Impact of forming gas annealing on ZnO-TFTs

ZnO-based thin film transistors (ZnO-TFTs) were fabricated by sputtering of ZnO on 200 nm SiO₂ on p⁺-Si substrates. Forming gas annealing (FGA) was carried out directly after deposition at 400°C, 450°C, and 500°C for 1h. TFTs annealed at 400°C exhibited a high threshold voltage (V_Th) of 11 V while those annealed at 500°C showed a low V_Th of -3 V. Saturation mobility (μ_sat) increased slightly with temperature to about 1 cm²/Vs. The effect of FGA at 450°C was also compared to oxygen (O₂) annealing at the same temperature and it was observed that V_Th was higher and saturation mobility was lower after oxygen annealing. It is concluded that an improvement of electrical properties with forming gas annealing is due to incorporation of hydrogen in the active ZnO layer. Hydrogen at oxygen vacancy sites bonds to four Zn atoms (i.e., H forms a multicenter bond) and acts as a shallow donor in ZnO.