Surface analysis at BESSY II: sharper insights into thin-film systems
The illustration shows how the APECS measurement works on a nickel single crystal with an oxidised surface. An X-ray beam ionises atoms, either in the nickel crystal or on the surface. The excited photoelectrons from the surface and from the crystal have slightly different binding energies. The Auger electrons make it possible to determine the origin of the photoelectrons.
© Martin Künsting /HZB
Interfaces in semiconductor components or solar cells play a crucial role for functionality. Nevertheless, until now it has often been difficult to investigate adjacent thin films separately using spectroscopic methods. An HZB team at BESSY II has combined two different spectroscopic methods and used a model system to demonstrate how well they can be distinguished.
Photoelectron spectroscopy (PES) enables the chemical analysis of surfaces and semiconductor layers. In this process, an X-ray pulse (photons) hits the sample and excites electrons to leave the sample. With special detectors, it is then possible to measure the direction and binding energy of these electrons and thus obtain information about electronic structures and the chemical environment of the atoms in the material. However, if the binding energies are close to each other in adjacent layers, then it is hardly possible to distinguish these layers from each other with PES.
A team at HZB has now shown how precise assignments can nevertheless be achieved: they combined photoelectron spectroscopy with a second spectroscopic method: Auger electron spectroscopy. Here, photoelectrons and Auger electrons are measured simultaneously, which gives the resulting method its name: APECS for Auger electron photoelectron coincidence spectroscopy (APECS).
A comparison of the binding energies determined in this way then allows conclusions to be drawn about the respective chemical environment and thus enables the finest layers to be distinguished. Using a single-crystal nickel sample, a very good model system for many metals, the team has now been able to show how well this works: The experimental data enabled the physicists to precisely determine the shift in the binding energy of the electrons, depending on whether they came from the thin oxidised surface or from the deeper crystal layers.
"At first, we were sceptical whether it would be possible to really extract a clear distinction from the data. We were excited to see such a distinct effect," says Artur Born, first author of the paper, who is doing his doctorate in Prof. Alexander Föhlisch's team.