Flexible electronics such as wearable electronics, electronic skins, implantable devices, and flexible displays are started influencing our lives in various aspects due to their lightweight, portability, and human-friendly interface. In these applications, nonvolatile memory (NVM) and synaptic devices are essential elements to implement high speed integrated circuits and neuromorphic computing at low power consumption. Perovskite-based ferroelectric (FE) materials such as Pb(Zr,Ti)O₃ (PZT), BaTiO₃ (BTO), and SrBi₂Ta₂O₉ (SBT) have been studied widely because of their fast switching speed, large polarization, and high dielectric constant. However, it is very difficult to integrate these materials with conventional semiconductor technology owing to their poor compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Therefore, hafnium (Hf) based FE-materials could be a suitable candidate for next-generation flexible electronics. Which exhibits immense prospects due to their excellent performance, CMOS compatibility, high scalability, and chemical simplicity with various dopants (Al, Zr, Gd, La, Sr, and Y). Among these, Zr-doped HfO₂ (Hf_0.5Zr_0.5O₂ (HZO)) has been attracted intensive attention because of its low annealing temperature and large remnant polarization.

We study the inverted staggered structured flexible HZO FE-TFTs on polyimide (PI) substrate with zinc oxide (ZnO) as the active layer. A 50 nm molybdenum (Mo) layer was deposited by sputtering and patterned to form the gate electrode. Then, the HZO precursor solution was spin-coated at 2000 rpm for 30 s and cured at 250 °C on a hot plate for 5 min, followed by UV/O₃ curing at 100 °C for 5 min in ambient air as a gate insulator (GI). Thereafter, the Al₂O₃ precursor solution was deposited as a capping layer on top of the HZO film using the same process steps but the rpm was 3500. The crystallization was performed by furnace annealing at 450 °C for 2 h in N₂ environment. The thicknesses of HZO and Al₂O₃ films are 20 and 15 nm, respectively. Afterwards, the ZnO (25 nm) channel layer was deposited using spray pyrolysis at 350 °C substrate temperature. The ZnO and GI films were patterned sequentially to form the active island and via-hole, respectively. Finally, a 50 nm thick Mo film was sputtered and patterned to form the source/drain (S/D) electrodes.

The origin of ferroelectricity in HZO GI was analyzed by grazing incidence X-ray diffraction (GI-XRD) measurement from film and drain current vs gate voltage (I_D vs V_GS), gate leakage current (I_G), and drain current vs drain voltage (I_D vs V_DS) characteristics measurements from TFT structures using Agilent 4156C semiconductor parameter analyzer. Channel width and length were 50 and 10 μm, respectively.

Fig. 1 (a) shows the schematic structure of an inverted staggered HZO FE-TFT on PI substrate. Fig. 1 (b) and (c) show the I_D vs V_GS characteristics with anticlockwise hysteresis of HZO and Al₂O₃ capped HZO TFTs on PI substrate. The Al₂O₃ capped HZO TFT exhibits clear ferroelectric property with a large memory window (MW) of ≈1.9 V. A thin Al₂O₃ capping layer on top of HZO enhances the microstructure and morphology of HZO even at a low annealing temperature of 450 °C and helps to induce the ferroelectricity in HZO GI. Fig. 1 (d) shows the photograph of the measurement setup of HZO FE-TFTs during electrical measurements under tensile stress at 2 mm. Fig. (e) and (f) exhibit the I_D vs V_GS and I_G vs V_GS characteristics with anticlockwise hysteresis of Al₂O₃ capped HZO FE-TFTs under different tensile stress, respectively. The I_D vs V_GS curves exhibit almost identical performance during forward and reverse bias at different bending radii without showing significant changes in the electrical characteristics. The MWs were evaluated under different bending radii and they show the almost same value of ≈1.9 V from flat state and down to 2 mm bent state. The corresponding I_G vs V_GS curves at different bending radii exhibit a clear butterfly shape with current peaks at about ±2 V. These findings indicate that the solution-processed flexible HZO FE-TFTs have high stability under mechanical stress and contain ferroelectricity even at 2 mm bending radius without deteriorating MW. Therefore, the low-cost solution-processed FE-HZO GI has significant potential to be employed in nonvolatile memory and synaptic devices for future flexible electronics.