JOURNAL OF APPLIED PHYSICS
VOLUME 95, NUMBER 11
1 JUNE 2004
Comparative x-ray absorption spectroscopy study of Co-doped SnO2 and TiO2 A. Lussier,a) J. Dvorak, and Y. U. Idzerda Department of Physics, Montana State University, Bozeman, Montana 59717
S. B. Ogale, S. R. Shinde, R. J. Choudary, and T. Venkatesan Department of Physics, Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742-4111
共Presented on 8 January 2004兲 We performed x-ray absorption spectroscopy measurements at the cobalt L 2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2 . Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2 , but not in TiO2 . © 2004 American Institute of Physics. 关DOI: 10.1063/1.1688655兴
The Cox Sn1⫺x O2⫺ ␦ samples were grown by pulsed laser ablation, with thicknesses of approximately 1500 Å, on R-plane sapphire substrates. The cobalt doping levels were x⫽5% and 8%, and ␦ stands for oxygen vacancies. More details on growth can be found elsewhere.13 The Co0.07Ti0.93O2⫺ ␦ samples were grown by pulsed laser deposition with similar thicknesses on STO or LAO substrates. Again, sample growth details can be found elsewhere.6 The XAS measurements were carried out at the MSU Materials X-ray Characterization Facility located on beamline U4B of the National Synchrotron Light Source. The cobalt XAS spectra, taken in total electron yield mode with linear polarized light with 0.4 eV energy resolution for small concentrations of cobalt in SnO2 , are similar to those of cobalt in TiO2 as can be seen in Fig. 1. This spectral shape is known not to be that of metallic cobalt,
Ferromagnetic semiconductors, if successfully synthesized, could contribute significantly to the field of spintronics.1 Early and ongoing efforts with diluted magnetic impurities in III–V semiconductors produced successful ferromagnetic semiconductors at low temperatures.2 In 2001, Matsumoto et al. reported on room temperature ferromagnetism in cobalt doped anatase TiO2 . 3 Since then, this material has been studied extensively by several groups. In particular, x-ray absorption spectroscopy 共XAS兲 measurements on cobalt doped TiO2 by Chambers et al.4 shed light on the cobalt atoms’ valence state and provide a basis for comparison with our spectra. The origin of ferromagnetism in this compound is still debated, with some authors claiming cobalt clustering,5 and others claiming full incorporation of cobalt atoms in the lattice.6 Additional insight into the ferromagnetism and magnetic dopant arrangement in the host lattice can be gained by comparing element-specific information from systems that are structurally related. To investigate these structural similarities, we present XAS measurement on Co-doped SnO2 and TiO2 films taken at the cobalt L 2,3 edges and the oxygen K edge. Undoped SnO2 is considered ‘‘the’’ prototype transparent conductor.7 It has high metallic conductivity and is optically transparent in the visible range. This makes SnO2 共with its alloy with In2 O3 ) technologically useful as a transparent electrical contact in such devices as flat-panel displays8 and solar cells.9 The conductivity has been explained theoretically in terms of high structural nonstoichiometry due to small defect formation energies of interstitial tin, and oxygen vacancies.10 Experimental results on dilute magnetic semiconductors, in particular Co-doped TiO2 , by other authors11,12 suggest that magnetic interactions in those materials are charge carrier mediated. In light of this, tin dioxide’s relatively high carrier concentration9 makes it a promising candidate as a ferromagnetic semiconductor.
FIG. 1. The XAS cobalt L 2,3 edge spectra for Co-doped SnO2 and TiO2 . The great similarity in features offers evidence for cobalt atoms occupying oxygen octahedral coordinated cation sites in both compounds. Below the 5% and 8% curves are shown the difference curves, for which the anatase 共7%兲 Co-doped TiO2 spectrum has been subtracted.
a兲
Author to whom correspondence should be addressed; electronic mail:
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© 2004 American Institute of Physics
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Lussier et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
FIG. 2. The XAS oxygen K edge spectra of Co-doped SnO2 and TiO2 . The t 2g band in the SnO2 samples is completely suppressed. The t 2g directionality is demonstrated by the feature suppression with changing photon incidence direction.
