Obvious ferroelectricity in undoped HfO2 films by chemical solution deposition
Although great achievements have been made in ferroelectricity of HfO2-based films by ALD method, the performance is strongly constrained by film thickness and dopant types. This work was the first time to realize ferroelectric properties in undoped HfO2 films by chemical solution deposition method, which breaking the above restrictions when using ALD method. The evolution of ferroelectricity in pure HfO2 films was studied over a wide range of thickness from 34 nm to 136 nm, without doping any other metallic elements. The HfO2 film with the thickness of 136 nm exhibited a large remnant polarization (Pr) of 22.56 μC/cm2 after a wake-up process of 105 cycles and could endure up to 107 switching cycles. Residual carbon from incomplete decomposition of organic matter in films and growth disruption of grains through layer-by-layer thermal treatment could lead to the reduction of grain size, which were beneficial to the formation of metastable ferroelectric o-phase. Oxygen vacancies existing at the interfaces between TiN bottom electrode and HfO2 film could be rearranged during cycling of electric field, thus inducing the appearance of ferroelectricity. The movement of domains under different tip bias by piezo force microscope verified the existence of ferroelectricity in undoped HfO2 films. This report therefore provides an inexpensive and feasible way to gain ferroelectricity in pure HfO2 films and paves the way toward other sensor applications beyond thickness limitation and doping system.