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Cite this:Nanoscale, 2019,11, 8110

Received 29th January 2019, Accepted 5th April 2019 DOI: 10.1039/c9nr00932a rsc.li/nanoscale

Controllable defect driven symmetry change and domain structure evolution in BiFeO

₃

with

enhanced tetragonality

Chao Chen,â,bChangan Wang,^cXiangbin Cai, ^dChao Xu,êCaiwen Li,â,b Jingtian Zhou,^fZhenlin Luo,^fZhen Fan, â,bMinghui Qin,âMin Zeng,â

Xubing Lu, âXingsen Gao, *âUlrich Kentsch,^cPing Yang,^gGuofu Zhou,^b,h Ning Wang, ^dYe Zhu,êShengqiang Zhou,^cDeyang Chen *â,b,hand Jun-Ming Liu ⁱ

Defect engineering has been a powerful tool to enable the creation of exotic phases and the discovery of intriguing phenomena in ferroelectric oxides. However, the accurate control of the concen- tration of defects remains a big challenge. In this work, ion implantation, which can provide controllable point defects, allows us to produce a controlled defect driven true super-tetragonal (T) phase with a single-domain-state in ferroelectric BiFeO3thinﬁlms. This point-defect engineering is found to drive the phase transition from the as-grown mixed rhombohedral-like (R) and tetragonal- like (MC) phase to true tetragonal (T) symmetry and induce the stripe multi-nanodomains to a single domain state. By further increasing the injected dose of the He ion, we demonstrate an enhanced tetragonality super-tetragonal (super-T) phase with the largest c/a ratio of∼1.3 that has ever been experimentally achieved in BiFeO3. A combination of the morphology change and domain evolution further conﬁrms that the mixed R/MC phase structure transforms to the single-domain-state true tetragonal phase.

Moreover, the re-emergence of the R phase and in-plane nanoscale multi-domains after heat treatment reveal the memory eﬀect and reversible phase transition and domain evolution. Ourﬁndings demonstrate the reversible control of R-Mc-T-super T symmetry changes (leading to the creation of true T phase BiFeO3 with enhanced tetragonality) and multidomain-single domain structure evolution through controllable defect engineering. This work also provides a pathway to generate large tetragonality (or c/a ratio) that could be extended to other ferroelectric material systems (such as PbTiO3, BaTiO3 and HfO2) which might lead to strong polarization enhancement.

As a consequence of strong couplings and complex interplay between defects and the lattice, spin, charge and orbital degrees of freedom in complex oxides, defect engineering provides a plethora of splendid possibilities for creating a wealth of exotic phases and emergent physical phenomena.^1–7In ferroelectric oxides, defect engineering is emerging as a powerful pathway to tailor structural transformation and manipulate material properties,^8–12 e.g., defect induced super-tetragonal PbTiO3and tetragonal-like BiFeO3 thin films with giant tetragonality and enhanced polarization by the introduction of PbO¹³and Bi2O3 impurities,^14–16respectively. The creation of novel domain states, the control of domain structure evolution, domain wall conduction and polarization switching as well as the enhancement of ferroelectric Curie temperature have also been realized through defect engineering.^17–22 However, these defects, introduced by changing growth con- ditions, cannot be precisely controlled. In recent years, the atomic-scale layer-by-layer growth techniques of oxide hetero- structures have enabled the controllable introduction of planar defects—interfaces, leading to new modalities arising in superlattices, such as our recent discovery of polar vortices in PbTiO3/SrTiO3superlattices⁸and domain configuration design in BiFeO3/La-BiFeO3superlattices.²³However, the post growth control of these planar defects is not allowed as they are

aInstitute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.

E-mail: [email protected], [email protected]

bGuangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics,

South China Normal University, Guangzhou 510006, China

cHelmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstr. 400, 01328 Dresden, Germany

dDepartment of Physics and Center for Quantum Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

eDepartment of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

fNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China

gSingapore Synchrotron Light Source, National University of Singapore, 5 Research Link, 117603, Singapore

hNational Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, China

iLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

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imparted during the growth process. Therefore, ion implantation, the standard technique to introduce accurately controlled defects post growth in semiconductor devices for the adjustment of electronic properties,^24–26 is used in this work to study the controllable defect engineering of structure trans- formations and domain evolution in ferroelectric BiFeO3thin films.

