Synthesis and Gas Sensitivity of Thin Chromium Oxide Films
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sis and Gas Sensitivity of Thin Chromium Oxide Films A. V. Almaeva, B. O. Kushnareva*, E. V. Chernikova, and V. A. Novikova a
Tomsk State University, Tomsk, 634050 Russia *e-mail: [email protected]
Received April 16, 2020; revised July 6, 2020; accepted July 20, 2020
Abstract—Thin polycrystalline Cr2O3 films were obtained using RF magnetron sputtering followed by annealing in air at T = 673 K. The obtained films had grain diameters within 40–70 nm and a bandgap width of 3.3 ± 0.2 eV. In a temperature range of 303–473 K, sensors based on these Cr2O3 films exhibited high response to NO2, H2, acetone and toluene vapors, while weakly responding to the presence of CH4 and CO and showing relatively weak dependence of the electric resistance on air humidity. A qualitative model of the influence of gases on the electrical properties of thin Cr2O3 films is proposed. Keywords: chromium oxide, thin films, magnetron sputtering, gas sensors. DOI: 10.1134/S106378502010017X
The interest in developing gas sensors based on metal-oxide p-type semiconductors including Cr2O3 is related to their low operating temperatures and weak dependence of the gas sensor properties on air humidity [1–5]. In the region of low operating temperatures (T = 423–573 K), thick polycrystalline Cr2O3 films and nanostructure arrays are characterized by high sensitivity to hydrocarbon vapors [6, 7]. With respect to response speed, production cost, and compatibility with microelectronic technology, the most attractive gas sensors are those based on thin Cr2O3 films manufactured by magnetron sputtering [8]. The creation of sensors based on thin polycrystalline Cr2O3 films with small grain sizes (rg) is hindered by the lack of models adequately describing the sensor effect. Solving this problem first requires studying the sensitivity of Cr2O3 to model gases with low molecular masses. In this context, the present work was devoted to the synthesis of thin Cr2O3 films and investigation of their gas sensing properties. Thin polycrystalline Cr2O3 films were obtained using RF magnetron sputtering of pure chromium (99.7% Cr) targets in oxygen–argon plasma with deposition onto 330-μm-thick sapphire substrates of 50-mm diameter. The substrate temperature during deposition was maintained on the level of room temperature. The working gas pressure and discharge power were maintained at 7 × 10–3 mbar and 70 W, respectively, and the oxygen content in O2 + Ar mixture was 56.1 ± 0.5 vol %. The substrate–target distance was 70 mm, and the film deposition time was 45 min. After deposition, the film was annealed in air
at 673 K for 3 h. The thickness of the obtained films was about 200 nm. Finally, platinum contacts on a Cr2O3 film surface were formed at a distance of 1.8 mm. The gas sensing properties of Cr2O3 films were studied in a vacuum chamber of 950-cm3 volume, through which a mixture of dry clean air and target gas could be pumped. The target gas content in the mixture was set by a gas mixture generator, while dry clean air was supplied from a special setup. The tota
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