Influence of Mg doping on the morphology and optical properties of ZnO films for enhanced H₂ sensing

Abstract

Highly oriented ZnO and Mg doped ZnO thin films were fabricated on Al2 O3 substrate by sputtering at room temperature. The effect of Mg doping on the structural, optical, and morphological properties of ZnO film was investigated. The intensity of (002) peak in X-ray diffraction measurements revealed the influence of Mg doping on the crystallinity and orientation of ZnO film. Photoluminescence (PL) results show that the Ultraviolet (UV) emission peak was shifted to lower wavelength side for Mg:ZnO film indicating the possibility for quantum confinement. UV-vis-NIR optical absorption revealed an improvement in optical transmittance from 70 to 85%, and corresponding optical band gap from 3.25 to 3.54 eV. Atomic force microscope (AFM) images revealed the nano-size particulate microstructure of the films. The surface topography of Mg doped ZnO film confirmed decreased grain size with large surface roughness and increased surface area, favorable for sensing. Pure ZnO and Mg doped ZnO film were used as active layer and tested for its sensing performance to hydrogen. Compared to undoped ZnO, 22 at.% Mg doped ZnO film showed much higher sensor response to H2 at a concentration as low as 200 ppm and at a lower operating temperature of 180°C. A linear sensor response was observed for H2 concentration in the range of 100-500 ppm.

Keywords: X-ray diffraction; atomic force microscopy; hydrogen sensor; sputtering; thin films.