Effect of annealing on the microstructural, optical and electrical properties of ZnO nanowires by hydrothermal synthesis for transparent electrode fabrication

ZnO nanowires were grown by hydrothermal synthesis and the effect of annealing on their physical properties was investigated by field-emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, spectrophotometry and Hall effect measurements. The nanowires adopted the c-axis [0 0 1] growth orientation of the underlying gallium and aluminium co-doped ZnO seed layer and their vertical alignment improved with annealing temperature up to 250 °C, above which it became random at 350 °C. This was attributed to the improvement in crystallinity up to 250 °C and its deterioration at 350 °C. Energy dispersive spectroscopy confirmed that the Zn:O atomic ratios for all samples were nearly stoichiometric. Average optical transmittances around 50–70% were observed in the visible region. The optimum electrical properties for transparent electrode fabrication were achieved at 250 °C, where the carrier concentration was maximum (5.5 × 10²⁰ cm⁻³) and electrical resistivity was minimum (1.1 × 10⁻² Ω cm).