Sputtering and crystalline structure modification of bismuth thin films deposited onto silicon substrates under the impact of 20-160 keV Ar ions

The sputtering of bismuth thin films induced by 20-160 keV Ar⁺ ions has been studied using Rutherford backscattering spectrometry, scanning electron microscopy and X-ray energy dispersive and diffraction spectroscopy. These techniques revealed increasing modifications of the Bi film surfaces with increasing both ion beam energy and fluence up to their complete deterioration under irradiation conditions E = 160 keV and φ = 1.5 × 10¹⁶ cm⁻², leaving isolated islands of preferred (0 1 2) orientation on the Si substrate. The observed surface morphology and crystalline structure evolutions are likely due to a complex interplay of interaction mechanisms involving both elastic nuclear collisions and inelastic electronic ones. The measured Bi sputtering yields versus Ar⁺ ion fluence for a fixed ion energy exhibit a significant depression at very low φ-values followed by a steady state regime above ∼2.0 × 10¹⁴ cm⁻². Measured sputtering yields versus Ar⁺ ion energy with fixing ion fluence to 1.2 × 10¹⁶ cm⁻² in the upper part of the yield saturation regime are also reported. Their comparison to theoretical model and SRIM 2008 Monte Carlo simulation predictions is discussed.     

Real-time X-ray studies of the growth of Mo-seeded Si nanodots by low-energy ion bombardment

Silicon surface evolution during room temperature low-energy (300, 500 and 1000 eV) normal incidence Ar⁺ ion bombardment in the presence of Mo seed atoms has been studied with real-time grazing-incidence small-angle X-ray scattering and ex situ atomic force microscopy. When a small amount of Mo atoms was supplied to the Si surface during ion bombardment, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (“dots”) coarsened with time until they reached a constant spatial wavelength. The longer length scale corrugations associated with kinetic roughening, however, continued to grow in amplitude during bombardment. The overall roughness is dominated by different corrugations at different times in the kinetic evolution, showing a complex behavior. The evolution of the kinetic roughening can be described by the Family-Vicsek scaling hypothesis, but measured scaling exponents are not in agreement with those of existing models.     

Frequency dependence studies on the interface trap density and series resistance of HfO2 gate dielectric deposited on Si substrate: Before and after 50 MeV Li ions irradiation

We report the first investigation of the frequency dependent effect of 50 MeV Li³⁺ ion irradiation on the series resistance and interface state density determined from capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics in HfO₂ based MOS capacitors prepared by rf-sputtering. The samples were irradiated by 50 MeV Li³⁺ ions at room temperature. The measured capacitance and conductance were corrected for series resistance. The series resistance was estimated at various frequencies from 1 KHz to 1 MHz before and after irradiation. It was observed that the series resistance decreases from 6344.5 to 322 Ω as a function of frequency before irradiation and 8954-134 Ω after irradiation. The interface state density D_it decreases from 1.12 × 10¹² eV⁻¹ cm⁻² before irradiation to 3.67 × 10¹¹ eV⁻¹ cm⁻² after ion irradiation and further decreases with increasing frequency.     

Temperature dependence of liquid Sn sputtering by low-energy He and D bombardment

Absolute sputtering yields of liquid tin from 240 to 420 °C due to irradiation by low-energy helium and deuterium have been measured. For ion energies ranging from 300 to 1000 eV, temperature enhancement of liquid tin sputtering was noted. These measurements were obtained by IIAX (the Ion-surface InterAction eXperiment) using a velocity-filtered ion beam at 45° incidence to sputter material from a liquid tin target onto deposition monitors. Sputtering yields from 500 eV ion bombardment at 45° incidence increase from 0.1 ± 0.03 and 0.019 ± 0.008 Sn particles/ion at room temperature, for He⁺ and D⁺ ions respectively, to 0.30 ± 0.12 and 0.125 ± 0.05 Sn particles/ion for 380 °C. Temperature enhanced sputtering has been seen in other liquid metals (namely lithium, tin-lithium, and gallium) using both ion beam and plasma irradiation.     

Ion irradiation effects on tungsten-oxide films and charge state effect on electronic erosion

We have studied electronic- and atomic-structure modifications of polycrystalline WO₃ films (bandgap of ∼3 eV) by ion irradiation. WO₃ films were prepared by oxidation of W films on MgO substrates and of W sheets. We find disordering or amorphization, the lattice expansion of ∼1.5% and bandgap increase of 0.2 eV after 90 MeV Ni ion irradiation at ∼3 × 10¹² cm⁻². A broad peak of optical absorption appears around 1.6 μm by ion irradiation. We also find that the erosion yield by high-energy ions with the equilibrium charge exceeds 10⁴ and that the erosion yield under ion impact with non-equilibrium charge (90 MeV Ni⁺¹⁰) is ∼1/5 of that with the equilibrium charge (89 MeV Ni⁺¹⁹). Effects of depth dependence of the ion mean charge on the erosion yields are discussed. The erosion yield by low-energy ions is also presented.     

