Study on the Fabrication of Ferroelectric Ultrathin Films and Nanostructures by MOCVD and Their Size Effects

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In order to investigate the size effect in ferroelectric thin films and nanoisland structures, PbTiO_3 ultrathin films and nanosilands were fabricated by MOCVD and their structureal nature and ferroelectric properties were studied.1) Epitaxial PbTiO_3 ultrathin films with thicknesses of 10-0.9nm were successfully grown on SrRuO_3/SrTiO_3 by MOCVD.2) Piezoresponse measurements using Piezoresponse Force Microscopy (PFM) and observation of ferroelectric polarization using scanning nonlinear dielectric microscopy (SNDM) evidently proved the presence of ferroelectricity in PbTiO_3 ultrathin films with a thickness of 1.3nm (3 mono layers (ML)). Ferroelectrcity was not observed in the 0.9nm (2.2 ML) -thick film. In our experiment, critical thickness was 1.3nm.3) PbTiO_3 and PZT nanoislands were successfully fabricated on Pt/SrTiO_3 substrates by self-assembly using MOCVD. Three types of islands (pyramidal-shaped, triangular-prism-shaped and square-shaped islands) were grown on Pt/SrTiO_3(111), (110) and (001), respectively.4) PFM measurement proved the presence of ferroelectrcity in the PbTiO_3 nanoisland with a width of 27nm and a height of 1.2nm.5) Phase transition temperatures of PbTiO_3 nanoislands on Pt/SrTiO_3(111) and (110) were measured by Raman spectroscopy. From Raman measurement, it was fond that phase transition temperatures in pyramidal-shaped islands and triangular-prism-shaped islands decreased as island size decreased.6) Crystal structure of the nanosiland on Pt/SrTiO_3(111) was investigated using Synchrotron X-ray diffraction. As island size decreased, the c-axis lattice constant decreased at sizes smaller than 100nm though the a-axis lattice constant did not changed.7) Nano-sized Au dots were successfully fabricated by self-assembly. Using these Au nanodots as a top electrode, I-V characteristics of PbTiO_3 nanosilands on Pt/SrTiO_3 were measured.