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  • New paper published in Applied Surface Science

New paper published in Applied Surface Science

NPL and partners new paper on Inelastic background modelling applied to hard X-ray photoelectron spectroscopy of deeply buried layers

NPL is delighted to announce, alongside the Henry Royce Institute at The University of Manchester, Scienta Omciron GmbH and the University of Southern Denmark, the acceptance of the paper HAXPES Lab ‘Inelastic background modelling applied to Hard X-ray Photoelectron Spectroscopy of deeply buried layers: a comparison of synchrotron and lab-based (9.25 keV) measurements’. This paper has been published in Applied Surface Science and represents an important milestone in the establishment of the Hard X-ray Photoelectron Spectroscopy (HAXPES) research methodology.

Measuring materials in thin buried layers, common in electronic materials, is a significant analytical problem which is usually addressed by destructive depth-profiling or cross-sectioning methods. Currently it is very difficult to measure the amount of material in a buried layer and the depth of that layer with nanometre precision in a non-destructive manner. In 2019, the Henry Royce Institute, strategic partners of NPL,  secured the world’s first high throughput HAXPES Lab instrument from Scienta Omicron following a successful installation at The University of Manchester enabling fast HAXPES measurements in the laboratory using a high-intensity liquid metal-jet Gallium X-ray source (9.25 keV). 

The information depth of XPS, a surface analysis method can be increased by using high-energy X-rays, a method known as HAXPES. By using X-rays with more than six times the energy of traditional XPS, the information depth is increased from a several nanometres to a few tens of nanometres. The team had the ambition to stretch this information depth much further by also analysing electrons which have lost energy as they travel through the material. In a proof-of-principle experiment using materials carefully constructed at NPL they demonstrated the ability to locate a thin layer of an OLED green emitter at depths of 200 nm beneath the surface of an organic film with approximately 1 nm precision.

The method developed using the Royce lab-based HAXPES instrument, and supported by NPL metrology can be applied to characterise a wide range of thin-film materials with applications ranging from electronics to medical devices and sensors. In these devices, layer thickness and interfacial properties are critical and the method has been shown to have sub-monolayer sensitivity even for deeply buried layers.

Applications for advanced materials research are expected across many of Royce’s core research areas including Atoms-to-Devices, Nuclear Materials, Materials for Demanding Environments, Energy Storage, Biomedical Materials, and 2D Materials. Prior to the development of the HAXPES Lab, this methodology was only available to users of synchrotron facilities such as the I09 beamline at Diamond Light Source, where work was carried out to help benchmark the lab system.

The application of HAXPES for advanced materials research requires standardisation, calibration and, for quantification of elemental concentrations, a library of relative sensitivity factors (RSFs) for every accessible core level up to 9000 eV. This paper uses high quality control materials and calculations of these RSFs which can now be applied to every element. This is key to establish HAXPES as a widely used and accepted quantitative analytical method. This paper, by a team led by Dr Ben Spencer, Dr Alex Shard, and Professor Wendy Flavell, therefore presents a significant advancement for HAXPES.

Dr Alex Shard, Principal Research Scientist and Head of Science for the Chemical and Biological Science Department at the National Physical Laboratory, says, “It has been a great pleasure to work with Royce, the University of Manchester, Scienta Omicron and the University of Southern Denmark in this ongoing HAXPES metrology project. The number of HAXPES instruments around the world is rapidly increasing, as is the demand for reliable and meaningful measurements. In the near future we plan to develop the metrology for lab-based HAXPES instruments enabling instrument calibration, accurate measurements and algorithms for faster and easier data analysis.”

Professor Wendy Flavell, Professor of Surface Physics and the Vice Dean for Research in the Faculty of Science and Engineering at the University of Manchester, says, “We were delighted to secure the world’s first HAXPES Lab system from Scienta Omicron in Royce, and this work marks our first major output demonstrating the capability and, importantly, standardising the technique so it can be applied to world-leading advanced materials science.”

Dr Ben Spencer, Senior Experimental Officer Royce, says, “This work shows just how far into the surface we can look using high energy X-rays non-destructively. It’s an important step towards the HAXPES Lab becoming a black-box tool, and in Royce we have already applied HAXPES to measure a wide range of important materials-- watch this space.”

Scienta Omicron CEO Johan Åman says, “We are very proud to have worked closely with the researchers of this paper. The development of relative sensitivities factors for the gallium source is important for moving what was a novel technique towards a standard technique. We expect that the HAXPES Lab will enable a wide range of experiments to be completed by users into the future, all while being in their home lab”.

We would like to acknowledge the authors and contributors to this work: B. F. Spencer, S. Maniyarasu, B. Reed, D. J. H. Cant, R. Ahumada-Lazo, A. G. Thomas, C. A. Muryn, M. Maschek, S. K. Eriksson, T. Wiell, T.-L. Lee, S. Tougaard, A. G. Shard, and W. R. Flavell.

Read the full paper here: https://doi.org/10.1016/j.apsusc.2020.148635

11 Jan 2021