Hafnia-Zirconate (HZO) system was chosen as the thin-film of choice. In the first part of FERHAZ the growth of this system was performed on various substrates: perovskites, hexagonal substrates wuth an aim to understand the effect of size and strain in the stabilization of ferroelectric (FE) phases. On SrTiO3 (STO) substrate, by using a correlated metal (La0.67Sr0.33MnO3) as the back electrode (image 1), we were able to demonstrate unequivocally a new FE rhombohedral phase in HZO [1], with the largest FE polarization in HZO. By systematically optmizing the growth on various substrates, susbequently we were able to understand and provide suitable crystallographic guidelines for the stabilization of this phase, understanding that the stability of this new phase relies on carefully engineering biaxial compressive strain, and a favorable crystallographic orientation at nanoscopic domain sizes (<10 nm). State-of-the-art aberration corrected microscopy techniques such as differential phase contrast STEM, provided unprecedented insights about the crystallographic polarization in these systems [3, 5]. (Image 2)
Taking lessons from these studies, next we were able to carefully engineer this new FE phase directly on Si, and study the FE properties of metal-insulator-semiconductor capacitors. Although it was not possible to engineer just a pure FE phase owing to the low strain that Si substrate provides to the films, the direction to optimize the growth in this way has been set [2].
From a device perspective, FE (and multiferroic) tunnel junctions have been fabricated with rhombohedral HZO tunnel layer sandwiched between Co and LSMO, both ferromagnetic metals [5]. Structure-property analysis of these devices and their failure mechanisms has been strudied through a combination of electrical characterization including memristive hysteresis, endurance, fatigue measurements, and atomic resolution STEM techniques and spectroscopy techniques such as iDPC-STEM, HAADF-STEM and energy dispersive analysis using the Themis Z microscope purchased at the University of Groningen. The influence of oxygen vacancy migration facilitated by LSMO is very crucial to explain the tunnel resistance properties observed in these devices [manuscrript under preparation].
In-situ biasing and heating microscopy protocols (sample preparation, and measurement set up)- which do not exist previously- were being designed to carry out operando studies of device operation inside the electron microscope. However, the COVID pandemic stopped the progress on this towards the end of FERHAZ, only to recently resume (after FERHAZ).
FERHAZ has resulted in 5 direct publications (thus far) in good journals, with ref [2, 5]. These articles have gained significant public interest. For e.g. the discovery of the rhombohedral phase for the first time is reported here:
https://www.eurekalert.org/pub_releases/2018-10/uog-nfb101918.php(öffnet in neuem Fenster) and inside the university media channels here:
https://www.rug.nl/sciencelinx/nieuws/2018/10/20181022_noheda?lang=en(öffnet in neuem Fenster)Our results on direct epitaxy of HZO on Si is explained in the ACS live slides platform here:
https://pubs.acs.org/doi/suppl/10.1021/acsaelm.9b00585/suppl_file/el9b00585_liveslides.mp4(öffnet in neuem Fenster) Our paper on multiferroic tunnel junctions was the editors suggestion in physical review applied in Sept 2019
In addition information about these works have been shared on social media platforms such as facebook and linkedin.
