Designing Morphotropic Phase Composition in BiFeO₃

Abstract

In classical morphotropic piezoelectric materials, rhombohedral and tetragonal phase variants can energetically compete to form a mixed phase regime with improved functional properties. While the discovery of morphotropic-like phases in multiferroic BiFeO3 films has broadened this definition, accessing these phase spaces is still typically accomplished through isovalent substitution or heteroepitaxial strain which do not allow for continuous modification of phase composition postsynthesis. Here, we show that it is possible to use low-energy helium implantation to tailor morphotropic phases of epitaxial BiFeO3 films postsynthesis in a continuous and iterative manner. Applying this strain doping approach to morphotropic films creates a new phase space based on internal and external lattice stress that can be seen as an analogue to temperature-composition phase diagrams of classical morphotropic ferroelectric systems.

Optical response of BiFeO3 films subjected to uniaxial strain

“…While the lattice parameter of the BFO/LAO film is already large in the as-grown strain state and increasing moderately with He doping, the lattice of the BFO/LSAT film shows an extraordinary expansion due to the phase transition towards the highly tetragonal T-like BFO polymorph. This response is in perfect agreement with previous work which demonstrated tailoring of morphotropic BFO phases with strain doping [19].…”

“…As an example, He ion implantation can modify oxygen octahedral rotations in perovskites [18], which can change orbital overlaps and consequently reduce electronic band gaps [35,36]. Similarly, strain doping has been shown to cause a series of structural phase transition in T-BFO that can be linked to a rotation of the polarization vector towards out of plane [19]. Most polar perovskites possess lower electronic band gaps in their tetragonal phase than in their rhombohedral phase.…”

“…Previous work demonstrated that low-energy He ion implantation can be used to introduce uniaxial strain in epitaxial oxide thin films [17,18]. Strain doping of BFO films recently showed it is possible to exert continuous control of morphotropic phase composition, i.e., the direct transformation of R-to T films postsynthesis [19]. Uniaxial out-of-plane lattice expansion induced by strain doping thereby offers an excellent opportunity to generate a full array of strain states that are otherwise impossible to achieve by biaxial in-plane strain via standard epitaxy.…”

Strain Engineering of Epitaxial Oxide Heterostructures Beyond Substrate Limitations

“…4c and 4d) reveals the realization of true T phase (not MC phase) BFO via this proposed method. Although the nearly tetragonal phase on LAO 58 and YSZ 59 substrates by biaxial strain and true T phase on LAO by chemical doping 49 or uniaxial strain [37][38][39] have been reported, here we obtain true T phase BFO using biaxial strain. RSM data of 50 nm-thick BFO on STO, NSO and LSAT (as shown in Fig S4, Fig S5 and Fig S6, respectively) further demonstrate the coexistence of R and T-like phases in agreement with AFM measurements (Fig.…”

Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

“…Due to its nobility, helium implants interstitially, without chemical substitution, thus inducing a "swelling" of the host material's unit cell volume [19]. Helium implantation is customarily achieved by laboratory ion sources [7,[20][21][22][23], similar to those used for wafer processing in semiconductor engineering. Another route-the one chosen in the present work-is to use a helium ion microscope as a way to implant ions locally.…”