Studies On Solution-Processed Oxide Gate Dielectric Layers For Thin-Film Transistors

Oxide thin film transistors(TFTs)have attracted much attention due to their advantages of ultra-low turn-off current,higher field-effect mobility,and good uniformity,which have application prospects in large-area,transparent,flexible,and energy-saving displays.At present,the industrial TFTs are all fabricated by vacuum method with expensive equipment,complex process and low material utilization.Compared with vacuum method,solution-processed method has many advantages,such as low cost,high material utilization,easy to control the doping ratio flexibly,and so on.High k gate dielectrics such as HfO₂,Al₂O₃ and ZrO₂ are prepared by solution-processed,which can reduce the operating voltage and improve the reliability of TFTs.In recent years,there have been many reports on solution-processed oxide TFTs with high field effect mobility(even>100 cm²V^-1s^-1).However,the capacitance uncertainty of the oxide gate dielectric fabricated by solution-processed will lead to the instability of the device and the overestimation of the mobility.Therefore,the experiments were carried out for different gate dielectric materials and preparation methods,and the film characteristics and electrical properties of the device were deeply explored.(1)Study on solution-processed of Al₂O₃ gate dielectric.Al₂O₃ gate dielectric film was fabricated by spin-coated and annealed at different temperatures and humidity.In ScO_x(In₂O₃:Sc₂O₃=98:2,wt%)active layer was fabricated by RF sputtering.The gate dielectric annealed at low temperature and high humidity exhibits a strong electric double-layer(EDL)polarization.There is a large capacitance C_i at low frequencies,and C_i decreases rapidly as the frequency increases.In order to further explore the influence of hydrogen ions on the device,The gate surface was treated with water before spin coating the precursor.The hysteresis voltage of oxide TFTs based on Al₂O₃without and with water treatment are 0.16 V and-0.24 V,respectively.The gate dielectric with water treatment shows stronger EDL polarization.TOF-SIMS was carried out to verify the presence of hydrogen in the film.It shows the hydrogen content gradually decreases from the Al₂O₃surface to the Al₂O₃/ITO interface.(2)Study on solution-processed of HfO₂ gate dielectric.HfO₂ gate dielectric film was fabricated by spin-coated and annealed at different temperatures.As the annealing temperature increases,the C_i of the gate dielectric decreases.It shows a counterclockwise hysteresis in the transfer characteristic curves,and the hysteresis voltage is less than 0 V and decreases with the increase of temperature.The HfO₂ dielectric annealed at low-temperature contains certain hydrogen ions or hydroxyl groups,and exhibits the certain EDL polarization.(3)Study on solution-processed of ZrO₂ gate dielectric.ZrO₂ gate dielectric film was fabricated by spin-coated and annealed at different temperatures.The C_i of the gate dielectric increases with the increase of annealing temperature,and does not show the obvious EDL polarization.The EDL polarization in ZrO₂ dielectric is weaker than that in Al₂O₃ and HfO₂dielectrics.The oxide TFTs based on Al₂O₃ dielectric layer is easily affected by the preparation process and preparation environment,and the change of hydrogen content in the preparation process will affect the device performance;the performance of TFTs based on ZrO₂ dielectric layer is less affected by the process environment;the performance of TFTs based on HfO₂dielectric layer is between the TFTs of Al₂O₃ and ZrO₂.The dielectric constant,band gap and polarization mode of the three kinds of high-k gate dielectric films are combined to provide the reference for the ratio,composition and preparation method of precursor solution.