As one of the very few known room temperature multifer- roic (ferroelectric and antiferromagnetic) materials,^27,28 BiFeO3exhibits rich phase diagrams which include bulk rhombohedral, rhombohedral-like, tetragonal-like and ortho- rhombic phases that have been theoretically predicted by density functional calculations and phase field simulations,^29–31 and experimentally achieved through strain engineering (e.g., epitaxial strain and hydrostatic pressure)^29–39 and defect engineering (such as substitutional defects and impurity defects),14,15,40–42leading to a large number of intriguing properties.^29,43–51 Among these, tetragonal-like BiFeO3

driven by epitaxial strain with a large c/a ratio (∼1.23) is of great interest due to its giant spontaneous polarization up to 150 µC cm⁻².⁴²Indeed, both theoretical calculations^52–56and experimental studies13,14,42,57–59have demonstrated that ferroelectric oxides with giant tetragonality (or c/a ratio) possess a large polarization as a consequence of their large dipolar moment. However, despite considerable eﬀort and dramatic progress in the research of tetragonal-like (Mc) BiFeO3, broad questions remain: (i) is it possible to stabilize tetragonal-like phase BiFeO3in thick films while rhombohedral-like (R) phase BiFeO3generally emerges due to strain relaxation in the films thicker than∼30 nm,²⁹(ii) is there a pathway to achieve the theoretically predicted true tetragonal (T) phase with a single domain state instead of the experimentally observed tetragonal-like Mc phase with a multi-nanodomain structure in pure BiFeO3, and (iii) could the c/a ratio (tetragonality) be further enhanced to obtain real super-tetragonal (super T) phase BiFeO3that would lead to a larger spontaneous polarization? These challenges to be addressed are the main focus of this study.

Ion implantation, as a controllable defect engineering route, has enabled the ability to continuously tailor the structure phase transition, electric transport, magnetic properties, and the band gap in epitaxial oxide thin films by driving the out-of-plane lattice expansion while leaving the in-plane lattice epitaxially locked to the substrate.^60–64 Very recently, it has also been demonstrated that He ion implantation can be used to control ferroelectric switching behaviors and enhance elec- trical resistivity in ferroelectric thin films (such as PbTiO3, PbZr0.2Ti0.8O3 and BiFeO3).^65–67 However, the use of He implantation to simultaneously control the phase transition and domain structure evolution in BiFeO3thin films remains elusive.

In this work, we start from BiFeO3films with a mixed rhombohedral-like (R) and tetragonal-like (Mc) phase. We have dis- covered that controlled point defects, introduced by He ion implantation, can be used to drive the R-Mc-T-super T phase transition, enabling the formation of a true super-tetragonal

phase in 70 nm-thick BiFeO3films with enhanced tetragonality (c/a ratio ∼ 1.3). Implanted He ions in the films enable not only the absence of the R phase but the transformation from the Mc structure to theoretically predicted true T symmetry.^52,53 Further increasing the dosages can continuously enhance the c-axis lattice parameter from 4.65 Å to 4.93 Å and the c/a ratio from 1.22 to 1.3 (the largest tetragonality has ever been experimentally obtained in BiFeO3). It is also found that the in-plane nanoscale stripe domains (multi- domains) in the as-grown sample evolve to a single domain state after He implantation, which further confirms the Mc to true T phase transition. Moreover, the annealing process has enabled the re-emergence of the R phase and in-plane stripe nanodomains, demonstrating the reversible control of phase transitionviaHe implantation.

We grew a series of epitaxial 70 nm-thick BiFeO3 films on LaAlO3 (001) substrates by pulsed laser deposition (PLD) at 680 °C under an O2pressure of 100 mTorr and cooled under 1 atm oxygen atmosphere. After deposition, He ion implantation was carried out at 8 keV with a fluence from 5 × 10¹⁴to 5 × 10¹⁵ions per cm². A combination of X-ray diﬀraction (XRD), reciprocal space mapping (RSM) and transmission electron microscopy (TEM) was used for the structural characterization.

Morphology changes and ferroelectric domain structures were determined using atomic force microscopy (AFM) and piezoresponse force microscopy (PFM).

The topography of the as-grown sample, as shown in Fig. 1a, was imaged by using the AFM measurement, revealing the coexistence of the striped dark contrast R phase and bright contrast MC phase matrix in the 70 nm-thick BiFeO3 film, which is consistent with previous work.²⁹The atomically flat terraces, one unit cell in height, confirm the high quality growth of the film, with a root mean square (RMS) roughness only of 0.2 nm. Interestingly, the striped R phase is greatly reduced after He implantation with the dose of 5 × 10¹⁴ions per cm², and it completely disappears with the implanted dose up to 5 × 10¹⁵ ions per cm², as presented in Fig. 1b and c, respectively. We infer that there is an R-Mc phase transition induced viaHe implantation. To verify this hypothesis, X-ray reciprocal space mappings (RSMs) are carried out. RSM (002) reflections in Fig. 1d show the coexistence of R and T-like phases. The three-fold and two-fold splits along (103) and (113) RSMs (Fig. 1g and h) further confirm that the tetragonal- like structure is Mc phase in the as-grown sample, in agreement with previous work.^31,35,39Surprisingly, the mixed R and Mc phases evolve to a true tetragonal structure with an enhanced c/a ratio after He implantation (Fig. 1e, i and j).

