Transparent p-type NiO:Al thin films as room temperature hydrogen and methane gas sensors

摘要

Transparent Al-doped NiO thin films with thickness of 100 nm were prepared by radio frequency (rf) sputtering technique in order to investigate their response to reducing gases such as hydrogen (H-2) and methane (CH4), at room temperature (RT). During deposition, the amount of doping [Al/(Al + Ni)] was 10%, while the O-2 content in the Ar-O-2 mixture in the plasma was 2.8%. These optimum values were set to obtain films with transmittance over 40% in the visible and optical energy band gap (Eg) of 3.65 eV. All films were polycrystalline, with a crystallite size of 7.9 nm and a lattice constant of 0.42 nm. Moreover, the films showed a smooth surface with root mean square (rms) roughness of 2.87 nm. The p-type electric behavior of the films was identified by current modulated ac-Hall effect measurements. The films were exposed to 100 ppm H-2 in synthetic air, at room temperature, showing a typical p-type behavior and a sensitivity of 68%, while the response and recovery times were found to be 388 s and 451 s, respectively. Subsequently, the films were also exposed on CH4 showing a sensitivity of 58%, with response and recovery times equal to 1373 s and 95 s, respectively.