Elemental Identification & Spectral Features

The most common first step when carrying out XPS analysis of a sample is to record a wide energy scan that ranges from 0 eV – 1000eV in binding energy. This is called a survey scan and is very important in determining the elemental composition of the sample. XPS spectra are usually displayed as a plot of intensity as a function of binding energy, with the intensity taken as the number of electron counts per second. From equation 2.4, the kinetic and binding energies have different signs, so the spectra are plotted with increasing binding energy from right to left. In other words, a low binding energy represents a high kinetic energy and a high binding energy represents a low kinetic energy. When the photoemission process occurs, the electrons are emitted at discrete energies which produce intense peaks in the spectra as shown in Figure 2.6 for a survey scan of the DETA SAM deposited on a native SiO2 substrate. The binding energy at which these peaks are positioned

allows the user to identify the elements present at the surface region as each element has a unique set of core level binding energies. As discussed previously, any scattered electrons that lose energy are emitted with a random energy and add to the rising background of the spectrum.

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Figure 2.6 Survey scan of DETA SAM deposited on SiO2 substrate with the principal core

levels labelled.

Also, commonly observed in survey spectra are X-ray induced Auger lines. Auger electron emission features arise when a core level electron transitions to a deeper empty energy level. The electron can then transfer energy to another electron in that energy level providing it with enough kinetic energy to be emitted from the surface. In this process the energy of the emitted electron is dependent only on the energy separation between the core levels. One can easily distinguish Auger lines from photoemission lines by changing the energy of the X-ray source. The Auger lines will always appear at the same energy position irrespective of the incident photon energy, while the photoemission peak will be shifted by the difference in photon energy between the Mg and Al anodes. Tables of measured and calculated

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binding energies exist for almost all elements. There are many books and websites available to aid in the identification process such as the NIST online database3,11_.

Once all elements have been identified from the survey scan, high resolution narrow energy scans are typically performed on the principle peaks seen in the survey scan for each element present. A standard non-monochromated XPS can be used to identify the presence of elements which are at atomic concentration levels greater than ~ 0.1 % - 1 %. Elements are generally identified by the presence of their most intense spectral feature, for example in the case of copper one would generally examine the Cu 2p peak. Although each element has its own unique set of core level binding energies, other elements may overlap with these positions. The Si 2p and the Co 3s peaks have binding energy positions of 99.8 eV and 101 eV respectively; as the two peaks are so close together it is not possible to resolve them. In this case, other core level peaks would be examined such as the Si 2s and the Co 2p.

Several different spectral features can also be present in the recorded spectra. These features can give more information on the sample including chemical states. Spectral features can arise for a variety of different reasons including instrumental, X-ray electron interactions and electron-electron interactions. When using a non- monochromated X-ray source, a satellite peak accompanies the main photoemission peak. These satellite peaks are caused by low intensity X-rays from Kα3,4 emission and is common from both anodes. The peaks created by these X-

rays are generally less than 10% of the main peak intensity and are shifted to between 8.4 – 10.1 eV lower binding energy from the main peak for the Mg anode and 9.8 – 11.8 eV lower binding energy from the main peak for the Al anode3_{. To}

remove these satellite peaks an X-ray monochromator must be used.

Quite often plasmon loss peaks are observed at higher binding energies from the main peak and are specific to clean metal or highly doped semiconductor surfaces. Plasmon loss peaks are caused by discrete energy loss due to collective oscillations

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of conduction electrons. Any photoemitted electron passing through these oscillations will experience an energy loss characteristic of the mode of oscillation of the conduction electrons. Typically, these peaks are seen at a binding energy ~10 eV greater than the main peak, with higher harmonics of the oscillations occurring at regular intervals above the bulk loss feature.

Shake up structures occur when photoelectrons lose energy through the promotion of valence electrons. An outgoing photoelectron can interact with a valence electron and “shake it up” to a higher energy level. The kinetic energy of the emitted photoelectron is then reduced resulting in a peak at a higher binding energy than the main peak. A very strong shake up peak is observed for CuO where energy loss is tied to a specific transition in the atom12_{. Figure 2.7 shows a Cu 2p spectrum for}

a CuO sample.

Figure 2.7 Difference between metallic Cu and CuO. The shakeup features of CuO are

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The shape of these shake up peaks allows for the distinction between different oxides species, as Cu2O does not possess these features and shake up peaks for

Cu(OH)2 have a different shape.