Incipient ferroelectrics based on hafnium oxide

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Ferroelectric (FE) materials are of high technological interest because of their extraordinary dielectric, electro-optic, pyro- and piezoelectric properties. Their fields of application range from actuators and sensors through to data and energy storage. Conventional ferroelectrics usually possess a so-called Perovskite structure and are comparably complex oxides as evidenced by lead zirconate titanate (PbZrxTi1-xO3, PZT) as one of the most important examples.Prior to the first funding period, the first observation of FE properties in 10 nm thin Si-doped HfO2 films was published by the applicants. This finding was especially surprising given the intense research devoted to HfO2 as ceramic material as well as for ultrathin high-permittivity dielectrics in semiconductor technology. The origin of this ferroelectricity remained unclear. An orthorhombic Pca21 phase was proposed to explain the behavior. The experimental proof could not be provided up to that point. Doping and mechanical stress were discussed to potentially account for the stabilization of the FE phase. Therefore, the experimental and theoretical investigations were aligned along the working hypothesis that pure HfO2 is an incipient ferroelectric which could overcome the inherently small barrier between the para- and the ferroelectric state promoted by the abovementioned levers. The report section contains a discussion of I) the stabilization and the experimental proof of the Pca21 phase; II) the suitability of different preparation methods with focus on the cost-efficient, flexible chemical solution deposition; III) the manifestation and aptitude of different dopants; IV) the previously only sparsely considered field cycling behavior of HfO2-based ferroelectrics.Goal of the extension proposal is to deepen the physical and theoretic understanding of the stabilization of the ferroelectric phase as well as of the mechanisms that account for the field cycling behavior in HfO2-based thinfilms. This is of outstanding relevance for both fundamental material science and future applications. Starting point for the proposed studies are the findings of the first funding period, which allowed a clear identification of relevant factors toward a physical multiscale model of the system HfO2/ZrO2. Particular aspects are intended to be critically discussed in comparison to the classic Perovskite ferroelectrics. Here, analogies will be helpful but also differences shall be emphasized. The work packages are arranged around the following subgoals: I) an understanding of the influence of surface energy; II) analysis of the phase transitions and the polarization reversal; III) study of dopants not utilized yet; IV) elucidation of the role of oxygen vacancies; V) working out the differences to classic perovskite ferroelectrics.