Electronic structure of Zn 1−x Co x O using photoemission and x-ray absorption
spectroscopy
S. C. Wi, J.-S. Kang, J. H. Kim, S.-B. Cho, B. J. Kim, S. Yoon, B. J. Suh, S. W. Han, K. H. Kim, K. J. Kim, B. S.
Kim, H. J. Song, H. J. Shin, J. H. Shim, and B. I. Min
Citation: Applied Physics Letters 84, 4233 (2004); doi: 10.1063/1.1756197
View online: http://dx.doi.org/10.1063/1.1756197
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APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 21
24 MAY 2004
Electronic structure of Zn1À x Cox O using photoemission
and x-ray absorption spectroscopy
S. C. Wi, J.-S. Kang,a) J. H. Kim, S.-B. Cho, B. J. Kim, S. Yoon, and B. J. Suh
Department of Physics, The Catholic University of Korea, Puchon 420-743, Korea
S. W. Han and K. H. Kim
Department of Physics, Gyeongsang National University, Chinju 660-701, Korea
K. J. Kim, B. S. Kim, H. J. Song, and H. J. Shin
Pohang Accelerator Laboratory (PAL), POSTECH, Pohang 790-784, Korea
J. H. Shim and B. I. Min
Department of Physics, POSTECH, Pohang 790-784, Korea
共Received 23 July 2003; accepted 28 March 2004; published online 7 May 2004兲
Electronic structures of homogeneous bulk samples of Zn0.9Co0.1O which do not exhibit diluted
ferromagnetic semiconducting 共DMS兲 behavior have been investigated using photoemission
spectroscopy and x-ray absorption spectroscopy. We have found that the Co ions in Zn1⫺x Cox O are
in the divalent Co2⫹ (d 7 ) states under the tetrahedral symmetry. Our finding indicates that the DMS
properties will not be produced when Co ions are properly substituted for Zn sites, implying that the
DMS properties observed in Zn1⫺x Cox O thin films are likely to be extrinsic. © 2004 American
Institute of Physics. 关DOI: 10.1063/1.1756197兴
Motivated by the recent theoretical calculations by Dietl
et al.1 which predicted the room-temperature diluted ferromagnetic semiconducting 共DMS兲 property in Mn-doped
ZnO, Zn1⫺x Tx O systems (T⫽3d transition-metal atom兲
have been investigated extensively. There were reports that
Zn1⫺x Mnx O epitaxial thin films exhibited ferromagnetic
properties2,3 and that Zn1⫺x Cox O films showed ferromagnetism with the Curie temperature T C ⬃300 K.4 However, the
reproducibility is somewhat questionable, and there are contradicting reports on Co-doped ZnO.4 – 6
In order to understand the magnetic interaction in
Zn1⫺x Tx O, it is essential to understand the electronic structure of the doped 3d T impurities in Zn1⫺x Tx O. Photoemission spectroscopy 共PES兲 is one of the powerful experimental
methods for providing direct information on the electronic
structures of solids. To our knowledge, only a few PES studies have been reported on Zn1⫺x Tx O: the PES study on
Zn1⫺x Mnx O 共Ref. 7兲 and the Co 2p core-level PES spectra
for Zn1⫺x Cox O. 6
In this study we have investigated the electronic structures of homogeneous bulk samples of Zn0.9Co0.1O and ZnO
using PES and x-ray absorption spectroscopy 共XAS兲. Differently from the film samples reported previously,4 magnetic
properties of our bulk Zn1⫺x Cox O samples revealed that the
Co–Co magnetic interaction is dominated by the antiferromagnetic coupling.8 Magnetic susceptibility at high temperature exhibits a typical Curie–Weiss behavior with negative
T C and the magnetization of Co ion is reduced with increasing the Co ion concentration reflecting an increase in average
antiferromagnetic interaction between Co ions. Hence the
present bulk samples are not diluted ferromagnetic semiconductors.
