Studies on laser-ablated bismuth layered ferroelectric thin films for non-volatile memory applications.

摘要

Pulsed-laser-deposition (PLD) technique was used to deposit bismuth-layered ferroelectrics, in particular SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) with and without dopants, on Pt/TiO₂/SiO₂/Si substrates. We have studied the influence of PLD process parameters and were able to orient the films along the polarization axis. The first part of this work was devoted to the optimization of PLD process parameters in order to orient the films along the (200) direction and to achieve best possible ferroelectric properties of SBT thin films for memory devices. The maximum remanent polarization for the films grown at 350°C and annealed at 750°C, was found to be about 11 μC/cm2 with a coercive field of 121 kV/cm.; The second part of this dissertation was devoted to further improvements in the remanent polarization in order to increase the charge storage density of the ferroelectric memory components. CaBi₂Ta₂O ₉ (CBT) thin films were grown with the similar process parameters and showed inferior ferroelectric properties. The partial incorporation of Ba and Ca at Sr-site of SBT thin films was performed to understand the effect of A-site cations. The switchable component of polarization was found to increase upon partial substitution of Ca at Sr-site. A systematic substitution of Sr up to 30% Ca was conducted to understand the structural modification and compositional dependent electrical properties of SBT thin films. Incorporation of Ca at Sr-site decreased the lattice parameter and increased the grain size. The relative permitivity of the SBT thin films were systematically decreased with Ca contents and was attributed to the lower permitivity of CBT system.; We have studied the bottom electrode effect on ferroelectric properties and dc-current transport mechanism of SBT thin films. We found, Schottky conduction was dominant in SBT thin films irrespective of the bottom electrode. SBN thin films showed enhanced ferroelectric properties with remanent polarization of 25.7 μC/cm2 and coercive field of 198 kV/cm. The current-time measurements determined the predominant electronic conduction process in SBN thin films. Finally, we have used Raman spectroscopy to study the lattice vibrational modes and phase transition behavior of SBT and modified SBT compounds. The decrease in the tolerance factor of SBT compound with the incorporation of Ca at Sr-site and Nb at Ta-site showed a pronounced structural distortion in the lattice and shifted the lowest transverse optical phonon mode towards higher wavemumbers, as a result increase the transition temperatures. (Abstract shortened by UMI.)