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

为了研究铁电薄膜和纳米岛结构中的尺寸效应，采用MOCVD方法制备了PbTiO_3/SrRuO_3薄膜和纳米SiO_2薄膜，并对其结构和铁电性能进行了研究。1）采用MOCVD方法在SrRuO_3/SrTiO_3上成功地生长了厚度为10-0.9nm的PbTiO_3/SiO_2薄膜。用扫描非线性介电显微镜（SNDM）观察了PbTiO_3薄膜的铁电极化，证实了PbTiO_3薄膜具有铁电性。在0.9nm（2.2ML）厚的膜中没有观察到铁电性。3）采用MOCVD自组装技术在Pt/SrTiO_3衬底上成功制备了PbTiO_3和PZT纳米岛。三种岛屿在Pt/SrTiO_3（111）、（110）和（001）上分别生长了四个岛状、三棱柱状和正方形的纳米晶，4）PFM测量证实了PbTiO_3纳米岛具有铁电性，其宽度为27 nm，高度为1.2nm。5）Pt/SrTiO_3（111）和（110）通过拉曼光谱测量。通过拉曼光谱的测量发现，随着岛的尺寸减小，纳米岛和三棱柱岛的相变温度降低。6）采用同步辐射X射线衍射仪研究了Pt/SrTiO_3（111）上纳米硅的晶体结构。随着岛尺寸的减小，c轴晶格常数在小于100 nm时减小，而a轴晶格常数没有变化。7）通过自组装成功制备了纳米级Au点。以这些Au纳米点为上电极，测量了Pt/SrTiO 3上PbTiO 3纳米银的I-V特性。

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.