Influence of the electrode materials on the electrical response of ZnO-based contact sensors

The sensitivity to CO and H₂ of heterocontacts between p-type La₂CuO₄ and n-type ZnO semiconducting oxides, and the humidity sensitivity of Au/ZnO heterocontacts have been studied. Heterocontacts were prepared by mechanically pressing sintered pellets of the two oxides, or sintered pellets of ZnO and Au sheets. As ohmic electrodes, three different materials were applied on ZnO pellets (Zn-containing Ag, In-and Ga-containing Ag, and Al pastes). This paper reports the influence of the electrode materials used as ohmic contact for ZnO on the chemical sensitive electrical response of both systems. For ZnO pellets with two ohmic electrodes, different resistivities were observed using the various electrode materials. Current-voltage (I-V) characteristics for single ZnO pellets were linear, showing ohmic behaviour. Little chemical sensitivities were observed for ZnO pellets. I-V curves for heterocontacts showed a rectifying character, due to a p-n diode behaviour for La₂CuO₄/ZnO contacts, and to a Schottky barrier behaviour for Au/ZnO. For both systems, the rectifying character was enhanced in the presence of reducing gases (CO and H₂) or humidity, respectively. The chemical response of the heterocontacts, which was due to the La₂CuO₄/ZnO and Au/ZnO interfaces, was dependent on the electrode materials applied on ZnO pellets, with the same trend for both systems. The greater the resistivity of the ZnO pellets, the larger the sensitivity of the heterocontacts. Slight differences in the potential barrier heights are induced by the different resistivities of the three electrode materials. The larger the band-bending at ZnO/metal or ZnO/p-type semiconducting oxide interfaces, the larger the influence of adsorbates, resulting in increased sensitivity.