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 (WOX) 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/WOX 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 (WOX NW) heterojunctions were fabricated by stress-induced method for WOX NW synthesis on Si MEMS structures. With this fabrication technique, the growth position of the WOX NWs can be controlled by patterning of the WOX 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 WOX-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/WOX 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.