Ion Beam Mixing Experiments Research Articles

On the lower energy limit of electronic stopping in simulated collision cascades in Ni, Pd and Pt

We have investigated the effect of the choice of kinetic energy threshold Tc for electronic stopping (Se) in molecular dynamics (MD) simulations of collision cascades. We find that while the choice of Tc has only a negligible effect in low energy cascades, it has a pronounced effect on the fraction of defects found in clusters in high energy cascades. In order to gain insight into the level of energy losses expected in cascades, we have simulated a large number of cascades in Ni, Pd and Pt, with primary knock-on atom (PKA) energies between 30eV and 200keV. Since the energy losses affect the total atomic displacements especially in high energy cascades, we use this measure to determine a reasonable value of Tc. By directly comparing the simulated mixing efficiency to ion beam mixing experiments, we conclude that a threshold of Tc=1eV is too low for simulations of high energy cascades in all materials considered. A value of Tc=10eV is more realistic, although for Pd and Pt, this results in a slight over-prediction of the mixing efficiency.

Glass forming abilities of the Fe–Ta–(Zr, Ti) alloys studied by thermodynamic calculation and ion beam mixing

Abstract Thermodynamic calculation and ion beam mixing experiments were carried out for five sets of multilayered films with compositions of Fe 40 Ta 20 (Zr x Ti 1 − x ) 40 ( x = 0, 0.25, 0.5, 0.75, 1). The results show that the experimental results agreed well with the prediction, i.e., unique amorphous phases could be formed in all the five sets of films, and by replacing Ti with appropriate amount of Zr, the glass forming ability of the alloy could be improved. Meanwhile, phase separation of amorphous phase was observed in the Fe 40 Ta 20 Zr 40 and other quaternary films. The possible mechanism of the amorphous phase formation and separation was discussed based on thermodynamic calculation and atomic collision theory.

Effects of Mo addition on the metallic glass formation of (ZrNi)100−xMox alloys

The formation of (ZrNi)100−xMox metallic glasses was predicated by thermodynamic calculation. It is found that the (ZrNi)100−xMox metallic glasses could be formed in the composition range of 0–64at.% Mo, and the minor addition of Mo could facilitate the formation of the (ZrNi)100−xMox metallic glasses. To verify thermodynamic prediction, ion beam mixing experiments were carried out. The results showed that no unique amorphous phase would be formed when the addition of Mo was higher than 60at.%, indicating that the thermodynamic prediction is reasonable. Besides, the possible formation mechanisms of these metastable phases were discussed in terms of phase competition and the atomic collision theory.

Effect of Y on the glass forming ability of the Fe–Nb system studied by ion beam mixing

Ion beam mixing experiments were carried out to investigate the effect of the Y addition on the glass forming ability and associated structural phase transformations of the Fe–Nb binary metal system. The results show that the addition of Y could extend the glass forming range of the Fe–Nb system from 25–75 at% Fe to 10–80 at% Fe, yet while the addition of Y exceeded 42 at%, the glass forming ability would be deteriorated. The effect of Y is considered to be attributed to the competition between the big size difference of the component metals and large positive heat of mixing of Y–Nb.

Synthesis of amorphous alloys and amorphous–crystalline composites in ternary Ni–Nb–Zr system by ion beam mixing

Six sets of Ni–Nb–Zr multilayered films were designed and prepared with the overall compositions of Ni63Nb28Zr9, Ni48Nb13Zr39, Ni29Nb60Zr11, Ni79Nb12Zr9, Ni31Nb31Zr38, and Ni31Nb11Zr58, and an ion beam mixing experiment was then conducted using 180 keV xenon ions. It is found that the Ni–Nb–Zr system is a readily glass-forming system, and both fully amorphous alloys and amorphous–crystalline composites could be synthesized. A detailed discussion was presented for the formation mechanism of the amorphous alloys and amorphous–crystalline composites. In addition, thermodynamic calculation predicted the glass-forming ability of the system, which is in good agreement with the IBM results. The experimental results and thermodynamic calculation obtained in present study are also supported e.g. by the observations of rapid solidification.

