ZnO/Si nanowires heterojunction array-based nitric oxide (NO) gas sensor with noise-limited detectivity approaching 10ppb

We report a ZnO/Silicon nanowire (ZnO/Si NWs) heterojunction array-based NO gas sensor operating at room temperature with an extremely high response (noise limited response similar to 10ppb). The sensor shows very high selectivity towards NO gas sensing and limited perturbation in response due to the presence of moisture. The sensor has been fabricated by using cost-effective chemical processing that is compatible with wafer-level processing. The vertically aligned Si NWs array has been made by an electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The electrical characteristic of the model was to fit the I-V data before and after exposure to gas and essential changes in electrical parameters were obtained. This was then explained based on a proposal for the mechanism of gas sensing. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW.