Ferroelectricity and resistive switching in BaTiO3_3 thin films with liquid electrolyte top contact for bioelectronic devices

We investigate ferroelectric- and resistive switching behavior in 18-nm-thick epitaxial BaTiO$_3$ (BTO) films in a model electrolyte-ferroelectric-semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. The BTO films are grown by pulsed laser deposition (PLD) on semiconducting Nb-doped (0.5 wt\%) SrTiO$_{3}$ (Nb:STO) single crystal substrates. The ferroelectric properties of the bare BTO films are demonstrated by piezoresponse force microscopy (PFM) measurements. Cyclic voltammetry (CV) measurements in EFS configuration, with phosphate buffered saline (PBS) acting as the liquid electrolyte top contact, indicate characteristic ferroelectric switching peaks in the bipolar current-voltage loop. The ferroelectric nature of the observed switching peaks is confirmed by analyzing the current response of the EFS devices to unipolar voltage signals. Moreover, electrochemical impedance spectroscopy (EIS) measurements indicate bipolar resisitive switching behavior of the EFS devices, which is controlled by the remanent polarization state of the BTO layer. Our results represent a constitutive step towards the realization of neuroprosthetic implants and hybrid neurocomputational systems based on ferroelectric microelectrodes

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures

Emergence and Evolution of Ferroelectricity in Oxide Heterostructures

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

Design and Manipulation of Ferroic Domains in Complex Oxide Heterostructures

The current burst of device concepts based on nanoscale domain-control in magnetically and electrically ordered systems motivates us to review the recent development in the design of domain engineered oxide heterostructures. The improved ability to design and control advanced ferroic domain architectures came hand in hand with major advances in investigation capacity of nanoscale ferroic states. The new avenues offered by prototypical multiferroic materials, in which electric and magnetic orders coexist, are expanding beyond the canonical low-energy-consuming electrical control of a net magnetization. Domain pattern inversion, for instance, holds promises of increased functionalities. In this review, we first describe the recent development in the creation of controlled ferroelectric and multiferroic domain architectures in thin films and multilayers. We then present techniques for probing the domain state with a particular focus on non-invasive tools allowing the determination of buried ferroic states. Finally, we discuss the switching events and their domain analysis, providing critical insight into the evolution of device concepts involving multiferroic thin films and heterostructures.ISSN:1996-194

Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation

Forthcoming low-energy consumption oxide electronics rely on the deterministic control of ferroelectric and multiferroic domain states at the nanoscale. In this review, we address the recent progress in the field of investigation of ferroic order in thin films and heterostructures, with a focus on non-invasive optical second harmonic generation (SHG). For more than 50 years, SHG has served as an established technique for probing ferroic order in bulk materials. Here, we will survey the specific new aspects introduced to SHG investigation of ferroelectrics and multiferroics by working with thin film structures. We show how SHG can probe complex ferroic domain patterns non-invasively and even if the lateral domain size is below the optical resolution limit or buried beneath an otherwise impenetrable cap layer. We emphasize the potential of SHG to distinguish contributions from individual (multi-) ferroic films or interfaces buried in a device or multilayer architecture. Special attention is given to monitoring switching events in buried ferroic domain- and domain-wall distributions by SHG, thus opening new avenues towards the determination of the domain dynamics. Another aspect studied by SHG is the role of strain. We will finally show that by integrating SHG into the ongoing thin film deposition process, we can monitor the emergence of ferroic order and properties in situ, while they emerge during growth. Our review closes with an outlook, emphasizing the present underrepresentation of ferroic switching dynamics in the study of ferroic oxide heterostructures

Research Data supporting "Comprehensive study of Raman optical response of typical substrates for thin-film growth under 633 nm and 785 nm laser excitation"

Names of data files correspond to the names of Figures in publication "Comprehensive Study of Raman Optical Response of Typical Substrates for Thin-Film Growth Under 633 nm and 785 nm Laser Excitation" Files: All other files contain .txt data exported from individual graphs in Figures. Please, use any of the following software to plot the data: - OriginLab (https://www.originlab.com/) - Spyder (https://www.spyder-ide.org/) ----------------------- The research project the data originated from: Raman spectroscopy is one of the most efficient and non-destructive techniques for characterising materials. However, it is challenging to analyse thin films using Raman spectroscopy since the substrates beneath the thin film often obscures its optical response. Here, we evaluate the suitability of fourteen commonly employed single-crystal substrates for Raman spectroscopy of thin films using 633 nm and 785 nm laser excitation systems. We determine the optimal wavenumber ranges for thin-film characterization by identifying the most prominent Raman peaks and their relative intensities for each substrate and across substrates. In addition, we compare the intensity of background signals across substrates, which is essential for establishing their applicability for Raman detection in thin films. The substrates LaAlO3 and Al2O3 have the largest free spectral range for both laser systems, while Al2O3 has the lowest background levels, according to our findings. In contrast, the substrates SrTiO3 and Nb:SrTiO3 have the narrowest free spectral range, while GdScO3, NGO and MgO have the highest background levels, making them unsuitable for optical investigations. The dataset usage instructions: Names of data files correspond to the names of Figures whose data they contain. Optical data for Figures 1-3, S1-S5 can be processed using any software for data analysis, such as Spyder (https://www.spyder-ide.org/). All optical data was pre-processed with background subtraction and baseline corrected (multi-polynomial fit of degree 2). The data collection methods: The optical and electrical data (files for Figures 1-3, S1- S5) were collected using self-made setup as described in: Materials and Methods, section 2 and supplementary data. The setup was operated with self-written software in Python 3. All data were collected at the University of Cambridge, Department of Materials Science & Metallurgy

Probing ferroic states in oxide thin films using optical second harmonic generation

Forthcoming low-energy consumption oxide electronics rely on the deterministic control of ferroelectric and multiferroic domain states at the nanoscale. In this review, we address the recent progress in the field of investigation of ferroic order in thin films and heterostructures, with a focus on non-invasive optical second harmonic generation (SHG). For more than 50 years, SHG has served as an established technique for probing ferroic order in bulk materials. Here, we will survey the specific new aspects introduced to SHG investigation of ferroelectrics and multiferroics by working with thin film structures. We show how SHG can probe complex ferroic domain patterns non-invasively and even if the lateral domain size is below the optical resolution limit or buried beneath an otherwise impenetrable cap layer. We emphasize the potential of SHG to distinguish contributions from individual (multi-) ferroic films or interfaces buried in a device or multilayer architecture. Special attention is given to monitoring switching events in buried ferroic domain- and domain-wall distributions by SHG, thus opening new avenues towards the determination of the domain dynamics. Another aspect studied by SHG is the role of strain. We will finally show that by integrating SHG into the ongoing thin film deposition process, we can monitor the emergence of ferroic order and properties in situ, while they emerge during growth. Our review closes with an outlook, emphasizing the present underrepresentation of ferroic switching dynamics in the study of ferroic oxide heterostructures.ISSN:2076-341

In-situ monitoring of epitaxial ferroelectric thin-film growth

In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.ISSN:0953-8984ISSN:1361-648

Novel diffraction strategies to unravel hidden order in ferroelectric thin films

ISSN:1662-534XISSN:1662-535