Mössbauer studies of iron doped poly(methyl methacrylate) before and after ion beam modification

High-energy MeV ions from accelerators are known to produce drastic modifications in polymers. The typical effects include chain scissions, crosslinks, molecular emission and double bond formation. Poly(methyl methacrylate) was doped with Fe(III) and irradiated with 95 MeV O⁷⁺ ions.⁵⁷Fe-Mössbauer studies were done on the doped samples before and after irradiation. Before irradiation, no Mössbauer absorption was observed. The irradiated samples showed a good Mössbauer absorption, which seems to indicate that there is a significant interaction between the metal ion and the polymer matrix. Two possibilities exist at these doses (～ 22 × 10¹² ions/cm): Fe(III) ions may be bridging the various polymer segments through crosslinking or amorphization of the sample leading to Fe-C bonding. Studies of FTIR, conductivity and glass transition temperatures on these samples support these observations.     

Structural and electrical properties of swift heavy ion beam irradiated Co/Si interface

Synthesis of swift heavy ion induced metal silicide is a new advancement in materials science research. We have investigated the mixing at Co/Si interface by swift heavy ion beam induced irradiation in the electronic stopping power regime. Irradiations were undertaken at room temperature using 120 MeV Au ions at the Co/Si interface for investigation of ion beam mixing at various doses: 8 × 10¹², 5 × 10¹³ and 1 × 10¹⁴ cm⁻². Formation of different phases of cobalt silicide is identified by the grazing incidence X-ray diffraction (GIXRD) technique, which shows enhancement of intermixing and silicide formation as a result of irradiation.I–V characteristics at Co/Si interface were undertaken to understand the irradiation effect on conduction mechanism at the interface.     

Ion-induced elongation of gold nanoparticles in silica by irradiation with Ag and Cu swift heavy ions: track radius and energy loss threshold

Systematic investigations of the energy loss threshold above which the irradiation-induced elongation of spherical Au nanoparticles occurs are reported. Silica films containing Au nanoparticles with average diameters of 15-80 nm embedded within a single plane were irradiated with 12-54 MeV Ag and 10-45 MeV Cu ions at 300 K and at normal incidence. We demonstrate that the efficiency of the ion-induced nanoparticle elongation increases linearly with the electronic energy transferred per ion track length unit from the energetic ions to the silica film. Ion beam shaping occurs above a threshold value of the specific electronic energy transfer. Three relevant regions are identified with respect to the original size of the Au nanoparticles. For 15 and 30 nm diameter particles, elongation occurs for electronic stopping power larger than 3.5 keV nm(-1). For Au nanoparticles with 40-50 nm diameter an electronic stopping power above 5.5 keV nm(-1) is required for elongation to be observed. Elongation of Au nanoparticles with 80 nm diameter is observed for electronic stopping between ～ 7-8 keV nm(-1). For all combinations of ions and energies, the ion track temperature profiles are calculated within the framework of the thermal spike model. The correlation between experimental results and simulated data indicates a thermal origin of the increase in the elongation rate with increasing the track diameter.     

Structural and electrical properties of swift heavy ion beam irradiated Fe/Si interface

The present work deals with the mixing of iron and silicon by swift heavy ions in high-energy range. The thin film was deposited on a n-Si (111) substrate at 10⁻⁶ torr and at room temperature. Irradiations were undertaken at room temperature using 120 MeV Au⁺⁹ ions at the Fe/Si interface to investigate ion beam mixing at various doses: 5 × 10¹² and 5 × 10¹³ ions/cm². Formation of different phases of iron silicide has been investigated by X-ray diffraction (XRD) technique, which shows enhancement of intermixing and silicide formation as a result of irradiation. I-V measurements for both pristine and irradiated samples have been carried out at room temperature, series resistance and barrier heights for both as deposited and irradiated samples were extracted. The barrier height was found to vary from 0·73–0·54 eV. The series resistance varied from 102·04–38·61 kΩ.     

Mixing in Au/Ge system induced by Ar<Superscript>+</Superscript> ion irradiation

Rutherford Backscattering Spectrometry (RBS) and Electrical Resistivity Measurements (ERM) were used to investigate the mixing of Au/Ge bilayer deposited onto glass substrate induced by Ar ions. Mixing was initiated by bombarding the sample with 400 keV ⁴⁰Ar⁺ beam with a fluence up to 1.2 × 10¹⁷ ions/cm² at a constant flux of 0.25 μA/cm². To assist the evaluation of the experimental results, all spectra were simulated using “RUMP” computer code. RBS results indicated that ion beam mixing led to a formation of AuGe₂ compound. The mixed region was noticed to increase with the gradual increase of Ar⁺ fluence. Results were also compared with current theoretical models used to describe the mixing process. The Bφrgesen thermal spike model was found to accurately predict the diffusion in Au/Ge interface. An increase in the electrical resistivity of the film was detected during Ar⁺ irradiation.     

