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Ms Anna Razumnaya

New materials for ferroelectric and multiferroic Terahertz devices

The objective of my research project is to develop the modeling of new materials for ferroelectric and multiferroic THz materials for further application in THz devices. The modeled structures will be considered in form of single layer thin films, superlattices structures made by a ferroelectric layer and a magnetic layer, and multilayer structures built up from alternating dielectric/ferroelectric and magnetic layers to optimize the magnetoelectric coupling between the layers.

        We live in the world where electronic devices play crucial role in every aspect of our life. To continue enhancing them we need new materials with remarkable capabilities. Among many materials, ferroelectrics possess properties useful for creation of high-performance micro- and nanoscaled functional devices. The prominent feature of ferroelectrics is the stability of spontaneous electrical polarization, which is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories and processing the information with incredible speed. In addition, ferroelectrics have higher values of dielectric constant as compared to other dielectrics and possibility to tune the polarization by the applied stress, the so-called piezoelectricity effect. It is worth to note that ferroelectric properties depend strongly on theirs structure, so scientists are not only developing new ferroelectric compounds, but also seeking for new structuring of materials, which allow precise control over ferroelectrics engineering, hence their properties. Modern nanotechnologies are vital here. Moving from bulk material to ultrathin films and assembling them into the periodic sandwiched structure of superlattices, one can expect not only the improuing of the existing properties, but also the emergence of the qualitatively new effects. Variation of the structural and synthesis parameters, like for instance the chemical composition of layers and their thickness allows the highest degree of tuning of the properties of these emergent artificial materials. However, the most exciting option is to control the superlattice functional properties through the strains, engineered by the proper selection of either the substrate type or the lattice mismatch of the composition layers.

Figure 1. Conceptual view of two- and three-layered superlattices deposited on MgO substrate and composed from Ba1-xSrxTiO3 (BST) and BaTiO3 (BT) (left), and Ba1-xSrxTiO3 (BST), BaTiO3 (BT) and SrTiO3 (ST) (right) correspondingly.

Figure 2. The unit cell of typical ferroelectric material, perovskite BaTiO3 compound. The grey sphere in the center depicts the Titanium atom, the yellow spheres at the corners correspond to the Barium atoms and the blue spheres to the Oxygen atoms.

Methods
        To investigate the properties of ferroelectrics we need the information about their structure and lattice dynamics. The x-ray diffraction provides us with structural information, whereas the Raman scattering elucidates the features of the lattice dynamics. Using x-ray diffraction, we can get the overall structural information of the investigated sample. We use the different geometries to capture all the details about the unit cells of the layers forming the superlattice and the structural quality of the whole object. In particularly, we use the so-called ω – 2θ geometry to discover the parameters of the deformed unit cells. The main challenge here is that the superlattice diffraction pattern is the superposition of patterns of individual layers. This makes it difficult to find out these parameters directly from the diffraction pattern. To discover the structure of the unit cells we need to calculate the theoretical diffraction pattern and then to fit it to the experimental one. The closer parameters used for theoretical pattern calculation to the real values the better is the fit. Such consideration also provides us with the information about the values of microstrains in the samples. Having the values of structural parameters, we can conclude about the stress between the individual layers.

Figure 3. Two-dimensional reciprocal maps of the BST thin film deposited on the (001)MgO substrate: 0KL plane (a) and H0L plane (b). The grid lines show the reciprocal lattice of the film. The reciprocal space maps illustrate the mono-oriented nature of the BST film. (c) The linear cross-section of 0KL plane as shown by the yellow dashed line. (d) The linear cross-section of H0L plane shown by the yellow dashed line. The vertical axes in the (c) an (d) panels correspond to d-space calculated values of interplanar spacings.

       High mechanical stresses and, hence, significant forced strains can bring about the formation of principally new phase states in ferroelectric superlattices, affect spontaneous polarization and permittivity, as well as substantially change the Curie temperature. Distortions of a crystal lattice appeared during the displacement of ions lead not only to changes in the structural parameters determined by X-ray diffraction but also to changes in the phonon spectrum of a crystal. Some lattice vibrations, in particular, the ferroelectric soft mode observed in the Raman spectra, are usually very sensitive to displacements of ions, which cause a change in the structure during the phase transition. The study of the behavior of soft modes is the main source of information on microscopic mechanisms of phase transitions in ferroelectrics, and the frequency of the soft mode is related to the static permittivity.
        Study of the temperature dependent Raman spectra allows us to find the temperature, where the soft mode vanishes, hence the phase transition temperature. It is of the great importance for industrial applications to shift such temperature as higher as possible, to have the stable functional ferroelectric properties at room temperatures and even higher. The second useful feature that can be improved after the Raman spectroscopy characterization is coming here. It was shown that in the superlattice with the higher temperature of phase transition the frequency of the soft mode is greater as compared to the sample in which phase transition happens at lower temperature. That’s because of two-dimensional stresses in one of the layers of superlattice is imposed by the alternating layers. Such stresses reduce the mobility of atoms in crystal lattice and phase transition occurs at higher temperatures.


Figure 4. Correspondance of crystal lattices in superlattices to their raman spectra. (Left) Nearly perfect match of layers in the two-layered BT/BST superlattice, the soft mode frequency (blue circles) is situated at about 75 cm-1. (Right). The layers of BT in the three-layered BT/BST/ST superlattice are compressed by surrounding layers, so they are stretched in the direction normal to the plane of substrate. Soft mode in the three-layered superlattice is at approximately 95 cm-1.
Devices and applications
       Among different possible applications of ferroelectric superlattices the most exciting is the possibility of their integration in the future information-processing circuits. Further scalability of modern semiconductor transistor is limited by the so-called thermodynamic Boltzmann tyranny, leading to the self-destructive heating of the ultra-small switchable transistors, that is difficult to overcome. The problem can be solved by replacing the gate-separating dielectric material by the ferroelectric one. Recent studies have shown that ferroelectric materials possess the negative capacitance. This effect reveals the lowering of the charge with the applied voltage. It is supposed that the use of ferroelectrics in transistor will lead to the reduction of the power consumption of the processing unit over the order of magnitude. However, the possible application of ferroelectrics is not limited to the processing purposes only. They are also useful for the information storage. Memory devices based on ferroelectrics are under the extensive research, because of theirs outstanding characteristics. Ferroelectric memory cells already presented on the market are much faster than their semiconductor alternatives and also demonstrate the great read/write cycles endurance. But the most intriguing feature of the ferroelectrics is the non-volatility of polarization switching. The data written in such memory as the spontaneous polarization units can be stored for a long period of time, hence the ferroelectric memory can operate as random access memory and data storage cell simultaneously.