Thickness scaling of ferroelectric Hf0.5Zr0.5O2 films: How microstructural evolution drives leakage current amplification

Scalability down to several nanometers is recognized as one of the main advantages of ferroelectric hafnium oxide films, because this property of the functional material is vital for the implementation of high-density and low-power ferroelectric memory. The reduction in energy consumption during thickness scaling occurs due to the reduction in coercive voltage, which determines the required operating voltage of the memory chip. However, this phenomenon is accompanied by an increase in leakage currents, which has the opposite effect. This work reveals the origin of leakage currents in ferroelectric Hf_0.5Zr_0.5O₂ films and their increase when the films are scaled from 10 to 5 nm through a combination of experimental techniques and theoretical calculations. Analysis of possible current transport mechanisms, spatial current distribution obtained by conductive atomic force microscopy, grain size measured by scanning electron microscopy, phase composition established by X-ray diffraction and the band gap of the Hf_0.5Zr_0.5O₂ structural polymorphs calculated using density functional theory shows that the dominant contribution to current transport comes from grain boundaries containing multiple charge carrier traps. In-plane and out-of-plane trap densities increase with decreasing thickness due to grain size reduction and redox reactions with electrode materials. These insights provide actionable guidelines for engineering HZO films and interfaces to suppress leakage, a pivotal advancement toward realizing scalable, energy-efficient ferroelectric memory technologies.