The material class of hafnium oxide-based ferroelectrics adds an unexpected and huge momentum to the long-known phenomenon of pyroelectricity. In this thesis, a comprehensive study of pyroelectric and electrocaloric properties of this novel ferroelectric material class is conducted. hafnium oxide is a lead-free, non-toxic transition metal oxide, and abundant in the manufacturing of semiconductor devices. The compatibility to existing fabrication processes spawns the possibility of on-chip infrared sensing, energy harvesting, and refrigeration solutions, for which this dissertation aims to lay a foundation. A screening of the material system with respect to several dopants reveals an enhanced pyroelectric response at the morphotropic phase boundary between the polar orthorhombic and the non-polar tetragonal phase. Further, a strong pyroelectric effect is observed when applying an electric field to antiferroelectric-like films, which is attributed to a field-induced transition between the tetragonal and orthorhombic phases. Primary and secondary pyroelectric effects are separated using high-frequency temperature cycles, where the effect of frequency-dependent substrate clamping is exploited. The piezoelectric response is determined by comparing primary and secondary pyroelectric coefficients, which reproduces the expected wake-up behavior in hafnium oxide films. Further, the potential of hafnium oxide for thermal-electric energy conversion is explored. The electrocaloric temperature change of only 20 nm thick films is observed directly by using a specialized test structure. By comparing the magnitude of the effect to the pyroelectric response, it is concluded that defect charges have an important impact on the electrocaloric effect in hafnium oxide-based ferroelectrics. Energy harvesting with a conformal hafnium oxide film on a porous, nano-patterned substrate is performed, which enhances the power output. Further, the integration of a pyroelectric energy harvesting device in a microchip for waste heat recovery and more energy-efficient electronic devices is demonstrated. High dielectric breakdown fields of up to 4 MV/cm in combination with a sizable pyroelectric response and a comparably low dielectric permittivity illustrate the prospect of hafnium oxide-based devices for future energy conversion applications.

Infrared Spectroscopy of Multiferroics Author: Veronica Goian Institute: Department of Dielectrics, Institute of Physics of the Academy of Sciences, Na Slovance 2, 182 21 Prague 8 Abstract: We have investigated numerous multiferroic and magnetoelectric materials mainly using infrared (IR) spectroscopy. Nevertheless, the studies were frequently combined with radio-frequency, microwave, THz, Raman and structural measurements provided by our colleagues, as well as by magnetic and elastic investigations, where we participated. Our main aim was the complex study of quantum-paraelectric antiferromagnet EuTiO3 in the form of crystals, ceramics and thin films. Near 300 K we have discovered an antiferrodistorive phase transition from cubic mPm3 to tetragonal I4/mcm structure in bulk EuTiO3 and explained its low-frequency dielectric properties by anomalous polar phonon behavior. Large and anisotropic magnetodielectric effect, which we found in EuTiO3, was successfully explained and experimentally confirmed by observation of tuning of phonon frequency with magnetic field. Our IR studies of tensile strained EuTiO3 thin films revealed a displacive ferroelectric phase transition near 250 K. Our American colleagues revealed the ferromagnetic order below 4.2 K in the same strained EuTiO3 thin film. In such way we have...

