Ion Beam-induced Mixing Research Articles

Swift Heavy Ion-Induced Reactivity and Surface Modifications in Indium Thin Films.

As an energetic ion traverses a target material, it loses its energy through the processes of electronic energy loss (Se) and nuclear energy loss (Sn). Controlled swift heavy ion (SHI) irradiation on solid targets produces its effects through both of these mechanisms, as a consequence of which modifications occur in the structure, surface morphology, and magnetic and optical properties, apart from ion implantation and ion-induced reactivity. A systematic investigation of these effects can be useful in developing standard protocols for creating desired effects in materials using specific ion beams. In this study, indium films of thickness 25 nm were deposited on silicon substrates and were subjected to 100 MeV O7+ and 100 MeV Si7+ ion irradiation, with the fluences varying from 1 × 1011 to 1 × 1013 ions/cm2. The pristine and SHI-irradiated films were then characterized using glancing incidence X-ray diffraction (GIXRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The motive was to identify the effects of irradiation with different ion species having large variations in electronic and nuclear energy losses. While the RBS results suggest that sputtering is extremely low and that there are no major changes in the film composition due to ion beam-induced mixing, the GIXRD analysis indicates that increasing the ion fluence reduces the crystallinity of the film for both the ions. Ion beam irradiation with O7+ ions, however, results in beam-induced reactivity, as the GIXRD scan shows characteristic peaks from indium oxide (In2O3), which become the predominant peaks at the highest fluence used here. Si7+ ion irradiation results in a narrowing of the particle size distribution on the surface, with no evidence of reactivity. SEM results indicate fusion and fragmentation of grains with the increase in the ion fluences, and AFM images reveal an increase in the surface roughness of a few percent when irradiated with both 100 MeV O7+ and 100 MeV Si7+ ions.

Design and production of tungsten-carbide rich coating layers

Ion beam induced mixing (IBM) can be used for making protecting coating layers at room temperature. We have studied the production of tungsten-carbide, having high strength and low friction, by IBM since this material is also a candidate for protective coatings. WC rich layers have been produced by irradiating C/W multilayer of various structures (with individual layer thicknesses from 10 to 20 nm) by noble gases using medium energy projectiles. The resulting alterations of the samples have been measured by Auger electron spectroscopy (AES) depth-profiling. TRIDYN simulations, with some parametrization, were applied to determine the elemental in-depth distribution after IBM; the compound formation was calculated by a simple model. The calculated and measured depth profiles were compared and excellent agreement has been found for a rich dataset differing in layer structures, projectiles, ion fluences and energies. The good agreement in a wide parameter range validates our procedure and allows the design of the WC-rich layers and also enables the significant decrease of the experimental work.

Nanoscale SiC production by ballistic ion beam mixing of C/Si multilayer structures

The ion beam-induced mixing process using Ar+, Ga+, and Xe+ ion irradiation has been used to form SiC rich layers on the nanometer scale at the interfaces of C/Si/C/Si/C multilayer structures. The SiC depth distributions were determined by Auger electron spectroscopy (AES) depth profiling and were compared to the results of analytical models developed for ballistic ion mixing and local thermal spike induced mixing. In addition, the measured SiC depth distributions were correlated to the Si and C mixing profiles simulated by the TRIDYN code which can follow the ballistic ion mixing process as a function of ion fluence. Good agreement has been found between the distributions provided by AES depth profiling and TRIDYN on the assumption that the majority of the Si (C) atoms transported to the neighboring C (Si) layer form the SiC compound. The ion beam mixing process can be successfully described by ballistic atomic transport processes. The results show that SiC production as a function of depth can be predicted, and tailored compound formation on the nanoscale becomes feasible, thus leading to controlled synthesis of protective SiC coatings at room temperature.

