Simple fabrication method of silicon/tungsten oxide nanowires heterojunction for NO₂ gas sensors

Heterojunctions, formed at the interface between two different materials, have attracted much attention as a gas-sensing material. In particular, Si/tungsten oxide (WO_X) heterojunctions are well known to be capable of gas detection at low temperature and to increase the sensitivity to and selectivity of NO₂. However, during the fabrication process of the Si/WO_X nanostructure-based sensor it is difficult to control the synthesis position of the nanostructures; hence, it is complicated or time consuming. In this work, semiconducting gas sensors based on n-type silicon/n-type suspended tungsten oxide nanowire (WO_X NW) heterojunctions were fabricated by stress-induced method for WO_X NW synthesis on Si MEMS structures. With this fabrication technique, the growth position of the WO_X NWs can be controlled by patterning of the WO_X seed film, and the NWs can be synthesized by simply heating the seed film for 20 min. In addition, all fabrication processes consist of batch-processes. Unlike conventional WO_X-based sensors, the resistance of this sensor is reduced in the presence of NO₂, an oxidizing gas, due to the band bending phenomenon of the Si/WO_X NW heterojunction. The fabricated sensor can detect 500 ppb of NO₂ and exhibits excellent selectivity to CO and toluene, which are exhaust gases, like NO₂. This selectivity will be particularly useful when using sensors to detect NO₂ in exhaust gases of automobiles or factories.