Function Of Ion Species Research Articles

Converting polycrystals into single crystals – Selective grain growth by high-energy ion bombardment

Common failure mechanisms in microelectronics such as electromigration, creep and fatigue can be positively influenced by microstructure optimization. In this paper a new mechanism of microstructure optimization in thin metal films is proposed. Post-deposition ion bombardment can produce an in-plane texture in originally highly fiber textured thin metal films by a selective grain growth process. In extreme cases the in-plane texture becomes as sharp as the out-of-plane fiber texture. A subset of grains oriented for ion channeling was found to grow significantly at the expense of the remaining grain fraction. We studied the selective grain growth as a function of ion species (N +, Ne +, Ar +), ion energy (1–3.5 MeV) and target temperature (liquid nitrogen to 400 °C). In a textured thin film the degree of preferred in-plane orientation can be strongly influenced by ion bombardment, and therefore this technique has the potential to become a powerful tool for the enhancement of reliability in micro- and nanosystems.

Production of Nanowires on Semiconductor Surfaces by Ion Bombardment

Low energy (typically in the keV regime) ion bombardment of some semiconductor materials can lead to the development of regularly spaced ripples on the surfaces, which with suitable doping procedures, can be employed as nanowires. Although these ripples are reminiscent of those induced by the action of wind on sand dunes or by water currents on the seabed, the mechanisms are different. The formation of the bombardment-induced ripples depends primarily on the material sub-strate. The average distance between the ripples, i.e. ripple wavelength is also a function of ion spe-cies, ion energy, angle of incidence, ion areic dose and substrate temperature. In this paper, some of the reported results on the effect of areic dose on ripples produced by noble gas, oxygen and nitrogen ion bombardment on selected semiconductor materials will be tested against Bradley-Harper theory. Some of the ripple properties can be explained by this analytic theory. The analysis suggests that room temperature bombardment of silicon might be transition regime between those of a low ion areic dose rate at high temperature and a high areic dose rate at low temperature bombardment. For a full understanding of bombardment-induced topography on semiconductor surfaces at room temperature, one will have to resort to the more advances theories, which can only be solved numerically, with all it associated difficulties.

HREM and Image Simulation of Short-Range-Order Domains in Ion Irradiated Zirconolite

Abstract Zirconolite (CaZrTi2O7) is an important phase proposed for immobilization of plutonium. Radiation effects in zirconolite were studied by 1 MeV Kr+ and 1.5 MeV Xe+ irradiation at various temperatures. Zirconolite became amorphous at temperatures below a critical temperature, Tc. The critical temperature was found to be a function of ion species: Tc - 654 K for 1 MeV Kr+ and 710 K for 1.5 MeV Xe+. The temperature dependence of amorphization dose is shown in FIG. 1. Above Tc, the specimen remained crystalline after prolonged irradiation (up to 3.6×l015 ions/cm2). However, the high-temperature irradiated zirconolite was transformed into the fluorite structure (as shown by the strong diffraction maxima in FIG. 2). In addition to the maxima from the fluorite structure, strong diffuse maxima were observed surrounding the Bragg position of pyrochlore superlattice (FIG. 2).

Formation mechanism of silver nanocrystals made by ion irradiation of [formula omitted] ion-exchanged sodalime silicate glass

Sodalime silicate glass surface layers were doped with up to 7.0 at.% Ag + ions by ion-exchange in a AgNO 3/NaNO 3 solution at 330–355°C. Ion irradiation using either 400 and 500 keV He, 1 MeV Ne or 2 MeV Xe was then used to induce the growth of metallic nanocrystals in the ion-exchanged region. The ion fluences ranged from 1.3×10 14 ions/cm 2 to 1.1×10 17 ions/cm 2 . X-ray and electron diffraction show small Ag nanocrystals with a broad size distribution, up to a diameter of 10–15 nm, after irradiation. Optical transmission measurements show the characteristic surface plasmon resonance of metallic Ag around 420 nm. The absorption resonance sharpens and increases in strength with increasing ion irradiation fluence, indicating that both nanocrystal size and volume fraction increase with irradiation fluence. Depending on ion fluence, up to ∼15% of the ion-exchanged Ag + ions is incorporated in nanocrystals. From a systematic comparison of the degree of nanocrystal formation as a function of ion species, fluence and energy, it is concluded that nanocrystal formation is mainly caused by the atomic displacement energy loss component of the incoming ion beam; the electronic energy deposition component is less efficient.