Mechanism of ion-induced mixing phenomena in Gold–Nickel bilayer on Si substrate

Gold/Nickel bilayer thin films deposited on Si(100) substrates are bombarded by 100 keV 40Ar+, 260 and 300 keV 84Kr2+ and 400 keV 132Xe3+ ions which deposit maximum energy across the Gold/Nickel interface and hence produce maximum atomic transport within two sides of the interface. However, due to the energy of the projectile ions some Si atoms have also gained energy, which, in turn, displaces the atoms. The atomic displacements caused by the projectile in the system has been analyzed using RBS, XRD, SEM/EDS and AFM/MFM techniques. The relative change in the variance of the intermixed region across the Au–Ni–Si interfaces excluding the irradiation-induced surface roughness has been calculated and it was observed that it increases linearly with ion fluence. The measured athermal mixing rates which vary between 3.7 and 6.9 nm4, have been explained by various existing phenomenological models. It shows that local or global thermal spike models are most suitable for explaining the ion beam-induced mixing of systems with strong thermochemical properties of the constituents. The mixing efficiency of this system has been calculated and found to be 1.2 nm5/keV.

ChemInform Abstract: Organometallic Routes into the Nanorealms of Binary Fe‐Si Phases

AbstractReview: 115 refs.

Organometallic Routes into the Nanorealms of Binary Fe-Si Phases

The Fe-Si binary system provides several iron silicides that have varied and exceptional material properties with applications in the electronic industry. The well known Fe-Si binary silicides are Fe3Si, Fe5Si3, FeSi, α-FeSi2 and β-FeSi2. While the iron-rich silicides Fe3Si and Fe5Si3 are known to be room temperature ferromagnets, the stoichiometric FeSi is the only known transition metal Kondo insulator. Furthermore, Fe5Si3 has also been demonstrated to exhibit giant magnetoresistance (GMR). The silicon-rich β-FeSi2 is a direct band gap material usable in light emitting diode (LED) applications. Typically, these silicides are synthesized by traditional solid-state reactions or by ion beam-induced mixing (IBM) of alternating metal and silicon layers. Alternatively, the utilization of organometallic compounds with reactive transition metal (Fe)-carbon bonds has opened various routes for the preparation of these silicides and the silicon-stabilized bcc- and fcc-Fe phases contained in the Fe-Si binary phase diagram. The unique interfacial interactions of carbon with the Fe and Si components have resulted in the preferential formation of nanoscale versions of these materials. This review will discuss such reactions.

Ion mixing at 20 keV: A comparison of the effects of Ga +, Ar + and CF 4+ ion irradiation

Medium-energy (some tens of keV) ion irradiation is frequently used in various technologies. It is well known that during this irradiation serious alterations are introduced to the material, changing its structure, composition, etc. While there are studies on the amorphization, no results have been reported on the medium-energy ion beam-induced mixing, however. In this work, we present Auger electron spectroscopy (AES) depth profiling measurements of Si/Cr multilayer samples, which were irradiated by various ions (Ga +, Ar +, CF 4 +) of 20 keV applying angles of incidence of 5° (Ga +), 65° (Ga +) and 75° (Ar +, CF 4 +). The ion beam-induced mixing at the Si/Cr interface (the broadening of the interface) was measured as a function of the removed layer thickness. The weakest and strongest ion mixing (for a given removed layer thickness) were found for CF 4 + and Ga + 5° irradiations, respectively. In the case of Ga + irradiation, the larger the angle of incidence the weaker the ion mixing. The extent of mixing does not correlate with the corresponding projected range. Comparison of the experimentally measured ion mixed profiles with those given by dynamic TRIM simulations gave poor agreement for Ar + and fails for Ga + irradiations, respectively.

Temperature-dependent changes in structural and magnetic properties of heavy ion irradiated nanoscale Co/Pt multilayers

We report the changes in 100 keV Kr ion-irradiated nanoscale Co/Pt multilayers at different temperatures and fluences. We observe irradiation-induced changes in their structural and magnetic properties. Ion beam-induced mixing across the interfaces leads to the formation of a CoPt ordered/disordered phase. A large increase in the coercivity is also observed when the irradiation is performed at room temperature. Such an enhancement in the coercivity is attributed to defect clustering and CoPt ordered/disordered phase formation. The mechanism of phase formation is explained in light of the heat of formation rule.

