Raman Spectroscopy as a Key Method to Distinguish the Ferroelectric Orthorhombic Phase in Thin ZrO2-Based Films

Inducing and detecting the polar orthorhombic phase are crucial for the establishment of ferroelectricity in HfO2- and ZrO2-based thin films. Unfortunately, commonly used structural characterization techniques such as grazing incidence angle X-ray diffraction (GIXRD) only partially allow an accurate detection of this crystalline phase, whose characteristic pattern almost coincides with the one of the tetragonal phase. As a consequence, phase determination is commonly based on peak deconvolution tracing the position of the main peak at 2 theta values of around 30 degrees, which can be assigned both to the t(101) and the o(111) plane directions and additionally be influenced by mechanical stress in the layers. Alternatively, epitaxial layers are required to differentiate the phase. Herein, using an integrated experimental-computational approach, it is shown how Raman spectroscopy can distinguish between the monoclinic, the tetragonal, and the orthorhombic phase of ZrO2. The Raman spectra calculated from first principles match the experimentally measured data and thus enable an unambiguous phase assignment. Therefore, Raman spectroscopy proves to be a powerful technique for discerning the three main crystalline phases in these materials. As demonstrated by the good agreement between structural and electrical data, it can therefore be used to predict ferroelectricity in the addressed layers.

Automated summary

This article discusses the importance of inducing and detecting the polar orthorhombic phase in HfO2- and ZrO2-based thin films. Traditional structural characterization techniques are not accurate enough, hence the use of Raman spectroscopy to differentiate different phases. By comparing experimental and computational Raman spectra, the phase assignment can be determined, allowing for the prediction of ferroelectricity in the layers.