Towards the limit of ferroelectric nanosized grains

Ferroelectric random access memories are non-volatile, low voltage, high read/write speed devices which have been introduced into the market in recent years and which show the clear potential of future gigabit scale universal non-volatile memories. The ultimate limit of this concept will depend on the ferroelectric limit (synonymous superparaelectric limit), i.e. the size limit below which the ferroelectricity is quenched. While there are clear indications that 2D ferroelectric oxide films may sustain their ferroelectric polarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone J M 1999 Appl. Phys. Lett. 75 856), the limit will be quite different for isolated 3D nanostructures (nanograins, nanoclusters).

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. Preparing highly diluted precursor solutions we achieved single separated ferroelectric grains with grain sizes ranging from 200 nm down to less than 20 nm.

For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the paraelectric phase which has no spontaneous polarization. Recent calculations (Zhong W L, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) and experiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J. Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4.2–20 nm.