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  4. 024: XPS for Beginners – Fundamentals 4

024: XPS for Beginners – Fundamentals 4

  • Created April 21, 2021
  • Author Mark Isaacs
  • Category HarwellXPS Beginners Course

For more information on some of the topics discussed in this video, check out some of our articles below:

Analysis depth and IMFP

Background types

Useful tool: IMFP calculator

Questions

1: The IMFP of Ag 3d electrons (Kinetic energy = 1119.7 eV) travelling through an unknown mixed oxide matrix overlayer is determined to be 2.42 nm. What is the information depth of this system?

Answer

Easy one to start with, we saw in the video that we can define the information depth as 3x the IMFP. So, in this case, our information depth is 7.26 nm!

2: This unknown sample is compared to a number of well defined standards. SiO2, TiO2 and Graphite overlayers are prepared atop a silver sample of equal thicknesses. If the IMFP of Ag 3d electrons through each is 3.18, 2.27 and 3.07 nm respectively, in what order should we expect to see an increasing intensity of our Ag 3d spectra (lowest to highest)?

Answer

A higher IMFP suggests we can see deeper into our material, we learned that IMFP may be affected by many factors including the atomic mass, so it is no surprise to see that TiO2, with it’s high mass, results in the most restricted path length of travelling photoelectrons. Higher path lengths result in a higher signal from substrate layers (in this case, Ag). This means that, in increasing order of signal intensity, we will see Ag/TiO2, Ag/unknown, Ag/SiO2 and Ag/graphite.

3. The IMFP of Si 2p photoelectrons travelling through a SiO2 overlayer, having been excited by various sources is given in nm below:

  • Mg Kα = 2.67
  • Al Kα = 3.09
  • Ag Lα = 5.6
  • Cr Kα = 9.34
  • Ga Kα = 14.83

Which sources would you require to analyse the substrate of the following systems?

  • 2 nm SiO2/Si
  • 6 nm SiO2/Si
  • 9 nm SiO2/Si
  • 13 nm SiO2/Si
  • 30 nm SiO2/Si

Answer

Again here, we’re going to work to our approximation that information depth = 3λ. Clearly the thicker the silica overlayer, the more photoelectrons we will lose to scattering processes, so let’s go through 1 by 1.

2nm and 6nm: The smallest ID will be that from Mg kα, which ≈ 8.01 nm, so clearly we can use any source to observe the substrate for these samples.

9nm: This is now thicker than we should be able to observe using Mg kα radiation, so we need to try others. The ID from Al kα is ≈ 9.27nm, so we should just be able to observe it, though it will likely be a weak signal.

13nm: This is clearly above the capabilities of Al kα radiation, so now we will move to Ag Lα (ID = 16.8 nm) to observe the substrate.

30nm: Finally we look at our thickest film. Cr kα radiation results in an ID ≈ 28.02, so we can see that even this will not be enough to see the substrate in this system. We must use the Ga kα source, with a whopping 44.49 nm ID!

In section 5, we explore the many additional peaks that may appear in our XPS spectra – shake-up peaks, satellites, plasmons – as well as discussing the asymmetry exhibited by certain peaks and look at what causes this.

Continue to Section 5

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