This applications note demonstrates the use of both conventional (Al Kα) and higher photon energy (Ag Lα) excited X-ray Photoelectron Spectroscopy (XPS) to characterise the solid electrolyte interphase layer formed on a graphite anode during charge/discharge cycles.  Samples were prepared in a glove box at the manufacturing site and transported to the XPS analysis laboratory using an air sensitive sample transport accessory.

Nitrogen-doped GaAs (GaAs_1-xN_x) is finding application in high efficiency solar cells and vertical cavity surface emitting lasers (VCSELs). Nitrogen doping has been shown to cause perturbation of the host band structure. Determination of chemical state and nitrogen concentration is pivotal in defining the material’s electronic structure. X-ray photoelectron spectroscopy (XPS) is a commonly used technique for surface characterisation of this dilute N doped material.
In this application note we contrast the use of Al Kα and Ag Lα as excitation sources for XPS analysis of GaAs_1-xN_x. The advantage of using the higher photon energy excitation source to ‘remove’ overlapping Auger transitions with the N 1s core-level photoemission peak is demonstrated. Also introduced are advantages relating to the increased sampling depth provided by using the Ag Lα excitation for characterisation of a thin-film material.

Small area X-ray Photoelectron Spectroscopy in the 10’s of micron range has become an important tool for the surface analyst. Two approaches to producing small area XPS information are currently used in commercial XPS instrumentation. One approach employs a micro-focused X-ray probe to limit the sample area illuminated by X-rays during measurement. The second approach, utilised in the AXIS range of spectrometers, is to use a system of electrostatic lenses and apertures to limit the acceptance area of the analyser. In both cases the analysed area on the sample surface is determined by measuring the distance required to receive a 20 to 80% signal increase when scanning the spot across a sharp edge. This is standard practice as described in the literature. 

Published work [1] has highlighted the fact that signal can be obtained over an area many times greater than the quoted spot size when recording XPS data using X-ray microprobe technology. This investigation utilises an approach similar to the aforementioned work to determine quantitatively the total area measured during small area XPS measurements using the virtual probe approach.

[1] U. Scheithauer, Surf. Int. Anal., 40, 706-709 (2008).

2D materials are becoming increasingly relevant in state of the art device development.  Using a combination of the surface sensitive techniques Inverse Photoelectron Spectroscopy (IPES) and Ultraviolet Photoelectron Spectroscopy (UPS), we demonstrate how the analyst can obtain information on the nature of the electronic properties for single and multi-layer stacks. In this example the properties of a thin layer of MoS₂ are investigated. Bulk MoS₂ is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of ~1.2 eV. However, with reduced thickness the bandgap increases with a confinement-induced shift of the indirect gap from the bulk value. Thickness/growth control is therefore critical to the performance of this material in next-generation devices.