Low temperature synthesis of ZnO nanowires on GAZO thin films annealed at different temperatures for solar cell application

Abstract

ZnO nanowires were hydrothermally grown on sputtered gallium and aluminium co-doped ZnO (GAZO) thin films as seed layers and the effect of seed layer annealing temperature on their microstructural and optoelectronic properties was investigated. Atomic force microscopy revealed an increase in the GAZO films' mean particle size and surface roughness with annealing temperature. The GAZO films exhibited a zincite phase with a preferred orientation along the (002) plane and their crystallinity improved with annealing temperature, resulting in low electrical resistivity. The nanowires' alignment improved with seed layer annealing temperature up to 150°C and then deteriorated at 250−350°C. This was attributed to the improvement in nanowire crystallinity up to 150°C and its deterioration at 250–350°C. Energy dispersive spectroscopy confirmed that the nanowires were closer to stoichiometric ZnO (1:1). Raman spectroscopy revealed the presence of few defects and slight residual tensile stress in the nanowires, in corroboration with XRD analysis. The films and nanowires exhibited average visible transmittances around 75–85% and 50–80%, respectively. The least electrical resistivity (7.0×10−3Ωcm) and highest figure of merit (1.02×10−2Ω−1) were obtained for the well-aligned nanowires grown on 150°C annealed films, demonstrating their suitability for solar cell transparent electrode fabrication.