Peculiarities in the formation of gold nanoparticles by ion implantation in stabilized zirconia

The investigation of bulk single crystals and sputter-deposited films of yttria-stabilized zirconia (YSZ) upon irradiation by gold (Au) ions with an average energy of 160 keV followed by postimplantation annealing revealed peculiarities in the formation of nanocrystalline metal (nc-M) particles in this matrix. In the case of irradiation to small doses (∼5 × 10¹⁵ cm⁻²), the optical absorption spectra of samples showed evidence of the formation of nanoclusters of matrix cations (nc-Zr). In these samples, postimplantation annealing at temperatures ∼700°C and above leads to the formation of nc-Au particles. Local elemental analysis of individual nc-M particles in the YSZ matrix irradiated to a dose of 4 × 10¹⁶ cm⁻² and annealed at 800°C showed the presence of metal nanoparticles with complex compositions including both implanted Au and matrix Zr atoms.     

Ion beam induced mixing at Co/Si interface

Ion beam mixing has emerged as a technique for understanding reactivity and chemistry at metal/Si interface and may find its applications in the field of microelectronics. We have investigated ion beam mixing at Co/Si interface induced by electronic excitation using 120 MeV Au⁺⁹ ion irradiation at different fluences, varying from 10¹² to 10¹⁴ ions/cm². Mixing was investigated by Rutherford Backscattering Spectroscopy (RBS) as a function of ion fluence and its mechanism across the interface is explained by the thermal spike model.     

Radiation damage study of MeV ions-implanted Nd:YVO4 crystal

Damage formation mechanism of Nd:YVO₄ implanted with MeV ions is investigated. MeV Si⁺ ions were implanted into Nd:YVO₄ crystal, and the lattice damage was measured using Rutherford backscattering spectroscopy/channeling (RBS/C) method. The damage creation kinetic indicates a significant contribution from electronic energy loss to the surface damage. A detailed analysis allows us to deduce the different contributions from electronic and nuclear stopping powers to the lattice damage production. An obvious difference in extent of damage from 1 MeV and 3 MeV Si⁺ implantations also implies that there exists a threshold value of the electronic energy deposition for damage formation. The exact value of threshold is obtained by comparison with the experimental data obtained from 3 MeV O⁺, F⁺ and Si⁺ implantation results, which turns out to be (1.7 ± 0.1) keV/nm.     

Ion beam-induced and thermal reactions at Fe:GaAs interface

Ion beam-induced and thermal reactions at Fe:GaAs interface are studied by using conversion electron Mössbauer spectroscopy and small angle X-ray diffraction measurements. A thin film of Fe (enriched to 30% in⁵⁷Fe Mössbauer isotope) was deposited in UHV environment on 〈100 〉 oriented semi-insulating GaAs substrates. Some of the samples were ion-mixed by 130 keV Ar⁺ ions at dose values of 3 × 10¹⁵ and 10¹⁶ ions/cm². The asdeposited and ion-mixed samples were annealed at different temperatures up to a maximum of 500° C. It was observed that ion mixing led to precipitation of disordered and/or defective binary phase along with ferromagnetic Fe₃GaAs ternary phase which upon vacuum annealing at 500°C for 1 h leads to a mixture of structurally well-defined Fe₃Ga, FeAs and FeAs₂ phases. The combined analysis of Mössbauer and X-ray data is shown to reveal the location of the phases below the sample surface. The mechanism for phase formation and associated reaction kinetics at Fe/GaAs interface is discussed in the light of the experimental results.     

Conductive nanoscopic ion-tracks in diamond-like-carbon

Highly energetic heavy ions with energies of 1 MeV/nucleon or more (e.g. 350 MeV Au ions) result in material modification in matter. The extremely high local energy deposition along the path leads to a material change within a nanoscopic cylinder of about 10 nm throughout the film thickness (up to 30 μm). In diamond-like carbon the material change results in conducting tracks embedded in the insulating material. This is due to a change in the bond structure to a higher sp² bonding content in the tracks and results in a conductivity change of up to four orders of magnitude. This paper discusses the conductivity mechanism in the 10 nm thick wires and presents a study of the conductivity dependence on the sp³-content in the diamond-like carbon and the used ion species. The conductive tracks are the basis of nanoscopic electronic devices made by irradiation of layered structures.     

Ar ion induced copper germanide phase formation at room temperature

The copper germanide phase Cu₃Ge which is emerging as an alternative material for making contacts and interconnects for semiconductor industry has been produced across the interface of Cu/Ge bilayers by ion beam mixing at room temperature using 1 MeV Ar ions. The dose dependence of the thickness of the mixed region shows a diffusion controlled mixing process. The experimental mixing rate and efficiency for this phase are 5·35 nm⁴ and 10·85 nm⁵/keV respectively. At doses above 8 × 10¹⁵ Ar/cm² the formation and growth of another copper rich phase Cu₅Ge has been observed. The present theoretical models are inadequate to explain the observed experimental mixing rate.     

