Growth and local structure of BiFeO₃thin films with giant tetragonality on SrRuO₃-buffered SrTiO₃(001) substrate by ion beam sputtering

Abstract

Rhombohedral (R-) and tetragonal-like (T-) BiFeO 3 (BFO) mixed-phase thin films and single R-phase BFO thin films have been grown on SrRuO 3buffered SrTiO 3 (STO) (001) substrate by ion beam sputtering. From X-ray diffraction study, it was found that T-BFO and R-BFO mixed-phase thin films can be grown at a low oxygen partial pressure of 1 mPa, and the lattice parameters of the T-BFO and R-BFO were a = 0.381 nm, c = 0.461 nm (c/a = 1.22), and a = 0.394 nm and c = 0.400 nm (c/a = 1.02), respectively. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and energy dispersive X-ray analysis (EDXA) mappings have revealed a clear T-BFO lattice and a-c domain in the middle of the thin films. However, Bi-rich and Fe-rich secondary phases were also found with the T-BFO phase. Therefore, it can be concluded that secondary phases such as Bi 2 O 3 or Bi-poor phase are needed for T-BFO formation on STO (001) substrate, and the a-c domain formation does not occur by stress relaxation but matching of the growth directions of different domains.

A multiferroic on the brink: Uncovering the nuances of strain-induced transitions in BiFeO3

“…In some cases, a bismuth oxide phase is manifest in XRD data 29,60 or through transmission electron microscopy (TEM) (Ref. 96 ). With this in mind, one may wonder what is the exact role of bismuth in forming the T' phase of BFO, even on appropriately lattice-matched substrates?…”

Strain evolution of epitaxial tetragonal-like BiFeO3 thin films on LaAlO3(001) substrates prepared by sputtering and their bulk photovoltaic effect

“…13,14) Recently, a metastable phase of BFO with a tetragonal-like structure with a large c=a ratio of ∼1.25 has been reported by many researchers. [15][16][17][18][19] This tetragonallike (T-) BFO has mainly been obtained under a strong compressive strain above 3.5%, for example, which can be induced by LaAlO 3 (LAO) (001) or YAlO 3 (001) singlecrystal substrates. The first-principles calculation predicts that T-BFO could show a spontaneous polarization as large as 150 µC=cm 2 .…”

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale

“…The different Ca solubilities in the two phases observed in this study are of importance for functional device design, as the secondary BO phase leads to the super-tetragonal BFO phase under the right growth conditions, which otherwise only occurs in films grown on substrates inducing a very strong compressive strain. ,− This allows the super-tetragonal phase to be grown on a wider range of substrates, including STO and even polycrystalline ones . In the Ca-doped thin-film sample of this study, the secondary BO phase, however, does not give rise to the super-tetragonal phase but maintains the so-called R-phase, which is monoclinic .…”

“…In the BFO system, Bi 2 O 3 (BO) is one of the secondary phases, which can form. − BO has several times been studied in BFO thin films by TEM techniques showing a variety of different nanostructures. − A lower substrate temperature than usual and a slower growth rate are reported to promote the formation of the secondary BO phase . The BO phase is very useful because it can be utilized to grow super-tetragonal BFO under low substrate strain conditions instead of needing a highly compressive strain, which allows growing the super-tetragonal phase on a wide variety of substrates rather than just a few suitable ones. ,− Super-tetragonal BFO instead of its normal rhombohedral counterpart has the advantage of having a much larger ferroelectric polarization of (130–150) μC/cm 2 instead of (80–100) μC/cm 2 . ,− …”