structure and functional properties of epitaxial PbZr xTI1-xO3 FILMS
Kurt Vergeer is PhD Student in the MESA+ research group Inorganic Materials Science. His promoter is Guus Rijnders.
The work described in this thesis is focused on the characterization and understanding of epitaxial, clamped, dense PbZrxTix-1O3 (PZT) films. An introduction to Piezoelectricity and Piezoelectric materials such as PZT is given in Chapter 1.
In Chapter 2 the fabrication of epitaxial films and how these films are characterized is explained. The fabrication steps needed to create a capacitor structure with an epitaxial PZT film on SrTiO3 (STO) and KTaO3 (KTO) single crystal substrates with a top and bottom conductive SrRuO3 (SRO) electrode using PLD are shown.
In Chapter 3 a thermodynamic model is developed, which is used to simulate properties of clamped PZT films throughout this work. It is shown that the new model, which allows for a more general domain interaction between different domains, is an improvement on existing models for predicting material characteristics. Free energy equations for single- and poly-domain films are given, which allow us to simulate the material properties of epitaxial, clamped, dense PZT films.
In Chapter 4 the free energy equations are used to analytically derive PZT films properties, which are compared to numerical calculations. Here it is shown that the equations can be used to derive the piezoelectric coefficients, stress, strain, polarization and crystal phase of PZT. An analytical approximation is given that provides a relationship between important parameters allowing for a more simplified prediction of material parameters.
Experimental work done on tetragonal poly domain PbZr40Ti60O3 films is described in Chapter 5. X-ray diffraction measurements done with an applied field are used to measure the intrinsic piezoelectric coefficients of individual domains. It is shown that the intrinsic piezoelectric coefficient of the domains in the PZT film is negative while the average piezoelectric coefficient of the film is positive. This result was predicted by the model.
An alternative explanation for the origin of high piezoelectric coefficients using the thermodynamic model is discussed in Chapter 6. The model is used to explain the high piezoelectric characteristics of both film and bulk PZT found at the morphotropic phase boundary (MPB) using the characteristics of the rhombohedral and the tetragonal crystal phase or the phase change between them. The model also predicts that in principle arbitrarily high piezoelectric coefficients can be obtained in defect free films. The value of the coefficient is highly dependent on the misfit strain, which can be tuned using different substrates.
In Chapter 7 the structure of the domains and domain walls (DWs) in tetragonal PZT is explored. Data obtained using transmission electron microscopy shows that 2D films exist out of a c/a, c’/a’ domain structure. X-ray diffraction and piezo force microscopy on 3D films show a complex c/a, c’/a’, c/b, c’/b’ domain structure which reconstructs when an electric field is applied or when the out-of-plane polarization is switched. The complex domain structure can be explained as a mixed micro- and macro-domain structure, that on average is described by the domain structure required by our new thermodynamic model.
