SCIENTIFIC HIGHLIGHTS

Ferroelectric BaTiO3 and doped-HfO2: epitaxy and control of polarization

The ferroelectric polarization of epitaxial oxide films is controlled by growth kinetics and by concurrent action of light and adsorbates

The classical methods to tune polarization of ferroelectric films require chemical substitutions or selection of a particular substrate. We have developed two alternative methods that are more flexible, permitting tailoring polarization without requiring changes in film composition or substrate. One strategy, illustrated with the paradigmatic ferroelectric BaTiO3, is based on imposing kinetic limitations during epitaxial growth. The balance between kinetics and thermodynamics fixes the amount of point defects in the deposited films. The defects produce lattice expansion of BaTiO3, determining the unit cell tetragonality and ultimately the ferroelectric polarization. This method, that allows obtaining a particular polarization without changing composition or substrate, is demonstrated with BaTiO3 films integrated epitaxially on Si(001) wafers [1].

The second strategy permits dynamic control of the polarization by using light radiation. Under illumination, photoinduced carriers in semiconductors and photodissociated adsorbates modify the electrostatic screening at the surface of ferroelectric films and modify the switchable polarization. We have shown that water-related adsorbates at the surface of BaTiO3 enable a substantial modulation (up to 75%) of the switchable remanent polarization by light. As ferroelectric perovskites hold promises of enhanced photovoltaic efficiency in solar cells and photocatalytic activity, these findings may get a far reaching relevance for novel applications of [2].

The recent discovery of ferroelectricity in doped HfO2 represents a breakthrough towards commercial permanent memories based in ferroelectrics. The metastable ferroelectric phase of HfO2 is obtained in polycrystalline films, and epitaxial ferroelectric HfO2 films have been rarely achieved. However, epitaxial films are needed for better understanding of the properties and prototyping devices of nanometric dimensions. We have stabilized the ferroelectric phase of Hf0.5Zr0.5O2 films in epitaxial films, which present high polarization that depends strongly on the thickness and demonstrating by the first time for epitaxial films absence of wake up effect, long retention and high endurance against fatigue. [3]

Authors:
Ignasi Fina,1 Jike Lyu,1 Fanmao Liu,1 Raul Solanas,1 Guillaume Sauthier,2 Andrew M. Rappe,3 Josep Fontcuberta,1 and Florencio Sánchez1

Affiliations:
1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Spain
3 Department of Chemistry, University of Pennsylvania, Philadelphia, USA

Publications:
[1] Tailoring Lattice Strain and Ferroelectric Polarization of Epitaxial BaTiO3 Thin Films on Si(001)
Scientific Reports 8, 495 (2018)
DOI: 10.1038/s41598-017-18842-5
[2] Control of the polarization of ferroelectric capacitors by the concurrent action of light and adsorbates
ACS Applied Materials & Interfaces 10, 23968-23975 (2018).
DOI: 10.1021/acsami.8b05751
[3] Robust ferroelectricity in epitaxial Hf1/2Zr1/2O2 thin films
Applied Physics Letters
113, 082902 (2018)
DOI: 10.1063/1.5041715

Figure:
(a) Sketch of a Pt/BaTiO3/La2/3Sr1/3MnO3 capacitor under illumination, with adsorbates in the Pt/ BaTiO3 interface.
(b) Dependence on deposition temperature of the remnant polarization of epitaxial BaTiO
3 films integrated epitaxially with Si(001).
(c) Epitaxial Hf
0.5Zr0.5O2 films with retention time longer than 10 years.

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