Accurate stopping power determination of ¹⁵N ions for hydrogen depth profiling by a unique combination of ion beams and synchrotron radiation

Hydrogen plays an important role in many areas of material sciences. For example, hydrogen is an abundant impurity in thin-film materials - depending on the deposition process - and has great influence on the chemical, physical and electrical properties of many materials. Therefore, hydrogen analysis is of particular importance and it is often necessary to obtain a depth profile of the H-concentration. Best-suited methods for depth-resolved hydrogen analysis are ion beam techniques such as elastic recoil detection (ERD) and nuclear reaction analysis (NRA). In principle, both methods can be performed as primary - reference material free - methods.

The method with the best depth resolution is NRA using the 6385 keV resonance of the ¹H(¹⁵N,αγ)¹²C nuclear reaction. The correct quantification of the depth scale in the measured hydrogen profiles essentially relies on accurate stopping power values. The most commonly used stopping power values are from the SRIM program [1]. Here, the stopping power for heavy ions is calculated using the stopping power for hydrogen and an effective charge. The parameters for the effective charge are based on a fit of experimental data. Due to the lack of reliable experimental data, the uncertainty in the stopping powers, thus obtained, can be quite large and is only known in the widest sense (as the average deviation between experiment and SRIM for all elements or all ions). Therefore, we present a new method to deduce these values from a unique combination of two techniques: NRA and X-ray reflectometry (XRR), also a primary method. This method is applied to the determination of the accurate stopping power of ~6.4 MeV ¹⁵N ions in H-containing amorphous Si-layers (a-Si:H).

The samples are prepared by magnetron sputtering of a-Si in an Ar/H₂-atmosphere on a Cr-layer, which is needed as contrast material for XRR. The energy loss in the layers is measured by NRA at HZDR and the H-content has also been determined by NRA [2]. The layer thickness, density and roughness are determined by XRR using synchrotron radiation. XRR measurements were performed at the electron storage ring BESSY at the hard X-ray beamline BAMline [3]. The beam was monochromatised to 10 keV using a Si[111]-double-crystal monochromator. The reflected photons of the θ-2θ-scans from the 6-circle goniometer are counted by a scintillation detector and a photodiode, respectively. Data analysis is performed by the IMD 4.1 software package [4].

The combination of results from NRA and XRR allows the accurate calculation of the mass stopping power independent of the density of the material. The results for the H-containing amorphous Si-layers are presented, including a discussion of the uncertainty budget. Stopping powers at different H concentrations, ranging from 6 at.% to 14 at.%, are determined with an overall uncertainty of 3%. The preliminary results show a reasonable agreement with the SRIM values, thereby increasing the confidence in the results obtained by NRA.
[1] J. F. Ziegler, J. P. Biersack and U. Littmark, “The Stopping and Range of Ions in Solids", Pergamon Press, New York, 2009, http://www.srim.org.
[2] D. Grambole et al, Nucl. Instr. Meth. B 210 (2003) 526.
[3] W. Görner et al., Insight-Non-Destr. Test. Cond. Monit. 48 (2006) 540.
[4] D.L. Windt, Comp. in Physics 12 (1998) 360.