Al-doping effects on structure, electrical and optical properties of c-axis-orientated ZnO:Al thin films

Al-doped ZnO (ZnO:Al) thin films were grown on glass substrates by sol–gel method, which were characterized by X-ray diffraction, electrical measurements and ultra-violet photoluminescence spectrometry. It is found that all the thin films have a preferential c-axis orientation. With increase of Al doping, the peak position of the (0 0 2) plane is shifted to the low 2θ values. A minimum resistivity of 6.2×10−4 Ω cm is obtained for the film doped with 1.5 mol% Al. The band gap is found to broaden with increasing dopant concentration. It is found from the PL measurement that near band edge (NBE) emission and deep-level (DL) emissions are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emissions of the thin films were depressed. As the concentration of Al increases, the peak of NBE emission has a blueshift to region of higher photon energy, which is coincident with those values calculated by the linear fit from the transmittance data.