Although the nearly tetragonal phase was obtained by biaxial strain,^68,69this is the first report to achieve a true T phase in high quality pure BiFeO3at room temperature, which has only been realized at high temperature or in chemical doping BiFeO3.^31,70,71Moreover, further enhancement of tetragonality (c/a ratio ∼1.29) is obtained as the implanted He dose increases to 5 × 10¹⁵ions per cm². Here we refer this tetragonal phase with an ultra-large c/a ratio to a true super-tetragonal (super-T) phase, which is diﬀerent from the previously

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reported super-tetragonal like phase that is the Mc phase in fact.29,33,35,39,72,73Typical X-ray θ−2θ scans show an obvious peak shift with the increasing implanted dosages, as shown in Fig. 1k, further confirming the Mc-T-super T symmetry

changes driven by this controlled defect engineering route.

Based on these XRD data (Fig. 1d–k), the dependence of the c/a ratio and c-axis with various He dosages is plotted in Fig. 1l. It shows the continuous enhancement of the c-axis Fig. 1 (a–c) Morphology evolution in mixed phase BiFeO₃(BFO) with the increase of implanted He dosages. (d–f ) RSM (002) reflections corresponding to (a–c) samples reveal the He implantation (defect engineering) induced phase transitions and elongated out-of-plane (c-axis). (g–h) Three-fold and two-fold splits along (103) and (113) RSMs confirm the tetragonal-like Mc phase in the as-grown samples. (i–j) Single peak feature of (103) and (113) RSMs in the He implanted sample indicates the creation of the true tetragonal phase. (k) Typical X-rayθ−2θscans of the as-grown and He implanted BiFeO₃thinfilms on LaAlO₃(LAO) substrates. (l) Dependence of the c/a ratio andc-axis lattice parameter with various He dosages.

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lattice parameter from 4.65 Å to 4.93 Å and the c/a ratio from 1.22 to 1.29. These are the largest values of thec-axis and c/a ratio which have ever been experimentally achieved in BiFeO3.

To understand the fine structure of these implanted films, atomic resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) studies were carried out.

Fig. 2a shows a dark field cross-sectional STEM image of the implanted film with 2.5 × 10¹⁵He per cm², not only confirming the thickness (∼70 nm) of BiFeO3 and the existence of a single phase (no contrast diﬀerences as prior work^29,71), but also revealing the maintenance of high-quality epitaxy with sharp interfaces after He implantation. A high-resolution Z-contrast image of the BiFeO3/LaAlO3 interface is shown in the bottom-right insets of Fig. 1a, illustrating the atomic-scale epitaxy between the BiFeO3 and LaAlO3 layers. This type of image is sensitive to variations in the atomic number and the intensity corresponds to the increasingZ number. Thus, the elements in decreasing order of brightness are Bi (83) and Fe (26), while O (8) is too light to be visible, which is confirmed in Fig. 1b using an ADF-STEM image and the corresponding EDS mappings. The selected area electron diﬀraction (SAED) patterns of implanted BiFeO3with He doses of 2.5 × 10¹⁵and 5 × 10¹⁵ions per cm²in Fig. 2c and d reveal the existence of a

single T phase (i.e., absence of the R phase) and the enhancement of c/a ratios to 1.27 and 1.30, respectively. Further atomic structure of these implanted films is shown in Fig. 2e and f, presenting the strongly elongated out-of-plane lattice parameter driven by He implantation with almost no changes of the in-plane lattice parameter constrained by the substrate.

Specifically, thec-axis changes from 4.65 Å to 4.93 Å and the c/a ratio is enhanced from 1.22 to 1.30, in contrast to the as- grown sample.

After successfully realizing the defect induced true super- tetragonal phase with giant tetragonality (c/a∼1.30) in BiFeO3

films, we next turn to study the domain structure evolution driven by this defect engineering eﬀect. AFM images shown in Fig. 3 again confirm the morphology evolution with the increase of He doses. In the meantime, we probe the corresponding domain structures using piezoresponse force microscopy (PFM). The uniform out-of-plane (OOP) PFM contrast and the in-plane (IP) nanodomains shown in Fig. 3a indi- cate the Mc structure with a multi-domain state in the as- grown film, in accordance with previous work.^39,74–76Both the implanted samples show the uniform OOP and IP contrasts (Fig. 3b and c), indicating the formation of a true T phase with a single domain state in agreement with our XRD and RSM

Fig. 2 (a) Darkfield cross-sectional TEM images of a He implanted 70 nm-thick BiFeO₃thinfilm and the BiFeO₃and LaAlO₃interface (insets in the bottom-right). (b) Angular darkfield (ADF)-STEM image of BiFeO₃and the corresponding EDS mapping. (c–d) Selected area electron diffraction (SAED) patterns of implanted BiFeO₃with He doses of 2.5 × 10¹⁵and 5 × 10¹⁵ions per cm², respectively. (e–f ) ADF-STEM atomic structure images correspond to the SAED samples in (c–d).

