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 (0.5wt%)-SnO2 target on various substrates (a-, m-, r-, and c-cut sapphire and quartz). All the films had a similar thickness (∼110nm), root-mean-square (rms) roughness (∼1.3nm), 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 (101), (002), and (101) 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 (0001) (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 (101) 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 (0001) 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.