Abstract In recent years ferroelectric memory devices have attracted much attention from the viewpoint of the next generation of highly integrated circuits. Research and development in dynamic random access memory (DRAM) using high dielectric constant films are extensive. However, DRAM is volatile memory, and it is desirable that nonvolatile memory should be developed. There are mainly two kinds of ferroelectric nonvolatile memories: a memory cell using a ferroelectric storage capacitor, and metal-ferroelectric-(insulator)-semiconductor FET (MFISFET). Especially, the latter is superior among memory devices since the memory is read out nondestructively. In practice, however, there are many challenges which have held back the progress in that direction, a major one being the difficulty of making an electrically switchable ferroelectric thin film on Si with good interface properties and long retention time. To overcome these problems, buffer layers are usually inserted between the ferroelectric layer and silicon substrate. The electrical properties of the MFIS memories with stacked gate configuration of ferroelectric Pt/SrBi2Ta2O9/Si3N4/p-Si (100) were investigated. 245nm-thick SBT thin films were spin-coated on the Si3N4/Si substrate followed by 1 min. rapid thermal crystallization annealing at the temperatures regime of 700~800¢J. In an attempt to operate memory at low voltage with sufficient large memory window, various ultra thin Si3N4 buffer layers in thickness of 3.5, 2, and 0.9nm were employed. The Si3N4 buffer layers were deposited by means of LPCVD with the exception of surface nitridation for 0.9nm SixNy thin film. From the results of C-V measurements, the memory window can be as large as 0.8V at the bias amplitude of 5 V for the sample with 0.9 nm SixNy buffer layer and 750¢J annealing temperature. Complete perovskite crystalline structure can also be affirmed by the spectra of X-ray diffraction measurements. The leakage current, which plays a very important role in the data retention, of Pt/SBT (245nm)/ Si3N4 (0.9nm)/p-Si (100) is as low as 2.5 x 10-8 A/cm2 at 200kV/cm. The 1010 write cycles and greater than 2hr retention time can be achieved. Optimization and scaling of SBT thin films are believed to be effective in pursuing extremely low voltage operation, high-density and liable 1T nonvolatile ferroelectric random access memories.

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In this study, the reactive rf magnetron sputtering was used to deposit Ba(Ti0.9,Zr0.1)O3 (BZT) ferroelectric thin films on Pt/SiO2/Si and SiO2/Si substrates, and MFMIS and MFIS structures are fabricated. The effects of various sputtering parameters on the characteristics of thin films, such as the oxygen concentrations, rf power and deposition time are discussed. The physical characteristics of BZT thin films were obtained by the XRD pattern and AFM morphology. The variations of crystallization and surface roughness of thin films were discussed. The electrical properties of BZT thin films deposited under various sputtering parameters are measured by the HP4284A and HP4156C. From the experimental results obtained, the optimal dielectric constant and leakage current density were 197 and 1.41¡Ñ10-7A/cm2, respectively, under the applied electrical field of 0.5 MV/cm. In addition, the coercive field and remanent polarization were 30 kV/cm and 7 £gC/cm2 from the P-E curves, respectively. In addition, the electrical characteristics of MFMIS and MFIS structures are discussed. Besides, the memory window and leakage current density of thin films deposited at various sputtering parameters on MFIS structure are also discussed.

Switchable polar properties of ferroelectric and multiferroic nanostructures are ideal to further diversify applications of mainstream semiconductors. Recent breakthroughs in Scanning Probe Microscopy (SPM) have enabled tailoring of polar domain structures at the nanoscale, which is critical to fabricate polarization-based devices. However, highly inhomogeneous electric fields of biased SPM-tips complicate polarization physics in ferroelectrics and multiferroics. Also, typical diffused phase transition in relaxor bulks originates from coupled inhomogeneities of intrinsic polar nanoregions (PNRs). In this thesis, anisotropic and time-dependent mechanisms were developed to study SPM-tip poled polarization switching in ferroelectric and multiferroic thinfilms. Moreover, frequency-related PNR thermodynamics and its effect on electrocaloric effect of locally disordered relaxors were modeled. Firstly, a three dimensional model was established to clarify tip-poling effect on ferroelectric domain nucleation and growth. The concept of “domain shape invariance” was confirmed through constant aspect ratio obtained for conic ferroelectric nucleus. This domain aspect ratio was found to abruptly decrease under the depolarization effect, saturating domain radius. Further increasing tipvoltage could drive longitudinal breakdown of already reverted domains throughout film thickness. Subsequently, tip-activated evolution of domain wall width in ferroelectric and multiferroic thinfilms was studied via extended Kittle’s law, which included anisotropic and dynamic effects arising from tip-fields. Our calculation results showed that wall width in LiNbO3 varied slightly in an initial stage, followed by a drastic change. This wall variation corresponded to three varying regions of coercive field. Besides, we highlighted three polarization switching modes in BaTiO3 - absence, activation and nonactivation mode. Importantly, distinct switching modes, i.e., breakdown mode of 71° domain switching and activation mode of 180°/109° switching, were revealed to fundamentally control filmorientation dependent multipolarization switching sequence in BiFeO3. Thirdly, Pauli’s mater theory was utilized to bridge microscopic evolution of PNRs and characteristic properties of Pb(Mg1/3Nb2/3)O3 (PMN) relaxors. Temperature dispersion and frequency dependence of PMN dielectric susceptibility were related to nonlinear PNR dynamics over a broad temperature interval. We could not validate PNR-volume predictions of percolation theory above the freezing temperature, but suggest a gradual saturation of PNR volume at lower temperatures. Besides, observed deviations of relaxor permittivity from the Curie-Weiss law were attributed to thermal effects on PNR dynamics and resultant polarization rotations. Furthermore, time-dependent PNR dynamics was proposed to study strong frequency dependence of typical relaxor behaviors. It was implied that frequency effect on PNR coercive field was governed by classic Merz’s-switching, leading to suitability of Vogel-Fulcher law for relaxors bulks. Last but not least, above-mentioned framework for PMN relaxors was incorporated with Landau-Ginzburg-Devonshire thermodynamics and Maxwell relation to better understand recently observed giant electrocaloric (EC) effect of relaxor thinfilms, which is promising for solid-state refrigeration. Three contributions were found to dominate relaxor EC response: temperature-dependent dielectric dispersion, inverse pyroelectric effect and thermally enhanced dielectric stiffness. We emphasized that the EC material with larger dielectric stiffness and smaller correlation length could extend its enormous EC response above Curie temperature. Finally, potential approaches, e.g., by manipulating shape, volume and density of PNRs, were suggested to engineer the EC enhancement in relaxor nanostructures. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

