Physical Characterization of Ruthenium-Doped Zinc Oxide Thin Solid Films Deposited by the Sol–Gel Technique: Gas-Sensing Performance in a Propane Atmosphere

Ruthenium-doped zinc oxide thin solid films [ZnO:Ru] were deposited on glass substrates using the sol–gel technique, by a non-alkoxide route. The effect of the film thickness of ZnO:Ru thin films on the gas sensing characteristics, complemented with morphological, structural, optical, and electrical properties were investigated. All the films were polycrystalline and the thickening of the films (ranged from 150 to 780 nm) shows an increase in the (002) preferential growth. Raman study confirms the presence of only ZnO wurtzite phase. A two-photon absorption effect was identified as a dominant physical mechanism of third order optical nonlinearity. The observed morphology of the films shows a constant porosity under different deposition conditions, as well as a variation in the crystallite size, ranging from 25 to 35 nm. Variation in the electrical resistance to propane gas as a function of film thickness is reported. Best gas sensing response was observed in samples deposited with film thickness less than 150 nm the highest sensitivity value obtained was of the order of 13. As a matter of fact, a variation in resistivity up to two orders of magnitude was observed, that makes films potentially adequate for gas sensors manufacturing.