INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-32, 2e étage, salle 201
Nick Barrett - SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette
Ferroelectric materials represent a potential technological breakthrough for post-CMOS electronics in terms of high density storage, rapidity and energy consumption. Non-invasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of Low energy (LEEM) and photoemission electron microscopy (PEEM) to charge, local potential, chemistry and electronic structure makes them useful tools for probing the near surface region of ferroelectric domains and domain walls. Following an introduction to LEEM and PEEM we will illustrate using two main examples how these techniques may be employed to study such systems. In the first example, we study the interaction of injected charge with polar twin walls in non-polar CaTiO3. The contrast between domain and domain walls vanishes upon electron irradiation. It is possible to recover the initial state, i.e. before charge injection, by annealing above 250°C. The ability to observe polarity at the nanoscale and to tune the surface charge at twin walls creates perspectives for the functionalization of polar twin walls in CaTiO3. In the second example, we present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO3 (001) single crystal. The n-type doping of the BaTiO3 is controlled by in-situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in and out-of plane polarization with two and fourfold symmetry, respectively. The results agree well with first principles calculations.
This work was supported by the ANR projects ANR-12-IS04-0001-01 CHEM-SWITCH and ANR-14- CE35-0019-01HREELM. We acknowledge Elettra for the provision of synchrotron radiation facilities.
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J. E. Rault et al, Phys. Rev. Lett. 111, 127602 (2013)