Saturable absorption in gold implanted fused silica

Metal nanocluster composite glasses (MNCGs) have been the subject of both experimental and theoretical investigation because of their peculiar optical properties. In particular, the enhanced third order optical nonlinearity could be exploited in the all-optical switching device technology. In the present work, we present some results on MNCG films prepared by ion implantation. Fused silica were implanted with Au⁺ of fluences 3 × 10¹⁶ and 1 × 10¹⁷ ions/cm² using an energy of 1.5 MeV. Optical absorption spectra of these samples have revealed prominent linear absorption bands at characteristic surface plasmon resonance (SPR) wavelength at and around 490 nm. Rutherford backscattering spectrometry (RBS) measurements reveal a Gaussian spatial distribution of Au ions. Third order optical nonlinear properties were studied by the Z-scan technique using a nanosecond laser. Z-scan measurements on the metal nanoclusters glass composites have revealed saturable absorption signifying the nonlinear responses.     

The improvement of low-resistance and high-transmission ohmic contact to p-GaN by Zn implantation

The electrical and optical characteristics of Zn⁺ ion-implanted Ni/Au ohmic contacts to p-GaN were investigated. After the preparation of Ni/Au electrode on the surface of p-GaN, the metal/p-GaN contact interface was doped by 35 keV Zn⁺ implantation with fluences of 5 × 10¹⁵-5 × 10¹⁶ cm⁻². Subsequent rapid thermal annealing of the implanted samples were carried in air at 200-400 °C for 5 min. Obvious improvements of the electrode contact characteristics were observed, i.e. the decrease of specific contact resistance and the increase of light transmittance. The lowest specific contact resistance of 5.46 × 10⁻⁵ Ω cm² was achieved by 1 × 10¹⁶ cm⁻² Zn⁺ implantation. The transmission enhancement of the electrodes was found as the annealing temperature rises. Together with the morphology and structure analyses of the contacts by scanning and transmission electron microscope, the corresponding mechanism for such an improvement was discussed.     

Radiolysis studies of aqueous κ-carrageenan

The effects on N₂O and N₂ gas on the radiation degradation yield of aqueous kappa (κ-) carrageenan were investigated. The G_d of solution saturated with N₂O solution was expectedly much higher than in air (1.7 and 1.2 × 10⁻⁷ mol J⁻¹). On the other hand, a lower G_d of 1.1 × 10⁻⁷ mol J⁻¹ was obtained from κ-carrageenan solution saturated with N₂.The rate constant of reaction of OH radicals with sonicated and irradiated κ-carrageenan were determined using e-beam pulse radiolysis. The rate constant of OH interaction with sonicated κ-carrageenan decreased with decreasing molecular weight. On the other hand, the OH interaction with irradiated κ-carrageenan decreased but did not vary significantly with decreasing molecular weight. Metal ion (Na⁺) induced conformational transition into helical form decreased the rate constant of OH reaction with κ-carrageenan. Likewise, the G_d in aqueous form was affected by the conformational state of κ-carrageenan. The helical conformation gave a lower G_d (7 × 10⁻⁸ mol J⁻¹) than the coiled conformation (G_d = 1.2 × 10⁻⁷ mol J⁻¹).     

Ion-induced electron emission monitoring of the structure and morphology evolution in HOPG

The temperature dependences of the ion-induced electron emission yield γ of highly-oriented pyrolytic graphite (HOPG) under high-fluence (10¹⁸-10¹⁹ ions/cm²) 30 keV Ar⁺ ion irradiation at ion incidence angles from θ = 0^o (normal incidence) to 80^o have been measured to trace both the structure and morphology changes in the basal oriented samples. The target temperature has been varied during continuous irradiation from T = −180 to 400 ^oC. The surface analysis has been performed by the RHEED and SEM techniques. The surface microgeometry was studied using laser goniophotometry (LGF). The dependences of γ(T) were found to be strongly non-monotonic and essentially different from the ones for Ar⁺ and N₂⁺ ion irradiation of the polygranular graphites. A sharp peak at irradiation temperature T_p ≈ 150 ^oC was found. A strong influence of electron transport anisotropy has been observed, and ion-induced microgeometry is discussed.     

Ion energy dependence of interface parameters of ion beam sputter deposited W/Si interfaces

W thin films and W/Si/W tri-layer samples have been deposited on c-Si substrates in a home-made ion beam sputtering system at 1.5 × 10⁻³ Torr Ar working pressure, 10 mA grid current and at different Ar⁺ ion energies between 600 and 1200 eV. Grazing incidence X-ray reflectivity (GIXR) measurements in specular and diffused (detector scan) geometry have been carried out on the above samples. The measured GIXR spectra were fitted with theoretically simulated spectra and the different interface parameters viz., interface width, interface roughness and interface diffusion have been estimated for both Si-on-W and W-on-Si interfaces in the above samples. The variation of the above interface parameters as a function of ion energy used for W sputtering has been studied.     

