Structural Studies of ZnO Calcined with Transition Metal Oxides
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Structural Studies of ZnO Calcined with Transition Metal Oxides Lori Noice1*, B. Seipel1**, Georg Grathoff2, Amita Gupta1,3, Peter Moeck1**, V. K. Rao3 Department of Physics, Portland State University, P.O. Box 751, Portland OR 97207-0751, * [email protected],** Oregon Nanoscience and Microtechnologies Institute, www.onami.org 2 Department of Geology, Portland State University, P.O. Box 751, Portland OR 97207-0751 3 Department of Material Science, Tmfy-MSE, The Royal Institute of Technology, Brinellvägen 23, Rm 224, SE 100 44 Stockholm, Sweden 1
ABSTRACT
Powder X-ray diffraction analyses of Mn-and Cu-doped ZnO powders calcined at 500˚C, show shifts in the wurtzite type semiconductor’s lattice constants and unit cell volume which correspond to the nominal concentrations of both transition metal dopants. Marked reductions in the a-lattice constant and unit cell volume for a small concentration of Cu dopants, which is not maintained upon increased Cu concentration, suggest a change in the copper ion hybridization state due to the dopant concentration. In all the samples, only ZnO and CuO phases were detected, aiding the ascertainment of any ferromagnetic response from the samples as arising from the formation of a true dilute magnetic semiconductor. INTRODUCTION
The prospects of a more advanced spintronic technology, which integrates intrinsic magnetic and electronic functionality into a single material, have provoked intensive research towards developing dilute magnetic semiconductors (DMS) with above room-temperature ferromagnetism [1]. In 2000, Dietl et al. employed the mean-field Zener model of ferromagnetism to predict above room temperature Curie temperatures (TC) for two semiconductors, GaN and ZnO, when doped with 5 % Mn and with a hole density of 3.5 x 1020 cm-3 [2]. Since that time, Mn-doped ZnO has been reported both paramagnetic and ferromagnetic, often in conjunction with equally incongruous ab initio calculations. This situation persists despite efforts to clarify the many pertinent issues of clustering, impurities and precipitates. So far, the various results regarding the ferromagnetism at room temperature of Mn-doped ZnO seem to defy convergence [3]. Perspective spintronic applications require the DMS to possess an electronic structure with spin-polarized state in its valence or conduction bands. These spin-polarized states are created through the long-range coupling of net electronic (spin + orbital) angular momentums of individual magnetic dopants [1,4]. Although most theories concerning ferromagnetism in semiconductors require mediation of this coupling by either n-type or p-type carrier states, Dietl’s prediction suggests holes to be the most effective method for attaining critical temperatures well above room temperature [2]. For the case of Mn-doped ZnO, however, no direct hole providing mechanism is present. The ZnO semiconductor is typically n-type when doped with TMs, and the unoccupied 3d states of the substitutional Mn2+ ion are well above the lowest ZnO conduction band [5,6].
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