The structural and optoelectronic properties of Ti-doped ZnO thin films prepared by introducing a Cr buffer layer and post-annealing

This work investigates the effects of Cr buffer layers and post-annealing on the properties of titanium-doped zinc oxide (TZO) thin films prepared by radio frequency magnetron sputter. All films had a (0 0 2) preferential orientation along the c-axis at 2θ ∼ 34°. The crystallinity, grain size, Hall mobility and carrier concentration of TZO films were enhanced by introducing a Cr buffer layer and post-annealing. The decrease in resistivity was mainly attributed to the increase in Hall mobility rather than carrier concentration. As a Cr buffer layer was inserted, the film resistivity decreased by 32% to 5.41 × 10−3 Ω cm while the energy band gap increased from 3.252 to 3.291 eV in comparison with that of the film deposited without the buffer layer. When the Cr-buffered films were annealed in a vacuum, the structural, electrical, and optical properties were improved with increasing annealing temperature. At an annealing temperature of 500 °C, the grain size, resistivity, and energy band gap attained the optimal values of 28.12 nm, 3.37 × 10−3 Ω cm and 3.357 eV, respectively. The average transmittance of TZO films in the visible region was between 75% and 84%, and it decreased with increase in the grain size. The decrease in transmittance is attributed to an increase in surface roughness due to the three-dimensional island grain growth during thermal annealing.