Les composés à base de matériaux ferroélectriques présentent un large éventail de propriétés d'un grand intérêt fondamental et industriel. Ils possèdent un couplage entre la polarisation, la contrainte, le champ électrique et la température et trouvent application dans les dispositifs à l'échelle nanométrique. Les ferroélectriques sont aujourd'hui par exemple déjà utilisés dans les condensateurs, les mémoires, les capteurs et les actionneurs. Afin de comprendre la relation entre leurs propriétés physiques exceptionnelles et leur structure, des méthodes numériques capables de simuler à l'échelle nanométrique sont souhaitées. Pour ce faire, le modèle d'ions polarisables (PIM) est appliqué, modèle dans lequel les ions sont considérés comme des espèces polarisables possédant des charges nominales. En regard des techniques de modélisation actuelles, l'utilisation de charges nominales devrait faciliter l'inclusion de diverses compositions et l'étude des défauts et des effets de surface. Les paramètres du PIM sont dérivés par une procédure d'ajustement sur des données de références obtenues par des calculs avec la théorie de la fonctionnelle de la densité (DFT). Pour une première étape vers la modélisation ferroélectrique avec PIM, l'accent est mis sur le développement d'un potentiel d'interaction pour le prototype ferroélectrique BaTiO3. BaTiO3 présente une séquence complexe de transition de phase (rhomboédrique, orthorhombique, tétragonale, cubique) qui est liée à de petites différences d'énergie de l'ordre de quelques meV/unité de formule. Pour cette raison, le développement d'un potentiel d'interaction pour BaTiO3 nécessite une grande précision pour décrire correctement l’équilibre entre les interactions à courte et à longue portée. Il a été démontré au cours de ce travail que des effets asymétriques du nuage d'électrons par rapport au noyau seraient nécessaires pour une représentation précise des forces à courte portée. Puisque de tels effets ne sont pas inclus dans le PIM, des erreurs de compensation dans la procédure d'ajustement entre les interactions à courte et à longue portée sont permises afin d'obtenir le meilleur ajustement global. Le PIM développé pour BaTiO3 reproduit plusieurs propriétés à température nulle. À température finie, le PIM prédit que la phase rhomboédrique sera stable jusqu'à 160K. Dans la plage de température comprise entre 160K et 210K, de fortes fluctuations de la polarisation et des paramètres de maille sont observées et aucune phase bien définie ne peut être distinguée. A partir de 210K, la phase cubique paraélectrique est atteinte. Le modèle PIM développé dans cette thèse peut être appliqué à des études à basse température dans la phase ferroélectrique rhomboédrique. Il peut donc être utilisé pour étudier les effets de surface ou des lacunes d'oxygène dans la phase rhomboédrique de BaTiO3 . / Ferroelectric based compounds present a wide range of properties which are from great fundamental and industrial interest on nanoscale. Ferroelectric based compounds possesses strong coupling between polarization, stress, electric field and temperature and are nowadays already used in capacitors, memories, sensors, and actuators. In order to understand the relationship between microstructure and the outstanding properties, numerical methods able to simulate at nanoscale are disired. For this propose, the Polarizable Ion Model (PIM) is employed that treats the ions as polarizable species with nominal charge. In comparison to current modelisation techniques, the use of nominal charges should facilitate the inclusion of various materials composition and the study of defect and surface effects. The pametrization of the model is derived by a fit on ab initio DFT reference calculations. For a first step towards ferroelectric modelling with PIM, the focus lies on the developpment of an interaction potential for the prototyp ferroelectric BaTiO3. BaTiO3 presents a complex phase transition sequence (rhombohedral, orthorhombic, tetragonal, cubic) that is related to small energy differences of the order of some meV/formula unit. Thus, the development of a reliable interaction potential requires high precision and a correct description of the balance between short range and long range interactions. It has been demonstrated during this work that for an accurate representation of the short range forces asymmetric size effects of the electron cloud with respect to the nucleus would be necessary. As such size effects are not included in the PIM, compensation errors in the fitting procedure between short range and long range interactions are allowed in order to obtain the best global fit. The developed PIM model reproduces several zero temperature properties of BaTiO3. At finite temperature the PIM predicts the rhombohedral phase to be stable up to 160K. In the temperature range between 160K and 210K strong fluctuations in polarization and cell parameters are observed and no well-defined phase can be distinguished. From 210K on, the average paraelectric cubic phase is reached.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19991号 / 工博第4235号 / 新制||工||1655(附属図書館) / 33087 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 田畑 修, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

<p>As a scalable and CMOS compatible novel ferroelectric material, the ferroelectric HZO thin film has been the promising material for various applications and continues to attract the attention of researchers. Achieving strong ferroelectricity and fast switching speed in ultrathin FE HZO film are crucial challenges for its applications towards scaled devices.</p> <p>The ferroelectric and anti-ferroelectric properties of HZO are investigated systematically down to 3 nm. The ferroelectric polarization, switching speed and the impact of ALD tungsten nitride electrodes are studied. Record high Pr on FE HZO and record high PS on AFE HZO are achieved with WN electrodes, especially in ultrathin sub-10 nm regime. The polarization switching speed of FE and AFE HZO, associated with C-V frequency dispersion, are also qualitatively studied. On the other side of the scaling limit, ferroelectric/dielectric stack superlattice structure is found to enhance the ferroelectricity in thick films which would have severely degraded.</p> <p>Ultrafast direct measurement on the transient ferroelectric polarization switching is used to study the switching speed in FE HZO with a crossbar MFM structure. Sub-nanosecond characteristic switching time of 925 ps was achieved, supported by the nucleation limited switching model. The impact of electric field, film thickness and device area on the polarization switching speed is systematically studied. The ferroelectric switching speed is significantly improved compared to previous reports and more importantly is approaching GHz regime, suggesting FE HZO to be competitive in high-speed non-volatile memory technology. Record fast polarization switch speed of 360 ps is obtained in sub-μm crossbar array FE HZO MFM devices. It also unveils that domain wall propagation speed in HZO is the limiting factor for switch speed and more aggressively scaled devices will offer much faster switch speed.</p> <p>The first experimental determination of nucleation time and domain wall (DW) velocity by studying switching dynamics of ferroelectric (FE) hafnium zirconium oxide (HZO) was performed. Experimental data and simulation results were used to quantitatively study the switching dynamics. The switching speed is degraded in high aspect ratio devices due to the longer DW propagation time or with dielectric interfacial layer due to the required additional tunneling and trapping time by the leakage current assist switch mechanism.</p>

No description available.

No description available.

No description available.

No description available.

No description available.