Emission of Rare Earth Ions Incorporated into Metal Oxide Thin Films and Fibres
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V3.6.1
Emission of Rare Earth Ions Incorporated into Metal Oxide Thin Films and Fibres
Ilmo Sildos, Sven Lange, Tanel Tätte, Valter Kiisk, M. Kirm1 and J. Aarik Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia 1 Institut für Experimentalphysik der Universität Hamburg, Luruper Chaussee 149, Hamburg 22 761, Germany ABSTRACT Photoluminescence (PL) of undoped and Sm-doped TiO2, ZrO2 and HfO2 thin films and fibers was investigated at temperatures ranging from 6 to 300 K. The thin films were grown by the atomic layer deposition (ALD) technique and doped by using the ion implantation method. The fibers were prepared by using the sol-gel method whereas an in-situ doping was used to obtain the required concentration of Sm3+ ions in the films. In undoped as well as doped materials, PL was efficiently excited via band-to-band transitions. The emission of undoped materials was attributed to the radiative recombination of self-trapped excitons (STE). In doped materials, intense emission of Sm3+ was recorded. It is proposed, that there exists a concurrence between the radiative recombination of bound excitonic states and the energy transfer to Sm3+ ions, particularly at lower temperatures. INTRODUCTION Transition metal oxides, especially ZrO2 and HfO2, have recently attracted great attention because of their potential applications in electronic devices [1,2]. These oxides can, however, also be applied in a number of optical devices [3–5] where dielectric materials with a relatively wide band gap and high refractive index are needed. The most interesting oxides for these applications are TiO2, ZrO2 and HfO2. The main optical parameters of those materials are relatively well known (Table I), however, their luminescence properties should still be studied in more detail. For instance, in important applications, such as optical amplifiers and lasers [5], high yield of luminescence is the most significant material parameter. For this reason, photoluminescence (PL) properties of Sm-doped thin films and fibers of TiO2, ZrO2 and HfO2 were investigated in this work. In order to better understand the emission and energy transfer mechanisms, the PL of undoped thin films was also studied. Table I. Band gap energies and refractive indices of various phases of TiO2, ZrO2 and HfO2. Material Phase Band gap, eV TiO2 amorphous 3.40 TiO2 tetragonal (anatase) 3.35 TiO2 tetragonal (rutile) 3.16 ZrO2 tetragonal 5.22 ZrO2 monoclinic 5.25 HfO2 amorphous – HfO2 monoclinic 5.65
Refr. index at 580 nm 2.45 2.53 2.82 – 2.21 1.95 2.16
References [6] [6] [6] [7] [8] [9] [10]
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EXPERIMENTAL DETAILS The TiO2, ZrO2 and HfO2 thin films were grown by using the atomic layer deposition (ALD) processes described elsewhere [6,8,9]. The films were deposited on Si(100) substrates and they were 150–200 nm in thickness. Structural data for as-grown films determined by using X-ray diffraction and reflection high-energy electron diffraction analysis are included in Table I. For further studies, TiO2 films with an anatase structure and HfO2
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