CONTRIBUTED P A P E R Ferromagnetism in ZnO- and GaN-Based Diluted Magnetic Semiconductors: Achievements and Challenges This paper reports on the current status of studies of optoelectronic devices that have been developed by doping with rare-earth and transition elements. By Vitaliy Avrutin, Member IEEE , Natalia Izyumskaya , U ¨ mit O ¨ zgu ¨r, Member IEEE , Donald J. Silversmith, Life Senior Member IEEE , and Hadis Morkoc ¸ ABSTRACT | Both GaN- and ZnO-based diluted semiconductors (DMSs) have recently attracted considerable interest fueled by theoretical predictions of ferromagnetic ordering in these materials above room temperature, making them especially attractive for spintronics. The intense experimental research that followed has revealed, however, a great deal of contro- versy. The local structure and magnetic behavior of GaN- and ZnO-based DMSs were found to depend strongly on a preparation technique and growth conditions for the materials of the same nominal composition and the reported results varied considerably from group to group. This problem highlighted clearly the lack of theoretical understanding of physical mechanisms underlying ferromagnetisms in these materials and the inadequacy of standard characterization techniques used to probe structural and magnetic properties of the DMSs. In this paper, we report on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs and make special impact on critical discussion of experimental methods employed for investigation of their magnetic and optical properties. KEYWORDS | Defects; diluted magnetic oxides; diluted mag- netic semiconductors (DMSs); GaN; local structure; spintro- nics; ZnO I. INTRODUCTION Diluted magnetic semiconductors (DMSs), or sometimes referred to as semimagnetic semiconductors, are semicon- ducting materials in which a fraction of the host cations is substitutionally replaced by magnetic ions. Much of the attention on DMS materials is due to their potential applications in what is dubbed as the ‘‘spintronics’’ devices, which exploit the spin in magnetic materials along with charge of electrons in semiconductors. Transition metals (TMs), which have partially filled d states (Sc, Ti, V, Cr, Mn, Fe, Co, and Ni; Cu has a completely filled d shell, however, acts as magnetic impurity in Cu 2þ charge state), and rare-earth elements, which have partially filled f states (e.g., Eu, Gd, Er), are used as magnetic ions in DMS. The partially filled d or f states contain unpaired electrons, in terms of their spin, which are presumably responsible for exhibiting magnetic behavior. To have any practical application in functional devices, DMS materials must exhibit ferromagnetism with the Curie temperature ðT C Þ above room temperature, which is naturally deemed to be the bottleneck issue. Besides, for some device applications, it is also desirable to have carrier-mediated ferromagnetism, so that the magnetic properties of the DMS can be manipulated through external means, i.e., by varying free-carrier concentration. Manuscript received April 28, 2009; revised February 17, 2010; accepted March 2, 2010. Date of publication May 24, 2010; date of current version June 18, 2010. V. Avrutin, N. Izyumskaya, U ¨. O ¨ zgu ¨r, and H. Morkoç are with the Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284 USA (e-mail: vavrutin@vcu.edu; nizioumskaia@vcu.edu; uozgur@vcu.edu; hmorkoc@vcu.edu). D. J. Silversmith is with Air Force Office of Scientific Research, Arlington (Ballston), VA 22203 USA (e-mail: donald.silversmith@afosr.af.mil). Digital Object Identifier: 10.1109/JPROC.2010.2044966 1288 Proceedings of the IEEE | Vol. 98, No. 7, July 2010 0018-9219/$26.00 Ó2010 IEEE