Effects Of Low-energy Ion Irradiation Research Articles

Phase composition of polycrystalline HfNx (0.45 ≤ x ≤ 1.60) and effects of low-energy ion irradiation on microstructure, texture, and physical properties

We have investigated the phase composition of HfNx as a function of x and the effects of low-energy ion irradiation on the microstructure and physical properties of polycrystalline layers grown on SiO2 at 350 °C by ultrahigh vacuum reactive dc magnetron sputtering of Hf in mixed N2/Ar discharges. X-ray diffraction and Rutherford backscattering spectrometry results show that the phases obtained in polycrystalline HfNx layers with increasing x are hcp-structure α-Hf:N (x ≲ 0.6); multiphase mixtures consisting of α-Hf, NaCl-structure δ-HfN, rhombohedral ɛ-Hf3N2, and/or ζ-Hf4N3 (0.6 ≲ x ≲ 0.9); δ-HfN single phase (0.9 ≲ x ≲ 1.3); and mixtures of δ-HfN and higher nitrides (x ≳ 1.3). HfNx layers with 0.9 ≲ x ≲ 1.2 grown under mild ion irradiation (incident ion energy Ei ≃ 7 eV and ion-to-Hf flux ratios Ji/JHf = 1−3) are underdense with mixed orientation, low in-plane stress, and rough surface morphology due to limited adatom mobilities resulting in kinetic roughening and atomic shadowing during film growth. However, the use of intense ion irradiation (Ei = 25 eV and Ji/JHf = 4−20) results in HfNx layers, which are fully dense with strongly 111-oriented texture, compressive in-plane stress, and smooth surfaces due to ion irradiation enhanced adatom surface mobilities. In addition, the latter films have lower resistivity and higher hardness. For stoichiometric δ-HfN layers, ρ decreases from 69.7 to 35.2 μΩ cm and H increases from 22.1 to 27.4 GPa, with increasing ion-irradiation intensity. However, for HfNx layers with 1.2 ≲ x ≲ 1.6, the correspondingly higher steady state atomic N surface coverages during deposition alter growth kinetics in favor of 001 texture with a fully dense structure and compressive in-plane stress.

Ion excitation and etching effects on top-surface properties of sp2 nanocrystallited carbon films

We proposed the low energy ion irradiation effects (ion excitation and ion etching) on the formation and the top-surface properties of sp2 nanocrystallited carbon films in electron cyclotron resonance (ECR) plasma sputtering system. In this work, the ion etching rate during film deposition was measured and a threshold voltage for physical etching was found to be about 35 V. Below the threshold voltage, the film growth was induced by ion excitation effect (ion exciting electrons of top-most atoms), as a result, the sp2 dominant nanocrystallite size in the films was large. When the ion energy exceeded the threshold voltage, the film growth was induced by ion etching effect (ion breaking covalent bond), and the nanocrystallite size was small confirmed by the transmission electron microscopy (TEM) observation and Raman analysis. Furthermore, the top-surface properties of sp2 nanocrystallite carbon films prepared with different ion irradiation effects were examined by using the atomic force microscopy (AFM). Results showed that the ion etched carbon films had smaller surface roughness, scratch depth and adhesive force compared with the ion excited carbon films. These findings shed light on the fabrication and applications of carbon based nanostructured materials.

Encapsulating Ion-Solid Interactions in Metal-Oxide-Semiconductor (MOS) Devices

We report on a measurement of low energy ion irradiation effects on as-grown films of ${\rm SiO}_2$ on a Si substrate. Beams of normally incident ${\rm Na}^ + $ ions with kinetic energies of 2 keV to 5 keV were focused onto $ \sim 1900 {\rm \AA}$ ${\rm SiO}_2$ films. Aluminum top metal contacts were subsequently deposited onto these targets such that irradiated regions and unexposed (pristine) regions of the target could be compared using capacitance–voltage (C–V) measurements of individual metal-oxide-semiconductor (MOS) devices. The C–V data reveal an energy-dependent shift in the flatband voltage ( ${V_{\rm FB}}$ ) that can be returned to its near-pristine value by a low temperature anneal. An increase in the density of interface states ( ${D_{\rm it}}$ ) inferred from the C–V curves is found to have a superlinear dependence on the incident kinetic energy. These data are consistent with previously observed UV radiation effects on MOS oxides, where transferred energy leads to electron-hole pair production and the diffusion and trapping of holes throughout the oxide. Our measured trapped hole densities are compared with calculated densities, which are based on the incident ion dose and the predicted ion implantation range, to arrive at a fractional yield for hole survival and measurement within an encapsulated MOS device.

