Strain and composition effects in epitaxial PZT thin films

The deposition of perovskite oxide thin films on silicon wafers is attracting great interest, since it promises the possibility of mass-production of thin film devices with functional mechanisms such as piezoelectricity, ferroelectricity, superconductivity, magnetism, and dielectricity. Piezoelectric Micro Electro Mechanical Systems (PiezoMEMS) using Pb(Zrx,Ti1-x)O3 (PZT) films on silicon are often viewed as forerunner in this trend, with promising applications such as inkjet printheads, piezoelectric micromachined ultrasound transducers and energy harvesters. The recently achieved high-quality epitaxial PZT films on silicon substrates by pulsed laser deposition offer the opportunity to study the fundamental relation between ferroelectric and crystallographic properties. This thesis studies the effects of strain and composition on the crystallographic and functional properties of PZT thin films. A wide range of strain values and compositions are achieveable and their mutual interactions are studied. In the used epitaxial PZT films, the e�ects that are not related to the intrinsic properties are minimized, such as grain boundaries, mixed orientations, space charge or misfit dislocations. The films are grown in a functional device geometry and are of a thickness that is regularly used in applications, such as PiezoMEMS. At this thickness, the films are relaxed from epitaxial strain. Owing to the high temperature deposition in pulsed laser deposition and subsequent cooling to room temperature, the thermal strain becomes the dominant parameter in determining the properties of the PZT heterostructures. An important aspect of PZT films and strongly dependent on the composition, is domain formation. For low Zr-content, the strain is relaxed by forming large tetragonal domains, while for PZT near the morphotropic phase boundary, nanoscale domains occur with an unknown stress relaxation mechanism. Several analytical and thermodynamical models may be used to link the crystallography to the functional properties.