Efficiency of thermoluminescent detectors to heavy charged particles

The efficiency of TLD-700 thermoluminescent detectors is studied for various ions as a function of energy. A new model for calculation of efficiency has been developed, based on the detector response to reference radiation, and the radial dose distribution of heavy ions only. No free parameters have to be used to calculate the thermoluminescence detector (TLD) efficiency as a function of ion species and energy. Comparison between model calculations and experimental results will be presented.

Surface concentration modification of PtPd alloys by noble gas ion sputtering

AbstractThe surface concentration of three PtxPd1 − x alloys (x = 0.3, 0.5 and 0.7) under ion bombardment was studied as a function of ion species and ion bombardment energy for the noble gas ions He, Ne, Ar and Xe in the 0.5–1.25 keV energy range. The surface composition was measured by low‐energy ion scattering spectroscopy (ISS) and Auger electron spectroscopy (AES). It was observed that Pd is preferentially sputtered from the surface of these alloys (as corroborated by previous studies). This effect was found to be greatest for the lightest ion (He) and most pronounced for the lowest energies used. Only a very weak dependence of the surface concentration on the energy of the heaviest ion (Xe) was detected. Comparing ISS and AES (true surface and near‐surface information respectively), it was also found that Pd is depleted in the near‐surface region. The depletion is ascribed to radiation‐enhanced diffusion and segregation and it is shown how the overlaid contour mapping of ISS and AES intensities, as a function of diffusion coefficient D and segregation energy ΔG, may be used to determine the radiation‐induced diffusion coefficient D as a function of ion species and ion energy.

Ion-beam-induced epitaxial crystallization of amorphous GaAs on GaAs(100)

Epitaxial crystallization of deposited amorphous GaAs layers on GaAs(100) up to the surface by bombardments with 400 keV Ar and 400 keV Kr has been successfully performed at a temperature range between 125 and 200°C. Properties of crystal growth were investigated as a function of ion species (Ar and Kr), energy (400 and 800 keV), ion dose, dose rate and substrate temperature by RBS channeling experiments. The growth rate has shown a nearly linear dependence on ion fluence. Ion bombardments below 100°C have induced further amorphization beyond the initial crystal/amorphous interface. On the scale of nuclear energy deposition density, bombardments with higher electronic excitation efficiency give a small increase of the growth rate. Ar bombardments have shown a strong dependence of the growth rate on dose rate, whereas Kr bombardments have revealed a weak dependence. An apparent activation energy of 0.13 ± 0.06 eV for the crystal growth was observed.

Implant-induced high-resistivity regions in InP and InGaAs

We have investigated the effects of ion bombardment on the electrical properties of intentionally doped InP and InGaAs grown by metalorganic molecular-beam epitaxy. The sheet resistivity and mobility of n+InP (Sn) and n+InGaAs (Sn) or p+InGaAs (Be) epilayers grown on semi-insulating InP substrates were measured as a function of ion species (O, B, H, or Fe), ion dose (1012–1015 cm−2), and post-implant annealing temperature (100–600 °C). In n+InP, the resistivity after bombardment goes through a maximum with annealing temperature, reaching a value of ∼106 Ω/⧠ for 0.5-μm-thick films after implantation with H or O and annealing at 200–300 °C. The as-grown resistivity is restored by annealing above 500 °C. Ion doses below 1012 cm−2 actually lead to a decrease in resistivity through the creation of shallow donor levels. By contrast, the implantation of Fe above a critical dose where the Fe density exceeds the dopant concentration leads to the formation of thermally stable, high-resistivity (>106 Ω/⧠) material. The temperature dependence of the resistivity shows an activation energy of 0.67 eV, which corresponds to the acceptor level of substitutional Fe in InP. Both n+InGaAs and p+InGaAs show somewhat similar behavior after implantation with maximum resistivities of ∼105 Ω/⧠ regardless of implant species. Once again for relatively low doses of O or H (below ∼1013 cm−2 in this case) there is creation of shallow defect levels that lower the resistivity of the material. The formation of these levels in InP has been investigated in more detail by measuring the depth-dependent carrier profile in implanted high-resistivity InP. The profile of the damage-induced centers is in close correlation with the nuclear energy deposition profile of the implanted ion in some cases, and with the profiles of stoichiometric excess due to unequal recoil of the lattice constituents in other cases.

