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ABSTRACT: The effect of high electronic energy deposition in amorphous germanium has been studied experimentally by Au irradiation with ion energies of up to 185 MeV and different angles of incidence and by molecular dynamics computer simulations. In both cases, the energy deposition leads to void formation accompanied by strong swelling of the amorphous germanium. The simulation results prove that the formation of the voids is mainly based on a shock wave mechanism and the swelling is determined by the competing processes of the formation and growth of voids on the one hand and the shrinking and annihilation of voids on the other hand. In full agreement between experiment and simulation, the amount of the swelling is a linear function of the total energy deposited into electronic processes and there exists a threshold value of the electronic energy loss per ion and depth for swelling. A comparison of the threshold values obtained by the experiment and the simulation suggests that approximately 20% of the energy deposited into electronic processes is converted into atomic motion.
Phys. Rev. B. 06/2011; 83(22).
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ABSTRACT: We report on the effects of swift heavy ion irradiation of embedded Pt nanocrystals (NCs), which change from spheres to prolate spheroids to rods upon irradiation. Using a broad range of ion irradiation energies and NC mean sizes we demonstrate that the elongation and dissolution processes are energy and size dependent, attaining comparable levels of shape transformation and dissolution upon a given energy density deposited in the matrix. The NC shape transformation remains operative despite discontinuous ion tracks in the matrix and exhibits a constant threshold size for elongation. In contrast, for ion irradiations in which the ion tracks are continuous, the threshold size for elongation is clearly energy dependent.
Journal of Physics D Applied Physics 03/2011; 44(15):155401. · 2.54 Impact Factor
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ABSTRACT: The structural properties and H desorption from embedded Pt nanocrystals (NCs) following irradiation with swift heavy ions were investigated as a function of energy and fluence. From x-ray absorption near-edge spectroscopy analysis, Pt–H bonding was identified in NCs annealed in a forming gas (95% N2 + 5% H2) ambient. The H content decreased upon irradiation and the desorption process was NC-size dependent such that larger NCs required a higher fluence to achieve a H-free state. Pt–H bonding and NC dissolution both perturbed the NC structural parameters (coordination number, bond-length and mean-square relative displacement) as determined with extended x-ray absorption fine structure measurements.
Journal of Physics D Applied Physics 03/2011; 44(15):155402. · 2.54 Impact Factor
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ABSTRACT: The temperature-dependent evolution of atomic vibrations in crystalline and amorphous InP has been studied using extended x-ray absorption fine-structure (EXAFS) spectroscopy. Measurements were performed at the In K edge for temperatures in the range of 20–295 K. In crystalline InP, the first nearest-neighbor (NN) EXAFS Debye-Waller factor, representative of the correlated mean-square relative displacement (MSRD) parallel to the bond direction, is considerably smaller than the uncorrelated mean-square displacement (MSD) determined from x-ray diffraction measurements. In contrast, the MSRD perpendicular to the bond direction agrees well with the MSD. This clearly demonstrates that vibrations of two neighboring atoms relative to each other are strongly reduced along the bond direction but are unhindered perpendicular to it, consistent with the well-known behavior of III-V semiconductors where bond bending is energetically favored over bond stretching. With increasing interatomic distance, the correlation of atomic motion quickly vanishes as manifested by increased EXAFS Debye-Waller factors. For the third NN shell the value closely approaches the MSD demonstrating the nearly uncorrelated motion of atoms only three shells apart. In the amorphous phase, only information about the first NN shell is accessible although the latter is now comprised of both P and In atoms. The EXAFS Debye-Waller factors are significantly higher than in the crystalline phase but exhibit a very similar temperature dependence. This results from strongly increased structural disorder in the amorphous phase whereas the thermally induced disorder is very similar to that in crystalline InP. A correlated Einstein model was fitted to the Debye-Waller factors yielding Einstein temperatures that vary as functions of the vibrational phase difference and reduced mass of the atomic pair, and represent a measure of the strength and thermal evolution of the corresponding relative vibrations.
Phys. Rev. B. 05/2009; 79(19).
