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

H? and NO, 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 degreesC. A TiO2 single layer sensor exhibited high resistance in air and low sensitivity to H-2 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 H-2 sensing performance. The sensitivity enhancement is considered to arise from the diffusion control of gaseous O-2 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. (C) 2001 Elsevier Science B.V. All rights reserved.