STUDIES ON FERROELECTRIC AND SWITCHING PROPERTIES OF SODIUM NITRITE-POLYMER COMPOSITE FILMS

Abstract

Ferroelectric materials have attracted a great deal of interest due to their potentiE applications in microelectronics especially in nonvolatile memory devices. The ferrc electric devices are usually operated at low voltages including nonvolatile memorie: Generally, ferroelectric ceramics in film form are suitable for the microelectronic ar plications. There are various fabrication methods such as sputtering, pulse laser dept sition (PLD), metal organic chemical vapor deposition (MOCVD), chemical solutio deposition (CSD), metal organic decomposition (MOD) and molecular beam epitax (MBE) have developed to form ferroelectric films. These ferroelectric ceramics ca also be produced in film form by dispersing in polymer matrix. The ceramic-polymc composite materials are emerging as a new class of electronic and dielectric material: The work in the thesis has been focussed on the ferroelectric and switching propel ties of sodium nitrite (NaNO2) composite films. The composite films of NaNO2 hav been produced with different polymers such as poly(vinyl alcohol) (PVA) and pol (vinylidene fluoride) (PVF). NaNO2 is a well known order-disorder type ferroelectri material at room temperature. The structural, thermal and morphological studi( have also been included in the thesis. An attempt has been made to correlate th structural, the morphological and the thermal properties with the ferroelectric an switching behavior of NaNO2 composite films. The work presented in the thesis hE been divided into various chapters as given below. Chapter 1 revises some of significant concepts needed to understand the work an Abstract iv survey of literature on the chosen topic. The physical and the electrical properties of NaNO2 studied in the past have been included. The current trends in the field of ferroelectric composite materials and their potential applications in various devices particularly in memory devices has been given. The material properties of NaNO2 and polymers used in the present studies for fabrication of the composite films have been given. The scope of the present work and its motivation has also been presented in this chapter. Chapter 2 introduces the experimental details of the fabrication of pure and com-posite films of NaNO2 and of various studies carried out for their structural and ferroelectric characterization. The ferroelectric composite films have been fabricated by the solvent-cast, the spray-deposition and the hot press methods. The technical details and specifications of various instruments involved in the present studies have been given. The circuit details and the procedure adopted for measuring different electrical properties such as the P-E hysteresis loop, the polarization switching and the capacitance-voltage (C-V) have been discussed. Chapter 3 contains the structural, morphological and thermal properties of the pure and the composite films of NaNO2. The x-ray diffraction (XRD) experiments have been performed to confirm the structural formation of NaNO2 in the composite film at different doping levels, at different deposition temperatures and with different polymers. The XRD scans have been useful in quantifying the crystallite strain present in the NaNO2: PVA composite films with different compositions. The XRD scans of 50 wt.% composite film were also taken at higher elevated temperatures. The sharp fall in intensity of XRD peak of plane(1 0 1) at 160 °C confirms the ferroelectric phase transition in the composite films. The fourier transform infrared (FTIR) spectroscopy of the NaNO2: PVA composite films with different compositions shows presence of Abstract crystallite strain. The field emission scanning electron microscope (FESEM) and atomic force mi-croscope (AFM) have been employed to obtain the morphological information i.e grain size, shape and porosity. The surface morphology of pure phases and different composite films are scanned. The images of FESEM showed the nanosize and uniform distribution of NaNO2 particles in 50 wt.% composition. The clusters of NaNO2 grains and porosity were observed in the pure NaNO2 films. The AFM and FESEM images of spray deposited films suggest that PVA polymer facilitate the homogenous distribution of spherical grains of NaNO2 with less porosity. The differential scanning calorimetry (DSC) thermograms of pure polymers; NaNO2 and different composite films were observed. The peaks have been identified and phase transition enthalpies were measured. The changes in the DSC scans of the composites have been compared with the pure components. The reduced enthalpies in the composite films are correlated to the structural changes and morphology. This chapter contains full analysis and correlation of the structural, morphology and thermal studies. The ferroelectric properties of the pure and the composite films of NaNO2 are discussed in Chapter 4. The hysteresis loops of the composite films have been taken under variety of conditions using the modified Sawyer-Tower circuit. The optimum ferroelectric properties of the NaNO2: PVA composite films have been found in 50 wt.