Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

18A/2004G002

Co-3d partial density of states of heavily Co-doped ZnO
Masaki KOBAYASHI*1, Yukiaki ISHIDA1, Jong-Il HWANG1, Takashi MIZOKAWA1,Atsushi FUJIMORI1, Kazutoshi MAMIYA2, Jun OKAMOTO3, Hiromasa SAEKI4,
Hitoshi TABATA4, and Tomoji KAWAI4
1
Department of Physics and Complexity Science and Engineering,
Univ. of Tokyo, Kashiwa 277-8561, Japan2
Photon Factory, Institute of Materials Structure Science, Tsukuba 305-0801, Japan
3
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
4
Institute of Science and Industrial Research, Osaka Univ., Ibaraki 567-0047, Japan
Introduction
Recently, it has been reported that transparent oxidebased diluted magnetic semiconductors (DMS’s),
especially ZnO-based DMS’s, show ferromagnetism with
Curie temperature above the room temperature (RT).
These DMS’s have attracted much attention as candidates
for room temperature ferromagnetic DMS’s [1]. Although
there are several reports on RT ferromagnetic Co-doped
ZnO, the ferromagnetism is still in dispute because of low
reproducibility and of the possibility of extrinsic originsof ferromagnetism such as Co-metal clusters.
In this work, we have succeeded in observing the Co3d partial density of states of Co 15%-doped ZnO DMS.
The PDOS have been compared with that of the Co 5%
one and cluster-model calculation.
Experiment
A Zn1-xCoxO (x=0.15) thin film was epitaxially grownon an α-Al2O3(0001) substrate by the pulsed laser
deposition technique using an ArF excimer laser. The
total thickness of the Zn1-xCoxO layer was ~200 nm on a50 nm ZnO buffer layer.
Co 3p → 3d resonant photoemission measurements
were performed at BL-18A. Spectra were taken in an-10
ultrahigh vacuum below 7.5x10 Torr. Photoelectrons
were collected with a VG CLAM hemispherical analyzer
in the angle integrated mode at RT. The total resolution of
the spectrometer including temperature broadening wasabout 200 meV. Sample surface was cleaned by cycles of+Ar -ion sputtering. Cleanliness of the sample surface waschecked by the absence of a high binding-energy (EB)shoulder in the O 1s spectrum and C 1s contamination by
x-ray photoemission spectroscopy with Al-Kα source(hν = 1486.6 eV).
Results and discussion
Ultraviolet absorption spectrum measured in the total
electron yield mode shows a clear Co 3p → 3d absorption
at about hν ~ 61 eV. Then, we choose an on-resonancespectrum taken at hν = 61.5 eV and off-resonance one athν = 60 eV. The top panel of Fig. 1 shows on- and offresonance spectra normalized to the integrated intensity inZnO between 0 and 9 eV. The Co 3d PDOS has beenobtained by subtracting the off-resonance spectrum fromthe on-resonance one. The PDOS shows a peak near the104

Users' Reportvalence band maximum and a satellite at EB ~ 7 eV as
shown in the bottom panel of Fig.1.
By applying configuration-interaction (CI) clustermodel analysis for the PDOS, electronic structure
parameters, namely, the ligand to 3d charge transfer
energy ∆, the d-d Coulomb interaction energy U and theSlater-Koster parameter (pdσ), were obtained to be thesame as in a previous report on Zn1-xCoxO (x = 0.05) [2].
This is consistent with XPS results (not shown). This
result implies that one can incorporate Co ions into ZnO
at least up to 15% without forming secondary phases
and/or Co-metal clusters. Since Zn1-xCoxO with low Co
concentration (<5%) showed RT ferromagnetism, it isexpected that heavily Co-doped ZnO shows stronger RT
ferromagnetism if it was supplied with enough carriers.

Fig. 1. Valence-band photoemission spectra. Top:On- (hν = 61.5 eV) and off-resonance (hν = 60 eV)
spectra normalized to the integrated valence band
intensity of the ZnO. Bottom: CI cluster-model
analysis for the Co 3d PDOS.
References[1] S. J. Pearton et al., Semicond. Sci. Technol. 19, R59(2004).
[2] M. Kobayashi et al., Phys. Rev. B 72, 201201 (2005).
* masaki@wyvern.phys.s.u-tokyo.ac.jp