Multi-order parameter coupling and phase identification in hafnia

Ferroelectric hafnia exhibits excellent size scalability and silicon compatibility, which makes it promising for future ferroelectric nanoscale computing devices. Unlike conventional ferroelectrics such as perovskites, whose ferroelectric phase transitions are described by a single polar order parameter, hafnia has a wide variety of phases, with complicated phase transitions involving multiple order parameters. From a theoretical perspective, this makes it difficult to understand the underlying reason(s) for hafnia’s unique properties and behavior. From an experimental perspective, this variety also complicates phase identification, characterization, and development of prototype devices. Here, we describe first-principles simulations and Landau-Ginzburg-Devonshire modeling to show that the multi-order parameter nature of hafnia is the key to understanding its unique phase transitions, domain wall structures and polarization switching behaviors. Due to the presence of multiple order parameters, domain walls of different configurations could be formed, and each of them shows distinct switching mechanisms. Moreover, the existence of non-polar order parameters also suggests the existence of special domain walls where the sign of polarization is preserved across it. These domain walls are topological defects that could commonly exists in hafnia thin-film and have distinct properties. Finally, infrared spectrum, electronic band structure and exciton absorption simulations are performed to help experimental phase identification. These works deepen our theoretical understanding of ferroelectric hafnia, suggest possible ways to enable easier polarization switching and lower coercive field, and provide guidance for experimental characterization.

 

This work was conducted in collaboration with Songsong Zhou, Jiahao Zhang, Von Braun Nascimento, and Atanu Samanta. We benefited from advice from Jon Ihlefeld. This material is based upon work supported by the USDOE, Office of Science, Office of Basic Energy Sciences EFRC program, Award Number DE-SC00211118.  Computational support was provided by the NERSC, a U.S. Department of Energy, Office of Science User Facility located at LBNL, operated under Contract No. DE-AC02-05CH11231.