Ferroelectric thin film materials

Since the early suggestion that ferroelectric thin film materials could be the high dielectric layer in the capacitor of the ultra large scale integrated dynamic random access memory devices (ULSI DRAMS) made by Parker and Tasch, there has been a great deal of research effort to deposit multi-component ferroelectric oxide thin films as well as more recent industrial activity. The term ferroelectric indicates the property of certain materials that have remnant... 

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. 

The following discussion separates pyroelectric materials into 3 groups intrinsic pyroelectrics which are operated well below Tc, dielectric bolometer materials which are operated close to Tc, but with an electrical bias applied and ferroelectric thin films. 

Ferroelectric thin films considerably gain in interest within the last couple of years due to their potential application in nonvolatile random-accessmemory devices (FeRAM). Among potential candidates, PbZr. n i, (>> (pzt) is one of the most promising materials because of its large remanent polarization and low coercive field. However, pzt is also well known for its poor fatigue behavior on metal electrodes [1,2] and occurrence of size effects [3-5] which are well due to the ferroelectric/electrode interface properties [1-5]. 

Another general class of solids that has been prepared as thin films using PLD is ferroelectric materials. A potentially useful characteristic of ferroelectric materials is that they can be polarized by an electric field and retain this polarization when the field is removed. In ferroelectric thin films it may be possible to exploit this polarization phenomenon to make sensors, displays, and memory devices. A number of techniques have been used to prepare ferroelectric thin films. However, it has been difficult to control the stoichiometry (and correspondingly the properties) of these materials using thermal and sputtering techniques. In part, the difficulty in maintaining correct stoichiometry is due to the volatility of a component in the material (e.g. Pb in PbTiOs). 

Ultraviolet Raman spectroscopy has emerged as a powerful technique for characterization of nanoscale materials, in particular, wide-bandgap semiconductors and dielectrics. The advantages of ultraviolet excitation for Raman measurements of ferroelectric thin films and heterostructures, such as reduced penetration depth and enhanced scattering intensity, are discussed. Recent results of application of ultraviolet Raman spectroscopy for studies of the lattice dynamics and phase transitions in nanoscale ferroelectric structures, such as superlattices based on BaTiOs, SrTiOs, and CaTiOs, as well as ultrathin films of BaTiOs and SrTi03 are reviewed. 

A number of factors can influence the behavior of ferroelectric thin films and multilayer stmctures with layer thickness at nanometer scale. One of the major factors is strain in epitaxial structures [15]. Recent demonstrations of huge strain effect on ferroelectric properties include changes in the phase diagram [16-22], dramatic enhancement of ferroelectric polarization, and increase of the ferroelectric phase transition temperature [23-27], induced ferroelectricity in non-ferroelectric materials like SrTi03 or KTa03 [28-33], or even simple rocksalt binary oxides like BaO ([34], theoretically predicted). 

A broad range of electronic ceramic materials have been prepared by CSD, but three material systems have dominated the field of ferroelectric thin films. These include the perovskites PbZr03-PbTi03 (lead zirconate titanate PZT), BaTi03-SrTi03 (barimn strontium titanate BST), and the layered perovskite SrBi2Ta209 (strontium bismuth tantalate SBT). The extensive solid solubility ranges... 

A complete review of the reported properties of ferroelectric thin films prepared by CSD is beyond the scope of this chapter. Suffice it to say that fabrication approaches from each of the three CSD categories noted above have been used to prepare high-quality films in a range of thicknesses. The dielectric response and ferroelectric hysteresis behavior have been widely reported and the reader is referred to References 12 and 13 for representative results. Despite space limitations, three aspects of CSD processing and film properties warrant consideration here. These are (i) the ability to prepare oriented films by CSD (ii) typical stress levels within the films and (iii) the general dielectric properties of the thin film materials compared to the corresponding bulk materials. 

As ferroelectric material we use poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). This copolymer is soluble in non-toxic reagents, for example 2-butanone. The preparation of organic and ferroelectric thin films via spin coating from solution is possible [13]. The polarisation field of P(VDF-TrFE) is relatively high, about 50 MV/m [14]. Here, a downscaling of the P(VDF-TrFE) film thickness into a range below 100 nm is necessary in order to use small bias voltages for polarisation. 

Xu. Y. H. Ferroelectric Materials and Their Applications, North-Holland, Amsterdam, 1991. Ferroelectric Thin Films (E. R. Myers and A. Kingon, eds.). Materials Research Society, Pittsburgh, (1990). 

The Materials Research Society (MRS) has offered a number of symposia under the title Ferroelectric Thin Films. 

Electrostatic Force Microscopy (EFM) allows to obtain information on the surface electrical properties of materials by measuring electric forces between a charged tip and the surface. It is particularly suitable for the study and manipulation of ferroelectric thin films with large surface charge. Interestingly, an EFM can also be used to study the surface properties of dielectric materials, that are polarized by the electric field of the tip. In this mode of operation, the EFM is sometimes called Polarization Force Microscope and can be used to study and image even air-liquid interfaces [64]. 

We have elaborated a simple model to explain this situation [83] [84]. Suppose we have a ferroelectric thin film of thickness I and d is the distance between the tip and the sample, S is the surface area of the tip front end, and U is the applied voltage between the tip and the sample. To model our system, we assume that the tip is covered with a thin layer of silver and that all the electric connections in our system are done through aluminium contacts. P5 is the spontaneous polarization of the material, <7 is the surface... 

Setter N, Damjanovic D, Eng L, Fox G, Gevorgian S, Hong S, Kingon A, Kohlstedt H, ParkN, Stephenson G et al (2006) Ferroelectric thin films review of materials, properties, and applications. J Appl Phys 100(5) 51606-51606... 

Pb(ZrxTii x)03 (PZT) ferroelectric thin films are studied for their high dielectric constant and ferroelectric properties (e.g. high remanent polarization). Material is an excellent material in bulk ceramic applications. The dielectric constant lOSSeo at 1 MHz frequency was reported for poly crystalline PZT (x=0.50) thin film, remanent polarizationPr = 21.5 x 10 Ccm and coercive field of 3.9 kVmm ... 

AIN non-ferroelectric thin films are studied for their potential use as pressure transducers, speakers and SAW devices (Turner et al. 1994). Polyciystalhne AIN is an important substrate ceramic material due to its high thermal conductivity and dielectric breakdown strength however it does not exhibit piezoelectric activity in the bulk form. When properly oriented, AIN film shows piezoelectric activity up to the high temperatures. For some of the material coefficients of AIN (and also ZnO) films see Gualtieri et al. (1994). 

From an applications standpoint, measurement of other properties, namely dielectric constant and dielectric loss, again measured using an impedance analyzer, and Tc, which may be determined from k versus T measurements or by temperature dependent X-ray diffraction, allows for calculation of material and device figures of merit. These parameters are important in determining the suitability of a material for a particular application. Recently, pyroelectric coefficients and device performance figure of merit have also been determined for solution-derived ferroelectric thin films. ... 

The successful development of these thin films for device applications requires that two goals be met (1) the preparation of materials with device quality characteristics and (2) for certain applications, successful integration of the thin film with underlying silicon circuitry, without degradation of circuitry performance characteristics. A number of analytical characterization techniques have been employed to study film preparation and thin film—device integration issues. Some of these techniques and their applicability in characterizing ferroelectric thin film device preparation will be briefly discussed. 

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