Towards the limit of ferroelectric nanosized grains

Abstract

Ferroelectric random access memories are non-volatile, low voltage, highread/write speed devices which have been introduced into the market in recentyears and which show the clear potential of future gigabit scale universalnon-volatile memories. The ultimate limit of this concept will depend onthe ferroelectric limit (synonymous superparaelectric limit), i.e. the sizelimit below which the ferroelectricity is quenched. While there are clearindications that 2D ferroelectric oxide films may sustain their ferroelectricpolarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone JM 1999 Appl. Phys. Lett. 75 856), the limit will be quite different forisolated 3D nanostructures (nanograins, nanoclusters). To investigatescaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO3(PTO) and Pb(ZrxTi1−x)O3grown by a self-assembly chemical solution deposition method. Preparing highlydiluted precursor solutions we achieved single separated ferroelectric grains withgrain sizes ranging from 200 nm down to less than 20 nm. For grains smallerthan 20 nm, no piezoresponse was observed and we suppose this couldbe due to the transition from the ferroelectric to the paraelectric phasewhich has no spontaneous polarization. Recent calculations (Zhong WL, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) andexperiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J.Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectricparticles decrease with decreasing particle size. From these experiments theextrapolated critical size of PTO particles was found to be around 4.2–20 nm.