The map obtained for a thin barium titanate film after using a machine-learning method to cluster the data measured by contact Kelvin probe force microscopy. This map can reveal detailed information about the distribution of ferroelectric domains and their respective polarization amplitude. Image: HZB.How do the properties of ferroelectric materials change when the dimensions of the material are greatly reduced, such as when used for nanoelectronic components? Experiments have shown that shrinking can have enormous effects on the pattern of ferroelectric polarization.
“When the dimensions are reduced, the ferroelectric domains can take on a very different shape with a spatial extension of only several nanometers," explains Catherine Dubourdieu, head of the Institute of Functional Oxides for Energy Efficient IT at Helmholtz Zentrum Berlin (HZB) in Germany. "The diversity of electrical structures on a nanocrystalline scale opens up a whole new exciting horizon both for the understanding of the physics of these objects and for their potential applications. One key challenge is to be able to visualize such tiny domains in a non-destructive way.”
Together with colleagues at Oak Ridge National Laboratory (ORNL), Dubourdieu and her team have now found a way to map the polarization pattern in thin ferroelectric layers precisely and non-destructively. To do this, the researchers took advantage of so-called contact Kelvin probe force microscopy (cKPFM) – a method for measuring a material's electromechanical response under an electrical bias. They report their findings in a paper in ACS Applied Electronic Materials.
To evaluate the enormous amount of data generated by mapping at pixel sizes as low as 8x8nm2, the HZB team applied a machine-learning method. This made it possible to spatially resolve ferroelectric domains of less than 10nm in size and of different polarization amplitudes. As a sample material, the HZB researchers used a thin layer of barium titanate (BaTiO3) in two crystalline forms: the so-called perovskite structure (one of the best-known ferroelectric materials) and the hexagonal structure, which is not ferroelectric at room temperature.
To check the reliability of their measurement method, the HZB and ORNL teams also analyzed the nanostructures using transmission electron microscopy (TEM). "The results of both experimental methods were in complete agreement," Dubourdieu says.
The scientists were also able to use this method to follow the evolution of the ferroelectric pattern as the sample was heated up to its paraelectric state. This opens up the possibility of investigating the temperature dependence of the ferroelectric domain distribution and observing how ferroelectric domains form spatially below the so-called Curie temperature.
“Our results create a promising new perspective to study a large variety of polarization patterns at the nanoscale,” says Dubourdieu. “This could lead, for example, to mapping the distribution of topological polar textures such as polar skyrmions, which have been shown to have dimensions of about 10nm. It could also be used to discriminate the polar domains from the non-polar ones in polycrystalline HfO2-based ferroelectric thin films, a type of materials intensively studied for their potential integration in current nanoelectronics.
“In the future, mapping ferroelectricity at the nanoscale with the help of machine learning will undoubtedly bring insights into phenomena occurring when dimensions are reduced and bring benefit for the integration of ferroelectrics into nanodevices.”
This story is adapted from material from Helmholtz Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.