Oxide heterostructures have emerged over the last decade as a promising platform for energy-efficient electronics. Among oxides, ferroelectric materials, owing to their spontaneous polarization that can be controlled by an electric field, stand out as natural memory elements for low-power devices. In devices, thin films of ferroelectric materials need to be prepared with a high degree of control over the ferroelectric response. Regions of a ferroelectric material where electric-dipole moments point in the same direction are referred to as domains. Domain structure is the key property in applications because it determines the switching path of polarization and possible polarization states.
The domain structure is commonly set already during the integration of a ferroelectric layer into a heterostructure. The two interfaces of a ferroelectric layer, the interface towards the substrate (bottom) and the interface towards the surface (top), predominantly govern the ferroelectric response. While the influence of the bottom interface is set already \textit{during} the deposition of the ferroelectric layer, the influence of the top interface evolves even \textit{after} the deposition of the ferroelectric layer. Understanding the ferroelectric response of a thin film thus requires understanding the interplay of the contributions of the two interfaces. However, disentangling the contributions of interfaces is difficult post-deposition, and conventional techniques for the characterization of ferroelectric materials cannot be used during the thin-film synthesis.
In this thesis, we use second harmonic generation as a non-invasive nonlinear optical tool for directly investigating the emergence and evolution of ferroelectricity in oxide heterostructures. We develop the use of in situ second harmonic generation to study ferroelectricity during the thin-film deposition in real time. We use this approach, which is unique in the world, to study ferroelectricity in two model systems: barium titanate and lead titanate.
In the first two projects, we unravel the dynamics of the polarization during the growth of ferroelectric-based heterostructures. We first study the dynamics of polarization during the integration of the ferroelectric layer into a prototypical device architecture of a capacitor. Surprisingly, we observe a polarization suppression during the deposition of the top electrode as a result of the transiently insufficient charge screening at the top interface. This insight enables us to stabilize a robust single-domain configuration in a ferroelectric-based capacitor.
We take this a step further to explore the interface-governed polarization in the second project. We observe only the influence of the bottom interface on the polarization direction during the growth and the influence of both interfaces on the polarization direction once the growth is halted. We establish the concept of competition and cooperation of interfaces in the setting of the polarization direction. We find that in the case of matching interface contributions, we can even stabilize a robust single-domain configuration in an unfavorable electrostatic environment.
In the next two projects, we move on to investigating complex arrays of dipole moments and ordered multi-domain structures in ferroelectric∣dielectric multilayers. Using the non-invasive optical characterization ex-situ, we detect phase coexistence and interlayer coupling of polarization in such multilayers. We furthermore manipulate an ordered multi-domain configuration forming at the nanoscale into stable single-domain regions using electric fields of a scanning-probe tip.
The results presented in this thesis demonstrate that the thin-film synthesis is the decisive point for setting the ferroelectric response that is observed post-deposition. The developed approach for monitoring polarization during the growth is thus essential for understanding and engineering polarization in ferroelectric-based heterostructures. Our observation of the emergence and evolution of ferroelectricity in oxide heterostructures not only complements the standard characterization, but allows access to previously overlooked polarization dynamics. The access to these transient polarization states that occur during the synthesis is instrumental in explaining the often unexpected ferroelectric response once the synthesis is completed. Moreover, based on the information obtained during the synthesis, we tune the growth process to stabilize the coveted robust single-domain polarization in the ultrathin regime. We furthermore show the potential of following the same approach in studying more complex ordering of dipole moments and open up a path towards the use of this approach operando. Ultimately, we provide new routes to engineer the domain structure in ferroelectric layers displaying improved functionalities
