Site Selective Magneto-Optical Studies of Eu ions in Gallium Nitride
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Site Selective Magneto-Optical Studies of Eu ions in Gallium Nitride Nathaniel Woodward1, Atsushi Nishikawa2, Yasufumi Fujiwara2, and Volkmar Dierolf1 1
Physics, Lehigh University, Bethlehem, Pennsylvania, U.S.A.
2
Division of Materials and Manufacturing Science, Osaka University, Osaka, Japan.
ABSTRACT We report site-selective studies of the Zeeman splittings that are observed for magnetic fields up to 6.6T for different Eu incorporation sites in GaN. Utilizing resonant excitation with visible light, we are able to distinguish the site and find for one center (Eu1) a splitting into five components as expected for C3v symmetry. The corresponding g-values are 1.66 and 1.90. The two lines of another center Eu2 each split into two levels corresponding to g-values of 1.9 and 2.84. Most surprisingly a third center, for which only one line is clearly identified, a g-value of 6.16 is found which is larger than can be explained for a 7F2 purely ionic Eu state. INTRODUCTION Eu ions in GaN have been the topic of intense spectroscopic studies due to their application for electroluminescence devices. Several different incorporation sites have been identified by different groups [1-4]. Most notable, two centers Eu1 and Eu2 have been studied for their excitation efficiency under above band-gap excitation and their relative numbers in samples that have been grown or thermally annealed in a different ways. In order to further characterize the spectroscopic properties of these centers, measurements under the application of magnetic fields are an excellent tool to test models such as for instance crystal field analysis, which could yield important information about the site symmetry. Measurement of the Zeeman splitting of optical spectra is especially important for Eu3+ for which no ground state EPR data can be obtained due to the 7Fo character of its ground state. Kachkanov et al. have reported first results [5], which only offered limited site-selectivity and hence are difficult to interpret. The purpose of this report is to provide site-selective Zeeman data under obtained under resonant excitation conditions, which yield the g-factors of all sublevels of the 7F2 state. The two samples used in our experiment were grown by OMVPE at 1050o C under 10 kPa of pressure. For more information on the growth of these samples refer to [6]. The detailed spectroscopic data were reported in Ref. 1 and 2. RESULTS For all of the magnetic measurements, we utilize Faraday geometry, where the polarization of the incident laser is perpendicular to the applied magnetic field. The emission is collected in backscattering geometry. We use a fiber-based optical access module, which is placed into a magnetic cryostat (manufacturer: Janis model: 9T Cryostat). As a fiber-coupled tunable laser source we use an Argon ion laser pumped dye
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laser (manufacturer: Coherent, model 590) that is tunable from 565nm to 600nm. For our Zeeman studies, we focus on the spectral range around 570nm in which the phononassisted 7Fo to 5D0 excitation transition are loc
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