Martin F. Sarott

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Sarott

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Martin F.

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0000-0001-9660-6346

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Publications 1 - 10 of 18

Ferroelectric Thin Films for Oxide Electronics
Müller, Marvin; Efe, Ipek; Sarott, Martin F.; et al. (2023)
ACS Applied Electronic Materials
Ferroelectric materials have set in motion numerous ultralow-energy-consuming device concepts that can be integrated into state-of-the-art complementary metal–oxide–semiconductor technology. Their nonvolatile, spontaneous electric polarization makes them promising candidates to control functionalities at the nanoscale with energy-efficient electric fields only. In this spotlight article, we start with a brief introduction to ferroelectric materials, the challenges involving the design of thin films and review the state-of-the-art of their integration into various electronic applications. Revolutionary in situ and operando diagnostic tools allowing the monitoring of the technology-relevant polarization state during the material design, or its operation will be detailed. Concepts such as chiral states in ferroelectrics and neuromorphic-type switching will be addressed to provide a comprehensive view on the evolution of ferroelectric states for the next generation of low-energy-consuming electronics. Finally, we discuss the most recent developments in the field, including the emergence of ferroelectricity at the nanoscale and in two-dimensional systems.
Origin of the Critical Thickness in Improper Ferroelectric Thin Films
Vogel, Alexander; Ruiz Caridad, Alicia; Nordlander, Johanna; et al. (2023)
ACS Applied Materials & Interfaces
Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.
Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics
Gradauskaite, Elzbieta; Meier, Quintin N.; Gray, Natascha; et al. (2023)
Nature Materials
Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in so-called dead layers. It is particularly problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane-polarized ferroelectric epitaxial buffer provides the continuity of polarization at the interface; despite its insulating nature, we observe the emergence of polarization in our out-of-plane-polarized model of ferroelectric BaTiO₃ from the very first unit cell. In BiFeO₃, the flux-closure approach stabilizes a 251° domain wall. Its unusual chirality is probably associated with the ferroelectric analogue to the Dzyaloshinskii–Moriya interaction. We, thus, see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of functionality. This could be a useful insight on the road towards the next generation of oxide electronics.
Bulk-suppressed and surface-sensitive Raman scattering by transferable plasmonic membranes with irregular slot-shaped nanopores
Wyss, Roman M.; Kewes, Günter; Marabotti, Pietro; et al. (2024)
Nature Communications
Raman spectroscopy enables the non-destructive characterization of chemical composition, crystallinity, defects, or strain in countless materials. However, the Raman response of surfaces or thin films is often weak and obscured by dominant bulk signals. Here we overcome this limitation by placing a transferable porous gold membrane, (PAuM) on the surface of interest. Slot-shaped nanopores in the membrane act as plasmonic antennas and enhance the Raman response of the surface or thin film underneath. Simultaneously, the PAuM suppresses the penetration of the excitation laser into the bulk, efficiently blocking its Raman signal. Using graphene as a model surface, we show that this method increases the surface-to-bulk Raman signal ratio by three orders of magnitude. We find that 90% of the Raman enhancement occurs within the top 2.5 nm of the material, demonstrating truly surface-sensitive Raman scattering. To validate our approach, we quantify the strain in a 12.5 nm thin Silicon film and analyze the surface of a LaNiO3 thin film. We observe a Raman mode splitting for the LaNiO3 surface-layer, which is spectroscopic evidence that the surface structure differs from the bulk. These results validate that PAuM gives direct access to Raman signatures of thin films and surfaces.
Imaging ferroelectric domains with a single-spin scanning quantum sensor
Huxter, William; Sarott, Martin F.; Trassin, Morgan; et al. (2023)
Nature Physics
The ability to sensitively image electric fields is important for understanding many nanoelectronic phenomena, including charge accumulation at surfaces and interfaces and field distributions in active electronic devices. A particularly exciting application is the visualization of domain patterns in ferroelectric and nanoferroic materials, owing to their potential in computing and data storage. Here, we use a scanning nitrogen-vacancy (NV) microscope, well known for its use in magnetometry, to image domain patterns in piezoelectric (Pb[Zr0.2Ti0.8]O3) and improper ferroelectric (YMnO3) materials through their electric fields. Electric field detection is enabled by measuring the Stark shift of the NV spin using a gradiometric detection scheme. Analysis of the electric field maps allows us to discriminate between different types of surface charge distributions, as well as to reconstruct maps of the three-dimensional electric field vector and charge density. The ability to measure both stray electric and magnetic fields under ambient conditions opens opportunities for the study of multiferroic and multifunctional materials and devices.