Polycrystalline Zn1⫺x Cox O samples (x⫽0 and 0.1兲 were
a兲
Electronic mail: [email protected]
synthesized using the standard solid-state reaction method.8
The x-ray diffraction 共XRD兲 analysis showed that all the
samples have the single phase of the wurtzite structure with
no impurity phase. Valence-band PES, XAS, and Co 2p
→3d resonant photoemission spectroscopy 共RPES兲 measurements were performed at the 2B1 and 8A1 beamlines of
the Pohang Accelerator Laboratory 共PAL兲. Samples were
cleaned in situ by repeated scraping with a diamond file and
the data were obtained at room temperature with the pressure
better than 4⫻10⫺10 Torr. The Fermi level E F 9 and the overall instrumental resolution 共FWHM兲 of the system were determined from the valence-band spectrum of a sputtered Au
foil in electrical contact with a sample. The FWHM was
about 0.1–0.4 eV between a photon energy h ␯ ⬃30 eV and
h ␯ ⬇600 eV. All the spectra were normalized to the incident
photon flux. The XAS spectra were obtained by employing
the total electron yield method. The experimental energy
resolution for the XAS data was set to ⬃0.1 eV at the Co 2p
absorption threshold (h ␯ ⬇700 eV). Core-level spectra and
the valence-band spectrum with h ␯ ⫽1486.6 eV were obtained by using the monochromatized Al K ␣ radiation with a
F-WHM of ⬃0.6 eV.
Figure 1 shows the valence-band PES spectra of
Zn0.9Co0.1O and ZnO over a wide several photon energy
range 25 eV⭐h ␯ ⭐1486.6 eV. Our valence-band PES spectra
of ZnO agree with the existing data in the literature.10
Valence-band spectra for ZnO and Zn0.9Co0.1O are rather
similar, showing two large structures, one sharp peak around
11 eV binding energy 共BE兲, and the other broad band between ⬃3 and ⬃7 eV BE. The former and latter peaks are
assigned as the Zn 3d and O 2p emissions, respectively.
They also reveal negligible emission near E F , reflecting that
both ZnO and Zn0.9Co0.1O are insulating.
The left panel of Fig. 2 compares the on-resonance
(h ␯ ⬇777.1 eV) and off-resonance (h ␯ ⬇776.4 eV) spectra
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4233
© 2004 American Institute of Physics
141.223.173.110 On: Thu, 16 Apr 2015 08:18:17
4234
Wi et al.
Appl. Phys. Lett., Vol. 84, No. 21, 24 May 2004
FIG. 1. Valence-band PES spectra of ZnO 共left-hand side兲 and Zn0.9Co0.1O
共right-hand side兲 for 25 eV⭐h ␯ ⭐1486.6 eV.
for Zn0.9Co0.1O in the Co 2 p 3/2→3d RPES, which are scaled
to each other at ⬃7 eV BE. The difference between the onresonance and off-resonance spectra can be considered as
representing the Co 3d partial spectral weight distribution
共PSW兲.11 The right panel of Fig. 2 compares the h ␯ ⫽1486.6
eV valence-band spectrum of Zn0.9Co0.1O to that of ZnO,
and the differences between them also reflect the effect of the
Co 3d states. In both cases, the differences 共shown at the
bottom兲 are very similar to each other, showing a peak
around ⬃3 eV BE and negligible emission near E F . 12 These
results provide evidence that the Co 3d states in Zn1⫺x Cox O
are located near the top of the O 2p valence band.
The extracted Co 3d PSW reveals almost no emission
between E F and 2 eV BE. This feature does not agree with
the calculated Co 3d partial density of states 共PDOS兲, obtained from the local spin-density approximation 共LSDA兲
band structure calculation.13 The calculated Co 3d PDOS for
Zn0.75Co0.25O shows a sharp Co 3d peak near E F . To resolve
this discrepancy, a more elaborate calculation is necessary,
which considers the nonstoichiometric composition. Further-
FIG. 3. Comparison of the Co 2p XAS spectrum of Zn0.9Co0.1O to those of
CoO (Co2⫹ ) 共Ref. 15兲, LiCoO2 (Co3⫹ ) 共Ref. 16兲, and Co metal 共Ref. 15兲.
