The microstructure, optical, and electrical properties of sol-gel-derived Sc-doped and Al-Sc co-doped ZnO thin films

Transparent conductive ZnO:Al-Sc (1:0.5, 1:1, 1:1.5 at.% Al-Sc) thin films were prepared on glass substrates by sol-gel method. The microstructure, optical, and electrical properties of ZnO:Sc and ZnO:Al-Sc films were investigated. Results show that Sc-doping alone obviously decreases grain size and degrades the crystallinity; there is an amorphous phase on the surface of ZnO grains; the transmittance spectra fluctuate dramatically with a large absorption valley at about 430-600 nm. However, Al-Sc co-doping can stabilize grain size and improve the microstructure; an average visible transmittance of above 73% is obtained with no large absorption valley; the amorphous phase does not appear. The optical band gaps of ZnO:Sc and ZnO:Al-Sc films (3.30-3.32 eV) are blue-shifted relative to pure ZnO film (3.30 eV). Hall effects show that the lowest resistivity of 2.941 × 10−2 Ω cm and the maximum Hall mobility of 24.04 cm2/V s are obtained for ZnO:Al-Sc films while ZnO:Sc films do not exhibit any electrical conductivity. Moreover, there is an optimum atomic ratio with Al to Sc of 1:0.5-1 at.%. Although the resistivities are increased compared with that of ZnO:Al film, the Hall mobilities are raised by one order of magnitude.