Photon Factory Activity Report 2016 #34 (2017)

BL-2A,BL-2B/2016G003

Electron Structure of Nd0.6Sr0.4FeO3-δ Thin Film with Oxygen Vacancies Prepared bySoft-X-ray Spectroscopy
Tohru Higuchi1,*, Wataru Namiki1, Makoto Minohara2, Koji Horiba2 and Hiroshi Kumigashirta2
1
Department of Applied Physics, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan2
Photon Factory, High Energy Accelerator Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
We have studied the electronic structure of the Nd0.6Sr0.4FeO3-δ (NSFO) thin film with oxygen vacancies on Al2O3 (0001)
substrate by soft-X-ray spectroscopy. The Fe 2p photoemission spectrum exhibits the mixed valence states of Fe2+ andFe3+. The electrical conductivity exhibits thermal activation-type behavior and increases with increasing film thickness.
The band gap of charge transfer type is in a good agreement with the activation energy estimated from the Arrhenius plot.
These results indicate the conducting carrier of the NSFO thin film is mainly electron, although the conductivity does notdepend on oxygen gas partial pressure.
1 Introduction
La0.6Sr0.4FeO3-δ (LSFO) bulk ceramic is well known aselectron -ion mixed conductor, which is useful forcathode electrode of solid oxide fuel cell (SOFC)
operating at intermediate temperature region.
The
cathode electrode with electron-ion mixed conduction iseffective for a high energy transform efficient of SOFC,
since the electrode reaction is activated at the three phaseinterfaces between the electrode surface, the interface ofelectrode and electrolyte, and electrolyte. However, theprepared thin film exhibited was lower conductivity thanthe bulk ceramics. Furthermore, the chemical stability
has not been also proved thus far.
In this study, we have prepared the Nd0.6Sr0.4FeO3-δ(NSFO) thin film by RF magnetron sputtering. If theNSFO thin film exhibits higher mixed conduction or highelectron conduction activated by oxygen vacancies, thethin film might be practical as cathode electrode of SOFC
with high chemical stability. In this paper, we discuss
about the conducting carrier of NSFO thin film atintermediate temperature region in terms of the electricalconductivity and electronic structure.
2 Experiment
The NSFO thin film was deposited on Al2O3 (0001)
substrates by RF magnetron sputtering using a ceramictarget. The RF power of the ceramic target was fixed at50 W. The flow rate of Ar gas, the deposition pressure,and substrate temperature were kept at 0.67sccm, 8.0×10-3
Torr and 700˚C, respectively. The film thickness was 42,
66 and 112 nm.
The crystalline quality of the NSFO thin films werecharacterized by X-ray diffraction with CuKα using
Rigaku Miniflex 600. The electrical conductivities werecharacterized by the AC impedance method using afrequency response analyzer (Solartron 1260) andamplifier (Solartron 1296) in dry atmosphere (Ar/O2 =4/1) in a temperature region from 700 to 300˚C. In orderto probe the conducting carrier, the conductivities weremeasured by changing the oxygen gas partial pressure(PO2). The measurement frequency region of electricalconductivity was from 32 MHz to 100 mHz.

The electronic structures were investigated byphotoemission spectroscopy (PES) and X-ray absorption
spectroscopy (XAS) measurements at room temperature.
These spectroscopic measurements were conducted at theundulator beamline BL-2A MUSASHI of the Photon
Factory, KEK. The XAS spectra were recorded in a total
electron yield mode. The PES spectra were acquired
using VG-Scienta SES-2002 hemispherical analyzer. The
resolutions of PES and XAS were set to be approximately
100 and 60 meV, respectively.
3 Results and Discussion
Figure 1 shows the Fe-2p3/2 core-level PES spectrum ofthe NSFO thin film. The Fe-2p3/2 core-level peak iscomposed of two structures originating from Fe2+ andFe3+ states as shown in the dashed line obtained byGaussian fitting. The ratio between these mixed valencestate is Fe2+:Fe3+=2:3. The mixed valence states of Fe2+
and Fe3+ may contribute to the oxygen vacancies. On theother hand, the satellite peak is observed at ~8.0 eV fromFe 2p3/2 main peak. The satellite peak originates from thecharge transfer between the O 2p valence band to theunoccupied Fe 3d state, indicating that the valence bandconsists of the O 2p state hybridized with the Fe 3d state.

