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XPS analysis of a N-doped GaAs thin film using Ag Lα excitation

Nitrogen-doped GaAs (GaAs1-xNx) 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 GaAs1-xNx. 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.

Group Array Analysis of a Combinatorial Thin Film System

The last few decades have seen rapid development in computational and theoretical tools for fabricating, simulating, and characterizing material systems. In this applications note the potential of surface characterization by X-ray photoelectron spectroscopy to provide rapid elemental and chemical state information is presented. The development of the group analysis array functionality is of significant importance in the application of XPS analysis to combinatorial materials discovery to allow the processing and display of large data sets. As will be demonstrated, group array analysis provides a more detailed understanding of the chemical distribution across the sample.

Quantitative Lateral Resolution of the Kratos AXIS Ultra and AXIS Nova XPS Instruments

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).

Building the energy level diagram of a 2D material using coincident UPS and IPES

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.

A Complimentary Spectroscopic Characterization of a Corroded Tinplate Sample with XPS and AES

A corroded tinplate sample was analysed by XPS and AES techniques using the Kratos AXIS Supra+ to gain an insight into the extent and mechanism of corrosion. XPS was firstly used to analyse the composition of the surface and from this, characterize the Sn 3d and Fe 2p oxidation states. The corroded area was then examined using SEM to pinpoint significant positions for further analysis with AES. The resulting data indicates the position at which corrosion is initiated and examines the extent of this corrosion across an area of the surface.

Eliminating core line/Auger peak overlap using different photon energies

GaN is one of many materials which are difficult to analyze with the conventional Al Kα X-ray source due to a strong overlap between the N 1s core line and the Ga LMM Auger series. This brings difficulties with accurate quantification and also chemical state assignment. In this applications note, GaN was analyzed using different X-ray excitation sources with the aim of shifting the binding energy position of the Ga LMM Auger series to prevent its interference with the N 1s region. Automation of changing between the Al Kα and Ag Lα excitation sources when using the Kratos AXIS Supra+ is an added benefit.

DOI : 10.5281/zenodo.431123

Sample rotation during depth profiling

The aim of this investigation was to characterise the performance of the gas-cluster ion source (GCIS) for the depth profiling of thick organic multi-layer materials. The sample is composed of 25 repeating units of polystyrene (PS) and polyvinylpyrrolidine (PVP) PS = 288 ± 1.3 and PVP = 328 ± 7 nm on a glass substrate. It has been considered challenging to depth profile thick samples (>1 micron) of soft materials due to the changes which occur under sus-tained X-ray irradiation and bombardment of charged projectiles, however, in this study we describe a methodology to eliminate these issues. The use of fast acquisition, snapshot spectroscopy and sample rotation during etching is presented.

A Multi-Technique Characterization of Polymer Materials with XPS, UPS and REELS

Using a combination of surface analysis tools, different polymer materials were analysed using the Kratos AXIS Supra+. A common issue with XPS analysis is that the C 1s envelope can look relatively similar for different polymer materials, making them difficult to distinguish using XPS alone. Plasmon features, such as the π-π* transition, which give information related to the sp2 content of a material are also concealed by shifts between different C chemical states. Here, a combination of XPS, UPS and REELS are used as complimentary tools to help understand the chemistry of several polymer materials.