% composition. The frequency dependence of hysteresis loop characteristics has been studied in the frequency range of 10 Hz - 1 kHz. The enhanced value of Pr was observed in the composite film in the measured frequency range. The dependence of Pr on the number of reversal cycles was also studied in 50 wt.% composition and of pure NaNO2 films. The stability of Pr in the composite shows improvement over the pure Abstract vi NaNO2 film. The improved values of the polarization and stability have been attributed to the reduced strain in the composite film. The phase transition temperature obtained from the temperature dependence of Pr in 50 wt.% composite film is in good agreement with the DSC measurements. The hysteresis loop characteristics of spray deposited NaNO2: PVA composite films have been studied at different substrate temperatures (T3). The optimum value of Pr was found at T3 = 200 °C. The change in Pr with T3 has been attributed to the grain size and porosity of the films. The contribution of 180° and 90° domains are estimated from the back switching calculations. The minimum value of back switching {1-(Pr/Ps)} suggests that the contribution of 180° domains is more in the composite films deposited at T, = 200 °C. The fatigue characteristics were also studied in the composite and pure NaNO2 films deposited at T, = 200 °C. The optimum Pr and improved fatigue characteristics of spray deposited composite films at T, = 200 °C over the pure NaNO2 films may be due to large structural distortion and less porosity. The differential dielectric constant { 1/e0(dP/dE)} versus E curves exhibit two coercive fields corresponds to 180° and 90° domains. The hysteresis loop, fatigue and dynamic dielectric behavior of NaNO2: PVF composite films has also been studied and analyzed. The optimized ferroelectric properties are correlated to the structural, thermal and morphological changes with the composition and deposition temperature. Chapter 5 deals with the study of switching kinetics with an attempt to extract the microscopic parameters like the dimensionality, the velocity, the nucleation rate and rate-limiting parameters which give better understanding of the physical processes involved in the polarization switching of the pure and the composite films of NaNO2 by the application of theoretical models. Theoretical aspects related to the switching models are also discussed. The switching response in NaNO2: PVA nanocomposite Abstract vii films have been studied using bipolar square pulses as a function of composition of NaNO2. The switching current data fitted well to the infinite-grain model (IGM) in the region t < is and to the finite-grain model (FGM) in the region t > ts, where is is switching time. The polarization current and nucleation rate are optimum in 50 wt.% composite film and have been discussed in terms of grain size and strain variations with the composition. The effect of applied field on the switching behavior has also been studied. The exponential field dependence of the domain wall velocity and the nucleation rate indicate that nucleation limited mechanism is responsible for switching phenomena in the pure and the composite films of NaNO2. The activation field is found to be less in the composite film as compared to the pure NaNO2 film. The time dependence of polarization reversal current of the spray deposited NaNO2: PVA composite films has been investigated. The switching current transients have been analyzed by considering domain growth limited process with the Lorentzian distribution function of characteristic domain growth times based on nucleatiOn limited switching (NLS) model. This model gives excellent agreement with the experimen-tal polarization reversal transients throughout the whole time range. The switching parameters were determined in the composite films deposited at different substrate temperatures (Ts). The minimum value of the activation field at T, = 200 °C makes the polarization reversal more probable. The peak value of the polarization current exhibits exponential dependence on the external applied field. Chapter 6 contains the capacitance — voltage (C—V) characteristics of the composite films. The C—V curves of NaNO2: PVA nanocomposite films exhibit butterfly features and are attributed to the switching of polarization in the composite films. The effect of NaNO2 composition on C-V characteristics was also investigated. The switched charge estimated from the area under the curve as a function of composition gave Abstract viii the optimum value at 50 wt.% composition. The temperature dependence of C—V characteristics of optimized composite has been studied at a fixed frequency of 100 kHz. The Curie temperature obtained from the C—V measurements is in good agreement with the ferroelectric peak temperature of the DSC scan. The G—V characteristics were also studied along with C—V characteristics and show the butterfly shape. The C—V characteristics of the spray deposited composite films are also included. The nonlinear dependence of capacitance on the bias voltage may be suitable for the development of electronic devices such as voltage controlled oscillators. Chapter 7 contains an overall summary, correlations, and conclusions derived from various studies like structural, thermal, morphological, ferroelectric, switching kinetics and C—V characteristics in the pure and the composite films of NaNO2.