Evaluation of Dielectric Function of Thermally-grown SiO2 and GeO2 from Energy Loss Signals for XPS Core-line Photoelectrons
Authors: Yamamoto Taishi , Ohta Akio , Ikeda Mitsuhisa , Makihara Katsunori , Miyazaki Seiichi 
Published: February 2020

A clear understanding of energy band profiles in semiconductor devices is of great importance to make a quantitative discussion on the carrier transport and the device operation. Bandgap energy (Eg) of interest materials has often been evaluated from the analyses of the photo-absorption and the energy loss signals for electrons (or photoelectrons). Photoemission spectroscopy such as X-ray photoelectron spectroscopy (XPS) has been widely used to evaluate the electronic structure and to determine the energy band profiles since it enables us to measure not only valence band structures but also Eg values of insulating films in addition to chemical bonding features and compositions at hetero-interfaces [1]. As for the Eg evaluation of dielectrics, the threshold energy of the energy loss signals for core-line photoelectrons has to be measured accurately. Since the energy loss signals can be converted into the dielectric functions [2], they give us the knowledge of optical constants and absorption coefficients (α) for the materials of interests.
In this work, we have studied the evaluation methodology of dielectric function using loss signals measured by XPS. After the conversion into a dielectric function from the measured energy loss signals for photoelectrons, optical constants were also calculated to improve the determination accuracy of Eg from the XPS analysis.
After wet-chemical cleaning of p-type Si (100) surface, a SiO2 layer with a thickness of 50 nm was grown by dry oxidation at 1000 ºC. Energy loss signals for Si 2p photoelectrons were measured using XPS utilizing monochromatized AlKα radiation (hν = 1486.6 eV). To obtain the inherent energy loss signals, the measured energy loss signals for Si 2p photoelectrons were deconvoluted into two components in accordance with the spin-orbit splitting of Si 2p core-line [3].
The energy loss spectra for Si 2p3/2 photoelectron measured from SiO2 at photoelectron take-off angle of 90º, 30º, and 15º were compared as shown in Fig. 1. Energy loss function (ELF) calculated from reported optical constant was also shown as a reference [4]. In each spectrum, zero-loss energy was defined by Si 2p3/2 core lines from SiO2, and the normalization of photoelectron intensity was made by the zero-loss peak. Energy loss signals were gradually increased with decreasing take-off angle for a surface sensitive analysis, which can be interpreted in terms of an increase in the energy loss signals by inelastic surface scatterings due to surface plasmon excitation. A significant difference between the measured energy loss signals and ELF calculated from reported optical constant was observed in the energy range from 10 to 20 eV. Therefore, the surface components of energy loss signals were crudely estimated by subtracting energy loss signals measured at 30º from the signals at 15º. Energy loss signals after subtraction of surface components agree well with ELF.
Providing the obtained energy loss signals for photoelectrons proportional to the imaginary part of -1/ɛ, where ɛ is dielectric function of SiO2, the real part of 1/ɛ can be calculated by Kramers-Kronig transformation [2]. From both the imaginary part of -1/ɛ and the real part of 1/ɛ, dielectric function (ɛ1, ɛ2) was derived as shown in Fig. 2. Note that, the obtained dielectric function was found to be fairly close to the reported value [4]. In addition, a Tauc plot of absorption coefficient (α) obtained based on the calculation of optical constant (n, k) from the dielectric function (ɛ1, ɛ2) shows an Eg of 8.9eV being consistent with reported Eg [1] for SiO2 as demonstrated in Fig. 3.
In summary, we have demonstrated the conversion into dielectric function for thermally-grown SiO2 from the energy loss signals for photoelectrons measured at different photoelectron take-off angles.

Acknowledgements:

This work was supported in part by Grant-in Aids for Young Scientists (A) No. 25709023 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by JSPS Core-to-Core Program of International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration.

References

[1] S. Miyazaki et al., Appl. Surf. Sci., 113/114 (1997) 585.
[2] R. F. Egerton, Electron Energy-Loss Microscopy in the Electron Microscope, 2nd ed.(1989) 243.
[3] F. J. Himpsel, et. al., Phys. Rev.b 38 (1988) 6084.
[4] E. D. Palik, Handbook of Optical Constants of Solids, 1 (1985).

Figure 1