Origin of ferroelectricity and anti-ferroelectricity via doping and oxygen vacancy effects in lanthanum-doped hafnium zirconium oxide thin films

Lanthanum-doped hafnium zirconium oxide (La:HZO) has demonstrated high potential to solve the endurance challenge of hafnia-based ferroelectric memory. To date, La:HZO has been experimentally reported to exhibit ferroelectricity (FE) with lower La concentration (<1 mol. %) but anti-ferroelectricity (AFE) with higher La concentration that exceeds 1 mol. %. Despite the improved endurance with the introduction of La dopants, there still lacks an in-depth understanding to explain the mechanisms behind FE to AFE transition. In this work, we theoretically studied the origin of FE and AFE characteristics in La:HZO thin films through first-principles calculations. The polarization switching process was also analyzed based on the Landau–Ginzburg–Devonshire theory. The calculation results reveal that the dominant phase in La:HZO changes from non-FE monoclinic to FE orthorhombic to AFE tetragonal phase with increasing the concentration of La dopants, which results in FE-AFE transition. During the process, the presence of oxygen vacancies also facilitates the formation of the tetragonal phase, which highlights the significant role of oxygen vacancies in phase stabilization. Our findings explain the physical origin of the narrow doping window in La:HZO and provide a theoretical basis for optimizing HZO-based ferroelectric devices by regulating the concentration of dopants and oxygen vacancies within the film.