Fig. 1 Fe 2p3/2 PES spectra of the NSFO thin film.

Figure 2(a) shows Arrhenius plots of the electricalconductivity of the thin films with 42, 66 and 112 nm
thicknesses. Each thin film exhibits thermal activation-

Photon Factory Activity Report 2016 #34 (2017)

(a)

(b)

Fig. 2 (a) Arrhenius plots of the electrical conductivity of thethin films with 42 nm, 66 nm, and 112 nm thicknesses. (b)Electrical conductivity of the thin films with 42 nm and 112 nm
thicknesses as a function of PO2 at 500°C.

type behavior, which corresponds to the conductivity ofsemiconducting region. Each activation energy (∆E) wascalculated from the slope of Arrhenius plot. The ∆E does
not much depend on the film thickness, although theelectrical conductivity increases with increasing the filmthickness.
Figure 2(b) shows the electrical conductivities of theNSFO thin films with 42 and 112 nm thicknesses as afunction of PO2 measured at 500˚C. In term of the defectchemical analysis, this result indicates the existence ofionic conduction since the conductivity does not dependon Po2. Therefore, the NSFO thin film has high electron
conduction rather than ionic conduction, because the ratioof Fe2+ valence state created by oxygen vacancies is~40% as shown in Fig. 1.
Figure 3 shows the PES and O 1s XAS spectra of theNSFO thin film corresponding to the valence band andconduction band, respectively. From the dipole selection
rule, it has been clarified that the O 1s-XAS spectrum
reflects the unoccupied Fe 3d state hybridized with the O
2p state. The spectral shapes and peak positions are ingood agreement with LaFeO3 thin film. Wadati et al has
reported the electronic structure by the LDA+U
calculation comparing with the PES and XAS spectra [1].
In this calculation, the CT energy, electron correlationenergy and p-d transfer integral have been set as ~2.0 eV,
6.0 eV and -1.9 eV, respectively. From the agreementswith the calculation and the spectra, Wadati et al has
concluded that the LaFeO3 is a CT type insulator [1].

Fig. 3 Valence band and conduction band of the NSFO thin film
obtained from PES and XAS spectra, respectively.

The energy separation between the top of the valenceband and the bottom of the conduction band reflects thevalue of energy gap (Eg). The EF locates at the bottom ofconduction band. The estimated Eg is ~0.5 eV, whichcorresponds to the ∆E of the electrical conductivity in Fig.
2(a), indicating the high electron conduction. On theother hand, the LSFO thin film prepared by PLD method
has the high metallic conduction without ionic conduction.
In fact, the LSFO thin film has no Eg and exhibits thehole-induced state at around EF below the t2g- and egsubbands of Fe 3d state in the conduction band. However,such a state is not observed in the NSFO thin film. Thismay contribute to the change in band structure with theoxygen vacancies in NSFO thin film or replacing Nd as
La of LSFO. Thus, we would stress that the NSFO thinfilm has the electron conduction, which is activated by alot of oxygen vacancies.
Acknowledgement
We would like to thank Ms. Chizuko Kudo and Mr.
Naoya Suzuki for their technical supports. This work wassupported by a Grant-in-Aid for Scientific Research(Grant No.16H02115) from the Japan Society for thePromotion of Science and the MEXT Element Strategy
Initiative to Form Core Research Center.
References[1] H. Wadati, D. Kobayashi, H. Kumigashira, K.
Okazaki, T. Mizokawa, A. Fujimori, K. Horiba, M.
Oshima, N. Hamada, M. Lippmaa, M. Kawasaki, andH. Koinuma , Phys. Rev. B 71, 035108 (2005)
Research Achievements
1. W. Namiki, T.Tsuchiya, M. Takayanagi, S. Furuichi, M.
Minohara, M. Kobayashi, K. Horiba, H. Kumigashira,T. Higuchi, J. Phys. Soc. Jpn. 80 (2017) 074704.
2. W. Namiki, M. Takayanagi, S. Furuichi, T. Tsuchiya, M.
Minohara, M. Kobayashi, K. Horiba, H. Kumigashira,and T. Higuchi, ECS Trans. 75 (2017) 83.
* higuchi@rs.kagu.tus.ac.jp