Layer thickness and interface dependent ferroelectric properties of HfO2 based heterostructures

The excellent ferroelectric properties of HfO2 based thin films enable their promising applications in next-generation ferroelectric devices, yet physical mechanism tuning their ferroelectric properties is still lacking. Here, four different stable t/o/t sandwich heterostructures containing middle, relaxed o-HfO2 layers, interfacial, and fixed terminated t-HfO2 or t-ZrO2 layers are built to explore the stacking, interfacial, and thickness effect on ferroelectric properties. Applying a (C:N) pathway to t/o/t with thickness below 2.35 nm, the calculated spontaneous ferroelectric polarization (Ps) and switching barrier (Ea) show nearly positive linear relationship with layer thickness, and Ps is mainly contributed by atomic displacements in o-HfO2 layers, which is also interfacial dependent. Accordingly, layer thickness ferroelectric phases for t/o/t heterostructures are further mapped out by the linear extrapolation method, and the critical thickness for the emergence of Ps (1.33–1.84 nm), reproduction of the Ps value (4.05–4.76 nm), and Ea value of bulk o-HfO2 (6.70–8.70 nm) are given. While introducing oxygen vacancies to interfacial layers, the calculated Ps and Ea values of ultrathin t/o/t are close to those of bulk o-HfO2. Our study could give guidance for HfO2 based ferroelectric devices in experiments.