PLZT films

PLZT in thin and thick film form can be exploited for a variety of applications including optical signal switching and modulation. An attraction of PLZT is its high electro-optic coefficient and in consequence low drive voltages to perform a particular function (c.f Table 8.1). The PLZT thin film switch described below is included to illustrate principles rather than because of very significant commercial importance. 

There are various thin-film optical switch principles based on interference, diffraction or total internal reflection (TIR). The TIR switch, the essentials of which are shown in Fig. 8.18, depends on TIR of light passing from one medium to another of lower refractive index. TIR occurs for small differences in refractive indices when the angle of incidence approaches 90°, i.e. for light almost parallel to the surface of the material with the lower refractive index. 

A suitable substrate is sapphire with the PLZT film deposited on an (0001) face. Because of the smallness of the mismatch (about 2%) between the 0-0 distances (287 pm in an (0001) face for sapphire and approximately 280 pm in a (111) face for PLZT) the PLZT grows epitaxially, with its (111) face registering with the sapphire (0001) face (c plane). 

Typical dimensions of the sputtered PLZT film are as follows thickness t about 0.35 /an light guide step height th about 0.05/mi light guide step width iw 

It is readily shown that the critical angle 6C for TIR is given by 

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Modification by acetylacetone is a powerful route, that allows precursor solutions to be stabilized. Interaction of titanium alkoxides with acetylacetone was extensively studied and reviewed in [1391,86]. Study ofreactions, occurring on interaction of Zr(OPrn)4 and Ti-Zr alkoxide mixture with acetylacetone, was performed in [1448] and allowed the authors to simplify the technique for preparation of precursor solution for PZT films application and to overcome the requirement of prolonged refluxing, which certainly decreases reproducibility. After dissolution of titanium and zirconium alkoxides in methoxyethanol, acetylacetone is added to form stable zirconium and titanium stock solutions. The introduction of acetylacetone allowed aqueous lead acetate (and lanthanum acetate for PLZT films) solutions to be added to mixed titanium and zirconium solutions. No reaction steps involving elevated temperatures or distillation or long reaction times are required. The solution could be used both immediately on mixing or after storage for several months. Such solutions were successfully used for application of ferroelectric films. 

Thin PLZT films deposited by, for example, sputtering are too thin (< 1 /im) to achieve the necessary retardation for transverse mode optical devices. There is, however, potential for films having thicknesses in the range typically 2-25 /mi. If these can be successfully and economically produced then they offer potential for a variety of devices including optical shutters, modulators and displays. 

The films can be grown by various methods (see Section 3.6.9). G.H. Haertling and G.C. Robinson [10] describe the production of 6 /nn thick (9/65/35) PLZT films on sapphire substrates by an automated repeated dipping/firing process. The dipping solution consisted of the acetates of the metal ions, La, Zr, Ti and Pb. The films were the basis of a successful shutter and display based on the principles described above (see Sections 8.21 and 8.33). 

FIGURE 32.25 Schematic of an image storage device using a field-induced polarized PLZT film. 

Deineka, A., Ghnchuk, M., Jastrabik, L., Suchaneck, G., Gerittch, G. Influence of surface and interface on PLZT film optical properties. Phys. Status Solidi (a) 175,443-446 (1999)... 

Figure 24.6 (a) Linear increase in PLZT film thickness as a function of the number of layers, (b) Electrode yield (percentage of top electrodes that were not shorted) versus the number of layers for different concentrations (adapted from Ref. [35]). 

Solution deposition processing has been used to prepare thin films (qv) of PbTiOg, PZT, PLT, PLZT, BaTiO, LiNbOg, PMN, PMN-PT,... 

Ferroelectric Thin-Film Devices. Since 1989, the study of ferroelectric thin films has been an area of increasing growth. The compositions studied most extensively are in the PZT/PLZT family, although BaTiO, KNbO, and relaxor ferroelectric materials, such as PMN and PZN, have also been investigated. Solution deposition is the most frequentiy utilized fabrication process, because of the lower initial capital investment cost, ease of film fabrication, and the excellent dielectric and ferroelectric properties that result. 

Numerous uses for PZT/PLZT thin films are under investigation. The device that, as of this writing, is closest to commercialization is a nonvolatile memory. This device, which utilizes a ferroelectric thin-film capacitor integrated onto siUcon circuitry, provides memory retention when the power is off because of the polarization retention of the ferroelectric capacitor. One and zero memory states arise from the two polarization states, — and +F, of the ferroelectric. Because PZT is radiation-hard, the devices are also of interest for military and space appHcations. 

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

Haertling, G. H. 1991. PLZT thin films prepared from acetate precursors. Ferro. 116 51-63. 

Haertling, G. H. 1992. An acetate process for bulk and thin film PLZT. Proc. 7th Inti. Symp. Appl. Ferro. (Urbana June 6-8). pp. 292-295. 

The substrate on which thick films are grown also has to be chosen with care since it influences grain orientation in the film, and particularly mechanical stresses arising because of thermal expansion mismatch. To avoid film cracking the film has to be maintained in compression requiring the substrate thermal expansivity to be greater than that of the PLZT. 

Compile a list of the various methods for producing thin films of PLZT on a suitable substrate pointing out the merits or otherwise of each. 

YSZ film exhibited good in- and out-of-plane alignment with a FWHM from the (220) plane of 3.37° 0.01 and a FWHM from the (200) plane of 2.37° 0.01. Numerous other materials have also been deposited by CCVD as epitaxial films, for example, Ta O, PLZT, BST,YBCO,LSC,YSZ. 

A.T. Hunt, J. Abbey, and H.G. Hornis, PZT, PLZT, ESC and Pt Thin Films Produced in the Open Atmosphere Using Combustion Chemical Vapor Deposition. Presented at the International Symposium on Integrated Ferroelectrics, Colorado Springs, CO, 1995. 

Table 1 Common Precursors Used for PZT and PLZT Thin Films...

Pbi j[l-ajf(ZrzTii z)i jf/403 (PLZT) Thin-film optical switches... 

FIGURE 32.26 An example of a PLZT thin film optical switch on sapphire. 

Several methods have been used to produce PLZT thin films. Try to find as many methods as you can and discuss the pros and cons of each. 

Let us consider the system of electric dipoles and other defects randomly distributed in the film paraelectric phase. Similarly to the random field model for bulk relaxor ferroelectrics [83], this phase is called Burns reference phase. For example, the relaxor ferroelectric Pbo,92Lao,osZro,65Tio,3503 (PLZT) (where La ions are the main sources of random field) is known to have the Burns phase simply as the paraelectric phase of PbZro,65Tio,3s03 (PZT). Latter phase exists at T > Tj, Td is so-called Burns temperature and Td = 1), where Tc is transition temperature form paraelectric to ferroelectric phase in PZT. The indirect interaction of electric dipoles via soft phonon mode of a host crystal tends to order the system and so to generate the ferroelectric phase in it [84]. However, the direct interaction of dipoles and other defects like point charges, try to disorder a system, transforming it into relaxor ferroelectric. 

Shur, V. 1996. Switching kinetics in normal and relaxor ferroelectrics PZT thin films and PLZT ceramics. Proc. l(f ISAF, Piscataway, NJ IEEE, pp 233-240. 

FIGURE 7.1.27 Configuration of image memory and display system with PLZT ceramics. (T), PLZT ceramics transparent electrode photoconductive film (4), metal electrode d), lead terminal (e), image mask. 

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