Contribution of core-loss fine structures to the characterization of ion irradiation damages in the nanolaminated ceramic Ti3AlC2

The effect of low-energy ion irradiation on the nanolaminated Ti3AlC2 is investigated by means of X-ray diffraction, transmission electron microscopy, electron energy loss and X-ray absorption spectroscopy. The chemical sensitivity and local order probing from core-loss edges provide new insights into the structural modifications induced under irradiation. From the analysis of the C K energy loss near-edge structure and Al K X-ray absorption near-edge structure by ab initio calculations, the influence of the layered structure of this compound on the irradiation damage is demonstrated, and damage is found to be preferentially localized in the aluminum planes of the structure. On the basis of comparisons between calculations and experimental spectra, a structural model is proposed for the irradiated state. This study emphasizes the utility of core-loss fine structure analysis to enhance understanding of ion irradiation-induced damage in complex crystalline materials.

The low-energy ion range in DNA

In fundamental studies of low-energy ion irradiation effects on DNA, calculation of the low-energy ion range, an important basic physical parameter, is often necessary. However, up to now a unified model and approach for range calculation is still lacking, and reported data are quite divergent and thus unreliable. Here we describe an approach for calculation of the ion range, using a simplified mean-pseudoatom model of the DNA target. Based on ion stopping theory, for the case of low-energy (⩽ a few keV) ion implantation into DNA, the stopping falls in the low reduced energy regime, which gives a cube-root energy dependence of the stopping (E1/3). Calculation formulas of the ion range in DNA are obtained and presented to unify the relevant calculations. The upper limits of the ion energy as a function of the atomic number of the bombarding ion species are proposed for the low-energy case to hold. Comparison of the results of this approach with the results of some widely used computer simulation codes and with results reported by other groups indicates that the approach described here provides convincing and dependable results.

Effect of Low-Energy Ion Irradiation on Synthesis of Hard and Superhard Films

This article summarizes briefly our recent research on low-temperature synthesis of TiN, TiN/Cu and TiN/Si films by using inductively coupled plasma assisted magnetron sputtering method. It is shown that the incorporation of high-flux low-energy ion irradiation during deposition strongly affects film growth, structure evolution, morphology and mechanical properties. A main attention is devoted to the synthesis of superhard nanocomposite films at a low deposition temperature. In both TiN/Cu and TiN/Si films the maximum hardness reaches a value higher than 40 GPa.

Low-energy ion irradiation effects on hydrogen absorption and desorption in carbon nanotubes

Due to their unique structure, which provides a large ratio of surface area to volume, carbon nanotubes (CNTs) may prove to be ideal materials for hydrogen storage. Typically, hydrogen is stored in CNTs by exposure of the material to a high-pressure H 2 atmosphere at various temperatures. The maximum hydrogen concentrations stored following this method and measured using Ion Beam Analysis do not exceed 1 wt.%. Introduction of defects by ion irradiation prior to high-pressure H 2 treatment, offers an alternative method to activate H adsorption and enhance the chemisorption of hydrogen. In this work, a low-energy H − beam at 13.5 keV was used to irradiate single-wall nanotubes (SWNTs) and to produce defects in SWNTs to enhance hydrogen adsorption. Hydrogen desorption measurements are presented, which compare the behavior in irradiated and as-prepared SWNTs after hydrogen storage treatment. These desorption measurements showed a large enhancement of hydrogen desorption for irradiated samples.

Development of preferred orientation in polycrystalline NaCl-structure δ-TaN layers grown by reactive magnetron sputtering: Role of low-energy ion surface interactions

We have investigated the effects of low-energy ion irradiation on texture evolution during growth of B1–NaCl-structure polycrystalline δ-TaN layers on SiO2 by ultra-high-vacuum reactive magnetron sputter deposition at 350 °C in mixed Ar+15%N2 discharges. In parallel sets of experiments, the ion-to-metal flux ratio Ji/JTa was varied from 1.3 to 10.7 while maintaining the incident ion energy Ei constant at 20 eV, and Ei was varied from 8.5 to 30 eV with Ji/JTa=10.7. All TaN layers, irrespective of Ji/JTa or Ei, were overstoichiometric with N/Ta=1.13±0.03. Layers grown with Ei=30 eV are multiphase consisting of hexagonal ε-TaN and δ-TaN, while those grown with Ei⩽20 eV are single-phase δ-TaN. With Ei=8.5 eV, the δ-TaN layers have a mixed 111/002/022/113 texture, even for film thicknesses t up to 500 nm. In contrast, δ-TaN layers deposited with Ei=20 eV initially exhibit competitive texture evolution until a single texture dominates at t≳200 nm. The preferred orientation of 500-nm-thick Ei=20 eV layers can be selectively and continuously varied from predominantly 111 to nearly complete 002 by increasing Ji/JTa from 1.3 to ⩾ 7.4. The change in texture is primarily due to an increased steady-state atomic N coverage, resulting from collisionally induced dissociative chemisorption of incident energetic N2+ ions, with increasing Ji/JTa. 111-oriented δ-TaN layers are underdense with both inter- and intracolumnar porosity and a tensile stress of ≃ 1.4 GPa, while 002-oriented layers are fully dense and have small compressive stresses, ≲ 0.7 GPa.

Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties

CrN layers, 0.5 μm thick, were grown on MgO(001) at Ts=570–775 °C by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mTorr. Layers grown at Ts⩽700 °C are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)CrN||(001)MgO with [100]CrN||[100]MgO. At higher temperatures, N2 desorption during deposition results in understoichiometric polycrystalline films with N fractions decreasing to 0.35, 0.28, and 0.07 with Ts=730, 760, and 775 °C, respectively. The surface morphologies of epitaxial CrN(001) layers were found to depend strongly on the incident ion-to-metal flux ratio JN2+/JCr which was varied between 1.7 and 14 with the ion energy maintained constant at 12 eV. The surfaces of layers grown with JN2+/JCr=1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal 〈100〉 directions. The mounds have an average peak-to-valley height 〈h〉=5.1 nm and an in-plane correlation length of 〈d〉=0.21 μm. The combination of atomic shadowing by the mounds with low adatom mobility results in the formation of nanopipes extending along the growth direction. Increasing JN2+/JCr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with 〈h〉=2.5 nm and 〈d〉=0.52 μm. Correspondingly, the nanopipe density decreases from 870 to 270 μm−2 to <20 μm−2 as JN2+/JCr is increased from 1.7 to 6 to 10. The hardness of dense CrN(001) is 28.5±1 GPa, but decreases to 22.5±1 GPa for layers containing significant nanopipe densities. The CrN(001) elastic modulus, 405±15 GPa, room-temperature resistivity, 7.7×10−2 Ω cm, and relaxed lattice constant, 0.4162±0.0008 nm, are independent of JN2+/JCr.

Molecular dynamics study of Cu deposition on Mo and the effects of low-energy ion irradiation

Molecular dynamics simulations were performed to study the growth of Cu films on a Mo(1 1 0) substrate, both by thermal evaporation and by ion-beam assisted deposition (100 eV Ar assistance). The growth mode of the Cu films goes to a 2D–3D–2D transition; the evolving microstructures are also discussed. The influence of the energetic ion impacts is significant, notably affecting wetting phenomena, but also leading to a different film structure. The effects of 75 eV He ion irradiation of a few completed films are also investigated, to help assessing the information on the defect state of the deposited films that is provided by thermal helium desorption experiments.

Influence of Low-Energy Ion Irradiation on Plasma Membrane Permeability of Cells

Effect of low-energy ion irradiation on plasma membrane permeability has been investigated by using electron spin resonance (ESR) spectroscopy of spin probe technique. The investigated system is plumule cells of wheat (Triticum aestivum L.) seeds implanted by 30 keV N+ ions. ESR spectra indicated that plasmalemma permeability is sensitive to low-energy ion irradiation. Ion irradiations with increasing fluences up to semi-lethal dose lead to gradual increase in plasmalemma permeability of the plumule cells. The possible factors relevant to the changes in membrane permeability are discussed in relation to the changes in the physical state and chemical nature of membranes.

Average energy deposited per atom: A universal parameter for describing ion-assisted film growth?

The average energy deposited per atom, 〈Ed〉=Ei(Ji/JMe), where Ei is the ion energy and Ji/JMe is the ratio of the accelerated-ion to deposited-thermal-particle fluxes incident at the growing film, has been shown to be one of a set of parameters useful for describing the effects of low-energy ion irradiation on film microstructure during ion-assisted deposition. Recently, however, 〈Ed〉 has often been treated as if it were a fundamental, or universal, parameter. We have carried out experiments in which Ei (20–100 eV) and Ji/JMe (1–10) were varied independently during the deposition, at constant temperature, of polycrystalline Ti0.5Al0.5N films onto amorphous SiO2 substrates by ultrahigh vacuum reactive magnetron sputtering in pure nitrogen. Ion-irradiation-induced changes in film microstructure, texture, phase composition, and nitrogen-to-metal ratio were found to follow distinctly different mechanistic pathways depending upon whether Ei or Ji/JMe was varied, resulting in quite different properties for the same value of 〈Ed〉. Thus, 〈Ed〉 is clearly not a universal parameter.

Modifications of structure and magnetism of Ce/Fe multilayers induced by irradiation with low-energy Ar ions

We demonstrate the strong effect of low-energy ion irradiation during the growth process on the structural and magnetic properties of Ce/Fe multilayers prepared by ion-beam sputtering in an UHV system. The heterostructures show good periodicity and sharp concentration profiles at the interfaces. For the non-irradiated samples, Fe grows in an amorphous structure below a critical Fe-layer thickness of 24 Å. In this low-thickness range, the spontaneous magnetization M S(0) and the ferromagnetic Curie temperature T C are anomalously small. Irradiation of the growing films reduces the critical thickness for amorphous growth of the Fe layers. The main consequence is the removal of the anomaly of M S(0) inherent to the amorphous Fe phase in the non-irradiated multilayers.