Channeling regimes in ion surface scattering

We report on surface channeling experiments of singly charged ions on single crystal surfaces of Pt(1 1 0) and Pd(1 1 0). Using a time-of-flight system installed in forward direction we analyze the energy distribution of the scattered projectiles. By variation of the primary energy and the angle of incidence we investigate effects of the perpendicular energy on the channeling features. The perpendicular energy is defined as E = E₀sin²ψ with ψ the angle of incidence. In combination with precise azimuthal rotations of the crystal, we are sensitive to axial channeling and obtain information about the limits of axial surface channeling. From a comparison with detailed trajectory calculations we find that axial channeling effects are most pronounced for a perpendicular energy between 5 and 20 eV. As a result, we obtain an exemplary channeling map for the interaction of nitrogen ions with the (1 × 2) reconstructed Pt(1 1 0) surface identifying different channeling regimes.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

Radiation enhanced diffusion of implanted platinum in silicon guided by helium co-implantation for arbitrary control of platinum profile

The in-diffusion of platinum into a low-doped n-type float zone silicon guided and enhanced by radiation damage produced by co-implantation of helium ions was investigated. The implantation of 1 MeV platinum ions at different doses ranging from 5 × 10¹¹ to 5 × 10¹² cm⁻² was used to produce a finite source for platinum diffusion. Single and multiple energy implantation of helium ions with energies 7, 9, 11 and 13 MeV introducing different profiles of radiation defects were applied to enhance and shape the diffusion of platinum atoms performed by 20 min annealing at 725 °C in vacuum. The distribution of in-diffused platinum was studied by monitoring the acceptor level of substitutional platinum (E_C − E_T = 0.23 eV) by deep level transient spectroscopy. Results show that the helium co-implantation significantly enhances platinum diffusion and allows its control up to the depths of hundreds of micrometers. The resulting Pt_s distribution is given by the profile of radiation damage produced by helium ions while the amount of in-diffused Pt_s can be controlled by the dose of platinum implantation. It is also shown that an extra annealing at 685 °C performed prior to helium implantation substantially increases the amount of in-diffused platinum.     

Lattice location and electrical conductivity in ion implanted TiO2 single crystals

Single crystals of TiO₂ (rutile) were implanted at room temperature with Ar, Sn and W ions applying fluences of 10¹⁵/cm² to 10¹⁶/cm² at 300 keV. The lattice location, together with ion range and damage distribution was measured with Rutherford Backscattering and Channeling (RBS-C). The conductivity, σ, was measured as a function of temperature. The implanted Sn and W atoms were entirely substitutional on Ti sites in the applied fluence region, where the radiation damage did not yet reach the random level. A large σ increase was observed for all implants at displacement per atom values (dpa) below 1. Above dpa = 1, σ reveals a saturation value of 0.3 Ω⁻¹ cm⁻¹ for Ar implants, while for W and Sn implants a further increase of σ up to 30 Ω⁻¹ cm⁻¹ was measured. Between 70 K and 293 K ln σ was proportional to T^−1/2, (Ar,W) and T^−1/4 (Sn), indicating that the transport mechanism is due to variable range hopping.     

Design of a magnetic spectrograph for surface, interface and thin-layer analysis

The design of a magnetic spectrograph of the QDQ type, to be coupled to a 6 MV Van de Graaff accelerator for the analysis with Rutherford backscattering and recoil spectroscopy of surfaces and thin layers, will be shown. Due to the high energy resolution of. the instrument (ΔE/E ≈ 2 × 10⁻⁴) the depth resolution is determined mainly by energy straggling and the combined effect of angular and lateral straggling, and is close to l monolayer near the surface. The mass discrimination of the spectrograph in combination with the energy dispersion of the detector in the focal plane makes possible the background-free detection of light (¹H−¹⁹F) atoms recoiling from a heavier substrate. The design of the total setup is such that channeling and channeling plus blocking experiments can be carried out, so that the position of light (or heavy) atoms on or in monocrystalline samples can be measured. The detector in the focal plane is position-sensitive in two dimensions so that with one setting of the spectrograph an energy range of 2% and an angular distribution of 5° (with a resolution of 0.1°) can be measured simultaneously. The opening angle in the energy-dispersive direction is 0.1°. The whole spectrograph, including the scattering chamber, can be rotated over 120° with respect to the beam line. For this purpose a bellows construction is made between the beam line and the scattering chamber, permitting the rotation while maintaining a vacuum of ≈ 10⁻¹⁰ Torr in the chamber.     

