In this review, we summarize solution-processed oxide thin-film transistors (TFTs) researches based on our fulfillments. We describe the fundamental studies of precursor composition effects at the beginning in order to figure out the role of each component in oxide semiconductors, and then present low temperature process for the adoption of flexible devices. Moreover, channel engineering for high performance and reliability of solution-processed oxide TFTs and various coating methods: spin-coating, inkjet printing, and gravure printing are also presented. The last topic of this review is an overview of multi-functional solution-processed oxide TFTs for various applications such as photodetector, biosensor, and memory.

In this letter, the ferroelectric (FE) properties of 5-nm-thick Hf_0.5Zr_0.5O₂ (HZO) films deposited by atomic layer deposition have been investigated. By reducing the HZO film thickness to 5 nm, low-voltage operation (1.0 V) of the HZO-based capacitor was achieved while maintaining a remnant polarization (P_r) of about 10 μC/cm² (i.e., 2P_r of 20 μC/cm²). Meanwhile, in order to form an orthorhombic phase, which is responsible for FE properties, a rapid thermal annealing process was performed after TiN top electrode deposition. The FE properties were realized after low temperature annealing (450 °C for 1 min), making them compatible with the back-end of the line. In addition, the low operating voltage and the suppression of an additional monoclinic phase formation by stress-induced crystallization induced a robust endurance (>10¹⁰ cycles at 1.2 V) of the 5-nm-thick HZO sample.

The discovery of ferroelectricity in HfO₂-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf_0.5Zr_0.5O₂ (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications.

In this Letter, a high-pressure annealing (HPA) process is proposed as a way to reduce the thermal budget of Hf_0.5Zr_0.5O₂ (HZO) thin films with ferroelectric behaviors. The low-thermal-budget process is essential for integrating ferroelectric devices in the back-end-of-line to provide more functionalities and effective memory area. For the HZO film annealed at 300 °C using the HPA process, an orthorhombic phase responsible for ferroelectric behavior was formed with a decrease in film thickness, resulting in a remanent polarization (P_r) of ∼13 μC/cm² (i.e., 2P_r of ∼26 μC/cm²). Meanwhile, when only the annealing time was increased at 300 °C without applying pressure, the HZO film did not crystallize and exhibited linear dielectric properties. Consequently, the HZO films (<10 nm) can be crystallized by applying pressure (15 atm) even at low temperatures of 300 °C, thereby obtaining ferroelectric properties.