SWITCHING AND TUNING PROPERTIES OF FERROELECTRIC CESIUM NITRATE - POLYMER COMPOSITE. FILMS

Abstract

The relationship between device performance and crystallographic microstructure is one of fundamental importance in materials science. Hysteretic and nonlinear dielectric behavior in ferroelectric ceramics has been of interest since 1950s. These materials have found many applications in various electronic devices such as nonvolatile electronic memories (e.g. FERAM) and tunable radio frequency integrated circuits (RFICs). Due to the requirement of low operating voltages in above applications the ferroelectric materials need processing in film form. Many ferroelectric materials such as barium titanate (BaTiO3), barium strontium titanate (BST), lead zirconate titanate (PZT), potassium nitrate (KNO3), sodium nitrite (NaNO2) and cesium nitrate (CsNO3) have been studied. The ferroelectric films of these materials are produced by various fabrication methods such as sputtering; pulse laser deposition (PLD), metal organic chemical vapor deposition (MOCVD), chemical solution deposition (CSD), metal organic decomposition (MOD) and molecular beam epitaxy (MBE). Recently, ferroelectric - polymer composite materials are emerging as a new class of electronic and dielectric materials. The present work is aimed to study the ferroelectric and switching properties of cesium nitrate (CsNO3) - polymer composite films and its applications in tunable RFICs. The composite films of CsNO3 and poly (vinyl alcohol) (PVA) are prepared by the solvent cast and the spray deposition techniques. The structural information of the pure and the composite films of CsNO3 were investigated by x-ray diffraction (XRD) experiments and all the XRD patterns exhibit the trigonal phase at room temperature. The temperature dependent XRD scans confirm the phase transition temperature in the pure CsNO3 and 50 wt. % composite films near 150°C. The structural distortion c/a ratio were also calculated and found highest in spray deposited film deposited at Ts = 200°C. The XRD spectra of potassium doped composite films show a polycrystalline perovskite structure without impurity phases which means that K - doping of CsNO3: PVA composite films forms single phase solid solution up to the doping concentration x = 12.5%, and mixed phase above this concentration. The KNO3 peaks along with CsNO3 peaks were observed above doping Abstract concentration x = 12.5%, which means some of the K+ ions are substituting the Cs+ ions in the unit cell of CsNO3. The surface morphological information such as grain size, shape and porosity of the pure and composite films prepared by different method have been studied using field emission scanning electron microscope (FESEM). The FESEM images show the uniform distribution of CsNO3 particles in 50 wt. % composition and the film deposited at Ts = 200°C. The clusters of CsNO3 grains and porosity were observed in the pure CsNO3 films and at higher deposition temperature. The EDX spectrum of the Csi_,,K,,NO3: PVA composite films contain the element peaks of Cs, N, 0, K and PVA which matched with wt. % of the CsNO3 and KNO3. The different phase transition temperatures in pure PVA, CsNO3 and different composite films were obtained by differential scanning calorimetry (DSC). The ferroelectric phase transition temperature obtained from the DSC scan in the composite films was matched with transition temperature obtained from the temperature dependence XRD. The optimum value of remanent polarization (Pr = 2.75 ttC/cm2) were obtained in the 50 wt. % CsNO3: PVA composite films prepared by the solvent cast method, Pr = 5 1.1C/cm2 of spray deposited CsNO3: PVA composite films deposited at Ts = 200°C and Pr = 9.6 1.1C/cm2 in the Csi_„K„NO3: PVA composite films with x = 12.5%. The improved properties have been attributed to well grown grains, less porosity and high structural distortion (c/a ratio). The phase transition temperature obtained from the temperature dependence of remanent polarization is in good agreement with the DSC and the temperature dependent XRD measurements. The polarization switching properties are very important because the net switched charge limits the resolving power and switching time limits the running speed of read-write memory devices. The switching response in 50 wt. % CsNO3: PVA composite films have been studied using bipolar square pulses. 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 exponential field dependence of the domain wall velocity and the nucleation rate indicate that nucleation limited mechanism is responsible for switching phenomena in the composite films. 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 and by FGM model. The NLS model gives excellent agreement with the experimental polarization iv Abstract reversal transients throughout the whole time range. The switching parameters were determined in the composite films deposited at different substrate temperatures (Ts). The peak value of the polarization current (im) exhibits exponential dependence on the external electric field. The effect of pulse width on the switching was also analyzed in ultrasonic spray deposited pure CsNO3 films. The switching parameter such as effective dimensionality, switching time, and nucleation rate were extracted using finite grain model (FGM). The effect of pulse width on switching shows that longer pulse width enhanced the switching current and nucleation rate and decreased the switching time. The switching kinetics were also investigated in K doped CsNO3: PVA composite films. The dielectric constant (8) versus bias voltage (V) characteristics of the pure and composite films of CsNO3 exhibit butterfly features and are attributed to the switching of polarization in the composite films. The initial rise in dielectric constant with bias voltage may be due to orientation of domains in the field direction. The maxima in £ -V curves occur in the vicinity of coercive field at which most of the domains switcheeyn the field direction. Beyond the peak value, the decrease in dielectric constant in the forward and reverse cycles is due to the reduction in domain moment. The important aspect is that the voltage corresponding to dielectric constant peaks are in fact the coercive voltages. The nonlinear dependence of the dielectric constant on the electric field can be used in the tunable microwave applications. The modelling of suspended microstrip line on multilayer ferroelectric film has been carried out to determine the variation in characteristic impedance and effective dielectric constant with normalized conductor width, film thickness and frequency. Effect of side wall grooves on characteristic impedance and effective dielectric constant were also studied. Electrical equivalent circuits for open end, gap, step and bend discontinuities in strip conductor were extracted from the data obtained in the full wave simulation. Full wave FEM (finite element method) based 3D simulator HFSS is used in this modelling. The parameters of lumped equivalent circuit for different discontinuities such as open end, gap, step and bend are calculated. The obtained results can be used for the design of passive and active circuit especially tunable component using this transmission line.