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

Abstract

H 2 and NO x 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 TiO 2 single layer sensor exhibited high resistance in air and low sensitivity to H 2 at every temperature. Stacking of an M-SnO 2 layer (M: noble metals, loading amount: 0.5 wt.%) over the TiO 2 layer led to a decrease in sensor resistance and to an increase in the sensitivity. In contrast, stacking of an M-SnO 2 layer over an In 2O 3 layer was less effective for improving the sensitivity. The difference in the stacking effect between TiO 2- and In 2O 3-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-SnO 2/TiO 2 heterolayer sensors, it was suggested that the conduction path changed from the bottom of the TiO 2 layer to that of an M-SnO 2 layer plus the specific TiO 2 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-SnO 2 upper layer. Among the metal oxides tested, WO 3 exhibited the highest sensitivity to both NO 2 and NO, but the NO sensitivity was about a tenth of the NO 2 sensitivity. Stacking of a SiO 2 layer on the WO 3 layer decreased the NO 2 sensitivity, while the NO sensitivity remained almost unchanged. In this case, therefore, the SiO 2 layer was suggested to act as a diffusion control layer for NO 2. These results suggest that the slide-off transfer printing is quite useful for processing of semiconductor heterolayer sensors.