was carried out at different temperatures in nitrogen ambient for 5 min. The FTIR studies show that the structures of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence and annealing temperature. It is found that the structures formed at lower ion fluences (∼1 × 10¹⁷ ions cm⁻²) are homogenous oxygen-rich silicon oxynitride. However, at higher fluence levels (∼1 × 10¹⁸ ions cm⁻²) formation of homogenous nitrogen rich silicon oxynitride is observed due to ion-beam induced surface sputtering effects. The Micro-Raman studies on 1173 K annealed samples show formation of partially amorphous oxygen and nitrogen rich silicon oxynitride structures with crystalline silicon beneath it for lower and higher ion fluences, respectively. The Ellipsometry studies on 1173 K annealed samples show an increase in the thickness of silicon oxynitride layer with increasing ion fluence. The refractive index of the ion-beam synthesized layers is found to be in the range 1.54-1.96.     

Evidence of nanostructure formation in Ge oxide by crystallization induced by swift heavy ion irradiation

Ge oxide films were irradiated with 150 MeV Ag ions at fluences varying between 10¹² and 10¹⁴ ions/cm². The irradiation-induced changes were monitored by FT-IR spectroscopy, atomic force microscopy, X-ray diffraction and photoluminescence spectroscopy. The FT-IR spectra indicate stoichiometric changes and an increase in Ge content on irradiation. X-ray diffraction shows a crystallization of the irradiated films and presence of both Ge and GeO₂ phases. The Ge nanocrystal size, as calculated from Scherrer’s formula, was around 30 nm. The morphological changes, observed in atomic force microscopy, also indicate formation of nanostructures upon ion irradiation and a uniform growth is observed for a fluence of 1 × 10¹⁴ ions/cm².     

Method to measure composition modifications in polyethylene terephthalate during ion beam irradiation

Matter losses of polyethylene terephthalate (PET, Mylar) films induced by 1600 keV deuteron beams have been investigated in situ simultaneously by nuclear reaction analysis (NRA), deuteron forward elastic scattering (DFES) and hydrogen elastic recoil detection (HERD) in the fluence range from 1 × 10¹⁴ to 9 × 10¹⁶ cm⁻². Volatile degradation products escape from the polymeric film, mostly as hydrogen-, oxygen- and carbon-containing molecules. Appropriate experimental conditions for observing the composition and thickness changes during irradiation are determined. ¹⁶O(d,p₀)¹⁷O, ¹⁶O(d,p₁)¹⁷O and ¹²C(d,p₀)¹³C nuclear reactions were used to monitor the oxygen and carbon content as a function of deuteron fluence. Hydrogen release was determined simultaneously by H(d,d)H DFES and H(d,H)d HERD. Comparisons between NRA, DFES and HERD measurements show that the polymer carbonizes at high fluences because most of the oxygen and hydrogen depletion has already occured below a fluence of 3 × 10¹⁶ cm⁻². Release curves for each element are determined. Experimental results are consistent with the bulk molecular recombination (BMR) model.     

Anisotropic deformation of polystyrene particles by MeV Au ion irradiation

MeV Au irradiation leads to a shape change of polystyrene (PS) and SiO₂ particles from spherical to ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. Sub-micrometer PS and SiO₂ particles were deposited on copper substrates and irradiated with Au ions at 230 K, using an ion energy and fluence ranging from 2 to 10 MeV and 1 × 10¹⁴ ions/cm² to 1 × 10¹⁵ ions/cm². The mechanisms of anisotropic deformation of PS and SiO₂ particles are different because of their distinct physical and chemical properties. At the start of irradiation, the volume of PS particles decrease, then the aspect ratio increases with fluence, whereas for SiO₂ particles the volume remains constant.