Symmetry and functional properties of clamped Pb(Zr0.6Ti0.4)O3 tuned by thermal strain and electrostatics effects
Due to the COVID-19 crisis the PhD defence of Sizhao Huang will take place (partly) online.
The PhD defence can be followed by a live stream.
Sizhao Huang is a PhD student in the research group Inorganic Materials Science (IMS). His supervisors are prof.dr.ir. G. Koster and prof.dr.ing. A.J.H.M. Rijnders from the Faculty of Science and Technology (S&T).
The aim of this thesis is to experimentally verify a phenomenological three-domain model for clamped ferroelectric thin films, along with its piezoelectric properties extensions specified in PbZrxTi1-xO3(PZT) thin films under different boundary conditions (strains, temperature or electrical field). The thin film samples are produced by pulsed laser deposition (PLD), and the effect of thermal misfit strain induced during the cooling process after deposition was studied. In contrast with the epitaxial strain in thin films (less than 50nm thick), thin films with typical device device thickness are frequently dominated by the thermal strain. The three-domain model results show an enhancement in piezoelectric properties of thermal strained PZT (composition x=0.6) in comparison with its bulk properties. Two strategies were followed to validate the three-domain model and the predicted properties. Moreover, I was able to derive a PZT film phase diagram.
First an introduction and experimental section, describe the details of the three-domain model and the most important model results, as well as tests of the experimental strategy. The PZT film was deposited on a Si wafer and made into a cantilever for a 4-points bending test in Chapter 2.
The experimental evidence presented in Chapter 4, RSMs showed a definitive confirmation of the PZT symmetry. However, for relaxed thin films, the interpretation of the RSMs should be different for the case of epitaxially strained films. In Chapter 3, a mathematical description for relaxed thin films was established with a significant contribution from Dr. E. Houwman. It includes the tetragonal phase, the rhombohedral phase and the A-type, C-type monoclinic phase. The calculations allowed me to explain the RSMs measurements in Chapter 4, and most interestingly pointed at a distorted C-type monoclinic symmetry for PZT-S. Furthermore, this description shows universal applications for all RSM interpretation, as the results are in agreement with other publications.
In the alternative strategy presented in Chapter 4, temperature dependent reciprocal space mapping (RSM) was used to identify the crystal symmetry of a PZT (x=0.6) film grown on a 70% PbMg1/3Nb2/3O3-30%PbTiO3 (PMN-PT) substrate, a SrTiO3 substrate and a CaF2 substrate. Due to the thermal strain, at room temperature, the PZT on PMN-PT (PZT-P) has the rhombohedral symmetry, while the PZT on STO (PZT-S) is monoclinic and the PZT on CaF2 (PZT-C) is single domain tetragonal. It was concluded that I successfully proved the validity of the three-domain model. However, based on the obtained results of the experiments, I propose further refinements of the phase diagram. The meta-stable, monoclinic phase for PZT-S was measured instead of the predicted tetragonal a/b/c phase.
As for the measurements of the properties, the threading dislocations in PZT films that are possibly created by misfit strain, limited the direct DC electrical field strength due to a high leakiness of the capacitors. Therefore, an ionic gel (IG) was used to induce an electrostatic field. The IG shows benefits such as being effectively an electronic insulator, but with a high ionic conductivity, and additionally a strong mechanical strength and transparency. A free-standing IG was prepared with sputtered gold as a top electrode, subsequently cut and stuck onto a functional film to complete the transistor, known as “cut and stick” process. The PZT-S was successfully reversibly switched by this method as was verified by PFM. Moreover, the IG poled PZT-C was in-situ measured by X-ray diffraction (XRD) under application of a variable voltage to be able to determine the out-of-plane piezoelectric coefficient. These results in Chapter 5 demonstrated the merits using an IG and its potential applications in leaky ferroelectric transistors or flexible wearable devices.
