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Table 6. Ferroelectric Thin Films and Devices [3]. Thin Film Deposition Process [3].

The microstructure of perovskite thin films is governed by the initial stage of the film's growth. The cooling conditions after deposition are also important when depositing ferroelectric thin films that have a controlled crystal structure. Ferroelectric materials with their two stable remanent polarization states are ideally suited for low write-power nonvolatile memories. Many ferroelectric materials with excellent properties exist, but most of them commonly have rather complicated crystal structures, which make it hard to be achieved and maintain when integrated into a semiconductor process.

There are several possibilities to integrate a ferroelectric material into a memory cell. The straightforward solution is to replace the dielectric in a transistor—one capacitor DRAM cell by a ferroelectric material.

The newly discovered ferroelectricity in hafnium oxide could change this picture since the material can be deposited by the well-established atomic layer deposition techniques, and thus, can easily realize three-dimensional integrated structures. However, the high coercive field currently limits the voltage scalability and the endurance of the ferroelectric hafnium oxide leaving research with some remaining challenges.

The second mainstream option of integrating a ferroelectric memory cell is to place the ferroelectric into the gate stack of a transistor to realize a FeFET. This option has been limited by the very high permittivity and the low coercive field of conventional ferroelectric material like PZT, SBT, and the likes. The inherent de-polarization field of such a structure also limits the nonvolatile retention, and the low coercive field dictates very thick layers to achieve a reasonable memory window.

Again, doped hafnium oxide can solve these issues, and the scaling gap between FeFETs and conventional logic was overcome very rapidly. This development places FeFETs as a very promising option for future embedded nonvolatile memories on the research and development roadmap. Moreover, the programming and erase mechanism allows the integration into a NAND type architecture opening a path toward high-density storage or storage class memories.

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Besides these options, two further possibilities exist that allow a resistive readout—the FTJ and the DW memory. Both options are still in the early development stage and significant innovations are required to realize real memory arrays. However, in the successful case the resistance sensing may have some scaling advantages over the other two were the polarization charge is read out directly FeRAM or indirectly FeFET. Finally, there exists a number of possible applications beyond the field of semiconductor memories.

Memory-in-Logic and neuromorphic computing being two fields that still are related to the traditional memory application but use a specific way of operating the device. The inherent variability of the switching in very small FeFETs can be used to realize a true random number generator. This would be one of the most important contributors to further scaling.

However, the domain structure of a ferroelectric is making the stabilization of the NC in a way that would allow logic operation still doubtful. Again ferroelectric hafnium oxide would be the material of choice to supply the necessary CMOS compatibility, and too little is known so far about the details of the domain structure and its reversal to give a fair assessment of the realization potential of this approach. Finally, ferroelectrics come with the piezo and pyroelectric effect, which have a potential for energy storage, energy harvesting as well as sensors and actuators.

Specifically, for the CMOS compatible hafnium oxide, this research field is still in its infancy, and many possibilities remain to be explored. In summary, ferroelectric materials have a great potential for nonvolatile memories but also some other device applications. The discovery of the ferroelectricity in doped hafnium oxide has widened the possible application fields in integrated devices again that were limited by the integration difficulties of perovskites or layered perovskite until a few years ago.

A ferroelectric material is normally in single crystalline or polycrystalline form and possesses a reversible spontaneous polarization over a certain temperature range.

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There is a critical temperature, called the Curie temperature, which marks the transition from the ordered to the disordered phase. At this temperature, the dielectric constant may reach values three to four orders of magnitude higher than in the disordered phase. The order—disorder phase transition involves the displacement of atoms so that crystals or crystallites exhibiting ferroelectric phenomena must be noncentrosymmetric.

This implies that a phase transition will induce a mechanical strain, tending to change not only the volume and the shape of the material body, but also the optical refractive index. Thus, ferroelectric materials exhibit not only ferroelectric phenomena, but also piezoelectric, pyroelectric, and electro-optic effects, which can be used for many technological applications.