Cu-Zr-Nb Crystalline-Amorphous Composites Investigated by Thermodynamic Calculation and Ion Beam Mixing Experiments

The formation of Cu-Zr-Nb metallic glass was predicted by thermodynamic calculation and then five Cu-Zr-Nb ternary metallic multilayered films were designed and prepared by electron depositing. The metastable supersaturated solid solutions, amorphous phase as well as their composites were able to be obtained in these Cu-Zr-Nb metallic multilayered films upon ion beam mixing. The observations provided a clew to improve the ductibility of the metallic glasses. Some possible interpretations were presented concerning the formation of the crystalline-amorphous composite.

Non-equilibrium alloy phase formation and transformation driven by ion beam mixing in the Fe-Hf-Nb multilayers

In the ion beam mixing experiments, eight Fe-Hf-Nb multilayered films, with overall compositions of Fe67Hf22Nb11, Fe67Hf11Nb22, Fe54Hf38Nb8, Fe54Hf30Nb16, Fe54Hf11Nb35, Fe25Hf67Nb8, Fe25Hf50Nb25 and Fe25Hf11Nb64, were irradiated by 200 keV xenon ions to doses ranging from 3×1014 Xe+/cm2 to 7×1015 Xe+/cm2. The results showed that unique amorphous phases were obtained at designed alloy compositions, falling in the favored glass-forming region deduced from three binary metal sub-systems. Interestingly, at some alloy compositions, the crystal-amorphous-crystal transformations were observed back and forth while varying the irradiation doses. In addition, at the alloy composition of Fe25Hf67Nb8, a metastable FCC phase was formed through an HCP-FCC structural phase transformation and it had a large lattice constant identified to be a=4.51 A. Besides, the formation mechanism of non-equilibrium alloy phases was also discussed in terms of thermodynamics of solids and atomic collision theory.

Synthesis of amorphous alloys and amorphous-crystalline composites in the Cu-Nb-Hf system by ion beam mixing

Seven sets of Cu-Nb-Hf multilayered films were designed and prepared with the overall compositions of Cu21Nb65Hf14, Cu33Nb49Hf18, Cu34Nb34Hf32, Cu34Nb10Hf56, Cu50Nb23Hf27, Cu58Nb10Hf32, and Cu70Nb8Hf22, and an ion beam mixing experiment was then conducted using 200 keV xenon ions. It is found that the Cu-Nb-Hf system is a metallic glass forming one, and the single amorphous alloys could be synthesized in the Cu-Nb-based alloys with less than 18 at.% of Hf as a third addition. Also, when the Hf concentration is greater than 18 at.%, i.e., at the compositions of Cu34Nb34Hf32, Cu34Nb10Hf56, Cu50Nb23Hf27, Cu58Nb10Hf32, and Cu70Nb8Hf22, ion beam mixing resulted in the formation of amorphous-crystalline composites, which might have better mechanical properties than single-phase glassy alloys. In addition, a detailed discussion was presented for the formation mechanism of the amorphous alloys and amorphous-crystalline composites.

Proposed truncated Cu–Hf tight-binding potential to study the crystal-to-amorphous phase transition

Proposed truncated Cu–Hf tight-binding potential was constructed by fitting the physical properties of Cu, Hf, and their stable compounds, i.e., Cu5Hf, Cu8Hf3, Cu10Hf7, and CuHf2. Based on the constructed potentials, molecular dynamics simulations were carried out to compare the relative stability of the crystalline solid solution and the disordered state. Simulation results not only reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing when the solute atoms exceeding the critical concentration, but also predict that the glass forming range (GFR) of the Cu–Hf system is 21–77 at. % Cu, which covers the GFRs determined by various metallic glass-producing techniques. Ion beam mixing experiments of the Cu–Hf system were conducted using 200 keV xenon ions and the results show that a uniform amorphous phase can be obtained in the Cu23Hf77 sample, matching well with the GFR determined by the interatomic potential, which, in turn, provides additional evidence to the relevance of the constructed Cu–Hf potential.

