ToF-E ERDA Research Articles

Time detector design for Time-of-Flight Elastic Recoil Detection Analysis (ToF-E ERDA)

Time-of-flight-energy elastic recoil detection analysis (ToF-E ERDA) is a uniquely powerful thin-film analytical technique because it can be used to measure simultaneously the depth distributions of all elements in a thin-film sample. The basic design of the time detectors which are generally based on the electron mirror has not changed significantly over 25 years or so. A classic electron optical study has been made to elucidate the factors that limit the time resolution of electron mirror time pick off detectors and how they are interrelated. Degradation of the time resolution is mainly associated with the electron divergence from an ideal straight trajectory and variations in the propagation of electromagnetic pulses over the anode surface. A set of equations that can be used to optimise time detector design are presented along with a new configuration that has an expected time resolution of 40 ps fwhm and overcomes most of the drawbacks of the conventional configuration.

Core-level spectra and binding energies of transition metal nitrides by non-destructive x-ray photoelectron spectroscopy through capping layers

We present the first measurements of x-ray photoelectron spectroscopy (XPS) core level binding energies (BE:s) for the widely-applicable group IVb-VIb polycrystalline transition metal nitrides (TMN’s) TiN, VN, CrN, ZrN, NbN, MoN, HfN, TaN, and WN as well as AlN and SiN, which are common components in the TMN-based alloy systems. Nitride thin film samples were grown at 400°C by reactive dc magnetron sputtering from elemental targets in Ar/N2 atmosphere. For XPS measurements, layers are either (i) Ar+ ion-etched to remove surface oxides resulting from the air exposure during sample transfer from the growth chamber into the XPS system, or (ii) in situ capped with a few nm thick Cr or W overlayers in the deposition system prior to air-exposure and loading into the XPS instrument. Film elemental composition and phase content is thoroughly characterized with time-of-flight elastic recoil detection analysis (ToF-E ERDA), Rutherford backscattering spectrometry (RBS), and x-ray diffraction. High energy resolution core level XPS spectra acquired with monochromatic Al Kα radiation on the ISO-calibrated instrument reveal that even mild etching conditions result in the formation of a nitrogen-deficient surface layer that substantially affects the extracted binding energy values. These spectra-modifying effects of Ar+ ion bombardment increase with increasing the metal atom mass due to an increasing nitrogen-to-metal sputter yield ratio.The superior quality of the XPS spectra obtained in a non-destructive way from capped TMN films is evident from that numerous metal peaks, including Ti 2p, V 2p, Zr 3d, and Hf 4f, exhibit pronounced satellite features, in agreement with previously published spectra from layers grown and analyzed in situ. In addition, the N/metal concentration ratios are found to be 25–90% higher than those obtained from the corresponding ion-etched surfaces, and in most cases agree very well with the RBS and ToF-E ERDA values. The N 1s BE:s extracted from capped TMN films, thus characteristic of a native surface, show a systematic trend, which contrasts with the large BE spread of literature “reference” values.Hence, non-destructive core level XPS employing capping layers provides an opportunity to obtain high-quality spectra, characteristic of virgin in situ grown and analyzed TMN films, although with larger versatility, and allows for extracting core level BE values that are more reliable than those obtained from sputter-cleaned N-deficient surfaces. Results presented here, recorded from a consistent set of binary TMN’s grown under the same conditions and analyzed in the same instrument, provide a useful reference for future XPS studies of multinary materials systems allowing for true deconvolution of complex core level spectra.

New developments in stopping power for fast ions

We discuss the available stopping power tables and codes, and new experimental methods. Among the semi-empirical stopping power tables, SRIM and MSTAR describe experimental data best. Of the recent theoretical stopping codes CasP, PASS, TCS and dielectric formalism, all describe some data well. PASS is the only theoretical code with an extended table for many ions and targets, but on the average, its predictive value at low energies is less than that of SRIM or MSTAR. The first-principle extended classical trajectory Monte Carlo method is very promising. The ToF-E ERDA arrangements have made it possible to produce accurate data for many ion–target combinations. A new liquid jet target method promises data for liquid water which is an important substance for medical dosimetry.