Atomic structure and disordering induced by 350 MeV Au ions in MgAl2O4

Microstructure change and atomic disordering in MgO · nAl₂O₃ (n = 1.1) irradiated with 350 MeV Au ions (S_e = 35 keV/nm) were investigated through transmission electron microscopy (TEM) and high angular resolution electron channeling X-ray spectroscopy (HARECXS) techniques. High resolution TEM revealed that each ion track maintains crystalline structure. The core region of ion track is found to reveal a lattice fringe with a half period of spinel matrix, suggesting the phase transformation from spinel to rock-salt structure. HARECXS analysis clearly showed progress of cation disorder at a significantly large region of 10 nm in diameter. These results are compared with the previous results of 200 MeV Xe ion irradiated spinel (S_e = 25 keV/nm). The structure of ion tracks is found to consist of three concentric circle structures: the defective core region (2 nm in diameter), strained region (5 nm) and cation disordered region (10–12 nm).     

Sputtering from spherical Au clusters by energetic atom bombardment

Using molecular-dynamics simulation, we study the effect of 100 keV Au atom bombardment of spherical Au clusters (radius R=40 Å), containing 15,784 atoms. Results range from projectile transmission with only few atoms sputtered to more or less complete cluster disintegration. During disintegration, besides major fragments of the original cluster, monatomics and a large number of clusters with sizes up to 100 atoms, and even beyond, are created. Angular and energy spectra of sputtered atoms show features of both collisional sputtering and evaporation: particle emission is isotropic with an additional contribution of preferential emission along [1 1 0] directions. Energy spectra show the high-energy E⁻² fall-off typical of linear-cascade sputtering plus an additional low-energy thermal component.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

Influence of the target thickness on the backward and forward electron emission characteristics induced by protons incident on thin carbon foils

The forward (γ_F) and backward (γ_B) electron emission yields have been measured for protons incident on thin carbon foils for incident energies between 2 and 7 MeV as a function of the target thickness. Comparisons with theoretical results obtained by Monte Carlo simulations are presented. In particular, the Meckbach factor R_γ = γ_F/γ_B is discussed.     

Surface excitation parameter for semiconducting III–V compounds

In the rapid development of mesoscopic science, the study of surface excitations in solids and overlayer systems plays a crucial role. The surface excitation parameter which describes the total probability of surface plasmon excitations by an electron traveling in vacuum before impinging on or after escaping from a semiconducting III–V compound has been calculated for 200–2000 eV electrons crossing the compound surface. These calculations were performed using the dielectric response theory with sum-rule-constrained extended Drude dielectric functions established by the fits of these functions to optical data. Surface excitation parameters calculated for InSb, InAs, GaP, GaSb or GaAs III–V compounds were found to follow to a simple formula, i.e. P_s = aE^−b, where P_s is the surface excitation parameter and E is the electron energy. These surface excitation parameters were then applied to determine the elastic reflection coefficient for electrons elastically backscattered from III–V compounds using the Monte Carlo simulations. Good agreement was found for the electron elastic reflection coefficient between calculated results and experimental data.     