Glass forming ability of the binary Cu–Hf system studied by thermodynamic calculation and ion beam mixing

Based on Miedema's semi-empirical model, a Gibbs free energy diagram is constructed for the Cu–Hf system and shows that the amorphous phase is favored to be formed over 8–94 at.% Hf, which is much broader than the experimentally determined 30–70 at.% Hf. Three Cu–Hf multilayered films were therefore designed with the overall compositions of Cu 93Hf 7, Cu 81Hf 19 and Cu 23Hf 77, respectively, and then an ion beam mixing experiment was conducted using 200 keV xenon ions to see whether the metallic glass forming range of the Cu–Hf system could be extended beyond the previously reported range. Interestingly, ion beam mixing results show that a uniform amorphous phase can be obtained in the Cu 23Hf 77 sample and the amorphous phase is also observed to coexist with crystalline Cu in the Cu 81Hf 19 sample. These results suggest that the metallic glass forming range of the Cu–Hf system could, at least, be extended to 77 at.% Hf, which falls in the range predicted by the Gibbs free energy diagram.

Ion beam mixing to study the metallic glass formation of the Cu–Zr system

An ion beam mixing experiment for Cu–Zr system was conducted and two supersaturated solid solutions were observed with compositions of 16 atom% Zr in Cu and 17 atom% Cu in Zr, respectively, which are much greater than almost nil found from the equilibrium phase diagram of Cu–Zr system. The observation indicates that Cu–Zr metallic glasses could possibly be obtained in composition range bounded by the two observed solid solubilities, i.e. 16–83 atom% Zr. Besides, a unique Cu 65Zr 35 metallic glass was obtained by ion beam mixing and its composition is very close to the so-called best composition referred in the literature.

Ion-induced transformations of a W–Si interface

Tungsten silicide formation in multilayer tungsten/silicon structure was investigated. The W–Si multilayers were deposited on thermally oxidized silicon wafers using the dual-target magnetron sputtering. Deposition of the whole stack of sublayers was carried out without breaking vacuum in order to eliminate contamination or oxidation of the interfaces between sublayers. Samples were annealed in the RTA furnace at temperatures ranging from 700 °C up to 1050 °C. Some of the structures were irradiated with argon ion beam before annealing. Reactions between sublayers were studied using SEM imaging of cross-sectional cleavages and by X-ray diffraction analysis. Influence of the irradiation with argon ion beam on structural transformations was investigated using RBS analysis. It has been found that tungsten silicide formation depends on the deposition sequence. The reaction was more effective on interfaces between silicon layer deposited on tungsten then on interface between tungsten deposited on silicon. Ion beam mixing experiment showed that ion–target interaction promotes formation of the WSi 2 phase.

Metastable phase formation and magnetic properties of the Fe–Nb system studied by atomistic modeling and ion beam mixing

With the aid of ab initio calculations, an n-body Fe–Nb embedded-atom potential is first constructed and then applied to study the crystal-to-amorphous phase transition through molecular dynamic simulations. The simulations determine that the glass-forming range of the Fe–Nb system is 18–83 at. % of Nb. In ion beam mixing experiments, five Fe–Nb multilayered films with overall compositions of Fe85Nb15, Fe75Nb25, Fe55Nb45, Fe25Nb75, and Fe15Nb85, respectively, are irradiated by 200 keV xenon ions to doses in the range of (1–7)×1015Xe+/cm2. The result shows that the Fe–Nb metallic glasses can be synthesized within a composition range of 25–75 at. % of Nb, matching reasonably well the theoretical prediction. Moreover, in the Fe55Nb45 sample, a fcc-structured alloy phase with a large lattice constant of a≈0.408 nm was obtained at a dose of 3×1015 Xe+/cm2 and the associated magnetic moment per Fe atom was measured to be 2.41μB. The observed magnetic moment is much greater than the initial value of 1.42μB in the bcc-Fe lattice and can thus serve as evidence confirming the high-spin ferromagnetic state of fcc Fe predicted by ab initio calculations. Interestingly, further irradiation induced phase separation in the Fe55Nb45 sample, i.e., irradiation to a dose of 5×1015 Xe+/cm2 results in the growth of a fractal pattern consisting of Fe72Nb28 nanoclusters embedded in Fe35Nb65 matrix. The formation mechanism of the metastable phases as well as that of the fractal pattern observed in the Fe–Nb system was discussed in terms of the atomic collision theory and the well-known cluster-diffusion-limited-aggregation model.

Amorphous phase formation, spinodal decomposition, and fractal growth of nanocrystals in an immiscible Hf–Nb system studied by ion beam mixing and atomistic modeling

In the equilibrium immiscible Hf–Nb system characterized by a positive heat of formation, five Hf–Nb metallic glasses with overall compositions of Hf84Nb16, Hf65Nb35, Hf45Nb55, Hf38Nb62, and Hf20Nb80 are obtained by ion beam mixing with properly designed Hf–Nb multilayered films, suggesting a glass-forming composition range of 16–80 at.% of Nb. For the special case of Hf45Nb55 located at the ridge point on the convex free energy curve, dual-glass phases are formed at a dose of 2×1015 Xe+/cm2, which results from a spinodal decomposition of the expected Hf45Nb55 amorphous phase. With increasing irradiation dose, fractal growth of nanocrystals (around 20 nm) appears in the major glass phase and the dimension is determined to be from 1.70 to 1.84 within a dose range of (4–7)×1015 Xe+/cm2. In atomistic modeling, a n-body Hf–Nb potential is first constructed with the aid of ab initio calculations. Applying the constructed potential, molecular dynamics simulations using the hcp and bcc solid solution models, reveals an intrinsic glass-forming range to be within 15–83 at.% of Nb, which is compatible with the ion beam mixing experiments. Moreover, the formation of the metallic glasses and the fractal growth in association with the amorphous spinodal decomposition are also discussed in terms of the atomic collision theory and cluster-diffusion-limited-aggregation model.

Atomistic mechanism of interfacial reaction and asymmetric growth kinetics in an immiscibleCu−Rusystem at equilibrium

An $n$-body potential is first constructed for the immiscible $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ru}$ system at equilibrium with the aid of ab initio calculations for obtaining some physical properties necessary to fit the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ru}$ cross-potential. Molecular dynamics simulations are then performed to pursue atomistic modeling of interfacial reaction between the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ru}$ metal layers. Using the solid solution model, simulations reveal that the Cu-based and the Ru-based solid solutions collapse at their respective critical solid solubilities, i.e., $10\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ of Ru and $20\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ of Cu, thus determining an intrinsic glass-forming range of the system to be within 10--$80\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%$ Ru, which matches well with the observations in ion-beam mixing experiments. Using the $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Ru}$ sandwich model, simulations clarify that the interfacial free energy is the major driving force for interfacial reaction, resulting in spontaneous solid-state amorphization and that when the interfacial free energy is completely consumed, the reaction terminates, thus determining a maximum amorphous interlayer to be $2.91\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, which is in good agreement with that predicted from a recently proposed thermodynamic and kinetic model. Kinetically, simulations further reveal that the growth of the amorphous interlayer features an asymmetric behavior, i.e., the growth advances faster toward the Cu lattice than toward the Ru side, because the critical solid solubility of Ru in Cu $(10\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%)$ is smaller than that of Cu in Ru $(20\phantom{\rule{0.3em}{0ex}}\mathrm{at}.\phantom{\rule{0.2em}{0ex}}%)$.

Nonequilibrium Solid Phase Formation Studied by Lattice Dynamics Calculation and Ion Beam Mixing in an Immiscible Co−Ag System

