Ferroelectrics LiNbO

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

New promising technologies for future electron-beam lithography applications based on pyroelectrically induced electron emission from LiNbOs ferroelectrics [22] were recently proposed [23], The developed system possessing micrometer scale resolution used 1 1 electron beam projection. The needed electron pattern was obtained by means of deposited micrometer-size Ti-spots on the polar face of LiNbOs. Another solution for the high resolution electron lithography may be found in nanodomain patterning of a ferroelectric template. 

Another method presented in this paper is the indirect eb method when the C -lace of a LiNbOs ferroelectric is preliminary coated by a highly defective layer of the amorphous photo-resist material pmma. The thickness of this dielectric layer is large enough to protect the LiNb03 from penetration of high energy electrons into the bulk. In the presented calculations and simulation a very limited number of electrons penetrated into the LiNbOs crystal, so most of the injected electron charge remains trapped in the pmma layer. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Interest in film growth of niobium oxides is driven by the importance of several ferroelectric materials that contain niobium. The most widely used precursor is Nb(OEt)s, which is a volatile liquid. Examples of materials that have been grown by MOCVD with Nb(OEt)s include LiNbOs, Pb-Ti-Nb-0, and KNbOs. Owing to the extreme reactivity of Nb(OEt)s with water, attempts have been made to identify precursors that are easier to handle. Examples of such precursors include Nb (tmhd) Oi Pr) 4 Nb (N-alkoxo-/3 -ketoiminato) (OEt) 3... 

When single-crystal substrates with a small lattice mismatch are used, sol-gel produces epitaxial films for a few ferroelectric systems. Although epitaxial growth of crystalline films from an amorphous layer has been observed in the amorphous silicon to silicon transformation, sol-gel epitaxy only began to emerge as a possible fabrication technique in the last few years. Hirano and Kato were the first to observe the epitaxial growth of LiNbOs on the sapphire (110) face [37]. Xu et al. [34,43] found the epitaxial growth of LiNbOs on the LiTaOs (110) face and the LiNbOa (006) face. Epitaxial KNbOs was reported... 

Amorphous LiNbOs films made by sol-gel processing were subjected to a series of characterizations [57]. It was found that an amorphous LiNbOs film obtained by heating the gel film at 100°C for 2 h showed P-E hysteresis with remnant polarization Pr = 10 pC/cm2 and coercive field Ec= 110 kV/cm. Electron diffraction of such film showed a diffuse ring pattern characteristic of an amorphous nature. These are shown in Fig. 6 in which the scale for E is 147 kV/cm division and that for P is 5.6 pC/cm2 division. Further measurement showed a pyroelectric coefficient of 8 pC/cm2 K at 28°C. Note that for singlecrystal LiNbOa, Pr = 50 pC/cm2 and the pyroelectric coefficient was reported to be 20 pC/cm2 K [1]. Further, a piezoelectric resonance was observed at similar frequency range for both amorphous and crystalline LiNbOa, characteristic of a ferroelectric material [57]. 

Ferroelectric lithium niobate (LiNbOs) has been of considerable interest because of its nonlinear optical properties. Conversion of infrared into visible radiation in LiNb03 crystals has been observed (Midwinter and Warner, 1967 Arutyunyan and Mkrtchyan, 1975). Electro-optic coefficients of LiNbOs have been determined for a wide range of frequencies ranging from the visible (Smakula and Claspy, 1967) to the millimeter-wave portion of the spectrum (Vinogradov et al., 1970). Other nonlinear optical properties such as photovoltaic effects (Kratzig and Kurz, 1977) and optically induced refractive index changes (Ashkin et al., 1966 Chen, 1969) have also been observed. 

Table 31.7 lists some other ferroelectric ceramics, although it does not include the large number of solid-solution phases that are ferroelectric. Many ferroelectric ceramics have a perovskite structure above 0c, but this is not a prerequisite. For example, LiNbOs has an ilmenite (FeTiOs) structure and Cd2Nb207 has a pyrochlore structure (the mineral pyrochlore is CaNaNb20eF). 

Lobov, A.I., Shur, V., Baturin, I.S., Shishkin, E.I, Kuznetsov, D.K., Shur, A.G., Bolbilov, M S., and Gallo, K. 2007. Field induced evolution of regular and random 2D domain structures and shape of isolated domains in LiNbOs and LiTaOs. Ferroelectrics, 341 1, 109-116. 

