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Title: Electronic Structure Theory
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Hybrid density functional calculations on Zn1-xCoxO

Spin electronics (or spintronics) is a relatively new field where the goal is to use the magnetic properties of the intrinsic spin of the electron in electronic devices. A search is underway for semiconducting materials which are ferromagnetic at room temperature and above, with the goal of using them in spintronic devices. Ideally the semiconductor should be one that is already used by the semiconductor industry. Zinc oxide doped with some transition metal ions shows ferromagnetism above room temperature. In this project we have used the B3LYP hybrid density functional to study bulk ZnO, defects in ZnO and Zn1-xCoxO. The hybrid functional results in a band structure for ZnO which is in good agreement with experiment judged by the agreement in the band gap (B3LYP 3.4 eV, experiment 3.5 eV) and the position of the Zn 3d levels just below the O 2p band.

ZnO

Band structure of ZnO using B3LYP hybrid functional

Ferromagnetic Zn1-xCoxO is prepared in thin films in which Co2+ is believed to mainly substitute for Zn2+. The concentration of Co2+ is usually below 10% (x=0.1) and when the dopant ions are uniformly distributed in the host material there is negligible exchange coupling of the ions for this concentration. The mediator of the exchange coupling interaction may be defects in the Zn1-xCoxO thin films. We studied the role of the O vacancy, Zn interstitial, Zn vacancy and codoped N substituting for O in promoting ferromagnetism in Zn1-xCoxO. The singly, positively charged O vacancy and codoped N promote exchange coupling according to our calculations. The electronic structure of ZnO with a single, neutral O vacancy in a 3x3x2 supercell is shown below.

ZnO_O

Band structure of ZnO with one O vacancy in various charge states. (Green) Bulk ZnO band structure in the 3x3x2 supercell. (Red) ZnO with O vacancy, note defect level at -5 eV induced by two dangling electrons in O vacancy. Inset shows O vacancy defect levels for neutral (dotted line), singly positively charged (red lines) and double positively charged (solid black line) vacancy.Zn_Co_O

Spin density for Zn1-xCoxO with two Co ions and a singly, positively charged O vacancy. Co ions are shown as black spheres, Zn ions as large grey spheres and O ions as small grey spheres. A diffuse spin density associated with the O vacancy can clearly be seen.

It is not surprising that the only defects which promote exchange coupling are magnetic themselves. Both the singly, positively charged O vacancy and the NO defect are spin-one-half defects. The spin density for Zn1-xCoxO with two Co ions and a charged O vacancy in a 3x3x2 supercell are shown below. However, pairs of singly charged O vacancies are metastable with respect to disproportionation into neutral and doubly charged O vacancies; these do not promote ferromagnetism. Questions remain, therefore, over the mechanism of promotion of ferromagnetism in Zn1-xCoxO. See our paper for more details.


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School of Physics, Trinity College Dublin, Dublin 2, Ireland.
Tel:+353 1 8961675, Fax:+353 1 6711759, e-mail: physics@tcd.ie
Contact: chris.smith@tcd.ie
Last updated: Mar 21 2012.

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