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ABSTRACT: Swift heavy ion (SHI) irradiation of amorphous Si (a-Si) at non-perpendicular incidence leads to non-saturable plastic flow. The positive direction of flow suggests that a liquid phase of similar density to that of the amorphous solid must exist and accordingly a-Si behaves like a conventional glass under SHI irradiation. For room-temperature irradiation of a-Si, plastic flow is accompanied by swelling due to the formation of voids and a porous structure. For this paper, we have investigated the influence of SHI irradiation at room temperature on amorphous Ge (a-Ge), the latter produced by ion implantation of crystalline Ge substrates. Like a-Si, positive plastic flow is apparent, demonstrating that liquid polymorphism is common to these two semiconductors. Porosity is also observed, again confined to the amorphous phase and the result of electronic energy deposition. Enhanced plastic flow coupled with a volume expansion is clearly responsible for the structural modification of both a-Si and a-Ge irradiated at room temperature with swift heavy ions.
Journal of Physics D Applied Physics 04/2009; 42(11):115402. · 2.54 Impact Factor
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ABSTRACT: The vibrational and thermal properties of embedded Pt nanocrystals (NCs) have been investigated with temperature-dependent extended x-ray absorption fine structure (EXAFS) spectroscopy. NCs of diameter 1.8-7.4 nm produced by ion implantation in amorphous SiO(2) were analysed over the temperature range 20-295 K. An increase in Einstein temperature (∼194 K) relative to that of a Pt standard (∼179 K) was evident for the smallest NCs while those larger than ∼2.0 nm exhibited values comparable to bulk material. Similarly, the thermal expansion of interatomic distances was lowest for small NCs. While the amorphous SiO(2) matrix restricted the thermal expansion of interatomic distances, it did not have a significant influence on the mean vibrational frequency of embedded Pt NCs. Instead, the latter was governed by finite-size effects or, specifically, capillary pressure.
Journal of Physics Condensed Matter 04/2009; 21(15):155302. · 2.55 Impact Factor
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ABSTRACT: The transformation of Au nanoparticles (NPs) embedded in SiO2 from spherical to rod-like shapes induced by swift heavy ion irradiation has been studied. Irradiation was performed with 197Au ions at energies between 54 and 185 MeV. Transmission electron microscopy and small angle x-ray scattering measurements reveal an energy dependent saturation width of the NP rods as well as a minimum size required for the NPs to elongate. The NP saturation width is correlated with the ion track diameter in the SiO2. NP melting and in-plane strain in the irradiated SiO2 are discussed as potential mechanisms for the observed deformation.
Applied Physics Letters 03/2009; 94(11):113107-113107-3. · 3.84 Impact Factor
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ABSTRACT: The growth and structure of Pt nanocrystals (NCs) formed by ion implantation in a- SiO <sub>2</sub> has been investigated as a function of the annealing conditions. Transmission electron microscopy and small-angle x-ray scattering measurements demonstrate that the annealing ambient has a significant influence on NC size. Samples annealed in either Ar, O <sub>2</sub> , or forming gas (95% N <sub>2</sub> : 5% H <sub>2</sub> ) at temperatures ranging from 500 ° C –1300 ° C form spherical NCs with mean diameters ranging from 1–14 nm. For a given temperature, annealing in Ar yields the smallest NCs. O <sub>2</sub> and forming gas ambients produce NCs of comparable size though the latter induces H chemisorption at 1100 ° C and above, as verified with x-ray absorption spectroscopy. This H intake is accompanied by a bond-length expansion and increased structural disorder in NCs of diameter ≫3 nm .
Journal of Applied Physics 03/2009; · 2.17 Impact Factor
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P Kluth, C S Schnohr,
O H Pakarinen,
F Djurabekova,
D J Sprouster,
R Giulian,
M C Ridgway,
A P Byrne,
C Trautmann,
D J Cookson,
K Nordlund,
M Toulemonde
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ABSTRACT: We report on the observation of a fine structure in ion tracks in amorphous SiO2 using small angle x-ray scattering measurements. Tracks were generated by high energy ion irradiation with Au and Xe between 27 MeV and 1.43 GeV. In agreement with molecular dynamics simulations, the tracks consist of a core characterized by a significant density deficit compared to unirradiated material, surrounded by a high density shell. The structure is consistent with a frozen-in pressure wave originating from the center of the ion track as a result of a thermal spike.
