Phase transformation of Er 3+ -doped Al 2 O 3 powders prepared by the sol-gel method
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Er3+-doped Al2O3 powders were prepared by the sol-gel method, using aluminum isopropoxide [Al(OC3H7)3]-derived Al2O3 sols. The phase content diagram for the Er-doped Al–O system with the doping concentration up to 20 mol% is presented for sintering temperature from 550 to 1250 °C. The different crystalline types of (Al,Er)2O3 phases—␥, , ␣—and two Er-Al-O phases—ErAlO3 and Al10Er6O24—were detected. The Er3+ doping in the Al2O3 phases has a significant effect on phase transformations for ␥ → and → ␣, indicating the enhancement for stability of the ␥ and phases. Compared with the ␥–Al2O3 phase, the ␥–(Al,Er)2O3 phase can be obtained at high temperatures up to about 1100 °C. The –(Al,Er)2O3 phase, together with the ␣–(Al,Er)2O3 phase, was still observed at 1200 °C under the high Er3+ doping concentration. The Er3+ doping suppressed the crystallization of the ␥ and phases. At a low Er3+ doping concentration from about 2 to 5 mol%, the ErAlO3 phase precipitated out around 1100 °C. The Al10Er6O24 phase was observed at all sintering temperatures; the precipitation was carried out to all Er3+ doping concentrations with increasing sintering temperatures up to about 1200 °C.
I. INTRODUCTION 3+
Among rare-earth ion-doped optical waveguides, Er doped optical waveguides have aroused considerable interest following the success of the Er3+-doped fiber amplifier, whose intra-4f transition around 1.53 m coincides with the low-loss window of standard silicabased optical fibers.1 The oxides of group III and IV elements have been used as Er3+-doped planar optical waveguide matrix material, including SiO2,2 Al2O3,3,4 Y2O3,1 TiO2,5 In2O3,6 etc. In particular, Al2O3 is an interesting matrix material for use in planar optical waveguides because the relatively high reflection index of Al2O3 (n ⳱ 1.64) waveguides cladded with SiO2 (n ⳱ 1.45) results in high confinement of the optical mode in the waveguides, leading to efficient pumping and amplification. In addition, the high contrast of reflection index allows for the use of small device structures.3,4 However, only a few deposition techniques have been reported as able to provide high-quality Al2O3 waveguide films. Furthermore, incorporation of Er in Al2O3 is not easy due to the large difference between ionic radius of Er and Al. Er3+-doped Al2O3 waveguide films were first fabricated from Er ion implantation into Al2O3 films deposited a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 10, Oct 2003
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using radio-frequency magnetron sputtering by van den Hoven et al.3 High-density Al2O3 films with homogeneous incorporation of Er have also been produced by pulsed laser deposition.7 It has been found that the Er3+ doping concentration has a significant influence on the optical properties of the Er3+-doped Al2O3 films. The photoluminescence intensity increases by a factor of 50 as the implanted Er peak concentration is increased from 0.03 to 3 mol%.3 It is well known that the
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