Tunneling Spectroscopy of Magnetic Impurities (=Mn,Fe,Co) in ZnO
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1035-L05-26
Tunneling Spectroscopy of Magnetic Impurities (=Mn,Fe,Co) in ZnO Takashi Tamura, Changman Kim, Yasushi Oikawa, and Hajime Ozaki Electrical Engineering and Bioscience, Waseda University, 3-4-1 Ohokubo, Shinjuku-ku, Tokyo, 169-8555, Japan ABSTRACT The first observations have been made on the electronic density of states of Mn-, Co- and Fe-doped ZnO by tunneling spectroscopy. For Mn- and Co-doped ZnO, rises in the density of states were observed around 3.2eV and 2.8eV below the bottom of the conduction band, respectively. In these cases, the Fermi level lay at the bottom of the conduction band. For Fe-doped ZnO, a lump of density of states with about 1.2eV width was observed around the Fermi level in the mid-gap. These experimental results were qualitatively consistent with the recent theoretical calculations. INTRODUCTION Diluted magnetic semiconductors (DMSs), in which a portion of atoms of nonmagnetic semiconductor hosts is substituted by magnetic transition-metal ions, have interested us in the spintronics which exploit both the spin and the charge of carriers. Most of the investigations of DMSs have focused on - semiconductors such as (Zn,Mn)Te and (Cd,Mn)Te [1], and semiconductors such as (In,Mn)As [2] and (Ga,Mn)As [3, 4]. However, these DMSs showed an antiferromagnetism or had a very low ferromagnetic Curie temperature Tc unsuitable for a practical application.
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Recently, ZnO has attracted attention as a candidate for the DMSs host materials which shows the room temperature ferromagnetism [5]. Many workers have investigated the 3dtransition-metal-doped ZnO and some of them reported ferromagnetism above room temperature [6,7], while others reported low ferromagnetic-ordering temperature [8,9], spin-glass [10] and paramagnetic behavior [11]. Thus there is a lot of controversy about the observed ferromagnetism. It is not yet clear whether the observed ferromagnetism is truly intrinsic or related to secondary phases such as magnetic clusters and 3d transition-metal oxides. Therefore, it is important to know the fundamental properties of magnetic impurities in ZnO in order to elucidate the origin of the ferromagnetism. Recently, some theoretical investigations have been reported on the electronic structures of 3d transition-metal ions doped in ZnO. Those results were in agreement with each other on the main points. The density of states (DOS) of both Mn and Co ions distributed just above the valence band edge and inside the conduction band [12-15]. The Fe ions showed a DOS near the Fermi level in the band gap of ZnO, in addition to those for Mn and Co ions. On the other hand, the experimental studies on the electronic structure of doped magnetic ions in ZnO are very few up to now. The purpose of this study is to investigate the electronic DOS by tunneling measurements and compare them with the recent theoretical results. In this study we have first observed the DOS of M-doped ZnO (M = Mn, Fe, Co) by tunneling spectroscopy which is one of the useful methods for analyzing the DOS directly with hig
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