NanoscaleFerroelectric and Piezoelectric Phenomena in Oxide

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NanoscaleFerroelectric and Piezoelectric Phenomena in Oxide Powered By Docstoc
					Nanoscale Ferroelectric and Piezoelectric Phenomena in Oxide Heterostructures
            NSF-Europe Collaborative Program, DMR - 0427815
                  V. Nagarajan, R. Waser* and R. Ramesh#
       *IWE Juelich, Germany and #University of California, Berkeley
                                                                                    1.5                                           In this Nugget we report on the study of
        150




                                                               d33(mV(raw units))
                                                                                                                                  nanoscale polarization relaxation
                                                                                    1.0

                                                                                    0.5


        100
                                                                                    0.0

                                                                                                                                  phenomena in polycrystalline PZT films.
d33 (pm/V)




                                                                                -0.5

                                                                                -1.0
             50                                                                           -9   -6   -3    0
                                                                                                      Voltage
                                                                                                              3   6   9
                                                                                                                                  Piezoresponse force microscopy (PFM)
              0                                                                                                                   images of the as-grown sample reveal
         -50
        -100
                                                                                                                          B C     grains with a range of contrast, from
                                                                                                                                  fully white to gray to fully black. This
        -150
            90                                                                                                                    local change in the contrast (magnitude)
             180                                                                                                                  of the piezoresponse from grain to grain
                                                                                                                          A
                                             360
               270
                                                              50nm
             An




                                    270                                                                                           is due to the crystallographic orientation
                              180
                                    le (θ
                 360                      )
                g




                         90                                                                                                       within each grain. PFM based relaxation
                  le




                              Ang
                   θ)(




                                                                                                                                  experiments show that the rate of
                                                                                                                                  relaxation is different for each grain,
                                                                                                                                  furthermore is strongly dependent on the
                                        d33( raw units)




                                                          1
                                                                                                                                  tilt of individual grain with respect to the
                                                                                                                                  polar axis. Strongly tilted grains show a
                                                                                                                                  much stronger decay of the polarization
                                                                                                                                  compared to polar axis oriented grains.
                                                              Gray 1                                                              This has significant implications for the
                                                              Gray 2
                                                              White                                                               stability of domains in polycrystalline
                                                              Black                                                               films used in memory devices
                                                          0
                                                                       0.01                                           0.1     1
                                                                                                                          5       Applied Physics Letters, submitted
                                                                          Time(*10 sec)
                                    Size Effects in Ferroelectric Heterostructures

                                                     (100)(011)




                                                              Polarization (µC/cm )
         SrRuO3                                                                       120        15nm




                                                              2
                                             (001)                                               50 nm         In this Nugget we show a
                                                                               60                160nm         systematic quantitative
                                                                                                               experimental and
                                                                                0                              theoretical
             PZT
                                                                              -60                              study of the thickness
                                                                                                               dependence of switched
                                                                             -120                              polarization in (001)
                                                                                                               epitaxial PZT films,
         SrRuO3                                        2 nm
                                                                               -2000 -1000 0         1000 2000 4 to 80 nm thick. A
                                                                                    Electric Field (KV/cm)     preliminary model based
                                                                                                               on a modified Landau
                  1.2   Norm  alized polarization                                 0                            Ginzburg approach
Order Parameter




                        Intrinsic
                  1.0                                                                                          suggests
                        M ehta                                                                    ζ=1
                                                                                                               that the nature of the
                                                                                  ln(1-T'c/Tc)



                        G hosez                                                  -1
                  0.8
                                                                                                               electrostatics at the
                  0.6                                                            -2                            ferroelectric–electrode
                                                                                                               interface plays a
                  0.4
                                                                                                               significant role in the
                                                                                 -3
                  0.2                                                                                          scaling of ferroelectric
                                                                                                               thin films.
                  0.0                                                            -4
                        1                      10                                   1      2      3      4     Applied Physics Letters,
                                       d/ξ                                              ln (Thickness)         85, 5225(2004)
                   Misfit Dislocation Induced Polarization Degradation in Ferroelectric Heterostructures
                           V. Nagarajan, H. Kohlstedt and R. Waser, S. P. Alpay and R. Ramesh
                         IWE, Juelich,Germany; University of Connecticut; University of California, Berkeley
                                                                                             We present a quantitative study of
                                                                                             the thickness dependence in
                             (52/48) (20/80)
                                                                                             epitaxial PZT films grown on
                                                                                             SrRuO3 buffered SrTiO3
                                                                                             substrates. Theoretical
                                                                                             calculations present a physical
                                                                                             picture for the effect of such
                                                                                             dislocations in suppressing
                                       SRO
                                    a = 3.93Å                                                ferroelectricity. Both the
                                                                                             polarization and the Curie
                                                                                             temperature are predicted to drop
                                                                                             rapidly around the core of a
                                                                                             dislocation, gradually improving
                                                                                             to the bulk value over a distance
                             x (PbTiO3)                                                      of 5-10 nm. This leads to highly
                                                                                             localized polarization gradients
             1.0     20/80                                       1.0                         and hence depoling fields, which
Normalized d33




                                                 Normalized ∆P




                     52/48                                                                   would destroy the long-range
             0.8
                                                                 0.8                         interactions necessary for
             0.6                                                                             ferroelectricity. Piezoresponse
                                                                 0.6
                                                                                             measurements show that d33 for
             0.4
                                                                 0.4                         the PZT(50/50) system begins to
             0.2                                                                             get suppressed at a thickness
                                                                 0.2
                                                                                             much greater than that in the
             0.0                                                 0.0                         lattice matched PZT(20/80) thin
                       10              100                             0    50 100 150 200
                       Thickness (nm)                                                        films.
                                                                           Thickness (nm)    Nature Materials , submitted