Structural and electrical anisotropy of (001)-, (116)-, and (103)-oriented epitaxial SrBi2Ta2O9 thin films on SrTiO3 substrates grown by pulsed laser deposition

Epitaxial SrBi2Ta2O9 (SBT) thin films with well-defined (001), (116), and (103) orientations have been grown by pulsed laser deposition on (001)-, (011)-, and (111)-oriented Nb-doped SrTiO3 substrates. X-ray diffraction pole figure and phi -scan measurements revealed that the three-dimensional epitaxial orientation relation SBT(001)parallel to SrTiO3(001), and SBT[1(1) over bar 0]parallel to SrTiO3[100] is valid for all cases of SET thin films on SrTiO3 substrates, irrespective of their orientations. Atomic force microscopy images of the c-axis-oriented SET revealed polyhedron-shaped grains showing spiral growth around screw dislocations. The terrace steps of the c-axis-oriented SET films were integral multiples of a quarter of the lattice parameter c of SBT (similar to 0.6 nm). The grains of (103)-oriented SET films were arranged in a triple-domain configuration consistent with the symmetry of the SrTiO3(111) substrate. The measured remanent polarization (2P(r)) and coercive field (2E(c)) of (116)-oriented SBT films were 9.6 muC/cm(2) and 168 kV/cm, respectively, for a maximum applied electric field of 320 kV/cm. Higher remanent polarization (2P(r)=10.4 muC/cm(2)) and lower coercive field (2E(c)=104 kV/cm) than those of SBT(116) films were observed in (103)-oriented SET thin films, and (001)-oriented SET revealed no ferroelectricity along the [001] axis. The dielectric constants of (001)-, (116)-, and (103)-oriented SBT were 133, 155, and 189, respectively. (C) 2000 American Institute of Physics.

Epitaxial growth of non-c-oriented ferroelectric SrBi2Ta2O9 thin films on Si(100) substrates

Epitaxial SrBi2Ta2O9 (SBT) thin films with well-defined (116) orientation have been grown by pulsed laser deposition on Si(100) substrates covered with an yttria-stabilized ZrO2 (YSZ) buffer layer and an epitaxial layer of electrically conductive SrRuO3. Studies on the in-plane crystallographic relations between SrRuO3 and YSZ revealed a rectangle-on-cube epitaxy with respect to the substrate. X-ray diffraction pole figure measurements revealed well defined orientation relations, viz. SBT(116)\\ SrRuO3(110)\\ YSZ(100)\\ Si(100), SBT[110]\\ SrRuO3[001], and SrRuO3[111]\\ YSZ[110]\\ Si[110].

Evidence for two-phase regions in BST thin films from capacitance-voltage data

The Curie-Weiss plots of reciprocal dielectric constant versus temperature, in Ba0.5Sr0.5TiO3 films grown onto SrRuO3 lower electrodes by pulsed-laser deposition, show two minima below film thicknesses of 280 nm. This double minima implies possible mixed phases in the thin films. A graphical plot of capacitance for decreasing dc voltage versus that of increasing dc voltage shows a well-defined triangular shape for both Pb(Zr0.4Ti0.6)O-3 and SrBi2Ta2O9 thin films. However, for a 175-nm-thick Ba0.5Sr0.5TiO3 thin film, the plot shows an overlapping of two triangles, suggesting mixed phases. This graphical method appears to be effective in detecting structural subtleties in ferroelectric capacitors.

Progressive loss of ferroelectricity under bipolar pulsed fields and experimental determination of non-switchable polarization in Au/Ba0.5Sr0.5TiO3/SrRuO3 thin-film capacitors

175 nm-thick Ba0.5Sr0.5TiO3 (BST) thin film fabricated by pulsed laser deposition (PLD) technique is found to be a mixture of two distributions of material. We discuss whether these two components are nano-regions of paraelectric and ferroelectric phases, or a bimodal grain-size distribution, or an effect of oxygen vacancy gradient from the electrode interface. The fraction of switchable ferroelectric phase decreases under bipolar pulsed fields, but it recovers after removal of the external fields. The plot of capacitance in decreasing dc voltage (C(Vdown arrow) versus that in increasing dc 61 voltage C(Vup arrow) is a superposition of overlapping of two triangles, in contrast to one well-defined triangle for typical ferroelectric SrBi2Ta2O9 thin films.

Patterning and switching of nano-size ferroelectric memory cells

A "top-down" approach using a-beam lithography and a "bottom-up" one using self-assembly methods were used to fabricate ferroelelectric Pb(Zr,Ti)O-3, SrBi2Ta2O9 and BaTiO3 nanostructures with lateral sizes in the range of 30 nm to 100 nm. Switching of single sub-100 nm cells was achieved and piezoelectric hysteresis loops were recorded using a scanning force microscope working in piezoresponse mode. The piezoelectricity and its hysteresis acquired for 100 nm PZT cells demonstrate that a further decrease in lateral size under 100 nm appears to be possible and that the size effects are not fundamentally limiting on increase density of non-volatile ferroelectric memories in the Gbit range.