Influence of bilayer high- <i>k</i> Al ₂ O ₃ and HfO ₂ gate insulators on the electrical performance of amorphous oxide thin-film transistors

Abstract In conventional amorphous InZnGaO (a-IGZO) thin-film transistors (TFTs), thermal SiO 2 has been used as gate insulator (GI). However, it cannot induce high on-current due to low-capacitance. It can also cause high tunneling when the device is scaled down, reducing reliability. Therefore, in this paper we fabricate bilayer high-dielectric-constant (high- k ) Al 2 O 3 and HfO 2 GI as an alternative material to thermal SiO 2 to enhance the performance of a-IGZO TFT. Al 2 O 3 and HfO 2 were fabricated by plasma enhanced atomic layer deposition and radio frequency sputtering, respectively. We also compared the electrical performance and chemical properties of high- k materials and thermal SiO 2 . As a result, bilayer high- k Al 2 O 3 and HfO 2 GI exhibited high density, high dielectric constant, and relatively low leakage current of ∼10 −11 A at 2 V. The a-IGZO TFT device fabricating the stacked high- k Al 2 O 3 and HfO 2 GI exhibited optimal device performance with saturation mobility ( μ sat ) of 7.03 ± 0.9 cm 2 V −1 s −1 , threshold voltage ( V th ) of −0.2 ± 0.2 V, and steeper subthreshold swing of 0.27 ± 0.05 (V dec −1 ). Our findings suggest an excellent compatibility and high-quality interface between a-IGZO and bilayer Al 2 O 3 /HfO 2 GI. Moreover, their higher capacitance enables high-performance low-voltage operation in TFT device.