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X-ray photoelectron spectroscopy(XPS) is a technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. An electron is ejected from an atomic energy level by an X-ray photon, mostly from an Al-Ka or Mg-Ka primary source, and its energy is analysed by the spectrometer. This technique is conducted under ultra high vacuum.

The quantity that is measured in XPS is the Kinetic Energy of the electrons that is coming out of the sample which depends on the energy of the primary X-ray source.

K.E=hv-B.-F

K.E=Kinetic Energy

hv=photon energy

B.E=binding energy

F=Work function of the Spectrometer

In the spectrum, a number of peaks appears on a background but the main information comes from the core level peaks and the Auger peaks. The nomenclature employed to describe XPS and AES features is based on the momenta associated with the orbiting paths of electrons around atomicnuclei, indicated by the quantum numbers n, l, j. And yet the translation intothe notation is different for both techniques.

 

Physics Behind XPS

A typical XPS spectrum is a plot of the number of electrons detected (sometimes per unit time) (Y-axis, ordinate) versus the binding energy of the electrons detected (X-axis, abscissa). Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed. These characteristic peaks correspond to the electron configuration of the electrons within the atoms, e.g., 1s, 2s, 2p, 3s, etc. The number of detected electrons in each of the characteristic peaks is directly related to the amount of element within the area (volume) irradiated. To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intensity (number of electrons detected) by a "relative sensitivity factor" (RSF) and normalized over all of the elements detected.

The photo-emitted electrons that have escaped into the vacuum of the instrument are those that originated from within the top 10 to 12 nm of the material. All of the deeper photo-emitted electrons, which were generated as the X-rays penetrated 1-- 5 micrometers of the material, are either recaptured or trapped in various excited states within the material.

 
Use of XPS

XPS can be used to measure the elemental composition of the surface , empirical formula of a pure substance ,elements that contaminate a surface, chemical or electronic state of each element in the surface , uniformity of elemental composition across the top surface (or line profiling or mapping), uniformity of elemental composition as a function of ion beam etching (or depth profiling)

Example of XPS:
X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films:
The chemical state of oxygen, aluminum and zinc in Al-doped ZnO (ZAO) films was investigated by X-ray photoelectron spectroscopy (XPS), as well as the transition zone of the film-to-substrate, by auger electron spectroscopy (AES). The results show that zinc remains mostly in the formal valence states of Zn2+. A distinct asymmetry in Al 2p3/2 photoelectron peaks has been resolved into two components, one is metallic Al and the other is oxidized Al. The depth profile of the two components revealed that metallic Al mainly exists in the thin surface layer. The close inspection of O1s shows that O1s is composed of three components, centered at 530.15±0.15, 531.25±0.20 and 532.40±0.15 eV, respectively. AES reveals an abrupt transition zone between the ZAO and quartz substrate.
Procedure:
The ZAO films were prepared by DC magnetron reactive sputtering of an alloy target of Zn/Al (98.5:1.5 wt.%) in an oxygen and argon mixture. Quartz glass substrates were used and rinsed in acetone, ethanol and distilled water, sequentially 6. The substrates were not intentionally heated during sputtering. The as-deposited films were annealed for 60 min in vacuum (the pressure is prior to 3×10-3 Pa) at different temperatures. All films have a hexagonal structure of bulk ZnO and a high-preferred orientation with the c-axis ((200) plane) perpendicular to the substrate, which was determined from the X-ray diffraction patterns. Both of the as-deposited and annealed films show an average transmittance of above 90% in the visible region. X-ray photoemission spectroscopy (XPS) and auger electron spectroscopy (AES) were performed using the LAS-3000 surface analysis system (RIBER, France). XPS measurements were carried out using Al-Ka X-rays (1489.6 eV, width 0.85 eV), the energy scale of the spectrometer has been calibrated with pure Cu samples, and the pressure in the XPS analysis chamber was ~1×10-7 Pa. In order to examine the chemical state of each element in film body and investigate the transition zone of film/substrate, some samples were etched by Ar+ bombardment (~5×10-5 Torr) with an energy of 2.5 kV and a current of 2.0 µA. The etching rate of the sample was 1.5 nm min-1. The position of the C1s peak was taken as a standard (with a binding energy of 285.0 eV). For the comparison of Al with ZAO films, we have also made XPS measurements on a piece of metal aluminum with a purity of 99.999% before and after etching in 20 min by Ar+ bombardment.
Results:
Zn 2p3/2 peaks:
The core line of Zn 2p3/2 shows a similar feature in both the as-deposited and annealed ZAO films. Fig. 1 gives the typical XPS data of Zn 2p3/2 in ZAO film before and after etching of 5 min. On the surface, the core line of zinc shows a little asymmetry, which was attributed to the presence of excess zinc in the films 5. However, the asymmetry of the zinc line disappears after an etching of 1 min (not shown here). Then, the core line of Zn 2p exhibited high symmetry (the open circle line) in the film body, indicating that most of the asymmetry feature of zinc resides only on the very thin layer. In all cases, the binding energy of Zn 2p3/2 remains at 1022.40±0.10 eV, which is larger than the value of Zn in bulk ZnO. It also confirms that the largest and the majority of Zn atoms remain, in all probed films, in the same formal valence state of Zn2+ within an oxygen deficient ZnO1-x matrix 3. No metallic Zn with a binding energy of 1021.50 eV 5 was observed, which confirms again that Zn exists only in the oxidized state. For both of the as-deposited and annealed films, the position of Zn 2p3/2 shows little variation with the increase in etching time, indicating the stable chemical state of Zn in the film body.

Fig 02:XPS data of Zn 2p3/2 in ZAO film before (circle) and after etching for 5 min (line), respectively.
O1s peaks:
The typical O1s peak in the surface can be consistently fitted by three nearly Gaussian, centered at 530.15±0.15, 531.25±0.20 and 532.40±0.15 eV, respectively, in both the as-deposited and annealed films (Fig. 4). The fitted results indicate that each resolved component has a FWHM lower than 2.0 eV, while an initial O1s peak commonly has a FWHM above 2.90 eV. The high binding energy component located at 532.40±0.15 eV is usually attributed to the presence of loosely bound oxygen on the surface of ZAO film, belonging to a specific species, e.g., -CO3, adsorbed H2O or adsorbed O23 and 5. It was also observed that this component disappeared after an etching of 15 min for the as-deposited ZAO film; while for the annealed films, the necessary etching time is about 10 min 6. It must be mentioned that this component cannot be completely removed, even by annealing the film in vacuum at a temperature of as high as 400°C (see Fig. 6).

FIg03:XPS data (circle: experimental; dot line: components) of as-deposited (a,c) and annealed (b,d) ZAO film before (a,b) and after an etching of 5 min (c,d), respectively.

References

1.http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy

2.http://webh01.ua.ac.be/mitac4/micro_xpsaes.pdf
3.http://www.sciencedirect.com/science/article/pii/S0169433299006017