Effects of thickness and annealing on the properties of Ti-doped ZnO films by radio frequency magnetron sputtering

Abstract

Ti-doped ZnO (TZO) thin films were prepared by radio frequency magnetron sputtering with a target containing 1.5 wt% TiO 2 on glass substrates at 300 °C and then thermally annealed in a hydrogen ambient. The structural, electrical, and optical properties of TZO films were investigated with respect to the variation of film thickness and annealing condition. X-ray diffraction analysis exhibited that all TZO films had a (0 0 2) peak at 2θ ∼34°, indicating that the films were hexagonal wurtzite structure and showed a good c-axis orientation perpendicular to the substrate. As film thickness increased from 30 to 950 nm, the crystallite size increased from 11.9 to 36.8 nm and the surface roughness increased from 0.57 to 1.78 nm. The film resistivity decreased from 3.94 × 10 −2 to 1.06 × 10 −3 Ω cm. To enhance the characteristics of TZO films for transparent conductive oxide applications, the films were subsequently annealed at temperatures ranging from 300 to 500 °C in hydrogen or argon ambient for various times. The results indicated that hydrogen annealing made film resistivity decrease more than argon annealing. The resistivity of the hydrogen-annealed film monotonically decreased with increasing annealing time up to 90 min. At the optimal annealing condition (400 °C, 60 min), the film resistivity decreased by 57% and the average optical transmittance in the visible wavelength range (400–700 nm) increased slightly as compared to the as-deposited films. The enhanced characteristics of the annealed TZO film are attributed to desorption of negative charged oxygen species and passivation of surface and defects at grain boundaries.