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In-situ second harmonic generation for emergent electronics in transition-metal oxides

Periodic Reporting for period 4 - INSEETO (In-situ second harmonic generation for emergent electronics in transition-metal oxides)

Reporting period: 2021-07-01 to 2021-12-31

The technology for the growth of thin oxide films or heterostructures is approaching the same level of atomic control as in the case of semiconductors. Yet, whereas semiconductors are usually described within the single-electron approach, the high electron densities in oxides can be the source of novel phases and functionalities. This includes phenomena like long-range magnetic or electric order, but also exotic behaviour like colossal magnetoresistance. Aside from multiple technological advantages in working with thin films instead of single crystals, one of their outstanding assets is the rich variety of functionalities that emerge when different constituents are combined into a multilayer heterostructure. The fascinating properties of oxide heterostructures continue to infatuate and drive materials scientists to master synthesis, understanding, and control of these systems with the goal of exploiting their properties in devices. The functionality of a heterostructure depends critically on the choice of constituents, design parameters and growth conditions. The iteration procedure is tedious: A heterostructure is grown and characterized and from small deliberate variations in subsequent batches conclusions are drawn on the parameters that determine quality and functionality. This process is greatly sped up if quality and functionality can be observed while the heterostructure is growing. So far, reflection high-energy electron diffraction is the only widely established technique for monitoring the structure and homogeneity of multilayers in-situ, while they are growing, and provide direct feedback information on how to optimise the growth process. However, any insight about the spin- and charge-related phenomena just constituting the extraordinary potential of oxide electronics are inaccessible using this technique. With our proposal we will introduce second harmonic generation (SHG) as new in-situ technique that allows us to track spin-and charge-related phenomena such as ferroelectricity, (anti-) ferromagnetism, insulator-metal transitions, domain coupling effects or interface states in a non-invasive way throughout the deposition process. We are pursuing two goals: first, to establish SHG as new in-situ property-monitoring tool characterization technique for physical vapor deposition which monitors strong spin-charge correlation effects while they emerge during growth; second, to apply in-situ SHG for tailoring novel functionalities in exemplary chosen types of transition-metal-oxide heterostructures of great current interest. These model systems are (i) proper ferroelectrics tuned to high-k dielectric response and improper ferroelectrics whose behavior is determined by the unusual nature of the polar state; (ii) compounds in which the interplay of strain and defects leads to novel and reversibly tunable states of matter; (iii) heterostructures with functionalities originating from the interaction across interfaces. All our objectives will allow us to answer fundamental questions of contemporary condensed-matter research. What are the mechanisms stabilizing the spontaneous polarization in textbook ferroelectrics, but also in those novel types of ferroelectrics that are presently discovered in multiferroics and other complex oxides? What types of new functionalities can be derived from these ferroelectrics? Answers to any of these questions will have great impact beyond specialist communities.
Objective 1: Setup and test of in-situ SHG – We have now real time information about the ferroelectric state of ferroelectric BiFeO3 thin films during the synthesis.We are continuously working on the optimization of the SHG signal intensity for each investigated materials by selecting the optimal probe laser wavelength and pulse duration as well as noise reduction. The growth optimization of each compound is also a time demanding task.
Objective 2.1: The emergence of ferroelectric order – The investigation of proper ferroelectric PbTiO3, BaTiO3 and BiFeO3 is now well advanced. We can detect in situ SHG for all these materials during the synthesis. We have determined the thickness at which the polarization emerges in each system and ways to deterministically tune it. We have been investigating the polarization dynamics during the film synthesis. We also improved the understanding of the mechanism involved in the formation of ferroelectric domains in thin films and monitor directly the domain nucleation process. The growth of improper ferroelectrics, mainly hexagonal manganites is now established. We optimized the integration of such thin films onto commercially available substrates and demonstrated the major impact of the substrate interface on the emergence of the polar state. Epitaxial strain induce a drop of the ferroelectric transition temperature of several hundreds of degrees Celsius.
Objective 2.2: New state of matter driven by strain and defects –We are currently working on the impact of strain on the domain formation and ferroelectric response of the films. In particular, in PZT thin films near the morphotropic phase boundary composition. We detect unconventional continuous tuning of ferroelectric polarization in specific strain states.
Objective 2.3: Functionalities at and across interfaces – We are investigating the ability of SHG to probe interface induced effects and their dynamics, such as the modification of the charge screening environment. We were able to probe in situ the emergence of a strong depolarizing field during the growth. A SHG signal evolution corresponding to an abrupt domain formation indicated such depolarizing field could be monitored in situ. We also established experimental routes bypassing domain formation due to depolarizing field in the model system ferroelectric capacitor. Using atomic termination control we can define the polarization orientation in the films. Using our in situ probe we identified mechanisms involving surface chemistry engineering for polarization enhancement in the ultrathin regime.
We demonstrated the ability to probe in situ during the growth the ferroic functionality of thin films. Our real time SHG approach is overcoming state of the art techniques using intense x-ray radiations and long acquisition time. We also benefit from the non invasive nature of the SHG probe which allows us to investigate out ultrathin layers without the need to place electrodes. Most importantly we revealed the richness of the polarization dynamics during films synthesis. We identified the signature of depolarizing field induced domain formation and the influence of the surfaces contribution to the final polarization state in the ultrathin layers. Finally we demonstrated the ability to monitor sub-unit cell symmetry breaking during the growth of layered ferroelectrics. We therefore establish our symmetry sensitive probe as a tool to investigate layer by layer growth of complex materials and potentially as a monitoring tool for structural defect formation in thin films. We received additional funding via the ERC proof of concept frame to develop a marketable prototype of our in situ probe.
In situ SHG