In general, ferroelectric materials also have electrically induced polarizations, but these are negligibly small compared to spontaneous polarization. For most practical cases dealing with ferroelectric phenomena, electrically induced polarization can be ignored.

More details about the properties and applications of ferroelectric materials are given in Ferroelectric Phenomena in Chapter 4. Mikolajick, in Encyclopedia of Materials: Science and Technology , A ferroelectric material is characterized by the presence of a remnant polarization at zero bias voltage and the ability to switch the direction of this polarization Damjanovic The two polarization directions of a ferroelectric material can be used to store digital information Auciello et al.

Since the polarization will remain in its preset direction at zero bias, the resulting memory will be nonvolatile in nature. The first attempts to build ferroelectric memories, FeRAM ferroelectric random access memory , were started in the s Anderson At that time bulk barium titanate BaTiO 3 was used as the ferroelectric material and the memory matrix consisted of a simple cross-point arrangement of bitlines and wordlines.

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But the inability to control disturbances in the cross-point arrangement and the fast wear-out of the ferroelectric barium titanate during cycling resulted in the discontinuation of these activities in the early s. A ferroelectric material is characterized by the presence of a remnant polarization at zero bias voltage and the ability to switch the direction of this polarization Damjanovic, The first attempts to build ferroelectric memories ferroelectric random access memory: FeRAM were started in the s Anderson, At that time bulk barium titanate BaTiO 3 was used as the ferroelectric material and the memory matrix consisted of a simple cross-point arrangement of bitlines BLs and wordlines.

But the inability to control disturbs in the cross-point arrangement and the fast wear-out of the ferroelectric barium titanate during cycling resulted in discontinuation of these activities in the early s. Biljana D. Ferroelectric materials might be inorganic and organic, but the metal oxides are most known and most used.

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Among them the ferroelectrics with perovskite crystal structure with the general chemical formula ABO 3 are by far the most technologically important class of ferroelectric materials. Some perovskites show nonferroelectric phase transitions. The SrTiO 3 exhibits a phase transition with linked rotations, or tilts, of the TiO 6 octahedra about the cubic [] direction. Tilting of the octahedra is a common feature of the phase transitions that occur in perovskites and can lead to very complex series of phase transitions, as has been observed for NaNbO 3. There is another very important class of oxides, named complex perovskites, where the A-sites or, more commonly, B-sites in the structure are occupied by ions of different valence in a fixed molar ratio.

Doping certain ferroelectric ceramics with electron donors e. Heating these ceramics through their Curie temperature causes a very large, reversible increase in resistivity by several orders of magnitude in some cases over a narrow range of temperature. This large positive temperature coefficient of resistance is widely exploited in electric-motor overload-protection devices and self-stabilizing ceramic heating elements. Another idea of recent interest is the ferroelectric tunnel junction, in which a contact made up by nanometer-thick ferroelectric film is placed between metal electrodes.

Liu, C. Ferroelectric materials exhibit major hysteresis loops during field-induced polarization switching. Hysteresis also occurs during a phase transition, which is dependent on the material composition, temperature and other factors. Minor hysteresis loops are displayed under unipolar loading, even though the electric field and stress cycles do not induce apparent polarization switching and phase transitions.

Domain and phase activities in ferroelectric materials contribute to non-linear and hysteretic electromechanical response and other reliability issues Lines and Glass, ; Lynch, ; Damjanovic, Hysteresis below the Curie temperature is mainly caused by domain wall movements Hardtl, Hysteresis is associated with energy loss and heat generation in the materials.

Heat generation is a serious issue in piezoelectric devices working at high frequencies such as ultrasonic motors. While the study has clear national features everywhere, its scholars share attention for bureaucracy and organization, for civil service and personnel systems, for policy and decision-making, for implementation, for budgeting, intergovernmental relations, etc. In this chapter, attention is given to how these general topics of the study can be mapped.

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