Fabrication, processing and characterization of thin film ZnO for integrated optical gas sensors
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1201-H05-14
Fabrication, processing and characterization of thin film ZnO for integrated optical gas sensors Eliana Kamińska1, Anna Piotrowska1, Iwona Pasternak1, Michał A. Borysiewicz1, Marek Ekielski1, Krystyna Gołaszewska1, Witold Rzodkiewicz1, Tomasz Wojciechowski2, ElŜbieta Dynowska2, Przemysław Struk3, Tadeusz Pustelny3 1
Institute of Electron Technology, Al. Lotników 32/46, 02-668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland 3 Silesian University of Technology, ul. Akademicka 2A, 44-100 Gliwice, Poland 2
ABSTRACT Zinc oxide layers deposited on quartz substrates by means of RF reactive magnetron sputtering with subsequent RTP annealing in a nitrogen flow at 400oC and in an oxygen flow at 500oC have been investigated in applications to waveguide structures. The ZnO films reveal a highly c-oriented columnar structure with a surface roughness of 4.3 nm. Annealing causes a significant increase of the lattice constant to the value of 5.210±0.001 Å suggesting the relaxation of the stress in the film. The annealing process causes a significant improvement of propagation properties of the fabricated waveguide structures in comparison to structures using as-deposited ZnO films. The minimal attenuation coefficient of the 630 nm thick films was found to be 2.8 and 3.0 dB/cm for TE0 and TM0 modes respectively.
INTRODUCTION Recently, optical gas sensors built of an optical waveguide with a sensing layer on top of it, integrated with input and output couplers attract considerable interest [1-3]. In this respect, ZnO with its unique combination of optical properties and sensing capabilities offers an opportunity to use it for both the waveguide and sensing elements. However, there are still important technological steps to be mastered in order to fabricate practical devices. The growth of nondoped material presents a special challenge. It is commonly recognized that intrinsic defects such as vacancies, interstitials and extended structural defects as well as residual impurities contribute to background n-type conductivity and lattice constraints in nominally undoped ZnO. The structural properties strongly determine the optical properties of ZnO thin layers and thus influence the feasibility of the use as waveguides [4-7]. The fabrication of unstressed, high quality non-doped ZnO material with well defined microstructure and small surface roughness for use in integrated optical sensors is the focus of this paper.
EXPERIMENTAL Zinc oxide layers were deposited on quartz substrates by means of RF reactive magnetron sputtering. A ceramic stoichiometric ZnO target (99.999% purity) was used in this study. The
sputtering processes were conducted in an oxygen-argon mixture. The working pressure pAr+O2 was equal 1×10-2 mbar, partial O2 pressure pO2 = 3×10-3 mbar. The deposition rate was 24 nm/min. The as grown ZnO films were annealed in a nitrogen flow at 400oC for 10 min. and subsequently in an oxygen flow at 500oC for 10 min. Rapid Thermal Annealing (RTA) was used
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