Gas sensing properties of semiconductor heterolayer sensors fabricated by slide-off transfer printing

H2 and NOx sensing properties of semiconductor heterolayer sensors fabricated by a slide-off transfer printing method on alumina substrates equipped with electrodes have been investigated at 200–600°C. A TiO2 single layer sensor exhibited high resistance in air and low sensitivity to H2 at every temperature. Stacking of an M-SnO2 layer (M: noble metals, loading amount: 0.5 wt.%) over the TiO2 layer led to a decrease in sensor resistance and to an increase in the sensitivity. In contrast, stacking of an M-SnO2 layer over an In2O3 layer was less effective for improving the sensitivity. The difference in the stacking effect between TiO2- and In2O3-based is considered to arise from a change in the electrical conduction path induced by fabrication of the upper layer, based on the resistance level of each sensing and stacked layer. In the case of M-SnO2/TiO2 heterolayer sensors, it was suggested that the conduction path changed from the bottom of the TiO2 layer to that of an M-SnO2 layer plus the specific TiO2 region just above the electrode and that the specific region dominated the sensor resistance and hence the H2 sensing performance. The sensitivity enhancement is considered to arise from the diffusion control of gaseous O2 by the M-SnO2 upper layer. Among the metal oxides tested, WO3 exhibited the highest sensitivity to both NO2 and NO, but the NO sensitivity was about a tenth of the NO2 sensitivity. Stacking of a SiO2 layer on the WO3 layer decreased the NO2 sensitivity, while the NO sensitivity remained almost unchanged. In this case, therefore, the SiO2 layer was suggested to act as a diffusion control layer for NO2. These results suggest that the slide-off transfer printing is quite useful for processing of semiconductor heterolayer sensors.