Biya Cai Manipulating the Structural and Electronic Properties of Epitaxial NaNbO3 Films via Strain and Stoichiometry

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Inhaltsangabe zu „Manipulating the Structural and Electronic Properties of Epitaxial NaNbO3 Films via Strain and Stoichiometry“ von Biya Cai

Due to their intriguing dielectric, pyroelectric, elasto-electric, or opto-electric properties,
oxide ferroelectrics are vital candidates for the fabrication of most electronics. However, these
extraordinary properties exist mainly in the temperature regime around the ferroelectric
phase transition, which is usually several hundreds of K away from room temperature.
Therefore, the manipulation of oxide ferroelectrics, especially moving the ferroelectric
transition towards room temperature, is of great interest for application and also basic
research.
In this thesis, we demonstrate this using examples of NaNbO3 films. We show that the
transition temperature of these films can be modified via plastic strain caused by epitaxial film
growth on a structurally mismatched substrate, and this strain can be fixed by controlling the
stoichiometry.
The structural and electronic properties of Na1+xNbO3+δ thin films are carefully examined by
among others XRD (e.g. RSM) and TEM and cryoelectronic measurements. Especially the
electronic features are carefully analyzed via specially developed interdigitated electrodes in
combination with integrated temperature sensor and heater. The electronic data are
interpreted using existing as well as novel theories and models, they are proved to be closely
correlated to the structural characteristics. The major results are:
- Na1+xNbO3+δ thin films can be grown epitaxially on (110)NdGaO3 with a thickness up to 140
nm (thicker films have not been studied). Plastic relaxation of the compressive strain sets
in when the thickness of the film exceeds approximately 10 – 15 nm. Films with excess Na
are mainly composed of NaNbO3 with minor contribution of Na3NbO4. The latter phase
seems to form nanoprecipitates that are homogeneously distributed in the NaNbO3 film
which helps to stabilize the film and reduce the relaxation of the strain.
- For the nominally stoichiometric films, the compressive strain leads to a broad and
frequency-dispersive phase transition at lower temperature (125 – 147 K). This could be
either a new transition or a shift in temperature of a known transition. Considering the
broadness and frequency dispersion of the transition, this is actually a transition from the
dielectric state at high temperature to a relaxor-type ferroelectric state at low temperature.
The latter is based on the formation of polar nano-regions (PNRs). Using the electric field
dependence of the freezing temperature, allows a direct estimation of the volume (70 to
270 nm3) and diameter (5.2 to 8 nm, spherical approximation) of the PNRs. The values
confirm with literature values which were measured by other technologies. ...

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