Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Zinc oxide (ZnO) is one of the most promising semiconducting metal oxides, particularly for gas sensing applications. Impurity doped ZnO has offered much improved sensing performance, as compared to its undoped counterpart. In this work, undoped as well as indium doped nanocrystalline ZnO thin films are synthesized by a low cost chemical solution deposition route. X-ray diffraction patterns of the synthesized films reveal preferential orientation along the (002) plane. The surface and cross section morphology has clearly changed with the variation of indium content. Optical transmittance values increase with the increase in indium concentration and the band gap energy is found in the range 3.210 eV to 3.221 eV. PL spectra reveal three characteristic peaks, owing to band to band transition and defect level emissions. The effect of indium doping on the electrical parameters of ZnO is analyzed through Hall effect measurements at room temperature. The gas sensing characteristics of these sensors offer good reproducibility and stability towards various reducing gases, with an enhancement of response% and lower detection limit as compared to undoped ZnO. A doping level of 3 wt% of indium in ZnO is found to give optimum response and the lowest detection limit of hydrogen of 1 ppm or even lower. However, further increase in the doping concentration resulted in reduced sensing performance. This is attributed to the gas sensing mechanism related to the substitution of In3+ ions at Zn2+ ion sites enhancing the number of free charge carriers at the optimum level of indium. Through exploring this gas sensing mechanism, it is argued that the sensor performance can be dramatically improved by tailoring the indium concentration in ZnO for its practical application in various sectors as an effective gas sensor.