For the equilibrium immiscible Co-Ag system, a proven realistic ab initio derived n-body potential is applied to study the nonequilibrium solid phase formation at three chemical stoichiometries of Co/Ag = 1:3, 1:1, and 3:1. To predict the structural stability, the elastic constants and the phonon spectra are calculated at the chosen stoichiometries with a total of eight hypothetical crystalline structures. The calculated results suggest that four compounds, that is, D0(3) CoAg3, B1 CoAg, B2 CoAg, and D0(3) Co3Ag, are unstable, as they all feature negative elastic constants as well as imaginary phonons, and that another four compounds of both fcc-type L1(2) and hcp-type D0(19) structures at chemical stoichiometries of Co/Ag = 1:3 and 3:1, respectively, may elastically be favored and therefore obtainable under some specific conditions. It is also found that all the calculated elastic constants and phonon spectra are coincident within the framework of the elastic theory. Moreover, the calculated elastic constants are in good agreement with those acquired directly from ab initio calculations, lending support to the validity of the ab initio derived n-body Co-Ag potential as well as its resultant elastic constants and the phonon spectra. Interestingly, some of the predicted nonequilibrium solid phases, that is, two hcp-type compounds at chemical stoichiometries of Co/Ag = 1:3 and 3:1, respectively, are indeed obtained in ion beam mixing experiments and their lattice constants determined by diffraction analysis are in good agreement with those from calculations.

Metastable Phase Selection of an Immiscible Au–W System Studied by ab initio Calculation, Molecular Dynamics Simulation and Ion-Beam Mixing

With the aid of ab initio calculation, the lattice constants and cohesive energies of some ordered metastable Au–W compounds are calculated and used in deriving an n-body Au–W potential under the embedded atom method. On the basis of the proven realistic Au–W potential, molecular dynamics simulations predict the phase selection over the entire composition: the metastable Au100-xWx phase in an fcc structure is more stable than in a bcc structure when 0≤x≤43, whereas the bcc structure becomes energetically favored when 43<x≤100. The prediction is in good agreement with the results obtained from ion-beam mixing experiments, i.e., the bcc and fcc solid solutions are formed in the W-enriched and Au-enriched sides, respectively.

Heavy ion induced intermixing at Ta/Si and Ta/SiO2 interfaces

Systematic ion beam mixing experiments have been performed on Ta/SiO 2 and Ta/Si interfaces. Tantalum was evaporated on silicon and SiO 2 /silicon substrates by means of molecular beam epithaxy. Samples were subsequently irradiated with 500 keV Si, 100 keV C and 100 keV O ions to different ionic fluences. The irradiated samples were studied by Rutherford backscattering spectrometry (RBS) to obtain the depth profiles and the interdiffusion parameters at the interfaces. Atomic force microscopy (AFM) was used to examine the surface roughness before and after irradiation. Ballistic mixing was found in both systems. Additional contributions to diffusion parameters were thermal spike effects (Ta/Si) and chemical reactions (Ta/SiO 2 ). Radiation enhanced diffusion was found to be the dominant process above room temperature in both systems.

Heavy ion induced intermixing at Ta/Si and Ta/SiO2 interfaces

Systematic ion beam mixing experiments have been performed on Ta/SiO2 and Ta/Si interfaces. Tantalum was evaporated on silicon and SiO2/silicon substrates by means of molecular beam epithaxy. Samples were subsequently irradiated with 500 keV Si, 100 keV C and 100 keV O ions to different ionic fluences. The irradiated samples were studied by Rutherford backscattering spectrometry (RBS) to obtain the depth profiles and the interdiffusion parameters at the interfaces. Atomic force microscopy (AFM) was used to examine the surface roughness before and after irradiation. Ballistic mixing was found in both systems. Additional contributions to diffusion parameters were thermal spike effects (Ta/Si) and chemical reactions (Ta/SiO2). Radiation enhanced diffusion was found to be the dominant process above room temperature in both systems.

Ion beam characterization and engineering of strain in semiconductor multi-layers

The objective of this work is to measure and engineer the strain in III–V compound semiconductor multi-layers using ion beams, which leads to spatial band-gap tuning for the integration of optoelectronics devices. Strained layer superlattices have been of unique interest due to their ability to tune the band gap, which depends on the strain at the interface. Therefore the strain measurements at the interface are of great interest. A large area two-dimensional positions sensitive Δ E− E detector telescope and a fully automated high energy channeling facility have been developed at NSC, New Delhi. An overview of the series of ERDA, RBS, Channeling, HRXRD and ion beam mixing experiments performed in this direction will be discussed in this paper.