Low-temperature fabrication of solution-processed InGaZnO thin-film transistors by three-layer gradient diffusion

In this study, we report a novel vertical diffusion method for fabricating solution-processed quaternary In-Ga-Zn-O (IGZO) (defined as vd-IGZO) thin-film transistors (TFTs). Compared with conventional IGZO (defined as con-IGZO) TFT annealed at 380°C, vd-IGZO thin film is to spin coat Ga₂O₃, ZnO and In₂O₃ binary oxide precursors consecutively and then anneal at 250°C. Finally, the IGZO film with a vertically gradient Ga, Zn, In diffusion is obtained and used as the channel layer of TFTs. The results showed that the novel method lead to an improvement in charge carrier mobility and an improvement in the on-off current ratio of IGZO TFTs along with an improved stability. Compared with those for con-IGZO TFTs, the mobility for vd-IGZO TFT reaches 2.81cm² V−¹ s⁻¹ from 0.88cm² V−¹ s⁻¹, the threshold voltage(Vth) changes from 3.7V to 0.3V, the sub-threshold swing (S.S) decreases from 0.51V/dec to 0.35V/dec, and the on-current/off-current (I_on/I_off) increases from 2.1 × 10⁶ to 2.3 × 10⁷. Besides, the positive bias stress (PBS), negative bias stress (NBS) and environmental stability are all improved to some extent. Hence this novel vertical diffusion method shows great potential to be applicable in the fabrication of flexible TFT.