Plasma-irradiated hafnia ferroelectrics for high-performance flexible thin film transistors

Abstract

Integrating unconventional HfO₂-based ferroelectrics in thin film transistors (TFTs) has proven effective in enhancing performance by stabilizing negative capacitance (NC). This is achieved by incorporating the ferroelectric in series with a high-permittivity dielectric as a passive TFT component. However, implementing this on flexible, temperature-sensitive substrates presents significant challenges. The primary focus in TFTs on flexible substrates is their fabrication with a considerably low thermal budget to avoid damaging the underlying substrate. Herein, we introduce an approach to stabilize the desired ferroelectric polar phase of hafnium zirconium oxide (Hf_0.5Zr_0.5O₂, HZO) through superficial plasma treatment in the argon environment. Plasma energizes Ar⁺ ions, whose bombardment induces oxygen vacancies, thereby stabilizing the desired orthorhombic phase at low temperatures. The IGZO-channel TFTs incorporating HZO/HfO₂ passive stacks exhibit a substantial enhancement in subthreshold swing (SS), achieving a 72 % reduction from 147 mV/dec to 41 mV/dec, along with a notable increase in on-state currents (I_on) compared to conventional TFTs utilizing only HfO₂ dielectrics. The field-effect mobility (μ) significantly improves from 5.7 ± 0.2 to 28.8 ± 6.2 cm²/V·s. Flexible TFTs fabricated on polyimide substrates also show excellent mechanical stability, maintaining consistent I_on even after 10,000 bending cycles. Moreover, these TFTs exhibit enhanced μ of 72 ± 13.5 cm²/V·s in the flat state and 33.9 ± 3.8 cm²/V·s under bending—both notably higher than those of TFTs without the HZO-assisted NC effect.