Yanwen Zhang

High-precision measurement of electronic stopping powers for heavy ions using high-resolution time-of-flight spectrometry

Abstract A new technical and analysis approach based on using time-of-flight (ToF) to determine energy loss has been developed and used to improve the precision of measuring heavy-ion electronic stopping powers from a continuous energy spectrum of particles provided by a typical elastic recoil detection analysis geometry. The particle energies entering and exiting the stopping foil are determined using ToF spectrometry data, with and without the stopping foil. The Si detector is only used to tag identical energies and screen out the extraneous components from the spectrum. This approach, which is applicable to continuous energy measurements, eliminates the well-known calibration problem of Si detectors associated with heavy ions that is shown to lead to a clear deviation in the measured stopping power. Consequently, the stopping powers and the energy dependence are determined with higher precision. In this study, the stopping powers of a number of heavy ions ( 3⩾ atomic number ⩽53 ) in amorphous C, Al and Au have been determined with an absolute uncertainty of less than 2.5%. In some energy regimes, data are provided for the first time. In other energy ranges, the present data exhibit good agreement with most existing data. SRIM stopping power values show a reasonable agreement with experimental data in most cases; however, some deviations from the measured values, up to 15%, are observed around stopping maximum.

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

Abstract Z1 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 Z1=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 Z1 effects in stopping foils to be performed with high-precision, showed that the dependence of the experimentally determined stopping number (Lexp) with the stopping parameter ξ clustered around a trend line with Z1 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.

Electronic stopping power of swift heavy ions in carbon

The mean stopping power of He, Be, C and Al in carbon has been studied in a continuous energy ranging from ∼100 to 800 keV per nucleon using a modified time of flight-energy elastic recoil detection analysis set-up. In order to eliminate the calibration problems of Si detectors associated with heavy ions, the time of flight section was used to measure recoil energy both before entering and after passing through the stopping foil. Consequently, the energy dependence of the stopping power was achieved with high precision. The effect of the foil thickness on the mean stopping power was investigated by tilting the stopping foil at different angles to the incoming particles to increase the effective foil thickness up to ∼22%. The stopping curves obtained at different tilting angles show a similar energy dependence with scattering less than ∼0.8% from each other, and no obvious thickness dependence is observed. Both the stopping power and the energy dependence are in good agreement with the literature data. Comparing with semi-empirical SRIM prediction, considerable discrepancies up to 6%, 10% and 8% in stopping values for Be, C and Al, respectively, are observed.

Influence of heavy ion irradiation damage on silicon charged particle detector calibration

Abstract The full analytical potential of heavy ion backscattering and elastic recoil detection analysis (ERDA) depends critically on establishing a reliable energy calibration. In order to make accurate measurements of thin film samples we have investigated the changes in the energy calibration of a Si p+-n-n+ charged particle detector subjected to heavy ion irradiation over 24 h in a time of flight-energy elastic recoil detection analysis (ToF-E ERDA) measurement. In this study a set of similar Al/ZrO2/Zr samples were analysed sequentially with 60 MeV 127 I 11+ ions. The calibration change for 16O, 27Al and 90–92,95,96Zr were monitored by tagging individual recoils with their energy derived from the ToF. The calibration parameters for a wider range of elements (Li–Ag) were measured before and after the sequential irradiation with O, Al, Zr and I atoms. The results show that the change in the calibration could be characterised by an increase in the energy interval spanned by one channel and a slight decrease in the channel zero energy. The calibration shift for a given projectile atomic number depends linearly on the fluence of heavy particles impinging on the detector and the consequential increase in detector leakage current. This indicates that for similar irradiation conditions, a correction to account for the calibration shift may be simply determined for each sample from the number of heavy recoil counts registered or from the change in leakage current. Furthermore, the silicon charged particle detector calibration depends on recoil atomic number both before, and after, the heavy ion irradiation. The fluence-induced calibration shift for different recoils can be described by a linear dependence on recoil atomic number.

Evolution and recrystallization of buried amorphous layers in Al22+ implanted 4H-SiC

Epitaxial 4H-SiC has been irradiated at 150 K with 1.1 MeV Al-2(2+) molecular ions to fluences ranging from 1.5 x 10(13) to 8.00 x 10(14) Al+ cm(-2). The evolution and isochronal recovery of the bu ...