The nonlinear properties of FLCs attracted considerable attention both from the fundamental and technical points of view [132-134]. Since, in ferroelectric phases the center of inversion is absent (Fig. 7.1), the amplitude of the optical second harmonic generation (SHG) ought to be fairly strong, especially in commercially available FLC mixtures with high spontaneous polarization [133]. Reference [134] shows the ways of designing new FLC substances with the increased molecular second-order hyperpolarizability comparable to that of solid electrooptical crystals such as LINbOs. 

Since the electro-optic tensor has the same symmetry as the tensor of the inverse piezoelectric effect, the linear electro-optic (Pockels) effect is confined to the symmetry groups in which piezoelectricity occurs (see Table 8.3). The electro-optic coefficients of most dielectric materials are small (of the order of 10 m V ), with the notable exception of ferroelectrics such as potassium dihydrogen phosphate (KDP KH2PO4), lithium niobate (liNbOs), lithium tantalate (LiTaOs), barium sodium niobate (Ba2NaNb50i5), or strontium barium niobate (Sro.75Bao.25Nb206) (Zheludev, 1990). For example, the tensorial matrix of KDP with symmetry group 42m has the form... 

Hirano S., Kato K. Processing of crystalline LiNbOs films with preferred orientation through an organometallic route. Solid State Ionics 1989 32/33 765-770 Hirano S., Yogo T., Kikuta K., Kato K., Sakamoto W., Ogasawara S. Sol-gel processing and characterization of ferroelectric films. Ceram. Trans. 1991 25 19-32... 

Figure 22-9. Ferroelectric P-E hysteresis loops ofLiNbOs thin films (170 nm). (a) Symmetric ferroelectric hysteresis loop of the Pt-eletrode/LiNbOs film/Au-electrode sample, having of 60 pbC/cm and Ecof23 k V/mm (Cheng etal., 1991) (b) Au-electrode/LiNbOs film/n-Si substrate sample, with the scale of X-axis is 27 kV/cm/div. And Y-axis 3.0 fxC/cm /div. (Xu and Mackenzie, 1992, with permission).

In Sol-Gel Optics, vol. I, J.D. Mackenzie D.R. Ulrich, eds. Proc. SPIE 1990 1328 450 55 Cheng Chih-Hsing, Xu Yuhuan, Cherry H.B., Tseng J., Um G., Mackenzie J.D. Piezoelectric properties of micro-machined cantilever PLZT thin films. Ferroelectrics 1999 232 159-164 Cheng Chih-Hsing, Xu Yuhuan, Mackenzie J.D. Photoelectric properties ofun-doped and Fe-doped LiNbOs films made by sol-gel method. Mater. Res. Soc. Symp. Proc. 1991 243 65 Cross L.E. Ferroelectrics 1987 76 241... 

Cheng W., Baudrin E., Dunn B., Zink J.I. Synthesis and electrochromic properties of mesoporons tungsten oxide. J. Mater. Chem. 2001 11 92-97 Cheng S.D., Kam C.H., Zhou Y., Lam Y.L., Chan Y.C., Sun Z., Gan W.S., Pita K. The microstructure dependence on processing temperature in sol-gel derived thin ferroelectric films of LiNbOs on Si02/Si substrate. Ferroelectrics 1999 231(1-4) 805-810... 

Clearly, ferroelectric and nonferroelectric pyroelectrics are possible, and the pyroelectric coefficient varies widely between the two groups. The pyroelectric coefficients for ferroelectrics such as BaTiOs, LiNbOs, and LiTaOs are —200, —83, and —176 pC/m /K, respectively, whereas for nonferroelectrics such as ZnO, tourmaline, and CdS the corresponding values are —9.4, —4.0, and —4.0 pC/m /K, respectively. ... 

In addition to reflective, colored, and AR coatings, more recent optical applications include contrast enhancement filters [15,21], cold mirrors [15-21], patternable thick films for diffraction gratings [41] and optical memory disks [41,42], and ferroelectric films [38-40,58-67] (PLZT, KNbOj, or LiNbOs) for optoelectronic and integrated optics applications [38-40],... 

Another class of ferroelectrics is the perovskites such as the titanates (BaTiOs, PbTiOs, SrTiOs, CaTiOs), the niobates (KNbOs, NaNbOs), the ilmentites, (LiNbOs, LiTaOs)), the ternary lead zirconate titanate (PZT), and the quaternary lead lanthanum zirconate titanate (PLZT). These materials undergo a displacive-type phase transformation at their Curie temperature. 

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