SYNTHESIS AND CHARACTERIZATION OF DOPED PZT, SBT AND SBN NANOCERAMICS FOR MEMORY DEVICES

Abstract

The current research is an effort to realize and solve the problems which could be encountered while fabricating a ferroelectric capacitor for high density FRAM cell. In a high density memory structure, the ferroelectric capacitor is fabricated on the top of a poly-silicon plug, which in turn is connected to the drain of the transistor. Such a configuration would result in constraints for processing of the ferroelectric material. In order to preserve the integrity of the transistor, the processing temperature of the ferroelectric capacitor should be as low as possible. Even at low processing temperatures, a ferroelectric capacitor should exhibit well defined ferroelectric properties. The ferroelectric capacitor should possess high remnant polarization so that the memory stored in these capacitors could be read in an unambiguous manner and the coercive field required to switch the memory from one state to the other state should be as low as possible. In this study, the effect of dopant (single or double of La, Bi, Nd, Nb, V) contents on phase-transition behavior, ferroelectric property, and crystal structure of lead zirconate titanate (PZT), strontium bismuth tantalate (SBT) and strontium bismuth niobate (SBN) ceramics was explored with the aid of dielectric permittivity (6) verses temperature, ferroelectric hysteresis loop, x-ray diffraction (XRD), and complex impedance analysis. PLBZT ceramics near the morphotropic phase boundary (52/48) were prepared by coprecipitation and sol-gel method. It was found that the formation of perovskite phase in the sol-gel derived PLBZT ceramics take place comparatively at a lower temperature (250 °C), which is an important tool in the property optimization and device performance. The average particle size from TEM ii measurement was found to be 48 nm and 31 nm for Sample-A (coprecipitation method) and Sample-B (sol-gel method) respectively. Both the phase-transition behavior and ferroelectric properties such as spontaneous polarization (Ps) showed a dependence on dopant contents. With increasing Bi content, the Curie temperature (Ta) and Emax increases as determined from dielectric constant versus temperature plot. Ferroelectric hysteresis measurements of doped PZT system studied for various dopants gives the range for obtaining the best ferroelectric hysteresis with individually or co-doped ceramic systems. 3 mole % Nb and Nd were the composition observed to have best ferroelectric hysteresis loop whereas x=1.0 i.e in PBZT was found to give optimum ferroelectric properties when studied in a series of La: Bi double doping. SBT and SBN were prepared by reaction sintering of powder mixtures of constituent oxides. With partial substitution of tantalum/niobium by vanadium cations (up to 15 at. %), the single-phase layered perovskite structure was preserved, and the sintering temperature of the system was significantly lowered (-200 °C) by using fast firing method. X-ray structural studies carried out on SBVN ceramics confirmed the existence of preferential orientation (c-planes) of the grains. V205 addition substantially improved the sinterability of SBT and SBN and enabled to achieve high density (95%) which was otherwise difficult in the case of pure SBN. Substituting Nb+5 by much smaller V+5 cations resulted in an increased "rattling space" leading to a higher curie temperature. Furthermore, the vanadium doping as well as addition of 4% Bi led to a significantly lower processing temperature without affecting the ferroelectric properties. The incorporation of vanadium into the layered perovskite structure resulted in a shift of the Curie point to higher temperatures from 455 to 475 °C, with 15 at. % vanadium doping as well as the incorporation of vanadium did not increase the loss tangent. 111 V205 was found to be an effective microstructure modifier and grain growth truncator for SBN ceramics