Chemical Solution Route for High‐Quality Multiferroic BiFeO₃ Thin Films

Abstract

Bismuth ferrite (BiFeO3) has recently become interesting as a room‐temperature multiferroic material, and a variety of prototype devices have been designed based on its thin films. A low‐cost and simple processing technique for large‐area and high‐quality BiFeO3 thin films that is compatible with current semiconductor technologies is therefore urgently needed. Development of BiFeO3 thin films is summarized with a specific focus on the chemical solution route. By a systematic analysis of the recent progress in chemical‐route‐derived BiFeO3 thin films, the challenges of these films are highlighted. An all‐solution chemical‐solution deposition (AS‐CSD) for BiFeO3 thin films with different orientation epitaxial on various oxide bottom electrodes is introduced and a comprehensive study of the growth, structure, and ferroelectric properties of these films is provided. A facile low‐cost route to prepare large‐area high‐quality epitaxial BFO thin films with a comprehensive understanding of the film thickness, stoichiometry, crystal orientation, ferroelectric properties, and bottom electrode effects on evolutions of microstructures is provided. This work paves the way for the fabrication of devices based on BiFeO3 thin films.

Surface Modification and Enhancement of Ferromagnetism in BiFeO3 Nanofilms Deposited on HOPG

“…Figure 3 a shows the image of the etched area by detecting Bi + and Fe + ions, the arrows show the places where these elements were mixed prior to the BFO phase: it is interesting that this phase was mainly formed on the surface of more exfoliated film sections, probably associated with oxidative reactions with the partial emission of carbon oxides. In the near-surface region, where the amount of Bi - prevails (green areas of Figure 3 ), the structure of Bi 1+x FeO 3 (x > 1) is formed [ 44 ]. There is a number of formal oxidation states displayed by bismuth in its oxides, in particular 3 + (Bi 2 O 3 ) and 5 + (Bi 2 O 5 ) (and mixed valence states 3/5 + (Bi 2 O 4 , Bi 4 O 7 )).…”

Chemical Solution Deposition of BiFeO3 Films with Layer-by-Layer Control of the Coverage and Composition

“…PLD and RF sputtering were demonstrated to produce high-quality epitaxial BFO films with a small concentration of the structural defects and low leakage current [10,12]. On the other hand, chemical solution deposition (CSD) is of great interest because it is more suitable commercially, cheaper and makes it possible to cover large-scale wafers [14,15]. Nevertheless, CSD films are not very reproducible in their ferroelectric properties and possess a high value of leakage current impeding their further implementation in the devices [14].…”

“…Unlike PLD and RF sputtering deposition techniques, the CSD pathway is realized in the three-step annealing procedure: gelation step at lower temperatures (70-300 • C), pyrolysis (300-350 • C), and crystallization step at high temperatures (450-750 • C) [15]. The leakage current in CSD films is significantly higher than in thin films produced by RF sputtering and PLD, which are denser and possess more stoichiometric composition [14]. This difference is caused by the long-time low-temperature treatment that affects the inter-particle diffusion between Fe 2 O 3 and Bi 2 O 3 , resulting in the formation of either pure-phase BiFeO 3 or secondary phases, typically, sillenite-like Bi 25 FeO 39 and mullite-like Bi 2 Fe 4 O 9 [16,17].…”

“…In CSD, the control of the atmosphere and temperature parameters at the crystallization stage allows obtaining BFO films with polarization-electric field loops weakly affected by the leakage current [20,21] and even (001)-, (110)-, and (111)-epitaxially grown films [22]. The epitaxially-grown films reveal higher values of remnant polarization and piezoelectric performance [14]. The last problem of the CSD pathway lies in the micro-structure of the films, which is often porous and non-uniform [23].…”

“…In the sol-gel route of CSD, in order to achieve the films with thicknesses larger than 100 nm, a multi-layer approach is used, when the thickness of the films is controlled by the sequential deposition of the layers via spin coating [14,15]. Since the sol concentration plays a crucial role in its stability, the layer-by-layer deposition is used to avoid agglomeration of the particles in the solution and to achieve thick enough films.…”