Light-Emitting and Structural Properties of Si-rich HfO 2 Thin Films Fabricated by RF Magnetron Sputtering
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Light-Emitting and Structural Properties of Si-rich HfO2 Thin Films Fabricated by RF Magnetron Sputtering D. Khomenkov1, Y.-T. An2, X. Portier2, C. Labbe2, F. Gourbilleau2 and L. Khomenkova3 1
Taras Shevchenko National University of Kyiv, Faculty of Physics, 4 Pr. Hlushkov, Kyiv 03022, Ukraine 2 CIMAP/ENSICAEN/UCBN, 6 Blvd. Maréchal Juin, 14050 Caen Cedex 4, France 3 V. Lashkaryov Institute of Semiconductor Physics at NASU, 41 Pr. Nauky, Kyiv 03028, Ukraine ABSTRACT Structural, optical and luminescent properties of Si-rich HfO2 films fabricated by RF magnetron sputtering were investigated versus annealing treatment. Pronounced phase separation process occurred at 950-1100°C and resulted in the formation of hafnia and silica phases, as well as pure silicon clusters. An intense light emission of annealed samples in visible spectral range was obtained under broad band excitation. It was ascribed to exciton recombination inside silicon clusters as well as host defects. To confirm the formation of Si clusters, the structures were codoped with Er3+ ions and effective light emission at 1.54μm was obtained under non-resonant excitation due to energy transfer from Si clusters towards Er3+ ions. The interaction of Si clusters, host defects and Er3+ ions under is discussed. INTRODUCTION During the last years high-k hafnia-based materials are mainly considered as alternative gate dielectrics to silicon oxide in complementary metal-oxide semiconductor technology (CMOS) [1]. Recently, nanomemory applications of Si-doped hafnia films were reported [2,3]. Besides promised electrical properties, HfO2 demonstrates enhanced hardness, high refractive index (almost 2.1 at 632 nm), high optical transparency in the ultraviolet-infrared spectral range, wide optical bandgap (~ 5.8 eV) and low phonon cut-off energy (~about 780 cm-1) offered low probability of phonon assisted relaxation. However, despite these advantages, optical applications of HfO2-based materials are not numerous. HfO2 films as optical coatings were investigated in [4,5]. The intrinsic 4.2-4.4 eV luminescence of pure HfO2 was ascribed to self-trapped exciton [6,7], whereas the 2.5-3.5 eV emission was attributed to different oxygen vacancies with trapped electrons [6]. Rare-earth doped HfO2 materials were also studied in [8,9], but the mechanism of the excitation of rare-earth ions and their interaction with the host defects were not clarified. It is worth to note that the development of rare-earth doped materials suffers from lower absorption cross-sections of rare-earth ions for 4f-4f transitions (10-18-10-20 cm-2) required high-power excitation sources. Meanwhile 4f-5d transitions have higher cross-section (~10-12 cm-2), but corresponding excitation levels belong to UV and vacuum UV spectral range, restricting many applications of these materials. Thus, to enhance an excitation of 4f-4f transitions a host mediated excitation via energy transfer is needed. Among different rare-earth elements, the Er3+ ion is one of the most popular due to its radiative transitions in the
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