Nickel-chromium interface resolution in Auger depth profiles

Auger depth resolutions for fine-grained crystalline Ni/Cr multilayers have been determined. Resolution is shown to be a function of ion species, ion energy, ion incident angle, and the presence of reactive species. Deterioration of interface resolution with depth is shown to be the result of cumulative surface roughness induced by the ion beam to an extent proportional to the ion velocity normal to the surface. The presence of a reactive species to create an amorphous surface layer is shown to inhibit the development of surface roughness. The best resolution was obtained using low energy Xe ions and large ion beam angles. The ultimate resolution is limited by the fabrication perfection of the standard, and not by the sputtering process or by the Auger mean free path length.

Heavy-ion induced adhesion of thin gold films to oxidized substrates of tantalum and silicon

Energetic heavy ion beams are capable of enhancing the adhesion of metallic films to a variety of substrates. Gold films (200–600 Å) evaporated onto substrates of tantalum and silicon (with native oxides) were bombarded with ions of 12C, 16O, 28Si, 35Cl and 58Ni at 2.85 MeV/nucleon. The threshold dose required to produce a peel strength greater than the Scotch-tape peel strength for a gold surface was measured as a function of ion species and angle of incidence. We observed the threshold dose to vary as the cosine of the angle with respect to normal incidence. The dependence on particle type for the AuTa system (with approximately 40 Å native oxide) was found tobe D th(cm −2) = 10 17( dE dx ) Au −3.0±0.2 for all particle beams where ( dE dx ) Au is the (MeV cm 2/mg) of the ion in gold. A substrate of 6000 Å of tantalum oxide gave identical results. The value of the exponent, −3.0±0.2, differs significantly from the value −1.6±0.2 reported earlier by Tombrello et al. for AuTa. The AuSi system is described by D th = 6 × 10 18( dE dx ) Au −4.1±0.3 . Auger analysis of the AuTa interface suggests a migration of the nativ result of ion bombardment. For the AuTa system, some irradiated regions that passed the Scotch-tape test just after bombardment were observed to fail when the test was repeated a few days later. Thicker gold films and higher doses resulted in longer adhesion times. These and other observations suggest that the post-irradiation decline of adhesion for AuTa is caused by diffusion of ambient atoms through the gold film rather than by an intrinsic time dependence of the adhesive forces.

Electron spin resonance study of ion-implanted polymers

The effect of ion implantation on the polymers PAN (polyacrylonitrile), PPO (poly 2,6-dimethylphenylene oxide) and PPS (p-polyphenylene sulfide) is studied using electron spin resonance. ESR measurements on these polymers were performed as a function of ion species and fluence in the temperature range 10 <T< 300 K. The unpaired carrier concentration increases with increasing fluence and is independent of the ion species used for implantation in this study (84Kr,80Br,75As,40Ar and14N). It is therefore concluded that the carrier concentration is related to the structural damage and not to chemical doping effects. From the shape of the ESR line, the ratio of the relaxation times for one-dimensional to three-dimensional spin diffusion is determined to be larger than 1000. The temperature dependence of the unpaired carrier concentration shows a strong deviation from a Curie law behavior, which can be explained by assuming that a defect band is formed with a bandgap due to strong Coulomb interaction between electrons on the defect sites.

Heavy ion measurements in the synchronous altitude region

The launch of the P78-2 (SCATHA) satellite in January 1979 has provided a new opportunity to study the energetic ion composition in the high altitude equatorial regions of the earth's magnetosphere. In particular detailed pitch angle distributions were obtained as a function of ion species and energy. The energies measured range from ∼ 90 to 250 keV/nucleon for Z ⩾ 2. Data are presented which were acquired in late March and early April 1979. The relative abundance of He and CNO nuclei are found to be ∼ 10 −2 and 5 × 10 −4 respectively at L ≅ 5.5. Only an upper limit on the relative abundance of Fe group nuclei of < 3 × 10 −7 was obtained. The angular distributions of the heavy ions was found to be very steep for B B 0 < 1.5 and then to flatten markedly.

