Domain wall engineering through controlling of point defects in BiFeO₃ ceramics

Session

PSA.3 - Semiconductors & Devices

Authors

prof Goran Drazic (2, 3), prof. Andreja Bencan (1), Ms. Maja Makarovic (1, 3), assist. prof. Hana Ursic (1), assist. prof. Matej Komelj (1), prof. Tadej Rojac (1)

Affiliations

1. Jozef Stefan Institute
2. National Institute of Chemistry
3. Jozef Stefan International Postgraduate School

Keywords

ceramics, conductivity, Cs corrected STEM, ferroelectrics, domain walls, multiferroics

Abstract text

Using atomic-scale STEM – EELS analysis of domain walls in differently prepared BiFeO₃ ceramics we correlated the type and concentration of accumulated point defects (oxygen and bismuth vacancies, Fe⁴⁺) with the electrical conductivity. We found that using certain synthesis conditions the degree of accumulation of defects and consequently electrical conductivity can be tailored.

 

Ferroelectric and ferroelastic domain walls (DWs) belong to planar topological defects limiting two adjacent domains with uniform polarization. Based on the crystal structure DWs form along various crystallographic planes. They may accumulate different ionic or electronic point defects and can be pinned by crystal defects. The accumulation of these defects that strongly influence properties like electrical conductivity is affected by the conditions of material synthesis or materials thermal treatment. These effects could be directly used in the so-called “domain-wall nanoelectronics” [1,2,3,4] or may indirectly influence the functional response of polycrystalline ferroelectrics [5]. To understand how to control the DW properties, thorough understanding of the structural details of DWs on the nano and atomic scales is needed. 

 

This work is the continuation of recently published work on BiFeO₃ [6] where we explained the extrinsic nature of the DW conduction mechanism dominated by the accumulation of charged defects (Bi vacancies and Fe4⁺) at the DWs. Using Cs corrected STEM and EELS, we will show in this presentation that through different material processing conditions, like temperature, partial oxygen pressure, and cooling rates, we may control the type, and concentration of defects (bismuth and oxygen vacancies, electron holes) at DWs and consequently tailor the local DW structure, thickness, unit-cell distortions across DW and thus DW conductivity. 

 

Using a similar methodology as in [6], we experimentally determined the type of defects and their effect on the DW thickness and lattice strain distribution inside and outside the DW region. By measuring the oxygen to cations displacements from ABF and HAADF STEM images in [100] and [110] zone axis, with a resolution down to ~5 pm we identified the position and the type (charged/non-charged) of DWs. We quantitatively determined strains by peak-pair analysis and identified point defects (such as Bi vacancies) from the relative intensity distribution of the atomic columns in high-angle annular dark-field images. We used EELS analysis to determine the chemical composition and the valence state of Fe ion at the atomic level. The DW’s local conductivity was analyzed using conductive atomic force microscopy. 

 

A strong correlation between the concentration of accumulated defects on DW and electrical conductivity was found. The degree of defects accumulation at the DW can be tuned by the processing cooling rate and partial oxygen pressure. DW conductivity of samples fired in an inert atmosphere is substantially reduced, similar effect exhibit samples fired above Curie temperature and quenched. In the work, these results will be explained and discussed. 

References

[1] J. Seidel et al., Nat. Mater. 8 (2009), p.229.

[2] P. Sharma et al., Sci. Adv. 3 (2017), e1700512. 

[3] G. Catalan et al., Rev. Mod. Phys. 84 (2012), p.119.

[4] J. Seidel, Nature Materials, 18 (2019), 3, p.188

[5] T. Rojac et al., Adv. Funct. Mater. 25 (2015), p.2099.

[6] T. Rojac et al., Nat. Mat. 16 (2017), p.322.

[7] This work was supported by the Slovenian Research Agency (P2-0393, P2-0105).