Thesis (M.S.)--Oregon State University, 2004. / Printout. Includes bibliographical references (leaves 90-94). Also available on the World Wide Web.

No description available.

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 167-180).

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 83-85). Also available via the Internet.

The advent of ferroelectric thin films with strong piezoelectric response has enabled the development of new nano- and micro-electromechanical systems (NEMS/MEMS) capable of large displacements at low voltage levels, aiming to be compatible with complementary metal oxide semiconductor industry. Key to all of these applications is the ability to process ferroelectric materials with maximized electromechanical coupling and to integrate them into the devices. With the continuous drive towards miniaturization of devices for piezoelectric and electronic applications, processing of ultrathin ferroelectric films with maintained large electromechanical coupling is essential to the development of high performance NEMS and MEMS. The piezoelectric response of ferroelectric thin films is profoundly affected by the texture and microstructural characteristics of the material and is severely reduced at sub-micron thickness ranges. For the first time, reproducible synthesis of dense, highly textured and phase-pure PZT thin films was achieved via chemical solution deposition. The consistent processing of ferroelectric thin films resulted in the elimination of the coupling effects of crystallographic anisotropy, porosity and in general microstructural characteristics on the functional properties of the films. This enabled effective study of the key parameters influencing the electromechanical response of the ferroelectric thin films, such as crystallite size (thickness dependence), chemical heterogeneities and substrate clamping. Reproducible synthesis of highly (100)-textured PZT ultrathin films enabled the study of the size effects on the dielectric and piezoelectric response of these films in the thicknesses ranging from 20 up to 260nm. Dielectric and piezoelectric responses of the films monotonically decreased in thinner films. For PZT films at MPB, a critical thickness, ~50nm was observed below which the extrinsic contributions to the dielectric responses of the films are heavily suppressed. After the study and acknowledgment of the severe reduction of the piezoelectric response in ferroelectric ultrathin film, several factors affecting piezoelectric response of ferroelectric films were studied in order to maximize the response especially at low film thickness ranges: chemical homogeneity, residual stresses and substrate clamping as well as using alternative material systems; relaxor ferroelectrics. In particular, a major part of the piezoelectric (and dielectric) response of the PZT has extrinsic sources such as domain or phase boundary motion and vibrations. Special attention was paid throughout this investigation into understanding extrinsic origins in PZT thin films and different approaches was utilized to further activate and enhance their contributions. Focusing on the chemical homogeneity of the ferroelectric films, Different routes were used to process ultrathin films (<200nm) with maintained functional properties. Superior piezoelectric properties - 40% higher piezoelectric response than in conventionally processed films - were achieved in highly (100)-oriented PZT superlattice-like films with controlled compositional gradient centered around MPB composition on Si substrates. Superlattice (SL) or heterolayered ferroelectric thin films consist of alternate layers of ferroelectric materials, or phases, with a compositional gradient normal to the substrate. The dynamic motion of “artificially created” phase boundaries between layer to layer tetragonal and rhombohedral phases participated in the extrinsic contributions to the films’ dielectric and piezoelectric response. This approach led to processing of 200 nm SL films with d33,f values as high as some of the best previously reported data for 1 to 2 µm-thick PZT films. Furthermore, comprehensive processing optimization was carried out on relaxor-ferroelectric PMN-PT thin films. Dense, highly (100)-textured PMN-PT films were synthesized exhibiting the highest d33,f coefficients reported so far in the literature (210pm/V) for corresponding thickness ranges. Control of the microstructural characteristics - texture and density – throughout the whole film thickness was necessary to obtain films with maximized functional properties. To study the effect of substrate clamping on the piezoelectric performance of the films, the Si substrate in PZT and PMN-PT films were back-side etched via dry etching in an inductively coupled plasma reactor. This approach is similar the final state of the films for MEMS applications, where the Si substrate is mostly removed in order to have a free-standing or semi-free standing ferroelectric membrane or cantilever. A giant enhancement in the piezoelectric d33,f coefficient of the substrate-released samples was observed with respect to the films on the virgin substrate. The response increased by at least one order of magnitude from ~75-200 pm/V (for different PZT film thicknesses ranging from 300nm to 1 µm) to ~1500 to 4500 pm/V at reduced Si thickness. Experimental observations in macroscopic dielectric and piezoelectric characterization and microscopic piezo-response force microscopy of the samples indicate larger extrinsic contributions, -possibly with domain dynamic source- to the functional responses of the films in back-side etched samples. A fundamental change in the pattern of the electromechanical activity of the grains between the released and clamped films was observed in the band-excitation piezo-force microscopy investigations; A breakdown of the clustered pattern in the electromechanical activity of the grains in the PZT film. This giant enhancement promises a new pathway for greatly improved electromechanical properties which has a huge potential to enable high performance future device applications.