12C5+ radiation effects in SR-86 track recording polymer

The samples of SR-86 polymer were irradiated with¹²C⁵⁺ ions of energy 5·0 MeV/u using fluences of 10¹¹−10¹⁴ ions/cm² at NSC Pelletron in a high vacuum scattering chamber. The optical studies show an increase in absorption of UV or IR in the shorter wavelength region (250–500 nm). The study also reveals that the increase in radiation dose extends the optical absorption region to longer wavelengths. It is observed that the bulk etch rate of this polymer is enhanced after heavy ion irradiation.     

Fractal character of in situ heat treated metal-compound semiconductor contacts

The heat treatment of metallized (Au) compound semiconductors (InP) was studied by in situ scanning electron microscopy combined with mass spectrometry. Correlation was found between the change in the surface morphology and the volatile component loss caused by the material interactions taking place during the heat treatment. Our experiments proved that the surface morphology can be characterized by its fractal dimension at the maximum value of the volatile component loss. In this paper the dependence of the fractal dimension of the surface pattern on the heat treatment temperature (in a given temperature range), on the volatile component loss and on metal thickness is described. Changes of the surface morphology on the analyzed samples begin at different temperatures. The evaluated patterns were created describing contour lines of the metal islands on the surface. This island formation known as balling-up phenomenon is due to the heating up of the samples. In the case of the 10 and 30 nm Au/InP(100) samples the fractal behavior appeared nearly at the same temperature (470^∘C α 490^∘C). The examined thick Au(85 nm)/InP(100) contact showed a fractal character at a lower (385^∘C) temperature. Although fractal dimension values could be obtained in a rather wide temperature range the surface had a real fractal character at only those temperatures where volatile component loss took place.     

Irradiation-induced gold silicide formation and stoichiometry effects in ion beam-mixed layer

The irradiation-induced silicide formation in ion beam-mixed layer of Au/Si(1 0 0) system was investigated by using 200 keV Kr⁺ and 350 keV Xe⁺ ions to fluences ranging from 8×10¹⁴ to 1×10¹⁶ ions/cm² at room temperature. The thickness of Au layer evaporated on Si substrate was ∼500 Å. Rutherford backscattering spectrometry (RBS) experiments were carried out to study the irradiation effects on the mixed layers. We observed that at the fluence of 1×10¹⁶ Kr⁺/cm² and starting from the fluence of 8×10¹⁴ Xe⁺/cm², a total mixing of the deposited Au layer with Si was obtained. RBS data corresponding to the fluences of 1×10¹⁶ Kr⁺/cm² and 8×10¹⁴ Xe⁺/cm² clearly showed mixed layers with homogenous concentrations of Au and Si atoms which can be attributed to gold silicides.The samples irradiated to fluences of 1×10¹⁶ Kr⁺/cm² and 1×10¹⁶ Xe⁺/cm² were also analyzed by X-ray photoelectron spectroscopy (XPS). The observed chemical shift of Au 4f and Si 2p lines confirmed the formation of gold silicides at the surface of the mixed layers. Au₂Si phase is obtained with Kr⁺ irradiation whereas the formed phase with Xe⁺ ions is more enriched in Si atoms.     

Thermal spikes in Ag/Fe and Cu/Fe ion beam mixing

Ion beam mixing has been studied since 1980, and since then a lot of experimental and theoretical work has been done and knowledge has been gathered. Nevertheless, there are still many fundamental aspects that need to be clarified and with that aim many experiments need to be performed. Copper and iron are miscible in the liquid state, while silver and iron are not. However, both systems are thermally immiscible in the solid state. In order to have an insight into the importance of mixing within thermal spikes during ion beam irradiation, we deposited Cu/Fe and Ag/Fe bilayers onto Si substrates and irradiated them at room temperature with 2 MeV Cu and 2.5 MeV Au ions. A combination of Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM) was used to analyze the atomic transport at the interface and the morphology changes of the samples. From the element profiles at the interface we conclude a mixing efficiency, which is indeed larger than the prediction of the ballistic model in the Cu/Fe system and smaller in the Ag/Fe system. Since ballistic mixing is expected in any case, we argue that demixing and phase separation in the Ag/Fe system occur in the thermal spike phase of the cascade as a consequence of the positive heat of mixing. Further mixing does occur in the thermal spike in the Cu/Fe system and they remain mixed even at the solid state because of the high cooling rate. In addition, ion irradiation induces a large surface roughening of the Ag and Cu top layers as proven by AFM. This effect is important for the correct interpretation of the results. Furthermore, this recrystallization affects also the interface, producing a rough interface, that appears in the RBS spectra as an atomic ‘diffusion’ at the interface.     