Analysis of high- k HfO 2 and HfSiO 4 dielectric films

The impact of primary ion bombardment conditions on SIMS depth profiles through HfO 2 high- k films was investigated. Independent of the sputter beam conditions, Hf depth profiles show the apparent presence of Hf throughout the film, as well as tailing of the Hf signal into the substrate. A long tail region of decay length ∼10 nm seen for reactive and inert sputter beams, largely independent of the energy, is inconsistent with simple ion beam-induced mixing. Chemical state depth profiles by XPS show that Hf transforms into metallic Hf 0 in the Si substrate, corresponding to the tail regions of SIMS profiles. SIMS and XPS backside profiles confirm that Hf is not initially present in the Si substrate. Hf chemical state changes observed in XPS sputter depth profiles depend on the film stoichiometry, as seen with HfSi x O y films of different composition.

Modifications of magnetic properties of Pt/Co/Pt thin layers by focused gallium ion beam irradiation

We show how the magnetic properties of the Pt/Co ultrathin film structure can be modified and even controlled under uniform irradiation by Ga+ ions at low fluence in the 20–100 keV range. A systematic magneto-optical study is presented for the Pt/Co(1.4 nm)/Pt(111) ultrathin-film structure. At ion fluences below D=1014 Ga+/cm2, the coercive field is steadily reduced when increasing the fluence. At large fluences, in the range D=(5–10)×1014 Ga+ ions/cm2, the magnetization of the Co layer drops rapidly and the film finally becomes paramagnetic at room temperature for D>2×1015 Ga+ ions/cm2. We demonstrate that these magnetic changes are related to the effect of ion-induced collisional intermixing of the Co/Pt interfaces, leading to the formation of stable Co–Pt alloys with varying composition across the interfaces. A simple model is derived to relate the ion beam-induced mixing to the changes in magnetic properties. The present work allows us to gain a quantitative understanding of previous experiments using a focused Ga+ ion beam to pattern magnetic properties of similar samples at a sub-100 nm scale.

Ion beam mixing in Al/Fe multilayered thin films

Ion beam-induced mixing of Al/Fe multilayered thin films was studied by means of electron microscopy and Auger electron spectroscopy. Ion mixing was performed with Xe +-ions at room temperature with doses ranging from 5 × 10 15 to 3 × 10 16 ions cm 2 . The Auger concentration depth profiles showed that atomic mixing occurred at the interface and that oxygen at the interface reduced the mixing efficiency. An amorphous phase, solid solutions and different metastable and stable crystalline phases were obtained after atomic mixing in dependence on initial composition and thermal annealing conditions. The amorphous phase was transformed into the quasicrystalline state with sharp diffraction spots.

Secondary ion emission from transition metals during exposure to oxygen and subsequent sputtering

The interaction of oxygen with three transition metals (Ti, Ni, and Nb) was studied by means of high fluence secondary ion mass spectrometry. It was established that at high oxygen coverages secondary ion intensities during O2 exposure and subsequent sputter removal exhibited an almost identical evolution, indicating that secondary ions are emitted from similar structures in both stages. During sputter removal all measured ion signals I followed approximately an exponential dependence on the primary ion fluence φ, with I α exp (-σ, φ). This behaviour can be attributed to the collisional relocation of oxygen atoms in the metal substrate. The parameter Ym/σi (where Ym is the sputtering yield of the metal) characterizes the extent of ion beam-induced mixing for a given oxygen-metal system. For the same element, Ym/σi was found to depend slightly on the ion species monitored. Mean values (in atoms/cm2) were 5.2 × 1016 for Ni, 7.4 × 1016 for Nb, and 1.4 × 1017 for Ti.

Secondary ion emission from transition metals during exposure to oxygen and subsequent sputtering

The interaction of oxygen with three transition metals (Ti, Ni, and Nb) was studied by means of high fluence secondary ion mass spectrometry. It was established that at high oxygen coverages secondary ion intensities during O 2 exposure and subsequent sputter removal exhibited an almost identical evolution, indicating that secondary ions are emitted from similar structures in both stages. During sputter removal all measured ion signals I followed approximately an exponential dependence on the primary ion fluence φ, with I α exp (-σ, φ). This behaviour can be attributed to the collisional relocation of oxygen atoms in the metal substrate. The parameter Y m /σ i (where Y m is the sputtering yield of the metal) characterizes the extent of ion beam-induced mixing for a given oxygen-metal system. For the same element, Y m /σ i was found to depend slightly on the ion species monitored. Mean values (in atoms/cm 2) were 5.2 × 10 16 for Ni, 7.4 × 10 16 for Nb, and 1.4 × 10 17 for Ti.