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data. Therefore, the multi-domain to single domain structure evolution further verifies the Mc-T phase transition (this obser- vation is distinct from the discovery of a multi- to a single- domain state change in (Ba,Ca)(Zr,Ti)O3under an electric field which shows the ferroelectric rotation process without any structural change⁷⁷). It is worth noting that although temperature-driven true T phase BiFeO3 has been reported,^70,71 the domain structure has not been captured, inhibited by the unstable PFM scans at high temperature. Here we first time obtain the PFM images that reveal the single domain state in true T phase BiFeO3.

To explore the reversibility of phase transition and domain evolution induced by He implantation, high temperature

annealing of the 5 × 10¹⁴He per cm² dosed BiFeO3 film was carried out at 550 °C under 1 atm oxygen atmosphere for 1 hour. A combination of the re-emergence of the dark-contrast R phase (Fig. 4a) and nanoscale stripe domains (Fig. 4c) indicates the reproduction of mixed R and Mc phases due to the release of He, which is confirmed with the (002), (103) and (113) RSMs shown in Fig. 4d, e and f, respectively. Schematics shown in Fig. 4g summarize the reversible control of the R-Mc- T-super T phase transition in BiFeO3viaHe ion implantation and high temperature annealing.

In summary, we have demonstrated the ability to control the defect-driven R-Mc-T-super T phase transition and multi- single domain evolution in BiFeO3thin films, enabling the cre- Fig. 3 Topography, out-of-plane (OOP) and in-plane (IP) PFM images (from left to right) reveal both the morphology and domain structure evolution, with the increasing He implantation doses, in 70 nm-thick BiFeO3thinﬁlms: (a) mixed phase morphology with IP PFM nanoscale stripe domains in the as-grown BiFeO3sample. (b–c) Morphologies and PFM images of implanted samples show the phase transition and domain structure evolution through He implantation with doses of 5 × 10¹⁴and 5 × 10¹⁵ions per cm², respectively.

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ation of the single-domain-state, true super-tetragonal phase with enhanced tetragonality (c/a∼1.30). The reversible phase transition and domain evolution have been revealed by high temperature annealing to release the implanted point defects.

Our findings open a door to achieve giant tetragonality (or c/a ratio) in ferroelectric thin films which could be extended to

other ferroelectric material systems (such as PbTiO3, BaTiO3

and HfO2) which might lead to strong polarization enhancement.

Moreover, the discovery of controlled defect engineered domain evolution in this work provides a future direction to create and manipulate exotic domain states (e.g., vortices and skyrmions) in ferroelectric and magnetic materialsviathis approach.

Fig. 4 (a) Topography, (b) OOP PFM and (c) IP PFM images after annealing the implanted BiFeO₃at 550 °C under 1 atm oxygen atmosphere for 1 hour. (d) (002), (e) (103) and (f ) (113) RSM reﬂections of the annealing sample shown in (a–c). (g) Schematics of the reversible control of the R-Mc- T-super T phase transition in BiFeO₃viaHe ion implantation and annealing which have been demonstrated in this work.

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Author contributions

D.C. conceived and designed the project. S.Z. proposed the ion implantation/irradiation approach. C.C and C.L. fabricated and characterized the thin films. C.W. and U.K. conducted the He implantation experiments. X.C., C.X., N.W. and Y.Z. carried out the STEM measurements. J.Z., Z.L. and P.Y. contributed to the XRD and RSM data interpretation. Z.F., M.Q., M.Z., X.L.

and G.Z. analyzed the PFM data. X.G. and J.-M.L. discussed the TEM and AFM data. D.C. and C.C. wrote the paper with contributions and feedback from all authors. All authors discussed the results and commented on the manuscript.

Con ﬂ icts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2016YFA0201002) and the National Natural Science Foundation of China (Grant No.

U1832104 and 11704130). The authors also acknowledge the financial support of the Natural Science Foundation of Guangdong Province (No. 2017A30310169). Y. Z. acknowledges the financial support from the Research Grants Council of Hong Kong (Project No. 15305718) and the Hong Kong Polytechnic University grant (Project No. 1-ZE6G). N. W.

acknowledges the financial support from the Research Grants Council of Hong Kong (Project No. C6021-14E). C. Wang thanks China Scholarship Council (No. 201606750007) for financial support. S. Z. acknowledges the financial support from the German Research Foundation (ZH 225/10-1). Ion irradiation was done at the Ion Beam Center (IBC) at the Helmholtz-Zentrum Dresden-Rossendorf. X. Gao and X. Lu acknowledge the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2014 and 2016, respectively.

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