Applied X-ray spectroscopy


Oxides everywhere

In April a team from the group including Curran, Aysha, Maria and Anna spent several days at beamline I09 at the Diamond Light Source for a combined SXPS/HAXPES experiment on a number of oxide materials, all relevant for electronic device applications.

We were hunting for both chemical state information and final state effects from core level spectra as well as for signatures of the electronic structure of the materials, including some 2D electron gases at buried interfaces. In order to avoid beam induced changes to the samples we used the defocussed setup at I09 and were rewarded with some intensely bright and large beam illumination on the samples (see image below). The electronic structure experiments needed extended acquisition times to obtain the needed signal quality, so we even had time to catch some fresh air and walk to the top of ISIS hill providing a great view of Harwell campus and the surrounding greenery.

This was also Maria’s first synchrotron experience. She is spending six months in the group as a visiting PhD student as part of her PhD, which she is undertaking at the University of Padua, Italy, under the supervision of Prof Alessandro Martucci. She works on crystallisation of solution-based metal oxides on temperature-sensitive substrates.


Where is my titanium going? Testing the stability of TiW barriers

Hot off the press! Check out our latest collaboration with colleagues from Infineon Technologies Austria, KAI and HarwellXPS, exploring the interface stability of TiW/Cu heterojunctions using SXPS and HAXPES. This work marks the second publication in a series by Curran Kalha on TiW diffusion barriers and continues a long and fruitful collaboration with beamline I09 at the Diamond Light Source.

Diffusion barriers are essential components in power semiconductor devices and are designed to isolate metallisation schemes from the semiconductor devices. The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier for copper metallisation schemes. However, little has been established regarding the chemical state of the TiW/Cu interface or the possible degradation mechanisms of the barrier during annealing.

In our recent paper in Journal of Applied Physics (the preprint is also on arxiv), we show that the TiW alloy is an excellent barrier for copper metallisation schemes, successfully isolating the copper after annealing for as long as 5 h at 400°C using both synchrotron-based SXPS and HAXPES. Under thermal stress the barrier starts to degrade via the out-diffusion of Ti, but using laboratory-based SXPS at HarwellXPS it is clear that the Ti quantity lost in the diffusion barrier does not significantly impact the performance of the barrier.

Stay tuned for more TiW research and the completion of Curran’s TiW trilogy (and maybe a prequel or origin story too).


Implementing inorganic materials in affordable, flexible biosensor platforms

Integrating inorganic materials, that show great potential for sensing application, into platforms that are suitable for the industrial production of cheap, non-invasive sensors is of great importance for their broad implementation. In our recent open access paper in Materials Research Express, we show the successful integration of copper oxide based electrodes for glucose sensing on printed circuit board (PCB) technology. Together with collaborators at the University of Bath led by Dr Despina Moschou we could show that direct oxidation on PCB compatible substrates is possible and how production parameters including annealing duration and temperature influence the surface morphology and chemistry as well as influencing the resulting electrochemical sensing properties.

The work in the paper is based predominantly on the Masters research of Shijia Liu and Ayse Ay, who did their Masters projects as part of their MSc in Advanced Materials Science and Engineering degrees in the group in the academic year 2017/18. The research also included the involvement of two UROP (Undergraduate Research Opportunities Programme) students, Qiaochu Luo and Xiangqi Hu.


Getting to the bottom of TiW

After spending a week at EMPA in Zurich, Switzerland, depositing high quality TiW thin films in February, Curran, Nathalie and Anna travelled to DESY, Hamburg, Germany, in the first week of March to collect HAXPES data on them. We were back at one of our favourite HAXPES beamlines, P22 at PETRA III, and the work was, as always, expertly supported by the local team of Dr Christoph Schlueter and Dr Andrei Hloskovsky.

In order to ensure that the samples where in the best possible condition for measurement we needed to apply quite an involved level of logistics including vacuum sealing, glove box transferring, and in-situ sputtering. This enabled us to measure the films in their truly metallic state without interference from surface oxidation and contamination. Although HAXPES enables to probe the bulk of a sample, overlying surface oxides can significantly influence and perturb the HAXPES spectral quality. We both explored the influence of Ti/W composition on the electronic structure as well as a challenging experiment to try and probe the buried interface between TiW and the underlying SiO2/Si substructure. We also had time to explore the local offerings of cake and caffeinated beverages.


Making mixed metal systems

In the second half of February Curran and Anna spent a week at EMPA, the Swiss Federal Laboratories for Materials Science and Technology, in Zurich, Switzerland, to deposit a range of TiW films for an upcoming HAXPES experiment at DESY, Hamburg, Germany. The samples will be used to increase our understanding of mixed metal barrier materials for power electronics.

This collaboration was made possible by the award of a UCL Global Engagement Fund (GEF), a funding route available to UCL academics that supports collaboration with colleagues based in other countries.

Our colleagues at EMPA, Dr Sebastian Siol and Dr Siarhei Zhuk were excellent hosts and shared their extensive knowledge on the deposition of such metal systems. In parallel, Curran was able to immediately characterise all deposited samples using a combination of X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). This provided a solid characterisation basis to finetune deposition parameters and achieve a high level of control over film thicknesses and composition.

