Near surface characteristics of highly sensitive metal oxide thin films
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Near surface characteristics of highly sensitive metal oxide thin films G. Kiriakidis Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas, P.O. Box 1527 Vasilika Vouton, 711 10 Heraklion, Crete, Greece ABSTRACT We present the gas sensing properties of InOx thin films deposited by dc sputtering and ZnOx thin films deposited both by dc sputtering and PLD. The sensitivity of the films towards ozone is correlated with the deposition parameters like film thickness, substrate deposition temperature and growth rate. Secondary Ion Mass Spectrometry (SIMS) analysis showed a noticeable deficit in oxygen in the top 5 nm for films in the “conducting” state, i.e., after UV exposure. Analysis of the sensing response for alumina-based transducers of InOx thin films revealed high sensitivity (less than 25 ppb) with fast and stable response towards ozone while ultimate sensitivity levels down to 10 ppb for ozone and 50 ppb for NO2 were achieved. Surface topography study of ZnOx films utilizing optical, AFM and SEM analyses has shown a distinct surface morphology variation correlated to the growth technique It is demonstrated that PLD leads to very rough surfaces with characteristic non-coordinated columnar features in contrast with the rather smooth surfaces obtained by sputtering.
INTRODUCTION Extended studies on transparent conducting oxide (TCO) nanocrystalline films have been conducted due to their numerous technological applications. TCOs show high infrared reflectance, high luminous transmittance, and good electrical properties. These characteristics allow TCOs to be widely used as transparent conductors in electro-optic modulators, liquid crystal displays, and solar cells [1,2]. The electrical properties of TCO thin films are strongly influenced by the presence of oxidizing gases. Gas molecules interact with the surface of the films inducing redox reactions to take place, which alter the films’ conductivity. Therefore, TCO thin films are good candidates for gas sensing applications. InOx in its stoichiometric form (In2O3) behaves as an insulator, while in its nonstoichiometric form (InOx), it appears to have semiconducting properties, showing high transparency in the visible region due to a wide gap of approximately 3.7 eV and high
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reflectivity in the infrared region of the electromagnetic spectrum [3]. Similarly, ZnO is a widebandgap oxide semiconductor with a direct energy gap of about 3.4 eV. As a consequence, both absorb UV radiation due to band-to-band transitions, while they can be used as transparent electrodes in solar cells and flat panel displays as well as gratings in optoelectronic devices, windows in antireflection coatings and optical filters [4,5]. Furthermore, InOx and ZnOx are used as semiconducting gas sensors due to their conductivity changes when exposed to oxidizing gases such as ozone. InOx sensors have been successfully applied for the detection of oxidizing gases such as O3 and NO2 in the concentration range between some ppb and ppm. Takada et al.
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