Effects of Al and Sn dopants on the structural and optical properties of ZnO thin films

Al, Sn doped ZnO nanocrystals were successfully synthesized onto glass substrates by the sol–gel processing. The structure and morphology of crystals were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of various doping concentrations on electrical and optical properties were investigated by 4-point probe device and UV–VIS spectroscopy, respectively. The carrier concentration and carrier mobility of the doped ZnO thin films were also calculated and discussed. The lowest resistivity, 2 × 10−3 Ω cm, was observed for Sn doped ZnO (TZO) thin films with Sn doping concentration of 2 at.%, with an average optical transmittance of 89.2% in the visible range. As to Al doped ZnO (AZO) thin films, the lowest resistivity was of 9 × 10−2 Ω cm, with a higher average transmittance (91.4%) obtained from the sample with Al doping concentration of 1 at.%. The X-ray photoelectron spectroscopy (XPS) study showed Al–O and Sn–O bonding in the synthesized ZnO thin films, which confirmed the substitution of Zn2+ by Al and Sn ions. Room temperature photoluminescence was observed for pure and Al, Sn doped ZnO thin films and the origin of the emissions was discussed.

► Al, Sn doped ZnO thin films were synthesized by sol–gel method.
► The effects of different doping concentrations were investigated and compared.
► 1 and 2 at.% was proved to be the optimum concentration for AZO and TZO, respectively.
► The origin of the photoluminescence emissions was discussed.
► The films showed high transmittance and low resistivity.