Kinetic Monte Carlo simulation reveals defect charge accumulation favoring ferroelectric phase formation in Hf1−<i>x</i>Zr<i>x</i>O2 thin films

Abstract

Charged oxygen vacancies are thought to be responsible for fatigue effects in HfO2- and ZrO2-based ferroelectrics, while also supporting the formation of the ferroelectric phase. We investigate the possible influence of intrinsic electric fields generated by predominantly doubly positively charged oxygen vacancies that accumulate near the electrode and violate charge neutrality. Our calculations are based on a meticulous kinetic Monte Carlo simulation, which simultaneously treats the movement of electrons, the diffusion of oxygen defects, and their interactions that change the charge state in a self-consistent manner. It is shown that with a realistic vacancy concentration, electric fields of several MVcm−1 can form within 2nm of the electrode, supporting the formation of the ferroelectric phase. This effect becomes significant for thin films below 10nm. The strength of the effect depends on the position of the defect energy level relative to the valence band offset. This dependency could be used to control the effect.