Crystalline quality of 3C-SiC formed by high-fluence C +-implanted Si

Carbon ions at 40 keV were implanted into (1 0 0) high-purity p-type silicon wafers at 400 °C to a fluence of 6.5 × 10 17 ions/cm 2. Subsequent thermal annealing of the implanted samples was performed in a diffusion furnace at atmospheric pressure with inert nitrogen ambient at 1100 °C. Time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) was used to investigate depth distributions of the implanted ions. Infrared transmittance (IR) and Raman scattering measurements were used to characterize the formation of SiC in the implanted Si substrate. X-ray diffraction analysis (XRD) was used to characterize the crystalline quality in the surface layer of the sample. The formation of 3C-SiC and its crystalline structure obtained from the above mentioned techniques was finally confirmed by transmission electron microscopy (TEM). The results show that 3C-SiC is directly formed during implantation, and that the subsequent high-temperature annealing enhances the quality of the poly-crystalline SiC.

Electronic stopping forces of heavy ions in metal oxides

Electronic energy loss of charged particles in materials is a fundamental process responsible for the unique response of materials in applications of advanced nuclear power, radiation detectors and advanced processing of electronic devices. In this study, stopping forces for ions in metal oxides of ZrO2, Ta2O5 and Nb2O5 have been determined using a time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) set-up. In transmission geometry, the energy loss of heavy ions in these thin metal oxide foils was measured over a continuous range of energies from a few 10keV/nucleon to over a thousand keV/nucleon using the ToF data that was tagged by a Si detector with and without the stopping foils. Comparisons are made with the SRIM-2003 (The Stopping and Range of Ions in Matter) predictions. While SRIM predicts He stopping force well, varying degrees of agreement with the measured stopping data up to 7% are observed and deviations around stopping maximum are evident for heavy elements.

Electronic stopping powers for Be, Ca and Ti in SiC

Energy loss of ions in matter is fundamental to many applications dependent on the transport of ions in matter. In spite of a long history of studies, the electronic stopping power is not adequately described over all ranges of ions, energies and targets, particularly in the case of heavy ions or compound targets. In this study, stopping powers for Be, Ca and Ti in SiC have been determined using a time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) set-up. In transmission geometry, the energy loss of heavy ions in a self-supporting SiC foil was measured over a continuous range of energies using the ToF data that was tagged by a Si detector with and without the stopping foil. By essentially calibrating the Si detector for each channel over the measured energy region based on TOF spectrometry, measurement uncertainties of less than 4% are achieved. In comparing with the experimental data, both the SRIM (stopping and range of ions in matter) code and the binary theory provide reasonable predictions.

Electronic stopping powers for He, Be and F ions in Au

The stopping powers for He, Be and F ions in Au are determined using a previously developed time-of-flight energy elastic recoil detection analysis (TOF-E ERDA) set-up. In transmission geometry, the energy loss of particles in a self-supporting Au foil is measured over a continuous range of recoil energies using the time-of-flight (TOF) spectrometer with uncertainty of less than 5%. The stopping powers are parameterized using a sixth order polynomial and compared with existing experimental data in the literature. In the energy regimes where experimental data exist, the present data exhibit good agreement with most data. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) are in reasonable agreement with much of the experimental data.

Measurements of the mean energy-loss of swift heavy ions in carbon with high precision

Z 1 dependent effects in heavy ion stopping around stopping maximum under conditions of equilibrium charge have been measured with a time of flight-energy elastic recoil detection analysis (ToF-E ERDA) set-up. The ToF section was used to tag individual recoil atoms with their energy prior to entering the stopping foil and the exit energy was measured subsequently using a Si p–i–n charged particle detector. The mean energy-loss and average energy of recoils have been determined for many elements with Z 1=3–26 in a continuous energy range from ∼0.2 to ∼0.9 MeV per nucleon and from ∼0.03 to 0.1 MeV per nucleon for 79Au. The stopping power and energy dependence were in good agreement with most literature data. However, considerable discrepancies in the energy dependence were observed with semi-empirical SRIM-2000 estimates around the stopping power maximum for some of the elements. This technique, which allows direct measurement of Z 1 effects in stopping foils to be performed with high-precision, showed that the dependence of the experimentally determined stopping number ( L exp) with the stopping parameter ξ clustered around a trend line with Z 1 dependent deviations that exceeded the estimated uncertainty. The deviations were more significant for large ξ and were 15% between 9Be and 7Li at ξ=10.

