Sol-Gel Drived Novel Hafnium-Indium-Zinc Oxide (HIZO)-Based Gas Sensor for Nitrogen Dioxide Detection

Abstract

Introduction Nitrogen dioxide (NOX) is a toxic gas which can cause inflammation of respiratory system and increase the risk of asthma even at low concentration [1]. The metal oxide semiconductor (MOS) is one of the most popular gas sensing materials because of their high sensitivity, fast response and low-cost fabrication. Recently, various types of metal oxide nano-structures such as nanospheres, nanowires and nanosheets are studied to enhance sensing properties by maximizing the surface-to-volume-ratio [2-3]. However, these materials have several requirements of complex synthetic processes, such as high temperature, vacuum processes, and so on. To overcome the limitations, solution-processed MOSs as the sensing materials are could be other options due to their simple fabrication process. In the present work, for the first time, we fabricated gas sensor using sol-gel drived novel hafnium-indium-zinc oxide (HIZO) which has good sensitivity and selectivity toward low concentration of NOX gas through a solution process. This study also investigates the effect of hafnium additions on IZO to enhancements in the gas response toward NOX. Method The 0.25 M HIZO sol-gel (from Merck) solutions were prepared by dissolving indium nitrate hydrate, zinc nitrate hydrate, and hafnium chloride in DI-water with a various composition ratio. The atomic ratio of In:Zn was fixed at 6.8:2.2 and the atomic percentages of Hf of the total amounts were 0, 5, 9, 15, 20 and 30 at. %. The prepared solutions were stirred thoroughly at 70 °C for 12 h. To fabricate interdigitated electrodes, platinum was sputtered on Si/SiO2 substrate with 10 μm distance. The sol-gel solutions were drop-coated on the interdigitated electrodes and dried at 100 °C. The samples were finally annealed at 350 °C for 2 h in air to evaporate impurities and densify films. Sensing measurements were done in a gas sensing system varying operating temperatures in the range of 100 °C through 250 °C using a power supply. The gas concentrations were controlled diluting target gases with dry air. We tested gas responses by monitoring changes in resistances of Rg/Ra, where Rg is a resistance during exposure to the target gas, and Ra is a resistance in air, respectively. Results and Conclusions Figure 1 (a) shows gas responses of IZO with and without 15 %-hafnium as varying temperature from 50 to 250 °C. The sensor of hafnium-added IZO exhibited a higher response (61.4) toward 1.6 ppm of NOX gas than that of IZO without hafnium especially at 100 °C. It might be attributed to oxygen vacancies increase due to addition of hafnium. Figure 1 (b) and (c) illustrate oxygen 1s spectrum acquired from XPS analysis for the IZO and hafnium-added IZO, respectively. The amount of oxygen vacancies increases with an incorporation of hafnium as can be compared in figure (b) and (c). The sensor also showed excellent sensitivity toward NOX gas until 16 ppb, selectivity toward NOX gas among various gas species such as hydrogen, acetone, and toluene, and stability to continuous measurements for a month. We successfully fabricated solution-processed HIZO-based gas sensors capable of detecting low concentrations of NOX gas operating at low temperatures. Acknowledgments J.-S. Park was partially supported by the Project (No. P0006858) of KIAT and MOTIE in Korea.