Novel hafnium oxide (HfO 2 )-based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite-based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.

The unexpected ferroelectric properties of nanoscale hafnia-zirconia are considered to be promising for a wealth of applications including ferroelectric memory, field effect transistors, and energy-related applications. However, the reason why the unexpected ferroelectric Pca2 phase can be stabilized has not been clearly understood although numerous extensive theoretical and experimental results have been reported recently. The ferroelectric orthorhombic phase is not a stable phase under processing conditions from the viewpoint of bulk free energy. Although the possibility of stabilization of the ferroelectric phase due to the surface energy effect has been theoretically suggested, such a theoretical model has not been systematically compared with actual experimental results. In this study, the experimental observations on polymorphism in nanoscale HfO-ZrO solid solution thin films of a wide range of film compositions and thicknesses are comprehensively related to the theoretical predictions based on a thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there were non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as the film stress, the role of a TiN capping layer, and the kinetics of crystallization are systematically studied. This work also reports on the evolution of electrical properties of the film, i.e. dielectric, ferroelectric, anti-ferroelectric, and morphotropic phase changes, as a function of the film composition and thickness. The in-depth analyses of the phase change are expected to provide an important guideline for subsequent studies.

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Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

In this study, the changes in the structural and electrical properties of ferroelectric Hf1-xZrxO2 films with various Zr contents (0.26-0.70) were systematically examined during electric field cycling, resulting in a "wake-up" effect. To quantify the degree of wake-up effect, a "variable" polarization as the difference between remanent and saturation polarization was suggested as a new parameter, which could be calculated by excluding the linear dielectric contribution from the total electric displacement. Here, the variable polarization value could be minimized for an optimized Zr content of 0.43, which was slightly lower than the value for the largest remanent polarization. The polymorphism in Hf1-xZrxO2 thin films is known to be complicated due to the relatively small energy differences between various phases, such as the monoclinic, tetragonal, and orthorhombic phases. The variations in the polarization-electric field characteristics and dielectric constant values could be qualitatively and quantitatively understood based on the competition of various polymorphs that are dependent on the Zr content. Furthermore, a schematic model for the spatial distribution of mixed phases was suggested for Hf1-xZrxO2 films with various Zr contents based on the experimental observations.

Even though many studies on the field cycling behavior of ferroelectric hafnium oxide have recently been published, the issue is still not fully understood. The initial increase of polarization during first cycles is explained by different theoretical and empirical approaches. Field-induced phase changes as well as oxygen vacancy diffusion from interfacial layers toward the bulk are discussed. Trapped charges as well as the mentioned oxygen vacancy diffusion might cause a shift of the hysteresis along the voltage axis called imprint. Even though various studies connect this effect to charge diffusion with progression of cycling, a final experimental proof for the origin of wakeup and imprint is still missing. Based on the comprehensive comparative study of hafnia-zirconia and iron-doped lead zirconate titanate ferroelectrics, it is verified that the diffusion of oxygen vacancies is the main cause for both imprint and wake-up. Moreover, it is shown that a local seed inhibition of ferroelectric domains is most likely responsible for the reduced ferroelectric response in pristine state.

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Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.