Thermal stability of conductive and transparent V-doped ZnO thin films

The thermal stability of electrical, optical and structural properties of vanadium-doped zinc oxide (VZO) thin films has been investigated after heat treatments up to 550 °C in N2 and O2 atmospheres. ZnO and VZO films were prepared by RF magnetron sputtering on quartz substrates at 200 °C. It was found that the resistivity of VZO films remains low at about 5 × 10− 4 Ω cm when heat treated at temperature up to 450 °C in both N2 and O2 atmospheres, while resistivity of the ZnO film increased remarkably due to the compensation of oxygen vacancies in O2 heat treatments over 400 °C. Optical transmittance of the as-grown VZO film in the visible range (400-1100 nm) was 20-30% and improved up to about 80% when heat treated at temperature up to 500 °C in N2 and O2 atmospheres. Decreased c-axis lattice constant in the VZO film was larger in N2 heat treatments than in O2 heat treatments. Based on these experimental results, we made the hypothesis that VO bonds similar to vanadium trioxide in the as-grown VZO film changed to those similar to vanadium pentoxide after heat treatments over 500 °C.