X-ray Photoelectron Spectroscopy and Materials

The second blog in the characterization mini-series is on X-ray photoelectron spectroscopy (XPS) which is another surface analysis technique. Before getting into the details of XPS, I would like to introduce my colleague Hong Piao who assembled this blog.

HungHong joined GE Global Research in 2004. She is presently a Surface Scientist in the Materials Characterization and Chemical Sensing organization, where she is involved in research and development in application of advanced methods of surface analysis, including XPS, Auger Electron Spectroscopy (AES) and synchrotron-based techniques, to the study of surface chemistry to support a wide variety of materials development programs for GE Global Research and GE business. Her particular areas of expertise include the techniques and applications of X-ray Photoemission Spectroscopy in materials science. She has played an important role in different projects, including thermotunneling project by having developed a thermionic emission and work function measurement for low work function materials; Energy Conversion –Advanced Technology SiC gate oxide development project in understanding the gate oxide and interface structure; the CdTe solar effort where the team is providing surface and interface analysis of the materials in order to help understand the correlation to device performance; and NIST superhydrophobic program by finding correlations between the surface chemistry/electronic structure of the materials and their hydrophobic properties in order to help identify the correct procedures for materials fabrication, surface treatments and testing conditions. In particular, her most recent interest has focused on the applications of synchrotron based hard X-ray photoelectron spectroscopy in materials characterization.

X-ray photoelectron spectroscopy (XPS)

1. What does the instrument do?

XPS analyzes top surfaces (<10 nm) of materials.

2. How does the instrument work?

X-ray photons interact with a material surface under high vacuum conditions and ejects electrons called photoelectrons. The photoelectrons are collected and analyzed to determine their kinetic energy.

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Visit to view the animation.

3. Why do we use the instrument?

XPS is often used to provide elemental, chemical state and structure information on the solid surface. Photoelectrons are emitted from all unique energy levels of the target atom. Any effect that may cause a perturbation of the energy levels of atoms near the surface will cause a concomitant variation in the XPS spectrum, i.e., chemical shifts in energy, thus the chemical shifts correlated with overall charge on the atom can be used to identify the local environment and chemical state of the element present in the materials being analyzed (see the figure below).

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The recent development of XPS instrumentation with near-micron spatial resolution has advanced its capability for elemental and chemical state imaging accompanied by small-area analysis.

The recent development of XPS instrumentation with near-micron spatial resolution has advanced its capability for elemental and chemical state imaging accompanied by small-area analysis.

Diagnosis of interfacial delamination in microelectronic devices

The top panel of the figure below shows an optical image of a part of a microelectronic device. A schematic of the pad structure of the device is also shown. The photo revealed the occurrence of delamination on some pads. The bottom panel of the figure shows the quantitative XPS images of all the key elements (In, Sn, Mo, C, O, Si and N). The localization of ITO (In and Sn) and Mo are clearly revealed. The images constructed from the two N 1s chemical states are shown on the left. Apparently, the formation of undesired Mo nitride at the interface resulting from the interaction of Mo with the N-containing polymer (polyimide and/or adhesion promoters) residues is found to degrade the adhesion between the ITO and Mo layers by acting as a release layer; Piao et al, Surf. Interface Anal. 39 (2007) 493–500.

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XPS can also provide information on the structure of valence electrons for the determination of valence/conduction band positions and work function measurements. A great effort has been devoted to the exploration of promising materials for water splitting under visible light irradiation in the past. The figure below shows a comparison of the band positions (only information on occupied states) determined by synchrotron-based XPS measurements. It is noted that Ta3N5 and SrTiO3 are essentially stable during photo-oxidation and photo-reduction of water. This indicates that these two materials are potential photo-catalysts for water splitting application; Piao et al, 2012GRC499 (Class 1).

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2 Comments

  1. Raul Rebak

    Congratulations

  2. Randy Dellwo

    Great post Vince,

    I particularly like the XPS mapping images.

    Randy