Nanoscale visualization of piezoelectric and ferroelectric properties in oxide heterostructures

Abstract

Piezoelectricity can be used in energy harvesters, actuators, ultrasound devices, etc., because it is possible to convert mechanical deformation into electrical signals or vice versa. In addition, the use of ferroelectric random access memory (FRAM) and FeFET (Ferroelectric Field Effect Transistor) as next-generation memory devices is expected, as they have spontaneous polarization and polar switching characteristics among piezoelectric bodies. In the utilization of these piezoelectric devices, the macroscopic performance of the device is greatly influenced by the material properties in the nanoscale and the electrical and mechanical performance of the piezoelectric and ferroelectric conforms to the mechanism at the atomic level, so an understanding of nanoscale properties is essential. In this study, we used various analysis tools based on scanning probe microscopy (SPM) for piezoelectricity and ferroelectricity of oxide materials to carry out imaging and character analysis on the nanoscale. First, in the case of piezoelectric oxide, ZnO was made of a 3D hollow nanostructure to improve both mechanical strain and piezoelectric properties of ceramic piezoelectric material. Mechanical properties were verified through the nano-indentation test, and piezoelectric properties were verified by using piezoresponse force microscopy (PFM). Secondly, we fabricate the ferroelectric zirconium doped hafnia (HZO) thin film, which is expected to be used as a next-generation ferroelectric memory device. The ferroelectric domain map and ferroelectric hysteresis were identified in nanoscale using SPM. We introduce a novel PFM analysis method that can successfully measure ferroelectric domain imaging and hysteresis curves by finding and solving the causes of PFM analysis of HZO thin films, which was not accessible due to low piezoelectric coefficients. Third, we visualized the wake-up and fatigue (endurance) phenomena of ferroelectric HZO, which are issues in utilizing HZO as memory devices. Through this, we understood the mechanism of the wake-up and endurance problems.