Cross-section for 14N(α, p0)17O reaction in the energy range 3.2–4.0 MeV

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Abstract

The cross-section for the 14N(α, p0)17O nuclear reaction was measured in the beam energy range between 3.2 and 4.0 MeV, at a laboratory scattering angle of 135° using a thin TiN film (95 nm) deposited on Si substrate. The accurate stoichiometry as well as the thickness of the TiN film and other potential impurities were characterized by ERD–TOF with a 40 MeV Cu8+ beam and RBS with an 800 keV He+ beam. The cross-section was evaluated at maximum intervals of 15 keV, with smaller intervals of less than 10 keV in the neighborhood of the resonance, from the simultaneously measured nuclear reaction spectra and Rutherford backscattering spectra. The total uncertainty of the cross-section is estimated to be less than 7%. As an application, the evaluated cross-section was used to simulate the NRA spectra obtained from a GaAs1−xNx sample with different beam energies. The nitrogen concentration results are in good agreement with the secondary ion mass spectrometry measurements.

Introduction

Among the several nuclear reactions that can be used to characterize nitrogen, such as 14N(d, α)12C, 14N(d, p)15N, 14N(α, p0)17O and 14N(α, γ)18F [1], [2], the 14N(α, p0)17O endothermic resonance nuclear reaction stands out to be a better choice in most of ion beam laboratories because other nuclear reactions activated by probing deuterium beam induce unwarranted long-term radiation background in the setup. In the past, Doyle et al. applied 14N(α, p0)17O reaction on the 14N analysis using microbeam [3] and later, Lin et al. measured the excitation curve at 135° from 3.4 to 4.0 MeV which showed a resonance at beam energy around 3.7 MeV [4]. However no absolute cross-sections were presented. More recently, Giorginis et al. measured the cross-section of 14N(α, p0)17O nuclear reaction at 135° with the beam energy in the range 4.0–5.0 MeV [5] and observed three resonance peaks with the most pronounced one being excited at 4.44 MeV. Our preliminary measurements of the cross-sections in the energy range 3.2–4.0 MeV revealed that the maximum of the cross-section at the resonance around 3.7 MeV is comparable to the peak value at 4.44 MeV mentioned earlier. Lower beam energy accounts for a better depth resolution and less energy spread due to the thinner stopping foil needed to stop the scattered ions. In addition, at lower beam energy, the contributions from other possible beam-induced reactions are expected to be lower.

In this paper, we report and discuss the cross-sections measurements for the 14N(α, p0)17O nuclear reaction at a laboratory scattering angle θlab = 135° in the α particle energy range 3.2–4.0 MeV. An example using the evaluated cross-sections to characterize the GaAs1−xNx epitaxial layer is presented at the end of the paper. The results of the simulation on NRA spectra are in good agreement with SIMS measurements.

Section snippets

Experimental

A commercial TiN thin film (∼95 nm) deposited on Si substrate was selected for the measurement of cross-section variation as a function of incident beam energy. In order to obtain the accurate information of the ratio of area densities of Ti and N as well as the thickness of the film, ERD–TOF with 40 MeV Cu8+ beams (carried out on a 6 MV tandem accelerator [6]) and RBS with 800 keV He+ respectively were carried out. During the RBS measurement, the samples were tilted 7° to avoid the channeling

Data analysis

The method we used to evaluate the cross-section from the experimental data is based on the comparison of integrated proton peaks in NRA spectra and scattered peaks from Ti in RBS spectra. They are expressed in the following formula:YNRA=QΩNRACNσNRAE0-ΔE2,YRBS=QΩRBSCTiσRBSE0-ΔE2,where YNRA and YRBS are the integrated yields of proton from 14N(α, p0)17O reaction in NRA spectra and scattered helium from Ti in RBS spectra, respectively. Q is the number of He2+ ions bombarding the sample. ΩNRA and Ω

Results and application

Fig. 1 shows the atomic ratio of Ti and average N in the film measured by ERD–TOF using 40 MeV Cu8+ beam. The film is homogeneous and CTi/CN = 1.00 ± 0.05. The thickness of the film is (960 ± 10) × 1015 at./cm2. A few percent of O were detected at the surface and at the interface between the TiN film and Si substrate by ERD–TOF. However, the error on the stopping power due to the O presence at surface is less than 1%. The stoichiometry and the thickness of the film were confirmed by RBS measurement using

Conclusions

The cross-section of the 14N(α, p0)17O endothermic nuclear reaction for He beam energy 3.2–4.0 MeV has been evaluated at laboratory scattering angle of 135°. The error of the measurement was estimated to be 7%. As an application, the cross-section was used to simulate the NRA spectra obtained from GaAs1−xNx sample with different beam energies near the resonance. The simulation results were in good agreement with SIMS measurements. The numerical differential cross-sections will be available soon

Acknowledgements

The authors are grateful to R. Gosselin and L. Godbout for their technical support, and J.-N. Beaudry and P. Desjardins for providing the GaAs1−xNx samples and SIMS results. This work was supported by the Natural Science and Engineering Council of Canada (NSERC) and NanoQuébec.

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