Defect evolution during ion bombardment of amorphous Si probed by in situ conductivity measurements

In this work we use in-situ conductivity measurements during ion irradiation as a sensitive probe of the defect structure of amorphous Si. Electronic transport in amorphous Si occurs by hopping at the high density ( 10²⁰ cm⁻³ eV⁻¹) of deep lying localized states introduced by the defects in the band gap. In-situ conductivity measurements allow to follow directly the defect generation and annihilation kinetics during and after ion bombardment of the material. Amorphous Si layers, patterned to perform conductivity measurements, were annealed at 500°C in order to reduce the defect density by about a factor of 5. Defects were subsequently reintroduced by high energy ion irradiation at different temperatures (77–300 K). During irradiation the conductivity of the layer increases by several orders of magnitude and eventually saturates. Turning off the beam results in a decrease of the conductivity by a factor of 2 in times as long as a few hours even at 77 K. The effects of different ions (He, C, Si, Cu, and Au) and different ion fluxes (10⁹–10¹² ions/cm² s) on these phenomena have been explored. These data give a hint on the mechanisms of defect production and annihilation and demonstrate a strong correlation between electrical and structural defects in amorphous silicon.     

Enthalpy and heat capacity of LiAlO2 between 298 and 1700 K by drop calorimetry

The enthalpy of γ-LiAlO₂ was measured between 403 and 1673 K by isothermal drop calorimetry. The smoothed enthalpy curve between 298 and 1700 K results in H⁰(T) − H⁰(298 K)=−37 396 + 93.143 · T + 0.00557 · T² + 2 725 221 · T⁻¹ J/mol. The standard deviation is 2.2%. The heat capacity was derived by differentiation of the enthalpy curve. The value extrapolated to 298 K is C_p,298=(65.8 ± 2.0) J/K mol.     

Elastic recoil detection analysis on the ANSTO heavy ion microprobe

The heavy ion microprobe at the Australian Nuclear Science and Technology Organisation is capable of focussing heavy ions with an ME/q² of up to 100 amu MeV. This makes the microprobe ideally suited for heavy ion elastic recoil detection analysis (ERDA). However, beam currents on a microprobe are usually very small, which requires a detection system with a large solid angle. We apply microbeam heavy ion ERDA using a large solid angle ΔE−E telescope with a gas ΔE detector to layered structures. We demonstrate the capability to measure oxygen and carbon with a lateral resolution of 20 μm, together with determination of the depth of the contamination in thin deposited layers.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

Layout of a nuclear microprobe beamline using a liquid metal ion source

The Bochum solenoid lens microprobe will be installed in a new configuration which can be fed both by the 4 MV Dynamitron tandem accelerator and by a new 500 kV accelerator of extremely low energy-spread (ΔE/E 10⁻⁵). Apart from a conventional duoplasmatron ion source, a commercial gallium liquid metal ion source (LMIS) will be implemented in this accelerator. Microprobe optics will benefit from the high brightness of the LMIS ( 10⁵ Am⁻²sr⁻¹eV⁻¹), thus enabling an increased lateral resolution. Ion optical ray tracing, simulating the accelerator tube and the solenoid lens, has allowed one to specify the beam parameters required at the accelerator entrance to yield a micrometer focus at the target position. Using this information and further simulations, the accelerator injection optics can be particularly optimized for the new microbeam line.     

Separation of Am(III) from Eu(III) by extraction based on in situ extractant formation of dithiocarbamate derivatives

A new solvent-extraction technique based on in situ formation of dithiocarbamate derivatives in order to separate Am(III) from Eu(III) was carried out. In this technique, the extractant is formed during the extraction course by the reaction between the organic materials, which are needed to synthesize the extractant. The synthesis of extractant in in situ extractant-formation method was carried out as follows. Di-substituted amine, such as dioctylamine (DOA), dibenzylamine (DBzA) and so on, and carbon disulfide (CS₂) were mixed in organic solvents, such as nitrobenzene, to produce dioctylammonium dioctyldithiocarbamate (DOA⁺·DODTC⁻), dibenzylammonium dibenzyldithiocarbamate (DBzA⁺·DODTC⁻), or so on. These organic solutions are also the organic phase in the solvent extraction, whereas the aqueous phase is 1.00 mol/dm³ (H, Na)NO₃ solution. The elements of Am(III) and Eu(III) were extracted into organic phase from different hydrogen ion concentrations of aqueous phase. The SF of Am(III)/Eu(III) is 3.2 × 10⁴ at pH_eq = 6.25 in DOA–CS₂/nitrobenzene system. This separation technique of Am(III) from Eu(III) by extraction based on in situ extractant formation has the following advantages. (a) It is unnecessary to take the chemical stability of extractant into account for storage purpose, and (b) Am(III) can be completely separated from Eu(III) by a single extraction procedure.     