which precludes extensive metallic clustering, at least to our probing depth of a few hundred angstroms. The cobalt L 2,3 edge spectra of the Co-doped SnO2 , and anatase TiO2 samples all share the same general features with only very slight differences in relative peak intensities. The strong similarities in the spectra can be seen from the difference spectra between the SnO2 and TiO2 spectra 共included on Fig. 1兲. The similarity we observe in XAS spectral shapes is indicative of a similar local environment 共structural and chemical兲 and oxidation state for cobalt atoms, regardless of whether they are doped in TiO2 or SnO2 . The cation substitutional sites in both compounds have the same distorted oxygen octahedral coordination14,15 suggesting that cobalt atoms sit at the substitutional site, in agreement with cobalt K-edge XAS of Co-doped TiO2 samples by Chambers et al.16 Our L-edge spectra indicate that cobalt atoms are most likely substitutional in SnO2 also. A marked difference is that cobalt doped in SnO2 leads to giant magnetic moments of the cobalt atoms,11 which is not observed for cobalt in TiO2 . Although the cobalt atoms have analogous nearest neighbor coordination in both compounds, we detect an important difference in their magnetic behavior. The oxygen spectra hint to a possible explanation. To reiterate, the ferromagnetism in these dilute compounds is generally accepted to be charge carrier mediated. Additionally, the charge carrier density is determined by oxygen vacancies,17 and therefore closely associated with oxygen. If the electronic states of oxygen in TiO2 and SnO2 , which are revealed in the XAS spectra, are significantly different, we might expect the magnetic properties of the compound will also be different. As can be seen in Fig. 2, although the cobalt spectra are similar, the oxygen spectra are dramatically different. Note that oxygen atoms in TiO2 or SnO2 have only about 5% of their cation neighbors replaced by cobalt. The XAS spectra of oxygen are therefore virtually unaffected by the cobalt, and the obvious spectral differences reflect the different properties of the host lattice. The first peak at 530.5 eV in the TiO2 oxygen spectrum represents the
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t 2g band. It is a sharp feature related to directional bonds and is completely suppressed in SnO2 . The bond directionality is made evident by comparison of the spectra taken on a Co-doped TiO2 sample with the photon incident beam normal to the surface, or at 60° with respect to the sample normal. The t 2 g feature is partly suppressed in the latter case. These differences indicate differences in the host oxide’s electronic properties, which would in turn affect the carrier mediated magnetic interactions and possibly lead to giant cobalt magnetic moments in SnO2 . Among the proposed explanations for the giant magnetic moment is unquenching of the cobalt orbital moments11 and our observation of the electronic variations for oxygen could be related to quenching. Another possible explanation lies in spin transfer on neighboring Sn atoms, a possibility that we are currently investigating. To summarize, our cobalt L 2,3 XAS measurements allowed us to demonstrate that doped cobalt atoms are in the same local environment in SnO2 as in TiO2 . These measurements also corroborate other published results that show cobalt atoms occupy substitutional titanium sites in TiO2 . Additionally, XAS measurements at the oxygen K edge reveal vastly different oxygen electronic states in the two compounds. In light of the role played by oxygen vacancies in charge carrier generation, and by virtue of the proposed role played by charge carriers in the magnetic behavior of these materials, the differing oxygen states suggest an explanation for the giant magnetic moment observed in SnO2 but absent in TiO2 . This work is supported by the National Science Foundation and the National Synchrotron Light Source is supported by the Department of Energy. Additionally, we would like to acknowledge support under NSF-MRSEC Grant No. DMR 00-80008 and DARPA SpinS Grant No. N000140210962. G. Schmidt and L. W. Molenkamp, J. Appl. Phys. 89, 7443 共2001兲. H. Ohno, Science 281, 951 共1998兲. 3 Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.-y. Koshihara, and H. Koinuma, Science 291, 854 共2001兲. 4 S. A. Chambers et al., Appl. Phys. Lett. 79, 3467 共2001兲. 5 J.-Y. Kim et al., Phys. Rev. Lett. 90, 017401 共2003兲. 6 S. R. Shinde, S. B. Ogale, S. Das Sarma, J. R. Simpson, H. D. Drew, S. E. Lofland, C. Lanci, J. P. Buban, and N. D. Browning, Phys. Rev. B 67, 115211 共2003兲. 7 C¸. Kilic¸ and A. Zunger, Phys. Rev. Lett. 88, 095501 共2002兲. 8 B. G. Lewis and D. C. Paine, MRS Bull. 25, 22 共2000兲. 9 H. L. Hartnagel, A. L. Jain, and C. Jagadish, Semiconducting Transparent Thin Films 共IOP, Bristol, 1995兲. 10 C¸. Kilic¸ and A. Zunger, Phys. Rev. Lett. 88, 095501 共2002兲. 11 M. Berciu and R. N. Bhatt, Phys. Rev. Lett. 87, 107203 共2001兲; J. Ko¨nig, H. Lin, and A. H. MacDonald, ibid. 84, 5628 共2000兲. 12 S. A. Chambers, Mater., Today 34, 共April 2002兲. 13 S. B. Ogale et al., Phys. Rev. Lett. 91, 077205 共2003兲. 14 L. Soriano, P. P. Ahonen, E. Kauppinen, J. Gomez-Garcia, C. Morant, F. J. Palomares, M. Sanchez-Agudo, P. R. Bressler, and J. M. Sanz, Monatsch. Chem. 133, 849 共2002兲. 15 F. M. F. de Groot, J. Faber, J. J. M. Michiels, M. T. Czyzyk, M. Abbate, and J. C. Fuggle, Phys. Rev. B 48, 2074 共1993-II兲. 16 S. A. Chambers, S. M. Heald, and T. Droubay, Phys. Rev. B 67, 100401共R兲 共2003兲. 17 S. A. Chambers, C. M. Wang, S. Thevuthasan, T. Droubay, D. E. McCready, A. S. Lea, V. Shutthanandan, and C. F. Windisch, Jr., Thin Solid Films 418, 197 共2002兲. 1 2
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