Although it was an intense week of work, there was still time to enjoy the finer side of life in Zurich, including sampling some local delicacies including Schnitzel (pictured below), Raclette and a very good amount of Swiss chocolate.


How to clean metals

In early February a team led by Curran, including Prajna, Yujiang and Anna, visited beamline I09 at Diamond Light Source in an attempt to collect soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) data on the pure metals titanium and yttrium. Sounds simple, but obtaining clean metal surfaces, and more importantly maintaining clean surfaces during measurement, is a real challenge for these two. Titanium in particular finds application as a so-called getter material, where its high reactivity is used to absorb stray molecules in vacuum chambers to achieve ultra-high vacuum (UHV) conditions. Great for vacuum chambers, not ideal when you need to keep a titanium surface free of adsorbates. After some not entirely successful previous attempts, a combination of ex-situ chemical etching, in-situ argon etching, and keeping the samples at a few hundred toasty degrees during measurement proofed to be the magical combination to obtain perfect metallic spectra. Such high quality reference datasets are crucial to aid the exploration and understanding of complex systems, where convoluted spectral data can prove to be a formidable challenge. This data will support Curran’s ongoing work on metallisation schemes for power electronics (read more about some of the work here and here) and further projects on energy materials and catalysts. As always the beamtime was masterfully supported by Pardeep Kumar. This was also the first synchrotron experience for Prajna and Yujiang.


FinEstBeAMS at MAX IV

At the end of January Aysha and Anna spent a week at the Swedish synchrotron Max IV in Lund using the solid state end station of the FinEstBeAMS beamline. Together with colleague Dr Matthias Kahk from the University or Tartu, Estonia, we used polarisation-dependent soft X-ray photoelectron spectroscopy (SXPS) to explore the electronic structure of bismuth vanadate (BiVO4) and the influence of different dopants on the system. This was Aysha’s first synchrotron experience.


SPIE Photonex 2021 – Glasgow

Nathalie, Curran and Anna ventured to Glasgow this week for the group’s first in-person conference since March 2020. Nathalie and Curran presented part of their PhD work in excellent contributed talks. Curran got people excited about studying the behaviour of TiW diffusion barriers in metallisation schemes for power electronics (see a recent paper) and Nathalie clearly showcased the importance of radiation dose when comparing the effects of X-rays on samples during diffraction and spectroscopy (see a recent paper and Diamond Light Source Science Highlight). Anna presented an invited talk on the influence of polymorphism in Ga2O3 trying to convince the audience that core level spectra are sensitive to local coordination environments (read all about it here).

It was great to get back to in-person talks and be inspired by an excellent line-up of speakers in the Photoemission Spectroscopy for Materials Analysis section organised by Robert Palgrave, Rosa Arrigo and Phil King.


Work on X-ray induced changes gets a shout-out in Diamond Light Source Science Highlights

Our recent work led by Nathalie Fernando on the effects X-ray irradiation has on small molecular crystals during X-ray diffraction and X-ray photoelectron spectroscopy has been chosen as a Diamond Light Source Science Highlight.

Nathalie and Anna had a great experience talking to the Diamond comms team, who put together an awesome article for the Science Highlights series, which has showcased work conducted at the synchrotron since 2014.

Schematic of the [M(COD)Cl]2 catalysts and the different ways in which they are affected by X-ray irradiation.


X-ray induced changes in small molecular crystals

Our most recent work, led by Nathalie Fernando, focuses on a subject matter typically considered a nuisance to many experimentalists who use X-rays. That is, the X-ray induced changes to the sample being probed. X-rays feature in many characterisation techniques today but are often (incorrectly!) regarded as non-destructive. In recent years, these unwanted X-ray-matter effects have been worsened by X-ray sources, both lab and synchrotron based, with ever increasing brightness. These X-ray induced phenomena have been widely studied in macromolecular crystallography, but unfortunately, the same cannot be said for small molecular crystals.

Recently published in the Journal of Physical Chemistry A, (available also as a preprint on ChemRxiV) our paper is the culmination of a huge collaborative effort comprising of crystallographers, spectroscopists, and theorists.

Here, we explore the effects of X-ray irradiation on two industrially important catalysts [Ir(COD)Cl]2 and [Rh(COD)Cl]2, using synchrotron-based X-ray diffraction and laboratory-based X-ray photoelectron spectroscopy. Both single crystal and powder X-ray diffraction were used to obtain a detailed understanding of the changes in structure and atomic positions upon irradiation. This was only possible through the close collaboration with an amazing team of crystallographers, including Dr Andrew Cairns (Imperial College London), Dr Claire Murray (Diamond Light Source), and Dr Amber Thompson (University of Oxford). Crystallography and spectroscopy, with density functional theory calculations, led by Dr. Laura Ratcliff (Imperial College London) and with the support of Nayera Ahmed, an MSci student in the group, provide insights into the structural, chemical, and electronic changes taking place within the samples upon X-ray irradiation. Crucially, these changes are studied with respect to X-ray dose, using the RADDOSE-3D software, with the support of Dr. Joshua Dickerson and Prof. Elspeth Garman at the University of Oxford. 

This work presents an important first step towards understanding the changes in small molecular systems due to X-ray irradiation, and we hope that the combination of techniques outlined, will form the basis of many future systematic X-ray damage studies on a wide range of important small molecular materials.