Pb-Based Ferroelectric Thin Films Prepared by MOCVD

Abstract

This Chapter focuses on the preparation and ferroelectric properties of Pb-based thin films and nanostructures. First, the growth behavior of MOCVD-deposited PbTiO₃ and PbZr _x Ti_1–xO₃ (PZT) thin films on various substrates, such as polycrystalline Pt(111)/SiO₂/Si(100), SrTiO₃(100) single crystals and epitaxial SrRuO₃(100)/SrTiO₃(100), has been investigated. SEM, AFM and TEM observations revealed that PbTiO₃ and PZT showed the V–W (Volmer–Weber) growth mode when they were grown on polycrystalline Pt-covered Si substrates. From AFM observations, it was found that PZT grown on etched SrTiO₃(100) with a surface terminated by TiO₂ and on the TiO₂ planes of annealed SrTiO₃(100) showed the S–K (Stranski–Krastanov) growth mode. The V–W growth mode was also observed for PZT on the SrO planes of annealed SrTiO₃(100). When PZT was grown on epitaxial SrRuO₃(100)/SrTiO₃(100), the S–K growth mode was observed.

Epitaxial PZT ultrathin films 20 nm thick were successfully grown on SrRuO₃(100)/SrTiO₃(100) with a terrace and step structure. Only when PZT films were grown on SrRuO₃ with a terrace and step structure did they exhibit good ferroelectric hysteresis loops, with remanent polarizations (P _r ) of 29 μC/cm² to 33 μC/cm² and coercive fields (E _c ) of 340 kV/cm to 370 kV/cm. A 15 nm-thick PZT film showing an unsaturated hysteresis loop was also studied. The ferroelectricity and local current flow of PZT ultrathin films with thicknesses smaller than 10 nm were investigated by SPM techniques.

V–W growth was observed for PbTiO₃ and PZT on epitaxial Pt/SrTiO₃ and Pt/MgO substrates. Pyramidal-shaped, triangular-prism-shaped and square-shaped PbTiO₃ and PZT self-assembled nanostructures were successfully prepared on epitaxial Pt/SrTiO₃(111), (110) and (100) substrates, respectively, demonstrating structural control. Piezoresponse measurements using SPM proved that PbTiO₃ and PZT nanoislands prepared on various substrates had ferroelectricity even before they became films. The minimum volume of a PbTiO₃ nanostructure was 1.9 × 10³ cm (38 nm width × 1.7 nm height). This result has led us to a new study and development, of self-assembled and self-organized ferroelectric nanostructure technology.