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FULLY DIGITAL (UNTIL FURTHER NOTICE) : PhD Defence Philip Lucke | Layer growth and hysteresis modelling of epitaxial lead zirconate titanate thin films

Layer growth and hysteresis modelling of epitaxial lead zirconate titanate thin films

Due to the COVID-19 crisis the PhD defence of Philip Lucke will take place online (until further notice).

The PhD defence can be followed by a live stream.

Philip Lucke is a PhD student in the research group XUV Optics. His supervisor is prof.dr. F. Bijkerk from the Faculty of Science & Technology (S&T).

Piezoelectric thin films are of interest for modern MEMS systems, for example used in applications like energy harvesters, actuators, adaptive optics or inkjet-printer heads. This thesis describes physics studies on the growth and modelling of PbZr0.52Ti0.48O3 (PZT) thin films. It contains a new description of hysteresis, loss and nonlinearity for the sub-coercive field strain and polarization response for epitaxial PZT thin films.

The thesis consists of three parts. The first part presents a small-scale demonstrator of an adaptive XUV optic based on nanoscale piezoelectric surface manipulation. The demonstrator consists of a PZT thin film with an array of top electrodes to selectively control the actuation. It is shown that by utilizing a resistive layer on top of the piezoelectric layer, it is possible to achieve a controlled, gradual surface profile between the electrodes. The resulting deformations range in the nm range and are laterally scalable from sub mm to the mm range, which makes it suitable for corrections of optics in the XUV range. To our knowledge this is the first presentation of a gradual surface modulation utilizing thin film piezoelectrics.

The second part describes the influence of a LaNiO3 (LNO) template layer on the growth and the functional properties of the PZT film subsequently grown on the LNO. We show that by tuning the deposition conditions, oxygen pressure and thickness of the template layer, the growth of the PZT film can be changed from a smooth dense film into a rough columnar film. This change of morphology changes the ferroelectric properties of the films and is explained by an increase of the roughness of the template layer to accommodate strain relaxation. The results clearly underline the importance of the template layer for the growth and the functional properties of PZT thin films.

The third part of the thesis introduces a new model, the so-called polarization rotation model, to describe the hysteresis, loss and nonlinearity in epitaxial PZT thin films of monoclinic and rhombohedral crystal symmetry. The model is based on the rotation of the polarization vector within the unit cell, which is accompanied by a viscous interaction of the domains. In contrast to the widely used Rayleigh model, the polarization rotation model takes the crystal symmetry of the material into account. It is shown that for monoclinic PZT films the experimentally observed strain and polarization response can not be described by the Rayleigh model, but is perfectly described by the polarization rotation model. Furthermore, the new model differentiates between the nonlinearity of the response and the loss due to viscous interaction. In addition, it is shown that by addition of a non-zero bias term to the model, it also describes the measured strain and polarization response for a DC bias of 20 kV cm-1, explaining the observed reduction of loss and nonlinearity of the polarization response.