“Electron spectroscopy is applicable for the analysis of all elements in the Periodic System.”
Kai Siegbahn, Nobel lecture 1981.
X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect and has its beginnings in the ground breaking work of Kai Siegbahn. It can non-destructively probe the chemical environments and electronic structure of matter, and since its invention it has been applied to a vast range of materials, including solids, liquids and gases. The most common variety of XPS uses soft X-ray sources, e.g. Al Kα, giving extremely surface sensitive results probing only the first few nanometres of a solid sample surface. In contrast, hard X-ray photoelectron spectroscopy (HAXPES) uses X-rays with energies up to 10 keV to excite photoelectrons, which leads to a substantial increase in information depth. This makes HAXPES extremely useful for a wide range of scientific problems. Firstly, it allows the investigation of realistic samples without the need of any prior surface preparation. Secondly, as it provides complementary bulk data to the surface data from XPS. And thirdly, and most importantly for devices, as it can probe buried layers and interfaces in structured samples.
“The interface is the device.”
Herbert Kroemer, Nobel lecture 2000.
In electronic devices, from classic Si semiconductors to advanced oxide memories and transparent transistors, all electric behaviour and device performance is directly connected to interfaces, either between different semiconductors or between semiconductors and dielectrics or metallisation. Whilst the Si/SiO2 interface of established Si-devices is relatively well understood, we greatly lack information and insight beyond this prototypical interface. In new device generations, which are aiming to deliver low power consumption and advanced device characteristics, new materials are used, often creating very complex interfaces. The lack of knowledge about these boundary regions limits the application and use of novel device generations.
HAXPES is ideally placed to shine a light on the hidden but vital components of devices, delivering a direct probe of chemical states and electronic nature of buried layers and interfaces. One limiting factor to the application of HAXPES to date, is that the great majority of systems is located at synchrotrons due to a combination of the necessity for very intense X-rays and radiation safety. Recently, however, the first laboratory-based instruments have been announced opening up this technique to a wider user community.
This talk will cover the latest in method and instrument development, as well as showcase experimental results from a range of technologically relevant systems, including multilayer structures used in transistors and memristors.