Physical Review Letters 11/2008; 101(17):175503. · 7.37 Impact Factor
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ABSTRACT: InP was amorphized by ion irradiation in two very different regimes: (i) 185 MeV Au irradiation, where the energy loss was predominantly via inelastic processes (electronic stopping), or (ii) Se irradiation in an energy range of 0.08–7 MeV, where elastic processes (nuclear stopping) were dominant. The structural parameters of the amorphous phase were determined for as-irradiated and thermally relaxed samples using extended x-ray absorption fine structure spectroscopy. Despite the fundamentally different energy transfer mechanisms, no significant difference in the atomic structure of the two amorphized samples was observed. We attribute this result to a common “melt and quench” process responsible for amorphization. In fact, the measured structural parameters for the amorphized samples, including the fraction of homopolar In-In bonding, were consistent with simulations of the amorphous phase produced by assuming a quench from the melt.
Phys. Rev. B. 02/2008; 77(7).
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ABSTRACT: In this paper we present preliminary yet promising results on the measurement of latent ion tracks in amorphous, 2 μm thick SiO2 layers using small angle X-ray scattering (SAXS). The tracks were generated by ion irradiation with 89 MeV Au ions to fluences between 3 × 1010 and 3 × 1012 ions/cm2. Transmission SAXS measurements show distinct scattering from the irradiated SiO2 as compared to the unirradiated material. Analysis of the SAXS spectra using a cylindrical model suggests a core–shell like density distribution in the ion tracks with a lower density core and a higher density shell as compared to unirradiated material. The total track radius of ∼48 Å is in very good agreement with previous experiments and calculations based on an inelastic thermal spike model.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 266:2994-2997. · 1.21 Impact Factor
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ABSTRACT: Damage formation in InP, GaP, InAs, GaAs, and the related ternary alloys Ga0.50In0.50P and Ga0.47In0.53As irradiated at room temperature with 185 MeV Au ions was studied using Rutherford backscattering spectroscopy in channeling configuration, transmission electron microscopy, and small-angle x-ray scattering. Despite nearly identical ion-energy loss in these materials, their behavior under swift-heavy-ion irradiation is strikingly different: InP and Ga0.50In0.50P are readily amorphized, GaP and GaAs remain almost undamaged and InAs and Ga0.47In0.53As exhibit intermediate behavior. A material-dependent combination of irradiation-induced damage formation and annealing is proposed to describe the different responses of the III-V materials to electronic energy loss.
Phys. Rev. B. 81(7).
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ABSTRACT: Silica-embedded Ge nanoparticles (NPs) of different sizes irradiated with swift heavy ions (SHIs) at a given energy may reportedly elongate along the incident ion direction, perpendicular to it, or not at all. Here, for a given NP size distribution, we have investigated the SHI energy dependence of the elongation process. Higher-energy irradiation generally yielded elongation along the ion track (as previously observed), but for lower-energy irradiation, elongation both parallel and perpendicular to the ion direction was observed. We demonstrate that NP size and electronic energy loss together govern the elongation process, reinforcing the proposed model where elongation perpendicular to the ion direction is only expected for Ge NPs bigger than the mean ion track diameter in silica. Here, a wide fluence range is also probed, enabling us to follow in more detail the transition from spherical unirradiated Ge NPs to Ge NPs elongated either parallel or perpendicular to the ion beam. X-ray absorption spectroscopy (XAS) measurements are utilized for the quantification of crystalline, amorphous, and oxidized environments around Ge atoms. Combining such results with transmission electron microscopy (TEM) observations shows the Ge NPs are rendered amorphous prior to elongation, potentially via a melt-and-quench process. Thereafter, stronger electron-phonon coupling in amorphous Ge compared to crystalline Ge may potentially influence the elongation process. The Ge NP amorphization occurs at lower fluences for higher irradiation energies, indicating electronic energy loss—and not ballistic effects—governs the amorphization. Subsequent to amorphization and elongation, TEM and XAS results also show the NPs gradually intermix with SiO2 and dissolve within the matrix as the irradiation fluence increases. We discuss the impact of such results in the ion beam tailoring of Ge NPs for technological applications.
