Ferroelectricity in Hf_0.5Zr_0.5O₂ Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations

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Abstract

Because of their full compatibility with the modern Si-based technology, the HfO₂-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional Hf_0.5Zr_0.5O₂-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine Hf_0.5Zr_0.5O₂ films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric Hf_0.5Zr_0.5O₂ layer during the wake-up process under an electrical stress.

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