Outstanding Ferroelectricity in Sol–Gel-Derived Polycrystalline BiFeO₃ Films within a Wide Thickness Range

Abstract

As a promising lead-free ferroelectric, BiFeO3 has a very large intrinsic polarization of ∼100 μC/cm2, enabling its great potential in electronic applications especially in a film format. In this sense, reliable ferroelectric properties are desired; however, pure-phase BiFeO3 films are notorious for their large leakage current, especially of those processed by using the sol–gel methoda facile and industrially scalable method for film preparation. In this study, a protection layer, which can be easily integrated in the sol–gel process, is used to ensure the acquirement of remnant polarization of ∼65 μC/cm2 in ∼200 nm BiFeO3 thin films, whereas O2 annealing can enhance that to ∼120 μC/cm2 in ∼400–700 nm films. Reliable ferroelectricity of BiFeO3 films on Si wafers within a wide thickness range was thus achieved. The obtained ferroelectricity is among the best-achieved properties to date of BiFeO3 films for both thin and intermediate thicknesses, including both chemically and physically derived. These results are helpful to advance potential use of sol–gel-processed BiFeO3 films in electromechanical devices with different desired thicknesses.

Reliable ferroelectricity in sol–gel‐derived BiFeO3 thin films below 200 nm

“…The better performance of pure BFO thin films by the sol–gel method has made significant progress in recent years, with comparable or even superior P r (though with larger E c ) than pure BFO thin films prepared by physical methods at a similar thickness range of ∼300 nm. In particular, our prior film protected by an amorphous surface (as adopted in the present work as shown in Figure 6G) 10,27 is similar to polymeric ferroelectrics with a high breakdown strength 52 . Polarization ( P – E ) hysteresis loops and leakage current plots of the BFO thin films annealed at 550°C without an amorphous layer are shown in Figure S5.…”

“…Oxygen continually flowed into the rapid thermal processing furnace during annealing. All samples containing an additional amorphous top layer are only pyrolyzed at 400°C (not crystallized) during the heat treatment of the last layer, without annealing treatment (above 500°C they would crystallize), as we have reported in our previous work 10 …”

“…Among ferroelectric thin‐film materials, bismuth ferrite BiFeO 3 (BFO is attractive due to its high remanent polarization (∼100 μC·cm −2 ), 8 but several problems prevent BFO thin films from being used in practical applications. For example, during the fabrication of BFO thin films, the crystallization temperature often leads to the volatilization of Bi, whereas the treatment atmosphere normally results in the valence transition of Fe ions and oxygen vacancies 9,10 . During the fabrication of films, secondary phases readily form.…”

“…For example, during the fabrication of BFO thin films, the crystallization temperature often leads to the volatilization of Bi, whereas the treatment atmosphere normally results in the valence transition of Fe ions and oxygen vacancies. 9,10 During the fabrication of films, secondary phases readily form. The above factors can significantly increase the leakage current of BFO thin films, which further deteriorates the ferroelectric properties.…”

Ultrahigh Ferroelectric and Piezoelectric Properties in BiFeO3–BaTiO3 Epitaxial Films Near Morphotropic Phase Boundary

“…During the deposition, the substrate temperature was kept at 600 °C under an oxygen partial pressure (PO 2 ) of 2–13 Pa. To eliminate the effect of thickness on the phase structure and electrical properties, the thicknesses of the prepared BF–BT films were kept at about 300 nm. It has been reported that when the film thickness exceeds a critical value (usually 100–200 nm), the thickness variation has little effect on the physical properties of the ferroelectric films. − Before depositing BF–BT films, SrRuO 3 films were prepared on the (001) oriented SrTiO 3 substrate at 650 °C under PO 2 = 13 Pa as the bottom electrode . The SrRuO 3 film showed an atomically flat surface with a root-mean-square roughness below 0.2 nm, providing an excellent surface for the epitaxial growth of the BF–BT films.…”