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

Periodic Reporting for period 2 - INSEETO (In-situ second harmonic generation for emergent electronics in transition-metal oxides)

Reporting period: 2018-07-01 to 2019-12-31

"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? Which new states of matter are introduced by strain or from controlled distributions of defects? How do strain and defects interact, possibly resulting in additional ""synergetic functionalities""? Are there new states of matter emerging at interfaces or by the interaction of constituents across interfaces? Answers to any of these questions will have great impact beyond specialist communities. In addition, all objectives either involve new states of matter with a potential for new types of devices. We are convinced that this will play an essential role in the leap towards the next generation of functional oxide heterostr"
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 tested the investigation of single layers and multilayers. The constructive (destructive) interference between the SHG light emitted from each ferroelectric component of the multilayer informs us on the parallel (antiparallel) relative polarization orientations of the layers within the full heterostructure. The SHG brings the control over a full ferroelectric superlattice polarization state.
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 are determining the thickness at which the polarization emerges in each system and ways to deterministically tune it. We are now investigating the polarization dynamics during the film synthesis. We reveal the impact of surfaces contributions to the final polarization state. We are scrutinizing surface synergetic effects which could potentially lead to enhanced ferroelectric properties. The growth of improper ferroelectrics, mainly YMnO3 is now established. We were We 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 focusing in understanding the role of strain in proper and improper ferroelectrics and learning how SHG in situ can help our strategy.
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.
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.. We submitted a funding application within the ERC proof of concept frame
In situ SHG