Mechanical-electrical-chemical coupling study on the stabilization of a hafnia-based ferroelectric phase

The metastable polar orthorhombic phase is believed to be the origin of the ferroelectricity of hafnia-based films.The adjustment of stain,oxygen vacancies and dopant during film deposition and the wake-up electric cycling are common strategies to induce the ferroelectricity in hafnia.However,none of them could independently render the ferroelectric phase to be the most stable phase from the theoretical calculation results.The exact external conditions to stabilize orthorhombic phase still remain elusive.In this paper,we investigate the effects of the type,distribution,concentration,and charge state characteristics of oxygen vacancies and the uniaxial strain on the crystal’energy,dielectric constant and spontaneous polarization(Ps);In addition,the impact of the applied electric field parallel to the Ps on the crystal’energy is explored by first-principles calculations.It is challenging to independently stabilize the ferroelectric phase of hafnia-based films by a single component owing to the rather strict conditions.Surprisingly,the ferroelectricity can be easily obtained when simultaneously considering the effects of oxygen vacancies,uniaxial strain,and applied electric fields,suggesting the extremely important mechanical-electrical-chemical coupling effects.This work provides an explanation for the typical wake-up phenomenon in hafnia and a guidance for film applications.