Influence of TiO2 buffer layer and post-annealing on the quality of Ti-doped ZnO thin films

Ti-doped ZnO (TZO) thin films were grown on soda lime glass (SLG) substrate without and with a TiO2 buffer layer by radio frequency magnetron sputtering and then annealed under vacuum at 450 and 500 °C for 20 min. The structural, electrical, and optical properties of TZO films were investigated. XRD analysis shows that all TZO films are highly textured along the c-axis and perpendicular to the substrate. The structural properties of TZO films are improved by controlling the annealing temperature and inserting a TiO2 buffer layer. When the films were annealed at 450 °C, the crystallinity increased, but it then decreased slightly with increase in annealing temperature from 450 to 500 °C. Due to superior crystallinity, TZO films annealed at 450 °C exhibited lower resistivity and higher average transmittances in the visible region. The improvements in crystallinity, resistivity and transmittance are more obvious when a TiO2 buffer layer was inserted. The decrease in resistivity is mainly attributed to an increase in Hall mobility rather than carrier concentration. When the TZO films deposited on bare SLG substrate, the energy band gaps was decreased after annealing at 450 and 500 °C due to the decrease in carrier concentration. However, the absorption edge of TZO films deposited on TiO2-buffered substrate was blue shifted, and the energy band gap was increased due to the increase of carrier concentration. In this study, the TZO film with optimal properties was grown on the TiO2-buffered substrate and post annealed at 450 °C, achieving a resistivity of 3.76×10−3 Ω-cm and an average transmittance above 85%. Therefore, it can be concluded that inserting a buffer layer at an early stage of film deposition to improve crystallinity can help achieve low resistivity, high transmittance, and high energy band gap in transparent conducting TZO thin films.