Pratyush Buragohain1,Sangita Dutta2,3,Haidong Lu1,Sebastjan Glinsek2,Hugo Aramberri2,Claudia Richter4,Pamenas Kariuki4,5,Terence Mittmann4,Takanori Mimura6,Takao Shimizu6,7,Hiroshi Funakubo6,Uwe Schroeder4,Emmanuel Defay2,Jorge Iniguez2,3,Alexei Gruverman1
University of Nebraska-Lincoln1,Luxembourg Institute of Science and Technology2,University of Luxembourg3,NaMLab gGmbH4,TU Dresden5,Tokyo Institute of Technology6,National Institute for Materials Science (NIMS)7
Pratyush Buragohain1,Sangita Dutta2,3,Haidong Lu1,Sebastjan Glinsek2,Hugo Aramberri2,Claudia Richter4,Pamenas Kariuki4,5,Terence Mittmann4,Takanori Mimura6,Takao Shimizu6,7,Hiroshi Funakubo6,Uwe Schroeder4,Emmanuel Defay2,Jorge Iniguez2,3,Alexei Gruverman1
University of Nebraska-Lincoln1,Luxembourg Institute of Science and Technology2,University of Luxembourg3,NaMLab gGmbH4,TU Dresden5,Tokyo Institute of Technology6,National Institute for Materials Science (NIMS)7
The discovery of ferroelectricity in hafnium oxide-based thin films represents a significant technological development due to their inherent advantages over perovskite ferroelectrics (FE) related to CMOS compatibility. While most reports in the literature have focused on the ferroelectric – structural and electrical – properties in this highly technologically important material, their electromechanical behavior has been relatively unexplored so far. In this work, we investigated the electromechanical behavior in lanthanum doped HfO<sub>2</sub> (La:HfO<sub>2</sub>) and yttrium doped HfO<sub>2</sub> (Y:HfO<sub>2</sub>) thin film capacitors, specifically focusing on the sign of the longitudinal piezoelectric coefficient, d<sub>33,eff</sub>, using piezoresponse force microscopy (PFM). Careful consideration of the extrinsic contributions to the PFM signal allowed the unambiguous interpretation of the PFM signal. We observed a negative d<sub>33,eff </sub>, consistent with recent theoretical predictions, in the La:HfO<sub>2 </sub>thin films of thicknesses less than 30 nm. Local mapping of the ferroelectric domain structure revealed ‘anomalous’ switching regions that exhibit a positive d<sub>33,eff</sub> under certain material conditions. In addition, we also observed the transformation of the sign of the d<sub>33,eff</sub> from positive in the pristine state to negative after electrical field cycling in Y:HfO<sub>2</sub> thin film capacitors. We discuss in detail the methodology adopted in this work that allowed us to differentiate the intrinsic electromechanical response from extrinsic contributions such as electrostatics or parasitic charge injection in PFM measurements.