Color Centers in Magnesium Doped Polycrystalline Alumina
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Color Centers in Magnesium Doped Polycrystalline Alumina L. R. Brock, K. C. Mishra, Madis Raukas, Walter P. Lapatovich and George C. Wei OSRAM SYLVANIA, Research and Development, Beverly, MA 01915, U.S.A ABSTRACT We have investigated color centers in MgO-doped polycrystalline alumina (PCA) using absorption, excitation, and emission spectroscopy. Most of the color centers that were reported in earlier studies of the crystalline material have been observed to be present in the polycrystalline material. The absorption spectral features observed in the PCA are attributed to various color centers; however, they are not sufficiently resolved to make unique assignments. Suitable combinations of excitation and emission spectroscopy and also measurements at low temperature were therefore used to identify most of the color centers in this material. Among the numerous color centers that we have identified in PCA are variations of electron centers including F, F+, F2+, F22+ and F+-Mg ((Vo•-MgAl’)x ). The most prominent oxygen vacancy related defect observed at room temperature was the F+-Mg center, with absorption bands located at 217 and 249 nm, and an emission band at 303 nm. This center can be thought of as being formed by association of an F+ center with a Mg defect. The single crystal sapphire samples containing no Mg show only F+ (Vo•) centers with 230 and 257 nm absorption bands, and a 328 nm emission band. Low temperature (22 K) fluorescence excitation measurements of PCA led to emission from F22+ center at 467 nm. Additionally, there is evidence that the observed 368 nm emission band could be attributed to the zero-phonon line associated with the F2+ center. INTRODUCTION The new material of choice for High Intensity Discharge (HID) arc tubes is polycrystalline alumina (PCA) [1,2]. For many lighting applications, the optical characteristics of the PCA material are comparable to that of fused silica. PCA can be sintered to translucency with the addition of MgO as a grain-growth inhibiting agent [3]. However, unlike fused silica that devitrifies at 1000 oC, the melting point of PCA exceeds 2000 oC [4]. This allows for a higher operation temperature of the arc tube, leading to improved lamp performance including higher efficacy, better color rendering, and smaller lamp to lamp color spread. Additionally, PCA has been shown to be more resistant to corrosion than fused silica, which may result in longer life for HID lamps. The envelope of HID lamp arc tubes, whether fused silica or PCA, must retain its translucency throughout the lamp life. One dominant source of lamp degradation is the loss of transmittance of the lamp envelope. This could be partially due to generation of color centers, which absorb visible radiation from the discharge. Here we study the optical properties of MgO-doped PCA using absorption, excitation, and emission spectroscopy, with particular emphasis on color centers. A color center is a point defect in a crystal lattice. For example, an F center is a color center resulting from an electron or electrons
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