Optical and Structural Investigations on Mn-Ion States in MOCVD-grown Ga 1-x Mn x N
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Optical and Structural Investigations on Mn-Ion States in MOCVD-grown Ga1-xMnxN Strassburg, Martin 1,2, Senawiratne, Jayantha 1, Hums, Christoph 1,4, Dietz, Nikolaus 1, Kane, Matthew H. 2,3, Asghar, Ali 2, Summers, Christopher J. 3, Haboeck, Ute 4, Hoffmann, Axel 4 , Azamat, Dmitry 4, and Gehlhoff, Wolfgang 4, Ferguson, Ian T.2 1
Georgia State University, Department of Physics and Astronomy, Atlanta, GA 30303, U.S.A. Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA 30332-0250, U.S.A. 3 Georgia Institute of Technology, Materials Science and Engineering, Atlanta, GA 30332-245, U.S.A. 4 Institut für Festkörperphysik, Technische Universität Berlin, D - 10623 Berlin, Germany. 2
ABSTRACT The incorporation of Mn into GaMnN epilayers by MOCVD growth was investigated. Samples with high Mn concentrations lead to room temperature ferromagnetism. In addition an absorption band around 1.5 eV was observed. Intensity and linewidth of this band scaled with the Mn concentration and with the room temperature (RT) saturation magnetization. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5 D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting also in a significant reduction of the RT magnetization. The pronounced sensitivity of the relative position of the Fermi level and 1.5 eV absorption band can be used to predict the magnetization behavior of the Ga1-xMnxN epilayers. The absence of doping-induced strain was observed by Raman spectroscopy. The structural quality, the presence of Mn2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions were observed. INTRODUCTION Recent predictions and the subsequent experimental confirmations of room temperature ferromagnetism in transition metal (TM) doped wide bandgap materials, such as Ga1-xMnxN has renewed interest in this subject area [1]. In order to make use of these materials for spintronic applications a free carrier-mediated ferromagnetism is to be achieved; however, the origin of ferromagnetism in Ga1-xMnxN and related materials is controversial as is discussed below. It has been predicted that ferromagnetism is facilitated by the interaction between the Mn2+ ions and the holes in the GaN valence band [1]. Preparation of p-type material would be required to shift the Fermi level in GaN towards the valence band [2]. Other theoretical predictions suggest that a Mn-induced impurity band provides a mechanism for effective-mass transport that can be exploited for carrier mediation [3,4]. Sato et al. [4] showed that the incorporation of Mn facilitates the formation of a sharp 5E impurity band and a broader 5T2 impurity band altering the electronic structure in the bandgap of Ga1-xMnxN. The broadening in the partially filled 5T2 band stabilizes the ferromagnetism via the double exchange interaction [5,6] provided the Fermi level is
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