Measurements of Si Ion Stopping in Amorphous Silicon

The stopping of Si-28 ions in polycrystalline Si foils has been measured over the energy range 0.1-3.3 MeV per nucleon. For the low energy interval (0.1-0.5 MeV per nucleon), time of flight-energy elastic recoil detection analysis method was used, whilst for the high energy region (1.2-3.3 MeV per nucleon) the energy loss in the same foil was measured using a Si p-i-n diode with the Si-28 ions directly incident on the foil following acceleration. Below the stopping maximum the results are in good agreement with literature data based on Doppler shift measurements of short nuclear lifetimes but are about 20%, smaller than the SRIM prediction. Above the stopping maximum the data are in agreement with SRIM within the limits of statistical uncertainty. (Less)

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

Abstract The approach to quasi equilibrium sputtering of transition elements Co, Er, V and Ni during high-fluence implantation of Si(1xa01xa01) 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.

Measurement and uncertainties of energy loss in silicon over a wide Z1 range using time of flight detector telescopes

The energy loss of projectiles with Z(1) in the range 3-26 has been experimentally measured in the 0.1-0.7 MeV per nucleon energy range in the same Si stopping foil of 105.5 mug cm(-2) thickness using a time of flight-energy (ToF-E) elastic recoil detection analysis (ERDA) setup. A detailed study of the experimental uncertainties for ToF-E and ToF-ToF-E configuration has been made. For ERDA configurations where the energy calibration is taken against the edge positions small uncertainties in the angle at which recoils are detected can introduce significant, absolute uncertainty. The relative uncertainty contribution is dominated by the energy calibration of the Si E detector for the ToF-E configuration and the position of the second ToF detector in ToF-ToF-E measurements. The much smaller calibration uncertainty for ToF-ToF-E configuration implies this technique is superior to ToF-E measurements with Si E detectors. At low energies the effect of charge changing in the time detector foils can become important. It was found that empirical stopping numbers which include the effect of effective-charge were more closely clustered about a trend line when plotted against the Bohr parameter than the corresponding Bethe parameter. Study of the deviations from the trend line showed evidence of a shell dependant effect that exceeded the anticipated maximum relative uncertainties. (Less)

Bandgap modification in GaInAs/InP quantum well structures using switched ion channelling lithography

A novel technique has been developed to lithographically modify GaInAs/InP quantum well (QW) structures. 10xa0MeV 69Ga3+ ions are implanted into a 40xa0nm InP/ 9xa0nm GaInAs/InPxa0(100) substrate along the [100] direction. A 50 nm thick Au mask switches a fraction of the aligned ions into dechannelling trajectories. The dechannelled ions enhance the ion beam induced intermixing of the GaInAs QW with the surrounding InP cladding layers. Low-temperature photoluminescence (PL) was used to study bandgap modification of the GaInAs QW after implantation for a fluence range of 4×1011-4×1013xa069Ga3+ ionsxa0cm-2. A larger bandgap energy shift was observed with PL under the Au mask (28xa0meV) compared to areas not covered with the Au mask (12xa0meV). Furthermore, the integrated PL intensity was found to be lower in the masked areas, which is consistent with a greater defect concentration in the regions under the Au mask.

Annealing behaviour of foreign atom incorporated Co-silicides formed by MEVVA implantation into SiO2/Si and Si3N4/Si structures

Abstract Si 3 N 4 / Si (1 0 0), SiO 2 / Si (1 0 0) and SiO 2 / Si (1 1 1) wafers were implanted by keV Co ions under technical conditions to form thin silicide surface films. A metal vapour vacuum arc (MEVVA) source was employed to produce a high fluence of Co ions that was just sufficient to remove the oxide/nitride overlayer by sputter erosion. This high-fluence implantation under technical vacuum introduces foreign atoms into the implanted layer. The behaviour of these impurities following ∼900°C heat treatment in a N2 atmosphere has been studied. Elemental redistributions, crystal recovery, phase formation and electric properties were analysed. The results show that the annealing leads to an increase in crystal ordering within the implanted layer and stable phase formation, as indicated by a reduction in the channeling signal and a decrease in electrical resistivity. In the case of nitride-coated samples, the heat treatment leads to a reduction in the residual carbon and nitrogen in the samples, while the oxygen content is largely unchanged. In contrast, a remarkable increase in the carbon and especially the oxygen contents is observed in the case of the samples with oxide overlayers, and the Co shows a tendency to spread to greater depths.