X-ray electron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is one of the most powerful and common chemical analysis techniques, especially for surface and interface analysis. XPS is based on the photoelectric effect in which the binding energy (E_B) of a core-level electron is overcome by a sufficient impinging soft X-ray photon, and the core-level electron is excited and rejected from atom, called photoelectron (Fig. 2.9) [6]. Determining the kinetic energy of photoelectron, i.e., binding energy EB will give meaningful chemical information of an analyzed sample.

Figure 2.9. Surface irradiated by sufficient energy X-ray photon beam will emit photoelectrons:

phenomenon (left) and principle schematic (right) [7, 8]

Historical

The photoelectric effect was first observed by Heinrich Hertz in the 1880s. He noticed that metal contacts in electrical systems, when exposed to light, exhibit an enhanced ability to spark [7]. The photoelectron effect was then confirmed and studied more details by the experiments of Hallwachs in 1888, and J. J. Thompson in 1899. Finally, in 1905, Einstein, using Planck‟s 1900 quantization of energy concept, correctly explained all these observations - photons of light directly transferred their energy to electrons within an atom, resulting in the emission of the electrons without energy loss [7]. By this quantum theory development, Einstein was awarded the Nobel Prize in 1921.

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The photoelectric effect has been then applied as an analytical method. In 1914, Robinson and Rawlinson reported recognizable gold photoemission spectrum. In 1951, Steinhardt and Serfass first applied photoemission as an analytical tool. Throughout the 1950s and 1960s, Kai Siegbahn developed the instrumentation and theory of ESCA to give us the method we use today. Siegbahn also coined the term „electron spectroscopy for chemical analysis‟ later modified by his group to „electron spectroscopy for chemical applications.‟ In 1981, Kai Siegbahn was rewarded for his contributions with the Nobel Prize in Physics [6, 7].

Principle

When a photon impinges upon an atom, one of following phenomena may happen: (1) photon can pass through with no interaction, (2) photon is scattered by an atomic orbital electron, and (3) photon interacts with an atomic orbital electron with total energy transfer to electron, leading to electron emission from atom (Fig. 2.9, right). If the photon is scattered, the phenomenon is referred to as “Compton scattering”. If the photon interacts with the electron, the phenomenon describes the photoemission process, a basic of XPS.

To let the core-level electron emits from atom, the impinged photon energy, h needs to be higher than the electron binding energy, EB. Electrons emitted from atoms by this

The emission of core-level electron will result in the rearrangement of atomic orbitals and the emission of Auger electron or X-ray photon as described in Fig. 2.10 [7]. Binding energy of the ejected photoelectron depends on the final state configurations after photoemission [6, 7].

The concept of the binding energy of an electron in an atom requires elaboration. A negatively charged electron will be bound to the atom by the positively charged nucleus.

The closer the electron is to the nucleus, the more tightly it is expected to be bound.

Binding energy will vary with the type of atom (i.e., a change in nuclear charge) and the

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addition of other atoms bound to that atom (bound atoms will alter the electron distribution on the atom of interest). Different isotopes of a given element have different numbers of neutrons in the nucleus, but the same nuclear charge. Changing the isotope will not appreciably affect the binding energy. Weak interactions between atoms such as those associated with crystallization or hydrogen bonding will not alter the electron distribution sufficiently to change the measured binding energy. Therefore, the variations in the binding energy that provide us with the chemical information content of XPS are associated with covalent or ionic bonds between atoms. These changes in binding energy are called binding energy shifts or chemical shifts.

Figure 2. 10. (a) The X-ray photon transfers its energy to a core-level electron leading to photoemission from the n-electron initial state. (b) The atom, now in an (n-1)-electron state, can reorganize by dropping an electron from a higher energy level to the vacant core hole. (c) Since the electron in (b) dropped to a lower energy state, the atom can rid itself of excess energy by ejecting an electron from a higher energy level. This ejected electron is referred to as an Auger electron. The atom can also shed energy by emitting an X-ray photon, a process called X-ray fluorescence

The varying of binding energy with the type of atom and the chemical shifts are the key features of XPS. With these features, XPS enables qualitative elemental identification the entire periodic table save H and He. Chemical bonds between atoms are also identified. Simple identification can be achieved by recording low resolution spectra over a broad binding energy range, called survey scans. Figure 2.11 shows a XPS survey scan of an ALD 1.5 nm Al2O3/InAs sample as an example. The inset shows the peak‟s separation of As-In and As-O bonds due to chemical shift in As 3d XPS spectra.

