Analysing indium and indium-based compounds using X-ray Photoelectron Spectroscopy is challenging due to only slight shifts in the binding energies of the commonly used In 3d5/2 core line. A recent paper shares a comprehensive set of reference data for indium and its compounds, covering the In 3d, 3p, and 4d core lines, the In MNN Auger signal, as well as relevant counter ion signals [1]. Valuable tools, such as the modified Auger parameter and chemical state (Wagner) plots, which aid in differentiating indium species are also discussed.
Figure 1 and Table 1 present average literature values for the In 3d5/2 core line, highlighting both the average and standard deviations. These values illustrate the apparent challenge in distinguishing between various indium compounds. Factors like natural line widths, line shapes, and potential errors in charge correction add further complexity to accurate speciation.
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| Figure 1. Average In 3d5/2 literature values for indium compounds. |
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| Table 1. Average In 3d5/2 literature values for indium compounds. |
Experimental data (Figure 2 and Table 2) presents a similar trend, underscoring that the In 3d5/2 core line alone is not enough to reliably distinguish between indium species. Notably, the In 3d5/2 core line of indium oxides shows variable asymmetry in line shape, which has led to differing interpretations in the literature. Some researchers attribute the high binding energy component to hydroxide or oxy-hydroxide species, while others suggest that it reflects electronic properties (screening effects). The current experiments [1] support the view that screening effects play an important role in this asymmetry. Excellent studies on these screening effects have been conducted by Körber [2] and Harvey [3].
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| Figure 2 In 3d spectra from [1]. |
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| Table 2. Experimental In 3d5/2 values from [1]. |
The In M4,5N4,5N4,5 transitions show a broader range of binding energy than the In 3d5/2 core level, making it better suited for accurate speciation, particularly by making use of the modified Auger parameter (Figure 3 and Table 3). In mixed-system analysis, i.e., a system containing multiple indium species, both the position and shape of the In M4,5N4,5N4,5 Auger electron signal can useful for speciation. Béchu and Fairley have provided an excellent discussion on the application of nonlinear and linear least-squares fitting methods to the In M4,5N4,5N4,5 signal, specifically for the oxidation of InSb [4]. Table 4 presents the fitting parameters needed to reproduce the M4,5N4,5N4,5 line shapes in order to fit complex experimental envelopes.
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| Figure 3. In MNN spectra for various indium compounds [1]. For reference, vertical lines indicating the kinetic energy (MNN) for metallic indium have been overlaid in each tile. Note that the additional signal present for InPO4 at 414.7 eV was due to Na contamination. |
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| Table 3. In M4N4,5N4,5 and modified Auger parameter values [1]. |
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| Table 4. In MNN Auger peak fitting parameters [1]. |
Considering the information presented above, a comprehensive interpretation of XPS data involving indium and its compounds should involve a combination of the available data, including survey spectra (i.e., stoichiometry), the In 3d5/2 and In M4N4,5N4,5 Auger spectra, as well as the relevant counterion spectra (see [1]). For systems containing multiple indium compounds, the position and shape of the M4,5N4,5N4,5 transition can offer a more accurate approach than using the 3d5/2 core line alone.
References:
[2] C. Körber (et al.), Phys. Rev. B, 81 (2010) 165207
[3] S.P. Harvey (et al.), J. Phys. D Appl. Phys., 39 (2006) 3959.
[4] S. Béchu and N. Fairley, J. Vac. Sci. Technol. A, 42 (2024) 013202.
