Fe-Centers in GaN as Candidates for Spintronics Applications
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Fe-Centers in GaN as Candidates for Spintronics Applications Enno Malguth, Axel Hoffmann1 , Matthew Phillips, and Wolfgang Gehlhoff1 Microstructural Analysis Unit, University of Technology, Sydney, NSW 2007, Australia 1 Technische Universit¨at Berlin, Institut f¨ ur Festk¨orperphysik, 10623 Berlin, Germany
ABSTRACT The potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 400 µm thick freestanding HVPE grown GaN:Fe crystals with different Fe-concentration levels ranging from 5×1017 cm−3 to 2×1020 cm−3 was studied by means of photoluminescence, photoluminescence excitation (PLE) and Fourier transform infrared (FTIR) transmission experiments. The Fe3+/2+ charge transfer (CT) level was determined to be at 2.86 ± 0.01 eV above the valence band maximum considerably lower than the previously reported value of 3.17 ± 0.10 eV. A bound state of the form (Fe2+ , hV B ) with a binding energy of 50 ± 10 meV has been established as an excited state of Fe3+ . FTIR transmission measurements revealed an internal (5 E—5 T2 ) transition of Fe2+ around 400 eV which, until now, was believed to be degenerate with the conduction band. Consequently, a second CT band was detected in PLE spectra. INTRODUCTION A possible realization of spintronic devices is to introduce transition metal (TM) ions into a wide band gap semiconductor [1]. Theoretically, in such a diluted magnetic semiconductor (DMS) the magnetic order of the localized magnetic centers is established by delocalized free carriers, resulting in room temperature ferromagnetism [2, 3]. In particular, free holes are shown to interact with the magnetic impurities via exchange interaction [3]. Therefore, in order to predict the feasibility of carrier-mediated spin coupling, it is crucial to know the electronic structure of the TM centers, which depends on the present charge state and is strongly influenced by the host crystal [4]. The exact position of the Fe3+/2+ charge transfer (CT) level within the bandgap is of particular interest. While extensive studies have been conducted on the promising GaN:Mn material system, e.g. [5], only little is known about the applicability of GaN:Fe for spintronics. The spinforbidden (4 T1 (G)—6 A1 (S)) transition of the Fe3+ center in GaN exhibits a structured near infra red (IR) luminescence at 1.299 eV with a 8 ms lifetime [6, 7]. By means of photoluminescence excitation (PLE) Heitz et al. identified higher excited states, 4 T2 (G) and 4 E(G), at 2.01 eV and 2.731 eV, respectively, as well as a shallow bound state of the form (Fe3+ ,e,h) at 2.888 eV [8]. After pinpointing the Fe3+/2+ CT level at (3.17 ± 0.10) eV, the authors drew three major conclusions [8]: (1) The binding energy of the bound state equals ≈ 280 meV. (2) With a likely energy of approximately 390 meV the Fe2+ (5 E—5 T2 ) transition is degenerate with the conduction band and cannot be observed. (3) The internal reference rule
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