Optical and gas sensing properties of Al-doped ZnO transparent conducting films prepared by sol–gel method under different heat treatments

Al-doped ZnO transparent conducting films were prepared on quartz substrate by the sol–gel method using zinc acetate dehydrate, aluminum nitrate nonahydrate, ethanol and monoethanolamine (MEA) under different heat-treating processes. The evolution of structural, morphological, optical and electrical properties was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), UV–vis spectrophotometer and four-probe instrument. The analyzed results indicate that the obtained films are of good crystal quality with the wurtzite-type ZnO structure and a highly c-axis preferred orientation. The thin films with the different surface morphologies and thicknesses were obtained under the different heat-treating processes. After post-heating treatment, the energy band gap decreases from 3.21 eV to 3.14 eV and the resistivity from 3.22 Ω cm to 1.42 Ω cm, while the average transmittance in the range of 400–700 nm increases from 82.9% to 86.7%. As-prepared Al-doped ZnO thin films were used directly as a sensing device to investigate the sensing properties in ethanol atmosphere. The home-built Al-doped ZnO thin film sensors showed high response, fast response, and recovery times toward ethanol gas, making these thin films not only interesting for sensor devices, but also for a number of related electrochemical applications.