Photon Factory Activity Report 2019 #37 (2020)

BL-27A/2019G032

Multi-atom resonant photoemission in silicon compounds

Yuji BABA and Iwao SHIMOYAMAAdvanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun,Ibaraki, 319-1195, Japan
arrows originate from the resonant excitation
corresponding to the dipole-allowed Si 1s→3p*transition.
Fig. 2 shows the wide-scan XPS spectrum for
SiO2 excited by 2000 eV photons. In addition to theO 1s peak originating from SiO2, the N 1s and B 1speaks due to the matrix BN powder were observedat almost equal intensity. Similar spectrum wasobtained for Si3N4.
Next, the intensity of these peaks were measured
by scanning X-ray energy around the Si K-edge. Fig.
3 shows the intensity of B 1s, N 1s and O 1s peaks
for SiO2 as a function of photon energy. The XAFS
spectrum is also shown in the lower part. Theintensity of the O 1s peak was reduced around theSi 1s→3p* resonance, which is due to the MAR. But
the intensities of the B 1s and N 1s peaks were1 Introduction
XAFS (EXAFS, XANES) is a powerful method todetermine electronic and geometrical structure ofmaterials. Owing to the element specific feature ofcore-level excitation, structures around a specificatom can be determined by XAFS even for a multicomponent solid. However, in the case of complex
materials composed of three or more elements,
XAFS has a limitation that the atomic species
adjacent to the analyzing atom cannot be specified.
Taking semiconductor materials like siliconcompounds as an example, structures aroundsilicon atom in binary solid such as SiOx [1] and SiNx
[2] has been extensively investigated by Si K-edge
XAFS. However, for ternary compounds like silicon
oxynitride (SiOxNy), the structural analysis by XAFS
has a limitation because atomic species adjacent tosilicon atom cannot be specified.
In the present study, multi-atom resonance (MAR)[3] in photoemission spectroscopy was applied tothe silicon compounds in order to investigate
whether species of the neighboring atom can bespecified or not by MAR.
2 Experiment
Two kinds of pellet samples were prepared. Onewas SiO2 and the other was Si3N4. The powder
sample was mixed with matrix material at a weightratio of 1:1. As matrix material, boron nitride (BN)power was used for SiO2, and cellulose powder wasused for Si3N4. The mixed powder was pulverized by
motor, and then pressed into a pellet at a pressureof 3 ton/cm2.
The experiments were performed at the BL-27A
soft X-ray station, Photon Factory. X-ray
photoelectron spectra (XPS) were taken withhemispherical electron energy analyser (VSW.
CLASS-100). The photon beam was irradiated at 55
deg. from the surface normal, and the electron takeoff direction was the surface normal. An electron
flood gun was used during the measurements.
XAFS spectra were measured by plotting a sample
drain current as a function of photon energy. Thesample current was normalized by the photon
intensity measured by the drain current of aluminium
mesh located in front of the sample.

Fig. 1: Si K-edge XAFS spectra for SiO2 and Si3N4.

3 Results and Discussion
Fig. 1 shows the Si Κ-edge XAFS spectra for SiO2and Si3N4. The most intense peaks indicated as

Fig. 2: XPS wide scan spectrum for SiO2.

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Although the precise mechanism of the observedchanges in photoelectron intensity is not clear, wetentatively consider the mechanism as follows.
Generally,a photoionization cross section of anatom “A” (σΑ) by X-ray irradiation is independently
defined for the atom even for a molecule “A-B”.
Therefore, the changes in photoelectron intensity ofthe atom “A” around the resonance at theneighboring atom “B” cannot be interpreted by theextension of the theory on single atom resonance tobinary molecule. However, at the energy of the coreto-valence resonance of the atom “B”, σΒ drasticallyincreases, which coherently induces the reduction ofσΑ. This means that we have to newly consider thephotoionization cross section of a molecule σΑ−Β, atthe resonance energy of one of the atoms.
Although further theoretical study is needed toclarify the precise mechanism of the present
phenomenon, it was experimentally demonstrated
that the intensity of O 1s (or N 1s) photoelectrons isreduced around the Si 1s→3p* resonance only when
the oxygen (nitrogen) atom is directly bonded tosilicon. Considering that the observed intensitychanges give information about the atomic species
neighboring an atom, the phenomena will be applied
to a method for structural analysis of multicomponent compounds such as silicon oxynitride(SiOxNy).

Fig. 3: Intensity of B1s, N 1s and O 1s peaks in
XPS spectrum for SiO2 as a function of photon
energy around the Si Κ-edge. XAFS spectrum isalso shown in the lower part in the same energy
scale.

almost constant regardless of the photon energy.
This is simply explained that neither boron nor
nitrogen atoms are bonded to silicon because boron
nitride was used as a matrix material.
Fig. 4 shows the intensity of N 1s and O 1s peaks
for Si3N4 as a function of photon energy. Similar tothe SiO2 result, the intensity of the N 1s peak wasreduced around the Si 1s→3p* resonance, which isdue to the MAR, because nitrogen is directly bondedto silicon. On the other hand, the intensity of the O
1s peak was almost constant regardless of thephoton energy. This is also explained that oxygenatoms are not bonded to silicon because O1s
originates from oxygen in cellulose that was used asa matrix material.

References[1] R. Sammynaiken, S.J. Naftel and T. K. Sham, J.
Appl. Phys. 92, 3000 (2002).
[2] P.R.J. Wilson et al., Nanoscale Res. Lett. 6, 168(2011).
[3] A.W Kay, C.S Fadley et al., J. Electron Spectrosc.
Relat. Phenom. 101-103, 647 (1999).
* baba.yuji@jaea.go.jp

Fig. 4: Intensity of N 1s and O 1s peaks in XPS
spectrum for Si3N4 as a function of photon energy
around the Si K-edge. XAFS spectrum is also shownin the lower part in the same energy scale.

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