Role of Defect Sites and Ga Polarization in the Magnetism of Mn-Doped GaN D. J. Keavney, 1 S. H. Cheung, 2 S. T. King, 2 M. Weinert, 2 and L. Li 2 1 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 2 Department of Physics, University of Wisconsin, Milwaukee, Wisconsin 53211, USA (Received 25 July 2005; published 12 December 2005) We report a study of the Mn local structure, magnetism, and Ga moments in molecular beam epitaxy grown Mn-doped GaN films. Using x-ray absorption spectroscopy and magnetic circular dichroism, we find two distinct Mn sites and a Ga moment antiparallel to Mn. First-principles calculations reproduce this phenomenology and indicate that Mn preferentially populates Ga sites neighboring N split interstitial defects. These results show that defects may strongly affect the Mn ordering and magnetism, and that the GaN valence band is polarized, providing a long-range ferromagnetic ordering mechanism for Ga 1x Mn x N. DOI: 10.1103/PhysRevLett.95.257201 PACS numbers: 75.50.Pp, 75.25.+z, 78.70.Dm Doped magnetic semiconductors have received consid- erable attention as part of the search for a new class of spintronic devices [1], with a primary goal of realizing a room-temperature ferromagnetic semiconductor [2 –5]. In Mn-doped III-V semiconductors, Mn nominally sub- stitutes for the group III atom and simultaneously provides a localized magnetic moment and a hole, leading to Mn 3d-host 4sp exchange and ferromagnetic ordering. Defect sites may also be present, and can play an important role in the distribution of Mn and its magnetic ordering. In GaAs, Mn interstitials strongly affect the Curie temperature T C and drive its dependence on post deposition annealing through hole compensation. However, the importance of defects, and the types of defects present, will be very much dependent on the host compound, hence what is known about GaAs may not be applicable even to other III-V compounds. Therefore, understanding of magnetism in doped semiconductors requires knowledge of defects and their interactions with the Mn site distribution and the 3d-4sp exchange. X-ray absorption spectroscopy (XAS) and x-ray mag- netic circular dichroism (XMCD), coupled with theoreti- cal understanding of realistic defect-induced band struc- tures, offer a way to resolve these issues. Recently Wu [6] compared calculated XAS and XMCD spectra in Ga 1x Mn x As with experimental data [7,8], and showed that the inclusion of Mn interstitial sites and substitutional- interstitial dimers can have a strong effect on the spectra. In addition, XMCD can detect induced moments at the group III and V sites, indicating how the host participates in the magnetic ordering [8]. Mn-doped GaN may offer the promise to go beyond cryogenic Curie temperatures. Theoretical work predicts T C well above 300 K [9,10], and experimental T C from 10 to 900 K have been reported [11,12]. Manganese is ex- pected to occupy Ga sites as a 2 ion, and Edmonds et al. [13] recently reported Mn L 3;2 x-ray absorption on mo- lecular beam epitaxy (MBE) grown Ga 1x Mn x N, confirm- ing the divalent state. However, d 4 and d 3 configurations have also been detected using other methods and under varying growth and codoping conditions [14,15], raising the possibility of variations of the electronic structure due to defects that could be causing the observed T C variations. Epitaxial GaN films are known for very high defect den- sities, and the role of these defects and of interstitial Mn in the magnetic ordering is not well known. In this Letter, we report an XAS and XMCD study of the Mn electronic structure and induced Ga moments in single- phase Mn-doped GaN films with varying Mn concentra- tions, together with first-principles calculations for a vari- ety of defects. We find that the XAS and XMCD data, including the observed small antiparallel Ga moment, can be explained consistently if Mn is preferentially bound to N split interstitials. Our results also suggest interstitial Mn does not exist in significant quantities in these samples. The Ga 1x Mn x N films were grown on 6H-SiC (0001) substrates by electron-cyclotron resonance plasma-assisted MBE [16]. The 60 nm GaN buffer layer was grown at 570  C. The subsequent 150 nm Mn-doped layers were grown at 500  C and with the addition of H 2 to the nitrogen plasma, which is found to suppress the formation of pre- cipitates [16]. Four films, 150 nm thick, were grown with Mn doping, x, of 1.5, 1.9, 3.6, and 4.5%, as determined by Mn=Ga peak ratio in energy dispersive spectroscopy. A pseudo (1  1) surface reconstruction was observed by reflection high-energy electron diffraction (RHEED) on the buffer layer, suggesting 2D growth and a Ga-terminated polar surface [17]. For the Mn-doped layer, spotty RHEED patterns indicate 3-dimensional under nitrogen rich con- ditions [16]. The XAS and XMCD measurements were taken at beam line 4-ID-C of the Advanced Photon Source [18]. The samples were mounted in a superconducting solenoid with the field along the x-ray beam propagation direction, with the beam at 20  above the film plane. The beam line resolution at the Mn L edge was set to 0:27 eV. Data were collected in total electron yield by monitoring the sample photocurrent. The photon polarization was re- PRL 95, 257201 (2005) PHYSICAL REVIEW LETTERS week ending 16 DECEMBER 2005 0031-9007= 05=95(25)=257201(4)$23.00 257201-1  2005 The American Physical Society