Recent advances in AMS of Cl with a 3-MV-tandem

Accelerator mass spectrometry (AMS) of ³⁶Cl (t_1/2 = 0.30 Ma) at natural isotopic concentrations requires high particle energies for the separation from the stable isobar ³⁶S and was so far the exclusive domain of tandem accelerators with at least 5 MV terminal voltage. Using terminal foil stripping and a detection setup consisting of a split-anode ionization chamber and an additional energy signal from a silicon strip detector, a ³⁶S suppression of >10⁴ at 3 MV terminal voltage was achieved. To further increase the ³⁶S suppression energy loss straggling in various counter gases (C₄H₁₀, Ar-CH₄ and C₄H₁₀-Ar) and the effect of “energy focusing” below the maximum of the Bragg curve was investigated. The comparison of experimental data with simulations and published data yielded interesting insights into the physics underlying the detectors. Energy loss, energy straggling and angular scattering determine the ³⁶S suppression. In addition, we improved ion source conditions, target backing materials and the cathode design with respect to sulfur output and cross contamination. These changes allow higher currents during measurement (³⁵Cl⁻ current ≈ 5 μA) and also increased the reproducibility. An injector to detector efficiency for ³⁶Cl ions of 8% (16% stripping yield for the 7+ charge state in the accelerator, 50% ³⁶Cl detection efficiency) was achieved, which can favorably be compared to other facilities. The memory effect in our ion source was also thoroughly investigated. Currently our measured blank value is ³⁶Cl/Cl ≈ 3 × 10⁻¹⁵ when samples with a ratio of 10⁻¹¹ are used in the same sample wheel and ³⁶Cl/Cl ≈ 5 × 10⁻¹⁶ if measured together with samples with a ratio of 10⁻¹² or below. This is in good agreement with the lowest so far published isotope ratios around 5 × 10⁻¹⁶ and demonstrates that 3 MV tandems can achieve the same sensitivity for ³⁶Cl as larger machines.     

Temperature-dependent surface modification of InSb(0 0 1) crystal by low-energy ion bombardment

Structural and compositional modification of InSb(0 0 1) single crystal surfaces induced by oblique incidence 2-5 keV Ar and Xe ion irradiation have been investigated by means of scanning tunneling and atomic force microscopies, and time-of-flight mass spectroscopy of secondary ion emission. In general, ion-induced patterns (networks of nanowires, or ripples) are angle of incidence- and fluence-dependent. Temperature dependences (from 300 to 600 K) of the RMS roughness and of the ripple wavelength have been determined for the samples bombarded with various fluences. Secondary ion emission from an InSb(0 0 1) surface exposed to 4.5 keV Ar⁺ ions has been investigated with a linear TOF spectrometer working in a static mode. Mass spectra of the sputtered In⁺, Sb⁺ and In₂⁺ secondary ions have been measured both for the non-bombarded (0 0 1) surface and for the surface previously exposed to a fluence of 10¹⁶ ions/cm². In⁺ and In₂⁺ intensities for the irradiated sample are much higher in comparison to the non-bombarded one, whereas Sb⁺ ions show a reversed tendency. This behavior suggests a significant In-enrichment at the InSb(0 0 1) surface caused by the ion bombardment.     

Scintillation light produced by low-energy beams of highly-charged ions

Measurements have been performed of scintillation light intensities emitted from various inorganic scintillators irradiated with low-energy beams of highly-charged ions from an electron beam ion source (EBIS) and an electron cyclotron resonance ion source (ECRIS). Beams of xenon ions Xe^q+ with various charge states between q = 2 and q = 18 have been used at energies between 5 and 17.5 keV per charge generated by the ECRIS. The intensity of the beam was typically varied between 1 and 100 nA. Beams of highly charged residual gas ions have been produced by the EBIS at 4.5 keV per charge and with low intensities down to 100 pA. The scintillator materials used are flat screens of P46 YAG and P43 phosphor. In all cases, scintillation light emitted from the screen surface was detected by a CCD camera. The scintillation light intensity has been found to depend linearly on the kinetic ion energy per time deposited into the scintillator, while up to q = 18 no significant contribution from the ions’ potential energy was found. We discuss the results on the background of a possible use as beam diagnostics, e.g. for the new HITRAP facility at GSI, Germany.     

