Ferroelectric yttrium-doped hafnium-zirconium dioxide (Y-HZO) thin films were fabricated by solution process on Pt/Ti/SiO2/Si substrates. Both metal–ferroelectric–metal (MFM) structures with Pt top electrode and metal–ferroelectric–semiconductor (MFS) structures with indium tin oxide (ITO) top electrode were fabricated and characterized. Solution-derived Y-HZO films annealed at 600 °C–800 °C showed ferroelectric properties which were confirmed by polarization–voltage loops (P–V) and capacitance–voltage (C–V) curves. The ferroelectric properties of Y-HZO were superior to those of undoped HZO, Y-doped HfO2 and undoped HfO2 samples. C–V curves showed clear butterfly loops with depletion of top ITO layer in the case of MFS, while no depletion was observed in the case of MFM structures. Large memory window was obtained for MFS structures and ferroelectric properties were observed even after high temperature annealing process during the solution deposition of ITO electrodes. These results suggest that a solution processed Y-HZO is a promising candidate for ferroelectric gate thin film transistor.

Ferroelectric yttrium-doped hafnium zirconium dioxide (Y-HZO) was fabricated on Pt/Ti/SiO2/Si substrate by the solution process under various annealing environments. A metal–ferroelectric–metal structure with Pt as the top electrode was fabricated and characterized. Samples annealed at 600–800 °C in a vacuum environment showed ferroelectricity, which was confirmed by the polarization–electric field and capacitance–voltage measurements. The ferroelectric properties were dramatically improved when samples were annealed in a vacuum at 800 °C due to the decrease in leakage current compared to the Y-HZO films annealed at 800 °C in oxygen and nitrogen.

Ferroelectric gate transistor (FGT) with yttrium doped hafnium-zirconium dioxide (Y-HZO) gate insulator and oxide channel with various thicknesses of In2O3 and ITO were fabricated by chemical solution deposition. First, ferroelectric properties of Y-HZO in the metal-ferroelectric-semiconductor structure with 5–22 nm thick In2O3 and 6–24 nm thick ITO, have been confirmed by polarization–voltage and capacitance–voltage (C–V) characteristics. The C–V curves showed clear butterfly loops showing the depletion of In2O3 and ITO layer. Secondly, the device performance of FGTs has been evaluated with various thicknesses of In2O3 and ITO channel layer. The fabricated FGTs exhibited typical n-channel transistor operation with a counterclockwise hysteresis loop due to the ferroelectric nature of the Y-HZO-gate insulator. It was found that FGT shows a low subthreshold voltage swing, high on/off drain current ratio of 106, large on current, and memory window.

Stability of ferroelectricity in hafnium–zirconium oxide (HZO) films deposited by sputtering and chemical solution deposition (CSD) has been investigated. After confirming the ferroelectricity of both sputtered HZO and CSD yttrium-doped HZO (Y-HZO) films, indium-tin-oxide (ITO) was deposited by sputtering on sputtered HZO or CSD Y-HZO layer to fabricate metal-ferroelectric-semiconductor (MFS) structure. It was found that the sputtered HZO films in the MFS structure became paraelectric after re-annealing in N2 which was confirmed by both X-ray diffraction pattern and electrical measurements. On the other hand, the CSD Y-HZO films showed ferroelectric nature even after re-annealing with a negligible monoclinic phase.

The impact of reduced pressure annealing on ferroelectric properties in hafnium-zirconium oxide (HZO) films deposited by sputtering has been investigated. It is demonstrated that the HZO films annealed at 600 °C at less than 100 Pa show excellent ferroelectric properties, whereas the HZO films annealed at more than 1000 Pa show degraded electrical properties. This is presumably due to the introduction of oxygen vacancies, which stabilize the ferroelectric orthorhombic phase. Thermal stability can be also improved for the HZO films annealed at reduced pressures. In addition, ferroelectric gate transistor operation is demonstrated using an HZO gate insulator annealed at 100 Pa with the indium-tin-oxide channel.