| AVS 54th International Symposium | |
| Magnetic Interfaces and Nanostructures | Thursday Sessions |
| Session MI-ThA |
| Session: | Magnetic Semiconductors II |
| Presenter: | T.C. Kaspar, Pacific Northwest National Laboratory |
| Authors: | T.C. Kaspar, Pacific Northwest National Laboratory T.C. Droubay, Pacific Northwest National Laboratory Y.J. Li, Pacific Northwest National Laboratory M.H. Engelhard, Pacific Northwest National Laboratory P. Nachimuthu, Pacific Northwest National Laboratory V. Shutthanandan, Pacific Northwest National Laboratory Z. Zhu, Pacific Northwest National Laboratory S.M. Heald, Argonne National Laboratory D.J. Keavney, Argonne National Laboratory C.A. Johnson, University of Washington D.R. Gamelin, University of Washington S.A. Chambers, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
Doping ZnO with transition metal ions may be a promising route to realize dilute magnetic semiconductors which are ferromagnetic above room temperature. Although several groups have reported room temperature ferromagnetism in both Co:ZnO and Mn:ZnO, significant controversy persists as to whether the observed ferromagnetism is intrinsic to doped ZnO or is due to extrinsic factors such as secondary phase formation. Of particular concern is the formation of ferromagnetic Co metal clusters in Co:ZnO, and potentially ferromagnetic Zn-Mn oxides in Mn:ZnO. The difficulty lies in the small quantity of secondary phase required to explain the observed weak ferromagnetism, often comprising less than 5% of the dopants (which themselves are generally only 10% or less of the total cations in the material). Conventional materials characterization techniques, such as x-ray diffraction (XRD) and transmission electron microscopy (TEM), can be insensitive to the small volume fraction of secondary phase involved, making detection difficult. Spectroscopic techniques, particularly x-ray absorption (XAS), can provide much more information on the charge state and local environment of the dopant. However, the detection limit at the K-edge is about 5% of the dopants for metal formation; oxide secondary phases can be more difficult to detect. A related issue concerns the determination of the location and role of p-type dopants in ZnO, since ferromagnetic ordering is only expected in Mn:ZnO when the material is p-type. Here we present a detailed study of Co:ZnO and Mn:ZnO thin films deposited by pulsed laser deposition. The ZnO quality and majority dopant behavior were probed by conventional characterization techniques such as XRD, TEM, and XAS, which indicated dopant substitution for Zn in ZnO. The possibility of a small fraction of secondary phase formation was investigated with several techniques including x-ray linear dichroism, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS) sputter depth profiling. In Co:ZnO, localized Co metal formation at the film surface under reducing conditions was not detectable by K-edge XAS but was clearly observed by XPS sputter depth profiling. The presence and location of the potential p-type dopants N and Li were investigated by secondary ion mass spectrometry and nuclear reaction analysis. The implications of secondary phase formation on ferromagnetism in Co:ZnO and Mn:ZnO will be discussed.