The data for CoO, LiCoO2 , and Co metal were shifted by ⫹0.1, ⫺3.64, and
⫺0.1 eV, respectively. The calculated Co 2p XAS spectrum for a Co2⫹ ion
in the T d symmetry is presented at the bottom.
more, the LSDA⫹U method incorporating the Coulomb interaction U between Co 3d electrons will be required because it is well known that the LSDA calculation does not
properly describe the electronic structures of insulators with
strongly correlated electrons.14
Figure 3 compares the Co 2p XAS spectrum of
Zn0.9Co0.1O with those of reference Co oxides, having the
formal Co valences of 2⫹共CoO兲 and 3⫹(LiCoO2 ), and that
of Co metal.15,16 Note that Co ions both in CoO and LiCoO2
are located at the octahedron (O h ) centers, while Co ions in
Zn1⫺x Cox O are located at the tetrahedron (T d ) centers. The
peak positions and the line shape of the Co 2p XAS spectrum depend on the local electronic structure of the Co ion,
providing the information on the valence state and the
ground-state symmetry of the Co ion.17,18 The Co 2p 3/2 and
2 p 1/2 spectral parts are clearly separated by the large 2p
core-hole spin–orbit interaction, and the core-hole lifetime
broadening is small, resulting in the sharp multiplet structures. The Co 2p XAS spectrum of Zn0.9Co0.1O looks similar
to that of CoO except for the absence of the low-h ␯ shoulder
(h ␯ ⬃776 eV), but quite different from those of LiCoO2
and Co metal. This observation indicates that Co ions in
Zn1⫺x Cox O might be in the divalent Co2⫹ valence states, but
not in the trivalent Co3⫹ valence states, and that the formation of the Co metal cluster in our Zn0.9Co0.1O samples can
be ruled out.
In order to estimate the ground-state valence states of Co
ions, we have analyzed the Co 2p XAS spectrum of
Zn0.9Co0.1O within the cluster model where the effects of the
multiplet interaction, the crystal field, and the hybridization
with the O p ligands are included.18 The calculated XAS
FIG. 2. Left-hand side: comparison of the Co 2p 3/2→3d on-resonance spectrum (h ␯ ⬇777.1 eV) and the off-resonance spectrum (h ␯ ⬇776.4 eV) for
spectrum is shown at the bottom of Fig. 3. We have found
Zn0.9Co0.1O, scaled at ⬃7 eV BE. Right-hand side: comparison of the
that including only one configuration, corresponding to
valence-band spectrum of ZnO and that of Zn0.9Co0.1O, obtained with
Co2⫹ (d 7 ), and the T d crystal field energy of 10D q ⫽0.6 eV
h
␯
⫽1486.6
eV.
For
both
cases,
the
difference
curves
are
shown
at
the
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yields a good fit for the measured 2p XAS spectrum. The
tom.
141.223.173.110 On: Thu, 16 Apr 2015 08:18:17
Wi et al.
Appl. Phys. Lett., Vol. 84, No. 21, 24 May 2004
analysis shows that it is not necessary to include the chargetransfer configuration, indicating that the hybridization between Co 3d and O p orbitals is small, in contrast the case of
Mn-doped ZnO.7 Based on the calculated XAS spectrum,
one can conclude that the doped-Co ions in Zn1⫺x Cox O are
divalent in the ground states (Co2⫹ ,S⫽3/2) in the T d symmetry with a small crystal field energy. Our finding suggests
that, in our samples, Co ions substitute properly for the Zn
sites with Co2⫹ valence states. In view of no long range
ferromagnetic order in our samples, the DMS properties reported in Co-doped ZnO films4,6 are likely to be extrinsic,
that is, to originate from Co clusters or other impurity phases
not found in bulk samples.
In conclusion, the electronic structures of bulk
Zn0.9Co0.1O and ZnO samples have been investigated by employing PES and XAS. According to the Co 2p→3d RPES
and the careful comparison between x⫽0 and x⫽0.1, the
Co 3d states in Zn0.9Co0.1O found to lie near the top of the
O 2p valence band with a peak around ⬃3 eV BE. The measured Co 2p XAS spectrum shows that the ground states of
Co ions in Zn0.9Co0.1O are divalent 共2⫹兲 under the T d symmetry, corresponding to the total spin of S⫽3/2 per Co ion.
Our finding suggests that the properly substituted Co ions in
Co-doped ZnO do not produce the DMS property with the
long range ferromagnetic order.
This work was supported by the KRF 共KRF-2002-070C00038兲 and by the KOSEF through the CSCMR at SNU
and the eSSC at POSTECH. The PAL is supported by the
MOST and POSCO in Korea.
4235
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