HAXPES depth profiling

GDOES (Glow-Discharge Optical Emission Spectroscopy) is a powerful technique for elemental depth profiling of materials. It is frequently applied in the industry; however, chemical and structural information is not obtained and the outermost ~100 nm layer may be difficult to probe correctly.

In some cases these inherent shortcomings of GDOES can be overcome with XPS (X-Ray Photoelectron Spectroscopy) profiling using ion sputtering in a standard laboratory. XPS is extremely surface-sensitive (~1 nm) and gives also chemical-shift information about the oxidation states and the chemical environment of the elements in the material. But the surface-sensitivity comes with a price, making it difficult to discriminate between the true signals from the material and surface contamination, and sputter-induced atomic rearrangements may also hamper the analysis. Such artifacts, together with the fairly long recording times, explain why XPS profiling is not very much used in the industry.

In recent years, the development of XPS analyzers for higher electron energies has paved the way for a new rapidly growing research field, HAXPES (Hard X-ray Photoelectron Spectroscopy), yielding the same excellent chemical data as standard XPS but with considerably larger probing depths suitable for sputter-free profiling and for probing of buried layers. By irradiating the samples with varying X-ray energies at state-of-the-art synchrotron-radiation sources, it is possible to distinguish contamination effects and study the change in the materials properties without sputtering from the outermost ~1 nm surface layer down to depths of ~50 nm.

Inspired by the convincing scientific results with HAXPES, we have, together with Prof. Håkan Rensmo’s group at Uppsala university and the solar-cell developer Solibro, investigated the feasibility of applying sputter-free HAXPES for industrial depth profiling of elements as a complement to GDOES in the outermost materials layers. Samples of CIGS-type materials were investigated at SOLEIL in Paris. HAXPES spectra were taken during one beam shift (a little over 8 hours) by using three different X-ray energies, and from the sample stochiometries and known electron cross-sections, we were able to deduce relative abundance trends in the outermost ~20 nm layer of elements such as Cu, Ag, Ga, In, Se, Rb, and Cs. One example of the resulting HAXPES data is shown below with GDOES data from the same sample. In this example the HAXPES and GDOES data are in fair agreement, verifying the validity of the GDOES profiles in the uncertain outermost layer. In other cases (not shown), HAXPES completes the picture in the outermost layer where reliable element data cannot be obtained with GDOES, giving key information for the development of the solar-cell production processes at Solibro.

HAXPES profiles for one sample (treated in three different ways) from the SOLEIL measurements.

GDOES profiles for the same sample. 1 s on the x-axis corresponds to ~6 nm.

We have also investigated the feasibility of using a proprietary approach for rapid sputter-free HAXPES profiling down to several micrometers or deeper. Spectra of an industrial nitrided titanium alloy sample (not from Solibro) were taken for two different X-energies (10 keV, 2.3 keV) at ten depths. A low-resolution X-ray mode was used to maximize the data acquisition speed. Data can be extracted from the 10 keV spectra supported by the reference information in the 2.3 keV spectra. The spectral contributions from the two different titanium nitride phases and the unreacted titanium alloy at different depths can be deduced from the relative intensities of Ti, Al, V, and N. Furthermore, which is not detectable with GDOES, chemical information can be deduced from the fine structure of the of the peaks (e.g., from Ti 2p at ~450 eV). To do this properly, the peaks of interest are resolved with a high-resolution X-ray mode. The used approach is also applicable for chemical depth profiling of, for instance, nitrided or carburized steel.

HAXPES overview spectra of a nitrided titanium sample using an X-ray energy of 2.3 keV
HAXPES overview spectra of a nitrided titanium sample using an X-ray energy of 10 keV

To conclude, we have demonstrated the feasibility of using synchrotron-based HAXPES for industrial elemental and chemical depth profiling. This feasability study were carried out at the end of 2018. The synchrotron tests at SOLEIL were financed by a grant from CALIPSOplus. For inquiries about HAXPES and XPS measurements at synchrotron facilities or lab-based XPS measurements, please contact Robert Moberg. For information about Solibro and Solibro’s interest in depth profiling, please contact Olle Lundberg.