Synthesis and characterization of high quality ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures abo
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Synthesis and characterization of high quality ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K Stephen Y. Wu1, H.X. Liu1, Lin Gu2, R.K. Singh1, M. van Schilfgaarde1, David J. Smith2,3, N.R. Dilley4, L. Montes4, M.B. Simmonds4, and N. Newman1 1
Dept. of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 852876006 2 Center for Solid State Science, Arizona State University, Tempe, AZ 85287 3 Dept. of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504 4 Quantum Design, San Diego, CA 92121 ABSTRACT Reactive MBE growth was used to synthesize ferromagnetic Cr-doped GaN and AlN thin films with Curie temperatures above 900K. 2% Cr-doped GaN and 7% Cr-doped AlN were found to have a saturation magnetization moment of 0.42 and 0.6 µB/Cr atom, indicating that ~14% and ~20% of the Cr, respectively, are magnetically active. Structural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) did not find evidence of a ferromagnetic secondary phase. Electrical characterization indicate that the resistivity of the Cr-doped GaN films depends exponentially on temperature as R=Roexp[(To/T)1/2], characteristic of variable range hopping. In contrast, Cr-doped AlN films are highly resistive. Local spin density functional calculations predict that Cr forms a deep level defect in both systems and the t2 level falls approximately at midgap. Our theoretical and experimental results indicate that ferromagnetism in Cr-doped GaN and AlN arises as a result of the double exchange mechanism within the partially filled Cr t2 band. INTRODUCTION Spintronics has been receiving a tremendous amount of attention lately due to its unique ability to utilize both the charge and the spin state of an electron [1]. This combination makes dilute magnetic semiconductors (DMSs) a very attractive material system for spin-based electronic applications. The initial observation of ferromagnetism in Mn-doped InAs and GaAs above 100K has since initiated great interest in related III-V systems that exhibit ferromagnetism at even higher temperatures [2]. However, in order to realize practical applications, spinpolarized populations must exhibit long lifetimes and diffusion lengths [3], and exhibit ferromagnetism above room temperature. Even though the Mn-doped III-V system has been receiving quite a bit of attention among the SPINS community to-date, we believe that the Cr-doped III-V system is preferred for a number of reasons. First, Cr has a lower vapor pressure than Mn, so it would be expected to have a larger sticking coefficient at the elevated substrate temperatures used during MBE (Molecular Beam Epitaxy) and CVD (Chemical Vapor Deposition) growth. The limited growth temperature used during deposition of other ferromagnetic Mn-doped III-V semiconductors to overcome the high volatility of Mn is presumably responsible for the observed poor crystalline quality and high levels of compensation in these materials [4]. Second, calculations using the LSDA (l
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