Ion induced high energy electron emission from copper

We present measurements of secondary electron emission from Cu induced by low energy bombardment (1-5 keV) of noble gas (He⁺, Ne⁺ and Ar⁺) and Li⁺ ions. We identify different potential and kinetic mechanisms and find the presence of high energetic secondary electrons for a couple of ion-target combinations. In order to understand the presence of these fast electrons we need to consider the Fermi shuttle mechanism and the different ion neutralization efficiencies.     

Fuel retention study in fusion reactor walls by micro-NRA deuterium mapping

Nuclear Reaction Analysis (NRA) with a ³He ion beam is a powerful analytical technique for analysis of light elements in thin films. The main motivation for ³He focused beam applications is lateral mapping of deuterium using the nuclear reaction D(³He,p)⁴He in surfaces exposed to a tokamak plasma, where a lateral resolution in the μm-range provides unique information for fuel retention studies.At the microprobe at the Jo?ef Stefan Institute typical helium ion currents of 300 pA and beam dimensions of 4 × 4 μm² can be obtained. This work is focused on micro-NRA studies of plasma-facing materials using a set-up consisting of a silicon partially depleted charge particle detector for NRA spectroscopy applied in parallel with a permanently installed X-ray detector, an RBS detector and a beam chopper for ion dose monitoring. A method for absolute deuterium quantification is described. In addition, plasma-deposited amorphous deuterated carbon thin films (a-C:D) with known D content were used as a reference.The method was used to study deuterium fuel retention in carbon fibre composite materials exposed to a deuterium plasma in the Tore Supra and TEXTOR tokamaks. The high lateral resolution of micro-NRA allowed us to make a detailed study of the influence of topography on the fuel retention process. We demonstrated that the surface topography plays a dominant role in the retention of deuterium. The deep surfaces inside the castellation gaps showed approximately two orders of magnitude lower deuterium concentrations than in areas close to the exposed surface.     

Study of surface activation of PET by low energy (keV) Ni and N ion implantation

Polyethyleneterephthalate (PET) has been modified by 100 keV Ni⁺ and N⁺ ions using metal ion from volatile compound (MIVOC) ion source to fluence ranging from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The increasing application of polymeric material in technological and scientific field has motivated the use of surface treatment to modify the physical and chemical properties of polymer surfaces. When a material is exposed to ionization radiation, it suffers damage leading to surface activation depending on the type. The surface morphology was observed by atomic force microscopy (AFM). That show the roughness increases with fluence in both the cases. The Ni particles as precipitation in PET were observed by cross-section transmission electron microscopy (XTEM). The optical band gap (E_g) deduced from absorption spectra; was calculated by Tau’c relation. Raman spectroscopy shows quantitatively the chemical nature at the damage caused by the Ni⁺ and N⁺ bombardment. The ration of I_D/I_G shows graphite-like structure is formed on the surface. A layer of hydrogenated amorphous carbon is formed on the surface, which has confirmed by XPS results also.     

Relationship between radiation damage anisotropy in MgO and YSZ single crystals and the Ion/Atom ratio deposition parameter in biaxially-textured MgO and YSZ thin films fabricated by ion beam assisted deposition

To elucidate the underlying physics of ion beam assisted deposition (IBAD), irradiation damage effects in magnesia (MgO) and yttria-stabilized zirconia (YSZ) were investigated. Ion irradiations were performed on MgO and YSZ single crystals of three low-index crystallographic orientations using 100 and 150 keV Ar⁺ ions over a fluence range from 1 × 10¹⁴ to 5 × 10¹⁶ Ar/cm². Damage accumulation was analyzed using Rutherford backscattering spectrometry combined with ion channeling. Damage evolution with increasing ion fluence proceeded via several characteristic stages and the total damage exhibited a strong dependence on crystallographic orientation. For both MgO and YSZ, damage anisotropy was maximal at a stage when the damage saturated, with the (1 1 0) crystallographic orientation being the most radiation damage resistant. The Ion/Atom ratio deposition parameter reported for IBAD of MgO and YSZ films was found to correlate with the damage plateau stage described above. Finally, the role of the Ion/Atom ratio is discussed in terms of radiation damage anisotropy mechanism.     

Properties of Na implanted targets

Ten types of ²³Na implanted targets have been fabricated for the purposes of investigating the effects of proton beam bombardment on the implanted sodium distribution. Targets were implanted at energies of E_Na = 10-30 keV using copper, tantalum, and nickel as host materials. Thin layers (100-200 Å) of chromium and gold were also evaporated over some of the targets to provide a protective layer for the implanted sodium. The ²³Na(p, γ) resonance at a lab proton energy of E_p = 309 keV was used to determine the implanted distribution. Successive resonance profile measurements are presented for each implanted target, and the concurrent loss of ²³Na resulting from beam bombardment is reported. The calculated temperature rise of the targets indicates that beam heating has a negligible effect on the implanted sodium distribution, and that the principal mechanism for ²³Na loss during beam bombardment is sputtering.