Valencies of Mn impurities in ZnO
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Valencies of Mn impurities in ZnO L. Petit1 , T. C. Schulthess1 , A. Svane2 , W.M. Temmerman3 , and Z. Szotek3 1
Computer Science and Mathematics Division, and Center for Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
2
Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark 3
Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK Abstract
We use the self-interaction corrected (SIC) local spin-density (LSD) approximation to investigate the groundstate valency configuration of Mn impurities in p-type ZnO. In Zn1−x Mnx O, we find the localized Mn2+ configuration to be preferred energetically. When codoping Zn1−x Mnx O with N, we find that four d-states stay localized at the Mn site, while the remaining d-electron charge transfers into the hole states at the top of the valence bands. If the Mn concentration [Mn] is equal to the N concentration [N], this results in a scenario without carriers to mediate long range order. If on the other hand [N] is larger than [Mn], the N impurity band is not entirely filled, and carrier mediated ferromagnetism becomes theoretically possible.
The design of diluted magnetic semiconductors (DMS), that apart from the well known electronic properties also have incorporated spin-functionality, is expected to play a major part in the development of the next generation of electronic devices. [1] In this respect, it is important that these materials remain ferromagnetic above room temperature. In Mn doped GaAs, where ferromagnetism is well established, the Curie temperature is TC ' 160 K. Ferromagnetism has been predicted theoretically to occur in a variety of Mn doped semiconductors, but there remains considerable disagreement as to the nature of the exchange mechanism and the magnetic order. ZnO crystallizes in the hexagonal wurtzite structure (lattice constants a0 =3.2495 ˚ ˚). Its wide band gap, in the near UV range (3.3 eV) makes it a A, and c0 =5.2069 A candidate for optoelectronic applications that rely on short wavelength light emitting diodes. As was shown by Fukumura et al., [2] solubility of Mn in the ZnO matrix is relatively high (x ≤ 0.35). The various experimental investigations of the magnetic order in Zn1−x Mnx O give contradictory results, ranging from spin glass behaviour [3] and paramagnetism, [4] to ferromagnetism at room temperature [5]. The very latest
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experimental study that we are aware of, finds no evidence for magnetic order, down to T=2 K. [6], and it has been suggested that the previously observed ferromagnetism is due to precipitates containing manganese oxides. [7] Codoping with N has so far revealed itself to be rather elusive. [8] With respect to theory, the agreement is that Zn1−x Mnx O, without additional carriers is not ferromagnetic. According to the Zener model approach by Dietl et al., [9] ferromagnetism in DMS’s originates from the RKKY-like interaction between the localized transition metal moments, and delocalized hole carriers. In Mn doped ZnO, the Mn impurities provid
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