H2 sensing characteristics of highly textured Pd-doped SnO2 thin films

The effects of the crystallographic orientation on the H2 gas sensing properties were investigated in highly oriented polycrystalline Pd-doped SnO2 films, which were obtained using rf magnetron sputtering of a Pd (0.5 wt%)-SnO2 target on various substrates (a-, m-, r-, and c-cut sapphire and quartz). All the films had a similar thickness (∼110 nm), root-mean-square (rms) roughness (∼1.3 nm), surface area, and chemical status (O, Sn, and Pd). However, the orientation of the films was strongly affected by the orientation of the substrates. The (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, and rather randomly oriented films were deposited on (0 0 0 1) (c-cut) Al2O3 and quartz substrates. In addition, the oriented Pd-doped SnO2 films were highly textured and had in-plane orientation relationships with the substrates similar to the epitaxial films. The (1 0 1) Pd-doped SnO2 films on ( 1   1   2 ¯   0 ) and ( 1   1 ¯   0   2 ) Al2O3 showed a considerably higher H2 sensitivity, and their gas response decreased with increasing sensing temperature (400–550 °C). The films deposited on ( 1   0   1 ¯   0 ) and (0 0 0 1) Al2O3 showed the maximum sensitivity at 500 °C. The comparison of the H2 gas response between undoped and Pd-doped SnO2 films revealed that the Pd-doping shifted the optimum sensing temperature to a lower value instead of improving the gas sensitivity.