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DF: Dielektrische Festkörper

DF 8: Dielektrische und ferroelektrische Schichten, Nanostrukturen und Rastermethoden

DF 8.1: Hauptvortrag

Donnerstag, 27. März 2003, 09:30–10:10, HSZ/403

The superparaelectric limit of ferroelectric nanosized grains — •Rainer Waser — RWTH Aachen University & FZJ Research Center Jülich

This overview will first outline the trends in RAM technologies with focus on the future development of Ferroelectric Random Access Memory (FeRAM) devices down to a cell size well below ≈ 0.1 µ m². To achieve Gigabit density, successful scaling of bit cell properties and solving the existing and new design issues is key. To investigate scaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO₃ (PTO) and Pb(Zr_xTi_1−x)O₃ grown by a self-assembly chemical solution deposition method. We achieved single separated ferroelectric grains with sizes ranging from 200 nm down to less than 10 nm. The grain size dependent domain configuration is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that the PTO grains in a dense film contain laminar 90^∘ domain walls whereas separated PTO grains show more complicated structures of mainly 180^∘ domain walls. Applying a voltage to the cantilever of the PFM in contact mode the local current of the sample was monitored enabling us to differentiate between conductive and highly isolating areas. Repeating the measurement at voltages higher than 200 mV lead to a dramatic increase in the conductivity and to a change in the topography. This result determines the maximum driving voltage for PFM measurements. For grains smaller than 18 nm no piezoresponse is measured and we suppose this could be the transition from the ferroelectric to the superparaelectric phase without spontaneous polarization.