H2 sensing properties in highly oriented SnO2 thin films

Highly oriented polycrystalline SnO2 films were deposited on the sapphire substrates with various orientations using rf magnetron sputtering, and the effects of the crystallographic orientation on the H2 gas sensing performance were investigated. The orientation of the SnO2 films was varied with the substrate orientation such that (1 0 1), (0 0 2), and (1 0 1) oriented films were grown on (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), and (1 1¯ 0 2) (r-cut) Al2O3 substrates, respectively. More than one preferred orientation was observed in the films deposited on (0 0 0 1) (c-cut) Al2O3, quartz, and SiO2 (20,000 Å)/Si substrates. All the films had a similar thickness (∼115 nm), root-mean-square (rms) roughness (∼1 nm), and surface area, and therefore the sensing performance of each film was little affected by the microstructure. The (1 0 1) SnO2 films grown on (1 1¯ 0 2) Al2O3 exhibited the highest H2 gas response (Ro/Rg) of ∼300 to 1.0% H2/air, and the other films showed an order of magnitude lower gas response. The chemical composition and surface state of the films were further examined by AES and XPS to find out the reasons for the different H2 gas response of the (1 0 1) films grown on (1 1 2¯ 0) and (1 1¯ 0 2) Al2O3.