The Origin of the Improvement of Dielectric Property in the Modulated Structures: Electronic Structure Study

Abstract

Some group of crystals possess the unusual characteristics of being permanently polarized within a given temperature range. Unlike the general piezoelectric classes, which produce a polarization under stress, the pyroelectrics develop this polarization spontaneously and form permanent dipoles in the structure. This polarization also changes with temperatureshence the term pyroelectricity comes. Pyroelectric crystals such as tourmaline and wurtzite are often called polar materials, referring to the unique polar axis existing within the lattice. The length of the polar axis (dipole moment) varies with temperature, changing sign as the temperature is either elevated or lowered. A subgroup of the spontaneously polarized pyroelectrics is a special category of materials known as ferroelectrics. Materials in this group are characterized as crystals that possess a spontaneous dipole, and this dipole is reversible by an electric field of some magnitude less than the electric breakdown of the material itself. Because of the empirical nature of determining the reversibility of the dipoles, as detected in a hysteresis loop measurement, one cannot predict the existence of ferroelectricity in a new material with much accuracy. We do see, however, that the basis for the existence of ferroelectricity rests primarily on structural (symmetrical) considerations. Generally, the dielectric constants of ferroelectric materials are hundreds times bigger than those of normal dielectric compounds. It is necessary to possess stronger dielectric properties when a material is applied for the phase shifters, tunable filters, capacitors, non-volatile ferroelectric random access memory (FeRAM) devices, dynamic random access memory (DRAM) devices and steerable antennas. Many scientists, therefore, have tried to increase the dielectric property of ferroelectric materials by substituting the preexisting metals with various transition metals, and/or by changing the synthetic processes. Several compounds, for example, lead zirconate titanates [Pb(Ti1-xZrx)O3, PZT] and lead lanthanum zirconate titanates [(Pb1-yLay)(Ti1-xZrx)O3, PLZT] have been successfully developed through the substitutional method. Both compounds exhibit better dielectric property than the intrinsic PbTiO3 does. In addition, a flood of research have been focused on the syntheses of new materials which are exhibiting high dielectric constants. Aurivillius phase compounds such as SrBi2Ta2O9(SBT) and SrBi2Nb2O9(SBN) are the typical examples. Differently from the substituted solution form mentioned above, the modulated structure form (see Figure 1) with two kinds of classical ferroelectric compounds in film type can be introduced. Modulated structure framework is an artificial crystal which is designed to deposit the alternating layers with more than two different materials. A ferroelectric perovskite is deposited onto a substrate first time and then the second compound is deposited onto the surface of the first one, and again the first one onto the surface of the second one, and so on. The modulated structure film is known to exhibit better physical properties compared to the individual materials. For example, ferroelectric modulated structure which is made of BaTiO3 and SrTiO3 shows higher voltage tunability and dielectric properties than BaTiO3 only or SrTiO3 only. Dielectric property of a material depends on the amount of structural distortions in the compounds. Withers and coworkers have suggested that the major cause of spontaneous polarization in the two-layer of ABO3 materials is the displacement of the A site cation in the perovskite block along the a-direction in the polar space group A21am. In addition, the distorted O-B-O chains which construct perovskite layers, are thought to be the origin of the polarization in the ABO3 perovskites. So the higher polarization is expected when the structural distortion is stronger. In the deposition of yttrium barium copper oxide (YBCO) films onto MgO substrates, it has been observed that the