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Figure 2.11. XPS survey scan of Al2O3/InAs sample, the inset shows As 3d spectra Figure 2.12 presents a simplified schematic diagram of an X-ray photoelectron spectrometer. The photons generated from the X-ray source impinge upon the sample, resulting in the ejection of photoelectrons from sample. The photoelectrons are collected by electron optics and directed into an electron energy analyzer where they are sorted by energy. The number of electrons per energy interval is then transduced to a current by an electron detector. The photocurrent is subsequently converted and processed into a spectrum by suitable electronics. The experiment is typically performed under ultra-high vacuum (UHV) conditions, about 10^-9-10^-11 torr. This high vacuum is needed in order to maintain sample surface integrity (the surface gas adsorption) and minimize the scattering of photoelectrons by others gas molecules [6]. Due to the relatively short inelastic mean free path in the irradiated material and the typical kinetic energies possessed by the photoelectrons, only the photoelectrons produced in the top several mono-atomic-layers of the sample are observed as their characteristics energies. Thus, the XPS is typically useful for surface and interface analysis.

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Figure 2.12. Schematic design of an X-ray photoelectron spectrometer

As discussed before, binding energy, EB of photoelectron is determined by measuring the kinetic energy of photoelectron, KE. To measure photoelectron KE for different kinds of materials, samples are placed in electrical contact with the spectrometer (in case sample is conductance), typically by grounding both the sample and the spectrometer. By this way, the Fermi level, E_F of both the sample and spectrometer is putted at the same energy level (Fig. 2.13). The binding energy, referenced to Fermi level is then determined through measured KE and the work function of spectrometer sp by the following equation (see Fig. 2.13):

  (2.5)

For the case of insulating samples, as photoelectrons are emitted after X-ray bombardment, the sample becomes positive charge and cannot compensate as conducting samples. As a result, the Fermi level of the sample and spectrometer may be different, leading to error in the binding energies calculated from the photoelectrons‟ kinetic energies. In this case, “charge neutralization” or “charge compensation” process is needed to compensate the accumulation positive charges. The use of flood guns which provide a source of low energy electrons to take place of the ejected photoelectrons is a common technique [6, 7]. Additionally, the differential charging may also be reduce through placing addition sources such as placing of metallic wall surrounding the sample, placing the sample on a conductive platform, and mixing the sample with another substance [6].

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Figure 2.13. The energy level diagram for an electrically conducting sample that is grounded to the spectrometer. The Fermi levels of the sample and spectrometer are aligned (EF(s) = EF(sp) ) so that EB is referenced with respect to EF. The measurement of EB is independent of the sample work function, φs, but is dependent on the spectrometer work function, φsp

Peak fitting

To detail the information from XPS spectra, the area and EB of each subpeak for a given orbital must be determined. Typically, the spacing between subpeaks is similar to peak width (~1 eV). Hence, it is rare when individual subpeaks are completely separated in an experimental spectrum. This requires the use of a peak fitting procedures to resolve the desired peak parameters. Parameters used in such procedures include the background, peak position, peak width, and peak shape (Gaussian, Lorenzian, asymmetric, or mixture thereof).

In this work, the XPSPEAK software, version 4.1 is used for the peak fitting (see Fig.

2.14). After loading the experiment data file, the background of spectrum is set. The most common method which is used to model the background (inelastic scattering) was developed by Shirley, called Shirley‟s background model. Others background models are Tougaard background model and linear background model. After setting background, adding/adjusting processes can procedure. The software provides some features which allow user to adjust the number of peaks as well as peaks‟ parameters such as peak type, peak position, FWHM, % Lorentzian-Gaussian, etc. The details about software as well as the guide of using is supported free and can be found online [9].

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Figure 2.14. The control windows of XPSPEAK software, version 4.1