Characterisation of Compact Discs using Time of Flight-Energy Elastic Recoil Detection Analysis

The challenge of meeting the ever increasing demands for low-cost information storage media with greater information storage density and rapid access has prompted development of sophisticated optical technologies, e.g., compact disc (CD) and digital video disc (DVD) in read only memory (ROM), recordable (R), rewritable (RW) and random access memory (RAM). Here the suitability of time of flight-energy elastic recoil detection analysis (ToF-E ERDA) to characterise optical storage media film has been investigated. High-energy heavy ions (48 MeV 81Br8+ and 60 MeV 127I10+) from a tandem accelerator were used to analyse CD-ROM, CD-R and CD-RW structures. Up to 9 elements, H, C, O Al, P, Co, Cr, Ge, and Sb/Te could be distinguished simultaneously. The results demonstrate the unique power of the technique for characterising the composition and depth profile of the optical multi-layers as well as the ingress and influence of foreign species.

Sputtering transients for some transition elements during high-fluence MEVVA implantation of Si

The approach to quasi equilibrium sputtering of transition elements Co, Er, V and Ni during high-fluence implantation of Si(1 1 1) using a metal vapour vacuum arc (MEVVA) source has been studied by time of flight-energy elastic recoil detection analysis (ToF-E ERDA) and scanning electron microscopy (SEM). The partial sputter yield of the implanted species was determined from the change in the content of the implanted species with the implanted ion fluence. The partial sputter yield of Co exhibits a step-like rise to ∼0.4 that might be associated with a rapid segregation of Co to the surface followed by a slow exponential-like increase. Er on the other hand follows an exponential approach to the quasi-equilibrium partial sputtering yield which is indicative of no strong buildup of Er within the sputter escape depth. Additional data for Ni and V suggest also an exponential approach to quasi-equilibrium sputtering.

Analysis of ferromagnetic removable hard disc media ageing by time of flight-energy elastic recoil detection analysis

Time of flight-energy elastic recoil detection analysis (ToF-E ERDA) has been applied to study the ageing of commercial thin film hard discs. Samples from a standard commercial removable hard disc were subjected to accelerated ageing heat treatment in normal humidity from 90°C up to 240°C in steps of 30°C for 3 h each. A test using a high humidity environment at 90°C up to 40 h was also performed. The ToF-E ERDA was carried out using a 60 MeV 127I 11+ ion beam incident at 67.5° to the surface normal, and detecting recoils at 45° to the ion beam direction by measuring their time of flight and energy in a detector telescope. The magnetic thin film structure was assigned to be a CN x /Co/N–Cr/Ni–P multilayer. At temperatures higher than 180°C the CN x coating layer detoriated with an increase of surface oxygen and an N content reduction in the N–Cr layer.

Foreign atom incorporation during metal silicide formation by ion beam synthesis

Depth distributions were measured of foreign atoms incorporated during the formation of thin silicide surface layers by ion beam synthesis. Si (111) wafers were implanted with Co and V ions using a pulsed Metal Vapour Vacuum Arc (MEVVA) ion source operated at 40 kV extraction voltage. Post-implantation analysis was carried out using Time of Flight-Energy dispersive Elastic Recoil Detection Analysis (ToF-E ERDA) with 60 MeV 127I 10+. The results showed that addition to the metal ions, carbon and oxygen were incorporated at at.% levels, with distributions that were peaked at the surface and extended into the implanted layer. The findings suggest that incorporation of C and O is significantly influenced by the degree of silicide formation, with more continuous and stoichiometric silicides corresponding to a lower incorporation of foreign atoms.

Stoichiometry and profiling of surface layers by means of TOF-E ERDA and RBS

An experimental method is established for stoichiometric analysis and profiling of the surface region of solids by adapting a TOF-E counter telescope to acquire spectra of the atomic mass and the kinetic energy of recoils. A 10 MeV 35Cl beam, delivered by a small tandem, bombards samples to eject the recoils. The samples include Ta2O5 films deposited onto Si substrates. Advantages and the limit of applicability of the present method are discussed.