Magnetic and Optical Properties of Eu-doped GaN
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Magnetic and Optical Properties of Eu-doped GaN Jennifer Hite1, G T Thaler1, J H Park2, A J Steckl2, C R Abernathy1, J M Zavada3, and Stephen Pearton4 1 Materials Science and Engineering, University of Florida, Gainesville, FL, 32611 2 Nanoelectronics Laboratory, University of Cincinnati, Cincinnati, OH, 45221 3 Electronics Division, US Army Research Office, Research Triangle Park, NC, 27709 4 Materials Science and Engineering, University of Florida, PO Box 116400,100 Rhines Hall, Gainesville, FL, 32611
ABSTRACT GaN films were doped with Eu to a concentration of ~0.12 at. % during growth at 800°C by molecular beam epitaxy, with the Eu cell temperature held constant at 470°C. All samples were post-annealed at 675°C. The films exhibited strong photoluminescence (PL) in the red (622 nm) whose absolute intensity was a function of the Ga flux during growth, which ranged from 35.4x10-7 Torr. The maximum PL intensity was obtained at a Ga flux of 3.6x10-7 Torr. The samples showed room temperature ferromagnetism with saturation magnetization of ~0.1-0.45 emu/cm3, consistent with past reports where the Eu was found to be predominantly occupying substitutional Ga sites. There was an inverse correlation between the PL intensity and the saturation magnetization in the films. X-ray diffraction showed the presence of EuGa phases under all of our growth conditions but these cannot account for the observed magnetic properties. INTRODUCTION There is continued interest in the properties of rare-earth doped wide bandgap nitrides for their potential application in optoelectronics devices such as visible lasers that can be grown on Si substrates [1-8]. The large bandgaps of GaN, AlN allow emission of higher energy rare earth transitions that are otherwise absorbed in smaller bandgap host materials such as GaAs. These materials may have application in visible displays or in white light systems that employ colorcombining techniques. Rare earth doping of GaN with Eu and Gd has also been reported to produce ferromagnetism, although the mechanism is not clear. Several groups have suggested that there is long range polarization of the area surrounding each Gd atom. There is a significant difference in the dopant concentrations needed to induce ferromagnetism in GaN between rare earths and the more conventional transition metals such as Mn. In the latter case, concentrations of 3-5 at.% are typically required and this is well above the solid solubility, requiring use of low growth temperatures or non-equilibrium incorporation methods such as ion implantation. By comparison, concentrations of 1016-1018 cm-3 of Gd and Eu appear sufficient to induce ferromagnetism and correspondingly large magnetic moments. This has the advantage that there is less compromise in the material quality through the use of lower impurity levels. In this paper we report on the synthesis of Eu-doped GaN by molecular beam epitaxy (MBE) and the optical and magnetic properties of the resulting films. Strong red (622 nm) photoluminescence is obtained along with
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