Influence of low-energy bombardment of an RF magnetron sputtering discharge on texture formation and stress in ZnO films

In an oxygen planar RF magnetron sputtering discharge, the time-averaged flux and energy of positive ions drifting out of the plasma and striking the substrate surface have been determined as a function of RF discharge power over a range of 100 to 1000 W, and as a function of chamber pressure from 0.2 to 6 Pa by measurement of ion-current density and time-averaged plasma sheath potential at the substrate. These data were related to the resulting crystal structure of the deposited ZnO films which had been studied in detail using well-known methods of X-ray diffraction. The impact energy of the positive ions bombarding the growing film varies from some 10 eV to close 50 eV depending on magnetron RF discharge power and oxygen pressure, respectively. The incident ion flux was found to be below 1× 10¹⁵ cm⁻²s⁻¹ up to 1 × 10¹⁶ cm⁻²s⁻¹, a value of the same order of magnitude as that for the condensing rate of sputtered ZnO species. The structural results obtained show that both the ion energy and the ion flux in the range mentioned above cause significant changes in the degree of crystallinity, preferred orientation and texture sharpness of the deposited ZnO films. Furthermore, positive ion bombardment during film growth has been found to alter the ZnO unit cell dimension up to 2% relative to the equilibrium bulk or powder value which is responsible for the formation of strong compressive residual stress of up to several GPa within the ZnO film. Following these results, one of the criterions for preparing highly c-axis oriented ZnO films with columnar grain structure is to decrease both the energy and the flux of the positive ion bombardment without decreasing the deposition rate of ZnO species. At a such slight-bombardment RF magnetron deposition the compressive residual stress of the ZnO film can be reduced towards zero.     

Electron emission induced by H and H incident on thin carbon foils: influence of charge changing processes

The forward and backward electron emission yields γ_F and γ_B have been calculated by Monte Carlo simulation for protons (H⁺) and hydrogen atoms (H⁰) (with energies between 25 keV and 5 MeV) incident on thin amorphous carbon foils. Direct electron excitations by the incident projectiles as well as electron excitations resulting from charge exchange processes undergone by H⁺ or H⁰ have been taken into account. For the latter, Auger and Shell processes have been considered. Subsequent electron transport has been considered in order to calculate the forward and backward electron emission yields γ_F and γ_B.     

High resolution channeling contrast microscopy and channeling analysis of SiGe quantum well structures

High resolution channeling contrast microscopy (CCM) and channeling measurements were carried out to characterize SiGe quantum well structures on micron thick graded layers (i.e. virtual substrates). The virtual substrates were grown by gas source molecular beam epitaxy at a pressure of 10⁻⁵ mbar and low pressure chemical vapor deposition at 10⁻² mbar on boron doped Si(0 0 1) substrates respectively. A homoepitaxial silicon buffer layer was grown prior to the deposition. The nominal structure is a 20 nm Si_0.75Ge_0.25 layer at the surface, followed by 10 nm pure Si, 500 nm Si_0.75Ge_0.25 and a 1000 nm thick graded SiGe (0–26%) layer. RBS was used to measure the depth profiles, and angular scans around the (1 0 0) axis were carried out to assess crystal and interface quality. CCM was used to acquire depth resolved images of micron-sized lateral inhomogenities (‘cross-hatch') present on both samples.