Onset of Multiferroicity in Prototypical Single-Spin Cycloid BiFeO₃ Thin Films
Dufour, Pauline; Abdelsamie, Amr; Fischer, Johanna; et al. (2023)
Nano Letters
In the room-temperature magnetoelectric multiferroic BiFeO3, the noncollinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single-domain ferroelectric and spin cycloid state in epitaxial BiFeO3 (111) thin films grown on orthorhombic DyScO3 (011). Comparing them with films grown on SrTiO3 (111), we identify anisotropic in-plane strain as a powerful handle for tailoring the single antiferromagnetic state. In this single-domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nm. This as-grown single-domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically controlled noncollinear antiferromagnetic spintronics.
Controlling the Polarization in Ferroelectric PZT Films via the Epitaxial Growth Conditions
Sarott, Martin F.; Bucheli, Ursina; Lochmann, Adrian; et al. (2023)
Advanced Functional Materials
The integration of thin-film ferroelectrics with reliable properties into oxide electronics requires accomplishing deterministic polarization states. Since ferroelectricity emerges during thin-film synthesis already, it is essential to elucidate how the interplay of different growth parameters affects the polarization. Here, the polarization of fully strained Pb(Zr₀.₂Ti₀.₈)O₃ (PZT) films is accessed in situ, during epitaxial growth. Surprisingly, it is found that the orientation of the out-of-plane polarization during growth may differ from the one after growth completion and it strongly depends on the substrate temperature and the oxygen partial pressure. Increasing the growth temperature and/or the oxygen partial pressure favors a uniform downward-oriented polarization, independent of the direction of polarization during growth. Specifically, for films with an emerging upward-oriented polarization, a polarization reversal and a downward-oriented polarization after cool-down is observed. The in situ measurements obtained by optical second harmonic generation (SHG) in conjunction with ex situ piezoresponse force microscopy (PFM) and X-ray diffraction (XRD) measurements point to the temperature- and pressure-dependent formation of a charged Pb defect gradient toward the film surface as the responsible mechanism for the polarization reorientation.
Improving the crystallinity and texture of oblique-angle-deposited AlN thin films using reactive synchronized HiPIMS
Patidar, Jyotish; Sharma, Amit; Zhuk, Siarhei; et al. (2023)
Surface and Coatings Technology
Many technologies, such as surface-acoustic-wave (SAW) resonators, sensors, and piezoelectric MEMS require highly-oriented and textured functional thin films. The best results are typically achieved for on-axis sputter geometries, but in some scenarios, this is not feasible, such as during co-deposition from multiple magnetrons or when coating substrates with high aspect ratios. Ionized physical vapor deposition (PVD) techniques such as HiPIMS can be used to accelerate the film-forming species onto the growing film using substrate-bias potentials, thus increasing ad-atom mobility and film texture. However, gas-ion incorporation can limit the feasibility of such synthesis approaches for defect-sensitive functional thin films. This work reports on the oblique-angle deposition of highly textured, c-axis oriented AlN (0002) films using reactive metal-ion synchronized HiPIMS. AlN thin films deposited using direct current magnetron sputtering (DCMS) and HiPIMS are discussed for comparison and the effect of ion irradiation through substrate biasing is investigated. We find that combining HiPIMS with a moderate substrate bias of only −30 V improves the crystalline quality and texture of the films significantly, while the process-gas incorporation and point defects formation can be further reduced by synchronizing the negative substrate bias potential to the Al-rich fraction of each HiPIMS pulse. In addition to a pronounced out-of-plane texture, the films show uniform polarization of the grains making this synthesis route suitable for piezoelectric applications. While the compressive stress in the films is still comparatively high, the results already demonstrate, that metal-ion synchronized HiPIMS can yield promising results for the synthesis of functional thin films under oblique-angle deposition conditions - even with low substrate-bias potentials.
Signatures of enhanced out-of-plane polarization in asymmetric BaTiO3 superlattices integrated on silicon
Chen, Binbin; Gauquelin, Nicolas; Strkalj, Nives; et al. (2022)
Nature Communications
In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO3/BaTiO3/SrTiO3 superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c-axis oriented BaTiO3 layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO3 single film and conventional BaTiO3/SrTiO3 superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO3 as the thickness of BaTiO3 increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.
Multilevel polarization switching in ferroelectric thin films
Sarott, Martin F.; Rossell, Marta D.; Fiebig, Manfred; et al. (2022)
Nature Communications
Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr0.52Ti0.48O3 films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

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Martin F. Sarott

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