Zinc Oxide Thin Films used as an Ozone Sensor at Room Temperature
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0915-R07-04
Zinc Oxide Thin Films used as an Ozone Sensor at Room Temperature Ana Claudia Pimentel, Alexandra Gonçalves, António Marques, Rodrigo Martins, and Elvira Fortunato Materials Science, FCT-UNL, Campus da Caparica, Caparica, na, 2829-516, Portugal
ABSTRACT In this paper we present results of intrinsic/non doped zinc oxide films deposited at room temperature by rf magnetron sputtering able to be used as a truly semiconductor on electronic devices like ozone gas sensors and ultra-violet detectors. The produced films are polycrystalline with a c-axis preferential orientation parallel to the substrate. The films’ resistivity varies from 4.0×10-2 Ωcm to 1.0×10-9 Ωcm, depending on the deposition conditions used (rf power density and oxygen partial pressure), which turns not affecting the optical properties (in average a transmittance of around 85 % and an optical band gap of about 3.44 eV, independent of the deposition conditions used). When exposed to UV light the sensor response based on these films may exceed more than 5 orders of magnitude, recovering to the initial state in the presence of ozone. The sensitivity of the films is improved when the oxygen partial pressure increases and the rf power density used decreases, due to changes on the structural properties of the films. INTRODUCTION ZnO is not new in the semiconductor field, for example, studies of its lattice parameter dating back to1935 [1]. It is a material with a high transmittance in the visible region (a wide band gap semiconductor (3.3 eV) [2]) and a high chemical, thermal and mechanical stability, with a hexagonal structure [3]. Nevertheless recently it is attracting the scientific community attention as can been seen from the surge of an elevated number of publications due to their potential use as an active and versatile semiconductor. This makes ZnO, particularly ZnO thin films in nowadays one of the most studied materials aiming its application in transparent electronic devices, such as an active layer in transparent TFTs [4], UV detectors [5] ozone detectors [6] and even as a sensor and actuator for diamond AFM probes [7]. Several techniques can be used to grow ZnO thin film, like metal organic chemical vapor deposition [8], spin coating [9], thermal oxidation [10], magnetron sputtering [11], among others. The great advantage of using the magnetron sputtering lies in the fact that we can grow high quality transparent polycrystalline ZnO thin films at room temperature. Besides that, it is also possible to obtain films with a wide range of resistivities, ranging from 10-2 to 109 Ω.cm. The electrical and optical properties of ZnO films are affected by the deposition conditions, such as working deposition pressure, the type of substrate, the temperature, the oxygen partial pressure during the deposition as well as the rf power density [12]. In this work we report results concerning the role of the rf power density and oxygen partial pressure on the electrical, structural and optical properties of ZnO thin films as well as the effect of the U
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