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

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 (Hf0.5Zr0.5O2, 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 (Ion) 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 Ion 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.