Electrically induced cancellation and inversion of piezoelectricity in ferroelectric HfZrO.

HfO-based thin films hold huge promise for integrated devices as they show full compatibility with semiconductor technologies and robust ferroelectric properties at nanometer scale. While their polarization switching behavior has been widely investigated, their electromechanical response received much less attention so far. Here, we demonstrate that piezoelectricity in HfZrO ferroelectric capacitors is not an invariable property but, in fact, can be intrinsically changed by electrical field cycling. HfZrO capacitors subjected to ac cycling undergo a continuous transition from a positive effective piezoelectric coefficient d in the pristine state to a fully inverted negative d state, while, in parallel, the polarization monotonically increases. Not only can the sign of d be uniformly inverted in the whole capacitor volume, but also, with proper ac training, the net effective piezoresponse can be nullified while the polarization is kept fully switchable. Moreover, the local piezoresponse force microscopy signal also gradually goes through the zero value upon ac cycling. Density functional theory calculations suggest that the observed behavior is a result of a structural transformation from a weakly-developed polar orthorhombic phase towards a well-developed polar orthorhombic phase. The calculations also suggest the possible occurrence of a non-piezoelectric ferroelectric HfZrO. Our experimental findings create an unprecedented potential for tuning the electromechanical functionality of ferroelectric HfO-based devices.