Accurate quasiparticle calculation of x-ray photoelectron spectra of solids

Tsubasa Aoki,Kaoru Ohno

doi:10.1088/1361-648x/aabdfe

Abstract

It has been highly desired to provide an accurate and reliable method to calculate core electron binding energies (CEBEs) of crystals and to understand the final state screening effect on a core hole in high resolution x-ray photoelectron spectroscopy (XPS), because the ΔSCF method cannot be simply used for bulk systems. We propose to use the quasiparticle calculation based on many-body perturbation theory for this problem. In this study, CEBEs of band-gapped crystals, silicon, diamond, β-SiC, BN, and AlP, are investigated by means of the GW approximation (GWA) using the full ω integration and compared with the preexisting XPS data. The screening effect on a deep core hole is also investigated in detail by evaluating the relaxation energy (RE) from the core and valence contributions separately. Calculated results show that not only the valence electrons but also the core electrons have an important contribution to the RE, and the GWA have a tendency to underestimate CEBEs due to the excess RE. This underestimation can be improved by introducing the self-screening correction to the GWA. The resulting C1s, B1s, N1s, Si2p, and Al2p CEBEs are in excellent agreement with the experiments within 1 eV absolute error range. The present self-screening corrected GW approach has the capability to achieve the highly accurate prediction of CEBEs without any empirical parameter for band-gapped crystals, and provide a more reliable theoretical approach than the conventional ΔSCF-DFT method.

Highlights

Since the invention of high resolution x-ray photoelectron spectroscopy (XPS) by Siegbahn et al [1–4], XPS has been widely used as a method to investigate the valence nature and chemical properties of atoms in variety of materials [5]
In XPS experiments of crystalline systems, the binding energies (BEs) are commonly measured from the Fermi level εsFpec of the spectrometer and defined as Eb = E(N − 1) − E(N) + φ, where φ is the work function given by φ = −εsFpec + εvacuum. (Here εvacuum is the vacuum level.) For metals, the reference value φ of the standard sample can be directly used to deduce the experimental core electron binding energies (CEBEs), because the Fermi level is automatically adjusted; there is no ambiguity in the experimental CEBE values
The point is that Ebcore and EbVBM has the same work function φ and their difference does not contain φ, and a firm definition of the CEBEs is possible by using this difference as shown in figure 1

Summary

Introduction

Since the invention of high resolution x-ray photoelectron spectroscopy (XPS) by Siegbahn et al [1–4], XPS has been widely used as a method to investigate the valence nature and chemical properties of atoms in variety of materials [5]. (CEBE) certainly reflects the nature of chemical bonds and local electronic structures involving the ionic valence and so on. Since these environmental effects are difficult to characterize quantitatively, a comparison with reliable reference data is always required [7], and the development of an accurate first-principles method to calculate CEBEs has been highly desired [2, 3, 7]. The most important point here is that, the binding energies (BEs) measured in XPS are directly related to the quasiparticle (QP) energies, which are defined as the difference between the total energy of the N-electron ground (initial) state, E(N), and the total energy of the (N − 1)-electron excited (final) state, E(N − 1) [8]

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