This study investigated the sensitivity dependence of thin-film-structured NO 2 gas sensors on the surface morphology of indium oxide-sensing membranes. The indium oxide-sensing membranes were deposited on quartz substrates by using a radio frequency magnetron sputtering system at various oxygen/(argon + oxygen) ratios to modify their In and O atomic percentage, surface morphology, and grain size in the membranes. According to the energy dispersive spectroscopy and x-ray diffraction results, the oxygen atomic percentage and the grain size of the indium oxide-sensing membranes increased and decreased with an increase in the oxygen/(argon + oxygen) ratio, respectively. Through atomic force microscopy, the average roughness of 50-nm-thick indium oxide films deposited at various ratios of oxygen/(argon + oxygen), namely 0% oxygen (pure argon), 20% oxygen, 60% oxygen, and 100% oxygen (pure oxygen), was found to be 0.80 nm, 0.98 nm, 2.68 nm, and 1.25 nm, respectively. The highest sensitivity of the various NO 2 gas sensors was observed at an operating temperature of 150°C because of the generated O - ions and the provision of sufficient energy to overcome the required activation energy. At the operating temperature of 150°C, the sensitivity tendency of the various NO 2 gas sensors was according to the surface roughness of the indium oxide-sensing membranes. Furthermore, the response and the recovery times of the various NO 2 gas sensors corresponded to the surface morphology of the indium oxide-sensing membranes. The response and the recovery times of the NO 2 gas sensors operated at 150°C under an NO 2 gas concentration of 100 ppm were 70 s and 364 s, respectively.
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