Growth and characterization of transition-metal and rare-earth doped III-nitride semiconductors for spintronics
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Growth and characterization of transition-metal and rare-earth doped III-nitride semiconductors for spintronics H. Asahi, S. Hasegawa, Y.K. Zhou and S. Emura ISIR, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT Transition metal (Cr) and rare-earth (Dd, Dy) doped III-nitride semiconductor bulk layers and superlattice (SL) structures are grown on sapphire (0001) substrates and GaN (0001) templates by plasma-assisted molecular-beam epitaxy. For the GaGdN/GaN and InGaGdN/GaN SL and Si co-doped samples, enhancement of magnetization and magnetic moment are observed, suggesting the carrier-mediated ferromagnetism. Low temperature growth of GaGdN can increase the Gd concentration and magnetization. Results for the Dy-doped GaN as well as the GaCrN/AlN/GaCrN tunnel magneto-resistance (TMR) diodes are also described. INTRODUCTION Diluted magnetic semiconductors (DMSs) have been gathering much interest from the industrial viewpoint because of their potentiality as a new functional material, which will open a way to fabricate novel functional semiconductor devices. For the device application, it is very important that the Curie temperature (Tc) of DMSs should be higher than room temperature. GaMnAs is best investigated and well established DMS [1]. However, the Tc for the GaMnAs is still ~ 160 K [2] and is much lower than room temperature. Theoretical calculations suggested that the transition metal-doped GaN will exhibit room temperature ferromagnetism [3, 4]. Sonoda et al reported the MBE growth of Mn-doped GaN and the observation of quite high temperature ferromagnetic (FM) characteristics with Tc as high as 940 K [5]. The observation of FM properties was also reported on the Mn implantation or solid state diffusion GaN [6, 7]. We have grown Cr-doped GaN and observed high temperature (> 400K) FM characteristics [1] as well as the photoluminescence (PL) emission [9]. The observation of PL emission at room temperature is important to fabricate practical spintronics devices that control charges, photons and spins; this is contrast to GaMnAs, where no PL emission was observed. The observation of high temperature FM characteristics was also reported by many other groups [10, 11]. The observation of FM was attributed to double exchange interaction by ab initio computations. However, the first principles Monte Carlo simulations suggested that the magnetic exchange interactions in wide band gap DMSs are effectively short range and the calculated Tc should low for the low concentrations of magnetic ions and in the absence of delocalized or weakly localized carriers [12]. Considering the spinodal decomposition into one dimensional high concentration of magnetic ions (Konbuphase), high temperature FM characteristics was simulated [13]. However, room temperature FM mechanism is still matter of on-going studies. For the MBE-grown Gd-doped GaN, we have first reported the observation of room temperature FM characteristics for the rare-earth doped GaN with Gd concentration of several % [14]. FM was observed even at
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