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X-ORIGINAL-URL:https://laboratoire-albert-fert.cnrs-thales.fr/
X-WR-CALNAME:Laboratoire Albert Fert
X-WR-CALDESC:CNRS - Thales
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UID:MEC-1102a326d5f7c9e04fc3c89d0ede88c9@laboratoire-albert-fert.cnrs-thales.fr
DTSTART:20261214T083000Z
DTEND:20261214T103000Z
DTSTAMP:20261023T090400Z
CREATED:20261023
LAST-MODIFIED:20261023
PRIORITY:5
SEQUENCE:24
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SUMMARY:PhD defense – Thomas Buttiens
DESCRIPTION:On Monday, December 14, at 9:30 h (TRT Auditorium), Thomas Buttiens will defend his PhD thesis titled “Ferroelectric Pb(Zr,Ti)O3 thin films for emerging spintronic and electronic devices: from room-temperature two-dimensional electron gases to tunnel junctions”, supervised by Manuel Bibes and Thomas Maroutian.\nJury members:\nBertrand Vilquin, professor at INSA Lyon, Reviewer\nMathias Opel, Researcher at Walther Meissner Institute (Garching), Reviewer\nNathalie Viart, Professor at IPMCS Strasbourg, Examiner\nLaurent Vila,  NELLOW, Grenoble, Examiner\nAbstract: \nThe rising energy consumption of information and communication technologies calls for emerging devices beyond CMOS, in which data is stored and processed in a non-volatile way. Ferroelectrics are strong candidates for this purpose: their remanent polarisation can be switched by an electric field at low energy and retained without power. The polarisation state can then be read in different ways. In ferroelectric tunnel junctions (FTJs), it sets the resistance of an ultra-thin barrier. In ferroelectric spin-orbit (FESO) devices, spintronic transistor-like devices with separate write and read paths, it controls the Rashba spin-orbit coupling of a two-dimensional electron gas (2DEG), so that the sign of the spin-to-charge conversion (SCC) follows the polarisation. Previous demonstrations relied on SrTiO3 (STO), whose ferroelectric-like state only exists at cryogenic temperatures. This thesis explores the ferroelectric Pb(Zr,Ti)O3 (PZT) as a platform for both device families, motivated by its large, room-temperature polarisation and by first-principles predictions of a strong, polarisation-dependent Rashba effect. Epitaxial PZT thin films were grown by pulsed laser deposition on SrRuO3 (SRO) bottom electrodes, on GdScO3, DyScO3 and SrTiO3 substrates. X-ray diffraction, reflectivity and atomic force microscopy confirm single-phase, strained films with flat surfaces, while piezoresponse force microscopy (PFM) and polarisation loops show robust switching, with a saturated polarisation up to 95 µC/cm² and an endurance exceeding 10^10 cycles. To induce a 2DEG, an ultra-thin aluminium layer was first deposited directly on PZT, as Al oxidises by pumping oxygen from the underlying oxide. On bare PZT, photoemission spectroscopy and electron microscopy show that this reaction reduces Ti but also produces undesired metallic Pb, which migrates into the AlOx layer, and the resulting films are insulating. Inserting a thin STO buffer between PZT and Al prevents Pb reduction. With optimised STO and Al thicknesses and a post-annealing step, a metallic 2DEG forms at the STO surface, with a sheet resistance of about 10 kΩ/sq, a carrier density of about 2×10^14 cm^-2 and a mobility of about 2 cm²/Vs. Polarisation loops measured using the 2DEG itself as top electrode demonstrate that it can coexist with a switchable ferroelectric polarisation. Finally, spin pumping from a ferromagnetic layer shows evidence of SCC in this 2DEG, the read-out mechanism of FESO devices. Using this platform for devices requires patterning, since the SRO electrode needed to switch the polarisation short-circuits the 2DEG in unpatterned samples. The fabrication of ferroelectric-gated Hall bars and FESO-type devices is then addressed. The main obstacles are identified: defect creation during etching, degradation of the 2DEG during lithography, and potential barriers at the interface between the electrodes and the 2DEG. These results provide guidelines for the fabrication of future devices. Finally, ultrathin PZT layers were grown to reach the tunnelling regime and integrated them into fabricate FTJs. PFM shows that ferroelectricity persists down to a few nanometres, which sets the barrier thickness. SRO/PZT/Co/Au junctions of different sizes were characterised by local probe techniques, and pulsed measurements show reproducible, non-volatile resistance switching. To enable statistical studies, an advanced patterning process compatible with automated probing was developed with SPINTEC, and is transferable to other ferroelectrics. Overall, this work establishes PZT heterostructures as a versatile platform for emerging ferroelectric devices: combined with an STO buffer, PZT hosts a room-temperature 2DEG coexisting with a switchable polarisation, a first step towards FESO devices, and at ultralow thickness, it provides non-volatile resistive switching in tunnel junctions.\n \n
URL:https://laboratoire-albert-fert.cnrs-thales.fr/en/events/phd-defense-maxime-ardisson-857-916-970-297/
CATEGORIES:PhD defence
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