Phys. Rev. B. 85(23).
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ABSTRACT: Swift heavy-ion (SHI) irradiation of amorphous germanium (a-Ge) layers leads to a strong volume expansion accompanied by a nonsaturating irreversible plastic deformation (ion hammering), which are consequences of the high local electronic energy deposition within the region of the a-Ge layer. We present a detailed study of the influence of SHI irradiation parameters on the effect of plastic deformation and structural modification. Specially prepared a-Ge layers were irradiated using two SHI energies and different angles of incidence, thus resulting in a variation of the electronic energy deposition per depth εe between 14.0 and 38.6 keV nm-1. For all irradiation parameters used a strong swelling of the irradiated material was observed, which is caused by the formation and growth of randomly distributed voids, leading to a gradual transformation of the amorphous layer into a sponge-like porous structure as established by cross-section scanning electron microscopy investigations. The swelling depends linearly on the ion fluence and on the value of εe, thus clearly demonstrating that the structural changes are determined solely by the electronic energy deposited within the amorphous layer. Plastic deformation shows a superlinear dependence on the ion fluence due to the simultaneous volume expansion. This influence of structural modification on plastic deformation is described by a simple approach, thus allowing estimation of the deformation yield. With these results the threshold values of the electronic energy deposition for the onset of both structural modification and plastic deformation due to SHI irradiation are determined. Furthermore, based on these results, the longstanding question concerning the reason for the structural modification observed in SHI-irradiated crystalline Ge is answered.
Phys. Rev. B. 83(5).
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ABSTRACT: Extended x-ray absorption fine structure spectroscopy was used to measure the interatomic distance distributions of the first three nearest-neighbor (NN) shells around Ga and In atoms in Ga1−xInxP. The first NN shell has a composition-dependent bimodal distance distribution with a relaxation parameter of ϵ=0.80±0.04 similar to other III-V ternary alloys. The second NN shell distance distribution remains multimodal, corresponding to the three different cation-cation pairs but is closer to the virtual-crystal approximation (VCA). The third NN shell mean distance is well approximated by the VCA although the distribution is significantly broadened. Predictive model calculations are discussed in detail where good agreement with experimental results is found. Like in Ga1−xInxAs, lattice mismatch is accommodated in Ga1−xInxP by both bond-length and bond-angle relaxations although primarily via the latter.
Phys. Rev. B. 78(11).
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ABSTRACT: Changes in the shape and size of Co, Pt and Au nanoparticles induced by swift heavy-ion irradiation (SHII) have been characterized using a combination of transmission electron microscopy, small-angle X-ray scattering and X-ray absorption near-edge structure. Elemental nanoparticles of diameters 2–15 nm were first formed in amorphous SiO2 by ion implantation and thermal annealing and then irradiated at room temperature with 27–185 MeV Au ions as a function of fluence. Spherical nanoparticles below a minimum diameter (4–7 nm) remained spherical under SHII but progressively decreased in size as a result of dissolution into the SiO2 matrix. Spherical nanoparticles above the minimum diameter threshold were transformed to elongated rods aligned with the ion beamdirection. The nanorod width saturated at an electronic energy deposition dependent value, progressively increasing from 4–6 to 7–10 nm (at 5–18 keV/nm, respectively) while the nanorod length exhibited a broad distribution consistent with that of the unirradiated spherical nanoparticles. The threshold diameter for spherical nanoparticle elongation was comparable to the saturation value of nanorod width. We correlate this saturation value with the diameter of the molten track induced in amorphous SiO2 by SHII. In summary, changes in nanoparticle shape and size are governed to a large extent by the ion irradiation parameters.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms.
