Obvious ferroelectricity in undoped HfO2 films by chemical solution deposition

Abstract

Although great achievements have been made in realizing ferroelectricity in HfO₂-based films by the ALD method, the performance is strongly constrained by film thickness and dopant types. This study was the first to realize ferroelectric properties in undoped HfO₂ films by the chemical solution deposition method, which broke the above restrictions while using the 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 a thickness of 136 nm exhibited a large remnant polarization (P_r) of 22.56 μC cm⁻² after a wake-up process of 10⁵ cycles, and could endure up to 10⁷ switching cycles. Residual carbon from the incomplete decomposition of organic matter in films and growth disruption of grains through layer-by-layer thermal treatment could lead to the reduction in grain size, which was beneficial to the formation of the metastable ferroelectric o-phase. Oxygen vacancies existing at the interfaces between the TiN bottom electrode and HfO₂ film could be rearranged during electric field cycling, thus inducing the appearance of ferroelectricity. The movement of domains under different tip bias by a piezo force microscope verified the existence of ferroelectricity in the 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 limitations and doping systems.