Ion beam mixing in amorphous silicon I. Experimental investigation

Atomic mixing in amorphous Si has been investigated by measuring the irradiation-induced spreading of very thin impurity layers embedded in vacuum-deposited Si as a function of ion species, dose, implantation temperature and impurity species. Samples consisting of ∼10 A ̊ layers of Ni, Ge, Pd, Sn, Sb, Pt and Au at depths of 200–600 Å were irradiated at temperatures from 77 to 523 K with 50–300 keV Ne +, Ar +, Kr + and Xe + ions to doses of about 10 14−10 17 ions cm −2 and analyzed by 1.5 MeV He + ion backscattering spectrometry. The morphologies of the samples before and after irradiation were examined by transmission and scanning electron microscopy. The effective diffusion lengths squared, Dt, were deduced from the increases in the variances of the backscattering signals from the impurity species. This quantity was found to vary linearly with the ion dose φ for Kr + doses ranging from 6.3 × 10 14 to 2.0 × 10 16 ions cm −2 for the markersthat were investigated (Ge, Sb and Pt). This suggests that Dt φ is a pertinent parameter to characterize the interaction of ion beams with amorphous Si. Dt φ was found to be independent of the temperature during irradiation from ∼80K to ∼300 K for Ni, Ge, Sn, Sb, Pt and Au markers. Mixing of Pd is strongly temperature-dependent over the entire range investigated. Our data are consistent with a linear dependence of Dt φ on ν R p , the total nuclear energy loss divided by the projected range.

Superconducting properties and structure of ion bombarded Nb layers

Abstract The influence of ion bombardment on the superconducting transition temperature Tc and the structure of thin evaporated niobium layers has been investigated as a function of ion species and layer purity. Irradiation through pure layers with neon ions and fluences of typically 1016 ions/cm2 leads to relatively small Tc decreases (δTc × 0.5 K), while in oxygen contaminated layers larger effects depending on oxygen concentration are observed. Homogeneous implantation of chemically active impurities (nitrogen, oxygen) also drastically depresses Tc reaching the detection limit of 1.2K at a concentration of 15 at. %N. The Tc depressions correlate with a lattice parameter expansion of the Nb bcc structure at a rate of about 0.1 %per 1 at. % impurity.

Influence of ion bombardment on the superconducting transition temperature of evaporated vanadium layers

The influence of ion irradiation and implantation on the superconducting transition temperature T c of evaporated vanadium layers has been investigated as a function of ion species and layer purity. Irradiations of pure layers with Ne + ions and fluences of typically 10 16 ions/cm 2 lead to small T c decreases ( ΔT c∼ 0.1–0.2 K). In oxygen contaminated layers however appreciable changes of T c(ΔT c ∼ 2–3 K ) are observed depending on oxygen content and ion fluence. Similarly, the direct implantation of chemically active impurities (N +, O +) leads to drastic changes of T c . Here T c decreases as a function of nitrogen concentration down to the detection limit of 1.2 K. From these results it is concluded that the T c changes are due to impurity stabilized disorder. Energy dependent chanelling analysis of nitrogen and neon implanted V and Mo single crystals has been performed to obtain additional information on the nature of this disorder.

Abstract: Recent advances in ion implantation

The progress in solid state technology is largely determined by our ability to control and modify certain materials parameters in a well defined manner. Semiconductors are a typical example, where minute traces of impurities in the order of parts per million and less will influence the electrical properties markedly.The concept of introducing such dopants into semiconductors by means of high energetic particles was discussed many years ago. In 1954, Shockley1 submitted a patent describing the ’’Forming of Semiconductor Devices by Ionic Bombardment.’’ As the semiconductor technology matured, developing shallower and shallower device structures, the control aspect became vitally important. And it is precisely in the field of control that ion implantation offers major advantages over standard diffusion technologies, e.g.,(a) Accurate control of the total amount of impurity transferred to the wafer by measurement of the accumulated charge during implantations.(b) A high degree of areal uniformity across the w...

Electrical conductivity of disordered layers in GaAs crystal produced by ion implantation

The electrical conductivity of disordered layers in GaAs produced by ion implantation has been studied as a function of ion species, dose level, energy of implanted ions, and anneal temperature. The resistivity of the implanted layer, ρ, at room temperature can be expressed by a single relation, ρ=const×NDD−3, even for different masses and energies of implanted ions, where NDD is the total number of displacements of host atoms per unit volume due to nuclear collisions with implanted ions. At a temperature above 180 K the resistivity is proportional to exp(B/T) and below 180 K it changes as exp(B/T1/4). The conduction mechanism of the implanted layers is discussed based upon the hopping conduction in disordered or amorphous semiconductors. The annealing behavior of the implanted layers was also studied. The activation energy for recovery of the resistivity ρ was about 0.35 eV, which is quite small compared with the migration energies of stray atoms in ordered crystals. These results indicate that the implanted layer is in a disordered state rather than in a crystalline state containing the point defects.