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

Abstract

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 2θ values of around 30°, 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.

Influence of Si-Doping on 45 nm Thick Ferroelectric ZrO2 Films

“…Unlike GIXRD, the o-and t-phase in thin films are easier to distinguish by Raman spectroscopy. 29 As shown in Figure 2b, the integrated area corresponding to the o-phase at a Raman shift of 340 cm −1 , normalized with respect to undoped ZrO 2 , drops from 100 to 0%. At the same time, the area related to the t-phase at a Raman shift of 265.8 cm −1 increases to 169% with increasing Si-doping from 0 to 20 ALD cycles.…”

“…Unlike GIXRD, the o- and t-phase in thin films are easier to distinguish by Raman spectroscopy . As shown in Figure b, the integrated area corresponding to the o-phase at a Raman shift of 340 cm –1 , normalized with respect to undoped ZrO 2 , drops from 100 to 0%.…”

“…The Raman intensities were calculated with the Placzek approximation, and the resulting peaks were added to account for the polycrystalline nature of the samples. The Raman active frequencies for the phases have been assigned …”

“…The Raman active frequencies for the phases have been assigned. 29 Figure 2a shows GIXRD results for ZrO 2 films with 0 to 20 ALD cycles of SiO 2 after 800 °C PMA. By incorporating up to 10 ALD cycles of Si dopants, the intensity of a peak for the (−111) plane of the P2 1 c monoclinic phase at 2θ = 28.5°d…”

Review on the Microstructure of Ferroelectric Hafnium Oxides

“…[105][106][107][108] As a result, the tetragonal phase fraction would decrease over cycling and with it the antiferroelectric-like behavior, for example, as recently observed via Raman spectroscopy. [109] However, investigations on the Curie temperature suggest temperatures significantly above room temperature (250-500 °C) [34][35][36][37][38] not supporting a stable tetragonal phase at the operating temperature. This has been further confirmed by recent results, which clearly indicate that the transition from tetragonal phase to the orthorhombic phase is irreversible and thus cannot be responsible for antiferroelectric-like behavior.…”