Irradiation effects in semiconductor

Samples of crystalline silicon, porous silicon, gallium arsenide and silicon diodes were exposed to 50–80 MeV silicon and oxygen ions in the fluence range of the order of 10¹³ to 10¹⁴ ions/cm². The irradiated samples were characterized to obtain information on the relative concentration and depth distribution of the induced defects. For comparison a few silicon diodes and crystalline silicon samples were also exposed to 6 MeV electrons. The main techniques used for the analysis of silicon samples were low angle X-ray diffraction, photo-luminescence spectroscopy and lifetime of minority carriers, whereas diodes were characterized on the basis of switching parameters. It is observed that a large number of defects are produced in the surface region of each of the irradiated semiconductor sample though the energy deposited in the surface region through electronic loss is three orders of magnitude greater than that of nuclear collisions.     

Photoluminescence and Raman studies in swift heavy ion irradiated polycrystalline aluminum oxide

Polycrystalline aluminum oxide is synthesized by combustion technique and XRD studies of the sample revealed the α-phase. The synthesized sample is irradiated with 120 MeV swift Au⁹⁺ ions for the fluence in the range from 1 × 10¹¹ to 1 × 10¹³ ions cm⁻². A broad photoluminescence (PL) emission with peak at ∼ 447 nm and two sharp emissions with peak at ∼ 679 and ∼ 695 nm are observed in pristine when sample was excited with 326 nm. However, in the irradiated samples the PL intensity at ∼ 447, 679 and 695 nm decreases with increase in ion fluence. The α-Al₂O₃ gives rise to seven Raman modes with Raman intensity with peaks at ∼ 253, 396, 417, 546, 630, 842, 867 cm⁻¹ observed in pristine. The intensity of these modes decreases with increase in ion fluence. However, the Raman modes observed at lower fluences are found to disappear at higher fluence.     

Formation of Ni2Si silicide in Ni/Si bilayers by ion beam mixing

We have studied ion mixing in Ni-Si(1 1 1) bilayers using noble gas ions. Thin Ni films of 45 nm thickness, deposited on a Si (1 1 1) substrate, were irradiated with 175 keV Kr and 110 keV Ar ions at the same fluence of 4×10¹⁶ ions/cm² at room temperature. The formation of the mixing and the elemental depth profile were investigated by Rutherford backscattering spectrometry. In the Ar irradiated sample, there was no structural change. On the other hand, we have noted the formation of Ni₂Si for the sample irradiated with Kr ions. X-ray diffraction measurements confirmed the formation of the Ni₂Si phase. The surface morphology of the Kr irradiated sample was also studied by scanning electron microscopy.     

Electron microscopy study of ion beam synthesized β-FeSi2

β-FeSi₂ embedded in a Si matrix was prepared by ion beam synthesis (IBS). Two step implantation, with energies 60 and 20 keV, of two different doses of the iron ions, 5 × 10¹⁵ and 5 × 10¹⁶ cm⁻¹, was performed. After the implantation, the samples were subjected to rapid thermal annealing (RTA) at 900 °C. The crystal structure of the resulting material was studied using cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM). The comparison of the XTEM images with the initial iron ions implantation profiles, simulated by SRIM (Stopping and Range of Ions in Matter) demonstrate that the process of IBS, followed by RTA, preserves the initial implantation profile, implying a negligible Fe atoms diffusion velocity in comparison with the one of the chemical reaction between Fe and Si. The XTEM images show that continuous β-FeSi₂ layers are fabricated when there is a stoichiometric region in the initial implantation profile. Fe concentration lower than the stoichiometric one in the whole implantation range results in formation of β-FeSi₂ nanocrystallites embedded in the Si matrix. The behavior of the absorption coefficient energy dependences, obtained from the optical transmittance and reflectance measurements, reflects the different crystal structures forming in the two types of samples.     

Influence of ion energy on damage induced by Au-ion implantation in silicon carbide single crystals

This article reports on the influence of the ion energy on the damage induced by Au-ion implantation in silicon carbide single crystals. 6H-SiC samples were implanted with Au ions at room temperature at two different energies: 4 and 20 MeV. Both Rutherford Backscattering spectrometry in channelling geometry (RBS/C) and Raman spectroscopy were used to probe the ion implantation-induced damage. Results show that the accumulated damage increases with the fluence up to the amorphization state. RBS/C data indicate that 4-MeV implantation induces more damage than 20-MeV implantation at a given fluence. This effect is attributed to nuclear collisions since the amount of damage is identical at 4 or 20 MeV when the fluence is rescaled in dpa. Surprisingly, Raman data detect more damage for 20-MeV implantation than for 4-MeV implantation at low fluence (below 10¹³ cm⁻²) where point defects are likely formed.